Code-division Multiple-access Cellular System Research Articles

The performance of direct-sequence spread spectrum with complex processing and quaternary data modulation

The receiver processing that is often employed for direct-sequence spread spectrum with quaternary data modulation operates separately on in-phase and quadrature components of the spread-spectrum signal. Previous results on complex quaternary sequences, which are also referred to as four-phase sequences, provide a strong motivation to employ alternative processing methods that permit the receiver to benefit from the correlation properties of these sequences. With this goal in mind we present receiver architectures that use complex processing and are capable of demodulating the signals proposed for the third-generation mobile cellular code-division multiple-access systems. The performance of the proposed receiver architecture with complex processing is shown to be determined by the complex aperiodic correlation functions for the spreading sequences. Comparisons are made with systems in which the quaternary sequences are derived from pairs of common binary spreading sequences and the receiver employs separate in-phase and quadrature processing. We find that in spite of their superior periodic crosscorrelation properties, the previously developed quaternary sequences do not provide significantly smaller error probabilities in multiple-access systems.

Performance analysis of channel-borrowing handoff scheme based on user mobility in CDMA cellular systems

Code-division multiple-access (CDMA) cellular systems use soft handoff. Although the capacity of CDMA systems is interference-limited in nature, channel shortages may occur because soft handoff uses several channels simultaneously. To cope with this problem, we propose an improved handoff method that borrows channels from stationary calls participating in soft handoff and allocates the borrowed channels to handoff requests by moving calls when a channel shortage occurs. Borrowing from stationary calls is possible because these calls do not undergo fast-fading and do not require receiver diversity. The proposed method is designed to avoid increased interference resulting from channel borrowing. The proposed channel-burrowing handoff scheme is analyzed in a situation involving both moving and stationary calls. A comparison is made between the performances of the typical IS-95-based handoff scheme and the proposed scheme. Numerical results show that the proposed scheme is better than the IS-95 scheme in view of the handoff refused probability, the handoff queuing delay, and total interference.

Reverse link capacity analysis of a CDMA cellular system with mixed cell sizes

The demands for mobile communication services are growing rapidly. In heavily populated areas, cell splits are unavoidable to increase the capacity of the cellular system. Cell splitting makes a cellular system have mixed cell sizes. For cell planning, it is necessary to analyze the reverse link capacity of a code-division multiple-access (CDMA) cellular system with mixed cell sizes. In this paper, we propose a method to calculate the reverse link capacity of a CDMA cellular system with mixed cell sizes. When a macro cell is split into three micro cells, as an example, we calculate the reverse link capacities for the three micro cells and the neighboring macro cells. The results show that as the radius of a micro cell decreases, the reverse link capacity of the micro cell increases, while those of the neighboring macro cells decrease.

Subspace-based SIR estimation for CDMA cellular systems

The signal-to-interference ratio (SIR) has been highlighted in the literature to be an efficient criterion for several radio resource management algorithms such as power control and handoff. We address the problem on how to obtain fast and accurate measurements of this quantity in a practical context. We develop a general SIR estimation technique for code division multiple access (CDMA) cellular systems, that is based on a signal subspace approach using the sample covariance matrix of the received signal. Analysis and simulation results for an IS-95 like system show that the SIR can be estimated to within 80% of the actual SIR after less than seven frames, or within 90% of the actual SIR after less than 15 frames. We also study a computationally less expensive SIR tracking algorithm based on updating the signal subspace. We show that the algorithm works well in the context of a rapidly time varying channel.

A parallel code acquisition technique for M-ary orthogonal signals in a cellular CDMA system

Addressed in this paper is a parallel code acquisition technique for M-ary orthogonal signals in a cellular code division multiple access (CDMA) system where pilot signal or preamble is not available for code acquisition. The proposed code acquisition system consists of parallel M-ary orthogonal demodulators each having M correlators. The system also employs two operating modes, i.e., search and verification modes with the decision variables formulated with successive maximums of M correlator outputs. The mean acquisition time is first derived for a multiple H/sub 1/ cell case and then the probabilities of detection, miss, and false alarm are obtained for frequency-selective Rayleigh fading environments. Also provided is a computationally efficient solution for detection performance analysis by using quantized decision variables. The performance of the proposed code acquisition technique is numerically evaluated when it is applied to the initial acquisition of the mobile station requesting hand over.

