Signature Quantization in Fading CDMA With Limited Feedback

In this work, we analyze the performance of a signature quantization scheme for reverse-link Direct Sequence (DS)- Code Division Multiple Access (CDMA). Assuming perfect estimates of the channel and interference covariance, the receiver selects the signature that minimizes interference power or maximizes signal-to-interference plus noise ratio (SINR) for a desired user from a signature codebook. The codebook index corresponding to the optimal signature is then relayed to the user with a finite number of bits via a feedback channel. Here we are interested in the performance of a Random Vector Quantization (RVQ) codebook, which contains independent isotropically distributed vectors. Assuming arbitrary transmit power allocation, we consider additive white Gaussian noise (AWGN) channel first with no fading and subsequently, with multipath fading. We derive the corresponding SINR in a large system limit at the output of matched filter and linear minimum mean squared error (MMSE) receiver. Numerical examples show that the derived large system results give a good approximation to the performance of finite-size system and that the MMSE receiver achieves close to a single-user performance with only one feedback bit per signature element

On rate capacity and signature sequence adaptation in downlink of MC-CDMA system

This dissertation addresses two topics in the MC-CDMA system: rate capacity and adaptation of users\u27 signature sequences. Both of them are studied for the downlink communication scenario with multi-code scheme. The purpose of studying rate capacity is to understand the potential of applying MC-CDMA technique for high speed wireless data communications. It is shown that, to maintain high speed data transmission with multi-code scheme, each mobile should cooperatively decode its desired user\u27s encoded data symbols which are spread with different signature sequences simultaneously. Higher data rate can be achieved by implementing dirty paper coding (DPC) to cooperatively encode all users\u27 data symbols at the base station. However, the complexity of realizing DPC is prohibitively high. Moreover, it is found that the resource allocation policy has profound impact on the rate capacity that can be maintained in the system. Nevertheless, the widely adopted proportional resource allocation policy is only suitable for the communication scenario in which the disparity of users\u27 channel qualities is small. When the difference between users\u27 channel qualities is large, one must resort to non-proportional assignment of power and signature sequences. Both centralized and distributed schemes are proposed to adapt users\u27 signature sequences in the downlink of MC-CDMA system. With the former, the base station collects complete channel state information and iteratively adapts all users\u27 signature sequences to optimize an overall system performance objective function, e.g. the weighted total mean square error (WTMSE). Since the proposed centralized scheme is designed such that each iteration of signature sequence adaptation decreases the WTMSE which is lower bounded, the convergence of the proposed centralized scheme is guaranteed. With the distributed signature sequence adaptation, each user\u27s signature sequences are independently adapted to optimize the associated user\u27s individual performance objective function with no regard to the performance of other users in the system. Two distributed adaptation schemes are developed. In one scheme, each user adapts its signature sequences under a pre-assigned power constraint which remains unchanged during the process of adaptation. In the other scheme, pricing methodology is applied so that the transmission power at the base station is properly distributed among users when users\u27 signature sequences are adapted. The stability issue of these distributed adaptation schemes is analyzed using game theory frame work. It is proven that there always exists a set of signature sequences at which no user can unilaterally adapt its signature sequences to further improve its individual performance, given the signature sequences chosen by other users in the system

Design of optimal equalizers and precoders for MIMO channels

Channel equalization has been extensively studied as a method of combating ISI and ICI for high speed MIMO data communication systems. This dissertation focuses on optimal channel equalization in the presence of non-white observation noises with unknown PSD but bounded power-norm. A worst-case approach to optimal design of channel equalizers leads to an equivalent optimal H-infinity filtering problem for the MIMO communication systems. An explicit design algorithm is derived which not only achieves the zero-forcing (ZF) condition, but also minimizes the RMS error between the transmitted symbols and the received symbols. The second part of this dissertation investigates the design of optimal precoders which minimize the bit error rate (BER) subject to a fixed transmit-power constraint for the multiple antennas downlink communication channels under the perfect reconstruction (PR) condition. The closed form solutions are derived and an efficient design algorithm is proposed. The performance evaluations indicate that the optimal precoder design for multiple antennas communication systems proposed herein is an attractive/reasonable alternative to the existing precoder design techniques

Transmitter adaptation for CDMA systems.

