WO2008086044A1 - Procédé de détection de probabilité maximale locale mis en oeuvre dans un système de communication - Google Patents

Procédé de détection de probabilité maximale locale mis en oeuvre dans un système de communication Download PDF

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Publication number
WO2008086044A1
WO2008086044A1 PCT/US2008/000485 US2008000485W WO2008086044A1 WO 2008086044 A1 WO2008086044 A1 WO 2008086044A1 US 2008000485 W US2008000485 W US 2008000485W WO 2008086044 A1 WO2008086044 A1 WO 2008086044A1
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bit
cdma
detector
user
bits
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PCT/US2008/000485
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English (en)
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Yi Sun
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Yi Sun
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/7097—Interference-related aspects
    • H04B1/7103—Interference-related aspects the interference being multiple access interference
    • H04B1/7105—Joint detection techniques, e.g. linear detectors
    • H04B1/71057—Joint detection techniques, e.g. linear detectors using maximum-likelihood sequence estimation [MLSE]

Definitions

  • the embodiments of the present invention relate to a CDMA multiuser detector, and more particularly, the embodiments of the present invention relate to a method of local maximum likelihood detection with bit extending and multiplexing to approach the optimum performance as well as the single-user performance in the high SNR regime in random densely and sparsely spread CDMA.
  • CDMA code division multiple access
  • the radio frequency spectrum suitable for wireless communications becomes more and more stringent and expensive.
  • GML global maximum likelihood
  • the GML detector minimizes the bit error rate (“BER"), which can be lower than the BER of the conventional MF by a factor of 1000 or more, but it is NP-hard in complexity and thus is prohibited in practical communication systems.
  • the liner multiuser detectors such as MF, decorrelator, and MMSE, have per-bit complexity liner to the number of users but their BER is far worse than that of the GML detector.
  • the nonlinear suboptimal detectors such as the PIC, PDA, SDR, coordinate ascent, and SDP, etc. — though lower than NP-hard — still have a complexity higher than liner and thus is not feasible for practical communication systems. Moreover, they still perform much worse than the GML detector.
  • phase information can be derived so as to generate a reference for coherent demodulation.
  • United States Patent Number 5,917,829 issued to Hertz et al. on June 29, 1999 in class 370 and subclass 47 teaches a method and apparatus for optimally decoding messages from a CDMA signal sent by a plurality of users and received asynchronously at one receiver using a minimal number of computations and minimal memory and processing resources. It is based on signal correlation, as well as to subsequent decoding by decorrelation, requiring a correlation period of only one symbol length, by providing for each user a pair of partial signatures sequences, with which the signal is correlated, deccorelating the results with the inverse of the cross-correlation matrix of all partial sequences, and combining the resultant partial symbol estimates to obtain final estimated symbol values.
  • the partial sequences are formed by separating each original sequence at a point corresponding to the estimated symbol boundary time, relative to an arbitrary correlation window, of one symbol length.
  • the method can be modified to also apply to the case that any of the signals is received over multiple paths, any path possibly undergoing Doppler shift.
  • an adaptive signal receiver including at least one blind detection unit arranged to be robust to asynchronous multiple access interference ("MAI").
  • the useful signal is detected using a user signature sequence including a fixed term and a complex adaptive part having a length extending over a number of samples within a defined observation window. Provision is made for updating automatically and periodically the complex adaptive part of the signature sequence.
  • United States Patent Number 6,654,365 issued to Sylvester et al. on November 25, 2003 in class 370 and subclass 342 teaches a maximum likelihood (“ML”) detector providing performance in the presence of multiple user interference (“MUI”), particularly performance of a multiuser receiver for asynchronous CDMA.
  • the detector can be implemented using a Viterbi algorithm.
  • An approximation to the ML detector provides a sparse-trellis search based on the structure of the ML detector.
  • the resulting detector which may be referred to as a reduced-complexity recursive detector ("RCRD"), has a dynamic structure allowing a trade-off between complexity and performance.
  • Use is made of a metric to define the trellis-structure and the M-algorithm to reduce the number of surviving paths.
  • the metric calculation is then repeated at decision points to provide soft-decision information for further signal processing, soft-decision decoding of an error-correction code, or iterative reception of the multiuser signal.
  • United States Patent Number 7,076,015 issued to Bhatoolaul et al. on July 1 1, 2006 in class 375 and subclass 365 teaches a method of detecting one of a set of preamble sequences in a spread signal.
