Asynchronous CDMA Systems Research Articles

Weyl Spreading Sequence Optimizing CDMA

This paper shows an optimal spreading sequence in the Weyl sequence class, which is similar to the set of the Oppermann sequences for asynchronous CDMA systems. Sequences in Weyl sequence class have the desired property that the order of cross-correlation is low. Therefore, sequences in the Weyl sequence class are expected to minimize the inter-symbol interference. We evaluate the upper bound of cross-correlation and odd cross-correlation of spreading sequences in the Weyl sequence class and construct the optimization problem: minimize the upper bound of the absolute values of cross-correlation and odd cross-correlation. Since our optimization problem is convex, we can derive the optimal spreading sequences as the global solution of the problem. We show their signal to interference plus noise ratio (SINR) in a special case. From this result, we propose how the initial elements are assigned, that is, how spreading sequences are assigned to each users. In an asynchronous CDMA system, we also numerically compare our spreading sequences with other ones, the Gold codes, the Oppermann sequences, the optimal Chebyshev spreading sequences and the SP sequences in Bit Error Rate. Our spreading sequence, which yields the global solution, has the highest performance among the other spreading sequences tested.

Adaptive Power Control for A-CDMA Systems Over K-Distributed Fading Channels

Energy adaptation for Asynchronous CDMA (A-CDMA) systems over Rayleigh and Rician fading distributions with and without diversity combining techniques has been discussed in literature. In this work, adaptive power control for A-CDMA systems over channels undergoing K-distribution is discussed in detail. K-distribution is a valid model to represent the envelope statistics of the backscattered echo, and is used to simulate the effects of a dense urban environment with the average received power level fluctuating slowly due to shadowing effects. Performance measures, such as probability density and cumulative distribution functions of (a) probability of error and (b) signal energy-to-noise ratio and, system parameters, such as fading margin, mean transmitter energy gain, and percentage points are computed for K-distribution channels as a function of probability of error specification and the probability of unsatisfactory operation.

The Autocorrelation Magnitude of Balanced Binary Sequence Pairs of Prime Period <inline-formula> <tex-math notation="TeX">$N\equiv 1(\bmod\ 4)$</tex-math></inline-formula> With Optimal Cross-Correlation

In this paper, for balanced binary sequence pairs (BBSPs for short) of period N ≡ 1(mod, 4), the optimality of their cross-correlation is determined. Next, a lower bound on the maximal out-of-phase autocorrelation magnitude is established for BBSPs of period N ≡ 1(mod, 4) with optimal cross-correlation. Furthermore, BBSPs with optimal cross-correlation whose maximal out-of-phase autocorrelation magnitude achieves the new lower bound are constructed based on the cyclotomy of order 4. These proposed sequences have a periodic total squared correlation value meeting the Ganapathy-Pados-Karystinos bound, and are therefore an option for use in asynchronous CDMA systems.

Performance Analysis of Two-Level Asynchronous Optical CDMA Systems Utilizing Wrapped OPPM

In this paper the performance and the capacity of two-level asynchronous incoherent optical CDMA (OCDMA) systems employing wrapped overlapping pulse position modulation (WOPPM) and optical AND logic gate receiver are analyzed. These analyses are performed by considering the multiple access interference as the main limiting factor. Because of the power limitation in some laser sources, the same-symbol-power scheme for all users is assumed. Optical orthogonal codes with the same length are assigned to the users of each class. Considering the throughput as constraint, the effects of the number of users in each class and the weight of low-weight users on the performance of each class are examined and the results are compared with the same-mark-power case. Finally, the effect of WOPPM signaling parameters such as overlapping index and total number of symbols on the error probability of each class is investigated in the presence and in the absence of throughput constraint. Our results show that when the multi-level technique is employed in WOPPM OCDMA systems, one can find solutions for some design problems that cannot be handled by conventional one-level methods. In addition to this benefit, employing this technique leads to a less complex transceiver structure.

Capacity Analysis of Asynchronous Optical CDMA Systems Utilizing Wrapped OPPM: Multiple Access Interference Limited Case

In this paper the capacity of asynchronous incoherent optical CDMA (OCDMA) systems employing wrapped overlapping pulse position modulation (WOPPM) for two types of receiver structures is analyzed. The analysis is performed in the presence of multiple access interference as the main limiting factor. Throughput analysis for uncoded systems utilizing WOPPM is conducted and a lower bound on the capacity for two types of correlator based receiver structures is derived. Furthermore, for a fixed code and a fixed maximum number of symbols, a numerical solution for maximizing the capacity is presented. Moreover, to minimize the probability of error when the throughput and the number of users are fixed, a numerical solution is introduced. Finally, for two common design problems, algorithms are proposed for achieving the system parameters well close to the optimal settings. The results of our different analyses for uncoded systems using WOPPM have been compared with unwrapped OPPM. Our results have also been compared with on-off keying (OOK) and pulse position modulation (PPM). The results of this work show that, for a fixed throughput, the performance of WOPPM is significantly superior to OOK and PPM.

