Direct Sequence Code Division Multiple Access Research Articles

Energy Efficient Communications in CDMA Networks: A Game Theoretic Analysis Considering Operating Costs

A game-theoretic analysis is used to study the effects of receiver choice and transmit power on the energy efficiency of communications in cellular, multi-hop, and peer-to-peer networks in which the nodes communicate using direct-sequence code-division multiple access (DS-CDMA). The unique Nash equilibrium of the game in which the network nodes can choose their receivers as well as their transmit powers to maximize the total number of bits they transmit per unit of energy spent (including both transmit and operating energy) is derived. The energy efficiencies resulting from the use of different linear multiuser receivers in this context are compared for the noncooperative game. Significant gains in energy efficiency are observed when the linear minimum mean-square error (MMSE) receiver is used instead of conventional matched filter receivers.

Deterministic Spreading Sequences for the Reverse Link of DS-CDMA With Noncoherent $M$-ary Orthogonal Modulation: Impact and Optimization

M-ary orthogonal modulation with noncoherent detection is an attractive scheme for the reverse link of direct-sequence code-division multiple access (DS-CDMA). The performance analysis of such DS-CDMA systems with random spreading sequences has been thoroughly studied. However, little satisfactory work has been done for systems with deterministic spreading sequences, regardless of their importance from the viewpoint of multiple-access interference reduction. To fill this void, we consider deterministic spreading sequences in this paper. We first present a detailed error performance analysis for the reverse link of DS-CDMA with -ary orthogonal modulation on both additive white Gaussian noise (AWGN) and multipath Rayleigh-fading channels. Then, we formulate the design of spreading sequences for the DS-CDMA system as a nonlinear discrete optimization problem. Two steps are taken to complete the task of sequence optimization: (1) A large code space is generated by permutating a single binary code matrix and imposing a Kronecker product structure on candidate code matrices, and (2) an evolutionary algorithm is applied to efficiently perform optimization. Numerical examples show that the optimized sequences considerably improve the system performance, especially when the number of users is relatively small or an AWGN channel is considered. In addition, our proposed approach can make a good tradeoff between code performance and search complexity.

Energy-efficient power control is (almost) equivalent for DS-CDMA and TH-UWB

A distributed, energy-efficient power control algorithm for the uplink of wireless data networks can be derived through the application of noncooperative game theory. The performance of two widely used multiple access schemes, namely direct-sequence code division multiple access (DS-CDMA) and time hopping ultrawideband (TH-UWB), is compared on the basis of their energy efficiencies. It is shown that TH-UWB provides definite yet negligible advantages in frequency-selective fading, and that TH-UWB and DS-CDMA perform identically in the presence of flat fading.

A Capacity Analysis Method for Uplinks in DS/CDMA Cellular Systems with Imperfect SIR-Based Power Control and Multipath Fading

The letter proposes a method to analyze the system capacity of uplinks with imperfect SIR-based power control in direct sequence/code division multiple access (DS/CDMA) cellular systems with the multipath fading. It considers the received signal-to-interference ratio (SIR) as a random variable due to imperfect SIR-based power control. Based on assumptions regarding the average received SIR, a set of linear equations for the average received power on each uplink is derived. An outage-based system capacity can be then obtained according to the feasibility of the average received power vector and the average bit error rate. Finally, by applying central limit theorem, a closed-form solution for system capacity is approximately derived. Results show that numerical results of the closed-form solution have a good match with simulation results, which supports the validity of the proposed capacity analysis method.

Return-Link Code Acquisition for 1-D and 2-D With DS-CDMA for High-Capacity Multiuser Systems

Acquisition of the code timing in a direct-sequence code-division multiple-access (DS-CDMA) system at the base station must take place before signal detection and decoding is possible. Code acquisition under severe multiple-access-interference (MAI) conditions with time-varying codes makes the task even more difficult. Inefficient designs lead to a large number of false alarms and/or missed detections. This requirement is needed for conventional single-antenna (1-D) designs, as well as for multielement antenna (2-D) designs. This paper details a powerful code-acquisition technique for the uplink of DS-CDMA systems under high-loaded situations for both 1-D and 2-D schemes, where the number of users is greater than the processing gain. Under this high-MAI condition, the DS-CDMA acquisition problem becomes very difficult, and conventional search methods simply fail. The method discussed utilizes soft data from a multiuser detector to reduce the interference received by the acquisition unit. Analytical performance is compared to simulation results in terms of the number of users, processing gain, interferer-signal power, cancellation factor, antenna configuration, and noise variance. Numerical results validate performance under realistic conditions with amplitude, phase, and frequency impairments.

