Direct Sequence Code Division Multiple Access Research Articles

Signal Processing by Generalized Receiver in DS-CDMA Wireless Communication Systems with Optimal Combining and Partial Cancellation

Symbol error rate (SER) of quadrature subbranch hybrid selection/maximal-ratio combining (HS/MRC) scheme for 1-D modulations in Rayleigh fading under employment of the generalized receiver (GR), which is constructed based on the generalized approach to signal processing (GASP) in noise, is investigated. diversity input branches are split into in-phase and quadrature subbranches. -ary pulse amplitude modulation, including coherent binary phase-shift keying (BPSK), with quadrature subbranch HS/MRC is investigated. GR SER performance for quadrature HS/MRC and HS/MRC schemes is investigated and compared with the conventional HS/MRC receiver. Comparison shows that the GR with quadrature subbranch HS/MRC and HS/MRC schemes outperforms the traditional HS/MRC receiver. Procedure of partial cancellation factor (PCF) selection for the first stage of hard-decision partial parallel interference cancellation (PPIC) using GR employed by direct-sequence code-division multiple access (DS-CDMA) systems under multipath fading channel in the case of periodic code scenario is proposed. Optimal PCF range is derived based on Price's theorem. Simulation confirms that the bit error rate (BER) performance is very close to potentially achieved one and surpasses the BER performance of real PCF for DS-CDMA systems discussed in the literature.

Reverse Link Performance of DS-CDMA Cellular Systems through Closed-Loop Power Control, Base Station Assignment, and Antenna Arrays in 2D Urban Environment

The interference reduction capability of antenna arrays, base station assignment, and the power control algorithms have been considered separately as means to increase the capacity in wireless communication networks. In this paper, we propose smart step closed-loop power control (SSPC) algorithm and base station assignment method based on minimizing the transmitter power (BSA-MTP) technique for direct sequence-code division multiple access (DS-CDMA) receiver in a 2D urban environment. This receiver consists of conjugate gradient adaptive beamforming and matched filter in two stages using antenna arrays. In addition, we study an analytical approach for the evaluation of the impact of power control error (PCE) on the DS-CDMA cellular systems. The simulation results indicate that the SSPC algorithm and the BSA-MTP technique can significantly improve the network bit error rate in comparison with conventional methods. Our proposed methods can also significantly save total transmit power and extend battery life in mobile units. In addition, we show that the convergence speed of the SSPC algorithm is faster than that of conventional algorithms. Finally, we discuss two parameters of PCE and channel propagation conditions (path-loss parameter and variance of shadowing) and their effects on the capacity of the system via some computer simulations.

Signal Processing by Generalized Receiver in DS-CDMA Wireless Communication Systems with Frequency-Selective Channels

The generalized receiver (GR) based on a generalized approach to signal processing (GASP) in noise is investigated in a direct-sequence code-division multiple access (DS-CDMA) wireless communication system with frequency-selective channels. We consider four avenues: linear equalization with finite impulse response (FIR) beamforming filters; channel estimation and spatially correlation; optimal combining; and partial cancellation. We investigate the GR with simple linear equalization and FIR beamforming filters. Numerical results and simulation show that the GR with FIR beamforming filters surpasses in performance the optimum infinite impulse response beamforming filters with conventional receivers, and can closely approach the performance of GR with infinite impulse response beamforming filters. Channel estimation errors are taken into consideration so that DS-CDMA wireless communication system performance will not be degraded under practical channel estimation. GR takes an estimation error of a maximum likelihood (ML) multiple-input multiple-output (MIMO) channel estimation and GR spatially correlation into account in computation of minimum mean square error (MMSE) and log-likelihood ratio (LLR) of each coded bit. The symbol error rate (SER) performance of DS-CDMA employing GR with a quadrature sub-branch hybrid selection/maximal-ratio combining (HS/MRC) scheme for 1-D modulations in Rayleigh fading is obtained and compared with that of conventional HS/MRC receivers. Procedure of selecting a partial cancelation factor (PCF) for the first stage of a hard-decision partial parallel interference cancellation (PPIC) of the GR employed in DS-CDMA wireless communication system is proposed. A range of optimal PCFs is derived based on the Price’s theorem. Computer simulation results show superiority in bit error rate (BER) performance that is very close to that potentially achieved and surpasses the BER performance of the real PCF for DS-CDMA systems discussed in literature.