Performance Analysis of Subspace Based Downlink Channel Estimation for W-CDMA Systems Using Chaotic Codes

This paper presents blind channel estimation for downlink W-CDMA system that employs chaotic codes and Walsh codes for spreading information bits of the multiple users. In a W-CDMA system, while transmitting over multipath channels, both intersymbol interference (ISI) as a result of Inter Chip Interference and multiple access interference (MAI) cannot be easily eliminated. Although it is possible to design multiuser detectors that suppress MAI and ISI, these detectors often require explicit knowledge of at least the desired users' signature waveform. Earlier work focused on a subspace based channel estimation algorithm for asynchronous CDMA systems to estimate the multiple users' symbols, where only AWGN channel was considered. In our work, we study a similar subspace-based signature waveform estimation algorithm for downlink W-CDMA systems, which use chaotic codes instead of pseudo random codes, that provide estimates of the multiuser channel by exploiting structural information of the data output at the base station. In particular, we show that the subspace of the (data+noise) matrix contains sufficient information for unique determination of channels, and hence, the signature waveforms and signal constellation. We consider Rayleigh and Rician fading channel model to quantify the multipath channel effects. Performance measures like bit error rate and root mean square error are plotted for both chaotic codes and Walsh codes under Rayleigh and Rician fading channels.

Performance Analysis of Optical CDMA Systems Utilizing Optical Encoding in Presence of Interference and Receiver Noises

Abstract In this paper, performance of incoherent and asynchronous optical CDMA (OCDMA) systems employing “optical encoding” is investigated and the results are compared with the results obtained when the encoding is performed in electrical domain. These investigations are performed for receiver structures that employ a single hard limiter (SHL), a double optical hard limiter (DHL) and also for the structures without a hard limiter (WHL). In addition, “electrical encoding” OCDMA systems using DHL in their receiver structures are investigated. In this work, conventional optical orthogonal codes are used and Gaussian approximation is assumed for the output of APD. Besides interference, other destructive effects such as APD and thermal noises are also taken into account. It is shown that when the number of users N is equal to or greater than pulsed laser's modulation extinction ratio Me , for all receiver structures (SHL, DHL and WHL) “optical encoding” systems are superior to their “electrical encoding” counterparts and this superiority becomes much more apparent in SHL and DHL receivers. We will also show that when N < Me , “optical encoding” WHL systems outperform their “electrical encoding” counterparts. Our results also indicate that the performance of “optical encoding” SHL systems approaches the performance of DHL systems as the transmitted power increases.

Subspace based downlink channel estimation for W-CDMA systems

Wideband code division multiple access (W-CDMA) is a third generation (3G) mobile wireless technology that promises much higher data speeds for mobile and portable wireless devices than those which are commonly offered in todayʼs market. In W-CDMA systems transmitting over multipath channels, both Inter Symbol Interference (ISI) as a result of Inter Chip Interference (ICI) and Multiple Access Interference (MAI) cannot be easily eliminated. Although it is possible to design multiuser detectors that suppress MAI and ISI, these detectors often require explicit knowledge of at least the desired usersʼ signature waveform. Torlak and Xu (1997) [1] proposed a subspace based channel estimation algorithm for Asynchronous CDMA (A-CDMA) systems to estimate the multiple usersʼ symbols. In our work, we study a similar subspace based channel estimation scheme for downlink W-CDMA systems that provide estimates of the subchannels of multiple users by exploiting the structural information of the data output. In particular, we show that the subspace of the (data + noise) matrix contains sufficient information for unique determination of channels, and hence, the signature waveforms and signal constellation. Performance measures like Bit Error Rate (BER) and Root Mean Square Error (RMSE) are plotted for Rayleigh and Rician fading channels.

Performance analysis of downlink W-CDMA systems in Weibull, Lognormal and Nakagami fading channels

Wideband Code Division Multiple Access (W-CDMA) is a third generation (3G) mobile wireless technology that promises much higher data speeds to mobile and portable wireless devices than those which are commonly offered in todayʼs market. In W-CDMA systems transmitting over multipath channels, both Inter Symbol Interference (ISI) as a result of Inter Chip interference (ICI) and Multiple Access Interference (MAI) cannot be easily eliminated. Although it is possible to design multiuser detectors that suppress MAI and ISI, these detectors often require explicit knowledge of at least the desired usersʼ signature waveform. Torlak and Xu proposed a blind estimation algorithm for Asynchronous CDMA (A-CDMA) systems to estimate the multiuser channels (Torlak and Xu, 1997 [1]). In our work, we study a similar blind channel estimation scheme for downlink W-CDMA systems that provides estimates of the multiuser channels by exploiting the structural information of the data output. In particular, we show that the subspace of the (data + noise) matrix contains sufficient information for unique determination of channels, and hence, the signature waveforms and signal constellations. Performance measures like Bit Error Rate (BER) and Root Mean Square Error (RMSE) are plotted for the Weibull and Lognormal fading channels. Estimated signal constellations under the Weibull, Lognormal and Nakagami fading channels are also plotted.

