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Due to congestion in wireless frequency use, it is difficult to assign an independent frequency bandwidth to broadband wireless communication systems based on the DS-CDMA (Direct Sequence Code Division Multiple Access) system. Therefore, operation of the DS-CDMA system on the existing narrowband wireless system is contemplated. However, the interference of the transmitted power of the existing narrowband wireless system on the DS-CDMA system has not been studied. There is a possibility that significant narrowband interference on the DS-CDMA system may occur. In this paper, a narrowband interference rejection system in DS-CDMA is proposed. By making use of prior information on the carrier frequency of the narrowband interference, a narrowband interference rejection process with little computational overhead is realized. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 87(3): 74–86, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10076 [ABSTRACT FROM AUTHOR]

Most blind source separation algorithms assume the channel noise to be Gaussian. This paper considers the problem of noncooperative, blind detection of synchronous direct-sequence code-division multiple-access communications (no knowledge of the spreading sequences or training data) in non-Gaussian channels. Three iterative algorithms with different performance and complexity tradeoffs are proposed. Simulation results show that they significantly outperform Gaussian-optimal blind source separation algorithms in non-Gaussian channels. The Cramér-Rao lower bound for this problem is computed, and the performance of the proposed algorithms is shown to approach this bound for moderate signal-to-noise ratios. [ABSTRACT FROM AUTHOR]

The transmission quality of mobile radio communications is strongly affected by fading with rapid fluctuations in the envelope and phase of the received signal, in addition to the effect of thermal noise. This paper considers a multiple access system (DS-CDMA system) using the spread spectrum technique with direct spread, and presents a method of deriving the approximate average bit error rate in a frequency-selective fading environment (the n-ray Rayleigh fading model), with the maximum Doppler frequency, the delay time of the delayed wave, and the number of stations simultaneously utilizing the channel as parameters. The two-wave model and the IMT2000 model are considered as examples, and the validity of the proposed method is demonstrated by the fact that the results obtained by the approximate formula agree well with the simulation results even if the parameters are varied. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 87(1): 90–100, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.1185 [ABSTRACT FROM AUTHOR]

A so-termed chip-interleaved block-spread (CIBS) code division multiple access (CDMA) system has been introduced for cellular applications in the presence of frequency selective multipath channels. In both uplink and downlink operation, CIBS-CDMA achieves multiuser-interference (MUI) free reception within each cell. This paper focuses on the cellular downlink configuration and compares CIBS-CDMA against the conventional direct-sequence (DS) CDMA system, which relies on a chip equalizer to restore code orthogonality and, subsequently, suppresses MUI by despreading. We provide a unifying framework for both systems and investigate their performance in the presence of intercell interference and soft-handoff operation. Extensive comparisons from load, performance, complexity, and flexibility perspectives illustrate the merits, along with the disadvantages, of CIBS-CDMA over DS-CDMA, and reveal its potential for future wireless systems. [ABSTRACT FROM PUBLISHER]

This paper deals with channel estimation in the uplink of a direct-sequence code-division multiple-access system operating in a multipath environment. The parameters of interest are the delays and the complex attenuations incurred by the signal echoes along the propagation paths. We propose an iterative approach for estimating the channel parameters of a new user entering the system. The method is based on the space-alternating generalized expectation-maximization algorithm and exploits a training sequence. In comparison to other estimation algorithms, it reduces a complicated multidimensional optimization problem to a sequence of one-dimensional problems. In addition, it can be effectively used in applications over fast-fading channels. Computer simulations are employed to assess the performance of the proposed scheme. It is found that it is resistant to multiuser interference and has accuracy close to the Cramer-Rao lower bound even with very short training sequences. [ABSTRACT FROM PUBLISHER]

Interference suppression at the receiver can be used to improve performance and capacity in the downlink of direct-sequence code-division multiple-access systems. In this paper, an interference suppression technique is developed which uses partial knowledge of spreading sequences to cancel a group of interfering signals. Specifically, knowledge of a complex scrambling sequence is used to project the desired signal away from the interference in the in-phase/quadrature (I/Q) complex plane. A maximum likelihood receiver formulation is used, treating both the desired signal and interference as being conditionally noncircular. A zero-forcing equalization approach is used to preserve orthogonality between signals from the same base station. The development of the technique is based on the assumption that the group of signals corresponds to another base station that does not transmit the signal of interest. However, the technique is also applied to the path diversity scenario (dispersion, soft handoff, transmit delay diversity), in which the group of interfering signals includes a desired signal component. The approach, referred to as I/Q projection, provides significant gains when performance is interference limited. [ABSTRACT FROM PUBLISHER]

