Celso de Almeida

CDMA Ad Hoc Networks in Multipath Fading Channels

This work analyses the effects of multipath diversity on the performance of wireless code division multiple access (CDMA) ad hoc networks. Expected progress per slot is used as the performance criterion for the capacity of the described model. The fading can unfavorably affect the data transmission in an wireless network. By the application of the well known rake receiver for CDMA systems, it is shown that multipath diversity significantly increases the CDMA ad hoc network performance, decreasing the harmful effects of the fading

On the Performance of MC-CDMA Cellular Systems Employing Multiuser Decorrelating Detector and Antenna Array

The mean bit error rate (BER) for the uplink of a multicarrier-code-division multiple access (MC- CDMA) cellular system is evaluated. The system employs a multiuser decorrelating detector in the receiver, a uniform antenna array at base stations and perfect power control. The scenario assumes the presence of multiple access interference (MAI), co-channel interference (CCI), additive white Gaussian noise (AWGN), exponential path-loss and a slow frequency-selective Rayleigh fading. Expressions to evaluate the mean BER of the system are obtained, which are function of the signal-to- noise ratio (Eb/No), the channel reuse factor (F), the number of users (Nu), the spreading factor (G) and the number of antennas in the array (Y). Monte Carlo simulations are carried out to verify the tightness of the analytical expressions. The results show that the system diversity order is YG - Nu + 1. In some scenarios, when the number of antennas is increased, the system can be overloaded (Nu>G) and the mean BER maintains reasonable values.

Performance analysis of MC-CDMA systems employing maximal ratio combining and uniform linear antenna array

This paper presents exact analytical expressions to evaluate the mean symbol error rate (SER) and the mean bit error rate (BER) of Multicarrier Code Division Multiple Access (MC-CDMA) systems using a correlation receiver with maximal ratio combining (MRC) and uniform linear antenna array at the base station. The performance is evaluated considering additive white gaussian noise (AWGN), Rayleigh fading, multiple access interference (own cell interference) and co-cell interference. Simulations using the Monte Carlo method are carried out, which are compared with the analytical expressions. An asynchronous uplink case with independent fading at different subcarriers is considered and BPSK and M-QAM modulations are used.

On the performance of maximal ratio combining and maximum likelihood detection in M-QAM/MC-CDMA systems

This paper compares the performance of Multi-carrier Code Division Multiple Access (MC-CDMA) system using a correlation receiver with maximal ratio combining (MRC) and the performance of MC-CDMA using maximum likelihood detection (MLD). For this purpose, firstly analytical expressions to evaluate the mean symbol error rate (SER) and the mean bit error rate (BER) of MC-CDMA systems with MRC are obtained. Then, simulations using the Monte Carlo method are carried out with the aim of evaluating the analytical expressions tightness and to compare the performance of MRC and MLD. The performance is evaluated considering additive white gaussian noise, Rayleigh fading channel, M-QAM modulation and the presence of inter-cell interference (multiple access interference) and co-cell interference. An asynchronous uplink scenario with independent fading at different subcarriers is considered. Also, multiple co-cell interferers are assumed.

Data transmission efficiency over CDMA systems employing RLP and FEC error control strategies on the radio link layer

In this article, the normalized throughput and the normalized average delay for the transport layer at the reverse link of multicellular CDMA systems are obtained. For the transport layer the Transport Control Protocol (TCP) with consecutive retransmissions control is employed. On the radio link layer three TCP error control strategies are evaluated and compared. The first strategy is based on the radio-link-protocol (RLP), which breaks down the TCP packets into smaller blocks and uses an error detecting procedure to protect those RLP blocks. The second strategy uses forward-error-control (FEC) procedure at the radio link layer. In this case, two kinds of FEC are compared: convolutional and turbo coding. The third one is a hybrid between the first and second strategies with coding at the RLP layer. A frequency-selective Rayleigh fading channel with shadowing and power control loop error is considered.