Spread Spectrum Visual Sensor Network Resource Management Using an End-to-End Cross-Layer Design

In this paper, we propose an approach to manage network resources for a direct sequence code division multiple access (DS-CDMA) visual sensor network where nodes monitor scenes with varying levels of motion. It uses cross-layer optimization across the physical layer, the link layer, and the application layer. Our technique simultaneously assigns a source coding rate, a channel coding rate, and a power level to all nodes in the network based on one of two criteria that maximize the quality of video of the entire network as a whole, subject to a constraint on the total chip rate. One criterion results in the minimal average end-to-end distortion amongst all nodes, while the other criterion minimizes the maximum distortion of the network. Our experimental results demonstrate the effectiveness of the cross-layer optimization.

A low-complexity adaptive receiver for DS-CDMA systems in unknown code delay environment

In direct-sequence code division multiple access (DS/CDMA) multiuser communication systems in multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) must be considered. The multipath channel characterizes the propagation effects including the timing offset and delays, etc. Traditionally, we use the delay-locked loop (DLL) code tracking loop to recover the timing delay. But DLL cannot work well in multipath environment. In this paper, we propose a low-complexity adaptive receiver to suppress ISI/MAI and solve the timing offset problems without using conventional DLL code tracking loop. The proposed receiver employs an adaptive filter whose weights are adapted using a block least-mean square error algorithm with fractional sampling. Simulations confirm the good performance, including learning curves and theoretical analysis of minimum mean-square error, of the proposed receiver. Copyright © 2010 John Wiley & Sons, Ltd.

DS-CDMA Cellular Systems Performance with Base Station Assignment, Power Control Error and Beamforming over Multipath Fading

The interference reduction capability of antenna arrays, base station assignment and the power control algorithms have been considered separately as means to increase the capacity in wireless communication networks. In this paper, we propose base station assignment method based on minimizing the transmitter power (BSA-MTP) technique in a direct sequence-code division multiple access (DS-CDMA) receiver in the presence of frequency-selective Rayleigh fading and power control error (PCE). This receiver consists of constrained least mean squared (CLMS) algorithm, matched filter (MF), and maximal ratio combining (MRC) in three stages. Also, we present switched-beam (SB) technique in the first stage of the RAKE receiver for enhancing signal to interference plus noise ratio (SINR) in DS-CDMA cellular systems. The simulation results indicate that BSA-MTP technique can significantly improve the network bit error rate (BER) in comparison with the conventional case. Finally, we discuss on three parameters of the PCE, number of resolvable paths, and channel propagation conditions (path-loss exponent and shadowing) and their effects on capacity of the system via some computer simulations.

Performance Comparison of Overlap FDE and Sliding-Window Chip Equalization for Multi-Code DS-CDMA in a Frequency-Selective Fading Channel

Recently, overlap frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion which requires no guard interval (GI) insertion was proposed for signal transmission using multi-code direct sequence code division multiple access (DS-CDMA) in a frequency-selective fading channel. Another promising equalization technique is time-domain sliding-window chip equalization (SWCE). In this paper, the bit error rate (BER) performances achievable with overlap FDE and SWCE are compared. It is shown that, by extending the fast Fourier transform (FFT) block size, overlap FDE can achieve almost the same BER performance as SWCE with much less computational complexity than SWCE.

Joint Angular and Time Diversity of Multi-Antenna CDMA Systems in Wireless Fading Channels

In this paper, a multi-antenna based receiver structure for direct sequence code division multiple access (DS/CDMA) system is proposed. The proposed scheme exploits the excellent time resolution of a CDMA RAKE receiver and uses an antenna array beamforming structure to resolve multipath returns in both angular and time domains. A much higher diversity gain than that based only on the time domain diversity can be achieved. This work suggests a new space-time diversity paradigm, namely angular-time diversity, which differs from traditional Alamouti-type space-time coded schemes. The impairments caused by multipath and multiuser interference are analyzed. The performance of the proposed receiver in multipath fading channel is explicitly evaluated. An expression for uncoded system bit error probability is derived. Simulation results show the performance improvement in terms of BER due to the use of multi-antenna in the receiver, and the results illustrate that the multi-antenna based receiver works effectively in resolving multipaths in both angular and time domains to achieve performance improvement due to angular and time diversity gain provided by the multi-antenna system.