Performance of multiantenna multicarrier direct-sequence code division multiple access using orthogonal variable spreading factor codes-assisted space–time spreading in time-selective fading channels

A downlink (base-to-mobile) multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) system employing multiple antennas at both the base station and each of the mobile terminals is investigated, when communicating over fast time-varying fading channels resulting in time-selective fading. In the considered multiantenna MC DS-CDMA, space–time spreading (STS) based on the family of orthogonal variable spreading factor (OVSF) codes is proposed in order to achieve the time diversity because of the time-selective fading, in addition to the transmit/receive diversity. In this contribution, the closed-form expressions for the single-user bit-error rate (BER) are derived, in order to gain insight into the achievable BER performance of the multiantenna MC DS-CDMA. The BER performance of the multiantenna MC DS-CDMA system is investigated, when communicating over correlated time-selective Rayleigh fading channels. The study and performance results show that the multiantenna MC DS-CDMA using OVSF codes-assisted STS constitutes a high-efficiency downlink space–time transmission scheme. It is capable of achieving the full diversity provided by the time-selective fading and multiple transmit/receive antennas. Furthermore, the proposed multiantenna MC DS-CDMA is capable of achieving a self-balance between the time diversity achieved and the multiuser interference suppression capability, when using low-complexity correlation detection.

Reducing dynamic power consumption in next generation DS-CDMA mobile communication receivers

Reduction of the power consumption in portable wireless receivers is important for cellular systems, including UMTS and IMT2000. This paper explores the architectural design-space and methodologies for reducing the dynamic power dissipation in the Direct Sequence Code Division Multiple Access (DS-CDMA) downlink RAKE receiver. At the algorithm level, we investigate the tradeoffs of reduced precision and arithmetic complexity on the receiver performance. We then present and analyse two architectures for implementing the reference and reduced complexity receivers, with respect to dynamic power dissipation. The combined effect of reduced precision and complexity reduction leads to a 37.44% power savings.

Asynchronous, Decentralized DS-CDMA Using Feedback-Controlled Spreading Sequences for Time-Dispersive Channels

We propose a novel asynchronous direct-sequence codedivision multiple access (DS-CDMA) using feedback-controlled spreading sequences (FCSSs) (FCSS/DS-CDMA). At the receiver of FCSS/DS-CDMA, the code-orthogonalizing filter (COF) produces a spreading sequence, and the receiver returns the spreading sequence to the transmitter. Then the transmitter uses the spreading sequence as its updated version. The performance of FCSS/DS-CDMA is evaluated over time-dispersive channels. The results indicate that FCSS/DS-CDMA greatly suppresses both the intersymbol interference (ISI) and multiple access interference (MAI) over time-invariant channels. FCSS/DS-CDMA is applicable to the decentralized multiple access.

FPGA Implementation of Block Parallel DF-MPIC Detectors for DS-CDMA Systems in Frequency-Nonselective Channels

Multistage parallel interference cancellation- (MPIC-) based detectors allow to mitigate multiple-access interference in direct-sequence code-division multiple-access (DS-CDMA) systems. They are considered serious candidates for practical implementation showing a good tradeoff between performance and complexity. Better performance is obtained when decision feedback (DF) is employed. Although MPIC and DF-MPIC have the same arithmetic complexity, DF-MPIC needs much more FPGA resources when compared to MPIC without decision feedback. In this letter, FPGA implementation of block parallel DF-MPIC (BP-DF-MPIC) is proposed allowing better tradeoff between performance and FPGA area occupancy. To reach an uncoded bit-error rate of10−3, BP-DF-MPIC shows a 1.5 dB improvement over the MPIC without decision feedback with only 8% increase in FPGA resources compared to 69% for DF-MPIC.

Single-user performance of direct-sequence code-division multiple-access using relay diversity and power allocation

In this contribution, the authors investigate the single-user-bound performance of a direct-sequence code-division multiple-access (DS-CDMA) system, where one source mobile terminal (MT) communicates with its base-station (BS) with the assistance of multiple relays. The authors assume that the communications channels experience both propagation pathloss and fast fading, and that the channels from the source MT to the BS and relays as well as that from the relays to the BS may experience different fast fading modelled correspondingly by Nakagami-m distributions. In the study, they assume both the single-user combining using maximal ratio combining and the multiuser combining, which are derived based on the maximum signal-tointerference-plus-noise ratio criterion. The bit-error-rate (BER) performance of the DS-CDMA is investigated associated with considering the locations of the relays as well as the power-allocation among the source MT and relays. From the study and simulation results, it can be shown that the achievable BER performance of the DS-CDMA depends on the locations of the relays and also on the power-allocation among the source MT and relays. When the relays of a source MT are chosen from a different area, the power-allocation should also be adjusted correspondingly in order to achieve the minimum BER. Furthermore, when optimum power allocation is assumed, the BER performance of the DS-CDMA can be significantly improved, when increasing the number of relays assisting the source MT.

