Multiuser Communication Systems Research Articles

Implication of LDPC Technique in Non-ideal Multiuser Communication System

The present paper analyzes the effect of LDPC coding on the performances of a CDMA DS-SS multiuser system. Two types of detectors are analyzed, namely the conventional matched filter and the MMSE ones, in the ideal case--when the spreading codes are perfectly orthogonal and in a non-ideal one--when the users are more or less intercorrelated. Regarding the communication channel, several cases have been taken into consideration, from pure AWGN or Gaussian Mixture Noise to Rayleigh and Rice fading with different parameters. A large number of simulations have been performed using the Monte Carlo technique and the results are summarized and compared in the final part. Based on those results a number of interesting conclusions are highlighted in the Conclusion section.

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...

Multiuser Receiver for Joint Symbol/Channel Estimation in Dual-Hop Relaying Systems

We consider the uplink of a multiuser cooperative dual-hop communication system, where users transmit signals towards a base station with the help of multiple relays. The base station is equipped with an antenna array, while each user and relay are single-antenna devices simultaneously accessing the channel using non-orthogonal codes. For this communication scenario, we propose a novel multiuser receiver for joint symbol and channel estimation by capitalizing on a tensor modeling of the end-to-end system. The proposed multiuser receiver allows the base station to jointly estimate users' transmitted symbols and the channels involved in the dual-hop communication. Identifiability conditions of the proposed receiver are provided along with their practical implications to system design. Simulation results corroborate the effectiveness of our multiuser receiver in terms of channel estimation accuracy and bit-error-rate.

Combined multiuser acoustic communication and localisation system for µAUVs operating in confined underwater environments

This paper describes a communication and localization system for micro-autonomous underwater vehicles (µAUVs) undertaking monitoring and mapping tasks in nuclear storage ponds. A distinctive feature of this underwater environment is the severe multipath propagation and long reverberation times. The paper describes how a common code division multiplexed signal format is used to mitigate the acoustic channel impairments and to achieve both communication and localisation. The system consists of an onshore base-station, equipped with a four-element distributed transceiver antenna, and underwater acoustic modems installed in the µAUVs. The system simultaneously provides localisation of a swarm of µAUVs, with addressed transmission of navigation commands from the base- station to µAUVs, and telemetry data transmission to the base-station from the µAUV.

A Reduced Complexity Algorithm for Multi User Detection in DS UWB Systems

UWB is the latest technology to cause huge advances in wireless communication, networking, and radar and positioning systems. Many novel algorithms for the multiuser detection have been proposed for a DS-UWB multiuser communication system. In this paper we have proposed a reduced complexity Multi User detector based on minimum mean square error (MMSE).The algorithm exploit the inherent multipath diversity and also mitigate the effects of both inter symbol interference(ISI) and multiuser interference (MUI) with the reduced complexity. Simulation results show that the algorithm perform better than the other known detectors with the same complexity in literature

Multiuser Multihop MIMO Relay System Design Based on Mutual Information Maximization

In this paper, we consider multiuser multihop relay communication systems, where the users, relays, and the destination node may have multiple antennas. We address the issue of source and relay precoding matrices design to maximize the system mutual information (MI). By exploiting the link between the maximal MI and the weighted minimal mean-squared error (WMMSE) objective functions, we show that the intractable maximal MI-based source and relay optimization problem can be solved via the WMMSE-based source and relay design through an iterative approach which is guaranteed to converge to at least a stationary point. For the WMMSE problem, we derive the optimal structure of the relay precoding matrices and show that the WMMSE matrix at the destination node can be decomposed into the sum of WMMSE matrices at all hops. Under a (moderately) high signal-to-noise ratio (SNR) condition, this WMMSE matrix decomposition significantly simplifies the solution to the WMMSE problem. Numerical simulations are performed to demonstrate the effectiveness of the proposed algorithm.

