Title of Invention  METHOD FOR DEMODULATING A CARRIER WAVE MODULATED USING A DIGITAL SYMBOL SEQUENCE 

Abstract  A method is described for demodulating a carrier wave that is modulated using a digital symbol sequence and that is transmitted over a noiseimpacted channel, the ideal edge shapes of possible transitions between two symbols being known and stored in memory (reference edges), and a received edge being scanned and digitalized using a scanning frequency which is a multiple of the frequency of the symbol sequence. According to the present invention, it is provided that for detecting a received and scanned edge, all scanning values are used for the calculation of Euclidean distances from at least two reference edges, and the reference edge having the lowest Euclidean distance is selected. In one preferred embodiment, it is provided that a Viterbi algorithm is applied not, as is the case in known CPM demodulation methods, to an estimated symbol sequence, but rather to a sequence of estimated edges, the specific Euclidean distances between the edge received in one symbol period and the reference edges being considered as the costs of one trellis branch of a Viterbi algorithm. 
Full Text  METHOD FOR DEMODULATING A CARRIER WAVE MODULATED USING A DIGITAL SYMBOL SEQUENCE The invention relates to a method for demodulating a carrier wave modulated using a digital symbol sequence and transmitted over a noiseimpacted channel, the ideal edge shapes of possible transitions between two symbols being known and stored in memory (reference edges), and a received edge being scanned and digitalized using a scanning frequency which is a multiple of the frequency of the symbol sequence. In particular, continuous phase modulation (CPM) is used to transmit (data) packets. In this context, by varying the phase angle (relation, position), a plurality of different symbols; e.g., four;, can be transmitted. Since modulation using squarewave (rectangular) pulaes all having the same symbol length would lead to a very broad spectrum, modulation is often carried out using pulses that extend over the length of two symbol periods and that are represented as cosineshaped to avoid steep edges, A modulation of this type is also known as 2RCCPM, From the conference proceedings, "K.'H. Tietgen, 'Numerical Modulation Methods Applied in the FD/TDMA System S 900 D,' Second Nordic Seminar on Digital Land Mobile Radio Communications, Stockholm, it is known not to use symbols themselves for modulation, but rather to numerically modulate the known edges between two Figure 1 shows the block diagram of a transmitter that is wellsuited to use the known method. In this context, the system parts shaded in gray operate in the high system clock. Figure 2, for a fourvalue symbol alphabet, shows the conceivable edges in the transition from one specific symbol to the next in a symbol sequence to be transmitted. The idea in the known modulation methods is to describe the instantaneous frequency (i.e., the phase derived from time) of a (for example) fourstep signal given a pulse shape of a length 2T as a sidebyside arrangement (consecutive alignment) of 16 possible edges fn(t) having length T. Instead of transmitting overlapping, shifted elementary pulses Σ1d(l)g(t  iT), is also possible to directly transmit a nonoverlapping sequence of edges ΣifN[t  iT), neighboring symbols d{I) and d[I + 1)T in each case deciding which of the 16 edges in interval iT To receive a data sequence modulated in this way, a receiver has been proposed in the seminar report, "D,E. Pfitzmann and H.p, Ketterling, A New CP4FSK Sampling Demodulator for the FD/TDMA System S 900 D,' Second Nordic Seminar on Digital Land Mobile Radio Communications, Stockholm, 1986." The same receiver is also known from German Patent 36 2 8 993 The mode of operation of the receiver described above is a method of the species. Using the receiver described above, a received edge is scanned and digicalized using a scanning frequency which is a multiple of the frequency of the symbol sequence. In the example cited, scanning is carried out between two symbols, for example, 16 times. The digitalised values are compared with each other and, in the event that the difference is small becween two consecutive values, are combined into a symbol average (center). Ey averaging consecutive values, it is attempted to elimunace the influence of noise. Classical CPM receivers and demodulation methods are also termed "discriminator Integrate & Dump," Therefore, in what follows, when a signal edge shape of the "raised cosine" lasting for two symbol lengths is used, the term 2RCdsI&D is used, being supplemented, as appropriate, by the addition 2st (i.e., twostep, two symbols) or 4st {i,e., fourstep, four symbols). In general, it is desirable for a receiver if each individual symbol is received in a highly reliable manner. In the classical CPM receiver method, it is necessary to filter the signal that has become noisy in passing