A simple algorithm for adjusting cell-site transmitter power in CDMA cellular systems

Traditional downlink power control methods for code-division multiple-access (CDMA) cellular systems allocate individual power levels to the different users of a given cell according to their relative needs, while assuming the amount of total power transmitted by each base to be constant. If this amount is not controlled, it is impossible to reduce unnecessary interference and, as a result, to achieve an optimal power control performance. In this paper, we address rather a simple convergent algorithm for adjusting the total power as well as allocating it for each user. The objective of this algorithm is to allocate the available power of each base to as many mobiles as possible that can enjoy sufficient transmission quality. In the appendix, we also provide an extended version of the algorithm, which relaxes an ideal assumption on soft handoff pertaining to existing works. Numerical results show that the proposed algorithm can be an alternative for CDMA downlink power control.

A combined OFDM-CsDMA approach to cellular mobile communications

We present a combined orthogonal frequency-division multiplexing (OFDM) and code-division multiple-access cellular system. It is derived from the OFDM model by introducing an extra code-shift division multiple access layer. This mitigates intercell interference without affecting intracell orthogonality and such property can be maintained in multipath environments. The new scheme is suitable for both up and down links. It retains the low-receiver complexity property of the OFDM system.

A spatial interpolation algorithm for the upsampling of uniform linear arrays

A method to spatially interpolate (upsample) a uniform linear array (ULA) originally sampled at less than the spatial Nyquist rate in the presence of noise is considered. The multichannel narrowband interpolation algorithm is based on a forward-backward linear prediction approach. The method interpolates N-1 virtual sensor outputs based on the outputs of N real sensors. This is done by solving a set of forward-backward linear prediction equations that relate the known (real) sensor outputs to the unknown (virtual) sensor outputs. Simulation results that validate the performance and usefulness of this method are presented in the context of code division multiple access (CDMA) mobile communications. The algorithm is applied to increase the user capacity and resolution and decrease the mutual coupling of ULA's used in conjunction with cellular CDMA systems.

The feasibility study of transporting IS-95 CDMA signals over HFC networks

This letter studies the feasibility of transmitting IS-95 code-division multiple-access (CDMA) wireless signals in hybrid-fiber-coax (HFC) networks. Capacity estimation of an HFC optical node-based CDMA cellular systems using a return-path channel model is provided. In addition to multiple-access interference in IS-95 CDMA networks, the return-path channel model takes into account ingress noise, impulsive noise, and microreflections in a coaxial cable transmission system; and laser diode clipping-induced intermodulation noise in an optical fiber link.

Performance study of close-loop power control algorithms for a cellular CDMA system

Performance of a reverse link code-division multiple-access (CDMA) system with fast close-loop power control algorithms is studied. It is found that if the fast close-loop power control algorithm functions effectively, the speed of the mobile unit is in the range such that its Doppler frequency is less than one tenth of the power control updating rate. This paper also proposes a new predictive power control algorithm with better performance in terms of system capacity than the conventional and adaptive step size algorithms. An increase in system capacity as high as 22% compared with the conventional algorithm can be achieved depending on the mobile velocity.

Performance of optimum transmitter power control in CDMA cellular mobile systems

Efficient power control is of great importance in the design of high-capacity cellular radio systems. Optimum power control scheme (OPCS), in the sense that it minimizes the outage probability, has been fully investigated for frequency-division multiple-access/time-division multiple-access (FDMA/TDMA) cellular systems. In this paper, optimum power control theory is introduced to code-division multiple-access (CDMA) cellular systems to derive upper performance bounds for all types of transmitter power control schemes. Simulation results indicate that the OPCS will increase the CDMA system capacity by more than 1.9 dB over the perfect average power control scheme under IS-95-defined radio condition.

Analysis of power control and its imperfections in CDMA cellular systems

We develop a general model to study the forward-link capacity of a code-division multiple access (CDMA) cellular system with power control. Using this model, two forward-link power control schemes (nth-power-of-distance power control scheme and optimum power control scheme) are examined. The increase in capacity by using power control has been studied. The capacities of the forward link are also compared with those of the reverse link with perfect power control. In the case of imperfect power control, an analytical model is presented to study the effect of power control error in a CDMA cellular system. The effect of the dynamic range of transmission, moreover, has also been analyzed.

A quality-based birth-and-death queueing model for evaluating the performance of an integrated voice/data CDMA cellular system

Code-division multiple access (CDMA) has been proven to be one of the promising mainstream multiaccessing techniques in future cellular mobile communication systems. A quality-based birth-and-death queueing model is developed for the purpose of evaluating the performance of a CDMA cellular that supports two-rate transmissions. Performance measures obtained include channel utilization and call blocking probability. Based on these results, three call admission schemes are compared.

