Multicode Direct-sequence Code-division Research Articles

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.

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.

Joint transmit/receive one-tap minimum mean square error frequency-domain equalisation for broadband multicode direct-sequence code division multiple access

Multicode direct-sequence code division multiple access (DS-CDMA) can flexibly support multimedia services with various data rates simply by changing the code multiplexing order. The use of simple one-tap frequency-domain equalisation (FDE) at a receiver is known to improve the bit error rate (BER) performance of multicode DS-CDMA in a severe frequency-selective fading environment. However, the BER performance improvement is limited due to the presence of the residual inter-chip interference (ICI). The authors propose a joint transmit/receive minimum mean square error (MMSE) FDE, which carries out one-tap transmit FDE and one-tap receive FDE jointly based on the MMSE criterion. The authors theoretically derive a suboptimal set of transmit and receive FDE weights and investigate the BER performance improvement by computer simulation in a freqeuncy-selective Rayleigh fading channel. The proposed scheme improves the received signal-to-interference plus noise ratio after despreading, and consequently, the BER performance can be significantly improved compared to the conventioal receive MMSE-FDE by (a) making the variations in the equivalent channel gain shallower to reduce the residual ICI for large code multiplexing order U and (b) allocating the transmit power to the frequencies having good condition to improve the received signal-to-noise ratio for a small U. This is conformed by the computer simualtion.

Successive Interference Cancelers for Multimedia Multicode DS-CDMA Systems Over Frequency-Selective Fading Channels

Noncoherent and coherent multicode direct-sequence code-division multiple access (DS-CDMA) systems with successive interference cancellation (SIC) for multimedia reverse links over frequency-selective fading channels are studied. Followed by a RAKE receiver, the SIC scheme is applied for combating the multiple access interference. The bit error rate (BER) using the SIC technique over Nakagami-m fading channels is derived. Simulation results show that the multicode DS-CDMA system with SIC has demonstrated better performance than that without SIC under the multipath fading environment, while their corresponding numerical results from performance analyses are also provided for verifications. Furthermore, the coherent receiver could achieve a more satisfactory BER than the noncoherent counterpart at the expense of synchronization.

Performance of Multicode DS/CDMA With Noncoherent $M$ -ary Orthogonal Modulation in the Presence of Timing Errors

This paper derives an accurate approximation to the bit error rate (BER) of multicode direct-sequence code-division multiple access (DS/CDMA) with noncoherent <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -ary modulation in wideband fading channels when timing errors are made at the receiver employing equal-gain combining (EGC). This reflects the practical scenario where the path delays are estimated imperfectly, leading to synchronization errors between the correlation receivers and the received signals. The analysis can be applied to any type of a fading distribution for both independent and correlated diversity branches. It is shown that the derived analytical expressions are in close agreement with the Monte Carlo system simulations, particularly in the case of small timing errors.

Iterative Overlap FDE for Multicode DS-CDMA

Recently, a new frequency-domain equalization (FDE) technique, called overlap FDE, that requires no GI insertion was proposed. However, the residual inter/intra-block interference (IBI) cannot completely be removed. In addition to this, for multicode direct sequence code division multiple access (DS-CDMA), the presence of residual interchip interference (ICI) after FDE distorts orthogonality among the spreading codes. In this paper, we propose an iterative overlap FDE for multicode DS-CDMA to suppress both the residual IBI and the residual ICI. In the iterative overlap FDE, joint minimum mean square error (MMSE)-FDE and ICI cancellation is repeated a sufficient number of times. The bit error rate (BER) performance with the iterative overlap FDE is evaluated by computer simulation.

Frequency-Domain Interchip Interference Cancelation for DS-CDMA Downlink Transmission

The use of frequency-domain equalization (FDE) based on minimum-mean-square-error criterion can significantly improve the bit-error-rate (BER) performance of orthogonal multicode direct-sequence code-division multiple access downlink signal transmission in a frequency-selective fading channel. However, the presence of residual interchip interference (ICI) after FDE produces the orthogonality distortion among the spreading codes, and the BER performance degrades as the number of multiplex order increases. In this paper, we propose a frequency-domain ICI cancellation scheme, in which the residual ICI replica in the frequency domain is generated and subtracted from each frequency component of the received signal after FDE. Three types of ICI cancelation scheme are presented, and the effectiveness of the proposed ICI cancelation scheme is confirmed by computer simulation

Space-Time Cyclic Delay Transmit Diversity for a Multi-Code DS-CDMA Signal with Frequency-Domain Equalization

