Multiuser Wireless Communication Systems Research Articles

Interference Alignment in Multi-user MIMO Systems

As a promising technique to obtain optimal degree of freedom (DoF) on multi-user wireless communication systems, interference alignment (IA) has been studied extensively in recent years. The most effective approach of IA is to deploy optima linear transceiver by exploiting spatial characteristics of wireless channels, and it is evaluated in measured multi-input multi-output (MIMO) interference channels. The number of alignment constraints and channel diversity are two main issues faced by IA. Moreover, imperfect, noisy, localized and possibly delayed nature of channel knowledge feedback to the transmitters remains also as the urgent problem to be solved. These challenges have excited ingenious solutions in the implementation of IA schemes. In this work, we present a review of existing techniques for IA in typical multi-user MIMO systems, and on this basis, consider some future research issues related to implement. The study covers IA's feasibility conditions, solution algorithms in MIMO interference channel and MIMO multi-cell networks, with an emphasize on the CSI feedback technology which remains as the major challenge of IA.

Variable Forgetting Factor RLS Algorithm-Based Channel Estimator for MC-IDMA Communication Systems

There are different types of technologies that are in use for multi-user wireless communication systems. In this article, a relatively new multi-user scheme, Multi-Carrier Interleave Division Multi...

Energy optimization in multiuser quantized feedback systems

This article explores the effects of quantization of feedback information on energy consumption in multiuser wireless communication systems. In order to optimize the energy consumption of the system, the article concentrates on the amount of transmit energy, the additional energy due to quantization and the probability of power outage. Closed form expressions for such parameters are obtained, where the impact of the number of quantization bits is explicitly outlined. An optimization problem is then formulated to find the optimum number of quantization bits able to minimize the consumption in the energy resources. Simulations demonstrate the good results obtainable with the presented optimization strategy, and provide effective validation of the analytic solution presented in the article.

Opportunistic Scheduling for Multi-User Two-Way Relay Systems with Physical Network Coding

This letter considers a multi-user wireless communication system employing physical network coding (PNC), where a pair of users communicates with each other via a relay. To attain reliable communication over time varying channels, we apply an opportunistic scheduling scheme for the PNC to both the broadcast channel phase and the multiple access channel phase. We propose two criteria for selecting users based on the channel norm and the minimum distance. Also, an efficient method to compute the minimum distance for superposed QPSK signals is introduced. Simulation results show that the proposed scheduling for PNC provides a significant improvement over conventional schemes.

Reconfigurable parasitic antennas for compact mobile terminals in multiuser wireless systems

Abstract This article considers the exploitation of parasitic antenna arrays in multi-user (MU) wireless communication systems by using their adaptive beamforming capabilities in order to improve the average system throughput. The use of parasitic arrays and especially the electrically steerable passive array radiator (ESPAR) antennas enables the design of terminals with a single RF front-end and reduced antenna dimensions, i.e., lightweight and compact mobile terminals. Although the beamforming capabilities of active element arrays at the receiver have been well investigated in the past, this article highlights the potentials of pattern reconfigurable parasitic arrays based on the beamspace representation of the ESPAR antenna. The advantages of using ESPAR at the receiving terminal are examined both in opportunistic beamforming and in MIMO broadcast channel MU systems, optimizing correspondingly the SNR or the SINR of the forward link.

An Efficient Algorithm for Performance Analysis of Multi-cell and Multi-user Wireless Communication Systems

Theoretical Bit Error Rate (BER) and channel capacity analysis are always of great interest to the designers of wireless communication systems. At the center of such analyses people are often encountered with a high-dimensional multiple integrals with quite complex integrands. Conventional Gaussian quadrature is inefficient in handling problems like this, as it tends to entail tremendous computational overhead, and the principal order of its error term increase rapidly with the dimension of the integral. In this paper, we propose a new approach to calculate complex multi-fold integrals based on the number theory. In contrast to Gaussian quadrature, the proposed approach requires less computational effort, and the principal order of its error term is independent of the dimension. The effectiveness of the number theory based approach is examined in BER and capacity analyses for practical systems. In particular, the results generated by numerical computation turn out in good match with that of Monte-Carlo simulations.