Full duplex random access for multi-user OFDMA communication systems

Random medium access control (MAC) plays a critical role in OFDMA wireless networks to support services with bursty traffic. However, the existing OFDMA random access protocols suffer from low efficiency due to the constraints in half-duplex communications. In this paper, the capability of full-duplex communications is leveraged to propose novel mechanisms such as full duplex carrier sensing, collision detection, and collision jamming. With these mechanisms, a full-duplex carrier sense multiple access with collision detection (FD-CSMA/CD) protocol is developed for OFDMA wireless networks. It is based on a hierarchical design: (1) with full-duplex communications, operation of subchannels is decoupled, and a CSMA/CD-like protocol is implemented in each subchannel as a random access scheme; (2) on top of random access per-subchannel, a simple but effective subchannel selection scheme is executed locally on each client to harvest multi-user diversity. Both theoretical analysis and simulations are carried out to validate the effectiveness and efficiency of our protocol.

Design and Implementation of Key Technologies of Carrier Synchronization in Wireless Communication System

Noise multiple and user interference are the two important influencing factors that carrier synchronization technology in the wireless communication system faces. On the one hand, the phase locked loop and data aided carrier synchronization device is faced with poor estimation precision and slow convergence speed. On the other hand frequency offset and channel estimation problem need to be solved in multi-input multi-output (MIMO) communication system collaboration under the condition of dry low signal to noise ratio. So research of carrier synchronization algorithm of low complexity, high precision in view of the low SNR and multi-user communication system is becoming an important issue. 1. Carrier synchronization technology in wireless communication system of low signal-to-noise ratio and multiuser

Extensive Survey on MIMO Technology using V-BLAST Detection Technique

Wireless communication using Multiple Input Multiple Output (MIMO) systems increases efficiency for a given total transmits power. Here investigate a novel approach to perform decodingorder optimization in the vertical Bell Labs layered space-time (V-BLAST) planning. Allowing for instantaneous transmission of multiple, independently encoded data streams, the V-BLAST transmit-receive method applies to the uplink of a multi-user communication system. A multi-antenna receiver decodes data streams consecutively, trade with inter-stream interference by means of cancelling and nulling; interference from before decoded streams is cancelled by subtraction, and residual interference is nulled by linear equalization. The order in which the streams are decoded can have a visible impact on system presentation. Multiple Input Multiple Output (MIMO) channels can offer high capability to wireless systems and the capacity increases linearly with the number of antennas. There are many schemes that can be applied to MIMO systems such as space time block codes, space time trellis codes, and the Vertical Bell Labs Space-Time structural design (V-BLAST). We study the broad-spectrum MIMO system, the general VBLAST structure, and the Successive Interference Cancellation (SIC) Zero-Forcing (ZF) detectors in this paper.

Real-Time Dynamic Spectrum Management for Multi-User Multi-Carrier Communication Systems} \newcommandargmaxoperatornamewithlimits{argmax

Dynamic spectrum management is recognized as a key technique to tackle interference in multi-user multi-carrier communication systems and networks. However existing dynamic spectrum management algorithms may not be suitable when the available computation time and compute power are limited, i.e., when a very fast responsiveness is required. In this paper, we present a new paradigm, theory and algorithm for real-time dynamic spectrum management (RT-DSM). Specifically, a RT-DSM algorithm is real-time in the sense that it can be stopped at any point in time while guaranteeing a feasible and improved solution. This is enabled by the introduction of a novel difference-of-variables (DoV) transformation and problem reformulation, for which a primal coordinate ascent approach is proposed with exact line search via a logarithmically-scaled grid search. The proposed algorithm is referred to as iterative power difference balancing (IPDB). Simulations for different realistic wireline and wireless interference-limited systems demonstrate its good performance, low complexity and wide applicability under different configurations.

Statistical Framework for Optimization in the Multi-User MIMO Uplink With ZF-DFE

We consider performance optimization in the uplink of a multiuser multiantenna communication system. Each user multiplexes data onto several independently encoded data streams, which are spatially precoded and conveyed over a fading narrowband multiple-input multiple-output (MIMO) channel. All users' data streams are decoded successively at the receiving base station using zero-forcing decision feedback equalization (ZF-DFE). We target the joint optimization of a decoding order and linear precoders for all users based on long-term channel information. For a class of general MIMO channel models, including the separable-correlation and double-scattering models, we show that the choice of precoder for a certain user does not affect the performance of the others. This leads to a particularly straightforward characterization of general user utility regions as a polyblock, or a convex polytope if time-sharing is allowed. We formulate the decoding-ordering problem under transmit-correlated Rayleigh fading as a linear assignment problem, enabling the use of existing efficient algorithms. Combining decoding ordering with single-user precoder optimization by means of alternating optimization, we propose an efficient iterative scheme that is verified numerically to converge fast and perform close to optimally, successfully reaping the benefits of both precoding and ordering in the MIMO uplink.

