Interference suppression and diversity for CDMA systems

In code-division multiple-access (CDMA) systems, due to non-orthogonality of the spreading codes and multipath channels, the desired signal suffers interference from other users. Signal fading due to multipath propagation is another source of impairment in wireless CDMA systems, often severely impacting performance. In this dissertation, reduced-rank minimum mean square error (MMSE) receiver and reduced-rank minimum variance receiver are investigated to suppress interference; transmit diversity is applied to multicarrier CDMA (MC-CDMA) systems to combat fading; packet combing is studied to provide both interference suppression and diversity for CDMA random access systems. The reduced-rank MMSE receiver that uses a reduced-rank estimated covariance matrix is studied to improve the performance of MMSE receiver in CDMA systems. It is shown that the reduced-rank MMSE receiver has much better performance than the full-rank MMSE receiver when the covariance matrix is estimated by using a finite number of data samples and the desired signal is in a low dimensional subspace. It is also demonstrated that the reduced-rank minimum variance receiver outperforms the full-rank minimum variance receiver. The probability density function of the output SNR of the full-rank and reduced-rank linear MMSE estimators is derived for a general linear signal model under the assumption that the signals and noise are Gaussian distributed. Space-time coding that is originally proposed for narrow band systems is applied to an MC-CDMA system in order to get transmit diversity for such a wideband system. Some techniques to jointly decode the space-time code and suppress interference are developed. The channel estimation using either pilot channels or pilot symbols is studied for MC-CDMA systems with space-time coding. Performance of CDMA random access systems with packet combining in fading channels is analyzed. By combining the current retransmitted packet with all its previous transmitted copies, the receiver obtains a diversity gain plus an increased interference and noise suppression gain. Therefore, the bit error rate dramatically decreases with the number of transmissions increasing, which in turn improves the system throughput and reduces the average delay

MMSE multiuser detection for array multicarrier DS-CDMA in fading channels

Published versio

Multi-carrier CDMA using convolutional coding and interference cancellation

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN016251 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Performance of asynchronous orthogonal multicarrier CDMA system in frequency selective fading channel

An asynchronous multicarrier (MC) direct-sequence (DS) code-division multiple-access (CDMA) scheme for the uplink of the mobile communication system operating in a frequency selective fading channel is analyzed. Bit error rate performance of the system with either equal-gain combining or maximum-ratio combining is obtained. Numerical results indicate that the system performs better than that of the conventional DS-CDMA system and another MC-DS-CDMA system.published_or_final_versio

Performance of a space-time coded multicarrier CDMA system in frequency-selective Rayleigh channel.

Ph. D. University of KwaZulu-Natal, Durban 2014.The increasing demand for wireless services requires fast and robust broadband wireless communication for efficient utilisation of the scarce electromagnetic spectrum. One of the promising techniques for future wireless communication is the deployment of multi-input multi-output (MIMO) antenna system with orthogonal frequency division multiplexing (OFDM) coupled with multiple-access techniques. The combination of these techniques guarantees a much more reliable and robust transmission over the hostile wireless channel. This thesis investigates the performance of a multi-antenna space-time coded (STC) multi-carrier code-division multiple-access (MC-CDMA) system in a frequency-selective channel using Gold codes as spreading sequences. Spreading codes are known to be central to the performance of spread spectrum systems, STC MC-CDMA systems inclusive. Initial phase of this research work investigates multiple-access performance of spreading codes for the communication system. The performance of different sets of Gold codes for increasing number of interfering users for up to a thousand users and eight different code lengths, ranging from 31 to 4095-chip Gold codes, were considered. Simulation results show that odd-degree Gold codes give better bit-error-rate performance than even-degree Gold codes. Whereas the odd-degree codes exhibited relatively marginal loss in performance when the system was loaded, their even-degree counterparts degraded rapidly in performance, resulting in early emergence of an error floor, culminating in premature system saturation. Furthermore in this thesis, software simulations were carried to investigate the performance of a direct-sequence (DS) CDMA system in a flat-fading Rayleigh channel, and a multi-carrier (MC) CDMA system in a frequency-selective channel using different sets of Gold. The results showed that in a flat-fading channel, the Gold codes provide a constant coding gain close to that obtainable in a Gaussian channel. The results also showed that the impact of longer spreading codes was more pronounced for the MC-CDMA system in a frequency-selective channel as indicated by significant lowering of error floors. Also, frequency diversity associated with the use of longer codes coupled with multi-carrier modulation makes the MC-CDMA system resilient to multi-path effects. Further still, this thesis investigated the performance of a space-time block-coded (STBC) CDMA system in a flat-fading channel. Results showed that at low signal-to-noise ratio, the coding gain provided by the codes surpasses the diversity advantage provided by the use of the multiple antennas. The results also showed that coding gain between no-diversity link and its Gold-coded counterpart is the same as that between the transmit-diversity link and its Gold–coded counterpart. The independence of the diversity advantage provided by multiple transmit antennas and the coding gain obtainable from the use of the spreading sequences enables the prediction of the performance of composite space-time block-coded CDMA systems. Performance of a STBC OFDM system as well as a STBC MC-CDMA system in frequency-selective channel was also investigated. Results showed that the combination of diversity gain from the use of multiple antennas, coupled with coding gain provided by the Gold codes of the CDMA system, plus the diversity gain resulting from frequency diversity of multi-carrier transmission and the spectrum-spreading by the CDMA makes the composite STBC MC-CDMA system resilient to channel fading. This fact is particularly the case for long codes. For example, with reference to the OFDM transmission, the results showed that a 511-chip Gold-coded STC MC-CDMA system provided a factor of about 3,786 reduction in error floor

OFCDM systems over fading channels

Along with the fast growing demand of information exchange, telecommunication systems are required to provide fast and reliable service to high-data-rate applications such as video conference, real-time broadcasting, and on-line gaming. In downlink transmission, orthogonal frequency and code division multiplexing (OFCDM) has been an attractive technique for high-data-rate applications. With two-dimensional spreading, in both time domain and frequency domain, OFCDM achieves diversity gains in multiuser scenarios. Moreover, the adjustable spreading factors (SF) give OFCDM systems the flexibility in transmission rate and diversity gain. In this thesis, we focus on the downlink of OFCDM communication systems. The performance of OFCDM systems is investigated over Ricean fading channels with Rayleigh fading as special case. Code division multiple access (CDMA) technique is used to support multiuser communications, where users can transmit at the same time using the same frequency with the help of code sequences. We compare different combining methods that are employed to achieve diversity gain. Moreover, channel correlation is examined to see its effect on the system performance. We also propose to combine multiple-input and multiple-output (MIMO) techniques, specifically space-time block coding (STBC), with OFCDM systems. By adding spatial diversity, a MIMO system can provide more reliable transmission compared to a single-input and single-output (SISO) system. The space-time scheme used in our study is Alamouti scheme [1], which employs N = 2 and M antennas at the transmitter side and receiver side respectively. In the thesis, we explain the system structure, transmission and detection methods, and system performance of such MIMO-OFCDM systems. In our study, the expressions of system bit error rate (BER) are considered under the condition that no multi-code interference (MCI) is present. The accuracy of the BER expressions is verified when compared with the simulated ones for both SISO and MIMO-OFCDM systems with different combining methods. These comparisons are carried over different channels and with different system parameters to explore the benefits of OFCDM based systems. Both analytical and simulation results show the large diversity gains achieved when incorporating STBC with OFCDM

Adaptive parallel interference cancellation receivers with diversity combining for multicarrier DS CDMA systems

Master'sMASTER OF ENGINEERIN