Transmitter diversity with antenna hopping for wireless communication systems

Transmitter diversity is an attractive means for improving the performance of the downlink in a cellular system. This paper investigates one of these methods, referred to as antenna hopping, which consists in transmitting consecutive bursts of information on different antennas. Provided that the antenna branches are exposed to different multipath fading, a gain is achieved. Results from simulation of the GSM TCH/FS (full rate speech channel) reveal that the gain of hopping on two uncorrelated antennas is 2.9 dB for low speed terminals in an urban environment, and 5.4 dB for an indoor environment with semi-stationary users. The maximal gain is shown to be reduced at high cross-correlation and difference in path loss between the antenna branches. Antenna hopping combined with frequency hopping is shown to further enhance performance. With antenna hopping on two antennas, the maximal gain can be obtained by using frequency hopping on only 4 frequencies instead of 8 without antenna hopping. Finally, a comparison is made with delayed transmitter diversity, which is shown to have no essential advantages over antenna hopping at low terminal speed, which is the situation where diversity is really needed.