An uplink CDMA system architecture with diverse QoS guarantees for heterogeneous traffic

The uplink access control problems for cellular code-division multiple-access (CDMA) systems that service heterogeneous traffic with various types of quality-of-service (QoS) and use multicode CDMA to support variable bit rates are addressed. Considering its distinct QoS requirements, class-I real-time traffic (e.g., voice and video) is differentiated from class-II non-real-time traffic (e.g., data). Connection-oriented transmission is achieved by assigning mobile-oriented code channels for class-I traffic, where each corresponding mobile needs to pass an admission test. Class-II traffic is transmitted in a best-effort manner through a transmission-rate request access scheme which utilizes the bandwidth left unused by class-I traffic. Whenever a mobile has class-II messages to transmit, the mobile requests code channels via a base station-oriented transmission-request code channel, then, according to the base station scheduling, the transmission is scheduled and permitted. Addressed are the admission test for class-I connections, transmission power allocation, and how to maximize the aggregate throughput for class-II traffic. The admission region of voice and video connections and the optimum target signal-to-interference ratio of class-II traffic are derived numerically. The performance of class-II traffic transmissions in terms of average delay is also evaluated and discussed.

Design of Frequency Hopping Cellular CDMA System

The demand for wireless communication has forced researchers to exploit digital technology to satisfy the increasing demand for service. This demand has led to severe capacity limitations in many areas and spread spectrum techniques are being considered as one means to support these services. Characteristics of spread spectrum Code Division Multiple Access (CDMA) make it ideal for application in cellular mobile radio network. In this paper the design methodology of a cellular CDMA system using non-coherent FH/FSK is presented. The CDMA capacity equation and the interference model for cellular environment are also presented. The FH-CDMA system is simulated and its performance is compared with the theoretical results.

Adaptive arrays for narrowband CDMA base stations

Base station antenna arrays are a promising method for providing significant capacity increases in cellular mobile radio systems. This paper examines receiver structures and algorithms to assess the potential capacity gains from the employment of multiple receiver antenna elements, of different sizes, for code division multiple access (CDMA) systems. It considers antenna arrays for the mobile to-base station or reverse link of a CDMA cellular system such as the IS-95 standard. It begins with an introduction to CDMA communication systems and also addresses the general topic of antenna array receivers. Channel modelling is then discussed, as this will influence the design of CDMA receivers. The specific form of receiver array processing algorithms is then discussed and some performance comparisons provided. Finally, the most important reason for implementing antenna array systems, the capacity gains which are achievable, is indicated.

Outage probability of cellular CDMA systems with space diversity, Rayleigh fading, and power control error

It is well known that the capacity of code-division multiple-access (CDMA) systems degrades rapidly with the increase in power control error. The capacity is also affected by small-scale fading such as Rayleigh fading and by the voice activity. The main contribution of this paper is to provide a new simple approximation to the outage probability of the uplink of a CDMA system utilizing an antenna array for space diversity, taking into account stochastic models of the power control error, small-scale fading, and voice activity.

Performance of antenna array receiver algorithms for CDMA

This paper considers a number of techniques that have been proposed for spatial filtering purposes in code division multiple access (CDMA) antenna array receivers. There are two main types of algorithms: (i) matched filter methods that maximise the signal power at the filter output and (ii) interference suppression algorithms that maximise the signal-to-interference plus noise ratio (SINR) of the filter output. Theoretical and simulation results are presented to compare the convergence performance, in terms of the attained SINR, for the algorithms in a CDMA cellular system. A method of trading convergence performance with complexity for algorithms using the maximum SINR approach is also discussed.

CDMA cellular systems performance with fading, shadowing, and imperfect power control

This paper addresses capacity estimation for cellular code-division multiple-access (CDMA) systems, assuming the IS-95 standard as a reference. Extending a previous analytical method (Viterbi et al., 1994), we obtain a sequence of bounds on capacity, and then we introduce an accurate approximation to reduce computation complexity. The analysis accounts for interference internal and external to the reference cell, fading, shadowing, and imperfect power control. Outage probability is expressed in terms of the characteristic functions (cf's) of the interference and imperfect power control random variables (RV's). The interference contributions are computed on the basis of a Poisson distribution for the number of users in a lognormally shadowed channel. Results are provided for different channel conditions and are validated against Monte Carlo simulations. A comparison against previously published CDMA capacity estimates is carried out, aimed at clarifying some controversial issues. It is also confirmed that large system capacity is achievable under tight power control.

Overview of radiolocation in CDMA cellular systems

Applications for the location of subscribers of wireless services continue to expand. Consequently, location techniques for wireless technologies are being investigated. With code-division multiple access (CDMA) being deployed by a variety of cellular and PCS providers, developing an approach for location in CDMA networks is imperative. This article discusses the applications of location technology, the methods available for its implementation in CDMA networks, and the problems that are encountered when using CDMA networks for positioning.