  • the method includes the steps of correlating the received spread signal with sequences of a first orthogonal Gold code ("OGC") set in accordance with a first fast transform to provide a preamble signal, correlating the preamble signal with the set of preamble sequences in accordance with a second fast transform to generate a set of index values, forming a decision statistic based on the set of index values, and selecting, as the detected one of the set of preamble sequences, a preamble sequence corresponding to the decision statistic.
  • GOC orthogonal Gold code
  • the forming step includes the steps of forming an initial decision statistic based on the relative maximum index of the set of index values, selecting the signal generated by the preamble sequence combined with the preamble signal corresponding to the initial decision statistic, adjusting, in one or more of amplitude and phase, the signal selected in the selecting step, and forming the decision statistic based on the adjusted signal.
  • another object of the embodiments of the present invention is to provide a method of local maximum likelihood detection with bit extending and multiplexing to approach optimum performance as well as the single-user performance in the high SNR regime in random densely and sparsely spread CDMA.
  • the method is based on local maximum likelihood ("LML") detectors and bit extending and multiplexing, and can approach optimum performance of a GML detector as well as the single-user performance in the high SNR regime, while average per-bit complexity is liner in the number of detected bits and thus is suitable for implementation in practical CDMA systems.
  • LML local maximum likelihood
  • the method does not change total RF bandwidth, data transmission rate, and transmission power for each user, while achieving GML and single-user performance.
  • the method includes the steps of: extending, at a transmitter, each bit by a factor of integer B to have BT h seconds where T h is the bit period; selecting randomly equiprobably, by each user, B extended spreading sequences, each of which having BN chips and a unit length; spreading an extended bit by an extended spreading sequence; transmitting, by each user, B extended bits, with the integer B being sufficiently large so as to allow a total number of transmitted bits BK to be greater than about five hundred; and, applying, at a receiver, a LAS detector or an LML detector to detect the transmitted BK bits.
  • the spreading sequence can be densely or sparsely spread. In a densely spread sequence, all chips are i.i.d.
  • each spares sequence has L « BN nonzero chips that are randomly located and equiprobably take on ⁇ 1/VZ! While the bit extending factor is the same for all users, each user can also have a different number of multiplexed bits to achieve a different and adjustable transmission rate.
  • the GML detector in CDMA communication systems is well-known to achieve a bit error rate (“BER") that is the lowest for all possible detectors and can be significantly lower than the BER of a matched filter, which is used in practical CDMA systems, say by a factor of over a thousand.
  • BER bit error rate
  • the GML detector is NP-hard, whose per-bit complexity exponentially increases as the number of users increases and thus is prohibited to be implemented in practical CDMA systems.
  • the developed family of likelihood ascent search (“LAS”) detectors, as well as the developed family of LML detectors achieved a BER monotonically decreasing and reaching the BER of the GML detector as the number of users increases to over five hundred in a random spreading CDMA system. This phenomenon has been verified recently by theoretical analysis.
  • the embodiments of the present invention construct a quasi-large random sequence CDMA (QLRS-CDMA) system as well as quasi — large sparse sequence CDMA (QLSS-CDMA).
  • QLRS-CDMA quasi-large random sequence CDMA
  • QLSS-CDMA quasi — large sparse sequence CDMA
  • the bit extending and multiplexing do not change data transmission rate, bandwidth, and transmission power of the original CDMA system.
  • the constructed CDMA system transmits a total of BK bits in each time, which corresponds to a CDMA system with BK users, with BK being arbitrarily large if B is sufficiently large.
  • BK is over five hundred, the LAS and the LML detectors can achieve the BER of the GML detector as well as single-user BER in the high SNR regime for practical CDMA systems.
  • each sparse sequence has L « N nonzero chips that are randomly located and equiprobably take on ⁇ 1 /vTT
  • the sequence can be initially assigned to each user corresponding to a practically short sequence or randomly chosen bit-by-bit corresponding to a practically long sequence.
  • bits are synchronized by some synchronization techniques, the approach can be applied to bit-asynchronous CDMA systems.
  • the chip-matched filter at the receiver outputs the following TV-dimensional vector:
  • S (S 1 , ..., s ⁇ ) is the matrix of spreading sequences, each of which is normalized to Il s
  • Il I 5
  • A diag( ⁇ ,, ..., A x ) is the diagonal matrix of user's received signal amplitudes
  • b E ⁇ -1, l ⁇ ⁇ ' is the vector of transmitted bits of K users
  • m ⁇ N(O, O 2 I ⁇ ) is an N- dimensional white Gaussian noise random vector.
  • the same statistic y can be also obtained by a standard matched filter (“MF”) bank matching the signature waveforms in the received CDMA signal. IfK is sufficiently large, say K >500, the CDMA system is called a large random spreading CDMA (“LRS-CDMA”) system.
  • MF matched filter
  • LRS-CDMA large random spreading CDMA
  • QLRS-CDMA quasi-large random sequence CDMA