through the real transition channel. Conventional filters operate as an integrator, for example, over two symbol periods, to average the noise. In the abovementioned 2RC signal shapes, integration leads to intersymbol interference,abbreviated as ISI. To avoid intersymbol interference, it is known to apply a Viterbi algorithm to the received symbol sequenceDisadvantageous in this context is the great computing effort that places correspondingly great demands on the hardware, Therefore; the present invention is based on the objective of indicating a demodulation method in which, based on a method according to the species, it is possible to have small computing effort while maintaining the error rate at as close to the same level as possible. In this manner, the design effort for a receiver is simplified without relinquishing the advantages of "Continuous Phase Modulation" (CPNI) such as low complexity and a compact spectrum, which, as was already mentioned, is of decisive importance with respect co good bandwidth utilization. The objective is achieved through the present invention in that, to detect a received and scanned edge, all of the scanning values are used, in each case, to calculate Euclidean distances from at least two reference edges, and the reference edge having the lowest Euclidean distance is selected. In contrast to the cited classical receivers, in a demodulation method of this type, instead of the symbols to be estimated, the transition edges between the symbols are made central to the detection process. These edges are scanned multiple times and are compared with the original edges. Subsequently; the decision is made in favor of the edge whose distance, in the Euclidean sense, from the received edge is the smallest. This decision is carried out edgebyedge independently of the neighboring edges, the decision hypothesis therefore not necessarily guaranteeing the continuity of the derivation of the phase of the CPM signal. Therefore, an inventive refinement is preferred in which there is a continuous phase, Therefore, according to the present invention, it is preferably provided that a Viterbi algorithm be applied to a number of consecutive edges, the respective Euclidean distances between the edges received in one symbol period and the reference edges being considered as the cost of a trellis branch of the Viterbi algorithm. The properties of the CPM modulation thus make it possible to carry out an equalization of the data signals very efficiently, The proposed maximumlikelihood estimation of the received data sequences using a Viterbi algorithm not on the symbol sequences but on the received edges has the consequence that a model can be used whose number of states is smaller by a factor of M in comparison to the classical equalizer. In this context, M designates the stepquantity of the signal. Therefore, the computing effort required can be kept small, as a result of which the hardware implementation becomes more attractive, as is described below. The present invention is described in greater detail below on the basis of the drawing. The contents of the drawing are as follows; Figure 1 depicts the block diagram of a transmitter using numerical modulation of a digital symbol sequence on a carrier wave, Figure 2 depicts a representation of the conceivable edges between two consecutive symbols, using a 2RC pulse and a fourstep symbol alphabet (Num2RC 4st) , Figure 3 depicts a block diagram of a receiver according to the present invention, Figure 4 depicts a table by analogy no Figure 2 for a twostep modulation, Figure 5 depicts a representation of a reference edge; an actually received sdge, and a trellis segment havin assigned costs, Figure 6 depicts an SER curve (Symbol Error Rate) for the twostep Num2RC, having and not having Viterbi detection, Figure 7 depicts a comparison according to Figure 6 for the fourstep Num2RC; having and not having Viterbi detection, Figure 8 depicts a comparison for the twostep case of the method according to the present invention having Viterbi detection of the edges and a classical C?M demodulation method (Integrate & Dump), and Figure 9 depicts a comparison according to Figure 8 for the fourstep case. Figure 1 shows a generally known transmitter for the numerical modulation of a digital symbol sequence. The baseband model of the transmitter is composed of a serialparallel transducer (converter) (S/?), a lookup table (LUX), in which essentially the contents of Figure 2 are stored, and a baseband FM modulator. The fourstep signal is extracted from the input bic scream by combining, in each case, two bits. As a result of a delay of one symbol period, a total of four bits then address the LUT depicted in Figure 1, containing the 16 symbol transitions fN(t) depicted in Figure 2, The number of interpolation points (supporting values) can be freely determined, the number 16 being proposed in the related art cited in the specification. This number is a compromise, because although