Frequency-domain equalization (FDE) can take advantage of the frequency-selectivity of the channel to improve the transmission performance in a frequency selective fading channel. To further improve the transmission performance, the transmit diversity technique can be used. Cyclic delay transmit diversity (CDTD) can strengthen the frequency-selectivity while space-time transmit diversity (STTD) can achieve the antenna diversity gain. In this paper, we propose a 4-antenna space-time cyclic delay transmit diversity (STCDTD), which is a combination of 2-antenna STTD and 2-antenna CDTD schemes, for orthogonal multi-code direct sequence code division multiple access (DS-CDMA) using FDE. We evaluate the BER performance and the throughput performance by computer simulation and compare them with the original CDTD and STTD schemes.

Precise analysis of bit-error probability for asynchronous multicode DS-CDMA systems

A precise bit-error probability (BEP) analysis method is derived for a multicode direct-sequence code-division multiple-access (DS-CDMA) system in an additive white Gaussian noise channel. The method is applicable to a multicode DS-CDMA system with an arbitrary number of multiple code sequences, and any selection of multiple code sequences. The precise method gives results that discriminate the effect of the selection of different multiple code sequences on the BEP, whereas Gaussian approximations (GAs) do not. Thus, the new method can be used to select the best multicode set for a given system, a task that cannot be achieved using GAs. A two-step analytical procedure enables deriving an explicit, compact form for the CF of the receiver decision statistic in a DS-CDMA system with an arbitrary number of multiple code sequences, and for any selection of multiple code sequences.

Adaptive Selection/Maximal-Ratio Combining Based on Error Detection of Multidimensional Multicode DS-CDMA

A novel two-dimensional diversity combining is proposed for the uplink in a single cell which employs multidimensional multicode direct-sequence code-division multiple-access signaling and two receive antennas. First, the signaling is combined with precoding to obtain a constant envelope signal that is suitable for the uplink, and the resulting error detection is applied to the diversity combining. Based on the error detection, an adaptive selection-combining/maximal-ratio combining (SC/MRC) is performed, for which initial data detection is made by the SC to avoid the combining loss of the very noisy paths. When the initial SC is unreliable (indicated by the error detection), the MRC is attempted to fully exploit the space and multipath diversity. The adaptive SC/MRC is generalized to further increase the diversity gain over the MRC, offered by the error-detection capability. Through analysis and simulation results, it is shown that the adaptive SC/MRC and its generalized diversity receiver outperforms the other schemes in terms of the symbol-error rate, and also its bit-error rate can be lower than that of the variable spreading factor scheme using a single code.

Performance of Multicode DS/CDMA With Noncoherent&amp;gt;tex&amp;lt;$M$&amp;gt;/tex&amp;lt;-ary Orthogonal Modulation in Multipath Fading Channels

This paper presents the performance analysis and simulation of a multicode direct-sequence code-division multiple-access system with noncoherent M-ary modulation, in a multipath fading environment. This type of transceiver is specified for the reverse link of the IS-95B and cdma2000 (radio configurations 1 and 2) systems and is intended to serve high-rate applications such as data transfer and video communications. While previous studies considered the analytical error performance of coherent multicode systems, little attention has been devoted in the literature to the noncoherent case. We provide concise and useful expressions for the interference terms as a function of the commonly used aperiodic cross correlation functions. After a statistical characterization of these terms, we make use of the Gaussian approximation (GA) in order to obtain the bit-error rate (BER). However, unlike some other analyses (for coherent detection) relying on the GA, in our derivation, we take into account the fact that all the codes transmitted by a mobile user fade in unison. As demonstrated via computer simulations, this fact is crucial to obtain a reliable estimate of the BER, especially when equal-gain combining (EGC) is used at the receiver. The analysis is also extended to include a simple closed-loop power control algorithm and hard handoff between multiple cells. In particular, we verify-for the multicode case-previous observations that the use of EGC allows improvement only for a certain range of values of the total interference seen at the receiver: When either the number of interfering users is too large, or too many codes are assigned to the high-rate user, the noncoherent combining loss becomes such that the use of many diversity branches can decrease the performance as compared to a system with little or no diversity.

