The Multiuser Detector (MUD) operates directly .the multipath channel is time invariant. Multiuser

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    CHAPTER 3

    MULTIUSER DETECTION

    3.1 INTRODUCTION

    The Multiuser Detector (MUD) operates directly on the received

    signal, sampled once or several times during each chip period. Such a

    receiver is relatively insensitive to errors in the estimate of the propagation

    delay and other system parameters. Another advantage is that, it can be

    implemented in such a way that only signals that are of interest need to be

    detected.

    Instead of tuning the receiver directly from the received signal, a

    linear filter model of the DS CDMA system is also derived. This

    multivariable model has the symbols transmitted by all users as input and

    the received chip sampled signal as output. This model is then used as a

    basis for the detector design. Both synchronization errors and multi-path

    propagation is incorporated in this model and unknown parameters can be

    estimated with relative ease. The filter model will be time invariant or

    slowly time varying in a system with short codes. When long codes are

    used, the filter model will change abruptly from symbol to symbol, when

    the multipath channel is time invariant. Multiuser detection deals with the

    development and application of joint demodulation and interference

    cancellation techniques for improved detection of a desired set of digital

    signals.

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    A differential matched filter which is used the differential

    information in the FIR filter coefficients mitigates the complexity. This

    complexity reduction is important for the multistage detector. The SNR is

    still maximized but the detector is not Maximum Likelihood (ML) due to

    the presence of Multiple Access Interference (MAI). The effectiveness of

    the conventional receiver is limited because it disregards the structure of

    MAI and is susceptible to disparities in the received powers of the different

    users signals. MUD jointly detecting the signal from different users has

    been under intense research as a potential method to improve the system

    performance of CDMA systems during the last ten years.

    The DS/CDMA receivers are divided into Single user and

    Multiuser detectors. A single user receiver detects the data of one user at a

    time whereas a multiuser receiver jointly detects several users information.

    Single user and multiuser receivers are also sometimes called as

    decentralized and centralized receivers respectively. In DS-CDMA systems

    all users concurrently share the same bandwidth. The users are

    distinguished by assigning to each user a unique code or signature

    sequence, whose bandwidth is much larger than that of the transmitted

    information. This code sequence is used to modulate the data stream.

    3.2 LITERATURE REVIEW

    In the linear multiuser detector for synchronous CDMA systems,

    the received signal is demodulated using an orthogonal multiuser receiver

    that is matched to a set of orthogonal vectors that are closest in a least

    squares sense to the signature vectors. This approach is equivalent to

    optimally whitening the noise component in the output of the decorrelator

    prior to detection reported by Eldar and Oppenheim (2002).

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    In an asynchronous CDMA environment, receiver has

    knowledge of the signature waveforms of all the users with white Gaussian

    noise. The received energies of the users are unknown to the receiver and

    whose BER is independent of the energy of the interfering users discussed

    by Lupas and Verdu (1990). A new real time, digital adaptive multiuser

    receiver structure over white Gaussian noise was designed. This receiver

    efficiently implements the decorrelating detector and adapted to incorporate

    decision feedback detector to further improvement in their detector

    performance (Dao Sheng Chen Roy 1994).

    The adaptive receiver has no knowledge of the signature

    waveforms and timing of other users. The receiver is trained by a known

    training sequence prior to data transmission and continuously adjusted by

    an adaptive algorithm during data transmission. The linear receiver is a

    standard single user detector receiver consisting of a matched filter with

    constant coefficients discussed by Rapajic and Vucetic (1994). Several

    multiuser detectors such as decorrelator, the two stage detector and the

    decision feedback detector for the Raleigh flat fading synchronous CDMA

    channel was compared. In the presence of channel mismatch, the

    decorrelator offers better performance than more complex decision

    feedback and two stage detectors performed by Duel-Hallen et al (1995).

    The performance analysis of the minimum mean square error

    linear multiuser detector is considered in an environment of nonorthogonal

    signaling and additive white Gaussian noise. The probability of error of this

    detector is better than the decorrelating linear detector for all values of

    normalized cross correlations dealt by Poor and Verdu (1997). Adaptive

    receivers depend on number of users, their received powers, spreading

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    codes and time delays. Two adaptive receiver architectures such as the

    sampled received signals are filtered and another in which the spreading

    codes of users is filtered considered by Teng joon lim and Sumit roy

    (1998).

    The adaptive Multiuser Detector which converges to the

    minimum mean square error detector without training sequences and which

    requires less information discussed by Howard C. Huang and Verdu (1998).

    The signal detection based on additive colored non Gaussian noise, and

    attention is focused on one-shot structures where detection of each symbol

    is based only on the received process during its corresponding interval by

    Poor and Verdu (1998). Lupas and Verdu (1989) proposed the performance

    achieved by linear Multiuser Detectors whose linear memory

    transformation is a generalized inverse of the matrix of signature waveform

    cross correlations.

    The multiuser receiver is quite effective in increasing capacity of

    users with higher data rate and lower target bit error rate for multirate

    CDMA system described by Guo and Aazhang (1999). The front-end

    Recursive Least Square-minimum mean square error filters can be

    implemented using systolic arrays to exploit massively parallel signal

    processing computation, and to achieve energy efficiency by Xiaodong

    wang and Poor (1998).

    Moshavi (1996) discussed with multiuser detection, where

    information about multiple users is used to improve detection of each

    individual user. The optimum receiver to detect the bits of multiple CDMA

    users has an exponential complexity in the number of active users using

    nonlinear programming relaxations define by Yener et al (2002).

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    Komninakis et al (2002) dealt with algorithm offers good

    tracking behavior for multiuser fading ISI channels at higher complexity

    than conventional adaptive algorithms. Fuxjaeger and Iitis (2004) proposed

    the receiver consisting of a parallel bank of extended Kalman filters extracts

    estimates of the timing, and the multipath coefficients. The cross coupled

    decision feedback detector is suggested to reduce both inter symbol

    interference and multiple access interference for an asynchronous

    multicarrier CDMA scheme suggested by Liu et al (2004).

    The partial cancellation technique and minimum mean square

    error multiuser detection techniques are used for interference cancellation

    and the performance of this receiver is evaluated in Rayleigh multipath

    fading channel by Kumpoopong et al (2005). Mitsuhiro Tomita

    et al (2005) concentrated on sequential detection technique for a multiuser

    receiver that is constructed over a CDMA system. In this system, the

    transmitter transmits a symbol made by spreading the spectrum with an

    enveloped sequence protected by guard sequences, and a receiver

    demodulates the core sequence part of the received symbol with either a

    decorrelating detector.

    The adaptive multiuser detector has two major disadvantages.

    First, it cannot be applied in systems with long codes. Second, in some

    cases the rate of convergence may be too slow. However, these two issues

    are used in tuning procedure.

    The other group of detectors is based upon Interference

    Cancellation (IC). The idea is to cancel the interference generated by users

    other than the desired user. The lower computation demand and hardware

    related structures are the major advantages of this strategy. This category

    includes Serial Interference Cancellation (SIC) and Parallel Interference

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    Cancellation (PIC). One of the most effective PICs comes from the iterative

    multistage method. The inputs of one particular stage are the estimated bits

    of the previous stage. After interference cancellation, the new estimations,

    which should be closer to the transmitted bits, come out to be fed into the

    next stage.

    Therefore in last several stages, the multistage detector will

    almost compute from the same input to generate the same output. This is a

    substantial waste of the computation power and it increases the system

    delay.

    To overcome these limitations, a modified linear multiuser

    receiver has been proposed for synchronous CDMA sy