Indian Patents. 253321:METHOD AND SYSTEM OF COMMUNICATIONS

Title of Invention	METHOD AND SYSTEM OF COMMUNICATIONS
Abstract	The present invention relates to high data rate communications, and more especially it relates to line of sight, LOS, multiple input multiple output, MIMO, communications links and antenna configuration for LOS MIMO links, particularly radio links and optical wireless links.

Full Text	TECHNICAL FIELD OP THE INVENTION The present invention relates to high data rate communica- tions, and more especially it relates to line of sight, LOS, multiple input multiple output, MIMO, links, such as radio links and optical wireless communications links. For reasons of simplicity elements receiving or emitting elec- tromagnetic fields are referred to as antenna elements as, e.g., light emitters and sensors are direct correspondences in light communications to antenna elements for radio wave communications. BACKGROUND AND DESCRIPTION OP RELATED ART High-speed wireline or fiber optic connections of backbone networks interconnecting nodes of a terrestrial radio ac- cess network are previously known. It is also known to in- terconnect radio base stations with microwave links provid- ing interconnections of moderate data rates. Increased antenna area of prior art microwave link antennas increases signal quality, but also increases irradiated mi- crowave power as does transmission power increases. An in- creased antenna area can be achieved by arranging a plural- ity of smaller area antenna elements in an array. Efficient modulations and signal constellations offer re- lieved power requirement, or improved performance if micro- wave power is maintained, as number of signal points in the signal constellation increases. American Patent Application US2003/0125040 discloses a sys- tem for multiple-input multiple-output (MIMO) communica- tion. A MIMO channel formed by NT transmit antennas and NR receive antennas is decomposed into Nc independent channels also referred to as spatial sub-channels, where Nc based on channel state information. American Patent Application US2002/0039884 reveals a radio communication system with a transmitter having a plurality of transmitter antennas and a receiver having at least one antenna. Thereby a plurality of paths with various charac- teristics are formed between the transmitter antennas and the at least one receiver antenna. Data is assigned one or more categories. Depending on categories and path charac- teristics, the data is mapped to one or more of the trans- mitter's parts and antennas. American Patent Application US2002/0039884 describes a ra- dio communication system with a transmitter having a plu- rality of transmitter antennas and a receiver having at least one antenna. Data tags indicate data importance or other requirements. Data is assigned one or more catego- ries. Depending on categories and path characteristics, the data is mapped to one or more of the transmitter's parts and antennas. 3rd Generation Partnership Project (3GPP): Technical Speci- fication Group Radio Access Network, Physical layer aspects of UTRA High Speed Downlink Packet Access {Release 4), 3G TS 25.848 V4.0.0, France, March 2001, describes MIMO open loop signal processing of MIMO transmitter and receiver in section 6.5. Bell Labs Technical Journal, autumn 1996: G. Foschini, "Layered Space- Time Architecture for Wireless Communica- tion in a Fading Environment When Using Multi-Element An- tennas" shows that under fading conditions with statisti- cally uncorrelated identically distributed propagation channels, the bandwidth constrained channel capacity of a MIMO channel, CMIMO. scales on average as where CSLSD is channel capacity of a SISO channel, and M and N are number of antenna elements at receiver and transmit- ter side, respectively. For a band limited (bandwidth B) AWGN {Additive White Gaussian Noise) channel the SISO chan- nel capacity equals where SNRSISO is the SISO channel signal to noise ratio. Figure 1 schematically illustrates N transmitter antenna elements T1, T2,..., Tw» and M receiver antenna elements «Ri, Rz,.», RM» in MIMO communications. Between the various transmitter and receiver antenna elements there are propa- gation channels «h11, h12,... h1m,-hm»-- The individual propagation channels, that are SISO (Single Input Single Output) channels, form a MIMO channel. C. Schlegel and Z. Bagley, "Efficient Processing- for High- Capacity MIMO Channels" submitted to JSAC, MIMO Systems Special Issue: April 23, 2002 reveals estimation of optimum channel capacity of a MIMO system for a known MIMO-channel described by channel matrix H by means of singular value decomposition, SVD. where U and V are unitary matrices, S is a. resulting diago- nal matrix with singular values in the main diagonal, and VK is a Hermitian transformed matrix V. A. Goldsmith, S. h. Jafar, K. Jindal, S. Vishwanath, "Ca- pacity Limits of MIMO Channels" IKES Journal on Se1. Areas in Coram., Vol. 21, No. 5, June 2003 provides results on ca- pacity gain obtained from multiple antennas in relation to channel information at receiver or transmitter, channel signal-to-noise ratio, and correlation between channel gains of each antenna element. The paper also summarizes results for MIMO broadcast channel, BC, and multiple access channel, MAC, and discusses capacity results for multicell MIMO channels with base station cooperation, the base sta- tions acting as a spatially diverse antenna array. In accordance with Goldsmith et al., the MIMO channel ca- pacity for flat fading channel conditions, in the case of equal number of antenna elements for transmitter and re- ceiver antennas, is assuming uncorrelated channels of the various sending an- tenna elements. P. Kyritsi, "MIMO capacity in free space and above perfect ground: Theory and experimental results" 13th IEEE Interna- tional Symposium an Personal, Indoor and Mobile Radio Com- munications 2002, vol.1, pp. 182- 186, Sept. 2002 studies the capacity potential for propagation in free space over perfect ground. Theoretical predictions are compared with measurements over an empty parking lot with nearly flat surface. None of the cited documents above discloses particular an- tenna configurations related to communications distance with line of sight, LOS, MIMO communications. SUMMARY OF THE INVENTION Next generation radio access networks are expected to be required to support peak user data rates in the order of magnitude of 30 Mbps - 1 Gbps. With a vast amount of base stations, it would be advantageous to interconnect base stations over radio links for flexibly connecting/discon- necting links of a mobile station active set of radio links with the base station as the mobile station moves. Present radio link solutions do not offer sufficient data rates of aggregate user data, as to/from a base station, including a plurality of high rate user data links at rea- sonable power levels for reasonably sized element antenna apertures. Consequently, there is a need of antennas of large aper- tures providing required data rates at reasonable transmis- sion power for reasonably sized element antenna apertures. It is consequently an object of the present invention to achieve an antenna configuration for line of sight communi- cation useful for providing low error rates at moderate transmission power within limits as may be required by due authorities. It is also an object to achieve a system flexible to dif- ferent transmission ranges and wavelength ranges. An object is also to offer high data rates for low trans- mission power levels as regards antenna properties. Another object is to achieve an antenna configuration adapted to particular communications distance and wave- length. Finally, it is an object to relieve the dependency on tan- gible interconnections, such as wire lines or optical fi- bers, for interconnection of base stations or other nodes of a telecommunications system. Such interconnections are generally associated with great initial investment costs and maintenance costs. These objects are met by a method and system of antennas configured for a particular communications distance over line of sight links providing multiple input multiple out- put communications links. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically illustrates N transmitter antenna elements and M receiver antenna elements in MIMO communica- tions. Figure 2 schematically illustrates a spherical wave-front background to the invention. Figure 3 illustrates example capacity versus SNRSISO for four-element LOS MIMO linear array, according to the inven- tion, and four-element linear array non-LOS MIMO. Figure 4 illustrates a square grid LOS MIMO antenna array, according to the invention. Figure 5 illustrates an example rectangular array with four rows and three columns of elements according to the invention. Figure 6 shows a linear LOS MIMO antenna array, according to the invention. Figure 7 illustrates an equilateral triangular antenna realization according to an embodiment of the invention. Figure 8 depicts a spatially oversampled antenna array, according to the invention. Figure 9 demonstrates hexagonal antenna element packing, according to the invention. Figure 10 has an antenna array with circular element pack- ing, according to the invention. Figure 11 displays a clustered directional hybrid with eight groups of clustered antenna elements for eight chan- nels MIMO, according to the invention. Figure 12 shows a clustered directional hybrid with four groups of clustered antenna elements for four channels MIMO, according to the invention. Figure 13 illustrates a clustered directional hybrid with two groups of clustered antenna elements for two channels MIMO, according to the invention. Figure 14 comprises plotted capacity per bandwidth vs. nor- malized SNR for MIMO communications with square grid LOS MIMO antennas for various levels of clustering at both re- ceiver and transmitter side, according to the invention. Figure 15 shows an LOS MIMO antenna array with director elements, according to the invention. Figure 16 depicts schematically an LOS MIMO antenna with a grid of interconnected rods or tensed wires to which the antenna elements are attached, according to the invention. Figure 17 illustrates a two-layer square grid LOS MIMO an- tenna with two layers of antenna elements each on a square grid. Figure 18 illustrates a realization according to the inven- tion with equal distances between all nearest neighboring antenna elements, the antenna elements being positioned to the vertices of a cube. Figure 19 illustrates a realization according to the inven- tion with equal distances between all nearest neighboring antenna elements, the antenna elements being positioned to the vertices of a tetrahedron. DESCRIPTION OF PREFERRED EMBODIMENTS In backbone networks based on wireless communications it is important to achieve capacity to handle data rates of ag- gregate traffic, where individual peak user data rates are in the order of 100 Mbps to 1 Gbps. Fixed fiber optical networks are not always applicable. They are often associated with great costs, provide little or no flexibility and occupy extensive ground space. Prior art Multiple Input Multiple Output, MIMO, communica- tions systems most commonly are designed to utilize scat- tering and, therefore, requires a scattering environment. The present invention is not dependent on such scatterers and suits line of sight communication very well. A theo- retical reason for this is its exploitation of spherical wave fronts and associated phase differences. Figure 2 schematically illustrates propagation paths and principles of the invention. Respective propagation paths «P11», «p12», P13 between a transmitter antenna «T1» and receiver antennas «R1», «R2», «R3» differ slightly in length due to a spherical wave front property of the transmitted signal. The small differences in path lengths «δ11», «δ12», «δ13» add to the communications distance D. With path pij as a reference for the communications distance, δij equals zero. I.e. when p11 is selected as reference then δ11-0. The an- tenna configuration according to the invention essentially maximizes MEMO channel capacity for great signal to noise ratios, SNR, in respect of the spherical wave front prop- erty for LOS communications. This is in contrast to, e.g., maximizing antenna directivity as illustrated in and ex- plained in relation to figure 14 below. With each MIMO sub-channel operating close to its maximum theoretical performance, according to the configuration of the invention, great performance gains are achieved. Figure 3 illustrates example capacity versus SNRSISO for LOS MIMO and non-LOS MIMO (fading uncorrelated channels) for a four-element linear array. For the comparison illustrated in figure 3, non-LOS MIMO array antenna elements are as- sumed to be placed such that the antenna elements experi- ence channels with no or negligible cross correlation. In a typical local scattering environment, this is achieved by placing the antenna elements separated by half a wave- length. The illustrated capacity of LOS MIMO is achieved for a system according to the invention. The gain of LOS MIMO as compared to non-LOS MIMO in terms of capacity in- crease or SNR gain is the vertical or horizontal difference between the curves, respectively. The SNR gain implies, e.g., increased noise immunity or reduced transmission power requirement. Radio Access Networks, RAM's, are frequently realized with base stations connected in series, with at least one of the serialized base stations serving as an anchor to the core network. Consequently, the individual links between base stations may carry data traffic of a plurality of base sta- tions. With individual peak user data-rates in the range of 100 Mbps - 1 Gbps required peak rates of data links be- tween base stations could be expected to be in the range of 1-100'6bps. Prior art radio data links is not known to provide data rates of more than one Gbps for the spectrum efficiency achieved with the invention. The major two reasons for this are that there are practical limits on signal constel- lation sizes, practical and regulatory constraints on available radio spectrum, and power limits. Prior art relies upon uncorrelated channels between the various antenna elements. This could e.g. be the case for channels fading due to scattering. The presumption, how- ever, normally does not hold for LOS communications over wireless links, such as e.g. radio links. However, the in- vention points out that exploitation of the spherical prop- erty of wave fronts results in ideal MIMO gain in absence of scatterers. According to the invention rectangular or square grid LOS MIMO antenna array and linear LOS MIMO an- tenna arrays are preferred, see figures 4 and 6 respec- tively. This does not exclude circular or hexagonal pack- ing