  • each user transmits a number of extended bits that are multiplexed.
  • the spreading sequences can be densely or sparsely spread. In a densely spread sequence, all chips are i.i.d. with mean zero and variance ⁇ /(BN), say equiprobably take on ⁇ ⁇ N BN. In a sparsely spread sequence, there are only a small number of nonzero chips and most chips are zero. The nonzero chips are randomly located and are i.i.d.
  • each sparse sequence has L « BN nonzero chips that are randomly located and equiprobably take on ⁇ 1/vTT
  • the sequences for each user can be assigned in two ways. One is that user k initially, randomly, and equiprobably selects B k sequences, and then in every transmission uses the same B k spreading sequences to spread B k multiplexed bits. The other is that a user newly, randomly, and equiprobably selects B k spreading sequences for every B k transmitted bits in an extended period. The former are practically called short sequences and the latter are practically long sequences.
  • the bit sequence of each user can be coded or uncoded. Then, during B bit periods, the chip MF at the receiver outputs the following Z?vV-dimensional real vector:
  • the CDMA system in equation (3) above where bit duration is extended by a factor of B and B k bits of user k are multiplexed, has the same number of users K, bandwidth W, data transmission rate ⁇ IT h , received signal amplitude A k for each user, and the system load a if B k — B for all k.
  • the total number of transmitted bits BK can be arbitrarily large as B increases.
  • B is sufficiently large so that BK > ⁇ 00, it is a QLRS-CDMA system.
  • a QLRS-CDMA system is identical to an LRS-CDMA system.
  • the QLRS-CDMA becomes the QLSS- CDMA.
  • LAS detectors 1 Two families of multiuser detectors for CDMA multiuser detection have been developed by applicant.
  • LML detectors 2 The family of LAS detectors contains many special instances, each operating in a sequence of updating modes.
  • WSLAS detectors A particularly interested class of LAS detectors in the family is called the wide- sense sequential LAS ("WSLAS") detectors that are also LML detectors with neighborhood size one and have an average per-bit complexity linear in the number of transmitted bits.
  • the WSLAS detectors include the eliminating-highest-error and fastest-
  • the embodiments of the present invention apply the family of LAS detectors and the family of LML detectors to the proposed QLRS-CDMA and QLSS-CDMA systems.
  • the GML detector in CDMA communication systems is well-known to achieve a bit error rate ("BER") that is the lowest for all possible detectors and can be lower than the BER of a matched filter, which is used in practical CDMA systems, by a factor of over a thousand.
  • BER bit error rate
  • the GML detector is NP-hard, whose per — bit complexity exponentially increases as the number of users increases, and thus is prohibited to be implemented in practical CDMA systems.
  • the developed family of likelihood ascent search (“LAS") detectors achieved a BER monotonically decreasing and reaching the BER of the GML detector as well as the single-user BER in the high SNR regime as the number of users increases to over five hundred in a random spreading CDMA system.
  • This phenomenon has been verified recently by theoretical analysis. It is important that the LAS and the LML detectors can be implemented with only an average per-bit complexity linear in the number of transmitted bits and therefore can be implemented in practical CDMA systems. Practical CDMA systems, however, usually have about fifty users or fewer, much less than five hundred.
  • the method extends transmitted bits and spreads sequences by a same integer factor of B.
  • Each user multiplexes and transmits B extended bits in a B bit period (in practice, each user can also multiplex a different number of bits to achieve different data transmission rate).
  • the bit extending and multiplexing do not change data transmission rate, bandwidth, and transmission power of the original CDMA system.
  • the constructed CDMA system transmits a total of BK bits in each time, which corresponds to a CDMA system with BK users, with BK being arbitrarily large if B is sufficiently large.
  • BK is over five hundred, the LAS and the LML detectors can achieve the BER of the GML detector as well as the single-user BER in the high SNR regime for practical CDMA systems.
  • the system can be a centralized cellular network or an ad hoc network.
  • communication can be either an up-link or a down-link.
  • the method of local maximum likelihood detection with bit extending and multiplexing to approach optimum performance as well as the single-user performance in the high SNR regime in CDMA comprises the steps of:
  • Step 1 Extending, at a. transmitter, each bit by a factor of integer B to have BT h seconds;
  • Step 3 Spreading an extended bit by an extended spreading sequence
  • Step 5 Applying, at a receiver, an LAS detector or an LML detector to detect the transmitted B x + B 2 + ... + B ⁇ bits, with many LAS and LML detectors being seen in the discussion herein.
  • the family of LAS detectors and the family of LML detectors have the characteristic that the BER in QLRS-CDMA (or QLSS-CDMA) systems, as well as LRS- CDMA systems, monotonically decreases as the total number of multiplexed bits increases — with a kept unchanged K/N. Moreover, when the total number of transmitted bits is greater than 500, the LML detectors — including the WSLAS detectors in the family of LAS detectors — approach the BER of the GML detector as well as the single- user BER in the high SNR regime. The latter means that these detectors achieve the single user bound as if there is no interference bit in the high SNR regime. More detailed theoretical analysis and simulation results can be found in footnote 8 .
