The performance of optical fiber direct-sequence spread-spectrummultiple-access communications systems

Because of the random nature of the photodetection process and the multiple-user interference, an exact analysis of avalanche photodiode (APD)-based optical code division multiple-access (CDMA) communications systems is intractable and quite often, Monte Carlo (MC) simulations which yield exact estimates of system performance in terms of bit error rates (BERs) require a prohibitive computational burden. A quick and accurate MC method for simulating APD-based optical CDMA systems is presented. In particular, a performance analysis of optical CDMA systems employing optical orthogonal and prime sequence codes is undertaken     

Monte Carlo Bayesian Signal Processing for Wireless Communications

Many statistical signal processing problems found in wireless communications involves making inference about the transmitted information data based on the received signals in the presence of various unknown channel distortions. The optimal solutions to these problems are often too computationally complex to implement by conventional signal processing methods. The recently emerged Bayesian Monte Carlo signal processing methods, the relatively simple yet extremely powerful numerical techniques for Bayesian computation, offer a novel paradigm for tackling wireless signal processing problems. These methods fall into two categories, namely, Markov chain Monte Carlo (MCMC) methods for batch signal processing and sequential Monte Carlo (SMC) methods for adaptive signal processing. We provide an overview of the theories underlying both the MCMC and the SMC. Two signal processing examples in wireless communications, the blind turbo multiuser detection in CDMA systems and the adaptive detection in fading channels, are provided to illustrate the applications of MCMC and SMC respectively.     

Multicarrier CDMA techniques for future wideband wireless networks

In this paper, multicarrier techniques are considered in the context of the future wideband wireless networks. An overview of the different access schemes based on a combination of code division and multicarrier techniques, such as Multi-Carrier Code Division Multiple Access (MC-CDMA), Multi-Carrier Direct Sequence CDMA (MC-DS-CDMA) and Multi-Tone CDMA (MT-CDMA) is presented first. Afterwards, MC-CDMA systems are considered for the downlink of high rate cellular networks. The performance of different mono-user detection techniques are first analytically estimated and then compared with the help of Monte Carlo simulations for a Rayleigh channel. Simulation results on the performance of mono-user and multi-user detection techniques are also provided for Bad Urban (BU) Hilly Terrain (HT) and Vehicular UMTS channels. With regard to the channel coding, convolutional codes and turbo-codes are considered. Among others, a new linear detection technique named Global Minimum Mean Square Error Algorithm (GMMSE) is compared to multi-user Parallel Interference Cancellation detection technique. It is shown that GMMSE offers good performance especially for non-full load systems. Thus, the efficiency of MC-CDMA as a very promising multiple access and robust modulation scheme is successfully demonstrated for the downlink of the future wideband mobile networks.     

Broadband CDMA techniques

A very high-speed wireless access of 100 Mb/s to 1 Gb/s is required for fourth-generation mobile communications systems. However, for such high-speed data transmissions, the channel is severely frequency-selective due to the presence of many interfering paths with different time delays. A promising wireless access technique that can overcome the channel frequency-selectivity and even take advantage of this selectivity to improve the transmission performance is CDMA. There may be two approaches in CDMA technique: direct sequence CDMA and multicarrier CDMA. A lot of attention is paid to MC-CDMA. However, recently it has been revealed that DS-CDMA can achieve good performance comparable to MC-CDMA if proper frequency domain equalization is adopted. This article discusses their similarities and performances. A major transmission mode in 4G systems is packet-based. Automatic repeat request combined with channel coding is a very important technique. Recent research activity on this technique is also introduced.     

Design and analysis of cellular mobile communications system basedon DQRAP/CDMA MAC protocol

Future third-generation mobile communication systems will need multi-access control (MAC) protocols suitable for multimedia code division multiple access (CDMA) radio communications. Distributed queueing random access protocol (DQRAP)/CDMA is a general purpose MAC protocol oriented to the CDMA environment. Analytical model expressions and computer simulations have shown its capacity to achieve near-optimum performance under heterogeneous traffic scenarios in a unicellular environment. A cellular environment has been designed to verify that DQPAP/CDMA maintains its near-optimum performance in a packet switched mobile communication system. A new handover technique based on the protocol is proposed to further improve the system performance     

Successive interference cancellation for multiuser asynchronousDS/CDMA detectors in multipath fading links