Blind Adaptive Receiver in Asynchronous CDMA Systems Based on Pre-processing Recursive Least Squares Constant Modulus Algorithm

该文提出一种基于预处理递归最小二乘恒模算法(PP-RLSCMA)的多径异步CDMA系统盲自适应接收技术。首先对接收信号进行自适应预处理，并分析了预处理器的复杂性和稳定性。预处理的目的是通过对干扰和噪声的部分抑制以提高恒模接收的性能，所提出的预处理方法只与多径信道的最大长度有关。鉴于统计恒模算法收敛速度慢的缺点，提出一种快速递归最小二乘恒模算法的盲自适应接收。仿真表明，该文算法的误码率及收敛性能比LCMMV，LCCMA好。

Population Declining Ant Colony Optimization Multiuser Detection in Asynchronous CDMA Communications

In this paper we present a population declining ant colony optimization (PDACO) multiuser detector for asynchronous CDMA communications. Ant colony optimization (ACO) algorithms have already been used in multiuser detection in CDMA systems; however, as the pheromone accumulates, we may not get a global optimum because it stops searching early. PDACO can enlarge searching range through increasing the initial population of the ant colony, and the population declines in successive iterations. So, the performance of PDACO is superior with the same computational complexity. PDACO is applied to multiuser detection in asynchronous CDMA systems over slowly multipath Rayleigh-fading channels in this paper. Via computer simulations it is shown that the performance of PDACO detector is much better in bit-error rate and near-far effect resistance than conventional detector, ACO detector and genetic algorithm detector, and is close the optimal multiuser detector.

SINR-maximizing spreading code allocation for non-linear serial interference cancellation

This paper considers the issue of spreading code allocation and receiver optimization in a CDMA-based multiuser data network. Assuming that non-linear serial interference cancellation is performed at the receiver, the problem of noncooperative SINR maximization with respect to each user's spreading code and uplink receiver is first considered. It is shown that iterative SINR maximization converges in a finite number of steps, and that the equilibrium resulting from noncooperative behavior is also Pareto-optimal. Next, the problem of optimal multiuser detection for a system using the above optimal spreading codes is also considered, and it is shown that, for such a system, maximum likelihood detection entails a computational complexity that is exponential in the ratio between the number of users and the system processing gain, thus providing huge savings with respect to the general case wherein the optimal receiver has a complexity exponential in the number of users. While initially considering a simple frequencyflat synchronous CDMA system, the obtained results are then extended to more general scenarios, such as asynchronous CDMA systems, multipath CDMA channels, and multiuser multiantenna systems. Finally, the issue of non-cooperative power minimization and choice of the spreading code subject to a QoS constraints is also considered, and comparisons with the corresponding cooperative solution are also discussed. Numerical results corroborate the validity of the theoretical analysis, and show that the use of non-linear interference cancellation coupled with waveform adaptation brings remarkable performance gains with respect to linear multiuser receivers.

A modified unscented filter for delay tracking in asynchronous CDMA systems

In this paper, we present a modification on delay estimation and tracking using the unscented filter for an asynchronous CDMA system. The algorithm is based on utilizing the minimum excess mean square error criteria (MEMSEC) in deriving the filter update equation. Computer simulations are presented to illustrate the new filter's performance. More specifically, the delay estimation errors for multipath multiuser CDMA systems are evaluated using the unscented and the proposed modified unscented filters. The results show that the modified unscented filter tracks the delay variations with less errors than the unscented filter.

BER analysis of space–time diversity in CDMA systems over frequency-selective fading channels

The performance of direct-sequence code division multiple access (DS-CDMA) using space-time spreading system, over frequency-selective fading channels, is investigated. The underlying transmit diversity scheme, previously introduced in the literature, is based on two transmit and one receive antenna. It was shown that when employed in flat fast-fading channels, the received signal quality can be improved by utilising the spatial and temporal diversities at the receiver side. We study the problem of multiuser interference in asynchronous CDMA systems that employ transmit/receive diversity using space-time spreading. To overcome the effects of interference, a decorrelator detector is used at the base station. Considering binary phase-shift keying transmission, we analyse the system performance in terms of its probability of bit error. In particular, we derive the probability of error over frequency-selective Rayleigh fading channels for both fast and slow-fading channels. For the fast-fading channel, both simulations and analytical results show that the full system diversity is achieved. On the other hand, when considering a slow-fading channel, we show that the scheme reduces to conventional space-time spreading schemes where the diversity order is half of that of fast-fading.

