Multiple Access Communication Systems Research Articles

Performance of multiuser detectors based on EM-like method for ultra-wideband communications systems in multipath fading channel

An adaptive multiuser detector (MUD) is proposed for direct-sequence ultra-wideband (DS-UWB) multiple access communication systems to suppress both multiple access interference (MAI) and inter-symbol interference (ISI). In this contribution, considering the MUD from a combination viewpoint, we proposed a MUD based on electromagnetism-like (EM) method, which applied the concept of EM search to Hopfield neural network (EMHNN) for solving optimization problems. We analyze the performance of the EMHNN MUD in multipath fading channel, and compare it with the optimum detector and several suboptimum schemes such as conventional, decorrelator detector (DD), minimum-mean-squared error (MMSE) and HNN MUD. Simulation results will demonstrate that the proposed EMHNN MUD, which alleviates the detrimental effects of the MAI problem, can significantly improve the system performance.

A generic construction of low correlation zone sequences based on interleaved technique

Low correlation zone (LCZ) sequences are useful in quasi-synchronous code-division multiple access (QS-CDMA) communication systems. In this paper, a generic construction of LCZ sequences based on interleaved technique is investigated. Firstly, the shift sequence is shown to correspond to two-tuple balanced d -form function essentially, which results in new shift sequence. Secondly, an optimal design of p 2 -ary sequences over the integer residue class ring Z p 2 is proposed, which improves the previous construction when p is an odd prime.

Multiuser Detection Based on Adaptive LMS and Modified Genetic Algorithm in DS-CDMA Communication Systems

In this paper, we present an efficient evolutionary algorithm for the multi-user detection (MUD) problem in direct sequence-code division multiple access (DS-CDMA) communication systems. The optimum detector for MUD is the maximum likelihood (ML) detector, but its complexity is very high and involves an exhaustive search to reach the best fitness of transmitted and received data. Thus, there has been considerable interest in suboptimal multiuser detectors with less complexity and reasonable performance. The proposed algorithm is a combination of adaptive LMS Algorithm and modified genetic algorithm (GA). Indeed the LMS algorithm provides a good initial response for GA, and GA will be applied for this response to reach the best answer. The proposed GA reduces the dimension of the search space and provides a suitable framework for future extension to other optimization algorithms. Our algorithm is compared to ML detector, Matched Filter (MF) detector, conventional detector with GA; and Adaptive LMS detector which have been used for MUD in DS-CDMA. Simulation results show that the performance of this algorithm is close to the optimal detector with very low complexity, and it works better in comparison to other algorithms.

Robust pre‐filtering based space division multiple access over multi‐path channels

AbstractA novel concept of time reversal division multiple access (TRDMA) is proposed recently as a low complexity, energy efficient wireless multi‐user access method. In this paper, we first generalize TRDMA as a pre‐filtering based space division multiple access communication system, where each user's signature waveform is derived from the information of channel impulse responses (CIRs) between basestation and location specific multiple mobile stations. We develop two pre‐filtering schemes: one is referred to as time reversal matched filter. The benefit of the time reversal matched filter is to fully collect energy at the sampling instant of the intended mobile station (spatial and temporal focusing). The second pre‐filter, which is referred to as zero‐forcing pre‐filter (ZFPF), is derived to meet the zero‐forcing criterion such that multi‐user interference is completely eliminated. However, because the uplink CIR is generally different from the downlink CIR, we present extensive analysis on the performance degradation of the ZFPF caused by this mismatched effect. Furthermore, to improve the robustness to mismatch, we propose a simple yet effective robust ZFPF. We investigate and compare the performances of both schemes under various scenarios in terms of power consumption and averaged signal‐to‐interference‐plus‐noise power ratio. It is shown in both analytical and simulation results that satisfying performance can be achieved by the proposed pre‐filtering system. Moreover, we demonstrate that the proposed robust‐ZFPF has improved the performance in mismatched condition. Copyright © 2013 John Wiley & Sons, Ltd.