A Blind Adaptive MMSE Multiuser Detector over Multipath CDMA Channels and its Analysis

Based on the refinement-only fast subspace tracking (RO-FST) algorithm to adaptively estimate the signal sub-space, a blind adaptive SVD-based minimum-mean-square-error (MMSE) multiuser detector is proposed in this paper for asynchronous direct-sequence code-division-multiple-access (DS-CDMA) systems over multipath fading channels. Numerical results show that the proposed detector gives low computational complexity and good performance in terms of signal-to-interference-plus-noise ratio (SINR) and bit-error-rate (BER). In addition, we apply the perturbation analysis of the proposed adaptive detector to derive the closed-form expressions for the mean-square error (MSE) and SINR. The analytical results are shown to match well with the simulation results.

Blind MMSE-Constrained Multiuser Detection

In this paper, blind multiuser detection is studied for a direct-sequence (DS) code-division multiple access (CDMA) system with unknown multipath channels. The receiver is designed to follow a minimum mean-square-error (MMSE) form parameterized by a channel-like constraint vector, which is then optimized by the conventional constant modulus algorithm in a much smaller dimensional space. It is analytically established that the constraint vector asymptotically converges to the channel vector of the user of interest. Correspondingly, the receiver converges to the desired user's MMSE receiver, ensuring detection of the desired signals. When the constraint vector is treated as a channel estimate, the performance of the channel estimator is analyzed using a perturbation technique. Simulation results show a satisfactory performance of the proposed approach, which is also observed to be superior to other approaches in many scenarios.

Blind channel estimation for multiple input multiple output uplink guard-band assisted code division multiple access systems with layered space frequency equalisation

Single carrier (SC) code division multiple access (CDMA) with block transmission has been shown to be more effective while utilising a low-complexity equaliser to combat frequency-selective fading channels, when compared with conventional direct sequence CDMA technology. It also has lower peak-to-average power ratio and lower frequency sensitivity compared with multicarrier CDMA. The authors propose two blind channel estimation methods for uplink multiple input multiple output SC-CDMA systems with block transmisssion-one is the subspace-based method and the other is the so-called autocorrelation contribution method (ACM). Both the methods provide close performance to the case with perfect channel knowledge at high signal-to-noise ratio (SNR) without any training data required. It is shown that ACM yields a better performance than the subspace method at a lower SNR, and a similar performance at a high SNR, with the advantages of avoiding rank determination and noise power estimation as in the subspace method. In addition, the authors integrate layered space frequency equalisation with blind channel estimation, which provides improved performance over the conventional linear equalisation, by employing successive interference cancellation.

Performance of a Complete Complementary Code-Based Spread-Time CDMA System in a Fading Channel

In this paper, we study the performance of complete complementary code-based spread-time code-division multiple access (CC-ST-CDMA) in frequency-nonselective and frequency-selective fading channels. The theoretical bit-error-rate (BER) expressions conditioned on the number of users and the sequence length (and the number of resolvable paths for multipath frequency-selective fading) are obtained. The theoretical approximation of the BER for the system using long-sequence-length complete complementary codes is investigated as well. The simulations are conducted over flat-fading and two-path frequency-selective fading channels and have been used to verify the theoretical analysis. The results show that the proposed CC-ST-CDMA system has a comparable BER performance with complete complementary code-based multicarrier direct-sequence code-division multiple access (CC-MC-DSCDMA) for a flat-fading channel. However, for a frequency-selective fading channel, the CC-ST-CDMA is found to be more resistant, resulting in about 2-3 dB better performance than the CC-MC-DSCDMA.

A Multicarrier-CDMA Space–Time Receiver With Full-Interference-Suppression Capabilities

This paper proposes a low-complexity multicarrier-code-division-multiple-access (MC-CDMA) space-time receiver with full-interference-suppression capabilities. First, we derive a complete model of the interference which takes into account multiple-access, intersymbol, and intercarrier interferences. Based on this model, we introduce a new multicarrier-interference-subspace-rejection (MC-ISR) receiver and analyze its performance in an unknown time-varying Rayleigh channel with multipath, carrier offset, and cross correlation between subcarrier channels. We also propose a realistic implementation of this receiver, which includes an efficient strategy for carrier-offset recovery in a multicarrier- and multiuser-detection scheme. In addition, based on the Gaussian assumption, we derive a link-/system-level performance analysis of the MC-ISR over the two MC-CDMA air-interface configurations, which are the multitone CDMA and the multicarrier direct-sequence CDMA, and validate it by simulations. The gains in the throughput, which are attainable by the MC-ISR, are significant and evaluated in this paper as functions of the air-interface configuration, the number of subcarriers, and the modulation order.

