Title of Invention

FAST SLEWING PSEUDORANDOM NOISE SEQUENCE GENERATOR

Abstract

One or more loadable PN generators are controlled by a DSP or microprocessor in conjunction with a free-running counter which maintains a reference offset count. The PN generator will typically be part of a finger or searcher. Each PN generator is comprised of a loadable linear feedback shift register (LFSR) or its equivalent, a loadable counter for maintaining an index of the state of that particular PN generator, and a slew control device capable of receiving a slew command and controlling the LFSR and index counter to enact an advance or a retard of a certain offset distance. The speed increase is effected by DSP control. A table of LFSR states and corresponding index numbers are stored in memory. These LFSR states will subdivide the total possible number of possible states. It is advantageous to evenly space the stored states around the PN circle. The fast slewing is enabled by the DSP in two-step process. First the PN generator is loaded such that it 'jumps' to the closest state using the table, then the PN generator's slew control is used to slew the rest of the way.

Full Text

FAST SLEWING PSEUDORANDOM NOISE SEQUENCE GENERATOR BACKGROUND OF THE INVENTION L Field of the Invention The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for generating a pseudorandom noise (FN) sequence with the capability of being rapidly slewed from one offset to another. IL Description of the Eeiated Art Pseudorandom noise (FN) sequences are commonly used in direct sequence spread spectrum communication systems such as that described in the IS-95 over the air interface standard and its derivatives such as IS-Vo-A and ANSI J-STD-008 (referred to hereafter collectively as the IS-95 standard; promulgated by the Telecommunication industry Association (riA) and used primanly within ceilular telecommunications systems. The I$"-95 standard incorporates code division multiple access (CDMA) signal modulation techniques to conduct multiple communications simultaneously over the same RF bandwidth. When combined with comprehensive power control, conducting multiple communications over the same bandwidth increases the total number of calls and other communications that can be conducted in a wireless communication system by. among other things, increasing the frequency reuse in comparison to other wireless telecommunication technologies. The use of CDMA techniques in a mulitiple access communication system is disclosed in U,S, Patent No. 4.901.307, entitled "SPREAD SPECTRUM COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", .and U.S. Patent No. 5,103,459. entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", both of which are assigned to the assignee of the present invention and incorporated by reference herein. FIG, 1 provides a highly simplified illustration of a cellular telephone system configured in accordance with the use of the IS-95 standard. During operation, a set of subscriber units 10a - d conduct wireless communication by establishing one or more RF interfaces with" one or more base stations 12a d using CDMA modulated RF signals. Each RF interface between a base station 12 and a subscriber unit 10 is comprised of a forward link signal transmitted from the base station 12, and a reverse link signal transmitted from, the subscriber unit. Using these RF interfaces, a communication with another user is generally conducted by way of mobile telephone switching office (MTSO) 14 and public switch telephone network (PSTN) 16. The links between base stations 12, MTSO 14 and PSTN 16 are usually formed via wire line connections, although the use of additional Kb or microwave links is also known. Each subscriber unit 10 communicates with one or more base stations 12 by utilizing a rake receiver. A RAKE receiver is described in U.S. Patent No. 5.109.390 entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELHPHOME SYSTEM", assigned to the assignee of the present invention and incorporated herein by reference. A rake receiver is typically made up of one or more searchers for locating direct and multipath pilot from neighboring base stations and two or more fingers for receiving and combining information signals from those base stations. Searchers are described in. co-opending U.S, Patent Application 08/316,177, entitled "MLT.TIPATH SEARCH PROCESSOR FOR SPREAD SPECTRUM lN/TL"LTIPLE ACCESS COMMUNICATION S\'STEMS", filed September 30, 1994 assigned to the assignee ot the present invention anct incorporated herein by reference. Searchers and fingers must be capable of generating the proper PN Frequences to match those generated at ihe base station. PN seoucnces are typically generated through the use of linear feedback shirt registers/ or LESRs. Inherent in the design of direct sequence spread spectrum communication systems is the requirement that a receiver must align its PN sequences to those of the base station. Each base station and subscriber unit uses the exact same PN sequences. A base