Forward-link performance of CDMA cellular system

The impact of the propagation path-loss exponent (γ) on the forward performance of a direct-sequence code-division multiple-access (DS-CDMA) cellular system is investigated. For this purpose, a simple analytical model based on the inverse power-of-distance γ law is developed. The problem of finding proper power-control factors is considered. According to γ, the proper power-control factors are estimated for optimum performance. With these factors, results show that the capacity is reduced by a half by changing γ from 4.5 to 2.5. For this range in γ, power control can approximately double the capacity compared to the case of no power control     

Reverse link capacity and interference statistics of a fixed-steppower-controlled DS/CDMA system under slow multipath fading

Power control (PC) on the reverse link of a direct-sequence code-division multiple-access system is essential to increase system capacity. Perfect PC eliminates fluctuations in the received signal level and hence reduces the required signal-to-interference ratio. However, a perfect PC algorithm tracks multipath fading accurately, which results in increasing the intercell interference level. A fixed step PC algorithm becomes almost perfect when the power command rate is too fast compared to the Doppler rate, which is the case for low-mobility users. We investigate the statistics of the intercell interference assuming users are moving slowly. These statistics are then used to find the system capacity. Three parameters that can affect the capacity are considered: the number of the fading process resolvable paths, the maximum transmitted power, and soft handoff     

OCDD/CDMA: a new DS/CDMA with orthonormal code diversity detection

The paper presents a new spread spectrum communication system called orthonormal code diversity detection (OCDD)/CDMA system based on the novel concept of orthonormal-basis diversity which is a generalization of the existing spread spectrum diversity concepts such as path diversity and frequency diversity. The OCDD/CDMA system is similar to the conventional DS/CDMA system in the transmitter structure, but is different in the receiver structure as it employs the extended orthonormal basis-function set which is the union of the Walsh basis-functions multiplied by the PN sequences and, optionally, their delayed replicas. The received signal is matched to the extended basis functions, and the matched signal components are combined together after individual channel compensation. The proposed OCDD/CDMA system exhibits the bit error performance which is much improved over the conventional DS/CDMA system using maximal ratio combing. In addition, it is robust to the chip timing error, which becomes more crucial in the future DS/CDMA systems having a higher data rate and smaller chip interval. From the simulation results, we confirm that the OCDD/CDMA system is a unique spread spectrum communication technique that can effectively increase the diversity utilization in the slowly fading channel, overcoming the inherent problems in the DS/CDMA and OFDM/CDMA systems     

Analysis of a pipelined successive interference cancellation schemefor a DS/CDMA system

The bit error rate (BER) expression for a proposed pipelined successive interference cancellation (PSIC) scheme in a direct sequence/code division multiple access (DS/CDMA) system is derived analytically. The proposed PSIC scheme is a pipelined modification of a successive interference cancellation (SIC) scheme. The numerical results show that the proposed PSIC scheme outperforms the SIC scheme in viewpoints of the BER and the decoding delay performances     

Performance analysis of a DS/CDMA system with noncoherent M-aryorthogonal modulation in Nakagami fading

The probability of error performance of a direct sequence code-division multiple-access (DS/CDMA) system employing noncoherent M-ary orthogonal signaling in a Nakagami multipath fading channel is analyzed. A RAKE receiver structure with square-law demodulation is used at the receiver. The multiple-access interference are modeled as Gaussian and expressions derived for the exact probability of error. The performance is also evaluated in terms of the number of users that can be supported by the system at a given probability of error. The effect of correlated fading on system performance is also investigated by considering two correlation models, which can be characterized by a single correlation coefficient ρ. In the first model, the correlation coefficient between any two diversity branches is constant. In the second model, it is assumed that the correlation coefficient between any two diversity branches decreases exponentially as the separation between them increases. For both models, it is found that the presence of correlation deteriorates system performance. The use of larger signal alphabets than binary modulation in conjunction with diversity reception provides a considerable performance improvement even in the presence of correlated fading     

A diversity combining DS/CDMA system with convolutional encodingand Viterbi decoding

Averaged diversity combining is applied to an asynchronous DS/CDMA system using convolutional encoding and Viterbi decoding. A cyclic redundancy check (CRC) code is included in the scheme to trigger retransmission requests. Multiple received packets are combined on a bit by bit basis to form a single, more reliable packet. The error correcting decoder operates on the combined packet, as opposed to the most recently received individual packet (e.g., as in a type-I hybrid ARQ protocol), substantially increasing the probability of acceptance with each additional transmission. We show that the proposed technique allows a significant increase in the CDMA system capacity, throughput, and reliability     