위성 시분할다중접속 통신시스템을 위한 실시간 자동이득제어기 설계 및 구현

일반적으로 위성 시분할다중접속 (TDMA, Time Division Multiple Access) 통신시스템 설계 시 채널감쇄를 반드시 고려 해야하며 위성 통신시스템은 급격한 채널환경 변화에서도 안정적인 네트워크 구성을 위해 실시간 자동 이득제어기 (AGC, Automatic Gain Control)가 필수적이다. 본 논문에서는 위성 TDMA 통신시스템에서 타임 슬롯에 실시간 적용이 가능한 AGC를 제안한다. 제안하는 실시간 AGC 성능을 확인하기 위해 위성 TDMA 시스템을 설계 및 구현하였고 제안하는 AGC 알고리듬을 적용하여 시스템의 비트 오율을 확인하였다. 이 결과는 추후 위성 TDMA 통신시스템의 안정성을 높이는데 유용하게 활용될 수 있을 것으로 예상된다. Channel attenuation must be taken into account when designing Time Division Multiple Access(TDMA) communication systems. In addition, Automatic Gain Control (ACG) is a requirement in satellite communication systems in order to form a stable network regardless of rapidly changes in channel environment. In this study, we present an AGC with possible real-time application to time slots in satellite TDMA communication systems. A satellite TDMA system was designed and implemented in order to test the performance of the proposed real-time AGC, and the system's BER (Bit Error Rate) was found by applying the proposed AGC algorithm. These results can be expected to be of high value in improving the stability of satellite TDMA communication systems in the future.

Performance Analysis of a Simple Subspace Projection-Based Channel Estimate Improver for Multicarrier CDMA Systems

This paper addresses pilot-assisted estimation offrequency-selective time-invariant channels in multicarrier Code Division Multiple Access communications systems (MC CDMA and MC DS-CDMA). Performance in terms of normalised mean square error (NMSE) is derived for two discrete channel frequency response estimators: a conventional estimator based on the minimum mean square error criterion, and an improved estimator exploiting subspace relationships between the frequency and impulse responses of the discrete channel. For MC DS-CDMA, NMSE performances of both estimators result in closed-form solutions. For MC CDMA, a closed-form solution is derived for the NMSE of the conventional estimator; upper and lower bounds are provided for the NMSE of the improved estimator. Furthermore, for the particular case of identically distributed discrete channels with common arbitrary power delay profile and uncorrelated weights, a closed-form expression for the NMSE of the improved estimator is also obtained. Numerical results illustrate the accuracy of the proposed NMSE expressions.

A Game-Theoretic Approach for Energy-Efficient Contention-Based Synchronization in OFDMA Systems

The purpose of this paper is to provide a novel energy-efficient perspective to the problem of contention-based synchronization in orthogonal frequency-division multiple-access communication systems. This is achieved by modeling the terminals and their corresponding receivers at the base station as economic and rational agents that engage in a noncooperative game. In the proposed game, each one trades off its available resources (transmit power and detection strategy) so as to selfishly maximize its own revenue (in terms of probability of correct detection) while saving as much energy as possible and satisfying quality-of-service requirements given in terms of probability of false alarm and timing estimation accuracy. The existence and uniqueness of the equilibrium of the game are studied. In particular, a necessary and sufficient condition on the system parameters is given for the equilibrium to exist. An iterative and distributed algorithm based on best-response dynamics (at the transmit side) and a practical parameter estimation (at the receive side) are proposed to achieve the equilibrium point. Numerical results are used to highlight the effectiveness of the proposed solution and to make comparisons with existing alternatives in terms of power consumption, synchronization time, and estimation accuracy.

Contribution to Synchronization and Tracking Modelisation in a CDMA Receiver

We propose and analyze a noncoherent receiver with PN code tracking for direct sequence code division multiple access (DS-CDMA) communication systems. We employ the delay-lock loop (DLL) architectures for the tracking stage. The choice of DLL parameters is studied with special focus on DS-CDMA communication systems and orthogonality conditions. We described the modeling and simulation of the NCO using hardware description language VHDL. Details of the VHDL implementation are shown.

H∞Channel Estimation for DS-CDMA Systems: A Partial Difference Equation Approach

In the communications literature, a number of different algorithms have been proposed for channel estimation problems with the statistics of the channel noise and observation noise exactly known. In practical systems, however, the channel parameters are often estimated using training sequences which lead to the statistics of the channel noise difficult to obtain. Moreover, the received signals are corrupted not only by the ambient noises but also by multiple-access interferences, so the statistics of observation noises is also difficult to obtain. In this paper, we will investigate theH∞channel estimation problem for direct-sequence code-division multiple-access (DS-CDMA) communication systems with time-varying multipath fading channels. The channel estimator is designed by applying a partial difference equation approach together with the innovation analysis theory. This method can give a sufficient and necessary condition for the existence of anH∞channel estimator.

