Variable energy adaptation for asynchronous CDMA communications over slowly fading channels

In this paper, a detailed theoretical analysis of variable energy adaptation in an asynchronous code division multiple access (A-CDMA) system is discussed. Rayleigh and Rician frequency-selective slowly fading channels are considered. The receiver, capable of measuring the received signal energy-to-noise ratio, provides the transmitter with the necessary signal-to-noise ratio measurement to control the transmitter energy through a noise-free feedback channel. System parameters such as fading margin and mean transmitter energy gain are calculated for both Rayleigh and Rician fading channels as a function of the probability of error specification and the probability of unsatisfactory operation.     

Adaptive rate coding using convolutional codes for asynchronous code division multiple access communications over slowly fading channels

This paper presents a method of code rate adaptation using punctured convolutional codes for direct sequence spread spectrum communication systems over slowly fading channels. A blind channel estimation technique is used to estimate the nature of the multi-user channel at the detector (before the decoder). The path gains obtained from the channel estimation technique are used to adapt the code rates. Punctured codes derived from a specific rate 1/2 (M = 4) mother code are used to provide error protection corresponding to the actual channel state. The upper and lower bounds on the bit error probability and the upper bound on the error event probability are derived for hard-decision and soft-decision decoding over Rayleigh and Rician fading channels. The throughput gains obtained using the adaptive scheme and the performances of the punctured codes are studied.