Spectral shaping of spreading sequences as a mean to address the trade-off between narrowband and multi-access interferences in UWB systems

This paper presents a way to cope with the need of simultaneously rejecting narrowband interference and multi-access interference in a UWB system based on direct-sequence CDMA. With this aim in mind, we rely on a closed-form expression of the system bit error probability in presence of both effects. By means of such a formula, we evaluate the effect of spectrum shaping techniques applied to the spreading sequences. The availability of a certain number of degrees of freedom in deciding the spectral profile allows us to cope with different configurations depending on the relative interfering power but also on the relative position of the signal center frequency and the narrowband interferer.

Anti-jamming performance analysis of optimal chaotic spread spectrum sequences

In order to improve the anti-jamming performance of Direct Sequence/Code Division Multiple Access (DS/CDMA) system based on chaotic sequences,an optimization algorithm for choosing chaotic sequences was proposed.The optimal improved Logistic-map chaotic sequences were applied to DS/CDMA system,and the anti-jamming performances were studied under different kinds of jamming such as single-frequency interference,partial-band interference,and impulsive interference.The results show that the optimal improved Logistic-map chaotic sequences have larger capacity,excellent anti-jamming ability,and lower Bit Error Rate (BER).Compared with the Gold sequence,with the same BER,SNR can be improved at least 2 dB,so they are more suitable for DS/CDMA system and anti-jamming technology.

Metodologia de Projeto e Implementação em DSP de Detectores Multiusuário Heurísticos DS/CDMA.

The growing number of users of mobile communications networks and the scarcity of the electromagnetic spectrum make the use of diversity techniques and detection/decoding efficient, such as the use of multiple antennas at the transmitter and/or receiver, multiuser detection (MuD – Multiuser Detection), among others, have an increasingly prominent role in the telecommunications landscape. This paper presents a design methodology based on digital signal processors (DSP – Digital Signal Processor) with a view to the implementation of multiuser heuristics detectors in systems DS/CDMA (Direct Sequence Code Division Multiple Access). Heuristics detection techniques result in near-optimal performance in order to approach the performance of maximum-likelihood (ML). In this work, was employed the DSP development platform called the C6713 DSK, which is based in Texas TMS320C6713 processor. The heuristics techniques proposed are based on well established algorithms in the literature. The efficiency of the algorithms implemented in DSP has been evaluated numerically by computing the measure of bit error rate (BER). Finally, the feasibility of implementation in DSP could then be verified by comparing results from multiple Monte-Carlo simulation in Matlab, with those obtained from implementation on DSP. It also demonstrates the effective increase in performance and system capacity of DS/CDMA with the use of heuristic multiuser detection techniques, implemented directly in the DSP.

Receiver Multiuser Diversity Aided Multi-Stage Minimum Mean-Square Error Detection for Heavily Loaded DS-CDMA and SDMA Systems

By investigating the conditions required for successive interference cancellation multiuser detectors (SIC-MUD) to achieve near-optimum BER performance, we propose and investigate a so-called receiver multiuser diversity aided multi-stage minimum mean-square error MUD (RMD/MS-MMSE MUD), which is operated in the SIC principles. The BER performance of the RMD/MS-MMSE MUD is investigated in association with both the direct-sequence code-division multiple-access (DS-CDMA) over either Gaussian or Rayleigh fading channels, and the space-division multiple-access (SDMA) over Rayleigh fading channels. Furthermore, we consider both full-load and overload scenarios in comparison with the spreading factor N of DS-CDMA and the number of receive antennas N in SDMA. Our studies show that the RMD/MS-MMSE MUD is highly efficient for both full-load and overload systems. Specifically, the RMD/MS-MMSE MUD for the full-load systems of moderate size is capable of attaining the BER performance similar to that of the optimum maximum likelihood MUD (ML-MUD). For the overload systems, it can allow a DS-CDMA or SDMA system to support K=2N users and still achieve much better BER performance than a corresponding DS-CDMA or SDMA system using conventional MMSE-MUD to support K=N users.