as a means to increase antenna elements surface density as illustrated in figures 9 and 10 respectively. In the hexagonal packing of figure 9 the respective distances be- tween (at most 6) nearest neighboring antenna elements «An- tenna element» are all essentially equal «d». Spatially oversampled and clustered antenna arrays, see figure 8 and 11 respectively, are preferred for some situations. Fig- ures 12 and 13 show some other clustered directional hy- brids for 16 antenna elements «Antenna element». With reference to figures 11-13, while the total number of antenna elements, N, are kept constant equal to 16 elements the respective number of groups of elements, k {1 the figures varies. In figure 11 there are eight groups with two antenna elements «Antenna element» each. Within each group the antenna elements ^Antenna element* are posi- tioned sufficiently close for signals to add coherently in phase, thereby generating a directivity gain. Figure 13 illustrates an example realization with four groups, each of four antenna elements «Antenna element >. In figure 13, an example for N=16 and k=2 is illustrated. In the figures each group of antenna elements «Antenna element > generates a MIMO sub-channel. With N/k antenna elements for each MIMO sub-channel on receiver and transmitter side, the total achievable gain is (N/k)2, since both sides contribute to the gain. If equivalent isotropic radiated power, EIRP, is at its maximum level allowed, the gain at transmitter side is achieved as a reduction of transmit power and not in in- creased received power or energy per symbol. Assuming an SNR gain of (N/k)2 fox grouped directional antennas with k groups, equations (1) and (2) transform into There are SNR ranges where MIMO communications with clus- tered elements antennas outperform MIMO with the same num- ber of antenna elements, not being clustered. As noted in figure 14 a channel capacity increase is achieved with clustering particularly for poor transmission conditions (small SNR). Figure 14, plots the channel capacity per bandwidth Ccluatered /B for MIMO communications with clustered antenna elements versus SNR «SNRSISO» normalized to SISO com- munications conditions, and where k is the number of clus- ters of antenna elements at transmitter and receiver ends, ke[l,N]. The figure illustrates performance for an example of 16 antenna elements according to equation (5), with SISO performance of N=1 antenna element antennas included for reference. Typically high SNR conditions prevail in short range commu- nications . Consequently, gain increase by unclustered MIMO communications with great number of antenna elements is preferred for short-range communications. For high SNR, the MIMO channel capacity in (4) is approxi- mate to where f is a monotonically increasing function of one vari- able and \|.\| denotes absolute value. (Equations (4) and (6) turn out to be maximized by the same maximizing channel ma- trix, H=Hopt.) The inventors observe that the channel ma- trix H can be separated into a Xronecker product of two ma- trices, Hv and Hh. where Hv is of dimension NVXNV and Hy is of dimension NhxNh, Nv being the number of vertical antenna elements and Nh be- ing the number of horizontal antenna elements. The deter- minant in equation (6) then rewrites A further observation according to the invention is that each of Hv and Hh can be separated where the determinants and that the matrices Hv12 and HH12 are Vandermonde matrices. In a final step of observing it is noted that In equations (13). and (14) , the maximum is attained for vertical and horizontal distances dv and dh, respectively, For a generalized rectangular grid array with Nh elements in each row and Ny elements in each column, communicating at a frequency corresponding to wavelength λ over a communica- tions distance D, the optimum antenna elements distances in equation (15) and (16) converts to antenna dimensions equal to Figure 5 illustrates an example rectangular array with four rows and three columns of elements, each row comprising an- tenna elements separated a distance dh, and each column com- prising antenna elements separated distance dv. According to the invention the preferred antenna element distances are determined in accordance with equations (15) and (16) . The dimension (WidthXHeight) of the antenna array is then In figure 6, for an optimum MIMO system and for a communi- cations distance D much greater' than element separation d the distance a=d(N-1) is specified by where the approximation in equation (20) holds for great number of antenna elements N. For N=16 antenna elements «Antenna element>, the approximation error is about 7%. Table 1 illustrates element separation, d, of a transmitter- receiver pair of linear MIMO antennas versus communications distance, D, at some example wavelengths, X, equal to 3 mm, 7.9 mm and 42 .9 mm. For the square grid LOS MIMO antenna array in figure 4, the distance a, corresponding to that of equation (19) for lin- ear arrays, is determined to where the approximation in equation (22) holds for great number of antenna elements N. For N=16 antenna elements «Antenna element>, the approximation error is about 33%. An important observation is that for the square grid LOS MIMO antenna array in figure 4 the distances a and d get relatively smaller in proportion to the fourth root of N, whereas for the linear array of figure 6 the distance de- pendency is proportional to the square root of N. With the antenna area A=a2, and using the approximation in equation (22), the MIMO channel capacity, CMiIMO=N.CSISO, ex- pressed in terms of channel capacity for a SISO system, CSISO, with the example design of figure 4 according to the invention is In figure 4 and equation (21) the antenna elements «Antenna elements are assumed to be electrically active elements, supplying a voltage or current to a receiver. However, as illustrated below basically the same distance relations hold for antenna elements being directors guiding received electromagnetic field to electrically active antenna ele- ments . Figure 7 illustrates an equilateral triangular antenna realization according to an embodiment of the invention. The antenna elements are all separated by dtd. Similarly to the rectangular realization in figure 5, the optimum an- tenna element separation of the equilateral triangular an- tenna structure with three antenna elements equals where D is communication distance and λ is communication wavelength. Figure 15 illustrates a realization with director elements «Director»- direct electromagnetic fields received and elec- tromagnetic fields to be transmitted, preferably with one director per electrically active antenna element «Active elements >. Preferably the directors «Director» are pure reflectors but can also be made of dielectric material. The supports «Supports» are designed not to shadow, or only have a small shadowing impact on, the electrically active antenna elements «Active elements>. The positioning of the directors is preferably in accordance with equation (19) and (21) for a linear and square grid LOS MIMO antenna re- spectively. The relevant distance d is essentially equal to the separation distance of the projection of the directors onto a plane, the plane being perpendicular to the LOS transmission path to the other receiver/transmitter end. Advantages achieved by the realization of figure 15 in addition to those mentioned above are, e.g., simplified wiring of the antenna elements and the antenna elements spanning a smaller distance range thereby being mechani- cally robust. Also, by adjusting the directors the elec- trically active antenna elements need not always be reposi- tioned even if communications distance changes. The dependency of α, A and dv/dh/d on D for an LOS MIMO an- tenna has practical implications, addressed by the inven- tion. An obvious solution to the problem of getting a, to the communications distance D, appropriately matched ele- ment distance, dv, dh, d, is to manufacture custom-made an- tennas. From a cost perspective, however, a more attrac- tive solution is manufacturing of a set of antenna models for MIMO communications, each designed for a range of com- munications distances D, and upon installation selecting an antenna model within the set that best matches the communi- cations distance. For frequency non-selective channels, SVD (singular value decomposition) provides robustness and close to optimum performance also with non-perfect matching of communications distance, D, and element separation, dv, dh, d. Another embodiment is realized by individually ad- justable antenna elements. Preferably this is realised by a grid «Grid» of interconnected rods or tensed wires to which the antenna elements «Antenna element» are attached as illustrated in figure 16. The wires or rods are pref- erably connected to a frame «Frame». Models that are elec- tromechanically adjustable comprise electromechanical mo- tors to which the rods are connected, such that the rods to which the antenna elements «Antenna element» are attached may move along the frame. A further embodiment of adapting an LOS MIMO antenna to communications distance D uses spa- tial oversampled antennas as schematically illustrated in figure 8 and activating the antenna elements by signal processing providing best performance at actual communica- tions distance. The particular element distribution may be varied, e.g. as illustrated in figures 9 and 10, An impor- tant issue of the invention is that active elements are distributed such that their mutual distances reflects com- munications distance (distance between transmit and receive antennas) and wavelength such that the spherical properties of the radio wave can be exploited. It is observed that as transmitter and receiver antennas form an antenna pair for a communications link, the respec- tive element distances dv, dh and d in e.g. equations (15) and (16) of an example transmitter antenna can be reduced (or increased) if the element distance of a corresponding example receiver antenna of the communications link is in- creased (or reduced) in proportion to the distance reduc- tions (or increase) of the transmitter antenna. Indexing distances of transmitter and receiver antennas by T and R, respectively, if respective element distances of a receiver antenna, dvR., dhR and dR, are reduced (or increased) in rela- tion to an initially determined distance dv, dh or d1 transmitter-side antenna-element distance, dvT, dhT and dr, should be increased (or reduced) in proportion thereto (in relation to dv, dh and d) , Consequently, the distances dv, dh in equations (15) and (16) are the geometrical averages of receiver and transmitter antenna element distances, respec- tively. The actual antenna dimensions in equations (17) and (18), of course, are determined by actual respective vertical and horizontal element distances. Correspondingly, also an- tenna dimensioning in equations (19) and (21) are deter- mined by actual distances, if adjusted as described above. At transmitter side equations (17), (18), (19) and (21) translate to equations (24), (25), (26) and (27) and correspondingly for receiver side, they translate to equations (28), (29), (30) and (31) where The invention does not only cover planar antenna configura- tions, but also three-dimensional configurations as illus- trated in figures 17-19. Figure 17 illustrates a two-layer square grid LOS MIMO antenna with two layers of antenna elements each on a square grid. Figures 18 and 19 illus- trate realisations with equal distance between all nearest neighboring antenna elements. In figure 18 the antenna elements are positioned to the vertices of a cube and in figure 19 the antenna elements are positioned to the verti- ces of a tetrahedron. Various embodiments of the invention also cover different realizations of signal processing at transmitter and re- ceiver ends. The processing is necessary for adaptation to prevalent channel conditions. At receiver or transmit side, determining channel singular values as described in relation to equation (3) and singular value decomposition can be achieved by digital signal processing of base band signals. If determined at transmitter side, information on channel matrix, H, need to be transferred from receiver side, or the channel matrix otherwise estimated at trans- mitter side, see figure. For a 2X2 channel matrix, singu- lar value decomposition can also be achieved by a 3-dB hy- brid to perform multiplication or weighting as need be, op- erating on high-frequency signals. Also, for channel ma- trices greater than 2X2 a generalization of a 3-dB hybrid, a Butler matrix directional coupler, may be used. A fur- ther embodiment realizes the processing by means of an ar- rangement of microstrip or waveguides, also operating on high-frequency signals. At receiver side, channel equali- zation requires processing. This processing can be per- formed by any of the processing realizations described for transmitter side, or received signal can be equalized by means of zero forcing, for which the received signal being multiplied by the inverse matrix of channel matrix H, or by means of minimum mean square error, MMSS, for which the mean square error is minimized, the various processing re- alizations giving rise to further embodiments. If there is multipath propagation, this is preferably in- corporated into the singular value decomposition at trans- mitter side through feedback information. Corresponding information can also be derived through channel reciprocity if the reverse direction channel matrix is determined at transmitter side (the transmitter side also comprising ra- dio receiver). Another solution comprises a self-tuning antenna, optimizing performance at receiver side, transmit- ter side or both. The antenna element positioning is then adapted to channel propagation properties corresponding to a measured channel matrix, H. This can be achieved by, e.g. a stochastic gradient algorithm. Particularly for fixed positioned antenna elements, they may require the an- tenna elements to be re-distributed for optimum perform- ance. For an electromechanically adjustable element an- tenna the optimization can be achieved by automatic posi- tion adjustments of the antenna elements. The different solutions to multipath propagation can also be combined. Preferably and in accordance with the invention, singular value decomposition is applied, to flat (frequency non- selective) fading channels. If a channel nevertheless is frequency-selective fading, the channel can be considered piecewise flat fading for sufficiently small frequency in- tervals. Such piecewise flat fading channels can, e.g., be achieved by dividing a given frequency range or bandwidth, using orthogonal-frequencies sub-carriers of sufficiently narrow one or more bandwidths for the one or more band- widths to be much less than the coherence bandwidth. One technique for achieving such sub-carriers is orthogonal frequency division multiplex, OFDM. The concept of the present invention combines well with other known means to increase throughput, such as transmis- sion at both vertical and horizontal polarization or trans- mission at left-hand and right-hand circular polarization, or different coding of different sub-channels depending on their respective channel quality, which further demon- strates the usefulness of the invention. Such combinations are also within the scope of this invention. Dimensioning has been expressed in relation to particular orientation, e.g. horizontal or vertical orientation, re- ferring to orthogonal directions, perpendicular to the di- rection of communications. However, this does not exclude rotation of receiver and transmitter antennas in a plane parallel to the antenna elements, with corresponding rota- tion of both antennas such that their mutual orientation is preserved. Despite somewhat inappropriate, the notation of vertical and horizontal is kept for reasons of simplicity. The invention is not intended to be limited only to the em- bodiments described in detail above. Changes and modifica- tions may be made without departing from the invention. It covers all modifications within the scope of the following claims. WE CLAIM: 1. A method of configuring an antenna, the method com- prising: constructing the antenna to comprise a plurality of antenna elements; and configuring the antenna for line of sight (LOS) communi- cation such that the separation of the antenna elements is set in relation to √Dλ/N where D is communications distance, λ is communication wavelength and N is num- ber of antenna elements. 