基于光生编码超宽带信号的多用户通信系统

基于光生编码超宽带信号的多用户通信系统

Noncoherent Trellis Coded Quantization: A Practical Limited Feedback Technique for Massive MIMO Systems

Accurate channel state information (CSI) is essential for attaining beamforming gains in single-user (SU) multiple-input multiple-output (MIMO) and multiplexing gains in multi-user (MU) MIMO wireless communication systems. State-of-the-art limited feedback schemes, which rely on pre-defined codebooks for channel quantization, are only appropriate for a small number of transmit antennas and low feedback overhead. In order to scale informed transmitter schemes to emerging massive MIMO systems with a large number of transmit antennas at the base station, one common approach is to employ time division duplexing (TDD) and to exploit the implicit feedback obtained from channel reciprocity. However, most existing cellular deployments are based on frequency division duplexing (FDD), hence it is of great interest to explore backwards compatible massive MIMO upgrades of such systems. For a fixed feedback rate per antenna, the number of codewords for quantizing the channel grows exponentially with the number of antennas, hence generating feedback based on look-up from a standard vector quantized codebook does not scale. In this paper, we propose noncoherent trellis-coded quantization (NTCQ), whose encoding complexity scales linearly with the number of antennas. The approach exploits the duality between source encoding in a Grassmannian manifold (for finding a vector in the codebook which maximizes beamforming gain) and noncoherent sequence detection (for maximum likelihood decoding subject to uncertainty in the channel gain). Furthermore, since noncoherent detection can be realized near-optimally using a bank of coherent detectors, we obtain a low-complexity implementation of NTCQ encoding using an off-the-shelf Viterbi algorithm applied to standard trellis coded quantization. We also develop advanced NTCQ schemes which utilize various channel properties such as temporal/spatial correlations. Monte Carlo simulation results show the proposed NTCQ and its extensions can achieve near-optimal performance with moderate complexity and feedback overhead.

Exploit Network Coding Over GF(2 q ) for Multi-user Cooperative Wireless Networks

In this paper, we present a decode-and-forward network coded (DFNC) scheme over GF(2q) for the multi-user cooperative communication systems. In particular, we consider a cooperative network with m users transmitting independent packets to the same destination. These users form a cooperation set to help each other by using linear network coding. We propose a coding coefficients construction method which can efficiently reduce the transmission overhead from m(q + log2m) to m bits compared with conventional random network coding. Furthermore, we propose a novel decoding algorithm—credit-based updating algorithm in order to improve the solvability of decoding set of equations at the destination. The proposed decoding algorithm is combined with channel decoding and is applied on symbol-level. It can fully make use of the error recovery property of network coding while conventional decoding algorithms (e.g., Gaussian elimination) overlook it. We theoretically analyze the diversity performance in terms of information outage probability, and the results show that diversity order of m + 1 can be achieved for a m-user cooperation system. Moreover, we conduct extensive simulations to show that DFNC outperforms other transmission schemes in terms of symbol error rate and achieves higher diversity order. We also demonstrate that the proposed decoding algorithm provides significant performance gain over conventional decoding algorithm.

Optimal Randomization of Signal Constellations on the Downlink of a Multiuser DS-CDMA System

In this study, the jointly optimal power control with signal constellation randomization is proposed for the downlink of a multiuser communications system. Unlike a conventional system in which a fixed signal constellation is employed for all the bits of a user (for given channel conditions and noise power), power control with signal constellation randomization involves randomization/time-sharing among different signal constellations for each user. A formulation is obtained for the problem of optimal power control with signal constellation randomization, and it is shown that the optimal solution can be represented by a randomization among (K+1) or fewer distinct signal constellations for each user, where K denotes the number of users. In addition to the original nonconvex formulation, an approximate solution based on convex relaxation is derived. Then, detailed performance analysis is presented when the receivers employ symmetric signaling and sign detectors. Specifically, the maximum asymptotical improvement ratio is shown to be equal to the number of users, and the conditions under which the maximum and minimum asymptotical improvement ratios are achieved are derived. Numerical examples are presented to investigate the theoretical results, and to illustrate performance improvements achieved via the proposed approach.