Performance of convolutional codes in asynchronous CDMA communications under imperfect phase-tracking conditions

In this paper, the performance of short constraint length convolutional codes in imperfect phase tracking conditions is discussed. Convolutional codes are used to provide error protection to a particular user in an Asynchronous CDMA (A- CDMA) system undergoing slow Rician fading. Maximum likelihood decoding with a Viterbi algorithm recovers the information symbols. A Phase Locked Loop (PLL) depicting the variations in the phase error for abrupt phase changes in the input signal is considered. Analytical bounds, which are useful in predicting the performance of the A-CDMA system are derived and plotted for the cases of infinite and finite channel memory. The upper bounds on the unconditional Bit Error Rate (BER) with Viterbi decoding are derived and plotted for the various convolutional codes considered. The simulated BERs are found to agree well with their upper bounds.

BLIND PARALIND SPACE-TIME MULTIUSER DETECTION FOR ASYNCHRONOUS CDMA SYSTEM

This paper links the asynchronous CDMA system signal detection problem to the parallel profiles with linear dependencies (PARALIND), and derives a deterministic blind PARALIND algorithm whose performance is close to nonblind space-time minimum mean square error method. The proposed algorithm has the better performance than space-time matched filter, supports small sample sizes, and even works well in array error condition. Most notably, it does not require knowledge of the direction of arrival and channel fading information.

MULTIUSER INTERFERENCE SUPPRESSION TECHNIQUES FOR DIRECT-SEQUENCE CDMA COMMUNICATION SYSTEMS

In this paper, several techniques for multi-user interference suppression from power-controlled direct-sequence CDMA systems are proposed and the emphasis is on the one-shot structure. The proposed schemes arise from the optimum detector that takes into account the existence of power-controlled chip-synchronous multi-user interference and white Gaussian noise. Restricting the multi-user interference to the power-controlled chip-synchronous case strongly reduces the complexity of the optimum detector. For more reduction in the complexity, some approximations of the obtained techniques are also considered. It is shown that in asynchronous CDMA systems and in the presence of white Gaussian noise, the proposed detectors outperform the correlator with comparable complexity

Trellis ternary line coding scheme for asynchronous optical CDMA systems

In this paper, we investigate the employment of a ternary line coding technique based on Ungerboeck's trellis-coded method in asynchronous optical CDMA systems. The ternary coding we use is predicated upon the equal-weight orthogonal (EWO) scheme. Each user transmits two mutually orthogonal signature sequences to represent “+1” and “−1”, respectively, and nothing is transmitted for “0”. The receiver employs a maximum-likelihood soft-decoder to select the path with minimum Euclidean distance as the preferred path. This trellis ternary coding scheme applies set partitioning with partially overlapping subsets to increase the free Euclidean distance, which considerably improves system performance. Furthermore, due to line coding technique, such scheme comprises sufficient clock information, and thus benefits for baseband timing extraction (i.e. clock recovery). Numerical results reveal that the proposed trellis ternary coding scheme can significantly reduce the error floor and allow more active users to be accommodated in the network.

Error Probability Distribution and Density Functions for Rayleigh and Rician Fading Channels with Diversity

This paper analyzes the distribution and density functions of the probability of error for Rayleigh and Rician fading channels with diversity. An expression for the signal-to-noise ratio is derived for an asynchronous CDMA (A-CDMA) system with diversity. The error probability distribution and density functions are derived and plotted for different mean energy-to-noise ratios.

Joint Iterative Multiuser Detection and Channel Estimation for Differentially Coded Asynchronous CDMA Systems

In this paper, we propose a receiver structure that performs joint iterative multiuser detection and channel estimation for the uplink of differentially coded asynchronous direct-sequence code-division multiple-access systems in unknown multipath channels. The overall transmission scheme for each user is the serial concatenation of a recursive systematic convolutional (RSC) encoder and a differential encoder separated by an interleaver. The proposed receiver consists of two stages. The first stage performs channel estimation, soft interference cancellation, and soft-input soft-output multiuser filtering, followed by the second stage, which consists of a bank of serially concatenated single-user channel decoders and differential decoders. The single-user iterative decoder for each user consists of a powerful combination of an RSC decoder and a differential decoder, which work together in an iterative fashion. In terms of channel estimation, at the first iteration, the multipath channel is blindly estimated by exploiting the orthogonality property between the signal and noise subspaces, whereas, for the next iterations, the channel is estimated using the soft estimates of coded bits of each user provided by the single-user iterative decoders in conjunction with the output of the multiuser detector. By exchanging soft information between the different stages, the receiver performance is improved through iteration. Simulation results demonstrate that the performance of the proposed iterative multiuser detector with the integrated channel estimator approaches the single-user bound at high signal-to-noise ratio.