The sum capacity of optimally allocated spreading sequences with and without power control is studied for synchronous code-division multiple-access channels subject to frequency flat fading. Continuing upon the recently derived characterization of sum capacity for asymmetric received powers with no power control at the transmitter, we present an asymptotic analysis of the sum capacity in the wideband limit of a large number of users and evaluate possible gains compared with the random spread sequences. Next, we discuss group orthogonal optimal sequence allocation and analyze its potential to decrease the complexity of the receiver while achieving the sum capacity of optimally allocated equal-energy sequences. For the power controlled case, we give a general characterization of sum capacity and identify the optimal power-control law and sequence allocation that achieves this capacity. These results are also analyzed in the asymptotic limit of a large number of users. [ABSTRACT FROM PUBLISHER]

We analyze the performance of a second-order linear multistage multiuser code-division multiple-access receiver. The receiver's filtered output is designed to converge to that of the linear minimum mean-squared error solution as the number of stages increase. Our analysis is based on a related second-order stationary iterative solution method. We derive the large system output signal to interference-plus-noise ratio for each stage. We use this result to perform a numerical optimization with respect to the two second-order parameters of our receiver. Within this iterative framework, we can achieve performance extremely close to the optimal linear multistage multiuser receiver. [ABSTRACT FROM PUBLISHER]

A robust iterative multicarrier code-division multiple-access (MC-CDMA) receiver with adaptive multiple-access interference (MAI) suppression is proposed for a pilot symbols assisted system over a multipath fading channel with frequency offset. The design of the receiver involves a two-stage procedure. First, an adaptive filter based on the generalized sidelobe canceller (GSC) technique is constructed at each finger to perform despreading and suppression of MAI. Second, pilot symbols assisted frequency offset estimation, channel estimation and a RAKE combining give the estimate of signal symbols. In order to enhance the convergence behavior of the GSC adaptive filters, a decisions-aided scheme is proposed, in which the signal waveform is first reconstructed and then subtracted from the input data of the adaptive filters. With signal subtraction, the proposed MC-CDMA receiver can achieve nearly the performance of the ideal maximum signal-to-interference-plus noise ratio receiver assuming perfect channel and frequency offset information. Finally, a low-complexity partially adaptive (PA) realization of the GSC adaptive filters is presented as an alternative to the conventional multiuser detectors. The new PA receiver is shown to be robust to multiuser channel estimation errors and offer nearly the same performance of the fully adaptive receiver. [ABSTRACT FROM PUBLISHER]

We determine the optimal adaptive rate and power control strategies to maximize the total throughput in a multirate code-division multiple-access system. The total throughput of the system provides a meaningful baseline in the form of an upper bound to the throughput achievable with additional restrictions imposed on the system to guarantee fairness. Peak power and instantaneous bit energy-to-noise spectral density constraints are assumed at the transmitter with matched filter detection at the receiver. Our results apply to frequency selective fading in so far as the bit energy-to-equivalent noise power spectral density ratio definition can be used as the quality-of-service metric. The bit energy-to-equivalent noise power spectral density ratio metric coincides with the bit-error rate metric under the assumption that the processing gains and the number of users are high enough so that self-interference can be neglected. We first obtain results for the case where the rates available to each user are unrestricted, and we then consider the more practical scenario where each user has a finite discrete set of rates. An upper bound to the maximum average throughput is obtained and evaluated for Rayleigh fading. Suboptimal low-complexity schemes are considered to illustrate the performance tradeoffs between optimality and complexity. We also show that the optimum rate and power adaptation scheme with unconstrained rates is in fact just a rate adaptation scheme with fixed transmit powers, and it performs significantly better than a scheme that uses power adaptation alone. [ABSTRACT FROM PUBLISHER]

In cellular mobile communications, the mobile station must detect the received level so as to determine the position of the mobile station. In a broadband DS-CDMA system, RAKE reception is usual since the paths can be separated. Thus, the detection accuracy should be improvable by the path diversity effect. This paper considers a DS-CDMA system, and the accuracy of the received level detection is analyzed theoretically and by computer simulation for the case in which both path diversity and antenna space diversity are applied. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 86(2): 51–60, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.1153 [ABSTRACT FROM AUTHOR]