Effects of multiple co-channel interferers on the performance of amplify-and-forward relaying with optimum combining, multiple relays and multiple antennas

Abstract Cooperative relaying is a method used mainly to improve cellular networks in terms of diversity gain and coverage extension. However, its performance is gravely affected by the co-channel interference (CCI), especially when a high channel reuse is needed. Optimum combining (OC) is the combining technique that maximizes the signal-to-interference-plus-noise-ratio (SINR) and eliminates the CCI, achieving the best diversity gain. In the present paper, the performance analysis of cooperative amplify-and-forward relaying by using OC is extended for a scenario with multiple co-channel interferers, multiple-antenna relays and multiple antenna-destination. Also, the techniques of multiple relay transmission (MRT) and relay selection with transmit antenna selection (RS-TAS) are evaluated and compared. An approximation of the SINR and the diversity gain are obtained for both transmission techniques. The moment-generating function (MGF) of SINR is derived in order to obtain the average bit-error rate (BER) of the system. Analytical results are validated using Monte-Carlo simulations, which shows that the proposed scenario combats the CCI in a non obvious way and obtains diversity gain in both transmission techniques. Moreover, the MRT technique cancels a greater number of interferers than RS-TAS, while RS-TAS achieves better spectral efficiency and obtains better performance for small number of interferers.

Mean Bit Error Rate Evaluation of MC-CDMA Cellular Systems Employing Multiuser-Maximum-Likelihood Detector

In this paper, we evaluate the uplink bit error rate (BER) of multicarrier-code-division-multiple-access (MC-CDMA) cellular systems with multiuser-maximum-likelihood detector (MU-MLD). The scenario considers perfect power control, own-cell interference (or multiple access interference—MAI), interference from co-cells (or co-cell interference—CCI), additive white Gaussian noise, exponential path-loss and slow frequency-selective Rayleigh fading. Upper bound expressions to evaluate the mean BER are obtained, which are functions of the signal-to-noise ratio (SNR), the number of users, the spreading factor, and the channel reuse factor. For BPSK and 4-QAM (quadrature amplitude modulation), closed-form expressions involving the analytical calculation of pairwise error probabilities are obtained. For 16-QAM and 64-QAM, we show that the number of pairwise error probabilities needed to obtain the BER can be simplified through the constellations symmetry. The procedure for obtaining this simplification is detailed. Also, by considering BER asymptotes, expressions for the SNR penalty due to MAI are derived. Monte Carlo simulations verify the analytical expressions accuracy. In the simulations, the MU-MLD is implemented through the sphere decoder algorithm with MMSE QR factorization. Results show that MC-CDMA does not lose diversity due to MAI. Moreover, CCI significantly affects the system performance. As a consequence, it produces floors in the BER curves.

Performance Analysis of Opportunistic Systems Employing Maximal Ratio Combining and Antenna Array

Exact and approximate expressions are presented in this paper to evaluate the performance of opportunistic systems employing maximal ratio combining (MRC) and antenna array in the receiver in terms of the mean bit error probability (BEP). The channel model considers the presence of additive white Gaussian noise (AWGN) and flat slow fading with Rayleigh distribution. Simulation results have shown that the developed equations to evaluate the performance of those systems are very tight. Two different transmission scenarios are proposed which combat significantly the fading effects. Comparing with ordinary systems, the opportunistic systems present an outstanding performance.

The impact of high-power interference in decode-and-forward cooperation with optimum combining and multi-antenna destination

The co-channel interference (CCI) is the principal factor of signal degradation in cellular networks, even more when a better frequency planning is required, because it results in a high-power CCI. On the other hand, the cooperative communication allows to create a virtual antenna array in order to obtain a diversity gain and combat the fading effects. In the present paper, the impact of a high-power CCI in a decode-and-forward (DF) wireless cooperative network using optimum combining (OC), K relays and N antennas at destination is analysed. Analytical expressions of average bit-error rate (BER) and upper bound for BPSK modulation are derived. It is proved that only one link is used to eliminate the high-power CCI, obtaining a diversity gain of N(K+1)−1.

Low complexity multiuser detection based on reliable samples

A sub-optimum multiuser detector which compares the received samples to reliability thresholds, in order to classify them as reliable or non-reliable, is proposed for CDMA systems. Each one of these classified samples receives a different management in the detection process. This technique presents a good trade-off between performance and complexity in relation to the optimum multiuser detector. In fact, for some values of the reliability thresholds, the proposed detector reaches the optimum multiuser detector performance with a much smaller complexity. Theoretical equations and results of simulation for the complexity and bit error rate are presented. The proposed theoretical expressions are very tight when compared to the respective simulation results.