Reverse Link Performance of DS-CDMA Cellular Systems through Closed-Loop Power Control and Beamforming in 2D Urban Environment

The interference reduction capability of antenna arrays and the power control algorithms have been considered separately as means to decrease the interference in wireless communication networks. In this paper, we propose smart step closed-loop power control (SSPC) algorithm in direct sequence-code division multiple access (DS-CDMA) receivers in a 2D urban environment. This RAKE receiver consists of conjugate gradient adaptive beamforming (CGBF) and matched filter (MF) in two stages and finally, the output signals from the MFs are combined and then are fed into the decision circuit for the desired user. Also, we present switched-beam (SB) technique for enhancing signal to interference plus noise ratio (SINR) in network. Also, we study an analytical approach for the evaluation of the impact of power control error (PCE) on DS-CDMA systems in a 2D urban environment. The simulation results indicate that the convergence speed of the SSPC algorithm is faster than other algorithms. Also, we observe that significant saving in total transmit power (TTP) are possible with our proposed algorithm. Finally, we discuss two parameters of the PCE and path-loss exponent and their effects on capacity of the system via some computer.

Adaptive reference code‐based MAI cancellation for DS‐CDMA

AbstractA computationally efficient and practically deployable adaptive reference code‐based multiple access interference (MAI) cancellation scheme, in which the conventional transmitter/receiver architecture is minimally modified only at the receiver (and/or transmitter) end, is proposed for direct sequence code division multiple access (DS‐CDMA) communication. Upon numerical and theoretical analyses, the proposed communications system is seen to always outperform the existing conventional communications system. The theoretical analyses and results as presented are generally useful and applicable to any situation wherein IS95 pseudo noise (PN) codes are employed towards multiple access. Copyright © 2009 John Wiley & Sons, Ltd.

Distributed power control algorithms for asynchronous CDMA systems in frequency-selective fading channels

In Code Division Multiple Access (CDMA) radio environments, the maximum number of supportable users per cell is limited by multipath fading, shadowing, multiple access interference and near-far effects which cause fluctuations of the received power at the base station. In this context, power control and signal detection are essential to provide satisfactory Quality of Service (QoS) and to combat the near-far problem in CDMA systems. In this paper, we raised the uplink power control problem for a generalize asynchronous direct-sequence (DS) CDMA system that explicitly incorporate into the analysis: (1) the propagation delays in the network (generally neglected in the literature), (2) the adverse effect of multipath fading for wireless channels, and (3) the asynchronous transmissions in the uplink channels. This framework is used to propose a distributed power control strategy enhanced with linear multiuser receivers. It is shown that through a proper selection of an error function, the nonlinear coupling among active users is transformed into individual linear loops. A Linear-Quadratic-Gaussian (LQG) power control strategy is derived and compared with other approaches from the literature. Simulation results show that the uplink channel variations do not destroy the stability of these power control structures. However, delays in the closed-loop paths can severely affect the stability and performance of the resulting feedback schemes. It is also shown that the use of multiuser detection at the base station can bring significant improvements to the performance of power control.

An Efficient Measure for Nonlinear Distortion Severity due to HPA in Downlink DS-CDMA Signals

This paper deals with the nonlinear distortion (NLD) effects of high power amplifiers (HPAs) on direct sequence-code division multiple access systems. Such a distortion drastically degrades the system performance in terms of bit error rate (BER) degradation and spectral regrowth. Much effort has been conducted to minimize NLD. A key requirement to do so is to define a certain measure for the HPA nonlinearity, which when reduced often allows NLD to also be reduced. Several measures were proposed such as peak-to-average power ratio, instantaneous power variance, and cubic metric. In this paper, we show that such measures are not closely related to NLD and their reduction does not always lead to optimum performance. Hence, we introduce an efficient measure, namely, nonlinearity severity (NLS), to characterize NLD effects, as an alternative to the existing measures. The NLS is characterized by having direct link to the system performance as it is formulated based on the signal characteristics contributing to BER performance and spectral regrowth. Additionally, a major advantage of the NLS measure is that it is linked to the IBO level allowing the possibility of improving performance at all IBO levels of interest.