Spatial filtering for pilot-aided WCDMA systems: A semi-blind subspace approach

This paper proposes a spatial filtering technique for the reception of pilot-aided multirate multicode direct-sequence code division multiple access (DS/CDMA) systems such as wideband CDMA (WCDMA). These systems introduce a code-multiplexed pilot sequence that can be used for the estimation of the filter weights, but the presence of the traffic signal (transmitted at the same time as the pilot sequence) corrupts that estimation and degrades the performance of the filter significantly. This is caused by the fact that although the traffic and pilot signals are usually designed to be orthogonal, the frequency selectivity of the channel degrades this orthogonality at the receiving end. Here, we propose a semi-blind technique that eliminates the self-noise caused by the code-multiplexing of the pilot. We derive analytically the asymptotic performance of both the training-only and the semi-blind techniques and compare them with the actual simulated performance. It is shown, both analytically and via simulation, that high gains can be achieved with respect to training-only-based techniques.

Transmitter adaptation in multicode DS-CDMA systems

The problem of transmitter adaptation in the form of adapting the spreading sequences and the transmission powers of different users for a multicode direct-sequence code division multiple access (DS-CDMA) system is considered. Particular attention is given to a distributed algorithm, which updates each pair of transmitter and receiver without information from other users. The transmitter adaptation problem and the algorithm are studied from the viewpoint of a single user, as well as the viewpoint of the whole system. The algorithm is shown to give either the optimal sequences or a choice of sequences that is close to the optimal one. Simulation results show that major improvement in performance can be obtained with the proposed transmission adaptation scheme. The effect of restricting the choice of sequences to polyphase sequences is also considered.

Uplink performance evaluation of multicode DS/CDMA systems in the presence of nonlinear distortions

This paper investigates the behavior of multicode direct sequence code division multiple access (DS/CDMA) systems in the presence of nonlinear distortions caused by high power amplifiers. The performance of the multicode system can be degraded due to a large variation in the envelope of the multicode signal. A technique that uses a small number of parallel transmitted sequences is proposed for combating the nonlinearity. This technique ran be modified to have a constant envelope for achieving further performance improvement. Comprehensive comparisons are made for different systems, based on both theoretical analysis and computer simulation. Our study shows that the technique employing a few parallel transmitted sequences, in conjunction with quasi-ML (maximum likelihood) detection, and the use of the modified scheme, are very effective for combating the nonlinearity.

Precoding for Minimization of Envelope Variations in Multicode DS-CDMA Systems

A multicode direct-sequence code-division multiple-access system experiences large envelope variations as a result of a sum of many independently spread signals. However, large envelope variations is problematic because it reduces the spectral efficiency, the efficiency of the power amplifiers and the performance. All these effects depend on the non-linear amplifiers that generally are used in handsets. It would of course be possible to use a linear amplifier but then the power efficiency is drastically reduced. In this paper we analyze the envelope variations of a multicode signal in terms of the crest factor and find that it increases as the square root of the number of used codes. As a consequence it is only possible to use the multicode scheme for a few parallel codes. To reduce the envelope variations a precoder is introduced. This precoder is a non-linear high-rate block code especially designed for the set of spreading codes used. However, the precoder can be made independent of the spreading codes if a user-specific spreading code is concatenated with a set of Hadamard or Conference sequences. The resulting spreadig codes are orthogonal. Also, the precoder is independent of the user-specific spreading code, and can thus be used for all users. After precoding the crest factor is significantly reduced and the performance, due to introduced coding gain, improved. Algorithms for the design of precoders with both reduced envelope variation and good performance are presented. Furthermore, simulations show that a precoded multicode system outperforms an uncoded multicode system in a single-user as well as in a multiuser environment.

Rate-compatible convolutional codes for multirate DS-CDMA systems

New rate-compatible convolutional (RCC) codes with high constraint lengths and a wide range of code rates are presented. These new codes originate from rate 1/4 optimum distance spectrum (ODS) convolutional parent encoders with constraint lengths 7-10. Low rate encoders (rates 115 down to 1/10) are found by a nested search, and high rate encoders (rates above 1/4) are found by rate-compatible puncturing. The new codes form rate-compatible code families more powerful and flexible than those previously presented. It is shown that these codes are almost as good as the existing optimum convolutional codes of the same fates. The effects of varying the design parameters of the rate-compatible punctured convolutional (RCPC) codes, i.e., the parent encoder rate, the puncturing period, and the constraint length, are also examined. The new codes are then applied to a multicode direct-sequence code-division multiple-access (DS-CDMA) system and are shown to provide good performance and rate-matching capabilities. The results, which are evaluated in terms of the efficiency for Gaussian and Rayleigh fading channels, show that the system efficiency increases with decreasing code rate.