a high number of incerpolation points is desirable to achieve good noise averaging, nevertheless a low number keeps the computing effort down. The parts of the transmitter that operate in the high system clock are shaded in gray. Figure 3 shows the receiver according to the present invention. The baseband model is composed of a bandlimiting low pass TP, a baseband FM demodulator having parasite AM suppression, an edge detector, and a parallelserial transducer (P/S). The parts of the receiver that operate in the high system clock are again shaded in gray. According to the present invention, in the edge detector, the determination of the edge that most likely corresponds to the transmitted edge is carried out in an edgebyedge manner. Figure 4 depicts, in the form of a table, the twostep case in conceivable edges, the twostep case being fundamental for the observation in Figure 5. Expansion to the fourstep case or that of an even higherstep proceeds by analogy with no limitation on generalisability. Figure 5, in the lafthand diagram, the solid line depicts a received, noisy edge, and the dotdash line depicts one of the reference edges according to Figure 4 [Num2RC 2st). The (for example) IS values determined for each edge are used according to the present invention to calculate the sum of the quadratic differences (Euclidean difference). On the basis of the smallest Euclidean difference (righthand side in Figure 5), the reference edge is identified which most closely corresponds to the transmitted edge. Each individual detected edge determines two data symbols: the output symbol and the final symbol, According to one preferred embodiment of the present invention, provision is made to assure the continuity of the detected symbol transitions using a MaximumLikelihoodSequence estimator (i.e., preferably using a Vicerbi algorithm). Therefore, as is depicted in Figure 5 on the righthand side on the basis of a trellis segment, according to the present invention, the distances of the received edge from all valid reference edge© are used as costs for the Viterbi algorithm. The Viterbi detector makes use of the following parameters; the trellis depth corresponds to stepquantity M of the signal (2RC pulse formation: memory of the length of one symbol period), the number of transitions is M. According to this, the path unification length should be chosen at > 5 symbols. For the purpose of illustrating the processes, the following description is limited to the twostep case without restricting the generalizability The state transitions and the edges that are generated in the process are represented in the table according to Figure 4. The calculation of the cumulative path costs is made in the familiar manner: in every current state, the path costs are calculated arising from the cost contributions of the branches of the trellis diagram leading to this state and their points of origin. The minimum is determined and is stored on the underlying branch. The cost contributions of the individual branches of a trellis segment are calculated  as was already explained  as distances of the current received and noisy edge from the corresponding edges determined by the model. The distances of the received edge from the possible edges are preferably calculated as the sum of the quadratic distances (Euclidean distance). However, it is also conceivable to use the sum of the absolute distances or a different suitable calibration. The calculation of the cost contributions of one edge is explained on the basis of a numerical example. Figure S depicts a 1 > 1 edge according to the table in Figure 4, the edge being distorted as a consequence of interference. The distances from, all possible reference edges are entered on the corresponding edges, and therefore the right half of the image represents the trellis segment that belongs to this received edge. The "distance" of the received edge is smallest from the 1 > 1 edge, and therefore, in this trellis segment, this represents the most favorable branch. After running through all of the trellis segments, the decision is made in the usual way in favor of one path in the trellis structure; the path having the minimal costs is selected, i.e., the path that is closest to the received edge sequence. The symbols are then derived from the selecced path using the model. The difference from the known Viterbi algorithms can be seen in the fact that the Viterbi algorithm is applied not to a symbol sequence but to an edge sequenceAs a result of the systemimminent redundancy; which is based an the fact that two neighboring edges have one symbol in common, the computing effort is significantly reduced. Although, when a classical Viterbi equalizer is used on symbol sequences; the ISI (intersymbol interference) can also be eliminated, nevertheless the memory length to be taken into account is increased to two symbol periods, which resulcs in an Mfold increase in the number of states and; as a result; a drastic