Spreading code sequence acquisition system and method that allows fast acquisition in code division multiple access (CDMA) systems

[0002] This application claims the benefit of U.S. Provisional Application 60/000,775 filed Jun. 30, 1995.

[0003] The present invention generally pertains to Code Division Multiple Access (CDMA) communications, also known as spread-spectrum communications. More particularly, the present invention pertains to a new system and method employing a new code sequence design for providing fast acquisition of a received spreading code phase in a CDMA communications system.

[0004] Recent advances in wireless communications have used spread spectrum modulation techniques to provide simultaneous communication by multiple users. Spread spectrum modulation refers to modulating a information signal with a spreading code signal; the spreading code signal being generated by a code generator where the period Tc of the spreading code is substantially less than the period of the information data bit or symbol signal. The code may modulate the carrier frequency upon which the information has been sent, called frequency-hopped spreading, or may directly modulate the signal by multiplying the spreading code with the information data signal, called direct-sequence (DS) spreading. Spread-spectrum modulation produces a signal with bandwidth substantially greater than that required to transmit the information signal. The original information is recovered at the receiver by synchronously demodulating and despreading the signal. The synchronous demodulator uses a reference signal to synchronize the despreading circuits to the input spread-spectrum modulated signal in order to recover the carrier and information signals. The reference signal may be a spreading code which is not modulated by an information signal. Such use of a synchronous spread-spectrum modulation and demodulation for wireless communication is described in U.S. Pat. No. 5,228,056 entitled SYNCHRONOUS SPREAD-SPECTRUM COMMUNICATIONS SYSTEM AND METHOD by Donald L. Schilling, which techniques are incorporated herein by reference.

[0005] One area in which spread-spectrum techniques are used is in the field of mobile cellular communications to provide personal communication services (PCS). Such systems desirably support large numbers of users, control Doppler shift and fade, and provide high speed digital data signals with low bit error rates. These systems employ a family of orthogonal or quasi-orthogonal spreading codes, with a pilot spreading code sequence synchronized to the family of codes. Each user is assigned one of the spreading codes as a spreading function. Related problems of such a system include: handling multipath fading effects. Solutions to such problems include diversity combining of multipath signals. Such problems associated with spread spectrum communications, and methods to increase capacity of a multiple access, spread-spectrum system are described in U.S. Pat. No. 4.901,307 entitled SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS by Gilhousen et al. which is incorporated herein by reference.

[0006] The problems associated with the prior art systems focus around reliable reception and synchronization of the receiver despreading circuits to the received signal. The presence of multipath fading introduces a particular problem with spread spectrum receivers in that a receiver must somehow track the multipath components to maintain code-phase lock of the receiver's despreading means with the input signal. Prior art receivers generally track only one or two of the multipath signals, but this method may not be satisfactory because the combined group of low power multipath signal components may actually contain far more power than the one or two strongest multipath components. The prior art receivers track and combine the strongest components to maintain a predetermined Bit Error Rate (BER) of the receiver. Such a receiver is described, for example, in U.S. Pat. No. 5,109,390 entitled DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM by Gilhousen et al. A receiver that combines all multipath components, however, is able to maintain the desired BER with a signal power that is lower than that of prior art systems because more signal power is available to the receiver. Consequently, there is a need for a spread spectrum communication system employing a receiver that tracks substantially all of the multipath signal components, so that substantially all multipath signals may be combined in the receiver, and hence reduce the required transmit power of the signal for a given BER.

[0007] Providing quality telecommunication services to user groups which are classified as remote. Such as rural telephone systems and telephone systems in underdeveloped countries, has proved to be a challenge in recent years. These needs have been partially satisfied by wireless radio services, such as fixed or mobile frequency division multiplex (FDM), frequency division multiple access (FDMA), time division multiplex (TDM), time division multiple access (TDMA) systems, combination frequency and time division systems (FD/TDMA), and other land mobile radio systems. Usually, these remote services are faced with more potential users than can be supported simultaneously by their frequency or spectral bandwidth capacity.

[0008] The problems associated with the prior art systems focus around reliable reception and synchronization of the receiver despreading circuits to the received signal. Since spreading code sequences in a communications system which supports a relatively large number of users may be very long with a corresponding long code period, one particular problem associated with prior spread spectrum receivers is to rapidly determine the correct code phase of a received spread spectrum signal. One solution of fast acquisition of the correct spreading code phase is to form spreading code sequences with specific characteristics which a receiver can derive from a particular received code phase.