Kwan Ho-yuet.Thesis (M.Phil.)--Chinese University of Hong Kong, 2000.Includes bibliographical references (leaves 84-[87]).Abstracts in English and Chinese.Chapter 1 --- Introduction --- p.1Chapter 1.1 --- An Overview on Transmitter Optimization --- p.1Chapter 1.1.1 --- Transmitter Precoding Methods --- p.2Chapter 1.1.2 --- Chip Waveform Optimization --- p.3Chapter 1.1.3 --- Signature Sequence Adaptation --- p.3Chapter 1.2 --- Receiver Optimization --- p.5Chapter 1.3 --- Nonlinear Optimization with Constraints --- p.6Chapter 1.3.1 --- Lagrange Multiplier Methods --- p.6Chapter 1.3.2 --- Penalty Function Methods --- p.7Chapter 1.4 --- Outline of Thesis --- p.8Chapter 2 --- Transmitter Adaptation Scheme for AWGN Channels --- p.10Chapter 2.1 --- Introduction --- p.10Chapter 2.2 --- System Model --- p.12Chapter 2.3 --- Adaptation Algorithm --- p.13Chapter 2.3.1 --- Receiver optimization --- p.14Chapter 2.3.2 --- Single-user transmitter optimization --- p.18Chapter 2.3.3 --- Decentralized transmission scheme --- p.20Chapter 2.4 --- Modification of the sequence adaptation algorithm --- p.25Chapter 2.5 --- Performance Evaluation --- p.28Chapter 2.5.1 --- Performance of the decentralized scheme --- p.28Chapter 2.5.2 --- System Capacity with Target SNR Constraints --- p.29Chapter 2.5.3 --- Performance of modified sequences --- p.31Chapter 2.6 --- Summary --- p.33Chapter 3 --- Transmitter Adaptation Schemes for Rayleigh Fading Channels --- p.34Chapter 3.1 --- Introduction --- p.34Chapter 3.2 --- Sequence Adaptation for MC-CDMA Systems --- p.36Chapter 3.2.1 --- Multi-sequence MC-CDMA systems --- p.36Chapter 3.2.2 --- Single Sequence MC-CDMA systems --- p.41Chapter 3.2.3 --- Performance Evaluation --- p.45Chapter 3.3 --- Sequence Adaptation for Wideband CDMA System in Fading Channels --- p.50Chapter 3.3.1 --- System Model and Algorithm Development --- p.50Chapter 3.3.2 --- Performance Evaluation --- p.56Chapter 3.4 --- Summary --- p.60Chapter 4 --- Practical Issues on Sequence Adaptation --- p.61Chapter 4.1 --- Introduction --- p.61Chapter 4.2 --- Preliminary --- p.62Chapter 4.3 --- Sequence Adaptation Algorithm with Perfect Estimation of SNR --- p.63Chapter 4.4 --- Performance Evaluation --- p.68Chapter 4.4.1 --- Typical Behaviour Analysis --- p.71Chapter 4.4.2 --- Average Performance Analysis --- p.72Chapter 4.5 --- Sequence Adaptation Algorithm with imperfect estimation of pre- vious state SNR --- p.75Chapter 4.6 --- Performance Evaluation --- p.77Chapter 4.7 --- Summary --- p.79Chapter 5 --- Conclusions and Future Works --- p.81Chapter 5.1 --- Conclusions --- p.81Chapter 5.2 --- Future Works --- p.83Bibliography --- p.8

Resource allocation in DS-CDMA systems with side information at the transmitter

In a multiuser DS-CDMA system with frequency selectivity, each userâÂÂs spreading sequence is transmitted through a different channel and the autocorrelation and the cross correlation properties of the received sequences will not be the same as that of the transmitted sequences. The best way of designing spreading sequences for frequency selective channels is to design them at the receiver exploiting the usersâ channel characteristics. By doing so, we can show that the designed sequences outperform single user AWGN performance. In existing sequence design algorithms for frequency selective channels, the design is done in the time domain and the connection to frequency domain properties is not established. We approach the design of spreading sequences based on their frequency domain characteristics. Based on the frequency domain characteristics of the spreading sequences with unconstrained amplitudes and phases, we propose a reduced-rank sequence design algorithm that reduces the computational complexity, feedback bandwidth and improves the performance of some existing sequence design algorithms proposed for frequency selective channels. We propose several different approaches to design the spreading sequences with constrained amplitudes and phases for frequency selective channels. First, we use the frequency domain characteristics of the unconstrained spreading sequences to find a set of constrained amplitude sequences for a given set of channels. This is done either by carefully assigning an already existing set of sequences for a given set of users or by mapping unconstrained sequences onto a unit circle. Secondly, we use an information theoretic approach to design the spreading sequences by matching the spectrum of each userâÂÂs sequence to the water-filling spectrum of the userâÂÂs channel. Finally, the design of inner shaping codes for single-head and multi-head magnetic recoding channels is discussed. The shaping sequences are designed considering them as short spreading codes matched to the recoding channels. The outer channel code is matched to the inner shaping code using the extrinsic information transfer chart analysis. In this dissertation we introduce a new frequency domain approach to design spreading sequences for frequency selective channels. We also extend this proposed technique to design inner shaping codes for partial response channels