There has been an increasing interest in the use of code-division multiple access (CDMA) in cellular mobile and wireless personal communications. The choice of such multiaccess technique is attractive because of its potential capacity increases and other technical factors such as privacy and multipath rejection capabilities. However, it is well known that the performance of CDMA can be significantly degraded due to cochannel interference (CI) and the near-far effects. We consider the performance of direct-sequence (DS)-based CDMA over fading channels that are modeled as slowly varying Rayleigh-fading discrete multipath channels. Specifically, we propose and analyze an adaptive multistage interference cancellation strategy for the demodulation of asynchronous DS spread-spectrum multiple-access signals. Numerical results show that the proposed multistage detector, which alleviates the detrimental effects of the near-far problem, can significantly improve the system performance     

Efficient CDMA wireless position location system using TDOA method

In this paper, a new position location (PL) estimation technique for CDMA wireless communications system is proposed. The proposed technique provides faster solution to the hyperbolic model and does not suffer from the convergence problem. The performance of the proposed PL technique has been tested by considering different system parameters, such as channel noise and variable position of the user. Test results show that the proposed PL technique can successfully determine the position of a wireless user under various challenging scenarios. Copyright © 2011 John Wiley & Sons, Ltd.     

Performance analysis of a hybrid CDMA/ISMA protocol for indoorwireless computer communications

A combination of direct sequence code division multiple access (DS-CDMA) and p-persistent inhibit sense multiple access (ISMA) protocols is proposed for indoor wireless communications (IWC). The combination of these two protocols is called the hybrid CDMA/ISMA protocol. The performance of the hybrid CDMA/ISMA protocol is analyzed with a Markov model considering differential phase shift keying (DPSK) modulation. The performance is measured in terms of throughput and delay. The performance is also evaluated using computer simulation of which the results are in close agreement with the Markov model results. The study of indoor wireless communications using a hybrid CDMA/ISMA protocol can be useful for various applications in research and design offices, medical data communication in hospitals, etc     

An analytical framework for the design and performance evaluation of realistic Aloha-CDMA systems

The random access scheme has been shown to be an efficient transfer mechanism of packet data in wireless environments for nonreal-time applications where messages with variable length are transmitted over power controlled code-division multiple-access (CDMA) channels. Most of the previous work in this area covered the analysis of the CDMA systems for fixed or exponential packet length, infinite population, and unlimited waiting and service time where the results mainly depend on the mean values of the traffic. In this paper, we remove these assumptions and analyze the behavior of the system for the more general and realistic case of finite population, finite sojourn time, and general packet length distribution. Specifically, we provide an analytical method to study the performance related parameters of the system, such as packet delay, packet loss, and throughput as well as the effect of packet length distribution on the system performance under a realistic environment. The obtained results demonstrate that for the packet length distributions with the same mean but different tail properties, the system behavior can change dramatically. In addition, we demonstrate that this study provides an analytical tool that can be used as the underlying framework for the support of a wide range of applications and management functions such as optimization of design parameters, integration of multimedia services, and anomaly detection in CDMA wireless networks.     

Bayesian method for NLOS mitigation in single moving sensor Geo-location

The problem of locating mobile sensors has received considerable attention, particularly in the field of wireless communications. It is well-known that the presence of non-line-of-sight (NLOS) errors in the geo-location problem leads to severe degradation in the localization performance. In this paper, we propose a robust Bayesian method to mitigate the NLOS errors in location estimation of a single moving sensor, whereby the localization is performed using time-of-arrival (TOA) measurements. This method is based on the Markov chain Monte Carlo (MCMC) approach. Numerical simulations results illustrate the promising results of our method in a mixed line-of-sight (LOS) and NLOS environment.     

A unified approach to the performance analysis of speed estimation techniques in mobile communication

Estimation of the mobile speed, or equivalently, the maximum Doppler frequency, is of importance in a variety of applications in wireless mobile communications. In this paper, a unified framework for the performance analysis of several major speed estimation techniques is presented, which allows a fair comparison between all the methods, analytically. Interestingly, it is proved that all these methods are equivalent, asymptotically, i.e., for large observation intervals. In addition, we have derived closed-from expressions for the bias and variance of a recently proposed covariance-based method. We have also introduced a new estimator which relies on the average number of maxima of the inphase component, and have calculated its variance, analytically. Our extensive performance analysis, supported by Monte Carlo simulations, have revealed that depending on the channel condition and the observation interval, one needs to use a crossing- or a covariance-based technique, to achieve the desired estimation accuracy over a large range of mobile speeds.     

