Adaptive rate coding for asynchronous code division multiple access communications over slowly fading channels.

摘要

This dissertation presents two methods of rate adaptation for Asynchronous Code Division Multiple Access (A-CDMA) systems over slowly fading channels: the first one using a Forward Error Correction (FEC) scheme, and the second one using a hybrid Automatic Repeat Request (H-ARQ) scheme. The work described incorporates rate adaptation schemes for direct sequence spread spectrum systems. New methods of providing optimum throughput for FEC and H-ARQ schemes have been developed. The optimum throughput of an FEC scheme is the maximum code rate used in the scheme that provides a specified level of performance. The optimum throughput of the H-ARQ scheme is the highest throughput that can be achieved by a particular starting code under retransmission delay constraints. The description and analysis of the mathematical model of the A-CDMA system over a Rayleigh or Rician fading multipath channel is presented. The Markov model of the A-CDMA channel is discussed in detail.; Modified blind multiuser channel estimation techniques using subspace and pilot-aided methods are illustrated. The existing simulation techniques for blind multiuser channel estimation have been modified to suit a multiuser system with a single user and multiple interferers. The combined effect of the interferers is lumped together in the form of MAI. Adaptive rate coding schemes are discussed for both FEC and H-ARQ techniques, using punctured and repetition convolutional codes respectively. The request for change in code rate and the request for retransmission of a single packet are made through separate feedback channels. The feedback channel for request of change in code rate is made at the detector itself.; Received signal constellations using both subspace and pilot-aided approaches are plotted. The signal constellations using multiple antennas show better results than those using a single antenna. The theoretical upper and lower bounds on the error event and bit error probabilities using both punctured and repetition convolutional codes are derived. The simulated results on the bit error probability using these codes are also plotted. The simulated results of the codes agree well with their theoretical upper and lower bounds. The performance of the codes is investigated for different number of users and different power levels of user 0.; The Markov model used for throughput analysis of the FEC scheme using punctured codes is discussed. The final throughput values obtained are in close agreement with the highest code rate of the punctured codes used in the simulation. The throughput analysis of the Generalized H-ARQ scheme using repetition codes is also presented. The final throughput values of the proposed H-ARQ scheme are found to be lower than the throughput values of the GH-ARQ scheme at low values of signal-to-noise ratio. However, for higher values of signal-to-noise ratio, the throughput of the proposed H-ARQ scheme with code combining exceeds that of the GH-ARQ scheme.