Complementary Coded Code-division Multiple Access Research Articles

Complementary Coded Identical Code Cyclic Shift Multiple Access Under Asynchronous Frequency-Selective Fading Channels

A complementary coded code-division multiple-access (CC-CDMA) system is built on the direct sequence (DS) spread spectrum technique, which can suppress multipath interferences and multiple-access interferences due to the ideal correlation property of complementary codes (CCs). However, the DS spread spectrum technique will cause poor bandwidth efficiency; the CC-CDMA system suffers from serious interferences due to the ideal correlation of CCs may not be reconstructed at the receiver after passing through asynchronous frequency selective fading channels. In this paper, we propose a complementary coded identical code cyclic shift multiple access (CC-ICCSMA) system under asynchronous frequency selective fading channels, which allows multiple data streams to transmit in parallel by sharing one spreading code for improving the bandwidth efficiency. Considering the impact of interferences, we propose an ICCS matched filterbank to tackle the interferences, which can decode the data under all paths and achieve a high multipath diversity gain. Simulation results show that the proposed CC-ICCSMA system can achieve higher bandwidth efficiency than that of traditional CC-CDMA and can outperform the traditional DS-CDMA schemes in terms of bit-error rates and transmission rates.

Space Time Regularized Zero Forcing in Downlink Code Division Multiple Access Systems with Complementary Codes

Complementary coded code-division multiple access (CC-CDMA) has been originated as one of the extremely robust multiuser access technique in designing high data rate systems with frequency diversity for futuristic wireless communication. The integration of multiple-input multiple-output (MIMO) systems with CC-CDMA offers space diversity gain in addition to the achievable frequency diversity in CC-CDMA systems. However, the performance of conventional chip level space–time receiver in MIMO CC-CDMA is deteriorated due to multiple access interference (MAI) and spatial interference under frequency selective fading channels. A zero-forcing receiver used for minimizing MAI provides unsatisfactory performance in interference limited environments due to amplification of noise and hence the use of regularized zero-forcing (RZF) receiver is proposed and investigated in this study to achieve a space–time interference cancellation (STIC) system. Simulation results are performed to reveal the significance of RZF-STIC in achieving performance gain than conventional equalization schemes, and at the same time enhancing frequency diversity and spatial diversity gain with less complexity at all system loads.

Successive interference cancellation in asynchronous CC-CDMA systems under Rician fading channels

The unprecedented frequency diversity gain offered by complementary coded code-division multiple access (CC-CDMA) makes it more attractive than conventional CDMA systems in frequency selective fading channels. CC-CDMA loses its interference-resilient capability due to non-ideal correlation characteristics of complementary codes (CC) under uplink asynchronous communications. The existence of multiple access interference (MAI) in asynchronous systems results in drastic reduction in error rate performance in CC-CDMA systems. In this paper, the analysis of successive interference cancellation (SIC) is proposed for CC-CDMA systems as an alternative to existing non-interference cancellation schemes in uplink communication. The average bit error probability expressions for single branch and M-branch CC-CDMA systems operating under Rician fading channels are derived using alternate integral expression of Q-function. The accuracy of obtained closed form error probability expressions are confirmed through the proposed joint SIC with maximal ratio combining and minimum mean square error combining techniques in uplink CC-CDMA systems. The significance of soft-decision compared to hard decision SIC for CC-CDMA systems is also analyzed for both equal power and extreme near-far conditions. Furthermore, the simulation results exposed that significant performance improvement is obtained under near-far situations in uplink asynchronous CC-CDMA systems overcoming the effects of MAI and achieving improved system capacity.

Interference Cancellation in CDMA Systems Employing Complementary Codes Under Rician Fading Channels

The ideal correlation properties of single user code division multiple access (CDMA) systems with complementary codes (CC) are lost by multiple access interference and near–far effects in downlink frequency selective fading channels. This severity in interference has motivated to develop ways to overcome the threats with suitable suboptimal interference cancellation schemes. In this paper, we propose successive interference cancellation (SIC) for downlink CDMA systems with CC to improve the system capacity and reduce error rate under near–far situations. Theoretical study and extensive simulations were conducted to verify the effectiveness of proposed SIC under frequency selective Rician fading channels in achieving frequency diversity gain, close to theoretical lower bound in complementary coded CDMA (CC-CDMA) systems.

Pre-equalization Schemes for MIMO CC-CDMA Systems Over Frequency-Selective Fading Channels

Complementary coded code-division multiple access (CC-CDMA) system offers many attractive features over traditional CDMA systems due to its potential to provide ideal correlation properties and interference resistance feature in multiuser communication. However, the presence of multiple access interference (MAI) is one of the main drawbacks in reducing system performance under frequency-selective fading channels. In this paper, time domain pre-equalization (TDPE) is considered for downlink CC-CDMA systems to resist MAI over frequency-selective fading channels with perfect knowledge of channel state information. Pre-equalization shifts the complicated signal processing to the base station reducing the receiver complexity and improving the battery lifetime at the mobile station. We perform a comparison study between multiple-input multiple-output CC-CDMA (MIMO CC-CDMA) with pre-equalization and traditional equalization schemes towards overcoming MAI. The comparison study revealed that TDPE at the transmitter enriches diversity gain and reduces MAI in MIMO CC-CDMA systems with less receiver complexity.