2. The method according to claim 1 wherein the antenna configuration maximizes multiple-input multiple-output (MIMO) channel capacity. 3. The method according to claim 1 wherein the antenna is a linear antenna. 4. The method according to claim 1 wherein the antenna is a square grid antenna. 5. The method according to claim 4 wherein N=n2 for n an integer greater than 1. 6. The method according to claim 1 wherein the antenna is a rectangular grid antenna. 7. The method according to claim 6 wherein the dimension of separation is horizontal dimension. 8. The method according to claim 6 wherein the dimension of separation is vertical dimension. 9. The method according to claim 1 wherein for a triangular grid antenna with three antenna elements, the separation of the antenna elements is set in relation to √Dλ/3, where D is communications distance and λ is communication wave- length. 10. A method of configuring an antenna, the method com- prising: constructing the antenna to comprise a plurality of clusters of antenna elements; and configuring the antenna such that the plurality of clusters of antenna elements are separated by a distance set in rela- tion to communications distance and the antenna con- figuration is three-dimensional. 11. The method according to claim 10 wherein the antenna is configured such that the plurality of clusters of antenna elements are separated by a distance set in relation to com- munication wavelength. 12. The method according to claim 10 wherein for a linear antenna the plurality of clusters of antenna elements are sepa- rated by a distance set in relation to √DΛ/L where D is com- munications distance, λ is communication wavelength and L is number of clusters. 13. The method according to claim 10 wherein for a square grid antenna the plurality of clusters of antenna elements are separated by a distance set in relation to where D is communications distance, λ is communication wavelength and L is number of clusters. 14. The method according to claim 13 wherein L=I2 for I an integer greater than 1. 15. The method according to claim 10 wherein the antenna elements within a cluster are separated by a distance smaller than the smallest distance between clusters. 16. The method according to claim 10 wherein the antenna configuration comprises two layers, where each layer com- prises a planar arrangement of antenna elements on a square grid. 17. The method according to claim 10 wherein the antenna configuration comprises antenna elements positioned equi- distant in a three-dimensional space. 18. The method according to claim 15 wherein the antenna elements are positioned to vertices of a cube. 19. The method according to claim 15 wherein the antenna elements are positioned to vertices of a tetrahedron. 20. The method according to claim 10 wherein the antenna elements are fed with signals processed according to singular value decomposition for a transmission channel over the communications distance. 21. The method according to claim 20 wherein the trans- mission channel considered is a flat fading sub-carrier. 22. The method according to claim 20 wherein the trans- mission channel considered is an OFDM sub-carrier. 23. The method according to claim 10 wherein the signals received from the antenna elements are processed according to zero forcing for a transmission channel over the commu- nications distance. 24. The method according to claim 10 wherein the signals received from the antenna elements are processed to mini- mize mean square error for a transmission channel over the communications distance. 25. The method according to claim 10 wherein signal pro- cessing of signals received or to be transmitted is performed at high-frequency. 26. The method according to claim 25 wherein the process- ing is performed by one or more 3-dB hybrids. 27. The method according to claim 25 wherein the process- ing is performed by one or more Butler matrix directional couplers. 28. The method according to claim 25 wherein the process- ing is performed by an arrangement of microstrip. 29. The method according to claim 25 wherein the process- ing is performed by an arrangement of waveguides. 30. The method according to claim 29 wherein the antenna configuration is a radio antenna configuration. 31. The method according to claim 29 wherein the antenna configuration is a configuration of sensors or actuators for optical communications. 32. An antenna configuration comprising: a plurality of antenna elements, wherein the plurality of antenna elements is configured for line of sight (LOS) communication such that separation of the antenna elements is set in relation to √Dλ/N where D is communications distance, λ is communication wavelength and N is number of antenna elements. 33. The antenna configuration according to claim 32 wherein the antenna configuration maximizes MIMO chan- nel capacity. 34. The antenna configuration according to claim 32 wherein the antenna configuration is a linear antenna configu- ration. 35. The antenna configuration according to claim 32 wherein the antenna configuration is a square grid antenna configuration. 36. The antenna configuration according to claim 35 wherein N=n2 for n an integer greater than 1. 37. The antenna according to claim 32 wherein the antenna elements separation is set in relation to √Dλ/N where D is communications distance, λ is communication wavelength and N is number of antenna elements in dimension of sepa- ration, for a rectangular grid antenna. 38. The antenna according to claim 37 wherein the dimen- sion of separation is a horizontal dimension. 39. The antenna according to claim 37 wherein the dimen- sion of separation is a vertical dimension. 40. The antenna according to claim 32 wherein the antenna elements separation is set in relation to √Dλ/3, where D is communications distance and λ is communication wave- length, a triangular grid antenna with three antenna elements. 41. The antenna configuration according to claim 32, the antenna configuration being three-dimensional. 42. The antenna configuration according to claim 32, the antenna configuration comprising two layers, where each layer comprises a planar arrangement of antenna elements on a square grid. 43. The antenna configuration according to claim 32, the antenna configuration comprising antenna elements posi- tioned equidistant in a three-dimensional space. 44. The antenna configuration according to claim 43, the antenna elements being positioned to vertices of a cube. 45. The method according to claim 43, the antenna ele- ments being positioned to vertices of a tetrahedron. 46. An antenna configuration comprising: a plurality of clusters of antenna elements wherein the antenna elements are configured such that the plurality of clusters of antenna elements are separated by a dis- tance set in relation to communications distance, com- munication wavelength and number of antenna ele- ments, constructing the antenna to comprise a plurality of antenna elements; and configuring the antenna for line of sight (LOS) communi- cation such that the separation of the antenna elements is set in relation to √Dλ/N where D is communications distance, λ is communication wavelength and N is num- ber of antenna elements. 47. The antenna configuration according to claim 46, the plurality of clusters of antenna elements being separated by a distance set in relation to √Dλ/L, where D is communications distance, λ is communication wavelength and L is number of clusters, and wherein the antenna configuration is a linear antenna configuration. 48. The antenna configuration according to claim 46, the plurality of clusters of antenna elements being separated by a distance set in relation to where D is communica- tions distance, λ is communication wavelength and L is num- ber of clusters and wherein the antenna configuration is a square grid antenna configuration. 49. The antenna configuration according to claim 48, L=I2 for I an integer greater than 1. 50. The antenna configuration according to claim 46, the antenna elements within a cluster are separated by a distance smaller than the smallest distance between the plurality of clusters of antenna elements. 51. The antenna configuration according to claim 46, one or more antenna element feeders being adapted to feed the antenna elements with signals processed according to singu- lar value decomposition for a transmission channel over the communications distance. 52. The antenna configuration according to claim 51, wherein the transmission channel considered is a flat fading sub-carrier. 53. The antenna configuration according to claim 51, wherein the transmission channel considered is an OFDM sub-carrier. 54. The antenna configuration according to claim 46, wherein one or more processing elements are adapted to process signals received from the antenna elements according to zero forcing for a transmission channel over the commu- nications distance. 55. The antenna configuration according to claim 46, wherein one or more processing elements are adapted to process signals received from the antenna elements to mini- mize mean square error for a transmission channel over the communications distance. 56. The antenna configuration according to claim 46, wherein one or more processing elements are adapted to process at high-frequency signals received or to be transmit- ted. 57. The antenna configuration according to claim 56, the one or more processing elements being one or more 3-dB hybrids. 58. The antenna configuration according to claim 56, the one or more processing elements being one or more Butler matrix directional couplers. 59. The antenna configuration according to claim 56, the one or more processing elements being an arrangement of microstrip. 60. The antenna configuration according to claim 56, the one or more processing elements being an arrangement of waveguides. 61. The antenna configuration according to claim 46, the antenna elements being electrically active elements. 62. The antenna configuration according to claim 46, the antenna elements being directors. 63. The antenna configuration according to claim 62, the directors being reflectors. 64. The antenna configuration according to claim 46, the antenna elements being arranged symmetrically in a circular pattern. 65. The antenna configuration according to claim 46, the antenna elements being arranged in a hexagonal pattern. 66. The antenna configuration according to claim 46, the antenna elements being mounted on position adjustable rods or wires. 67. The antenna configuration according to claim 66, the position adjustable rods or wires being electromechanically adjustable. 68. The antenna configuration according to claim 67 wherein the adjustable position is adaptive to propagation channel properties corresponding to a measured channel matrix. 69. The antenna configuration according to claim 46, the antenna configuration being adapted to a predetermined range of communications distances. 70. An antenna configuration, the antenna configuration comprising a plurality of antenna elements, of which a subset forms an active set of antenna elements, the active antenna elements forming an antenna configuration that is configured for line of sight (LOS) communication such that separation of the antenna elements is set in relation to communications distance, communication wavelength and number of antenna elements. 71. The antenna configuration according to claim 70, wherein the antenna configuration is a radio antenna configu- ration. 72. The antenna configuration according to claim 70, wherein the antenna configuration is a configuration of sen- sors or actuators for optical communications. 73. A. communications system comprising: an antenna having a plurality of antenna elements, wherein the antenna is configured for line of sight (LOS) com- munication such that separation of the antenna elements are set in relation to √Dλ/N where D is communications distance, λ is communication wavelength and N is num- ber of antenna elements. 74. The communications system according to claim 73 wherein the separation of antenna elements are set different for a first and a second antenna, the two antennas operating in pair, such that the geometrical average of the elements dis- tance of the first, antenna, d1 and the elements distance of the second antenna, d2, is the effective antenna elements distance. 75. The communications system according to claim 73, wherein the antenna is one of a linear antenna, a square grid antenna and a rectangular grid antenna and the separation of the antenna elements is set in relation to √Dλ/N where D is communications distance, λ is communication wavelength and TNT is number of antenna elements. 76. The communications system according to claim 75 wherein N=n2 for n an integer greater than 1. ABSTRACT METHOD AND SYSTEM OF COMMUNICATIONS The present invention relates to high data rate communications, and more especially it relates to line of sight, LOS, multiple input multiple output, MIMO, communications links and antenna configuration for LOS MIMO links, particularly radio links and optical wireless links.