Time—frequency iterative multiuser detection in time—frequency‐domain spread multicarrier DS‐CDMA systems over Nakagami‐m fading channels

AbstractIn this paper, we propose and study a novel Time—frequency‐domain iterative multiuser detector (TF‐IMUD) for the multicarrier direct‐sequence code‐division multiple‐access (DS‐CDMA) systems employing both time (T)‐domain and frequency (F)‐domain spreading, which, for brevity, are referred to as the TF/MC DS‐CDMA systems. This novel TF‐IMUD consists of a set of T‐domain soft‐input soft‐output MUDs (SISO‐MUDs) and a set of F‐domain SISO‐MUDs, where information is exchanged between the T‐domain and F‐domain through two multiuser interference (MUI) cancellation units. The T‐domain and F‐domain SISO‐MUDs are operated under the maximum likelihood (ML) principles, and they are facilitated to be implemented using parallel signal processing techniques. In this paper, the complexity of the TF‐IMUD and the bit error rate (BER) performance of the TF/MC DS‐CDMA systems employing the proposed TF‐IMUD are investigated and also compared with the other existing MUD schemes, including the optimum ML‐MUD and the joint/separate minimum mean‐square error (MMSE)‐MUDs. Our study and simulation results show that the TF‐IMUD is capable of achieving nearly the same BER performance as the optimum ML‐MUD, but at significantly lower complexity. Copyright © 2010 John Wiley & Sons, Ltd.

FREQUENCY DOMAIN EQUALIZATION TECHNIQUES FOR MULTICODE DS-CDMA IN FREQUENCY SELECTIVE RAYLEIGH FADING CHANNEL

Orthogonal multicode direct sequence code division multiple access (DS-CDMA) has the flexibility in offering high data rate services. However, in a frequency-selective fading channel, the bit error rate (BER) performance is severely degraded since the orthogonality among spreading codes is partially lost. In this paper, frequency-domain equalization (FDE) and space antenna diversity combining are applied to orthogonal multicode DS-CDMA in order to restore the code orthogonality and improve the BER performance of the system. Two methods of FDE are considered, the first method based on fast Fourier transform (FFT), while the second method based on circulant matrices. Moreover, a channel interleaving method, called chip interleaving is used to improve the performance of multicode DS-CDMA system.

Subblock Equalization and Code Averaging for DS-CDMA Receivers

As third-generation (3G) cellular communication systems evolve to higher data rates, channel equalization becomes an important tool in maintaining receiver performance in dispersive channels. Because these systems employ direct-sequence code-division multiple-access (DS-CDMA), they pose unique challenges in equalizer design. In this paper, linear equalization (LE) and decision feedback equalization (DFE) designs are developed based on subblock equalization, in which a limited form of maximum-likelihood (ML) joint detection is used to detect symbols within a subblock. A pseudo-ML formulation is used, treating interference from symbols outside the subblock of interest as noise. This formulation leads to a feedforward filter (FFF) whose coefficients depend on the spreading codes of the symbols. These spreading codes change every symbol period, due to long-code scrambling, requiring constant recomputation of the FFF coefficients. To reduce complexity, code averaging is used, resulting in a code-independent periodically varying transversal filter. Another design is obtained by constraining the transversal filter to be time invariant. Simulation results for the high-speed packet-access uplink show that code averaging introduces a small loss in subblock equalization performance and that interesting gains over conventional LE can be obtained.

An Adaptive Multiuser Detection Algorithm for CDMA Systems Using Antenna Diversity

Based on the minimum mean squared error (MMSE) between the data stream and the linear combiner output, a new multiuser detection (MUD) algorithm that combines space–time (ST) processing and antenna array on direct-sequence CDMA signals is proposed. The proposed ST-MUD algorithm is proved to be equivalent to two existing MMSE-based ST-MUD algorithms, and the theoretical BER performances for all the three algorithms are the same. The most attractive feature of the new ST-MUD algorithm is based on the fact that the new method does not require explicit estimation of channel and signaling information. This avoids any channel estimation error, and the method is thus more robust and more accurate than the other two ST-MUD algorithms in practical implementation. Adaptation of the proposed ST-MUD algorithm is implemented by using training sequences. Performance of this new multiuser detector is compared with that of two existing MMSE multiuser detectors and the conventional single-user space–time rake receiver through simulations. The proposed ST-MUD algorithm provides a performance better than existing algorithms and is especially suitable for practical CDMA systems.