Performance of a turbo coded multicarrier DS/CDMA system withnonuniform repetition coding

This paper presents a turbo coded multicarrier direct sequence code division multiple access (DS/CDMA) system, where the outputs of a turbo encoder are repetition coded at multiple rates and transmitted in parallel over a number of subchannels. A performance bound useful in the so-called error floor region is obtained for Rayleigh fading channels when different diversity orders are given to each turbo code symbol. Simulation results are also provided for the low signal-to-noise ratio (SNR) region where the bound is not applicable. It is observed that the error floor can be lowered, with some performance loss in the low SNR region, by applying nonuniform repetition coding to the turbo code symbols     

Analysis of a simple successive interference cancellation scheme ina DS/CDMA system

Compensating for near/far effects is critical for satisfactory performance of DS/CDMA systems. So far, practical systems have used power control to overcome fading and near/far effects. Another approach, which has a fundamental potential in not only eliminating near/far effects but also in substantially raising the capacity, is multiuser detection and interference cancellation. Various optimal and suboptimal schemes have been investigated. Most of these schemes, however, get too complex even for relatively simple systems and rely on good channel estimates. For interference cancellation, estimation of channel parameters (viz. received amplitude and phase) is important. We analyze a simple successive interference cancellation scheme for coherent BPSK modulation, where the parameter estimation is done using the output of a linear correlator. We then extend the analysis for a noncoherent modulation scheme, namely M-ary orthogonal modulation. For the noncoherent case, the needed information on both the amplitude and phase is obtained from the correlator output. The performance of the IC scheme along with multipath diversity combining is studied     

A decision feedback decorrelator for a dual rate synchronous DS/CDMA system

A dual rate synchronous DS/CDMA system provides service to low bit rate and high bit rate users. In a fixed duration interval, a low rate user transmits one bit while a high rate user transmits M bits. The differences in the bit transmission rates result in different processing gains for each class of user. In this paper, we propose a decision feedback decorrelator for the dual rate synchronous DS/CDMA system which uses modified correlators and initiates the bit decision process at the end of the high rate bit interval. Each step of the decision process is executed by utilizing the decisions of all previously decoded users. This dual rate decision feedback receiver (evaluated by simulation) is found to outperform two types of decorrelators for dual rate CDMA systems. It is also observed that as the interferers grow stronger relative to the desired user, the performance of the decision feedback receiver for decoding the desired user approaches the single user bound. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interference suppression for DS/CDMA

A direct-sequence spread spectrum (DS/SS) receiver for suppressing multiple-access interference in direct-sequence code-division multiple-access (DS/CDMA) communication systems is introduced. The proposed receiver does not require knowledge of other users' spreading codes, timing, or phase information. Moreover, the receiver allows the number of taps to be chosen independently of the processing gain and, hence, is easily applicable to CDMA systems employing either a small or a large processing gain. Performance analysis, including average probability of error and signal-to-noise ratio, is provided, and results are presented for systems varying from lightly loaded (for example, eight user/255 chip) to heavily loaded (for example, 50 user/200 chip). Performance results indicate that the proposed receiver outperforms the linear correlation receiver and, in many cases, it does so by a considerable margin     

主动重发的DS／CDMA ALOHA

针对DS/CDMA ALOHA系统,本文提出了一种主动复发方案,并对主动重发时延为无限和有限值的情况分别进行了分析和研究,研究结果表明,主动重发时延为一个时隙时即可获得接近理想情况的性能。从理论计算和仿真实验的结果可以看到,这种新方案可以有效地改善DS/CDMA ALOHA协议的性能。     

Performance analysis of a convolutional coded DS/CDMA system in Nakagami fading channels

In this paper, we investigate the performance of convolutional coded DS/CDMA with noncoherent M-ary orthogonal modulation operating in a multi-user environment over a slow and frequency nonselective Nakagami-m fading channel with additive white Gaussian noise (AWGN). An expression for the pairwise error probability that can be used to compute the upper bound of coded system is first derived. Performance of the DS/CDMA system with and without the convolutional codes is then presented. We have observed that convolutional codes can compensate for the degradation quite well in multi-user situations over Nakagami fading channels with AWGN. For the case of an extreme fading, however, it has been seen that the convolutional code reaches its limit to improve the overall system performance as the number of users increase. This revised version was published online in August 2006 with corrections to the Cover Date.     