In a broadband DS-CDMA system, control of transmitter power is indispensable in order to solve the distance problem of mobile stations, in which the transmitter power of mobile stations is adaptively controlled so that the received power at the base station remains constant. In a DS-CDMA system, the paths can be separated and RAKE reception can be applied, which helps to reduce the transmitted power as well as the error in transmitter power control. This paper considers the case in which transmitter power control by feedback is applied to a DS-CDMA system, and analyzes the effects of control parameters such as the control time interval, the maximum amount of control, and the control step width, on the error of the transmitter power control, considering path and antenna diversity. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 85(12): 100–111, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.1145 [ABSTRACT FROM AUTHOR]

The call admission control (CAC) belongs to the category of resource management. Since the radio spectrum is very scarce resource, CAC is one of the most important engineering issues for mobile communications. In this paper, we propose a CAC scheme for direct sequence code-division multiple-access cellular systems supporting mobile multimedia communications services. There are multiple call classes in multimedia services and the required signal-to-interference ratio (SIR) varies with call classes. Call admission decision in the proposed scheme is based on SIR measurement. We take account of the traffic asymmetry between uplink and downlink, which is the most important characteristic of multimedia traffic. In addition, the proposed scheme guarantees the priority of handoff call requests over new call requests. We evaluate the performance of the proposed scheme using Markov analysis. The performance measures which we focus on are the system throughput and the blocking probabilities of handoff calls and new calls. The outage probability of a call in progress is also calculated, which is the probability that the measured bit energy-to-noise density ratio of the call is smaller than the required value for maintaining adequate transmission quality. We present some numerical examples with practically meaningful parameter values and, as a result, show that the proposed CAC scheme can operate well in the mobile multimedia systems such as the International Mobile Telecommunications-2000 (IMT-2000) systems. [ABSTRACT FROM PUBLISHER]

A novel one-shot decorrelator for asynchronous CDMA systems is developed. Compared with existing one-shot decorrelator, it can reduce complexity and has better performance while eliminating all MAI. This decorrelator is shown to be near-far resistant in both AWGN and fading channel. [ABSTRACT FROM AUTHOR]

The downlink capacity of frequency division duplex (FDD) based DS-CDMA system can be improved if multielement antenna array is equipped at the base station. In this paper, a novel two-path downlink beamforming (TPDB) scheme is proposed to reduce multiple access interference as well as providing two-order path diversity in downlink. An analysis model is presented for the capacity evaluation of multirate DS-CDMA system with base station antenna array, and the capacity performance of the TPDB is compared with other schemes based on this model. The comparison results show that the TPDB would be a promising candidate for the downlink transmission if both performance and implementation complexity are considered. Moreover, a simple algorithm is proposed for the steering vector estimation in the TPDB, and the robustness of this estimation algorithm in the presence of fading and interference is also confirmed by computer simulations. [ABSTRACT FROM AUTHOR]

A DS-CDMA system with both a circuit-switching scheme for voice transmission and a packet-switching scheme for data transmission is effective in multimedia communication required to transmit a variety of media. Although the Slotted-ALOHA system is believed to be an effective packet-switched system from the perspectives of transmission efficiency and interference power, only open loop transmit power control is used because of the difficulty in using closed loop transmit power control in the ALOHA system. In this paper, we analyze the performance of open loop transmit power control that uses a power margin to obtain the quality required even in packet-switched terminals, and propose wireless packet transmission with variable processing gain control as the method to achieve high-speed transmission by more effectively using the free channels. The results of computer simulations showed that the system capacity of circuit-switched terminals is not damaged when an appropriate power margin is set, and the additional effect of the packet-switched terminals is increased. In addition, the proposed system can greatly shorten the average delay time of packet-switched terminals. © 2001 Scripta Technica, Electron Comm Jpn Pt 1, 84(10): 16–26, 2001 [ABSTRACT FROM AUTHOR]

Wideband DS-CDMA must have path diversity reception (RAKE reception) because the received waves are separated into multiple paths as the spread bandwidth widens. Analysis of the reception characteristics which takes into account the actual path characteristics is essential to produce an optimal design for RAKE reception. In this paper, we illustrate the received level characteristics when the number of correlators is limited and the received levels are combined by a RAKE receiver based on a specific path selection scheme in a practical antenna diversity reception model having received level correlation between the antennas and different antenna gains. © 2001 Scripta Technica, Electron Comm Jpn Pt 1, 84(10): 36–54, 2001 [ABSTRACT FROM AUTHOR]