[0009] For example, prior art systems employ a method in which a code generator produces a pseudorandom code of length N, divides the code in half to generate two new codes with code period N/2, and multiplies the data with each code for transmission over an In-phase and Quadrature channel. The receiver only searches for the occurrence of the short code period on the I or Q channel. The advantage of the system is that the number of users supportable with codes of length N can be transmitted with a bandwidth necessary to support codes of length N/2. Such a system is described in U.S. Pat. No. 5,442,662 entitled CODE-DIVISION MULTIPLE-ACCESS COMMUNICATIONS SYSTEM PROVIDING ENHANCED CAPACITY WITHIN LIMITED BANDWIDTH to Fakasawa et al. with is incorporated herein by reference.

[0010] Another method and apparatus for producing a composite code for fast acquisition in a CDMA system may employ a code that is made to appear more complex by the use of one or more masking codes. The composite code generator comprises a plurality of component code generators. The composite codes are used to modulate in-phase and quadrature channels. A receiver has enhanced speed of acquisition because of the shorter time needed to search for composite codes in the quadrature channel, and the plurality of component codes of the in-phase channel are derived from the codes used in the quadrature channel. Such a system is described in U.S. Pat. No. 5,022,049 entitled MULTIPLE ACCESS CODE ACQUISITION SYSTEM to Abrahamson et al. which is incorporated herein by reference.

[0011] In related CDMA systems, a two-tier ciphering method ensures security by cycling code masks. A pseudorandomly generated code key is used to select one of a plurality of scrambling masks. A variant of this method uses orthogonal code hopping or random code hopping. A CDMA system can be viewed as encoding an information signal into blocks of L code symbols, and each block is then encoded with a scrambling mask of length L. A system of this type is described in U.S. Pat. No. 5,353,352, entitled CALLING CHANNEL IN CDMA COMMUNICATIONS SYSTEM to Dent et al. which is incorporated herein by reference.

[0012] Rapid acquisition of the correct code phase by a spread-spectrum receiver is improved by designing spreading codes which are faster to detect. The present embodiment of the invention includes a new method of generating code sequences that have rapid acquisition properties by using one or more of the following methods. First, a long code may be constructed from two or more short codes. The new implementation uses many code sequences, one or more of which are rapid acquisition sequences of length L that have average acquisition phase searches r=log2L. Sequences with such properties are well known to those practiced in the art. The average number of acquisition test phases of the resulting long sequence is a multiple of r=log2L rather than half of the number of phases of the long sequence.

[0013] Second, a method of transmitting complex valued spreading code sequences (In-phase (I) and Quadrature (Q) sequences) in a pilot spreading code signal may be used rather than transmitting real valued sequences. Two or more separate code sequences may be transmitted over the complex channels. If the sequences have different phases, an acquisition may be done by acquisition circuits in parallel over the different code sequences when the relative phase shift between the two or more code channels is known. For example, one of two sequences may he sent on an In phase (I) channel while the other is sent on the Quadrature (Q) channel. To search the code sequences, the acquisition detection means searches the two channels, but begins the (Q) channel with an offset equal to one-half of the length of the spreading code sequence. With a code sequence length of N, the acquisition means starts the search at N/2 on the (Q) channel. The average number of tests to find acquisition is N/2 for a single code search, but searching the (I) and phase delayed (Q) channel in parallel reduces the average number of tests to N/4. The codes sent on each channel may be the same code, with the code phase in one channel being delayed with respect to the other channel, or they may be different code sequences.

[0014] FIG. 1 is a block diagram of a typical code division multiple access communication system spreading code acquisition detector of the prior art.

[0015] FIG. 2 is a block diagram of the spreading code acquisition detector of the present invention.

[0016] In a CDMA communication system where they are a number of users, each user's signal is coded using a unique code sequence. Consequently, a receiver can detect the signal coming from a particular user. The first step in establishing a communication link with a user is to acquire the received spreading code phase. Typically, this process includes determining the phase (shift) of the observed sequence.

[0017] Communication is not possible until the proper spreading code phase has been determined. The invention described here in a new method of designing the code sequences such that a receiver can rapidly determine the received code sequence phase.