Full Text

TECHNICAL FIELD OP THE INVENTION
The present invention relates to high data rate communica-
tions, and more especially it relates to line of sight,
LOS, multiple input multiple output, MIMO, links, such as
radio links and optical wireless communications links. For
reasons of simplicity elements receiving or emitting elec-
tromagnetic fields are referred to as antenna elements as,
e.g., light emitters and sensors are direct correspondences
in light communications to antenna elements for radio wave
communications.
BACKGROUND AND DESCRIPTION OP RELATED ART
High-speed wireline or fiber optic connections of backbone
networks interconnecting nodes of a terrestrial radio ac-
cess network are previously known. It is also known to in-
terconnect radio base stations with microwave links provid-
ing interconnections of moderate data rates.
Increased antenna area of prior art microwave link antennas
increases signal quality, but also increases irradiated mi-
crowave power as does transmission power increases. An in-
creased antenna area can be achieved by arranging a plural-
ity of smaller area antenna elements in an array.
Efficient modulations and signal constellations offer re-
lieved power requirement, or improved performance if micro-
wave power is maintained, as number of signal points in the
signal constellation increases.
American Patent Application US2003/0125040 discloses a sys-
tem for multiple-input multiple-output (MIMO) communica-
tion. A MIMO channel formed by NT transmit antennas and NR
receive antennas is decomposed into Nc independent channels

