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Group-based approaches to space-time multiuser detection in WCDMA
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We investigate a chip rate implementation of the MMSE multiuser receiver in the case of uplink UMTS context where data are scrambled by long codes (Gold sequences). The main problem is the resolution of a high dimension linear system involving a band-diagonal auto-correlation matrix. Using a Cholesky (or LDU) decomposition and taking advantage of the multiuser correlation expression, we show that the complexity of this chip rate MMSE is similar to that of the more classical symbol rate MMSE receiver. Then, a theoretical equivalence between these two receivers is demonstrated. Finally, simulation results are presented which show the performance gains of the MMSE receiver when compared to the RAKE receiver.
In this letter, optimal power allocation and capacity regions are derived for groupwise successive interference cancellation (GSIC) systems operating in multipath fading channels, under imperfect channel estimation conditions. It is shown that the impact of channel estimation errors on the system capacity is two-fold: It affects the receiver performance within a group of users, as well as the cancellation performance (through cancellation errors). An iterative power allocation algorithm is derived, based on which it can be shown that that the total required received power is minimized when the groups are ordered according to their cancellation errors, and the first detected group has the smallest cancellation error. Performance/complexity tradeoff issues are also discussed by directly comparing the system capacity for different implementations: GSIC with linear minimum-mean-square error (LMMSE) receivers within the detection groups; GSIC with matched filter (MF) receivers; multicode LMMSE systems; and simple all MF receivers systems.
Spatial antenna diversity has been important in improving the
radio link between wireless users. Historically, microscopic antenna
diversity has been used to reduce the fading seen by a radio receiver,
whereas macroscopic diversity provides multiple listening posts to
ensure that mobile communication links remain intact over a wide
geographic area. In later years, the concepts of spatial diversity have
been expanded to build foundations for emerging technologies, such as
smart (adaptive) antennas and position location systems. Smart antennas
hold great promise for increasing the capacity of wireless
communications because they radiate and receive energy only in the
intended directions, thereby greatly reducing interference. To properly
design, analyze, and implement smart antennas and to exploit spatial
processing in emerging wireless systems, accurate radio channel models
that incorporate spatial characteristics are necessary. In this
tutorial, we review the key concepts in spatial channel modeling and
present emerging approaches. We also review the research issues in
developing and using spatial channel models for adaptive antennas
Third-generation mobile radio systems use time division-code
division multiple access (TD-CDMA) in their time division duplex (TDD)
mode. Due to the time division multiple access (TDMA) component of
TD-CDMA, joint (or multi-user) detection techniques can be implemented
with a reasonable complexity. Therefore, joint-detection will already be
implemented in the first phase of the system deployment to eliminate the
intracell interference. In a TD-CDMA mobile radio system,
joint-detection is performed by solving a least squares problem, where
the system matrix has a block-Sylvester structure. We present and
compare several techniques that reduce the computational complexity of
the joint-detection task even further by exploiting this block-Sylvester
structure and by incorporating different approximations. These
techniques are based on the Cholesky factorization, the Levinson
algorithm, the Schur algorithm, and on Fourier techniques, respectively.
The focus of this paper is on Fourier techniques since they have the
smallest computational complexity and achieve the same performance as
the joint-detection algorithm that does not use any approximations.
Similar to the well-known implementation of fast convolutions, the
resulting Fourier-based joint-detection scheme also uses a sequence of
fast Fourier transforms (FFTs) and overlapping. It is well suited for
the implementation on parallel hardware architectures
The third-generation mobile radio system UTRA that has been
specified in the Third Generation Partnership Project (3GPP) consists of
an FDD and a TDD mode. This paper presents the UTRA TDD mode, which is
based on TD-CDMA. Important system features are explained in detail.
