Direct-sequence Code-division Multiple-access Network Research Articles

Exclusion and Guard Zones in DS-CDMA Ad Hoc Networks

The central issue in direct-sequence code-division multiple-access (DS-CDMA) ad hoc networks is the prevention of a near-far problem. This paper considers two types of guard zones that may be used to control the near-far problem: a fundamental exclusion zone and an additional CSMA guard zone that may be established by the carrier-sense multiple-access (CSMA) protocol. In the exclusion zone, no mobiles are physically present, modeling the minimum physical separation among mobiles that is always present in actual networks. Potentially interfering mobiles beyond a transmitting mobile's exclusion zone, but within its CSMA guard zone, are deactivated by the protocol. This paper provides an analysis of DS-CSMA networks with either or both types of guard zones. A network of finite extent with a finite number of mobiles and uniform clustering as the spatial distribution is modeled. The analysis applies a closed-form expression for the outage probability in the presence of Nakagami fading, conditioned on the network geometry. The tradeoffs between exclusion zones and CSMA guard zones are explored for DS-CDMA and unspread networks. The spreading factor and the guard-zone radius provide design flexibility in achieving specified levels of average outage probability and transmission capacity. The advantage of an exclusion zone over a CSMA guard zone is that since the network is not thinned, the number of active mobiles remains constant, and higher transmission capacities can be achieved.

Distributed power control with multiuser detection for asynchronous DS-CDMA networks subject to time-delays

If transmission power is increased in a direct-sequence code-division multiple-access network, interference increases, thus reducing network capacity. In this paper, we address these opposite goals of reducing transmission power and increased network capacity by jointly considering power control and multiuser detection. Since propagation delay occurs inevitably during data communication, we present an analysis by considering also that the interference measurement is not available at the transmitter instantaneously but with some time-delay, something that is generally neglected in the literature. This framework is used to investigate the design of a distributed power control strategy enhanced with linear multiuser receivers in an asynchronous uplink channel, subject to multipath and quality of service constraints. Thus, a cross-layer solution that combines a Linear-Quadratic-Gaussian power control scheme with a linear multiuser receiver is proposed to compensate for the round-trip delay and the time-varying channel characteristics in the communication link. Simulation results are used to show the advantages of such scheme in terms of saving energy, increasing network capacity and robustness against propagation delays.

On the Outage Probability of SIR-Based Power-Controlled DS-CDMA Networks With Spatial Poisson Traffic

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this paper, we present a novel and rather simple method for calculating reverse-link outage probability in signal-to-interference ratio (SIR)-based power-controlled direct-sequence code-division multiple access (DS-CDMA) systems. We obtain outage probability for a single-service system using the <emphasis emphasistype="boldital"> interference-to-received-signal-power ratio</emphasis>. We extend our method to the multiservice case using the <emphasis emphasistype="boldital">uplink load factor</emphasis>, which is the ratio of the interference to the interference plus noise. Unlike most of the previous works, we employ the 2-D Poisson point process to model the spatial distribution of mobile users in the network coverage area. We approximate the intercell-interference-to-received-signal-power ratio and uplink load factor using their cumulants and obtain the outage probabilities for single- and multiservice cases. Our approximations are verified using simulations. </para>

Adaptive Closed-Loop Power Control Algorithms for CDMA Cellular Communication Systems

Power control has been widely studied and shown to be crucial for the capacity and performance of direct-sequence code-division multiple-access (DS-CDMA) systems. Practical implementations typically employ fast closed-loop power control, where transmitters adjust their transmit powers according to commands received in a feedback channel. The loop delay resulting from the measurements, processing, and transmission of the power control commands can result in oscillations of the transmission powers and lead to degradation in the system performance. In this paper we present new adaptive closed-loop power control algorithms that are able to alleviate the effect of the loop delay. The algorithms are based on self-tuning controllers designed for a log-linear model of the power control process. We carried out computational experiments on a DS-CDMA network using the distributed constrained power control (DCPC) as a reference algorithm. Practical versions of the algorithms are also provided and they were compared with the fixed-step power control (FSPC) algorithm employed in the IS-95 and WCDMA systems. The numerical results indicate that our algorithms can significantly improve the radio network capacity without any increase in power control signaling.

An Efficient Algorithm for Determination of the Optimum Base-Station Assignment in Cellular DS-CDMA Systems

An algorithm is proposed that finds the optimum assignment of mobile users to base stations, and its associated transmission powers, in a cellular direct-sequence code-division multiple-access network, with a computational complexity that grows polynomially with the number of users and base stations. The algorithm detects infeasible situations and allows the inclusion of power constraints. Its performance is analyzed in terms of complexity and system capacity.