also referred to as spatial sub-channels, where
Nc based on channel state information.
American Patent Application US2002/0039884 reveals a radio
communication system with a transmitter having a plurality
of transmitter antennas and a receiver having at least one
antenna. Thereby a plurality of paths with various charac-
teristics are formed between the transmitter antennas and
the at least one receiver antenna. Data is assigned one or
more categories. Depending on categories and path charac-
teristics, the data is mapped to one or more of the trans-
mitter's parts and antennas.
American Patent Application US2002/0039884 describes a ra-
dio communication system with a transmitter having a plu-
rality of transmitter antennas and a receiver having at
least one antenna. Data tags indicate data importance or
other requirements. Data is assigned one or more catego-
ries. Depending on categories and path characteristics,
the data is mapped to one or more of the transmitter's
parts and antennas.
3rd Generation Partnership Project (3GPP): Technical Speci-
fication Group Radio Access Network, Physical layer aspects
of UTRA High Speed Downlink Packet Access {Release 4), 3G
TS 25.848 V4.0.0, France, March 2001, describes MIMO open
loop signal processing of MIMO transmitter and receiver in
section 6.5.
Bell Labs Technical Journal, autumn 1996: G. Foschini,
"Layered Space- Time Architecture for Wireless Communica-
tion in a Fading Environment When Using Multi-Element An-
tennas" shows that under fading conditions with statisti-
cally uncorrelated identically distributed propagation

channels, the bandwidth constrained channel capacity of a
MIMO channel, CMIMO. scales on average as

where CSLSD is channel capacity of a SISO channel, and M and
N are number of antenna elements at receiver and transmit-
ter side, respectively. For a band limited (bandwidth B)
AWGN {Additive White Gaussian Noise) channel the SISO chan-
nel capacity equals

where SNRSISO is the SISO channel signal to noise ratio.
Figure 1 schematically illustrates N transmitter antenna
elements T1, T2,..., Tw» and M receiver antenna elements «Ri,
Rz,.», RM» in MIMO communications. Between the various
transmitter and receiver antenna elements there are propa-
gation channels «h11, h12,... h1m,-hm»--
The individual propagation channels, that are SISO (Single
Input Single Output) channels, form a MIMO channel.
C. Schlegel and Z. Bagley, "Efficient Processing- for High-
Capacity MIMO Channels" submitted to JSAC, MIMO Systems
Special Issue: April 23, 2002 reveals estimation of optimum
channel capacity of a MIMO system for a known MIMO-channel
described by channel matrix H by means of singular value
decomposition, SVD.

where U and V are unitary matrices, S is a. resulting diago-
nal matrix with singular values in the main diagonal, and
VK is a Hermitian transformed matrix V.

A. Goldsmith, S. h. Jafar, K. Jindal, S. Vishwanath, "Ca-
pacity Limits of MIMO Channels" IKES Journal on Se1. Areas
in Coram., Vol. 21, No. 5, June 2003 provides results on ca-
pacity gain obtained from multiple antennas in relation to
channel information at receiver or transmitter, channel
signal-to-noise ratio, and correlation between channel
gains of each antenna element. The paper also summarizes
results for MIMO broadcast channel, BC, and multiple access
channel, MAC, and discusses capacity results for multicell
MIMO channels with base station cooperation, the base sta-
tions acting as a spatially diverse antenna array.
In accordance with Goldsmith et al., the MIMO channel ca-
pacity for flat fading channel conditions, in the case of
equal number of antenna elements for transmitter and re-
ceiver antennas, is

assuming uncorrelated channels of the various sending an-
tenna elements.
P. Kyritsi, "MIMO capacity in free space and above perfect
ground: Theory and experimental results" 13th IEEE Interna-
tional Symposium an Personal, Indoor and Mobile Radio Com-
munications 2002, vol.1, pp. 182- 186, Sept. 2002 studies
the capacity potential for propagation in free space over
perfect ground. Theoretical predictions are compared with
measurements over an empty parking lot with nearly flat
surface.
None of the cited documents above discloses particular an-
tenna configurations related to communications distance
with line of sight, LOS, MIMO communications.

SUMMARY OF THE INVENTION
Next generation radio access networks are expected to be
required to support peak user data rates in the order of
magnitude of 30 Mbps - 1 Gbps. With a vast amount of base
stations, it would be advantageous to interconnect base
stations over radio links for flexibly connecting/discon-
necting links of a mobile station active set of radio links
with the base station as the mobile station moves.
Present radio link solutions do not offer sufficient data
rates of aggregate user data, as to/from a base station,
including a plurality of high rate user data links at rea-
sonable power levels for reasonably sized element antenna
apertures.
Consequently, there is a need of antennas of large aper-
tures providing required data rates at reasonable transmis-
sion power for reasonably sized element antenna apertures.
It is consequently an object of the present invention to
achieve an antenna configuration for line of sight communi-
cation useful for providing low error rates at moderate
transmission power within limits as may be required by due
authorities.
It is also an object to achieve a system flexible to dif-
ferent transmission ranges and wavelength ranges.
An object is also to offer high data rates for low trans-
mission power levels as regards antenna properties.
Another object is to achieve an antenna configuration
adapted to particular communications distance and wave-
length.

Finally, it is an object to relieve the dependency on tan-
gible interconnections, such as wire lines or optical fi-
bers, for interconnection of base stations or other nodes
of a telecommunications system. Such interconnections are
generally associated with great initial investment costs
and maintenance costs.
These objects are met by a method and system of antennas
configured for a particular communications distance over
line of sight links providing multiple input multiple out-
put communications links.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 schematically illustrates N transmitter antenna
elements and M receiver antenna elements in MIMO communica-
tions.
Figure 2 schematically illustrates a spherical wave-front
background to the invention.
Figure 3 illustrates example capacity versus SNRSISO for
four-element LOS MIMO linear array, according to the inven-
tion, and four-element linear array non-LOS MIMO.
Figure 4 illustrates a square grid LOS MIMO antenna array,
according to the invention.
Figure 5 illustrates an example rectangular array with
four rows and three columns of elements according to the
invention.
Figure 6 shows a linear LOS MIMO antenna array, according
to the invention.
Figure 7 illustrates an equilateral triangular antenna
realization according to an embodiment of the invention.

Figure 8 depicts a spatially oversampled antenna array,
according to the invention.
Figure 9 demonstrates hexagonal antenna element packing,
according to the invention.
Figure 10 has an antenna array with circular element pack-
ing, according to the invention.
Figure 11 displays a clustered directional hybrid with
eight groups of clustered antenna elements for eight chan-
nels MIMO, according to the invention.
Figure 12 shows a clustered directional hybrid with four
groups of clustered antenna elements for four channels
MIMO, according to the invention.
Figure 13 illustrates a clustered directional hybrid with
two groups of clustered antenna elements for two channels
MIMO, according to the invention.
Figure 14 comprises plotted capacity per bandwidth vs. nor-
malized SNR for MIMO communications with square grid LOS
MIMO antennas for various levels of clustering at both re-
ceiver and transmitter side, according to the invention.
Figure 15 shows an LOS MIMO antenna array with director
elements, according to the invention.
Figure 16 depicts schematically an LOS MIMO antenna with a
grid of interconnected rods or tensed wires to which the
antenna elements are attached, according to the invention.
Figure 17 illustrates a two-layer square grid LOS MIMO an-
tenna with two layers of antenna elements each on a square
grid.