Moreover, an overview of the system architecture and the radio interface
protocols is given. Furthermore, the physical layer of UTRA TDD is
explained, and the protocol operation is described
Although the multistage interference cancellation detector is
simple in structure, its performance degrades when the number of active
users becomes large. In some cases, the performance is even worse than
that without cancellation, due to the lack of the exact knowledge of the
interfering signal in cancellation. Partial interference cancellation
suggested by Divsalar and Simon (see IEEE Trans. Commun., vol.46,
p.258-68, 1998) tries to remedy this weakness by reducing the cost of a
wrong interference estimation through a weight in each stage. This paper
presents an adaptive multistage structure based on the partial
interference cancellation approach. In this structure, the weights are
obtained by minimizing the mean-square error between the received signal
and its estimate through a least mean square (LMS) algorithm. The
resulting weights contain reliability information for the hard decisions
made in the previous stage. Neither a training sequence nor a pilot
signal is needed in the proposed scheme, and its complexity is much
lower than that of linear multiuser detectors. Simulation results show
that the proposed scheme can outperform some of the existing
interference cancellation methods in both the additive white Gaussian
noise (AWGN) and the multipath fading channels
The implementation of advanced DS-CDMA receivers based on
multiuser detection principles is becoming a reality thanks to the
combination of an improved understanding of the theoretical basis of
multiuser detection and advances in digital, mixed-signal, and RF
technologies. Due to their lower complexity, subtractive interference
cancellation approaches are attractive for the practical implementation
of multiuser detection. In a parallel interference cancellation
receiver, it is practical to use the soft outputs of a matched filter
bank for amplitude estimation. A bias arises in the decision statistics,
however, due to imperfect estimation and interference cancellation. In
this paper, the source of the bias is explicitly recognized, and a
partial interference cancellation scheme that mitigates the negative
effects of biased estimation and significantly improves system
performance is proposed. A practical real-time algorithm that
significantly reduces the implementation complexity of this scheme
without sacrificing performance is then derived. To facilitate a
software radio implementation, the signal processing complexity of the
approach is characterized. The real-time processing algorithm is tested
via implementation in software on a floating-point general-purpose DSP.
The prototype includes a flexible software-based architecture which
performs IF sampling and uses digital downconversion prior to baseband
processing. The hardware test setup is described, and the results are
presented and compared with simulation and analytical results. The
experimental results confirm the simulation and analytical results which
show large performance gains over the conventional matched filter
The paper addresses Rayleigh fading, primarily in the UHF band, that affects mobile systems such as cellular and personal communication systems (PCS). The paper itemizes the fundamental fading manifestations and types of degradation. Two types of fading, large-scale and small-scale, are described. Two manifestations of small-scale fading (signal dispersion and fading rapidity) are examined, and the examination involves two aspects: time and frequency. Two degradation categories are defined for dispersion: frequency selective fading and flat fading. Two degradation categories are defined for fading rapidity: fast and slow. A mathematical model using correlation and power density functions is presented. This model yields a symmetry to help view the Fourier transform and duality relationships that describe the fading phenomena
The group decision feedback (GDF) detector is studied in this letter. Given the maximum group size, a grouping algorithm is proposed. It is shown that the proposed grouping algorithm maximizes the symmetric energy of the multiuser detection system. Furthermore, based on a set of lower bounds on asymptotic group effective energy (AGEE) of the GDF detector, it is shown that the proposed grouping algorithm, in fact, maximizes the AGEE lower bound for every group of users. The theoretical analysis of the grouping algorithm enables the offline estimation of the computational cost and the performance of a GDF detector. The computational complexity of a GDF detector is exponential in the largest size of the groups. Simulation results are presented to verify the theoretical conclusions. The results from this letter can be applied to the decision feedback detector by setting the maximum group size to one.
Multiuser decision-feedback detectors (DFDs) for direct-sequence code-division multiple access, based on the minimum mean-squared error (MMSE) performance criterion, are described. Both successive and parallel feedback (interference cancellation) with hard decisions are considered. An iterative DFD is presented, which consists of cascaded DFDs, each performing successive cancellation. The two-stage DFD achieves the single-user bound in the absence of error propagation, and performs significantly better than an MMSE DFD with parallel feedback. The filter structures are generalized to include finite impulse response feedforward and feedback matrix filters, which account for asynchronous users and intersymbol interference. The effect of error propagation is illustrated through simulation. Both uncoded and coded performance results are presented. Although error propagation can significantly degrade performance, the DFDs still offer a significant performance gain relative to linear MMSE detection.
We consider the convergence in norm of several iterative
implementations of linear multiuser receivers, under the assumption of
long random spreading sequences. We find that asymptotically, linear
parallel interference cancellation diverges for systems loads of greater
than about 17%. Using known results from the theory of iterative
solutions for linear systems we derive optimal or near-optimal
relaxation parameters for parallel (first- and second-order stationary,
Chebyshev) and serial cancellation (successive relaxation) methods. An
analytic comparison of the asymptotic convergence factor for the various
methods is given. Simulations are used to verify results for finite size
systems
Linear parallel interference cancellation (PIC) schemes are
described and analyzed using matrix algebra. It is shown that the linear
PIC, whether conventional or weighted, can be seen as a linear matrix
filter applied directly to the chip-matched filtered received signal
vector. An expression for the exact bit-error rate (BER) is obtained,
and conditions on the eigenvalues of the code correlation matrix and the
weighting factors to ensure convergence are derived. The close
relationship between the linear multistage PIC and the steepest descent
method (SDM) for minimizing the mean squared error (MSE) is
demonstrated. A modified weighted PIC structure that resembles the SDM
is suggested which approaches the minimum MSE (MMSE) detector rather
than the decorrelator. It is shown that for a K-user system, only K PIC
stages are required for the equivalent matrix filter to be identical to
the the MMSE filter. For fewer stages, techniques are devised for
optimizing the choice of weights with respect to the MSE. One unique
optimal choice of weights is found, which will lead to the minimum
achievable MSE at the final stage. Simulation results show that a few
stages are sufficient for near-MMSE performance
In this paper, we describe linear successive interference
cancellation (SIC) based on matrix-algebra. We show that linear SIC
schemes (single stage and multistage) correspond to linear matrix
filtering that can be performed directly on the received chip-matched
filtered signal vector without explicitly performing the interference
cancellation. This leads to an analytical expression for calculating the
resulting bit-error rate which is of particular use for short code
systems. Convergence issues are discussed, and the concept of
ε-convergence is introduced to determine the number of stages
required for practical convergence for both short and long codes
3rd generation radio systems will be increasingly developed, deployed and operated in the years to come. TDD is one of two main approaches to implementing these 3G systems, so that there will be an increasing need for the engineering community to learn quickly and comprehensively about the TDD technology. As 3G systems become popular, the topics will no doubt be introduced to academic curricula and will also provide a basis for future research. This book provides comprehensive coverage of TDD. It is essentially a Radio Access Network technology and the book embraces the structure of the radio interface as well as the user equipment and network equipment. In addition, Wideband TDD also covers the connection of the TDD Radio Access Network to the 3G Core Network and public switched networks (PSTN) as well as public and private packet networks (Internet and Intranet). Services, applications and performance are also addressed. Finally, TDD is compared with other radio access technologies, namely FDD, TD-SCDMA and WLAN. TD-SCDMA is the Narrowband version of TDD in 3G, and WLAN standards address wireless computer communications. Although there are a number of books published on 3G and UMTS, most of the focus of these books has been on FDD component of 3G. Wideband TDD: Describes all aspects of TDD in a single comprehensive manner Addresses TDD technology, TDD systems and the TDD market place Discusses deployment scenarios and Radio Resource Management for TDD Provides a comparison of TDD with other radio access technologies, namely FDD, TD-SCDMA and wireless LANs This will prove an essential addition to the bookshelf of professional communication and software engineers, development engineers, technical marketing professionals, researchers in industry, wireless equipment vendors such as Siemens, Nokia and InterDigital, operators and service providers. It will also provide a comprehensive overview of TDD for postgraduates who are taking advanced courses in Mobile Wireless communications.
Preface Acknowledgements 1. Direct solution methods 2. Theory of matrix eigenvalues 3. Positive definite matrices, Schur complements, and generalized eigenvalue problems 4. Reducible and irreducible matrices and the Perron-Frobenius theory for nonnegative matrices 5. Basic iterative methods and their rates of convergence 6. M-matrices, convergent splittings, and the SOR method 7. Incomplete factorization preconditioning methods 8. Approximate matrix inverses and corresponding preconditioning methods 9. Block diagonal and Schur complement preconditionings 10. Estimates of eigenvalues and condition numbers for preconditional matrices 11. Conjugate gradient and Lanczos-type methods 12. Generalized conjugate gradient methods 13. The rate of convergence of the conjugate gradient method Appendices.
Optimum signal processing in the receiver of future cellular CDMA mobile radio systems requires the estimation of the actual channel impulse responses on the basis of training signals which are embedded into the transmitted signal. Up to now, channel estimation in the case of the uplink, in which each user signal passes through a different radio channel and in which a different training signal is needed for each user turns out as so expensive that the application of CDMA in spite of its advantages appears questionable. In this paper, a channel estimation method is proposed and analyzed in which the different training signals of all users are time displaced versions of one and the same basic signal. Thus a low cost realization of the channel estimator in the form of a correlator becomes possible.
A spread-spectrum multiple-access (SSMA) communication system is treated
for which both spreading and error control is provided by binary PSK
modulation with orthogonal convolutional codes. Performance of
spread-spectrum multiple access by a large number of users employing
this type of coded modulation is determined in the presence of
background Gaussian noise. With this approach and coordinated processing
at a common receiver, it is shown that the aggregate data rate of all
simultaneous users can approach the Shannon capacity of the Gaussian
noise channel.
The system capacity and performance of multicarrier code-division multiple-access (MC-CDMA) communication systems can be significantly enhanced by jointly employing MAP-based multiuser detection (MUD) and channel decoding techniques. In this paper, a group-oriented soft iterative MUD based on the combination of smart antennas and iterative MAP-based MUD is presented. The proposed method is featured as a novel technique for further increasing the system capacity and performance. In this method, all the users are first grouped into several groups according to their impinging direction of arrivals (DOAs). All users with similar DOAs are classified into the same group and then low-complexity MAP-based iterative MUD is employed in each group. Because spatial filtering cannot suppress all the interference between the groups, interference cancellation among the groups is used prior to MUD within each group. It is shown that the proposed group-oriented soft iterative MUD algorithm can significantly reduce the computational complexity compared with the conventional optimal MAP-based MUD schemes. It is also demonstrated that the performance of the proposed algorithm can approach that of a single-user coded MC-CDMA system with an antenna array in additive white Gaussian noise and frequency selective fading channels.
To improve the bandwidth efficiency of current and future generations of wireless cellular systems, several techniques exist such as beamforming (BF) and multiuser detection (MUD). To reduce the complexity of the MUD, it has been proposed to separate users within a cell into mutually exclusive groups and perform MUD independently in each group. Inter-group interference (IGI) is reduced by using beamforming. In this work, the group optimal MMSE linear receiver for BF based space-time multiuser detector (ST-MUD) receivers (BF-STMUD) is derived. The complexity of this structure is compared to that of the full ST-MUD. It is shown through computer simulations that with proper grouping, an important reduction in complexity can be achieved with almost insignificant loss of performance.
We study the effect of cancellation order on bit error rate (BER) performance of a code division multiple access (CDMA) system employing a linear successive interference cancellation (SIC) receiver in both an additive white Gaussian noise (AWGN) channel and a Rayleigh fading channel. Four different ordering schemes are studied. We show that changing the cancellation order can lead to a significant change in the BER performance.
In this work, we consider a linear group polynomial expansion successive interference cancellation (GPE-SIC) detector in a synchronous CDMA system. It is a hybrid detector, which combines parallel and successive cancellation techniques in order to extract the advantages of both of the schemes. Benefiting from the fact that even if the cross-correlation matrix of the system is not diagonal-dominant, we can force the cross-correlation matrix of users within the same group to be diagonal-dominant by suitable grouping and approximate the decorrelator/MMSE detector by a low-complexity polynomial expansion detector. This approximation is very accurate if the cross-correlation matrix of users within the same group is diagonal-dominant. Simulation results showed that the (GPE-SIC) detector has the same performance as the linear group decorrelator successive interference cancellation (GDEC-SIC) detector but with lower computational complexity.
In this paper, we presented the new interference suppression method using groupwise decorrelation and groupwise interference cancellation for W-CDMA downlink. The codes are grouped by the correlation property between mother code and child code in OVSF code tree. By grouping, the proposed receiver structure has relatively low computational complexity and does not requires any information about interference users(code, data, amplitude). Using interference decorrelation, the proposed receiver has almost the same performance of single user situation.
We compare the BER performance of WCDMA/TDD based optimum combining and beamforming in different angular spread environments. In previous studies, the received signals at the array antenna are independent by separating the antenna distance and applied MMSE, MSINR, MRC to compare the performance under CCI (cochannel interference). In addition they theoretically and experimently proved the results they have simulated. With the premise of the results they suggested we perform a simulation to compare the BER performance of optimum combining and beamforming in using the MMSE algorithm under different users (intracell interference) and Eb/No. For the simulation, we chose the WCDMA/TDD standard. Through the simulation results, the uncorrelated receive signals at the array antenna can fight the fading for the sacrifice of the beamforming to the wanted signal direction. In simulation, two types of array structures were assumed. One is λ/2 and the other is 10λ. Also, the MMSE and MRC beamforming algorithms are compared under different angular spread channel environments. For the BER performance it is shown that the beamforming effect of maximum directivity to the wanted user is less than that of the space diversity effect. Therefore, there are two factors of the antenna distance and channel environment to get the receive signals uncorrelated. In WCDMA/TDD mode the cell is small (rich scattering environment) and the users are restricted so the optimum combining method is suggested as an effective method.
Codirectional or clustered mobile users can be received using smart antennas at the base station (BS) and a multiuser detector can be used to isolate their respective signals. We consider the scenario of clustered users in a WCDMA system and compare the performance of beamforming and switched beams antennas, along with multiuser detection strategies (decorrelating detector, DD, and minimum mean square error, MMSE, detector) for their reception at the BS. We show via simulations that the switched beams technique along with a multiuser detector is a better strategy than beamforming along with a multiuser detector, when the number of users in a cluster increases.
The capacity of MC-CDMA communication systems can be significantly
enhanced by employing multiuser detection (MUD) techniques. A group
oriented MUD based on the combination of smart antennas and multiuser or
joint detection is presented. The proposed method is featured as a novel
technique for further increasing the system capacity and performance. In
this method, all the users are first divided into several groups
according to their impinging direction of arrivals (DOA). All users with
similar DOAs are classified into the same group and then MUD is employed
in each group in parallel. Because spatial filtering cannot suppress all
the interference between the groups, interference cancellation among
groups is used prior to MUD within the groups. It is shown that the
proposed group oriented MUD algorithm can significantly reduce the
computational complexity and processing time delay compared with the
conventional MUD schemes. Furthermore, it is demonstrated that
significant performance improvement can be achieved
Integrating an adaptive antenna array (AAA) with multiuser
detection (MUD) is a new research area aimed at improving the system
performance and capacity. A new dynamic groupwise successive
interference cancellation array receiver scheme for DS-CDMA systems is
proposed and compared with other schemes. In this scheme, AAA
beamforming based on direction of arrival (DOA) estimation is integrated
with multistage interference cancellation (IC). Simulation results
illustrate that our method outperforms existing interference cancellers
for DS-CDMA systems in a multipath Rayleigh fading channel environment
This contribution presents some simulation results obtained by a
WCDMA base station equipped with an adaptive antenna system. The results
show that the performance gain obtained by an advanced antenna system
could be substantial compared to an ordinary sector covering system.
Migrating some existing sites/sectors to adaptive antenna systems can be
seen as one attractive solution that increases capacity in a coverage
area without condensing the system by building additional cell sites. To
find out where this grow-on-site migration strategy is most beneficiary,
it is important to evaluate the systems by taking typical user scenarios
into account. The user scenarios includes spatially distribution of
users as well as the expected user behavior related to the actual
application (or user data rate). It has been shown that the gains using
adaptive antennas for the investigated scenarios of non-uniformly
distributed mixed service users are substantial
In this work, groupwise successive interference cancellation
scheme for multirate CDMA signals is proposed. The proposed detector
operation is based on the minimum mean square error (MMSE) criterion. It
has the promise of decision feedback interference cancellation for
superior performance compared to the linear MMSE detection at the cost
of a moderate increase in complexity. In groupwise detection, users are
grouped according to their processing gain or data rate, i.e., users
with the same processing gain or data rate are grouped together, then
detected. Users with the lowest processing gain or highest data rate are
detected first, neglecting the presence of the other users in the
system. Interference between the groups is cancelled in a successive
order. The BER of the groupwise successive MMSE interference canceler is
compared with the BER multirate linear MMSE detector and multirate
decorrelating detector. The results show that the groupwise successive
MMSE interference canceler yields better performance than the multirate
linear MMSE detector and multirate decorrelating detector. When the
system is highly loaded, the groupwise successive MMSE interference
canceler can offer a better performance than the multirate linear MMSE
detector and the multirate conventional detector
We propose a switched beamforming system combined with multiuser
detectors (MUD) in order to suppress the multiple access interference
that is coming from the directions close to the desired user's signal.
Since a multiuser detector, in the proposed system, works only for one
beam signal, the required processing delay is significantly reduced
compared to the conventional MUD. We have simulated the proposed system
and our simulation results show that the proposed interference canceller
achieves high SINR in a heavy interference channel, which a conventional
spatial filtering system cannot achieve
A multi-stage partial parallel interference cancellation receiver
is proposed for the multi-rate DS-CDMA system which adopts the multiple
processing gain scheme. In each stage of the proposed receiver, partial
cancellation is performed with an adaptive decision threshold which is
computed from the matched filter outputs of the previous stage. The BER
performance of the proposed receiver is obtained by simulation in a
Rayleigh fading channel. It is shown that the proposed receiver achieves
smaller BER than the multi-stage parallel interference cancellation
receiver and extended group-wise successive interference cancellation
receiver in the multi-rate DS-CDMA system
Beamforming (BF) with multiple receiving antennas significantly improves the bit error rate (BER) performance for a DS-CDMA system in a flat Rayleigh fading environment. However, BF fails to suppress multiple access interference (MAI) completely, which results in an irreducible BER floor. Furthermore, soft-decision Viterbi decoding (VD) with BF further lowers the BER floor. The BER performance difference between the minimum mean square error (MMSE) linear multiuser detector (MUD) and zero forcing (ZF) MUD increases with the eigenvalue spread of the code correlation matrix. BF with MUD performs better than MUD alone, and BF+VD is much more effective at combating MAI than MUD+VD for low to moderate SNRs. In this regime, the performance of BF+VD approaches that of BF+MUD+VD, which clearly offers the best performance for all SNRs.
In this paper, we consider a matrix-algebraic approach to linear
group-wise successive interference cancellation. As for both successive
and parallel cancellation, it is shown that the general linear
group-wise scheme corresponds to a one-shot linear matrix filtering.
Furthermore, it is shown that if the general structure converges, it
converges to the decorrelator. Four structures are considered for the
group detector, namely the matched filter, the parallel canceller, the
decorrelator and the MMSE detector, and the equivalent one-shot filters
are derived. Conditions for convergence related to the grouping are
studied and it is shown that group detection with a decorrelator, which
has conventional successive interference cancellation as a special case,
always converges. Through numerical examples, it is illustrated that the
performance for all the structures behaves like the performance for the
linear successive canceller where a minimum BER is reached before
convergence to the performance of the decorrelator occurs. The matched
filter group canceller achieves the lowest minimum BER but also has the
slowest convergence
Cellular networks today are interference-limited and only becomes increasingly so in the future due to the many users that need to share the spectrum to achieve high-rate multimedia communication. Despite the enormous amount of academic and industrial research in the past 20 years on interference-aware receivers and the large performance improvements promised by these multi-user techniques, today's receivers still generally treat interference as background noise. In this article, we enumerate the reasons for this widespread scepticism, and discuss how current and future trends increases the need for and viability of multi-user receivers for both the uplink, where many asynchronous users are simultaneously detected, and the downlink, where users are scheduled and largely orthogonalized; but the mobile handset still needs to cope with a few dominant interfering base stations. New results for interference cancelling receivers that use conventional front-ends are shown to alleviate many of the shortcomings of prior techniques, particularly for the challenging uplink. This article gives an overview of key recent research breakthroughs on interference cancellation and highlights system-level considerations for future multi-user receivers.
Parallel interference cancellation (PIC) is considered a simple yet effective multiuser detector for direct-sequence code-division multiple-access (DS-CDMA) systems. However, system performance may deteriorate due to unreliable interference cancellation in the early stages. Thus, a partial PIC detector in which partial cancellation factors (PCFs) are introduced to control the interference cancellation level has been developed as a remedy. Although PCFs are crucial, complete solutions for their optimal values are not available. We consider a two-stage decoupled partial PIC receiver. Using the minimum bit error rate (BER) criterion, we derive a complete set of optimal PCFs. This includes optimal PCFs for periodic and aperiodic spreading codes in additive white Gaussian channels and multipath channels. Simulation results show that our theoretical optimal PCFs agree closely with empirical ones. Our two-stage partial PIC using derived optimal PCFs outperforms not only a two-stage, but also a three-stage full PIC.
In this paper, we compare several optimization methods for solving the optimal multiuser detection problem exactly or approximately. The purpose of using these algorithms is to provide complexity constraint alternatives to solving this nondeterministic polynomial-time (NP)-hard problem. An approximate solution is found either by relaxation or by heuristic search methods, while the branch and bound algorithm is used to provide an exact solution. Simulations show that these approaches can have bit-error rate (BER) performance which is indistinguishable from the maximum likelihood performance. A tabu search method is shown to be an effective (in terms of BER performance) and efficient (in terms of computational complexity) heuristic when compared to other heuristics like local search and iterative local search algorithms. When the number of users increases, the tabu search method is more effective and efficient than the semidefinite relaxation approach.
The minimum mean-squared error (MMSE) receiver is a linear filter which can suppress multiple access interference (MAI) effectively in direct-sequence code-division multiple-access (CDMA) communications. An antenna array is also an efficient scheme for suppressing MAI and improving the system performance. In this letter, we consider an adaptive MMSE receiver in conjunction with beamforming in CDMA systems employing an antenna array. The proposed structure is featured as a low complexity receiver, which adapts the MMSE filter coefficients and the beamforming weights simultaneously. However, it does require the channel state information and the direction of arrival (DOA) of the desired user signal. As a result, we propose two adaptation methods to perform joint channel estimation and signal detection without any training sequence. It is demonstrated that the two proposed methods achieve similar bit-error-rate performance. More importantly, their performance degradation compared with the case with perfect channel information is small.
In this work, we introduce a new linear group-wise SIC multi-user detector that can converge to either the decorrelator or the least minimum mean-square error (LMMSE) detector. We study the convergence behavior of the proposed scheme and show that the latter is equivalent to the block Gauss-Seidel iterative method if the group-detection scheme used is the decorrelator detector. Moreover, we prove that the latter is convergent if the group-detection matrix is positive definite. Our simulation results are in excellent agreement with the proposed theory
The reverse link (mobile to base) of a cellular direct-sequence
code-division multiple-access system is considered with the aim of
increasing system capacity while avoiding system complexity. The
capacity of a conventional (matched filter) receiver can be increased by
employing sectorised antenna arrays or multiuser detection techniques.
Results confirm that the potentially complex approach of beamforming
followed by multiuser detection further increases capacity when the
uplink transmissions are asynchronous for both additive white Gaussian
noise channels and Rayleigh fading multipath channels. A method by which
the computational cost of multiuser detection applied after beamforming
can be significantly reduced without greatly affecting performance is
proposed. Bit detection techniques based on partitioning the users into
spatial equivalence classes by beamforming and then employing standard
multiuser detection techniques within each class are investigated.
Classes of users can be determined so that multiple-access interference
terms corresponding to users in different classes are of little
significance. These terms can be discarded before applying multiuser
detection, thus reducing computation
The linear successive interference canceler (LSIC) is a multiuser detector that separates code-division multiple-access (CDMA) signals in a multistage manner. In each stage, a user is detected, and its contribution is regenerated and canceled from the input of that stage. A user's spreading sequence is employed for despreading and respreading, and the magnitude of the despreader output is used as the amplitude estimate to reconstruct that user's signal. This paper describes a generalized version of the LSIC (GLSIC) that employs various types of linear filters for the despreading and respreading operations. We analyze the bit error rate (BER), asymptotic multiuser efficiency (AME), and the mean and variance of the amplitude estimates of the GLSIC. It is shown that for certain choices of linear filters, complete cancellation of a user can be achieved, irrespective of the reliability of the symbol estimates. We also demonstrate that, from a BER and AME viewpoint, it is not beneficial to use a linear canceler with a decorrelator or a minimum mean-square-error (MMSE) receiver.
We propose novel space-time multistage and iterative receiver
structures and examine their application in code division multiple
access (CDMA) mobile communication systems. In particular we derive an
expression for weighting coefficients in parallel interference
cancellers (PICs) in a system with a large number of users, where
decision statistics bias is pronounced. We further examine the
parameters in this expression and show how to obtain a practical partial
cancellation method that allows on-line estimation of the weighting
coefficients. In the proposed multistage PIC, the coefficients are
calculated by using only the variances of the detector outputs. We also
examine an iterative PIC and observe that this receiver has similar
limitations as the multistage PIC. The application of the novel parallel
interference cancellation strategy in the iterative receiver structure
results in a spectacular system capacity improvement with a negligible
complexity increase relative to the standard iterative receiver. The
performance of the proposed receivers is further enhanced by receiver
adaptive array antennas and space-time processing
Previously developed blind techniques for multiuser detection in
code division multiple access (CDMA) systems lead to several near-far
resistant adaptive receivers for demodulating a given user's data with
the prior knowledge of only the spreading sequence of that user. In the
CDMA uplink, however, typically the base station receiver has the
knowledge of the spreading sequences of all the users within the cell,
but not that of the users from other cells. In this paper, group-blind
techniques are developed for multiuser detection in such scenarios.
These new techniques make use of the spreading sequences and the
estimated multipath channels of all known users to suppress the
intracell interference, while blindly suppressing the intercell
interference. Several forms of group-blind linear detectors are
developed based on different criteria. Moreover, group-blind multiuser
detection in the presence of correlated noise is also considered. In
this case, two receiving antennas are needed for channel estimation and
signal separation. Simulation results demonstrate that the proposed
group-blind linear multiuser detection techniques offer substantial
performance gains over the blind linear multiuser detection methods in a
CDMA uplink environment
Compensating for near/far effects is critical for satisfactory
performance of DS/CDMA systems. So far, practical systems have used
power control to overcome fading and near/far effects. Another approach,
which has a fundamental potential in not only eliminating near/far
effects but also in substantially raising the capacity, is multiuser
detection and interference cancellation. Various optimal and suboptimal
schemes have been investigated. Most of these schemes, however, get too
complex even for relatively simple systems and rely on good channel
estimates. For interference cancellation, estimation of channel
parameters (viz. received amplitude and phase) is important. We analyze
a simple successive interference cancellation scheme for coherent BPSK
modulation, where the parameter estimation is done using the output of a
linear correlator. We then extend the analysis for a noncoherent
modulation scheme, namely M-ary orthogonal modulation. For the
noncoherent case, the needed information on both the amplitude and phase
is obtained from the correlator output. The performance of the IC scheme
along with multipath diversity combining is studied
A spread-spectrum multiple-access (SSMA) communication system is
treated for which both spreading and error control is provided by binary
PSK modulation with orthogonal convolution codes. Performance of
spread-spectrum multiple access by a large number of users employing
this type of coded modulation is determined in the presence of
background Gaussian noise. With this approach and coordinated processing
at a common receiver, it is shown that the aggregate data rate of all
simultaneous users can approach the Shannon capacity of the Gaussian
noise channel
Direct-sequence code-division multiple access (DS-CDMA) is a
popular wireless technology. In DS-CDMA communications, all of the
users' signals overlap in time and frequency and cause mutual
interference. The conventional DS-CDMA detector follows a single-user
detection strategy in which each user is detected separately without
regard for the other users. A better strategy is multi-user detection,
where information about multiple users is used to improve detection of
each individual user. This article describes a number of important
multiuser DS-CDMA detectors that have been proposed
This paper introduces an improved nonlinear parallel interference
cancellation scheme for code-division multiple access (CDMA) that
significantly reduces the degrading effect on the desired user of
interference from the other users that share the channel. The
implementation complexity of the scheme is linear in the number of users
and operates on the fact that parallel processing simultaneously removes
from each user a part of the interference produced by the remaining
users accessing the channel the amount being proportional to their
reliability. The parallel processing can be done in multiple stages. The
proposed scheme uses tentative decision devices at the multiple stages
to produce the most reliably estimated received data for generation and
cancellation of user interference. Simulation results are given for a
multitude of different situations, in particular, those cases for which
the analysis is too complex
We consider interference suppression for direct-sequence
spread-spectrum code-division multiple-access (CDMA) systems using the
minimum mean squared error (MMSE) performance criterion. The
conventional matched filter receiver suffers from the near-far problem,
and requires strict power control (typically involving feedback from
receiver to transmitter) for acceptable performance. Multiuser detection
schemes previously proposed mitigate the near-far problem, but are
complex and require explicit knowledge or estimates of the interference
parameters. In this paper, we present and analyze several new MMSE
interference suppression schemes, which have the advantage of being
near-far resistant (to varying degrees, depending on their complexity),
and can be implemented adaptively when interference parameters are
unknown and/or time-varying, Numerical results are provided that show
that these schemes offer significant performance gains relative to the
matched filter receiver. We conclude that MMSE detectors can alleviate
the need for stringent power control. In CDMA systems, and may be a
practical alternative to the matched filter receiver
A linear group-wise successive interference canceller in a synchronous code-division multiple-access system (CDMA) is considered in this paper. The proposed hybrid detector that combines successive and parallel cancellation techniques makes use of advantages offered by the two techniques. The convergence of the hybrid interference cancellation (HIC) detector is guaranteed by an adjustable parameter that depends upon the largest eigenvalue of the system's transition matrix. Since the largest eigenvalue is difficult to estimate, an upper bound is necessary for successful convergence. For this reason, a new upper bound for the maximum eigenvalue of the system's transition matrix was developed. Moreover, a new ordering and grouping algorithm that results in a higher convergence speed is proposed. Simulation results show that a significant improvement in performance is obtained using this technique.