Performance analysis of non-data aided SBIB receivers for CDMA systems

In this paper, a non-data aided desired signal-blocked interference-blocked (SBIB) receiver with enhanced multiple access interference (MAI) suppression is proposed for direct-sequence code-division multiple access (DS-CDMA) systems over a multipath fading channel. Without using pre-known data, the proposed receiver can offer a similar performance as a non-blind maximum signal-to-interference-plus-noise ratio (MSINR) receiver, in which both the interference-plus-noise correlation matrix and the channel estimation are assumed to be correct. Performance analysis is carried out to examine the efficacy of the proposed receiver. Computer simulation results then confirm the correctness of the theoretical analysis and demonstrate that the proposed blind receiver can successfully resist MAI and outperform the conventional receivers.

Performance analysis of an adaptive two-stage PIC CDMA receiver in AWGN channels

Parallel interference cancellation (PIC) is considered a simple yet effective multiuser detector for direct-sequence code-division multiple-access (DS-CDMA) systems. However, its performance may deteriorate due to unreliable interference cancellation in the early stages. Thus, a partial PIC detector, in which partial cancellation factors (PCFs) are introduced to control the interference cancellation level, has been developed as a remedy. Recently, an interesting adaptive multistage PIC algorithm was proposed. In this scheme, coefficients combining the channel responses and optimal PCFs are blindly trained with the least mean square (LMS) algorithm. The algorithm is simple to implement, inherently applicable to time-varying environments, and superior to the non-adaptive type of partial PICs. Despite its various advantages, its performance has not been theoretically analyzed yet. The contribution of this paper is to fill the gap by analyzing an adaptive two-stage PIC in AWGN channels. We explicitly derive the analytical results for optimal weights, weight-error means, and weight-error variances. Based on these results, we finally derive the output bit error rate (BER) for each user. Simulation results indicate that our analytical results highly agree with empirical ones.

Power and Rate Adaptations in Multicarrier DS/CDMA Communications over Rayleigh Fading Channels

We consider power and rate adaptations in multicarrier (MC) direct-sequence code-division multiple-access (DS/CDMA) communications under the assumption that channel state information is provided at both the transmitter and the receiver. We propose, as a power allocation strategy in the frequency domain, to transmit each user's DS waveforms over the user's sub-band with the largest channel gain, rather than transmitting identical DS waveforms over all sub-bands. We then adopt channel inversion power adaptation in the time domain, where the target user's received power level maintains at a fixed value. We also investigate rate adaptation in the time domain, where the data rate is adapted such that a desired transmission quality is maintained. We analyze the BER performance of the proposed power and rate adaptations with fixed average transmission power, and show that power adaptation in both the frequency and the time domains or combined power adaptation in the frequency domain and rate adaptation in the time domain make significant performance improvement over the power adaptation in the frequency domain only. We also compare the performance of the proposed power and rate adaptation schemes in MC-DS/CDMA systems to that of power and rate adapted single carrier DS/CDMA systems with RAKE receiver.

A General Construction of OVSF Codes With Zero Correlation Zone

In this letter, a general construction of orthogonal variable spreading factor (OVSF) codes with zero correlation zone (ZCZ) property is presented based on orthogonal sequence sets and -complementary sets. It is shown that the proposed general construction includes the conventional binary and ternary OVSF codes as special cases and can accommodate more flexible rate assignment in direct sequence code division multiple access (DS-CDMA) systems.

Design and Performance of Spread Spectrum Karhunen–Loeve Transform for Direct Sequence CDMA Communications

In this letter, the eigen-analysis-based optimal design method of orthogonal block transform, the so-called Karhunen-Loeve transform, is used to generate a new family of spread spectrum codes. These spread spectrum and varying power orthogonal codes might find their application in direct sequence code division multiple access (CDMA) communications. The basis functions of the proposed transform are jointly spread in the time and frequency domains. This property is in contrast with the traditional optimal block transforms maximizing energy compaction due to their frequency localized basis magnitude functions desired in source coding and spectral processing applications. Bit-error rate (BER) performance of the proposed CDMA spreading codes under both additive white Gaussian noise (AWGN) and Rayleigh flat fading channel conditions are compared with the industry standard codes like Walsh and Gold families. It is shown in this letter that the proposed technique allows us to design orthogonal spreading codes with flexible lengths that perform comparable to or better than the Gold and Walsh codes.