Figure 18 illustrates a realization according to the inven-
tion with equal distances between all nearest neighboring
antenna elements, the antenna elements being positioned to
the vertices of a cube.
Figure 19 illustrates a realization according to the inven-
tion with equal distances between all nearest neighboring
antenna elements, the antenna elements being positioned to
the vertices of a tetrahedron.
DESCRIPTION OF PREFERRED EMBODIMENTS
In backbone networks based on wireless communications it is
important to achieve capacity to handle data rates of ag-
gregate traffic, where individual peak user data rates are
in the order of 100 Mbps to 1 Gbps.
Fixed fiber optical networks are not always applicable.
They are often associated with great costs, provide little
or no flexibility and occupy extensive ground space.
Prior art Multiple Input Multiple Output, MIMO, communica-
tions systems most commonly are designed to utilize scat-
tering and, therefore, requires a scattering environment.
The present invention is not dependent on such scatterers
and suits line of sight communication very well. A theo-
retical reason for this is its exploitation of spherical
wave fronts and associated phase differences.
Figure 2 schematically illustrates propagation paths and
principles of the invention. Respective propagation paths
«P11», «p12», P13 between a transmitter antenna «T1» and
receiver antennas «R1», «R2», «R3» differ slightly in length
due to a spherical wave front property of the transmitted
signal. The small differences in path lengths «δ11», «δ12»,
«δ13» add to the communications distance D. With path pij as

a reference for the communications distance, δij equals zero.
I.e. when p11 is selected as reference then δ11-0. The an-
tenna configuration according to the invention essentially
maximizes MEMO channel capacity for great signal to noise
ratios, SNR, in respect of the spherical wave front prop-
erty for LOS communications. This is in contrast to, e.g.,
maximizing antenna directivity as illustrated in and ex-
plained in relation to figure 14 below.
With each MIMO sub-channel operating close to its maximum
theoretical performance, according to the configuration of
the invention, great performance gains are achieved.
Figure 3 illustrates example capacity versus SNRSISO for LOS
MIMO and non-LOS MIMO (fading uncorrelated channels) for a
four-element linear array. For the comparison illustrated
in figure 3, non-LOS MIMO array antenna elements are as-
sumed to be placed such that the antenna elements experi-
ence channels with no or negligible cross correlation. In
a typical local scattering environment, this is achieved by
placing the antenna elements separated by half a wave-
length. The illustrated capacity of LOS MIMO is achieved
for a system according to the invention. The gain of LOS
MIMO as compared to non-LOS MIMO in terms of capacity in-
crease or SNR gain is the vertical or horizontal difference
between the curves, respectively. The SNR gain implies,
e.g., increased noise immunity or reduced transmission
power requirement.
Radio Access Networks, RAM's, are frequently realized with
base stations connected in series, with at least one of the
serialized base stations serving as an anchor to the core
network. Consequently, the individual links between base
stations may carry data traffic of a plurality of base sta-
tions. With individual peak user data-rates in the range

of 100 Mbps - 1 Gbps required peak rates of data links be-
tween base stations could be expected to be in the range of
1-100'6bps.
Prior art radio data links is not known to provide data
rates of more than one Gbps for the spectrum efficiency
achieved with the invention. The major two reasons for
this are that there are practical limits on signal constel-
lation sizes, practical and regulatory constraints on
available radio spectrum, and power limits.
Prior art relies upon uncorrelated channels between the
various antenna elements. This could e.g. be the case for
channels fading due to scattering. The presumption, how-
ever, normally does not hold for LOS communications over
wireless links, such as e.g. radio links. However, the in-
vention points out that exploitation of the spherical prop-
erty of wave fronts results in ideal MIMO gain in absence
of scatterers. According to the invention rectangular or
square grid LOS MIMO antenna array and linear LOS MIMO an-
tenna arrays are preferred, see figures 4 and 6 respec-
tively. This does not exclude circular or hexagonal pack-
ing as a means to increase antenna elements surface density
as illustrated in figures 9 and 10 respectively. In the
hexagonal packing of figure 9 the respective distances be-
tween (at most 6) nearest neighboring antenna elements «An-
tenna element» are all essentially equal «d». Spatially
oversampled and clustered antenna arrays, see figure 8 and
11 respectively, are preferred for some situations. Fig-
ures 12 and 13 show some other clustered directional hy-
brids for 16 antenna elements «Antenna element».
With reference to figures 11-13, while the total number of
antenna elements, N, are kept constant equal to 16 elements
the respective number of groups of elements, k {1
the figures varies. In figure 11 there are eight groups
with two antenna elements «Antenna element» each. Within
each group the antenna elements ^Antenna element* are posi-
tioned sufficiently close for signals to add coherently in
phase, thereby generating a directivity gain. Figure 13
illustrates an example realization with four groups, each
of four antenna elements «Antenna element >. In figure 13,
an example for N=16 and k=2 is illustrated. In the figures
each group of antenna elements «Antenna element > generates
a MIMO sub-channel. With N/k antenna elements for each MIMO
sub-channel on receiver and transmitter side, the total
achievable gain is (N/k)2, since both sides contribute to
the gain. If equivalent isotropic radiated power, EIRP, is
at its maximum level allowed, the gain at transmitter side
is achieved as a reduction of transmit power and not in in-
creased received power or energy per symbol. Assuming an
SNR gain of (N/k)2 fox grouped directional antennas with k
groups, equations (1) and (2) transform into

There are SNR ranges where MIMO communications with clus-
tered elements antennas outperform MIMO with the same num-
ber of antenna elements, not being clustered. As noted in
figure 14 a channel capacity increase is achieved with
clustering particularly for poor transmission conditions
(small SNR). Figure 14, plots the channel capacity per
bandwidth Ccluatered /B for MIMO communications with clustered
antenna elements versus SNR «SNRSISO» normalized to SISO com-
munications conditions, and where k is the number of clus-
ters of antenna elements at transmitter and receiver ends,
ke[l,N]. The figure illustrates performance for an example
of 16 antenna elements according to equation (5), with SISO

performance of N=1 antenna element antennas included for
reference.
Typically high SNR conditions prevail in short range commu-
nications . Consequently, gain increase by unclustered MIMO
communications with great number of antenna elements is
preferred for short-range communications.
For high SNR, the MIMO channel capacity in (4) is approxi-
mate to

where f is a monotonically increasing function of one vari-
able and |.| denotes absolute value. (Equations (4) and (6)
turn out to be maximized by the same maximizing channel ma-
trix, H=Hopt.) The inventors observe that the channel ma-
trix H can be separated into a Xronecker product of two ma-
trices, Hv and Hh.

where Hv is of dimension NVXNV and Hy is of dimension NhxNh,
Nv being the number of vertical antenna elements and Nh be-
ing the number of horizontal antenna elements. The deter-
minant in equation (6) then rewrites

A further observation according to the invention is that
each of Hv and Hh can be separated

where the determinants

and that the matrices Hv12 and HH12 are Vandermonde matrices.
In a final step of observing it is noted that

In equations (13). and (14) , the maximum is attained for
vertical and horizontal distances dv and dh, respectively,

For a generalized rectangular grid array with Nh elements in
each row and Ny elements in each column, communicating at a
frequency corresponding to wavelength λ over a communica-
tions distance D, the optimum antenna elements distances in
equation (15) and (16) converts to antenna dimensions equal
to

Figure 5 illustrates an example rectangular array with four
rows and three columns of elements, each row comprising an-
tenna elements separated a distance dh, and each column com-
prising antenna elements separated distance dv. According
to the invention the preferred antenna element distances

are determined in accordance with equations (15) and (16) .
The dimension (WidthXHeight) of the antenna array is then

In figure 6, for an optimum MIMO system and for a communi-
cations distance D much greater' than element separation d
the distance a=d(N-1) is specified by

where the approximation in equation (20) holds for great
number of antenna elements N. For N=16 antenna elements
«Antenna element>, the approximation error is about 7%.
Table 1 illustrates element separation, d, of a transmitter-
receiver pair of linear MIMO antennas versus communications
distance, D, at some example wavelengths, X, equal to 3 mm,
7.9 mm and 42 .9 mm.

For the square grid LOS MIMO antenna array in figure 4, the
distance a, corresponding to that of equation (19) for lin-
ear arrays, is determined to

where the approximation in equation (22) holds for great
number of antenna elements N. For N=16 antenna elements
«Antenna element>, the approximation error is about 33%.
An important observation is that for the square grid LOS
MIMO antenna array in figure 4 the distances a and d get
relatively smaller in proportion to the fourth root of N,
whereas for the linear array of figure 6 the distance de-
pendency is proportional to the square root of N.
With the antenna area A=a2, and using the approximation in
equation (22), the MIMO channel capacity, CMiIMO=N.CSISO, ex-
pressed in terms of channel capacity for a SISO system,
CSISO, with the example design of figure 4 according to the
invention is

In figure 4 and equation (21) the antenna elements «Antenna
elements are assumed to be electrically active elements,
supplying a voltage or current to a receiver. However, as
illustrated below basically the same distance relations
hold for antenna elements being directors guiding received
electromagnetic field to electrically active antenna ele-
ments .
Figure 7 illustrates an equilateral triangular antenna
realization according to an embodiment of the invention.
The antenna elements are all separated by dtd. Similarly to

the rectangular realization in figure 5, the optimum an-
tenna element separation of the equilateral triangular an-
tenna structure with three antenna elements equals
where D is communication distance and λ is communication
wavelength.
Figure 15 illustrates a realization with director elements
«Director»- direct electromagnetic fields received and elec-
tromagnetic fields to be transmitted, preferably with one
director per electrically active antenna element «Active
elements >. Preferably the directors «Director» are pure
reflectors but can also be made of dielectric material.
The supports «Supports» are designed not to shadow, or only
have a small shadowing impact on, the electrically active
antenna elements «Active elements>. The positioning of the
directors is preferably in accordance with equation (19)
and (21) for a linear and square grid LOS MIMO antenna re-
spectively. The relevant distance d is essentially equal to
the separation distance of the projection of the directors
onto a plane, the plane being perpendicular to the LOS
transmission path to the other receiver/transmitter end.
Advantages achieved by the realization of figure 15 in
addition to those mentioned above are, e.g., simplified
wiring of the antenna elements and the antenna elements
spanning a smaller distance range thereby being mechani-
cally robust. Also, by adjusting the directors the elec-
trically active antenna elements need not always be reposi-
tioned even if communications distance changes.
The dependency of α, A and dv/dh/d on D for an LOS MIMO an-
tenna has practical implications, addressed by the inven-
tion. An obvious solution to the problem of getting a, to
the communications distance D, appropriately matched ele-