Modified SUGAR/DS: a new CDMA scheme

The authors prove by analysis the possibility of gaining a minimum of 2 dB in signal-to-noise ratio by just, splitting the users to three or more groups and identifying each one by an orthogonal waveform (on top of his short Gold code) in a PSK/DS spread-spectrum network. User signals may arrive in a code-asynchronous fashion at the receiver, however, it is shown that by using the scheme the average code cross-correlation is minimal compared to the classic code division multiple access (CDMA) system. Both the chip-synchronous and asynchronous cases are investigated and the uniform and optimal cases of dividing the users into orthogonal groups are analyzed. The superior bit error and network data throughput results in the different fading and forward error correction environments make the system a strong candidate for competitive domestic applications     

Optimal SIR for an hybrid cellular DS/SFH CDMA system

A hybrid direct‐sequence/slow frequency hopping code‐division multiple‐access (DS/SFH CDMA) system operating in a Rayleigh fading environment is described and its spectral efficiency is estimated in terms of the theoretically achievable average channel capacity (in the sense of information theory) per user. The analysis covers the operation over a broadcast cellular time‐varying link and leads to a simple novel closed‐form expression for the optimal average spread signal‐to‐interference power ratio (SIR) received by each user based on the maximization of the achieved spectral efficiency. Copyright © 2007 John Wiley & Sons, Ltd.     

DS/CDMA closed-loop power control with adaptive algorithm

Closed-loop power control in the reverse link of a DS/CDMA system is analysed. The transmitter/receiver based on IS-95 and the radio propagation channel under mobile communication environments are modelled and a new power control algorithm is proposed. This algorithm utilises the burst error characteristics of the channel and can be directly applied to the current IS-95 system. The new power control algorithm, with respect to the SIR (signal to interference ratio), increases the service quality, and finally, enhances the system capacity     

DS/CDMA的数据检测技术

介绍了DS／CDMA移动通信系统降低误码率和提高系统容量关键技术之一的数据检测技术。     

Performance enhancement of DS/CDMA system using chaotic complex spreading sequence

In this paper, a novel direct sequence/spread spectrum (DS/SS) communication system is proposed. This scheme exploits the two-dimensional complex valued chaotic Ikeda map as the spreading sequences. With this double spreading DS/SS system, the effect of multiple access interference can be mitigated by choosing the spreading sequences with appropriate cross-correlation properties. The performance of the system is assessed and demonstrated in a multiuser environment by means of computer simulation with additive white Gaussian noise, Rayleigh fading, and selective fading channel conditions. These studies reveal that the proposed system significantly outperforms the Gold code DS/SS-BPSK system in synchronous channel conditions. In asynchronous case, the improvement is substantial for low signal-to-noise ratios.     

Forward-link soft-handoff in CDMA with multiple-antenna selection and fast joint power control

We consider forward-link soft-handoff with multiple antenna selection and fast joint power control at high data rates in a cellular code-division multiple-access network, where signals are directed to a mobile station (MS) from antennas located at the same or different base stations. The total power transmitted to any mobile is divided among the active antennas selected according to the momentary channel conditions so as to maximize the signal-interference ratio at each MS. Multiple-antenna selection is used to mitigate the effects of both short- and long-term fading, and achieve the best soft-handoff with respect to system capacity and complexity. To achieve capacity gains with soft-handoff, we derive optimum handoff thresholds corresponding to the optimum handoff region in different cell environments. Numerical results demonstrate that under high Doppler spread and large handoff-delay conditions, the proposed soft-handoff employing two transmit antennas and the optimum handoff threshold achieves a significant gain in microcell environments, but not in macrocell environments.     

Capacity of DS/CDMA communication systems with optimum spectraloverlap

We consider frequency overlapped DS/CDMA (FO/CDMA) communication systems where a number of DS/CDMA systems share frequency bands with adjacent systems. We analyze the multiple-access interference from adjacent systems with arbitrary amount of frequency overlap and compare the capacity of FO/CDMA with that of single wideband CDMA (WCDMA). The optimum amount of overlap increases with the number of overlapped systems. However the maximum capacity is obtained when the optimum number of systems are overlapped by half null-to-null bandwidth. It is shown that the FO/CDMA yields higher capacity in the case of relaxed bit error rate requirement and low decay rate of multipath intensity profile. Otherwise, WCDMA is superior     

DS/CDMA系统中的多用户检测技术

多用户检测技术可抑制多址干扰（ＭＡＩ）,从而提高ＣＤＭＡ系统的容量和性能。介绍了多用户检测器的分类和自适应算法、盲算法在多用户检测中的应用。