Adaptive Quantization on a Grassmann-Manifold for Limited Feedback Beamforming Systems

In this paper we examine delay limited adaptive quantization on the Grassmann manifold of 1-dimensional subspaces in n-dimensional space. Due to strict delay limits, vector quantization over multiple time instants cannot be applied to exploit the temporal correlation of the source signal. Instead, a vector predictive quantizer is proposed that combines prediction and differential quantization algorithms to achieve an efficient quantization of the correlated Grassmannian source. The proposed predictor is based on adaptive finite impulse response filters to adjust to the temporal statistics of the source signal. It is shown that the prediction error in the tangent space associated with the Grassmann manifold behaves approximately Gaussian, provided its norm is sufficiently small. The proposed quantization algorithm is applied to channel state information quantization in multi-user beamforming wireless communication systems. Large throughput gains are demonstrated in comparison to memoryless quantization, due to reduced multi-user interference.

User Activity Tracking in DS-CDMA Systems

In modern multiuser communication systems, users are allowed to enter or leave the system at any given time. Thus, the number of active users is an unknown and time-varying parameter, and the performance of the system depends on how accurately this parameter is estimated over time. The so-called problem of user identification, which consists of determining the number and identities of users transmitting in a communication system, is usually solved prior to, and hence independently of, that posed by the detection of the transmitted data. Since both problems are tightly connected, a joint solution is desirable. In this paper, we focus on direct-sequence (DS) code-division multiple-access (CDMA) systems and derive, within a Bayesian framework, different receivers that cope with an unknown and time-varying number of users while performing joint channel estimation and data detection. The main feature of these receivers, compared with other recently proposed schemes for user activity detection, is that they are natural extensions of existing maximum a posteriori (MAP) equalizers for multiple-input-multiple-output communication channels. We assess the validity of the proposed receivers, including their reliability in detecting the number and identities of active users, by way of computer simulations.

Optimal Detector Randomization for Multiuser Communications Systems

Optimal detector randomization is studied for the downlink of a multiuser communications system, in which users can perform time-sharing among multiple detectors. A formulation is provided to obtain optimal signal amplitudes, detectors, and detector randomization factors. It is shown that the solution of this joint optimization problem can be calculated in two steps, resulting in significant reduction in computational complexity. It is proved that the optimal solution is achieved via randomization among at most min{K,Nd} detector sets, where K is the number of users and \Nd is the number of detectors at each receiver. Lower and upper bounds are derived on the performance of optimal detector randomization, and it is proved that the optimal detector randomization approach can reduce the worst-case average probability of error of the optimal approach that employs a single detector for each user by up to K times. Various sufficient conditions are obtained for the improvability and nonimprovability via detector randomization. In the special case of equal crosscorrelations and noise powers, a simple solution is developed for the optimal detector randomization problem, and necessary and sufficient conditions are presented for the uniqueness of that solution. Numerical examples are provided to illustrate the improvements achieved via detector randomization.

Method of improvement of structural security of transmission of information in multi-user access communication system

Currently, the design of confidential communications multi-user access systems encourages a particular interest of transmission methods, which provide not only an increase of channel capacity, but also improve the security of transmitted signals. Transmission security is one of the important indicators of noise immunity and it determines the ability of a system to resist the actions aimed at detection of a signal and scanning of its parameters. The article proposes a method of forming a group signal based on a plurality of multi-level timer signal designs with their division according to the level. To improve the structural security of information signals of individual channels, the system uses multi-level timer signals, which are then combined into a group signal. In order to reduce the effectiveness of unauthorized access means for the procedure of decoding of an intercepted signal the method of division of timer signal constructions according to the level is used. The purpose of the article is to develop a method of forming a group signal based on a plurality of multi-level timer signal designs with their division according to the level. The proposed method of forming signal designs can be recommended for the construction of confidential multi-user access systems, in which additional measures are taken to protect the information on the first level of the reference model OSI

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.