Optimal resource allocation in multiservice CDMA networks

This paper addresses the problem of dynamic resource allocation in a multiservice direct-sequence code-division multiple-access (DS-CDMA) wireless network supporting real-time (RT) and nonreal-time (NRT) communication services. For RT users, a simple transmission power allocation strategy is assumed that maximizes the amount of capacity available to NRT users without violating quality of service requirements of RT users. For NRT users, a joint transmission power and spreading gain (transmission rate) allocation strategy, obtained via the solution of a constrained optimization problem, is provided. The solution maximizes the aggregate NRT throughput, subject to peak transmission power constraints and the capacity constraint imposed by RT users. The optimization problem is solved in a closed form, and the resulting resource allocation strategy is simple to implement as a hybrid CDMA/time-division multiple-access strategy. Numerical results are presented showing that the optimal resource allocation strategy can offer substantial performance gains over other conventional resource allocation strategies for DS-CDMA networks.

An equicorrelation-based multiuser communication scheme for ds-cdma systems

An equicorrelation-based multiuser communication (ECBMC) scheme for direct-sequence code-division multiple-access (DS-CDMA) systems is presented. The ECBMC receiver has low computational complexity that is comparable to that of the conventional detector. By using the equality of cross correlations, the ECBMC scheme can completely eliminate multiple-access interference (MAI) in a synchronous single-path DS-CDMA network. The system performance is independent of the number of active users. The scheme is extended to include the effects of multipath fading. It is able to suppress a major portion of the MAI. This proposed ECBMC scheme is quite attractive for an MAI-dominant environment.

Power control and capacity analysis for a packetized indoor multimedia DS-CDMA network

This paper proposes a packetized indoor wireless system using direct-sequence code-division multiple-access (DS-CDMA) protocol. The indoor radio environment is characterized by slow Rayleigh fading with or without lognormal shadowing. The system supports multimedia services with various transmission rates and quality of service (QoS) requirements and allows for seamless interfacing to asynchronous transfer mode (ATM) broadband networks. All packets are transmitted with forward error correction (FEC) using convolutional code for voice packets and Bose-Chaudhuri-Hocquenghem (BCH) code for data packets with an automatic retransmission request (ARQ) protocol and for video packets without ARQ. A queueing model is used for servicing data transmission requests. A power control algorithm is proposed for the system, which combines closed-loop power control with channel estimation to give the best performance. The cell capacity of each traffic type and various multimedia traffic configurations in both single-cell and multiple-cell networks are evaluated theoretically under the assumption of perfect power control. The effect of power control imperfection on the capacity using the proposed power control algorithm is investigated by computer simulation.

On the analytical evaluation of closed-loop power-control error statistics in DS-CDMA cellular systems

This paper proposes an analytical study that aims at evaluating the power-control error statistics in wireless direct-sequence code-division multiple-access (DS-CDMA) cellular systems based on an ideal variable step closed-loop power-control scheme. In particular, the cumulative distribution function and the correlation coefficient of the power-control error are derived through a first-order Taylor expansion of the received signal envelope. A novel power-control scheme that exploits the autocorrelation properties of the fading is also proposed, and its performance is analyzed in terms of power-control error statistics. Rayleigh and Rice frequency-selective channel models, which involve the use of a diversity RAKE receiver at the base station, have been taken into account. The proposed analytical approach specifically applies to CDMA systems. A method that aims at estimating the capacity of a DS-CDMA cellular network is also given.

Dynamic spreading gain control in multiservice CDMA networks

In this paper, we consider a direct-sequence code division multiple access (DS-CDMA) network consisting of a single radio access point and a collection of wireless terminals. The network offers two classes of service: class-1 (real-time) and class-2 (reliable). We are interested in studying the effect of dynamic spreading gain control (SGC) on the dynamics of multiple access interference (MAI), spectral efficiency, and the quality of service (QoS) experienced by each service class. We first consider a time-slotted system in which class-2 terminals operate in a random access fashion. We show that under optimal (throughput maximizing) dynamic SGC: (1) the optimal retransmission probability is equal to one, and (2) the optimal spreading gain increases linearly, or equivalently, the transmission rate decreases inverse linearly, as the MAI level increases. We then model the system as a continuous-time finite-source queueing system with processor sharing, and obtain an explicit (closed-form) expression for the stationary distribution of the number of active class-1 and class-2 terminals, that is, the MAI level. This distribution is used to derive expressions for various QoS measures and define a capacity or admissible region. The results obtained by simulation and analysis are in extremely close agreement. This work contributes to a better understanding of the relationships between QoS, multiple access interference, and allocation of radio resources in DS-CDMA networks.