station distinguishes itself from other base stations bv inserting a unique offset in the generation of its PN sequences. In IS-95 systems, all base stations are offset by an integer multiple of 64 chips- A subscriber unit communicates with a base station by assigning at least one fingerr to that base station. An assigned finger must insert the appropriate offset into its PN sequence in order to communicate with that Coimter 220 is also controlled by LFSR^EN. It is used to keep track of which state \he LPSR is m by generating an index which is labeled LFSRXOUNT. All of the components are reset by a common reset, which aligns LFSR_ COUNT and LFSR„STATE to predetermined positions. Since LFSR-EM controls both counter 220 and LFSR 210, and both either advance together or don't advance at ail, LFSR.COUNT can be used to determine where LF5R-STATE is in the PN sequence. FIG. 3 shows a conceptual timing diagram which illustrates an advance slew. Signal chip_clk depicts the chip rate. Signals BSl and BS2 represent the PN sequences of two different base stations. Each sequence advances through the same sequence of states, labeled SO, S1,.,., but as described above the base stations are distinguishable based on the offset between their respective PK sequences- The offset shown in this example is only 2 chips. As stated above, IS-'95 base stations are always offset by integer multiples of 64 chips. The offsets chosen have no material effect on slewing functionality, signal LFSR represents the state of the LF5R which would When LFSR-EN begins asserting again at the regular chip rate, LFSR will be aligned with BSl. The LFSRs in fingers must be slewed in several situations during normal communications. One situation occurs when finger assignment is performed: each finger must be assigned to a position where a searcher located a pilot. Short slews may be performed when a finger assigned to a multipath signal is reassigned to a stronger nearby muitipath signal. More generally though LFSRs will have to be slewed over large offsets. Fingers may be reassigned from one base station to neighboring base stations located at large offsets from the first. After a subscriber wakes up from a power-conserving sleep mode, fingers will generally need to be relocated. In most situations, it will be advantageous to minimize the time required to slew a finger since during slewing a finger is not useful for communications. The The speed increase is effected by DSP control. A table of LFSR states and corresponding index numbers are Stored in memory. These LFSR states will subdivide the total possible number of possible states. It is advantageous to evenly space the stored states around the FN circle. The fast slewing is enabled by the DSP m a two-step process. 'First the FN generator is loaded such that it "jumps" to the closest state using the table, then the PN generator's slew control is used to slew the rest of the way. When a finger is to be slewed to a particular offset, the position determined by adding that offset to the current value of the free-running reference counter is calculated- The nearest location and the corresponding LFSR state are retrieved from the table. Simultaneously, th.e index is loaded into the particular finger's counter and the LFSR state is loaded into the particular finger's LFSR. Once the LFSR and counter have been loaded, they In the exemplary embodiment, there are 2'45 possible states. The DSP stores 16 PN states and corresponding index values which are evenlt spaced around the PN circle (2048 chips apart). The exemplary PN generator can slew at a rate of 7 chips per chip time while advancing, or 1 chip per chip time while retarding. The maximium slew time is then 256 chip times plus the time it takes the DSP to cause the jump. Any location in any 2048 chip span can be reached within 256 chips by jumping to the nearest stored location before it and advancing at a rate of 7 chips per chip or jumping to the nearest stored location behind it and retarding at a rate of i chip per chip. Contrast this with the prior art method of an average of 2>13 chip times. By increasing the state storage by a factor of two, the maximum slew span decreases to 1024 and the maximum slew time decreases by a factor of two FIG. 7 is a flow diagram detailing the steps to perform the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A block diagram configured in accordance with the present invention is shown in FIG. 5. LFSR 52 will reset counter 510, whose output,_ FREE-COUNT, will, be used as a common reference. The reset is used for other initialization purposes as well For example, it is convenient to reset slew control 540. Counter 510 will serve as a time reference. Its output, FREE-COUNT, counts through the number of states in the FN sequence (2-15 in the exemplary embodiment) at a rate of one state per chip period. Note that no enable signal attaches to this counter; it is free running. It is not a subsequently be introduced into LFSR-STATE and the accompanying index LFSR-COUNT. FIG. 