  • the average per-bit complexity for the family of LAS detector is lower than 0.79BK, which is linear in the number of multiplexed and transmitted bits BK, and for the LML detector with neighborhood size J is at the order of the number of combinations selecting J out of BK.
  • the LML detectors with neighborhood size one including the WSLAS detectors — are particularly useful because while achieving the BER of the GML detector, they have a complexity linear in the number of transmitted bits and thus is suitable for implementation in practical CDMA systems.
  • the embodiments of the present invention are a significant innovation for wireless-communications. They will be adoptable in the standards of next generation wireless communications that will be popularly used worldwide.
  • the 3 rd generation standard of wireless communications is being commercialized, but no multi-user detection technique is properly used. As the number of wireless customers is ever increasing, however, multi-user detection technique must be ultimately used in the next generations standard, i.e., 4 th and 5 th .
  • the embodiments of the present invention are a strong candidate for the multi-user detection in the next generation wireless systems.
  • the embodiments of the present invention are the core technique. They target the next generation of wireless communications with high data rate transmissions.
  • bit flip rate (BFR) is defined as the number of bit flips divided by the total number of detected bits. Then the per-bit complexity is equal to the BFR times BK. U Correspondingly, if BFR is a constant regardless of BK, then the complexity is linear in BK. In all simulations, only BK is given, and thus the results are applicable to any pair of integers B and K with the given BK.
  • the BER's and the BFR's for five samples of spreading sequences are estimated and are shown with their averages in the drawing, which will be discussed in more detail below. As reference, the BER's of the MMSE-DF and the SIC detectors are estimated in simulations. The BER's of the MF, decorrelator, MMSE, and GML detectors in the LRS-
  • Y. Sun "A family of likelihood ascent search multiuser detectors; an upper bound of bit error rate and a lower bound of asymptotic mulituser efficiency" submitted to IEEE Trans. Commun. (ArXiv:071 1.3867); Y. Sun, "A family of likelihood ascent search multiuser detectors: approach to single-user performance by random sequences in CDMA," submitted to IEEE Trans. Commun. (ArXiv:071 1.3867).
  • CDMA limit are also shown based on the result found in footnote l2 . In all figures showing BER, the initial vector of the GPLAS detectors is MF, while the BFR is shown with both the random vector and the MF, respectively, being the initial vector. Given a fixed set of spreading sequences, all these suboptimal detectors including the LAS detectors are linearly complex.
  • the BER' s of the GPLAS detectors monotonically decrease.
  • the group size is relatively small — say the WSLAS detector — the BER' s achieve the BER of the GML detector when BK is greater than one thousand.
  • the average per-bit complexity of all the GPLAS detectors shown in FIGURE l(b) is less than 0.65BK with the random initial vector and only less than 0.23BK with the MF initial detector. In contrast, the complexity is the higher 1.5BK for the MMSE-DF and the NP-hard O(2 BK l ⁇ BKj) for the GML detector.
  • the GPLAS detectors perform much better than the other suboptimal detectors.
  • the complexity of the GPLAS detector is less than 0.79BK with a random initial vector and less than QAlBK with the initial MF detector.
  • the initial detector has no affect on the BER of the GPLAS detectors when BK ⁇ - 1 OO.
  • the MF initial increases the transition a by about 0.05 (i.e. from 1.1 to 1.15).
  • the SLAS detector with updating bits cyclically bit by bit is employed.
  • the initial detector is the MF.
  • the BER of the fixed point for each of the SLAS detectors is reported.
  • BK ⁇ 128, completely random spreading sequences are used such that a set of sparse sequences is randomly selected in each transmission.
  • BK > 128, five sparse sequences with L nonzero chips are randomly selected and fixed for all transmissions, and the BER' s for five samples are estimated and shown together with their averages in figures.
  • the number of additions per bit counted from the core operation is also estimated.
  • BK 0.8 bits/s/Hz
  • SNR 11 dB.
  • the complexity in additions per bit monotonically increases with increasing BK and L but is saturated with respect to BK for small L ( ⁇ 16). Hence, using sparse sequences with L
  • the complexity of the SLAS detector is insensitive to SNR and the value can be seen from FIGURES 4 (b) and 5 (b).
  • the WSLAS detectors approach almost the same performance of the SLAS detector since both are LML detectors.
  • CDMA monotonically decreases and the complexity increases as L increases.
  • L 16
  • the BER of the QLSS CDMA is already very close to the BER of the QLRS CDMA, approaching the single-user performance in the high SNR regime; and the complexity is significantly reduced by several orders compared with QLRS CDMA. Since the core operation of LAS detector is the update of likelihood gradient, 10 the sparse sequences can significantly reduces the complexity of the LAS detector.