Open-loop power control performance in DS-CDMA networks with frequency selective fading and non-stationary base stations

In this paper, we study the performance of a simple and easy-to-implement distributed power control strategy applicable to direct sequence code division multiple access (DS-CDMA) networks. The scheme makes use of the received power measurements made on the forward link at individual mobile units to control the transmit powers on their reverse links. The algorithm, which effectively compensates for the slowly varying distance and shadow losses (due to their high correlation on both forward and reverse links), attempts to minimize the effect of fast multipath fading by averaging it out. We adopt a quasi-analytic approach to estimate the reverse link capacity performance of an open-loop power control scheme in both a single cell and a multi-cell environment, and we do this for both a fixed base station and a moving base station scenario. Non-stationary base stations are typical in tactical and emergency communications scenarios where the base stations could be mounted on moving platforms (e.g., tanks, jeeps, unmanned airborne vehicles). We estimate the capacity degradation, when base stations move relative to other cells, as a function of the amount of cell overlap and the standard deviation of the power control error. We also provide a comparison of the performance of the open-loop power control strategy with that of a closed-loop power control strategy.

Adaptive interference cancellation for DS-CDMA systems using neural network techniques

The objective of this study is to apply and investigate a neural network-based decision feedback scheme for interference suppression in direct sequence code division multiple access (DS-CDMA) wireless networks. It is demonstrated that a decision feedback functional link equalizer (DFFLE) in combination with an eigenvector network can closely approximate a Bayesian receiver with significant advantages, such as improved bit-error ratio (BER) performance, adaptive operation, and single-user detection in a multiuser environment. It is assumed that the spreading codes of the interfering users will be unknown to the receiver. This detector configuration is appropriate for downlink communication between a base station and a mobile user in a digital wireless network. The BER performance in the presence of interfering users is evaluated. The improved performance of such a DFFLE receiver for CDMA is attributed to the nonlinear decision boundary it evaluates for the desired user. The receiver structure is also capable of rapid adaptation in a dynamic communications scenario for which there is entry/exit of users and imperfect power control. The convergence performance and error propagation of the DFFLE receiver are also considered and exhibit reasonable promise for third generation wireless DS-CDMA networks.

Common packet data channel (CPDC) for integrated wireless DS-CDMA networks

A common packet data channel (CPDC) architecture is proposed to support bursty, packet-based services in direct sequence code-division multiple-access (DS-CDMA) integrated wireless access networks. The architecture employs an asynchronous transfer mode (ATM) strategy in the forward CPDC link and a spread ALOHA-type random access strategy in the reverse CPDC link. A congestion control algorithm using base station broadcast and portable terminal random delay call reattempt is described. A performance analysis of the CPDC architecture and algorithms is carried out, and formulas for the bit error rate, blocking probability, system delay time, transmission time, and waiting time for packet data calls are derived. The interference caused by a CPDC to stream services in the network is determined, and the capacity of a CPDC is evaluated in terms of the number of packet data subscribers that can be served with a specified grade of service (GOS).

Performance analysis of hybrid ARQ protocols in a slotted direct-sequence code division multiple-access network: jamming analysis

An examination is made of the performance of type-I hybrid ARQ (automatic repeat request) protocols in a slotted direct-sequence CDMA (code-division multiple access) network operating in a hostile jamming environment. The network consists of an arbitrary number of transceivers arranged in a paired-off topology. The traffic arrival process is derived by means of a Markov model. Throughput-delay expressions are derived in terms of the channel cutoff rate and capacity. The effects of jammer state information are discussed. Network design parameters are identified and their dependency on system parameters is examined in detail. It is shown that, for a given population size, traffic intensity, and bit energy/jammer noise ratio, there is an optimal probability of retransmission, code rate, and processing gain that maximizes network performance in the presence of worst-case pulse jamming. >

A performance analysis of DS-CDMA and SCPC VSAT networks

Spread-spectrum and single-channel-per-carrier (SCPC) transmission techniques work well in very small aperture terminal (VSAT) networks for multiple-access purposes while allowing the Earth station antennas to remain small. Direct-sequence code-division multiple-access (DS-CDMA) is the simplest spread-spectrum technique to use in a VSAT network since a frequency synthesizer is not required for each terminal. An examination is made of the DS-CDMA and SCPC Ku-band VSAT satellite systems for low-density (64-kb/s or less) communications. A method for improving the standard link analysis of DS-CDMA satellite-switched networks by including certain losses is developed. The performance of 50-channel full mesh and star network architectures is analyzed. The selection of operating conditions producing optimum performance is demonstrated. >