Performance analysis of forward link beamforming techniques forDS/CDMA systems using base station antenna arrays

The performance of DS/CDMA systems using forward link beamforming and RAKE diversity combining is investigated analytically in frequency selective fading channels, whereas previous studies have resorted to Monte Carlo simulations in flat fading channels. The capacity of DS/CDMA systems is considerably improved by employing forward link beamforming     

Scheduling with base station diversity and fairness analysis for the downlink of CDMA cellular networks

Efficient packet scheduling in CDMA cellular networks is a challenging problem due to the time variant and stochastic nature of the channel fading process. Selection diversity is one of the most effective techniques utilizing random and independent variations of diverse channels to improve the performance of communication over fading channels. In this paper, we propose two packet scheduling schemes exploiting base station selection diversity in the downlink of CDMA cellular networks. The proposed schemes rely on the limited instantaneous channel state information (CSI) to select the best user from the best serving base station at each time slot. This technique increases the system throughput by increasing multiuser diversity gain and reducing the effective interference among adjacent base stations. Results of Monte Carlo simulations are given to demonstrate the improvement of system throughput using the proposed scheduling schemes. In addition, we investigate fairness issue of wireless scheduling schemes. Due to different characteristics of wireless scheduling schemes, the existing fairness indexes may result in misleading comparison among different schemes. We propose a new fairness index to compare the overall satisfaction of the network users for different scheduling schemes. Copyright © 2006 John Wiley & Sons, Ltd.     

A Unified Approach to the Performance Analysis of Speed-Estimation Techniques in Mobile Communication

The estimation of the mobile speed, or equivalently, the maximum Doppler frequency, is of importance in a variety of applications in wireless mobile communications. In this paper, a unified framework for the performance analysis of several major speed-estimation techniques is presented, which allows a fair comparison between all the methods, analytically. Interestingly, it is proved that all these methods are equivalent, asymptotically, i.e., for large observation intervals. In addition, we have derived closed-form expressions for the bias and variance of a recently proposed covariance-based method. We have also introduced a new estimator that relies on the average number of maxima of the inphase component, and have calculated its variance, analytically. Our extensive performance analysis, supported by Monte Carlo simulations, have revealed that depending on the channel condition and the observation interval, one needs to use a crossing- or a covariance-based technique, to achieve the desired estimation accuracy over a large range of mobile speeds.      

Efficient estimation of cell blocking probability for ATM systems

The assessment of the cell loss performance of networks using asynchronous transfer mode (ATM) via Monte Carlo simulation incurs an enormous computational burden due to the need to estimate an event that has a very small probability of occurrence. Although importance sampling (IS) techniques have been proven useful in simulations of rate events related to bit error rate in digital communications and false alarm rate in radar systems, its application to ATM queuing problems with correlated input traffic has yet to be demonstrated. It is established that significant computational savings can be obtained using IS for correlated traffic by using regenerative properties of the underlying system and biasing the conditional arrival process. The results show that IS can reduce the computational burden by more than three orders of magnitude. Extensions of the methodology to more complex arrival processes are discussed. The foundation for applying IS to ATM systems given can be used to study congestion control as well as networks of ATM queues in the future     

Capacity analysis of spectrally overlaid multiband CDMA mobilenetworks

The future (third-generation) mobile communications system will provide not only voice and low-speed data services, but also video and high-speed data services. It will eventually have the capability for wireless multimedia communications. As a radio-access technique for such a system, the code-division multiple access (CDMA) has been considered in many potential third-generation systems including CODIT and the proposal of Ohno, Sawahashi and Adachi (see Proc. IEEF VTC'95, Chicago, IL, p.779-83, 1995). We model those CDMA systems as the spectrally overlaid multiband CDMA (SOM-CDMA) system. In an SOM-CDMA system, several CDMA (sub)systems with different spreading bandwidths are spectrally overlaid with each other. The reverse-link capacity of an SOM-CDMA system is calculated, and some numerical results are presented with discussion. As a result, we demonstrate how much capacity the SOM-CDMA has and how we can achieve an optimal allocation of frequency and power between (sub)systems. The results can be used as a guideline in designing the radio network of SOM-CDMA systems as well as a means to assess the performance of given systems. The results can be applied to any CDMA system with an SOM-CDMA concept, including CODIT and the proposal of Ohno et. al     