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Abstract

L'invention concerne un procédé de détection de probabilité maximale locale présentant une extension et un multiplexage de bits permettant de s'approcher d'une performance optimale, ainsi que de la performance 'utilisateur unique' dans un régime SNR élevé, dans un AMRC aléatoirement étalé de façon dense et éparse. Le procédé repose sur des détecteurs de probabilité maximale locale ('LML') ainsi que sur l'extension et le multiplexage de bits, et permet de s'approcher de la performance optimale d'un détecteur GML, ainsi que de la performance 'utilisateur unique' dans le régime SNR élevé, tandis que la complexité moyenne par bit est linéaire dans le nombre de bits détectés et est ainsi appropriée à une mise en œuvre dans des systèmes AMRC pratiques.
PCT/US2008/000485 2007-01-13 2008-01-14 Procédé de détection de probabilité maximale locale mis en oeuvre dans un système de communication WO2008086044A1 (fr)

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102035610A (zh) * 2010-12-15 2011-04-27 南京邮电大学 基于初始状态向量控制的全反馈神经网络盲检测方法
CN102035609A (zh) * 2010-12-15 2011-04-27 南京邮电大学 基于复数连续全反馈神经网络的信号盲检测方法
CN102035769A (zh) * 2010-11-24 2011-04-27 南京邮电大学 基于复数离散全反馈神经网络的相移键控信号盲检测方法
CN105978616A (zh) * 2016-04-18 2016-09-28 重庆大学 结合遗传性质的大规模mimo系统las信号检测算法

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US20050265430A1 (en) * 1995-06-30 2005-12-01 Interdigital Technology Corporation System for using rapid acquisition spreading codes for spread-spectrum communications

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US5341318A (en) * 1990-03-14 1994-08-23 C-Cube Microsystems, Inc. System for compression and decompression of video data using discrete cosine transform and coding techniques
US20040252668A1 (en) * 1995-06-30 2004-12-16 Interdigital Technology Corporation Code division multiple access (CDMA) communication system
US20050265430A1 (en) * 1995-06-30 2005-12-01 Interdigital Technology Corporation System for using rapid acquisition spreading codes for spread-spectrum communications

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102035769A (zh) * 2010-11-24 2011-04-27 南京邮电大学 基于复数离散全反馈神经网络的相移键控信号盲检测方法
CN102035769B (zh) * 2010-11-24 2013-05-08 南京邮电大学 基于复数离散全反馈神经网络的相移键控信号盲检测方法
CN102035610A (zh) * 2010-12-15 2011-04-27 南京邮电大学 基于初始状态向量控制的全反馈神经网络盲检测方法
CN102035609A (zh) * 2010-12-15 2011-04-27 南京邮电大学 基于复数连续全反馈神经网络的信号盲检测方法
CN105978616A (zh) * 2016-04-18 2016-09-28 重庆大学 结合遗传性质的大规模mimo系统las信号检测算法

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