ment distance, dv, dh, d, is to manufacture custom-made an-
tennas. From a cost perspective, however, a more attrac-
tive solution is manufacturing of a set of antenna models
for MIMO communications, each designed for a range of com-
munications distances D, and upon installation selecting an
antenna model within the set that best matches the communi-
cations distance. For frequency non-selective channels,
SVD (singular value decomposition) provides robustness and
close to optimum performance also with non-perfect matching
of communications distance, D, and element separation, dv,
dh, d. Another embodiment is realized by individually ad-
justable antenna elements. Preferably this is realised by
a grid «Grid» of interconnected rods or tensed wires to
which the antenna elements «Antenna element» are attached
as illustrated in figure 16. The wires or rods are pref-
erably connected to a frame «Frame». Models that are elec-
tromechanically adjustable comprise electromechanical mo-
tors to which the rods are connected, such that the rods to
which the antenna elements «Antenna element» are attached
may move along the frame. A further embodiment of adapting
an LOS MIMO antenna to communications distance D uses spa-
tial oversampled antennas as schematically illustrated in
figure 8 and activating the antenna elements by signal
processing providing best performance at actual communica-
tions distance. The particular element distribution may be
varied, e.g. as illustrated in figures 9 and 10, An impor-
tant issue of the invention is that active elements are
distributed such that their mutual distances reflects com-
munications distance (distance between transmit and receive
antennas) and wavelength such that the spherical properties
of the radio wave can be exploited.
It is observed that as transmitter and receiver antennas
form an antenna pair for a communications link, the respec-

tive element distances dv, dh and d in e.g. equations (15)
and (16) of an example transmitter antenna can be reduced
(or increased) if the element distance of a corresponding
example receiver antenna of the communications link is in-
creased (or reduced) in proportion to the distance reduc-
tions (or increase) of the transmitter antenna. Indexing
distances of transmitter and receiver antennas by T and R,
respectively, if respective element distances of a receiver
antenna, dvR., dhR and dR, are reduced (or increased) in rela-
tion to an initially determined distance dv, dh or d1
transmitter-side antenna-element distance, dvT, dhT and dr,
should be increased (or reduced) in proportion thereto (in
relation to dv, dh and d) , Consequently, the distances dv, dh
in equations (15) and (16) are the geometrical averages of
receiver and transmitter antenna element distances, respec-
tively.
The actual antenna dimensions in equations (17) and (18),
of course, are determined by actual respective vertical and
horizontal element distances. Correspondingly, also an-
tenna dimensioning in equations (19) and (21) are deter-
mined by actual distances, if adjusted as described above.
At transmitter side equations (17), (18), (19) and (21)
translate to equations (24), (25), (26) and (27)

and correspondingly for receiver side, they translate to
equations (28), (29), (30) and (31)

where

The invention does not only cover planar antenna configura-
tions, but also three-dimensional configurations as illus-
trated in figures 17-19. Figure 17 illustrates a two-layer
square grid LOS MIMO antenna with two layers of antenna
elements each on a square grid. Figures 18 and 19 illus-
trate realisations with equal distance between all nearest
neighboring antenna elements. In figure 18 the antenna
elements are positioned to the vertices of a cube and in
figure 19 the antenna elements are positioned to the verti-
ces of a tetrahedron.
Various embodiments of the invention also cover different
realizations of signal processing at transmitter and re-
ceiver ends. The processing is necessary for adaptation to
prevalent channel conditions. At receiver or transmit
side, determining channel singular values as described in
relation to equation (3) and singular value decomposition
can be achieved by digital signal processing of base band
signals. If determined at transmitter side, information on
channel matrix, H, need to be transferred from receiver

side, or the channel matrix otherwise estimated at trans-
mitter side, see figure. For a 2X2 channel matrix, singu-
lar value decomposition can also be achieved by a 3-dB hy-
brid to perform multiplication or weighting as need be, op-
erating on high-frequency signals. Also, for channel ma-
trices greater than 2X2 a generalization of a 3-dB hybrid,
a Butler matrix directional coupler, may be used. A fur-
ther embodiment realizes the processing by means of an ar-
rangement of microstrip or waveguides, also operating on
high-frequency signals. At receiver side, channel equali-
zation requires processing. This processing can be per-
formed by any of the processing realizations described for
transmitter side, or received signal can be equalized by
means of zero forcing, for which the received signal being
multiplied by the inverse matrix of channel matrix H, or by
means of minimum mean square error, MMSS, for which the
mean square error is minimized, the various processing re-
alizations giving rise to further embodiments.
If there is multipath propagation, this is preferably in-
corporated into the singular value decomposition at trans-
mitter side through feedback information. Corresponding
information can also be derived through channel reciprocity
if the reverse direction channel matrix is determined at
transmitter side (the transmitter side also comprising ra-
dio receiver). Another solution comprises a self-tuning
antenna, optimizing performance at receiver side, transmit-
ter side or both. The antenna element positioning is then
adapted to channel propagation properties corresponding to
a measured channel matrix, H. This can be achieved by,
e.g. a stochastic gradient algorithm. Particularly for
fixed positioned antenna elements, they may require the an-
tenna elements to be re-distributed for optimum perform-
ance. For an electromechanically adjustable element an-

tenna the optimization can be achieved by automatic posi-
tion adjustments of the antenna elements. The different
solutions to multipath propagation can also be combined.
Preferably and in accordance with the invention, singular
value decomposition is applied, to flat (frequency non-
selective) fading channels. If a channel nevertheless is
frequency-selective fading, the channel can be considered
piecewise flat fading for sufficiently small frequency in-
tervals. Such piecewise flat fading channels can, e.g., be
achieved by dividing a given frequency range or bandwidth,
using orthogonal-frequencies sub-carriers of sufficiently
narrow one or more bandwidths for the one or more band-
widths to be much less than the coherence bandwidth. One
technique for achieving such sub-carriers is orthogonal
frequency division multiplex, OFDM.
The concept of the present invention combines well with
other known means to increase throughput, such as transmis-
sion at both vertical and horizontal polarization or trans-
mission at left-hand and right-hand circular polarization,
or different coding of different sub-channels depending on
their respective channel quality, which further demon-
strates the usefulness of the invention. Such combinations
are also within the scope of this invention.
Dimensioning has been expressed in relation to particular
orientation, e.g. horizontal or vertical orientation, re-
ferring to orthogonal directions, perpendicular to the di-
rection of communications. However, this does not exclude
rotation of receiver and transmitter antennas in a plane
parallel to the antenna elements, with corresponding rota-
tion of both antennas such that their mutual orientation is
preserved. Despite somewhat inappropriate, the notation of
vertical and horizontal is kept for reasons of simplicity.

The invention is not intended to be limited only to the em-
bodiments described in detail above. Changes and modifica-
tions may be made without departing from the invention. It
covers all modifications within the scope of the following
claims.

WE CLAIM:
1. A method of configuring an antenna, the method com-
prising:
constructing the antenna to comprise a plurality of antenna
elements; and
configuring the antenna for line of sight (LOS) communi-
cation such that the separation of the antenna elements is
set in relation to √Dλ/N where D is communications
distance, λ is communication wavelength and N is num-
ber of antenna elements.
2. The method according to claim 1 wherein the antenna
configuration maximizes multiple-input multiple-output
(MIMO) channel capacity.
3. The method according to claim 1 wherein the antenna is
a linear antenna.
4. The method according to claim 1 wherein the antenna is
a square grid antenna.
5. The method according to claim 4 wherein N=n2 for n an
integer greater than 1.
6. The method according to claim 1 wherein the antenna is
a rectangular grid antenna.
7. The method according to claim 6 wherein the dimension
of separation is horizontal dimension.
8. The method according to claim 6 wherein the dimension
of separation is vertical dimension.
9. The method according to claim 1 wherein for a triangular
grid antenna with three antenna elements, the separation of
the antenna elements is set in relation to √Dλ/3, where D is
communications distance and λ is communication wave-
length.
10. A method of configuring an antenna, the method com-
prising:
constructing the antenna to comprise a plurality of clusters
of antenna elements; and
configuring the antenna such that the plurality of clusters of
antenna elements are separated by a distance set in rela-

tion to communications distance and the antenna con-
figuration is three-dimensional.
11. The method according to claim 10 wherein the antenna
is configured such that the plurality of clusters of antenna
elements are separated by a distance set in relation to com-
munication wavelength.
12. The method according to claim 10 wherein for a linear
antenna the plurality of clusters of antenna elements are sepa-
rated by a distance set in relation to √DΛ/L where D is com-
munications distance, λ is communication wavelength and L
is number of clusters.
13. The method according to claim 10 wherein for a square
grid antenna the plurality of clusters of antenna elements are
separated by a distance set in relation to where D is
communications distance, λ is communication wavelength
and L is number of clusters.
14. The method according to claim 13 wherein L=I2 for I an
integer greater than 1.
15. The method according to claim 10 wherein the antenna
elements within a cluster are separated by a distance smaller
than the smallest distance between clusters.
16. The method according to claim 10 wherein the antenna
configuration comprises two layers, where each layer com-
prises a planar arrangement of antenna elements on a square
grid.
17. The method according to claim 10 wherein the antenna
configuration comprises antenna elements positioned equi-
distant in a three-dimensional space.
18. The method according to claim 15 wherein the antenna
elements are positioned to vertices of a cube.
19. The method according to claim 15 wherein the antenna
elements are positioned to vertices of a tetrahedron.