increase in the computing effort. In contrast thereto, the computing effort in the method according to the present invention, i.e., the Num2RC rsceiver, is smaller by a factor of M. In the fourstep case, for example in the NumSRC dem.odulation according to the present invention using Viterbi detection; only one quarter of the computing effort is necessary/ in comparison to the classical CPM receiver method. Figures 6 through S depict the advantages that are achieved by che method according to the present invention. Figure 6 depicts a comparison of the demodulation method according to the present invention (Num2RC 2st} with the same method having the additional application of a Viterbi algorithm on edge sequences according to Figure 5 (Num2RC 2st vit) . Figure 7 depicts an analogous comparison for the fourstep case. On the vertical axis, the error race is indicated (SER, Symbol Error Rate) . The comparison of Figure 6 and Figure 7 indicates that already in the fourstep case, a significant reduction of the error rate can be observed. Figures 8 and 9 depict a comparison of the method according to the present invention, having additional Viterbi detection of the received edges, with a classical CPM demodulation method, in which an integration is carried out over a symbol interval (period), i.e., the socalled "Integrate & Dump," to achieve an average of the channel noise, a Viterbi algorithm being additionally used on the detected symbols to identify the most probable symbol sequence and thus to reduce intersymbol interference. The Figures show that the error rate of the method according to the present invention is slightly worse both in the twostep as well as in the fourstep case. However, in this context, it should be noted that in the twostep case according to Figure 8, the computing effort comes to only one half, whereas in the fourstep case, according to Figure 9, only one quarter of the computing effort is required. Therefore, especially in the fourstep case, it is possible to design receivers using a method according to the present invention, which are produced on the basis of significantly lower hardware costs and which are therefore more costeffective to manufacture. Patent Claims 1. A method for demodulating a carrier wave that is modulated using a digital symbol sequence and that is transmitted over a noiseimpacted channel, the ideal edge shapes of possible transitions between two symbols being known and stored in memory (reference edges) , and a received edge being scanned (sampled) and digitalized using a scanning [sampling) frequency which is a multiple of the frequency of the symbol sequence, characterized in that, for detecting a received and scanned edge, all scanning values are used for the calculation; in each case, of Euclidean distances with respect to at least two reference edges, and the reference edge having the lowest Euclidean distance is selected. 2. The method as recited in Claim 1, characterized in that a Viterbi algorithm is used on a number of sequential edges, the Euclidean distances between the edge, received in one symbol period, and the reference edges being considered as the costs of one trellis branch of the Viterbi algorithm, 3. The method as recited in Claim 1 or 2, characterized in that, in a scanning frequency of n/T and a signal stepquantity of M, the reference edges are described by a total of nM3 stored values. 4, The method as recited in one of the preceding claims, characterized in that the modulation is carried out as a CPM (continuous phase modulation). 5. The method as recited in Claim 4, characterized in that the modulation includes a numerical formation of the derivation and of the phase of one cosine signal (2RCCPM) extending over two symbol periods. 6_ The method as recited in one of the preceding claims. characterised in that the trellis depth of the Viterbi algorithm corresponds to the stepquantity M of the signal (Ncrelling := M) . 6. A method for demodulating a carrier wave substantially as herein described with reference to the accompanying drawings. Dated this 6 day of November 2000 

inpct2000609cheabstract.pdf
inpct2000609checlaims filed.pdf
inpct2000609checlaims grand.pdf
inpct2000609checorrespondence others.pdf
inpct2000609checorrespondence po.pdf
inpct2000609chedescription complete filed.pdf
inpct2000609chedescription complete grand.pdf
inpct2000609chedrawings.pdf
inpct2000609cheform 1.pdf
inpct2000609cheform 19.pdf
inpct2000609cheform 26.pdf
inpct2000609cheform 3.pdf
inpct2000609cheform 5.pdf
Patent Number  210541  

Indian Patent Application Number  IN/PCT/2000/609/CHE  
PG Journal Number  50/2007  
Publication Date  14Dec2007  
Grant Date  08Oct2007  
Date of Filing  06Nov2000  
Name of Patentee  M/S. ROBERT BOSCH GMBH  
Applicant Address  Postfach 300220 D70442 Stuttgart  
Inventors:


PCT International Classification Number  H04L25/06  
PCT International Application Number  PCT/DE1999/001041  
PCT International Filing date  19990407  
PCT Conventions:
