Collision mitigation schemes for orthogonal code hopping multiplexing

Abstract

An orthogonal code hopping multiplexing (OCHM) scheme has been proposed as a novel statistical multiplexing scheme for orthogonal downlink to accommodate more low-activity bursty users than the number of orthogonal downlink codewords. However, OCHM may cause collisions or perforations among symbols, which degrade the performance of channel decoding when the perforation probability is high. Therefore, we propose several methods to mitigate the performance degradation due to the collisions or perforations in OCHM. First, we propose new log-likelihood ratio (LLR) conversion schemes inserting simple conversion functions between channel demodulator output and channel decoder input in order to improve the decoding performance in various perforation environments. Second, we disperse a collision to many symbols instead of removing a symbol by orthogonal code spreading. Third, we obtain the optimal code rate for varying the traffic load in OCHM, and propose an adaptive code rate control scheme. Finally, we lower symbol collision probability by discarding or delaying excessive frames whose number is larger than a threshold value. The above methods yield a decrease in the required $E_b/N_0$ at the transmitter and save system power.