20. The method according to claim 10 wherein the antenna
elements are fed with signals processed according to singular
value decomposition for a transmission channel over the
communications distance.
21. The method according to claim 20 wherein the trans-
mission channel considered is a flat fading sub-carrier.
22. The method according to claim 20 wherein the trans-
mission channel considered is an OFDM sub-carrier.
23. The method according to claim 10 wherein the signals
received from the antenna elements are processed according
to zero forcing for a transmission channel over the commu-
nications distance.
24. The method according to claim 10 wherein the signals
received from the antenna elements are processed to mini-
mize mean square error for a transmission channel over the
communications distance.
25. The method according to claim 10 wherein signal pro-
cessing of signals received or to be transmitted is performed
at high-frequency.
26. The method according to claim 25 wherein the process-
ing is performed by one or more 3-dB hybrids.
27. The method according to claim 25 wherein the process-
ing is performed by one or more Butler matrix directional
couplers.
28. The method according to claim 25 wherein the process-
ing is performed by an arrangement of microstrip.
29. The method according to claim 25 wherein the process-
ing is performed by an arrangement of waveguides.
30. The method according to claim 29 wherein the antenna
configuration is a radio antenna configuration.
31. The method according to claim 29 wherein the antenna
configuration is a configuration of sensors or actuators for
optical communications.

32. An antenna configuration comprising:
a plurality of antenna elements,
wherein the plurality of antenna elements is configured for
line of sight (LOS) communication such that separation
of the antenna elements is set in relation to √Dλ/N where
D is communications distance, λ is communication
wavelength and N is number of antenna elements.
33. The antenna configuration according to claim 32
wherein the antenna configuration maximizes MIMO chan-
nel capacity.
34. The antenna configuration according to claim 32
wherein the antenna configuration is a linear antenna configu-
ration.
35. The antenna configuration according to claim 32
wherein the antenna configuration is a square grid antenna
configuration.
36. The antenna configuration according to claim 35
wherein N=n2 for n an integer greater than 1.

37. The antenna according to claim 32 wherein the antenna
elements separation is set in relation to √Dλ/N where D is
communications distance, λ is communication wavelength
and N is number of antenna elements in dimension of sepa-
ration, for a rectangular grid antenna.
38. The antenna according to claim 37 wherein the dimen-
sion of separation is a horizontal dimension.
39. The antenna according to claim 37 wherein the dimen-
sion of separation is a vertical dimension.

40. The antenna according to claim 32 wherein the antenna
elements separation is set in relation to √Dλ/3, where D is
communications distance and λ is communication wave-
length, a triangular grid antenna with three antenna elements.
41. The antenna configuration according to claim 32, the
antenna configuration being three-dimensional.
42. The antenna configuration according to claim 32, the
antenna configuration comprising two layers, where each
layer comprises a planar arrangement of antenna elements on
a square grid.
43. The antenna configuration according to claim 32, the
antenna configuration comprising antenna elements posi-
tioned equidistant in a three-dimensional space.
44. The antenna configuration according to claim 43, the
antenna elements being positioned to vertices of a cube.

45. The method according to claim 43, the antenna ele-
ments being positioned to vertices of a tetrahedron.
46. An antenna configuration comprising:
a plurality of clusters of antenna elements wherein the
antenna elements are configured such that the plurality
of clusters of antenna elements are separated by a dis-
tance set in relation to communications distance, com-
munication wavelength and number of antenna ele-
ments,
constructing the antenna to comprise a plurality of antenna
elements; and
configuring the antenna for line of sight (LOS) communi-
cation such that the separation of the antenna elements is
set in relation to √Dλ/N where D is communications
distance, λ is communication wavelength and N is num-
ber of antenna elements.
47. The antenna configuration according to claim 46, the
plurality of clusters of antenna elements being separated by a
distance set in relation to √Dλ/L, where D is communications
distance, λ is communication wavelength and L is number of
clusters, and wherein the antenna configuration is a linear
antenna configuration.
48. The antenna configuration according to claim 46, the
plurality of clusters of antenna elements being separated by a

distance set in relation to where D is communica-
tions distance, λ is communication wavelength and L is num-
ber of clusters and wherein the antenna configuration is a
square grid antenna configuration.
49. The antenna configuration according to claim 48, L=I2
for I an integer greater than 1.
50. The antenna configuration according to claim 46, the
antenna elements within a cluster are separated by a distance
smaller than the smallest distance between the plurality of
clusters of antenna elements.
51. The antenna configuration according to claim 46, one
or more antenna element feeders being adapted to feed the
antenna elements with signals processed according to singu-
lar value decomposition for a transmission channel over the
communications distance.

52. The antenna configuration according to claim 51,
wherein the transmission channel considered is a flat fading
sub-carrier.
53. The antenna configuration according to claim 51,
wherein the transmission channel considered is an OFDM
sub-carrier.
54. The antenna configuration according to claim 46,
wherein one or more processing elements are adapted to
process signals received from the antenna elements according
to zero forcing for a transmission channel over the commu-
nications distance.
55. The antenna configuration according to claim 46,
wherein one or more processing elements are adapted to
process signals received from the antenna elements to mini-
mize mean square error for a transmission channel over the
communications distance.

56. The antenna configuration according to claim 46,
wherein one or more processing elements are adapted to
process at high-frequency signals received or to be transmit-
ted.
57. The antenna configuration according to claim 56, the
one or more processing elements being one or more 3-dB
hybrids.
58. The antenna configuration according to claim 56, the
one or more processing elements being one or more Butler
matrix directional couplers.
59. The antenna configuration according to claim 56, the
one or more processing elements being an arrangement of
microstrip.
60. The antenna configuration according to claim 56, the
one or more processing elements being an arrangement of
waveguides.
61. The antenna configuration according to claim 46, the
antenna elements being electrically active elements.
62. The antenna configuration according to claim 46, the
antenna elements being directors.
63. The antenna configuration according to claim 62, the
directors being reflectors.
64. The antenna configuration according to claim 46, the
antenna elements being arranged symmetrically in a circular
pattern.
65. The antenna configuration according to claim 46, the
antenna elements being arranged in a hexagonal pattern.
66. The antenna configuration according to claim 46, the
antenna elements being mounted on position adjustable rods
or wires.
67. The antenna configuration according to claim 66, the
position adjustable rods or wires being electromechanically
adjustable.

68. The antenna configuration according to claim 67
wherein the adjustable position is adaptive to propagation
channel properties corresponding to a measured channel
matrix.
69. The antenna configuration according to claim 46, the
antenna configuration being adapted to a predetermined
range of communications distances.
70. An antenna configuration, the antenna configuration
comprising a plurality of antenna elements, of which a subset
forms an active set of antenna elements, the active antenna
elements forming an antenna configuration that is configured
for line of sight (LOS) communication such that separation of
the antenna elements is set in relation to communications
distance, communication wavelength and number of antenna
elements.
71. The antenna configuration according to claim 70,
wherein the antenna configuration is a radio antenna configu-
ration.
72. The antenna configuration according to claim 70,
wherein the antenna configuration is a configuration of sen-
sors or actuators for optical communications.
73. A. communications system comprising:
an antenna having a plurality of antenna elements, wherein
the antenna is configured for line of sight (LOS) com-
munication such that separation of the antenna elements
are set in relation to √Dλ/N where D is communications
distance, λ is communication wavelength and N is num-
ber of antenna elements.
74. The communications system according to claim 73
wherein the separation of antenna elements are set different
for a first and a second antenna, the two antennas operating in
pair, such that the geometrical average of the elements dis-
tance of the first, antenna, d1 and the elements distance of the
second antenna, d2, is the effective antenna elements distance.
75. The communications system according to claim 73,
wherein the antenna is one of a linear antenna, a square grid
antenna and a rectangular grid antenna and the separation of
the antenna elements is set in relation to √Dλ/N where D is
communications distance, λ is communication wavelength
and TNT is number of antenna elements.
76. The communications system according to claim 75
wherein N=n2 for n an integer greater than 1.

ABSTRACT

METHOD AND SYSTEM OF COMMUNICATIONS
The present invention relates to high data rate communications,
and more especially it relates to line of sight, LOS, multiple
input multiple output, MIMO, communications links and antenna
configuration for LOS MIMO links, particularly radio links and
optical wireless links.

METHOD AND SYSTEM OF COMMUNICATIONS

Documents:

Inventors:

PCT Conventions: