Optimal sequences, power control, and user capacity of synchronousCDMA systems with linear MMSE multiuser receivers

There has been intense effort in the past decade to develop multiuser receiver structures which mitigate interference between users in spread-spectrum systems. While much of this research is performed at the physical layer, the appropriate power control and choice of signature sequences in conjunction with multiuser receivers and the resulting network user capacity is not well understood. In this paper we will focus on a single cell and consider both the uplink and downlink scenarios and assume a synchronous CDMA (S-CDMA) system. We characterize the user capacity of a single cell with the optimal linear receiver (MMSE receiver). The user capacity of the system is the maximum number of users per unit processing gain admissible in the system such that each user has its quality-of-service (QoS) requirement (expressed in terms of its desired signal-to-interference ratio) met. This characterization allows one to describe the user capacity through a simple effective bandwidth characterization: users are allowed in the system if and only if the sum of their effective bandwidths is less than the processing gain of the system. The effective bandwidth of each user is a simple monotonic function of its QoS requirement. We identify the optimal signature sequences and power control strategies so that the users meet their QoS requirement. The optimality is in the sense of minimizing the sum of allocated powers. It turns out that with this optimal allocation of signature sequences and powers, the linear MMSE receiver is just the corresponding matched filter for each user. We also characterize the effect of transmit power constraints on the user capacity     

Linear multiuser receivers in random environments

We study the signal-to-interference (SIR) performance of linear multiuser receivers in random environments, where signals from the users arrive in “random directions.” Such a random environment may arise in a DS-CDMA system with random signature sequences, or in a system with antenna diversity where the randomness is due to channel fading. Assuming that such random directions can be tracked by the receiver, the resulting SIR performance is a function of the directions and therefore also random. We study the asymptotic distribution of this random performance in the regime where both the number of users K and the number of degrees of freedom N in the system are large, but keeping their ratio fixed. Our results show that for both the decorrelator and the minimum mean-square error (MMSE) receiver, the variance of the SIR distribution decreases like 1/N, and the SIR distribution is asymptotically Gaussian. We compute closed-form expressions for the asymptotic means and variances for both receivers. Simulation results are presented to verify the accuracy of the asymptotic results for finite-sized systems     

Convergence of linear interference cancellation multiuser receivers

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     

MAP multiuser detection with soft interference cancellation for UTRA-TDD receivers

A new multiuser detector (MUD) exploiting the MAP algorithm is proposed for channels affected by severe multipath and is then suitable for time-division duplexing (TDD)-CDMA receivers. After coherent combining the received multipaths, a multidimensional MAP non-linearity is employed to compute step-by-step the a posteriori probabilities (APPS) of the transmitted data for each user separately and then their expected values conditional to the available observations. These are then employed for soft ISI and MAI removal from the received sequences so that the APPS can be more reliably re-computed. The procedure is iterated in a multistage structure until final decisions are taken. From the comparison with other existing techniques (interference cancellation, zero-forcing) the proposed reduced MAP-MUD receiver exhibits better performance and equal or minor complexity.     

Efficient adaptive receivers for joint equalization and interference cancellation in multiuser space-time block-coded systems

This paper develops low-complexity adaptive receivers for space-time block-coded (STBC) transmissions over frequency-selective fading channels. The receivers are useful for equalization purposes for single user transmissions and for joint equalization and interference cancellation for multiuser transmissions. The receivers exploit the rich code structure of STBC codes in order to deliver recursive-least-squares (RLS) performance at least-mean-squares (LMS) complexity. Besides reduced complexity, the proposed adaptive receivers also lower system overhead requirements.     

On the capacity of multiuser MIMO networks with interference

Maximizing the total mutual information of multiuser multiple-input multiple-output (MIMO) systems with interference is a challenging problem. In this paper, we consider the power control problem of finding the maximum sum of mutual information for a multiuser network with mutually interfered MIMO links. We propose a new and powerful global optimization method using a branch-and-bound (BB) framework, coupled with a novel reformulation-linearization technique (RLT). The proposed BB/RLT guarantees finding a global optimum for multiuser MIMO networks with interference. To reduce the complexity of BB/RLT, we propose a modified BB variable selection strategy to accelerate the convergence process. Numerical examples are also given to demonstrate the efficacy of the proposed solution.     

Iterative multiuser receivers for CDMA channels: an EM-basedapproach

Maximum-likelihood detection for the multiuser code-division multiple-access (CDMA) channel is prohibitively complex. This paper considers new iterative multiuser receivers based on the expectation-maximization (EM) algorithm and related, more powerful “space-alternating” algorithms. The latter algorithms include the SAGE algorithm and a new “missing parameter” space-alternating algorithm that alternately updates individual parameter components or treats them as probabilistic missing data. Application of these EM-based algorithms to the problem of discrete parameter estimation (i.e., data detection) in the Gaussian multiple-access channel leads to a variety of convergent receiver structures that incorporate soft-decision feedback for interference cancellation and/or sequential updating of iterative bit estimates. Convergence and performance analyses are based on well-known properties of the EM algorithm and on numerical simulation     

Noncoherent MMSE multiuser receivers and their blind adaptiveimplementations

Three noncoherent minimum mean-squared error (NMSE)-based multiuser receivers are proposed for multipulse modulation. These receivers have a common MMSE prefilter and are followed by one of three phase-independent decision rules. The simplest decision rule selects the maximum magnitude of the MMSE filter outputs, and the other two account for the second-order statistics of the residual multiple-access interference that remains after MMSE filtering. Blind adaptive algorithms are then proposed for the three noncoherent MMSE receivers. The common adaptive algorithm for the MMSE prefilter, which is based on the stochastic approximation method, is shown to converge in the mean-squared error sense to the nonblind NMSE prefilter. Our convergence analysis yields new insight into the tradeoff between the rate of convergence and the residual mean-squared error. The noncoherent blind receivers obtained here do not require the knowledge of the received signals of any of the interfering users, and are hence well-suited for distributed implementation in cellular wireless networks or in communication systems that must operate in noncooperative environments     

Iterative multiuser interference reduction: turbo CDMA

We view the asynchronous random code division multiple-access (CDMA) channel as a time-varying convolutional code. We study the case where the users encode their data, and, therefore, the single user transmitters and the CDMA channel appear as the concatenation of two coding systems. At the receiver we employ serial turbo decoding strategies. Unlike conventional turbo codes where both the inner and outer code may be selected, in our case, the inner code is due to the CDMA channel which we assume to be random. Nevertheless, the decoding system resembles the decoder of a serial turbo code and single-user performance is obtained even for numbers of users approaching the spreading code length     

Optimum receivers and low-dimensional spreaded modulation for multiuser space-time communications

The jointly optimum receiver is obtained for multi- user communications in a frequency nonselective Rayleigh-fading channel with N/sub T/ transmit antennas per user and N/sub R/ receive antennas. Based on a general analysis of quadratic receivers in zero-mean complex Gaussian vectors, asymptotically tight expressions (for high signal-to-noise ratio (SNR)) for the pairwise error probabilities are derived. Subsequently, it is shown that N/sub T/-dimensional single-user signaling suffices to provide full diversity order N=N/sub T/N/sub R/ for all the users. In other words, the presence of other users does not increase the minimum dimension required beyond what is needed for the single-user space-time channel. For the special case of low-rank "code-division multiple-access (CDMA)" signaling with N/sub T/=1 and provided the signatures of any two users are linearly independent, it is shown that the error probability of a K-user system asymptotically approaches single-user-like performance for every user. Remarkably, therefore, an increase in the number of users, and hence an increase in the aggregate spectral efficiency, does not require the users to transmit with more power to achieve single-user-like performance asymptotically. A signal design algorithm is proposed to illustrate this point and examples are given. These results are then generalized to the multiple transmit antenna case. A new (N/sub T/+1)-dimensional signaling strategy is proposed for the multiuser channel that leverages existing single-user space-time signal designs while ensuring full diversity order and single-user-like performance asymptotically for every user.     

Differential and coherent decorrelating multiuser receivers for space-time-coded CDMA systems

Previously, we proposed a differential space-code modulation (DSCM) scheme that integrates the strength of differential space-time coding and spreading to achieve interference suppression and resistance to time-varying channel fading in single-user environments. In this paper, we consider the problem of multiuser receiver design for code-division multiple-access (CDMA) systems that utilize DSCM for transmission. In particular, we propose two differential receivers for such systems. These differential receivers do not require the channel state information (CSI) for detection and, still, are resistant to multiuser interference (MUI) and time-varying channel fading. We also propose a coherent receiver that requires only the CSI of the desired user for detection. The coherent receiver yields improved performance over the differential receivers when reliable channel estimates are available (e.g., in slowly fading channels). The proposed differential/coherent receivers are decorrelative schemes that decouple the detection of different users. Both long and short spreading codes can be employed in these schemes. Numerical examples are presented to demonstrate the effectiveness of the proposed receivers.     

Blind adaptive DS/CDMA receivers for multipath and multiuser environments

This paper proposes several blind adaptive receivers to eliminate multiple-access interference (MAI), intersymbol interference (ISI), and interchip interference (ICI) in direct-sequence code-division multiple access (DS/CDMA) downlink multiuser systems. We use the following concepts to formulate the cost function: 1) the variance of the despreading output approaches to the variance of the desired signal and 2) the discreteness property of the input signal. The proposed approaches are called variance-oriented approaches (VOAs). The VOA is then applied to three proposed receiver structures, especially the generalized sidelobe canceller (GSC) scheme that is generally the concept of spatial domain in beamforming system, to eliminate the MAI by one particular constraint in temporal domain. Besides, by this constraint, GSC filter possesses the property of global convergence in multipath environment once the channel estimation is appropriate. Simulation examples are shown to demonstrate the effectiveness and comparison of the proposed blind adaptive receivers.     

Joint array combining and MLSE for single-user receivers inmultipath Gaussian multiuser channels

The well-known structure of an array combiner along with a maximum likelihood sequence estimator (MLSE) receiver is the basis for the derivation of a space-time processor presenting good properties in terms of co-channel and intersymbol interference rejection. The use of spatial diversity at the receiver front-end together with a scalar MLSE implies a joint design of the spatial combiner and the impulse response for the sequence detector. This is faced using the MMSE criterion under the constraint that the desired user signal power is not cancelled, yielding an impulse response for the sequence detector that is matched to the channel and combiner response. The procedure maximizes the signal-to-noise ratio at the input of the detector and exhibits excellent performance in realistic multipath channels     

上行多用户MIMO系统中的用户与天线联合选择方案

在多用户MIMO系统中,基站所能同时进行通信的用户数受到基站和用户端天线数的限制,随着用户数的增加,系统的性能反而会降低,因此,用户选择技术就成为一种改善系统性能的有效技术.在本文中,针对上行多用户MIMO系统提出了一种低复杂度的用户选择算法.为获得更大的系统性能,在用户选择的基础上,进一步提出了一种用户与天线联合选择算法.这两种算法在极大地简化计算复杂度的条件下,提供了与最优算法几乎相同的性能.     

Real-time algorithms and architectures for multiuser channelestimation and detection in wireless base-station receivers

This paper presents algorithms and architecture designs that can meet real-time requirements of multiuser channel estimation and detection in future code-division multiple-access-based wireless base-station receivers. Sophisticated algorithms proposed to implement multiuser channel estimation and detection make their real-time implementation difficult on current digital signal processor-based receivers. A maximum-likelihood based multiuser channel estimation scheme requiring matrix inversions is redesigned from an implementation perspective for a reduced complexity, iterative scheme with a simple fixed-point very large scale integration (VLSI) architecture. A reduced-complexity, bit-streaming multiuser detection algorithm that avoids the need for multishot detection is also developed for a simple, pipelined VLSI architecture. Thus, we develop real-time solutions for multiuser channel estimation and detection for third-generation wireless systems by: (1) designing the algorithms from a fixed-point implementation perspective, without significant loss in error rate performance; (2) task partitioning; and (3) designing bit-streaming fixed-point VLSI architectures that explore pipelining, parallelism, and bit-level computations to achieve real-time with minimum area overhead     

Analysis and design of interleavers for iterative multiuser receivers in coded CDMA systems

We deal with the design of interleavers in a coded code-division multiple-access (CDMA) scenario, where at the receiver an iterative turbo-like structure to perform multiuser detection is employed. The choice of the interleavers affects both the maximum-likelihood (ML) performance and the impact of the suboptimality of the iterative receiver. First, heuristic criteria of goodness for a set of interleavers, each assigned to a given active user, are introduced and motivated. One of these criteria is based on the intersection between the equivalent codes seen after the interleavers for each user pair. The design rules are valid for any kind of channel code. In particular, when the channel code used by every user is a terminated convolutional code, a very simple design rule, in the subset of congruential interleavers, is specified. The suitability of an interleaver set to iterative decoding is also treated. The analysis leads to a design rule which is shown to have great importance on the performance of a turbo-like receiver. Numerical results assess the validity of the derived design rules by showing that, for iterative multiuser receivers and reasonable block lengths, the suitability to iterative decoding is more important than the performance optimization.     

Ergodic capacity and outage probability optimization for secondary user in cognitive radio networks under interference outage constraint

This paper considers a cognitive radio network where a secondary user (SU) coexists with a primary user (PU). The interference outage constraint is applied to protect the primary transmission. The power allocation problem to jointly maximize the ergodic capacity and minimize the outage probability of the SU, subject to the average transmit power constraint and the interference outage constraint, is studied. Suppose that the perfect knowledge of the instantaneous channel state information (CSI) of the interference link between the SU transmitter and the PU receiver is available at the SU, the optimal power allocation strategy is then proposed. Additionally, to manage more practical situations, we further assume only the interference link channel distribution is known and derive the corresponding optimal power allocation strategy. Extensive simulation results are given to verify the effectiveness of the proposed strategies. It is shown that the proposed strategies achieve high ergodic capacity and low outage probability simultaneously, whereas optimizing the ergodic capacity (or outage probability) only leads to much higher outage probability (or lower ergodic capacity). It is also shown that the SU performance is not degraded due to partial knowledge of the interference link CSI if tight transmit power constraint is applied.     

Soft Bayesian recursive interference cancellation and turbodecoding for TDD-CDMA receivers

A novel receiver is proposed for CDMA links with recursive turbo coding, based on a soft-input soft-output multistage interference canceler (IC) and an iterative turbo decoder (TD). After performing IC and TD a first time, the coder state estimates available from the two soft-output Viterbi algorithms in the last TD iteration are exploited to calculate the data bits and the redundancy bits. These are employed for preliminary hard cancellation in the first iteration of a supplementary IC run, and then the TD is run again. An alternative solution with soft feedback is obtained by approximating the reliabilities of the redundancy bits with those of the coder state estimates, and using them for preliminary soft cancellation. Simulation results show the effectiveness of the proposed solutions     

Selective interference alignment and neutralization in coordinated multipoint using multiuser MIMO

In wireless communication, the concept of coordinated multipoint (CoMP) transmission is more attractive, and it transpired the notion based on interference management techniques. Interference alignment (IA) and interference neutralization (IN) methods can substantially align and neutralize the interference signals. The existing work in the CoMP transmission using multiuser multi-input and multi-output (MU-MIMO) had long-held problems such as specific limit of intercell (ICI) and cochannel interference (CCI) cancelation that contain low performance in cell-edge users. The proposed framework of the transmission signal in selective interference alignment and neutralization (SIAN) CoMP MU-MIMO system transmits multiple data streams in multipath by using downlink coordination between base stations and receiver side. This work contributes the individual perspectives to implement the IA and IN to align and cancel the interfered signals at the receiver side. Once the perspectives mentioned above are executed, zero-forcing (ZF) and rechanneling filter are applied on the receiver side to eliminate residual ICI. In addition, spectral efficiency significantly improves the achievable data rate and enhances the performance of cell-edge users and reduces the CCI at the receiver side. Furthermore, the antenna configuration signals are decoded to get the exact version of interference-free signals with null path loss of signal transmission. The effectiveness of the proposed framework analyzes and verifies the numerical evaluation of the achievable degree of freedom. Finally, the simulation demonstrates the comparison of the proposed CoMP scheme with MIMO ZF and non-CoMP schemes, which significantly improve the performance of spectral efficiency for cell-edge users.