Joint detection, channel estimation, and interference cancellation in downlink MC-CDMA communication systems using complex-valued multilayer neural networks

Data detection in the presence of interference is one of the main challenges in multicarrier code division multiple access (MC-CDMA) communication systems. In this paper, a new detection technique for downlink MC-CDMA systems is proposed. This technique uses complex-valued multilayer neural networks at the receiver side. With the new definition for desired responses (±(1+J) instead of ±1, where $$ J = \sqrt {{ - 1}} $$ ), the convergence rate is increased (in the training process) regarding to saturation of imaginary output and the performance is increased because of increasing Euclidean distance of output neuron inputs in two states of desired outputs (with factor of $$ \sqrt {2} $$ ). The performance of the proposed method is improved further by considering two various saturation coefficients (in the activation function of output layer) in the training and test processes. Since the last performance improving lead to low convergence rate, this effect is compensated by correcting the coefficient of training rate in the output layer. Simulation results confirm the high convergence rate, low computational complexity, and also good performance of the proposed method in wide range of SNRs.

Probability Distribution of Multiple-Access Interference in Chaotic Spreading Codes based on DS-CDMA Communication System

This paper is devoted to the evaluation of the probability distribution of the multiple access interference in asynchronous Direct Sequence Code Division Multiple Access (DS CDMA) communication system based on chaotic spreading sequences. Statistical properties of such interference in a system with K users causing the multiple-access interference are obtained by a computer simulation. Simulation is carried out for the spreading sequences produced by the logistic map; the length of sequences: N = 7, 31, 63 and the number of users correspondingly: K = (3, 8, 11); (11, 32, 65); (64, 129). Ill. 3, bibl. 8, tabl. 2 (in English; abstracts in English and Lithuanian). DOI: http://dx.doi.org/10.5755/j01.eee.123.7.2380

Optimal packet scheduling in a multiple access channel with energy harvesting transmitters

In this paper, we investigate the optimal packet scheduling problem in a two-user multiple access communication system, where the transmitters are able to harvest energy from the nature. Under a deterministic system setting, we assume that the energy harvesting times and harvested energy amounts are known before the transmission starts. For the packet arrivals, we assume that packets have already arrived and are ready to be transmitted at the transmitter before the transmission starts. Our goal is to minimize the time by which all packets from both users are delivered to the destination through controlling the transmission powers and transmission rates of both users. We first develop a generalized iterative backward waterfilling algorithm to characterize the maximum departure region of the transmitters for any given deadline T. Then, based on the sequence of maximum departure regions at energy arrival instants, we decompose the transmission completion time minimization problem into convex optimization problems and solve the overall problem efficiently.

Multi-user detection in ultra-wideband multiple-access communications systems using an efficient heuristic algorithm

This paper investigates learning and achievable bit error rate (BER) performance of ultra-wideband (UWB) systems that use intelligent multiuser detector (MUD) when communicating over UWB channels that experience both multiuser interference (MUI) and intersymbol interference (ISI), in addition to multipath fading. Multiple access interference (MAI) degrades performance of conventional single user detector in UWB systems. Due to high complexity of the optimum multiuser detector, suboptimal multiuser detectors with less complexity and reasonable performance have drawn considerable attention. By taking advantage of heuristic values and collective intelligence of tabu search with Hopfield neural networks (TAHNN), the proposed detector offers almost the same BER performance as a full-search-based optimum multiuser detector does, while greatly reducing computational complexity. To evaluate performance and robustness of our proposed TAHNN based MUD, we experiment with a number of test problems. Computational results show that our proposed TAHNN in almost all cases outperforms other foregoing heuristics applied to this paper. Key words: Ultra-wideband, Tabu search method, Hopfield neural networks, multiuser detection.

Multi-user detection in ultra-wideband multiple-access communications systems using an efficient heuristic algorithm

This paper investigates learning and achievable bit error rate (BER) performance of ultra-wideband (UWB) systems that use intelligent multiuser detector (MUD) when communicating over UWB channels that experience both multiuser interference (MUI) and intersymbol interference (ISI), in addition to multipath fading. Multiple access interference (MAI) degrades performance of conventional single user detector in UWB systems. Due to high complexity of the optimum multiuser detector, suboptimal multiuser detectors with less complexity and reasonable performance have drawn considerable attention. By taking advantage of heuristic values and collective intelligence of tabu search with Hopfield neural networks (TAHNN), the proposed detector offers almost the same BER performance as a full-search-based optimum multiuser detector does, while greatly reducing computational complexity. To evaluate performance and robustness of our proposed TAHNN based MUD, we experiment with a number of test problems. Computational results show that our proposed TAHNN in almost all cases outperforms other foregoing heuristics applied to this paper. Key words: Ultra-wideband, Tabu search method, Hopfield neural networks, multiuser detection.

