Abstract: A new hybrid automatic repeat request(HARQ) system is constructed by combining the sub-packet scheme with the averaged diversity combining technique. The optimal number of sub-packets of the new HARQ system is evaluated according to the characteristics that the signal to noise ratio is increased by a factor equal to the retransmission times in the Code Division Multiple Access(CDMA) system. In the new HARQ system the diversity combination of data packet and its copies in the retransmissions is converted to the diversity combination of the same sub-packets in the one data packet and no retransmission is needed, which brings the advantages of real time communication, omitting the complicated retransmission control and a mass of memory. Also the method of making combination before being despread avoids the inefficient combination due to the spread and restriction of memory in type -Ⅰ HARQ. In addition, the method of adding tail bits to each sub-packet improves the performance of the new HARQ. Numerical and simulation results show the new HARQ system has a much higher throughput than the type-Ⅰ HARQ and the evaluated optimal number of sub-packets is in agreement with the simulated results.

Key words: CDMA, combination, sub-packet system, HARQ