The robustness of the subspace method for blind signature waveform estimation with respect to channel order is analyzed in asynchronous DS-CDMA systems theoretically. Theoretical analysis and simulation results show that the overestimating of the channel order will lead to the degradation of the quality of the estimated signature waveform. So we should adopt the channel order as small as possible. [ABSTRACT FROM AUTHOR]

Multistage interference cancelers are deemed to be one of the most realizable schemes for asynchronous DS-CDMA. While limited to SD (Soft Decision)-based parallel multistage cancellation, this paper covers (1) theoretical relations introduced between suppressing factors and interference canceling gains, and (2) computer simulation results confirming the validity of the theory. There are two ways of setting suppressing factors. One is to set common values for all stages, and another is to set an individual value for each stage. This paper also discusses the differences between the two types of setting suppressing factors. © 2000 Scripta Technica, Electron Comm Jpn Pt 2, 83(12): 1–11, 2000 [ABSTRACT FROM AUTHOR]

This paper describes a model of a system with wideband CDMA that is a proposal of user access for future UMTS. This model has been implemented using OPNET tool. The model enables network architecture, radio interface, mobile station motion, call generation, signalling, etc. The examples of simulation results of call and handover blocking probability are also presented in this paper.

A microcellular DS-CDMA radio link design methodology and an evaluation method are proposed in this paper. The normalized link capacity (link capacity/process gain) and maximum average transmitting power of a mobile station are analyzed for the reverse link in a wideband propagation environment. The bit error rate (BER) is derived for an exponential delay profile with the parameter of normalized transmission bandwidth (transmission bandwidth B × delay spread τs). The average interference noise power of the other users from their own cells and that from other cells are derived based on power control error and the standard deviation of short-term fading. From these results, we derive the normalized link capacity and the maximum average mobile station transmitting power. The transmitting power is reduced by more than 40% for Bτs≤ 2 by combining space and path diversity. The link capacity is proportional to the number of antenna branches. © 1999 Scripta Technica, Electron Comm Jpn Pt 1, 82(9): 51–60, 1999 [ABSTRACT FROM AUTHOR]

This paper discusses the problem of coexistence of DS-CDMA (direct sequence code division multiplex access) and TDMA (time division multiple access) systems where their signals are transmitted in the same frequency band. The DS-CDMA signal is robust against narrowband interference, but there is a limit to its immunity against interference. This paper proposes, from such a viewpoint, a method using a complex multirate filter bank, as a way to reject TDMA signals, which act as narrowband interference to the DS-CDMA signal, in the coexistence system. By the proposed method, a notch filter can be composed with an arbitrary frequency resolution at an arbitrary frequency, by constructing the complex version of the conventional multirate filter bank. It is shown that, by eliminating the TDMA signal by the proposed method, the bit error rate of the DS-CDMA signal is improved, indicating that the proposed system is useful in the coexistence problem. © 1999 Scripta Technica, Electron Comm Jpn Pt 1, 82(5): 34–41, 1999 [ABSTRACT FROM AUTHOR]

An overview of some of the research at UCSD on wideband CDMA is presented. Initially a comparison is given of the performance of wideband and narrowband CDMA, and then previous results on both multicarrier CDMA and multiple access interference suppression are presented. The advantages of wideband CDMA are demonstrated, as are some key attributes of some of the signal designs for CDMA, such as the use of multicarriers [ABSTRACT FROM PUBLISHER]

This paper considers the DS-CDMA with an imperfect transmit power control and analyzes the relationship between the reverse-link capacity and the average transmit power, considering the space/path (RAKE) diversity. A simple formula for calculation is derived. Using the space diversity, an instantaneous large transmit power can be avoided when the transmit power is controlled. It is shown that, at that time, the interference from other cells can be reduced and the reverse-link capacity can be increased. Especially, when the spreading bandwidth is narrow and the single-path reception is used, the space diversity is shown to be necessary. It is known in CDMA that the link capacity can be increased by sectorizing the cell. In the practical antenna, however, where the directivity of the antenna overlaps with other sectors, there can be interference from adjacent sectors. A calculation formula is derived considering the antenna directivity. Using that formula, the reverse-link capacity and the average transmit power are examined precisely for the case where the pilot symbol interpolation QPSK coherent detection is used in the data modulation/demodulation. Finally, the effect of the shadowing correlation is investigated. © 1997 Scripta Technica, Inc. Electron Comm Jpn Pt 1, 80 (4): 72–82, 1996 [ABSTRACT FROM AUTHOR]