Prediction of Pilot Pollution in High-Rise Buildings Covered by Outdoor Base Station

In Code Division Multiple Access (CDMA) communication systems, the pilot pollution in high-rise building area is an increasingly prominent and long-standing problem, which seriously affects Quality of Service (QoS) and User Experience (UE). This paper presents a predicting method of pilot pollution in high-rise building area covered by Outdoor Base Station (OBS).The method, which is based on Operation Maintenance Center (OMC) and location (LOC) data, can predict pilot pollution severity occurred in high-rise and pertinently recommend a prioritization scheme, by analyzing the relationship of pilot strength, distance and angle between target area and OBS. The validity and feasibility of this prediction method have been verified by Call Quality Test (CQT). Operators Capital Expenditure (CAPEX) and Operating Expense (OPEX) could be greatly reduced with the efficient and convenient prediction.

Simple Code Switching Scheme for Security Enhancement of OCDMA Communication System

A simple code switching scheme for security enhancement of an optical code-division multiple-access (OCDMA) communication system is proposed. The scheme avoids the replacement of all the encoders and decoders in the system and all users' codes are changed by only tuning the center wavelength of the laser source. Each en/decoder contains multiple codes which are nonoverlapping in the spectrum. The scheme is demonstrated in a proof-of-principle transmission experiment. There are three codes on the encoder and decoder pairs, one 2-D code and two 1-D bipolar gold codes with chip number of 63 and 127, respectively. All three codes are decoded successfully after 12.7-km single-mode fiber link.

A New Approach to Random Access: Reliable Communication and Reliable Collision Detection

This paper applies Information Theoretic analysis to packet-based random multiple access communication systems. A new channel coding approach is proposed for coding within each data packet with built-in support for bursty traffic properties, such as message underflow, and for random access properties, such as packet collision detection. The coding approach does not require joint communication rate determination either among the transmitters or between the transmitters and the receiver. Its performance limitation is characterized by an achievable region defined in terms of communication rates, such that reliable packet recovery is supported for all rates inside the region and reliable collision detection is supported for all rates outside the region. For random access communication over a discrete-time memoryless channel, it is shown that the achievable rate region of the introduced coding approach equals the Shannon information rate region without a convex hull operation. Further connections between the achievable rate region and the Shannon information rate region are developed and explained.

Performance Analysis of SSMA Communication Systems With Spreading Sequences of Markov Chains: Large Deviations Principle Versus the Central Limit Theorem

In this paper, we consider asynchronous spread spectrum multiple access (SSMA) communication systems with spreading sequences of Markov chains. We show that, given phase shifts and time delays, the empirical mean of the multiple access interference (MAI) for such systems satisfies the conditional large deviations principle (LDP) with the convex, good rate function. By refinement of the large deviations analysis, we also obtain exact asymptotics of bit-error probabilities in such systems. Comparing theoretical expressions of bit-error probabilities with experimental results, we confirm that for too small numbers of users compared to the lengths of spreading sequences, the central limit asymptotics become invalid, but the large deviations asymptotics turn out to be relevant.

Code Hopping Communications for Anti-Interception with Real-Valued QZCZ Sequences

With the real-valued sequences, a new kind of Code Hopping Code Division Multiple Access (CH-CDMA) communication systems is proposed for anti-interception. While the sequence parameters are the same, the number of the real-valued sequence sets are much more than those of the traditional integer sequence, therefore, the codes for hopping are no longer in serious deficiency as they used to be, which makes the performance of anti-interception improved greatly. Moreover, using real-valued sequences with Quasi-Zero Correlation Zone (QZCZ) may eliminate the Inter-Symbol Interference (ISI) and Multi-Access Interference (MAI) so as to overcome the far-near effect and improve the performance of anti-interference.

Generalized Discrete Fourier Transform With Nonlinear Phase

Constant modulus transforms like discrete Fourier transform (DFT), Walsh transform, and Gold codes have been successfully used over several decades in several engineering applications, including discrete multi-tone (DMT), orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA) communications systems. In this paper, we present a generalized framework for DFT called Generalized DFT (GDFT) with nonlinear phase by exploiting the phase space. We show that GDFT offers sizable correlation improvements over DFT, Walsh, Oppermann and Gold codes, leading to better performance in all multi-carrier communications scenarios investigated. We also highlight how known constant modulus orthogonal transforms are special solutions of the proposed GDFT framework. Moreover, we introduce practical design methods offering computationally efficient implementations of GDFT as enhancements to DFT. We conclude the paper with examples of communications applications where GDFT is shown to outperform DFT and other known constant modulus bases.