Novel schemes of CQI Feedback Compression based on Compressive Sensing for Adaptive OFDM Transmission

In multi-user wireless communication systems, adaptive modulation and scheduling are promising techniques for increasing the system throughput. However, a mass of wireless recourse will be occupied and spectrum efficiency will be decreased to feedback channel quality indication (CQI) of all users in every subcarrier or chunk for adaptive orthogonal frequency division multiplexing (OFDM) systems. Thus numerous limited feedback schemes are proposed to reduce the system overhead. The recently proposed compressive sensing (CS) theory provides a new framework to jointly measure and compress signals that allows less sampling and storage resources than traditional approaches based on Nyquist sampling. In this paper, we proposed two novel CQI feedback schemes based on general CS and subspace CS, respectively, both of which could be used in a wireless OFDM system. The feedback rate with subspace CS is greatly decreased by exploiting the subspace information of the underlying signal. Simulation results show the effectiveness of the proposed methods, with the same feedback rate, the throughputs with subspace CS outperform the discrete cosine transform (DCT) based method which is usually employed, and the throughputs with general CS outperform DCT when the feedback rate is larger than 0.13 bits/subcarrier.

Partial Equalization for MC–CDMA Systems in Non-Ideally Estimated Correlated Fading

Multicarrier code-division multiple access (MC-CDMA) can support high data rates in next-generation multiuser wireless communication systems. Partial equalization (PE) is a low-complexity technique for combining the signals of subcarriers to improve the achievable performance of MC-CDMA systems in terms of their bit error probability (BEP) and bit error outage (BEO) in comparison with maximal ratio combining, orthogonality restoring combining, and equal-gain combining techniques. We analyze the performance of the multiuser MC-CDMA downlink and derive the optimal PE parameter expression, which minimizes the BEP. Realistic imperfect channel estimation and frequency-domain (FD) block-fading channels are considered. More explicitly, the analytical expression of the optimum PE parameter is derived as a function of the number of subcarriers, number of active users (i.e., the system load), mean signal-to-noise ratio (SNR), and variance of the channel-estimation errors for the aforementioned FD block-fading channel. We show that the choice of the optimal PE technique significantly increases the achievable system load for the given target BEP and BEO.

A useful lemma for capacity analysis of fading interference channels

This paper presents a simple new expression for the exact evaluation of averages of the form E [ln (1+x <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> +...x <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> /y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> +...+y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</sub> +1)], where x <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> ,..., x <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> , y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> ..., y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</sub> are arbitrary non-negative random variables, in terms of the joint moment generating functions of these random variables. Application examples are given for the ergodic capacity evaluation of some multiuser wireless communication systems which are difficult to solve by the known classical methods.

Diffusion approximation for a heavily loaded multi-user wireless communication system with cooperation

A cellular wireless communication system in which data is transmitted to multiple users over a common channel is considered. When the base stations in this system can cooperate with each other, the link from the base stations to the users can be considered a multi-user multiple-input multiple-output (MIMO) downlink system. For such a system, it is known from information theory that the total rate of transmission can be enhanced by cooperation. The channel is assumed to be fixed for all transmissions over the period of interest and the ratio of anticipated average arrival rates for the users, also known as the relative traffic rate, is fixed. A packet-based model is considered where data for each user is queued at the transmit end. We consider a simple policy which, under Markovian assumptions, is known to be throughput-optimal for this coupled queueing system. Since an exact expression for the performance of this policy is not available, as a measure of performance, we establish a heavy traffic diffusion approximation. To arrive at this diffusion approximation, we use two key properties of the policy; we posit the first property as a reasonable manifestation of cooperation, and the second property follows from coordinate convexity of the capacity region. The diffusion process is a semimartingale reflecting Brownian motion (SRBM) living in the positive orthant of N-dimensional space (where N is the number of users). This SRBM has one direction of reflection associated with each of the 2 N −1 boundary faces, but show that, in fact, only those directions associated with the (N−1)-dimensional boundary faces matter for the heavy traffic limit. The latter is likely of independent theoretical interest.

Time-Slotted Round-Trip Carrier Synchronization for Distributed Beamforming

Transmit beamforming is an energy-efficient wireless communication technique that allows a transmitter with two or more antennas to focus its bandpass signal in an intended direction. The idea of transmit beamforming has been extended to networks of single-antenna cooperative transmitters that pool their antenna resources and behave as a ldquodistributed beamformer.rdquo Unlike conventional transmit beamforming, however, the carriers of the single-antenna transmitters in a distributed beamformer are each synthesized from independent and imperfect local oscillators; carrier phase and frequency synchronization among the transmitters is necessary to ensure that a beam is aimed in the desired direction. In this paper, a new time-slotted round-trip carrier synchronization protocol is proposed to enable distributed beamforming in multiuser wireless communication systems. Numerical results for a distributed beamformer with two transmitters show that the parameters of the round-trip synchronization protocol can be selected such that a desired level of phase accuracy and reliability is achieved during beamforming and such that the synchronization overhead is small with respect to the amount of time that reliable beamforming is achieved. The impact of mobility on the performance of the round-trip carrier synchronization protocol is also shown to be small when the synchronization timeslots are short.