Adaptive antenna technology is now regarded as one of the key system components in future generation cellular networks. This paper considers the performance of a DS-CDMA system which employs adaptive antennas at the base station sites of both microcell and mixed micro/umbrella cell operational environments. A microcellular ray tracing propagation tool has been employed in order to ascertain site specific propagation data. Results for a typical small cell environment highlight both the propagation characteristics and the behaviour of the adaptive antenna, as well as the sensitivity of the system upon parameters such as the mutual coupling effect. This work does not assume idealised propagation conditions or antenna radiation patterns for the small cell analysis. In summary, the results indicate that microcellular base stations employing adaptive antenna signal processing can offer a significant capacity enhancement and improved channel characteristics when compared with their omnidirectional counterparts. In addition, it is shown that umbrella cells overlaying microcells within the same RF bandwidth allocation, do not impact upon system capacity providing that adaptive antenna technology is employed at the umbrella cell site. [ABSTRACT FROM AUTHOR]

A direct-sequences code division multiple access system has been accepted as a digital cellular standard (IS–95) in North America [1]. This digital cellular standard employs a powerful rate 1/2, constraint length 9, convolutional code in its forward link. It is well-known that in a Rayleigh fading channel the performance of a channel code depends very heavily on the interleaving depth and the relative variations of the channel characteristics. In slow fading channels, since the input symbols to the channel decoder are highly correlated, the bit-error-rate at the output of the channel decoder may be unacceptably high. Interleavers of large dimensions can reduce the correlation of the input signal to the channel decoder at the expense of an intolerable delay. In this paper we examine the performance of the IS–95 system, at the mobile receiver, for different channel fade rates. Also, we present a simple time diversity technique which employs multiple receive antennas. The multiple receive antennas in this case generate a fast fading effect and thus improves the performance of the channel decoder significantly. [ABSTRACT FROM AUTHOR]

DS-CDMA is recognised as one of the foremost mobile radio access techniques today. However, to date, there has been scant information on the system implementation and the many problems facing radio engineers. In this contribution, the design of the downlink (base to mobile) for a wideband (8.192Mchips/sec) DS-CDMA air interface is described along with practical results gained from its successful implementation in a full DS-CDMA system demonstrator developed under the U.K. DTI/EPSRC LINK programme. The aim is to provide a practical insight into CDMA and its application as a candidate for future personal communication systems. Synchronisation was one of the major challenges with the downlink and the techniques developed as part of this work are described fully. [ABSTRACT FROM AUTHOR]

Abstract Code division multiple access (CDMA) using direct sequence (DS) spread spectrum modulation provides multiple access capability essentially thanks to the adoption of proper sequences as spreading codes. The ability of a DS-CDMA receiver to detect the desired signal relies to a great extent on the auto-correlation properties of the spreading code associated to each user; on the other hand, multi-user interference rejection depends on the cross-correlation properties of all the spreading codes in the considered set. As a consequence, the analysis of new families of spreading codes to be adopted in DS-CDMA is of great interest. This article provides results about the evaluation of specific full-length binary sequences, the De Bruijn ones, when applied as spreading codes in DS-CDMA schemes, and compares their performance to other families of spreading codes commonly used, such as m-sequences, Gold, OVSF, and Kasami sequences. While the latter sets of sequences have been specifically designed for application in multi-user communication contexts, De Bruijn sequences come from combinatorial mathematics, and have been applied in completely different scenarios. Considering the similarity of De Bruijn sequences to random sequences, we investigate the performance resulting by applying them as spreading codes. The results herein presented suggest that binary De Bruijn sequences, when properly selected, may compete with more consolidated options, and encourage further investigation activities, specifically focused on the generation of longer sequences, and the definition of correlation-based selection criteria.

We consider the problem of training-based code-timing estimation for the asynchronous direct-sequence code-division multiple-access (DS-CDMA) system. We propose a modified large-sample maximum-likelihood (MLSML) estimator that can be used for the code-timing estimation for the DS-CDMA systems over the resolvable multipath channels in closed form. Simulation results show that MLSML can be used to provide a high correct acquisition probability and a high estimation accuracy. Simulation results also show that MLSML can have very good near-far resistant capability due to employing a data model similar to that for adaptive array processing where strong interferences can be suppressed.