[0018] Generally, in a spread spectrum communication system, the receiver does not initially know the received spreading code phase. A particular system may “guess” at a spreading code phase and attempt to despread the received signal. If the despread signal is despread, the receiver declares synchronization, but if the signal is not despread, the receiver adjusts the locally generated code phase to a new value (“guess”) and repeats the test. An exemplary acquisition system is shown in FIG. 1.

[0019] Referring to FIG. 1, the received signal r(t) is applied to a multiplier 101 and multiplied by the locally generated code sequence signal c(t) to attempt to despread the received signal r(t). The signal produced after the despreading either corresponds the received signal power plus noise power is the locally generated code phase is synchorinzed to the received spreading code phase, or corresponds to noise power only if the locally generated code phase is not synchronized to the received spreading code phase. Since the despread signal is a narrowband signal compared to the bandwidth of the spreading code, the output signal of the despreader is applied to the filter 102, which can be a bandpass or matched filter. The output despread signal is applied to the energy detector 103, which is used to measure the despread signal power.

[0020] The decision logic 104 compares the despread signal power to a predetermined threshold value V_τ to decide whether the locally generated code phase is synchronized to the received signal. The decision logic provides a decision value to the control logic which determines whether synchronization is achieved. If there is synchronization, the search is stopped, but if synchronization has not been achieved, the control logic (105) adjusts the code phase of the locally generated signal c(t) by sending the appropriate code phase adjustment signal s(t) to the spreading code waveform generator (106).

[0021] The appropriate code phase adjustment signal s(t) is determined by search technique implemented in the control logic (105). Existing systems typically employ serial search techniques, which are well known in the art. Using these techniques, each code phase is searched one at a time in sequence. Other search techniques may be used, such as a Z-search method by which each code phase before and after a chosen code phase is searched alternatively, each test increasing the phase shift of the tested code phase from the initial chosen code phase. This technique is commonly used to resynchronize a system which has temporarily lost code phase synchronization.

[0022] the method of one embodiment of the present invention uses a transmitted spreading code sequence (a long sequence) which is generated using two short sequences. The long sequence (the new code sequence) is formed by repeating one of the short sequences according to a predetermined method defined by the second short sequence. For example, if the first short sequences is 0110 and the second short sequence is 1100, and if the predetermined method is such that the first sequence is repeated as it is for each 1 in the second sequence and inverted for each 0 in the second sequence, the long sequence is 0110 0110 1001 1001. In another method the first short sequence is repeated as it is when the bit value in the second sequence does not change, and the first sequence is inverted when the bit value in the second sequence changes from 1 to 0 or 0 to 1. In this example, the long sequence would be 0110 0110 1001 0110.

[0023] A specific embodiment of the applicant's invention uses maximal length sequences (m-sequences). The m-sequences are generated using shift register circuits as is well known in the art. These sequences have the important property that if a shift register of length r is used, the period of the m-sequence is N=2′−1, and so r-bit portion of the m-sequence repeats in a period (each r bit section occurs only once in a period). The implication of this property is that, when the second short sequence described above is an m-sequence, the acquisition circuit needs to search only r=log₂N phases of the sequence instead of N phases, which makes acquisition much faster.

[0024] For example, if the first short sequence has a length of 511 code periods or chips, and the second short sequence (m-sequence) has a length of 1023 code periods. Then the long sequence (the final sequence is of length 511×1023=522753. Since 1023=2¹⁰−1, the acquisition circuit will acquire the code in at most 511×10=5110 phases instead of 522753 phases. Therefore the worst-case acquisition is over one hundred times faster.

[0025] It may be desirable for the short code phase to have boundaries which are aligned with information symbols that are transmitted through the channel. Because symbols are typically represented by 2ⁿbits, symbol boundaries will occur on even-numbered bit boundaries. As described above, the short code has a length of 511 code periods. In order for the short code to be aligned with symbol boundaries when the symbols each include 2ⁿbits, it may be desirable to concatenate another bit, either 1 or 0 onto the 551 511 length first short sequence to form a 512 short code. In this instance, the length of the long sequence would be 512×1023=523776 code periods. Alternatively, the second short sequence may be extended to be an even number of code periods. For example, if the second short sequence were extended to 1024 bits, the length of the long sequence would be 511×1024=523264 code periods.

[0026] To further decrease the acquisition time, one embodiment of the invention transmits complex valued spreading code sequences (In-phase (I) and Quadrature (Q) sequences) in a pilot spreading code signal, rather than transmitting real valued sequences. Two or more separate code sequences may be transmitted over the complex channels. If there is a known phase shift between the codes, an acquisition may be done in parallel over the different code sequences.

