COMBINED EFFECTS OF PHASE SWEEPING TRANSMITTER DIVERSITY AND CHANNEL CODING

A two-branch phase sweeping transmitter diversity scheme that can produce forced fast fading at the receiver is proposed for improving the effect of channel coding on bit error rate (BER) performance in very slow multipath fading environments. While the proposed transmitter diversity requires two transmit antennas, a single receive antenna can be used. Hence, it is most applicable for digital paging systems that require very simple receivers. The digital modulated signal is transmitted simultaneously from the two spatially separated antennas, with the carrier for one antenna being phase modulated by the phase sweeping function eta(t). Two sweeping functions are considered: eta(t) = DELTA-THETA sin(2-pi-f(H)t) (sinusoidal) and 2-pi-f(H)t (linear). An analysis shows that when bit interleaving of m-bit depth is used, it is required that DELTA-THETA > pi and mnf(H)T > 1/2 (sinusoidal) or >1 (linear) where n is the codeword length of the error correction code and T is the bit length of the digital transmission. When 32-kbps transmission with m = 10-bits interleaving and BCH (23,12) error correction code (n = 23) are used, f(H) = 70 Hz is sufficient for the sinusoidal phase sweeping function. Therefore, bandwidth expansion due to phase sweeping is negligible. Experimental results on transmitter diversity with the sinusoidal phase sweeping function are presented for 32 kbps quarternary differential phase shift keying (QDPSK) with differential detection.