Adaptive direct sequence code division multiple access (DS-CDMA) systems based on sequence optimization

In this work, we investigate issues on the application of a physical layer technique, sequence optimization, in code division multiple-access (CDMA) communication systems. The objective of sequence optimization is to construct a set of optimal sequences so that users in the system are supported with target signal-to-interference ratios (SIRS) using a minimum amount of total transmission power. We propose two joint transmitter-receiver adaptation schemes that make use of the optimal sequences for CDMA systems over the additive white Gaussian noise (AWGN) channel. In both schemes, training symbols are sent in blocks so that spreading sequences and minimum mean squared error (MMSE) receiver weights are adapted simultaneously. In the first scheme, spreading sequences are updated at the receivers in a distributed manner, and are fed back to the base-station to modulate the next block of data. In the second scheme, spreading sequences are updated at the base-station in a centralized manner using a minimum amount of feedback information from the receivers. Our second objective concerns sequence optimization in CDMA systems over fading channels. We propose two sequence allocation schemes, a minimum-ETSC scheme for MMSE signal reception and a minimum-TSC scheme for matched-filter signal reception, to use in a CDMA forward link over slow flat fading. The MMSE receiver weights of the minimum-ETSC sequences turn out to be the same as those of the matched-filter receiver. There is strong indication that the minimum-ETSC scheme, which has better power efficiency than the minimum-TSC scheme, may be the optimal scheme when the matched filter is used for signal reception. In addition, Lagrangian multiplier searching results indicate that the minimum-ETSC scheme is effective and consumes only slightly more power than the minimum power searched under most channel conditions when the MMSE receiver is used. Adaptive structure and algorithms in combination with channel estimation are also presented to facilitate the implementation of the minimum-ETSC sequences in practical systems. We also investigate sequence optimization in CDMA systems with multipath. We solve the optimization problem for the case of point-to-point transmission, in which all the receivers are co-located and undergo the same multipath channel. Finally, we propose a type-I hybrid ARQ protocol to make use of the optimal sequences to allow simultaneous transmission of multiple packets over a CDMA link. The transmitted packets are classified into two classes, namely newly arrived packets and retransmission packets. A resource allocation algorithm is responsible for determining the numbers and the target SIRs for each class of packets to be transmitted in each time slot so that the link throughput is maximized under a delay constraint. Numerical results obtained from both the Markov analysis and Monte Carlo simulations are used to evaluate the performance of the proposed protocol.