[0027] In this embodiment, one sequence is used to modulate the In phase phase(I) carrier while the other phase modulates the Quadrature (Q) carrier. To search the code sequences, the acquisition detection means searches the two channels simultaneously. If there is no phase shift between the two code phases, the acquisition means begins the search on the (I) channel at the beginning of the code sequence, but begins the (Q) channel with an offset equal to one-half of the spreading code sequence length. For this example, the acquisition means may search either channel beginning at any particular phase, as long as the search of the other channel begins by offsetting the search by a predetermined code sub-period. For example, with a code sequence length of N, the acquisition means start the search at N/2 on the (Q) channel. The average number of tests to find acquisition is N/2 for a single code search, but searching the (I) and phase delayed (Q) channel in parallel with an initial offset of N/2 code periods, reduces the average number of tests to N/4. The codes sent on each channel may be the same code, the same code sequence but delayed in one channel, or different code sequences.

[0028] An exemplary embodiment of a receiver which uses the fast acquisition sequences of the present invention is shown in FIG. 2. The received signal r(t) is demodulated by the synchronous In-phase demodulator 201 and by the synchronous Quadrature modulator 202 to produce in phase phasechannel signal r_r(t) and quadrature channel signal r_Q(t).

[0029] For the in phase phasechannel signal r_r(t), the locally generated code sequence begins searching the received in phase phasechannel with the long code spreading code sequence using a predetermined initial code phase. After despreading in multiplier 203, the in-phase signal is applied to a bandpass, envelope or matched filter 207 to produce a despread. Next, the energy detector 209 generates a measure of the signal power in the in-phase channels and applies this measure to decision logic 211. The decision logic 211 compares the despread signal energy with the predetermined threshold V_π with three possible outcomes. First, the measured energy level may indicate that the code phase of the locally generated despreading code sequence from the quadrature phasechannel spreading code generator 205 corresponds to acquisition of the correct code phase of the long code sequence. In this instance, the control logic 215 provides long code synchronization signals to spreading code generators 205 and 206 to lock the code phase of the generator 205 and to adjust the generator 206 to the offset code phase. Second, the measured energy level may indicate that the locally generated code phase corresponds to acquisition of the short code phase, in which case the control logic 215 provides short code synchronization signals to the spreading code generators 205 and 206, and initiates the next series of tests. These tests adjust the locally generated code sequence signal phases by the length of the short code instead of by the period of one code sequence value until synchronization of the long code is found. Third, the measured energy level may indicate that the locally generated code phase does not correspond to synchronization of either the long or short code, in which case the control logic continues the serial search by adjusting the phases of the locally generated code sequences by one code sequence period for each successive test.

[0030] The system operates in the same way for the quadrature channel signal r_Q(t). The locally generated code sequence has a phase which is offset by one-half of a code period of the locally generated code sequence used to despread the in phase channel signal r_r(t). After despreading in multiplier 204, bandpass, envelope or matched filtering in the filter 208, and measuring the despread quadrature signal power in the energy detector 210, the decision logic 212 compares the signal to a predetermined threshold V_rQto determine one of three possibilities. First, whether the code phase of the locally generated despreading code sequence from the quadrature channel spreading code generator 206 corresponds to acquisition of the correct code phase of the long code sequence, in which case the control logic 215 provides the long code synchronization signals to spreading code generators 206 and 205 to lock and adjust their respective code phases. Second, whether the locally generated code phase corresponds to acquisition of the short code phase. As with the in-phase channel, in this instance, the control logic 215 provides short code synchorinzation signals to the spreading code generators 205 and 206, and performs the next series of tests by adjusting the locally generated code sequence signal phases by the length of the short code until synchronization of the long code is found. Third, whether the locally generated code phase doe s not correspond to synchronization of either the long or short code in which case the control logic continues the serial search by adjusting the locally generated code sequences phases by one code sequence period for each successive test.

[0031] Further, the control logic 215 may adjust the threshold values V_π and V_rQto greater values when the short code is detected on either the in-phase or quadrature channels to increase the probability of detection and decrease probability of false detection.

[0032] While the invention has been described in terms of an exemplary embodiment, it is contemplated that it may be practiced as outlined above with modifications that are within the scope of the following claims.