Multiple antenna MMSE based downlink precoding with quantized feedback or channel mismatch

In this paper, we consider the downlink of a multiuser wireless communication system with multiple antennas at the base station and users each with a single receive antenna. It is known that when channel state information (CSI) is available at the transmitter a large performance gain can be achieved. In a system employing time-division duplexing (TDD), CSI can be obtained at the base station if there is reciprocity between the forward and reverse channels. CSI can also be conveyed from the users to the base station via a limited-rate feedback channel in a frequency-division duplexing (FDD) system. In any case, channel estimation errors are inevitable due to the presence of background noise in the estimated signal and due to the finite number of feedback bits used in a limited-rate feedback system model. In this paper, we first consider the general case when partial CSI is available at the transmitter. We derive an MMSE based precoding technique that considers channel estimation errors as an integral part of the system design. Using rate-distortion theory and the generalized Lloyd vector quantization algorithm, we then specialize our results for the more practical limited-rate feedback system model. Compared to previously proposed precoding techniques such as channel inversion and regularized channel inversion, it is shown that the proposed precoding technique significantly improves the average bit error rate (BER) in the system. Furthermore, the performance of the proposed technique is investigated in the high signal-tonoise ratio (SNR) regime. It is shown that the proposed technique suffers from a ceiling effect that asymptotically limits the system performance.

Multiuser MIMO system using block space–time spreading and tensor modeling

In this paper, we consider a point-to-multipoint downlink multiuser wireless communication system, where a multiple-antenna base station simultaneously transmits data to several users equipped with multiple receive antennas. The transmit antenna array is partitioned into transmission blocks, each one being associated with a given user. Space–time spreading is performed within each block using a transmit antenna subset. We formulate block space–time spreading using a tensor modeling. We show that the tensor-based block space–time spreading model has the distinguishing feature of modeling a multiuser space–time transmission with different spatial spreading factors (diversity gains) as well as different multiplexing factors (code rates) for the users. The space–time spreading structure is chosen to allow a deterministic multiuser interference (MUI) elimination by each user. A block-constrained tensor model is then presented for the received signal, which is characterized by fixed constraint matrices that reveal the overall space–time spreading pattern. At each receiver, blind joint channel and symbol recovery is performed using an alternating least squares algorithm. Simulation results illustrate the performance of the proposed transceiver model in terms of bit-error-rate, channel/symbol estimation accuracy and link-level throughput.

Design and Analysis of High-Capacity Anti-Collusion Hiding Codes

A new methodology for the design of high-capacity anti-collusion hiding codes in a large-scale fingerprint-based traitor tracing system is proposed in this work. We consider a hiding code of MN bits, where M bits are used as the user ID and N bits are the length of the spreading codes. Since each user is assigned one out of 2 M ID numbers and one out of N spreading codewords, the total number of users is equal to 2 M N. To accommodate an even larger number of users, we propose a shifted spreading scheme that shifts the spreading codeword circularly by a certain amount. By allowing P shifts (with P mod N), the total number of users increases from 2 M N to 2 M NP. When multiple users perform a weighted collusion attack, we show that the task of identifying colluders and their attack weights can be formulated as a user detection and channel estimation problem in a multiuser wireless communication system with a multipath fading channel. For the latter, there exist code design techniques that choose the spreading codes carefully so as to reduce multiaccess interference of users with different spreading codewords effectively. By exploiting this analogy, we develop an anti-collusion code called OSIFT (Orthogonal Spreading followed by the Inverse Fourier Transform). We compare several hiding codes in a fingerprinting system consisting of hundreds of colluders. It is demonstrated by computer simulation that, when the OSIFT code is adopted, colluders and their attack weights can be found by the proposed fingerprinting system more accurately.

Linear precoder optimization for ARQ packet retransmissions in centralized multiuser MIMO uplinks

We investigate the joint optimization of automatic repeat request (ARQ) and MIMO diversity for multi-user wireless communication systems. Exploiting MIMO for diversity gain and ARQ for error correction, our goal is to better integrate MIMO and ARQ in the transceivers to improve multi-user uplink performance. Specifically, we design optimal linear precoders for ARQ retransmissions by multiple wireless uplink terminals equipped with MIMO transmitters and we rely on joint detection of multiple retransmissions at the receivers. To better utilize ARQ diversity, we derive a set of progressive linear ARQ precoders for flat fading multiple-access channels based on the criterion of maximizing the mutual information of multiuser (re)-transmissions. We propose two efficient iterative numerical algorithms to determine the optimal precoders.

