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Reverse link capacity of a direct-sequence code-division multiple-access (DS-CDMA) system in a multicell environment has been studied recently, and significant capacity improvements due to the use of beamforming have been observed. However, system performance with beamforming will be affected by several impairments, such as direction of arrival estimation errors, array perturbations, mutual coupling, and signal spatial spreads. In this paper, reverse link performance of CDMA systems with beamforming under these impairments (imperfect beamforming) is investigated. A simplified beamforming model is developed to evaluate the system performance in terms of user capacity, bit-error rates (BER), and outage probabilities. Both signal-to-interference-ratio-based power control and strength-based power control are considered in this paper. The capacity and BER degradations due to different impairments are shown, and outage probabilities under different power control schemes are examined. [ABSTRACT FROM AUTHOR]

A complete complementary code based direct sequence code division multiple access (CC/DS-CDMA) system has been proposed recently as a potential candidate for beyond third generation (B3G) wireless communications. This paper addresses the issues that design of efficient code assignment schemes should be based on a flexible physical layer support, which is extremely important for emerging cross-layer designs in future wireless applications. The study in this paper considers a CC/DS-CDMA system with multiple time slots, three traffic classes and two dynamic code-flock assignment schemes, namely random assignment (RA) and compact assignment (CA). Simulation results show that the CC/DS-CDMA system has variable capacity property (VCP), which is sensitively affected by different code-flock assignment schemes. In general, CA can offer lower blocking probability, whereas RA can offer a larger mean system capacity and higher throughput when offered traffic is heavy. [ABSTRACT FROM AUTHOR]

This paper proposes an approach to improve turbo code performance on the space time transmit diversity (STTD) code-division multiple-access (CDMA) forward link through better estimation of the channel state information (CSI) required by the mobile turbo decoder. An improved CSI estimation scheme that accounts for the unique nature of CDMA STTD intracell interference is presented. It is compared to a standard CSI estimation scheme that assumes all STTD interference and noise can be represented as additive white Gaussian noise at the mobile receiver input. An STTD CDMA forward link simulation shows that link performance is significantly improved when the mobile turbo decoder uses the more accurate CSI provided by improved estimation. However, the degree of improvement depends on the turbo decoding algorithm, mobile velocity, and whether the base station is using STTD or single-antenna transmission. [ABSTRACT FROM AUTHOR]

This paper presents a coherent demodulation scheme that reduces the multiple-access interference (MAI) considerably in a band-limited direct-sequence code-division multiple-access system. The despreading signal is obtained from a combination of the conventional despreading signal and its weighted early and late versions that are used in the delay-locked loop (DLL). It is shown that with appropriate choice of the weighting parameter of the early-late signals of the DLL, the signal-to-interference-plus-noise ratio of the decision variable is increased, which leads to improvement in the bit-error performance and hence the potential increase in the system capacity by about 25%. The analyses are performed in the frequency domain. The effect of imperfect synchronization on the system performance is also analyzed. [ABSTRACT FROM AUTHOR]

This paper deals with channel acquisition and tracking in the uplink of a multicarrier code division multiple access (MC-CDMA) system. The transmission medium is characterized by multipath propagation, and the goal is to estimate the channel frequency response and the noise-power of each active user. Channel acquisition is performed jointly with the noise-power estimation following two different approaches. The first assumes independently faded subcarriers, while the second exploits the fading correlation across the signal bandwidth to improve the system performance. Both schemes are based on the maximum likelihood (ML) criterion and exploit some training blocks carrying known symbols. Channel tracking is pursued through least mean square (LMS) techniques using data decisions provided by a partial parallel interference cancellation (PPIC) receiver. Numerical results are given to highlight the effectiveness of the proposed methods and to present comparisons with other existing solutions. [ABSTRACT FROM AUTHOR]

An adaptive carrier interferometry (CI) scheme is proposed for multicarrier code-division multiple-access (MC-CDMA) systems where it is assumed that there is a feedback channel between the receiver and the transmitter. By exploiting the additional degree of freedom in selecting the amplitudes of the subcarriers in accordance with the channel condition, the proposed scheme attains a significant performance gain over the conventional CI-MC-CDMA systems in which a constant amplitude is set for all the carriers. Two adaptation strategies, namely 1) local adaptation and 2) global adaptation, are considered for estimating the appropriate subcarrier amplitudes at the receiver in the proposed systems. Both single-user adaptation, where the other users do not adapt, and multiuser adaptation, in which all users adapt, are investigated. A further advantage of the proposed scheme is that it eliminates the peak-to-average power ratio problem present in the conventional CI-MC-CDMA systems. [ABSTRACT FROM AUTHOR]

Compact high-temperature superconducting filters have high selectivity and sensitivity. Combined with a cryocooled low-noise amplifier, they significantly reduce the effective noise factor of a cellular base-station receiver. Their impact on wireless code-division multiple-access networks is examined using an analytical analysis and numerical simulations. Potential benefits of the superconducting technology include increased capacity utilization and coverage efficiency. Numerical simulations also address the question of how the network should be optimized to realize these benefits. [ABSTRACT FROM AUTHOR]

This study deals with the effects of the cross/intermodulation (CM/IM) on the communication performance when both Tx (transmit) leakage and external single tone interference pass through LNA in code division multiple access (CDMA)/frequency division duplex (FDD) transceiver. The CM/IM powers happen because of the nonlinearity of the LNA and these get work as interference to the desired signal. The CM/IM interference powers are analyzed with the frequency change of external single tone interference in Rx (receive) band. The relationship between these interference powers, input third-order intercept point (IIP3), and the dependence of the allowable noise figure (NFallowable) on the IIP3 are presented in this study. If IIP3 is large enough for CM/IM interference power spectrum slightly to overlap with the Rx band, the interference power in Rx band gets smaller but NFallowable increases as IIP3 is increased. Thus, the bit-error-rate (BER) performance is degraded. On the other hand, when the interference power spectrum largely overlaps with the Rx band, it is important to increase the IIP3 to get the better BER performance since the performance improvement by the linearity growth is larger than the performance degradation caused by the increment of NFallowable. [ABSTRACT FROM AUTHOR]