6 depicts a timing diagram of a more rapid advance slew than the one described in the discussion of prior art above. Similar to the discussion regarding FIGS. 3 and 4, BSi and BS3 represent the index ot the PN sequence used by two different base stations in communication with a subscriber unit whose PN sequence index is shown by LFSR. Note that BS3 is offset by five chips from BSI. The clock rate controlling LFSR is six times the chip rate, (This example is for demonstration, the exemplary embodiment employs a clock of eight times the chip rate, and, as specified by 15-95. all base stations are mnltiple of 64 chips apart.) Initially, LFSR is aligned with BSI- il advances in Sequence once per chip, as indicated when LFSR-EN is asserted. For this coniiguration, a maximum advance of five chips per chip can be accomplished, and is shown by the portion of LFSR^EN labeled "5 Chip Advance". Here LFSR_EN is assexted for an entire chip period. The LFSR state increments once per clock cycle. When not slewing, the LFSR should comparin the system offset of the base station and comparing thar with the difference between FREE-COUNT and the LFSR-COUNT of the finger communicating with that base station. For the purposes of this discussion In generic it is advantageous to select the jump locations to optimize for the shortest maximum slew time. which corresponds to selecting locations evenly spared around the FN circle, Other choices for jump locations can be made, and then the maximum slew time will be a function of the largest distance on the PN circle between two Stored jump locations. For certain distributions of actual base station offsets in a. system, this type of spacing, while increasing maximum slew time, may lower average slew time. Myriad varieties of spacing pattems are foreseeable and fail within the scope of the present invention. Another minor optimization for minimizing slew time would be to calculate the average number of chips that FREE-COUNT advances while the DSP is processing the jump and-add that number to the Z calculation. During most slews this will not have any effect since the residual slewing is performed by slew control 540, but occasionally the additional value will CLAIMS 1. A fast slewing PN sequence generator comprising: a DSP; a loadable LFSR for producing PN states and for receiving load values from said DSP; a loadable index counter for providing an index of said LFSR state and for receiving load values from said DSP; a controliable slew control for adjusting rate of state change in said loadable LFSR and correspondingly in said loadable index counter in response lo commands from said DSP; a reference counter for providing a reference rotate to said DSP; and a state table for storing a. subset of PN slavtes and their corresponding index values capable of being retrieved by said DSP. 2. The fast slewing PN sequence generator of Claim 1 wherein fast slewing to a desired offset is carried out by said DSP in steps comprising: a) determining load values from said state table for simulataneously loading into said loadable LFSR and said loadable index counter based on said reterence stare and said desired offset; b) simultaneously loading said load values; c) determining a residual slew value from the resultant offset and the desired offset; and d) commanding said controllable slew control to produce said desired offset by performing slewing based upon said residual slew. value 3. A method for performing fast slewing PN generation comprising the steps of: a) finding an index value and a corresponding PN generator state in a state table based upon a desired offset and a reference state value; b) simultaneously loading a PN generator with said PN generator stare and said index value. c) performing any required residual slew based upon current readings or said reference slate value and said PN generator index values. 4. A method for performing fast slewing PN generation comprising the steps of: a) determining a desired offset; b) reading a reference state; c) calculating an estimated absolute state position by adding said desired offset to said reference state; d) funding a nearest index value contained in a state table; e) retrieving an LFSR state corresponding to said nearest index value from said stare table: f) sirnultaneously loading said nearest index value into an index counter and said IFSR state into an LFSR; g) simultaneously reading an updated reference state and the current index of said index counter; h) calculating a residual slew value by summing difference said updated reference state added and said desired offset and subtracting from that sum said current index, arid i) directing a slew control to perform said residual slew. 5. A fast slewing PN sequence generator substantially as herein described with reference to the accompanying drawings. 6. A method for performing fast slewing PN generation substantially as herein described with reference to the accompanying drawings.

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