Cross layer optimization of downlink power allocation in multi-user wireless communication systems

In multi-user wireless communication systems, dynamic power allocation is an important means to deal with the time-varying nature of the physical and network layers. However, the current layer optimization approach to power allocation cannot achieve the global optimum of the overall system performance. To solve this problem, a cross-layer optimization framework is presented for downlink power allocation, which takes both the channel and buffer states into account. A cross-layer optimization problem is formulated to optimize the total throughput with queue length and power constraints. An analytical solution and a low complexity dynamic programming algorithm, which are referred as water-filling in cellar (WFIC) policy, are presented to optimize the downlink power allocation. Finally, simulation results are presented to demonstrate the potential of the proposed method.

Exploiting Multiuser Diversity With Imperfect One-Bit Channel State Feedback

It has been well recognized that significant throughput gains can be leveraged in multiuser wireless communication systems by exploiting multiuser diversity with a smart scheduler. This scheduler collects channel state information (CSI) from all users and allocates the resources to the user(s) experiencing favorable channel conditions. However, for a frequency-division-duplex system with a large number of users, how to efficiently collect the required CSI will be a challenging task, especially when the feedback links are of limited capacity. In this paper, we propose a scheduling algorithm to exploit multiuser diversity with possibly imperfect one-bit channel state feedback. The basic idea is to define a threshold lambda and let each user report one-bit information to the scheduler about the comparison between its measured channel fading level and lambda. Correspondingly, the scheduler uses these feedback bits to classify all users into two sets and assigns the channel to one user belonging to the set experiencing favorable channel conditions. Several implementation schemes are developed by attacking the optimization of lambda under different system configurations, covering both the case when the one-bit feedback is perfect and those when the one-bit feedback is imperfect. Computer simulations show that when the user number is large, say, more than ten users, the proposed scheduling supports significantly larger data rate over the round-robin scheduling, while in comparison with the optimum scheduling with complete CSI, the performance loss is limited if the one-bit feedback is of high reliability. In addition, our studies show that we can effectively enhance the robustness against feedback imperfectness by incorporating the feedback reliability into optimization of lambda

A Covariance Shaping Framework for Linear Multiuser Detection

A new class of linear multiuser receivers, referred to as the covariance shaping multiuser (CSMU) receiver, is proposed, for suppression of interference in multiuser wireless communication systems. This class of receivers is based on the recently proposed covariance shaping least-squares estimator, and is designed to minimize the total variance of the weighted error between the receiver output and the observed signal, subject to the constraint that the covariance of the noise component in the receiver output is proportional to a given covariance matrix, so that we control the dynamic range and spectral shape of the output noise. Some of the well-known linear multiuser receivers are shown to be special cases of the CSMU receiver. This allows us to interpret these receivers as the receivers that minimize the total error variance in the observations, among all linear receivers with the same output noise covariance, and to analyze their performance in a unified way. We derive exact and approximate expressions for the probability of bit error, as well as the asymptotic signal-to-interference+noise ratio in the large system limit. We also characterize the spectral efficiency versus energy-per-information bit of the CSMU receiver in the wideband regime. Finally, we consider a special case of the CSMU receiver, equivalent to a mismatched minimum mean-squared error (MMSE) receiver, in which the channel signal-to-noise ratio (SNR) is not known precisely. Using our general performance analysis results, we characterize the performance of the mismatched MMSE receiver. We then treat the case in which the SNR is known to lie in a given uncertainty range, and develop a robust mismatched MMSE receiver whose performance is very close to that of the MMSE receiver over the entire uncertainty range.

Quantum signal processing

In this article we present a signal processing framework that we refer to as quantum signal processing (QSP) (Eldar 2001) that is aimed at developing new or modifying existing signal processing algorithms by borrowing from the principles of quantum mechanics and some of its interesting axioms and constraints. However, in contrast to such fields as quantum computing and quantum information theory, it does not inherently depend on the physics associated with quantum mechanics. Consequently, in developing the QSP framework we are free to impose quantum mechanical constraints that we find useful and to avoid those that are not. This framework provides a unifying conceptual structure for a variety of traditional processing techniques and a precise mathematical setting for developing generalizations and extensions of algorithms, leading to a potentially useful paradigm for signal processing with applications in areas including frame theory, quantization and sampling methods, detection, parameter estimation, covariance shaping, and multiuser wireless communication systems. We present a general overview of the key elements in quantum physics that provide the basis for the QSP framework and an indication of the key results that have so far been developed within this framework. In the remainder of the article, we elaborate on the various elements in this figure.