Exploitation of Historical Signals in Large Packet-Switched CDMA Random Access Systems

– This paper proposes an approach to exploitation of retransmission diversity embedded in multiple packet transmissions of a large CDMA random access system. In the approach, the interference signals caused by packets that have been successfully demodulated previously are first subtracted thus reducing interference. Second, all signals received in slots when the desired packet was transmitted are jointed used thus obtaining retransmission diversity gain. Stationary distribution of the number of transmissions for a packet to succeed, throughput, delay, and spectral efficiency are analyzed by applying large CDMA techniques. The theoretical results are confirmed in simulations. We consider that a very large number of K potential users randomly access a basestation through a bit-synchronous CDMA channel. Each user has a sufficiently long buffer to store arrived packets. Packets are transmitted in a first-in-first-out order. At the front end, the packet that is currently served in transmission is called the current packet. All other packets in the buffer are packets waiting for transmissions. We assume that the new packet arrival rate at each user is sufficiently high so that there will be always a packet in the buffer waiting for transmission. Time is packet-slotted though the result can be extended to unslotted systems. In each slot, a user transmits a packet of L bits, which is coded with error detecting code. Therefore, the receiver at the basestation is able to detect if the demodulated packet is in error. If the demodulated packet is erroneous, the basestation informs through another reliable channel the user the failure of transmission and then the user will retransmit the packet. The retransmission continues until the basestation successfully demodulated the packet, followed with an acknowledgement message informing the user the success. Knowing the success of transmission, the user removes the successfully transmitted packet from the buffer and transmits another packet. While the error detecting code and the feedback channel take up certain spectrum, we will not take them into account in analysis of spectral efficiency.

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