Bayesian Monte Carlo multiuser receiver for space-time codedmulticarrier CDMA systems

We consider the design of optimal multiuser receivers for space-time block coded (STBC) multicarrier code-division multiple-access (MC-CDMA) systems in unknown frequency-selective fading channels. Under a Bayesian framework, the proposed multiuser receiver is based on the Gibbs sampler, a Markov chain Monte Carlo (MCMC) method for numerically computing the marginal a posteriori probabilities of different users' data symbols. By exploiting the orthogonality property of the STBC and the multicarrier modulation, the computational complexity of the receiver is significantly reduced. Furthermore, being a soft-input soft-output algorithm, the Bayesian Monte Carlo multiuser detector is capable of exchanging the so-called extrinsic information with the maximum a posteriori (MAP) outer channel code decoders of all users, and successively improving the overall receiver performance. Several practical issues, such as testing the convergence of the Gibbs sampler in fading channel applications, resolving the phase ambiguity as well as the antenna ambiguity, and adapting the proposed receiver to multirate MC-CDMA systems, are also discussed. Finally, the performance of the Bayesian Monte Carlo multiuser receiver is demonstrated through computer simulations     

Performance analysis of digital lightwave systems using efficientcomputer simulation techniques

Analytically-based methods for evaluating the performance of digital lightwave systems in terms of bit error rates (BERs) are extremely difficult to develop without making restrictive assumptions. A Monte Carlo simulation approach can offer an attractive alternative. However, for typical optical systems, this approach would require an excessive amount of computer time. Importance sampling (IS) is a variance reduction method which can substantially increase the computational efficiency of Monte Carlo simulations. This paper presents an IS method to efficiently evaluate the BERs of direct-detection optical systems employing avalanche photodiode (APD) receivers. Specifically, using a heuristic argument based on large deviations theory, a large class ℱ of exponentially twisted sampling distributions for the APD-based receiver is developed. It is then demonstrated that when used as a sampling distribution, the “optimized” exponentially twisted distribution of large deviations theory is the most efficient distribution among the sampling distributions in ℱ. Further, it is demonstrated that such a distribution would estimate the performance of optical systems with a high degree of accuracy to warrant its possible use as a powerful and flexible tool in computer-aided design, analysis and modeling of fiber-optic transmission systems     

A decorrelating RAKE receiver for CDMA communications overfrequency-selective fading channels

Previous studies showed that multiuser detection in code-division multiple-access (CDMA) communications can be performed without explicit knowledge of users' channel characteristics in a frequency-selective fading environment. However, the computations of these blind approaches are an order of magnitude higher than existing adaptive minimum output energy (MOE) receivers which require at least knowledge of the desired user's channel response. Although the high-complexity problem can be alleviated by constrained adaptive filtering, the tradeoff is a significant drop in receiver performance, especially when the multipath pattern is time varying. In this paper, we present an adaptive receiver for CDMA communications over frequency-selective, and possibly time-varying, wireless channels. A salient feature of the new receiver is that it has complexity and performance comparable to that of the well-known MOE receivers, and yet requires no knowledge of the desired user's channel characteristics     

The study of a BOCC m-ary spread spectrum scheme

M-ary spread spectrum technique has been found wide applications in wireless communications, but it needs too many orthogonal spreading codes, and its despreading/demodulation is quite complex computationally, which limit its wider applications. This paper proposes a novel scheme for Code Division Multiple Access （CDMA） communication systems based on M-ary spread spectrum, where only one prototype spreading code is assigned to each user and the codes for different users are orthogonal or quasi-orthogonal with each other. The M spreading codes of each user to represent K bits data are generated by circularly shifting the assigned code and reversing its polarity. The spreading codes generated like that are called as BiOrthogonal Cyclic Codes （BOCCs）. At the receiver of the system, a transform domain matched-filter implemented by means of Fast Fourier Transform （FFT） is employed to despread and demodulate the received signals, which has very low computational complexity. The results of simulation experiments and bit-error performance analysis show that the proposed scheme is practical and very useful in many cases.