Error Rate Analysis of SIMO-CDMA with Complementary Codes Under Multipath Fading Channels

Broadband communication systems of the current generation are likely to offer higher bit rates for delivering high-speed multimedia services to the end users. The achievable capacity and data rate of wireless communication systems are limited to fading channels varying with time, leading to multiple access interference (MAI) and multipath interference (MPI). In this paper, we investigate downlink single-input multiple output transmission for complementary coded code-division multiple access (CC-CDMA) systems working in channels with multipath fading. Here, parallel interference cancellation is employed for analysis of CC-CDMA with different frequency domain equalization schemes to eliminate MPI and MAI over multipath fading channels. Error rate analysis for CC-CDMA employing receiver diversity is assessed using simulations under varying channel parameters. Further, we compare different equalization schemes to show the superiority of regularized zero forcing in reducing the error rate of CC-CDMA systems.

Joint Pre-Equalization and Adaptive Combining for CC-CDMA Systems Over Asynchronous Frequency-Selective Fading Channels

Owing to ideal correlation properties of complementary codes (CCs), a complementary coded code-division multiple-access (CC-CDMA) system outperforms traditional CDMA systems in terms of its interference–resistance capabilities. However, it may lose its interference–resistance feature and suffer performance degradation over asynchronous frequency-selective fading channels due to the fact that it is incapable of reconstructing ideal complementary cross-correlation functions at a receiver. Based on the analysis on combining and detection algorithms designed for CC-CDMA systems, this paper proposes a joint pre-equalization and adaptive combining scheme. Pre-equalization is used at a transmitter to either enhance diversity gain or compensate channel selectivity leveraged by a controlling parameter $\zeta$ , whereas a recursive least square (RLS) adaptive combining algorithm is employed at a receiver to determine the optimal combining coefficients, aiming at minimizing detection errors under different channel conditions. Simulation results show the superiority of the proposed scheme in terms of bit error rate (BER) and its adaptability to varying channel states.

Performance of multicarrier complementary-coded CDMA under frequency-selective Nakagami-m fading channels

Multicarrier complementary-coded code division Multiple Access (MC CC-CDMA) is becoming an attractive multiple access technique for high data rate transmission in future wireless communication systems. MC CC-CDMA systems transmitting over frequency-selective channels suffer from multiple access interference (MAI) owing to non-ideal correlation properties of complementary codes (CC). MC CC-CDMA with equalization has recently gained much attention for its ability to offer an excellent performance than traditional systems in frequency-selective fading channels. In this paper, the authors present an analytical study and investigation of MC CC-CDMA downlink system using different combining schemes. The use of parallel interference cancellation (PIC) under frequency-selective Nakagami-m fading channels is also analyzed. A comparison among different combining schemes is provided to show the impact of PIC with minimum mean square error combining (MMSEC) and maximal ratio combining (MRC) on the performance of MC CC-CDMA system. The analytical and simulation results show that the combination of general combining schemes with PIC provides an efficient solution to suppress MAI in downlink MC CC-CDMA system than conventional MC-CDMA systems using Walsh codes under frequency-selective channels.

Multi-User Interference Cancellation in Complementary Coded CDMA with Diversity Gain

This letter analyzes the performance of complementary coded code division multiple access (CC-CDMA) systems in frequency selective fading channels. Based on the evaluation of various combining algorithms for the correlation functions of M element sequences of a CC, a per-carrier parallel interference cancellation scheme based on minimum mean square error combining (MMSEC) is proposed for CC-CDMA to achieve a frequency diversity gain and multi-user interference resistance. Both analytical and simulation results verify the superiority of the proposed scheme. It is shown that combining algorithms and correlation properties of element sequences have a significant impact on the performance of CC-CDMA systems in frequency selective fading channels.

Code-Hopping Multiple Access Based on Orthogonal Complementary Codes

Code-hopping multiple access (CHMA) is a newly emerging multiple-access technique with the potential to offer high capacity. Unfortunately, orthogonality among user signals in existing CHMA schemes can be preserved only in synchronous channels under an assumption that neither multipath interference (MI) nor multiple-access interference (MAI) exists. Exploiting their ideal orthogonality, we apply orthogonal complementary codes to CHMA systems to overcome problems with existing CHMA schemes. In particular, we will show that the application of orthogonal complementary codes can significantly improve the performance of a CHMA system due to its unique collision-resistant capability. To deal with destructive collisions among spreading codes in CHMA systems, convolutional coding is employed in complementary-coded CHMA systems. The properties and bit error rate (BER) performance of the proposed system are analyzed for both uplink and downlink applications, where the system may simultaneously suffer MI and MAI. Simulation results show that the complementary-coded CHMA with channel coding can provide much higher capacity than complementary-coded code-division multiple access (CDMA) systems in varying operational scenarios.