The matrix inversion for the maximum likelihood (ML) channel estimation requires high complexity for the direct-sequence code-division multiple-access (DS-CDMA) systems. The prime motivation of the paper is to propose channel estimators that achieve mean square error (MSE) performance of ML channel estimator in an iterative manner without any matrix inversion. Therefore, two computationally efficient solutions to the problem of ML channel estimation are proposed.We compare the both algorithms in terms of the number of used iteration and show that the proposed algorithms converge the same MSE performance of the ML estimator as the increasing number of iterations. [ABSTRACT FROM PUBLISHER]

A differential space-time block code (DSTBC) modulation scheme is used to improve the performance of DS-CDMA systems in fast time-dispersive fading channels. The resulting scheme is referred to as the differential space-time block code modulation for DS-CDMA (DSTBC-CDMA) systems. The new modulation and demodulation schemes are especially studied for the down-link transmission of DS-CDMA systems. We present three demodulation schemes, referred to as the differential space-time block code Rake (D-Rake) receiver, differential space-time block code deterministic (D-Det) receiver, and differential space-time block code deterministic de-prefix (D-Det-DP) receiver, respectively. The D-Det receiver exploits the known information of the spreading sequences and their delayed paths deterministically besides the Rake type combination; consequently, it can outperform the D-Rake receiver, which employs the Rake type combination only. The D-Det-DP receiver avoids the effect of intersymbol interference and hence can offer better performance than the D-Det receiver.

The problem of decentralized detection in a sensor network subjected to a total average power constraint and all nodes sharing a common bandwidth is investigated. The bandwidth constraint is taken into account by assuming non-orthogonal communication between sensors and the data fusion center via direct-sequence code-division multiple-access (DS-CDMA). In the case of large sensor systems and random spreading, the asymptotic decentralized detection performance is derived assuming independent and identically distributed (iid) sensor observations via random matrix theory. The results show that, even under both power and bandwidth constraints, it is better to combine many not-so-good local decisions rather than relying on one (or a few) very-good local decisions. [ABSTRACT FROM PUBLISHER]

A code-constrained constant modulus (CCM) design criterion for linear receivers is investigated for direct sequence code division multiple access (DS-CDMA) in multipath channels based on constrained optimization techniques. A computationally efficient recursive least squares (RLS) type algorithm for jointly estimating the parameters of the channel and the receiver is developed in order to suppress multiaccess (MAI) and inter-symbol interference (ISI). An analysis of the method examines its convergence properties and simulations under nonstationary environments show that the novel algorithms outperform existent techniques. [ABSTRACT FROM PUBLISHER]

This letter presents an analytical model for evaluating the bit error rate (BER) of a direct sequence code division multiple access (DS-CDMA) system, with M-ary orthogonal modulation and noncoherent detection, employing an array antenna operating in a Nakagami fading environment. An expression of the signal to interference plus noise ratio (SINR) at the output of the receiver is derived, which allows the BER to be evaluated using a closed form expression. The analytical model is validated by comparing the obtained results with simulation results. [ABSTRACT FROM PUBLISHER]

Packet combining with adaptive retransmission control (ARC) in DS-CDMA random access networks has been investigated. The proposed system uses diversity packet combining while adaptively adjusting the retransmission probability of backlog users to optimize packet retransmissions. A soft decision DS-CDMA analysis is used to derive the optimum channel input for ARC. Simulation results show that packet combining with ARC can significantly increase the throughput and maintain the performance very close to its optimum in the high traffic region. [ABSTRACT FROM PUBLISHER]

A general framework to assess the joint effect of pulse shaping and spreading sequence statistics on asynchronous DS-CDMA is developed. With this, the effect of three possible pulse shapes (rectangle, band-limited rectangle, and square-root raised-cosine) associated with three possible spreading sequences (i.i.d., chaos-based, and optimum) is analyzed. Improvements up to 16% in users capacity are shown over traditional systems. [ABSTRACT FROM PUBLISHER]

In this letter, we propose an adaptive step-size algorithm for the adaptive interference canceller (AIC) in the space-time coded DS-CDMA system. To improve the performance of the fixed step-size AIC (FS-AIC), the regular adaptive step-size algorithm is extended in complex domain and applied to the joint AIC and ML decoder scheme. Simulation results show that the joint adaptive step-size AIC (AS-AIC) and ML decoder scheme using the proposed algorithm has better performance than not only the conventional ML decoder but also the joint FS-AIC and ML decoder scheme without much increase of the decoding delay and complexity. [ABSTRACT FROM PUBLISHER]