Diversity is employed to improve the performance of a communication system in a fading environment. In a direct sequence spread spectrum (DSSS) system the signal bandwidth is usually larger than the coherence bandwidth of the channel. In such wide bandwidth systems, the transmitted signal energy is received via several delayed multipaths, and a RAKE receiver can be employed to obtain the benefits of diversity. The delay spreads encountered in indoor radio channels are very small. Therefore, in such channels, a moderately l q e bandwidth (e.g. 1 MHz) direct sequence spread spechum system wlll not give rise to this form of diversity. In this paper we propose a novel form of diversity which is useful especially in situations where frequency diversity is not available. This technique uses multiple transmit antennas and the antennas axe weighted by suitably determined time-varying phase offsets. It is shown that with this technique, M transmit antenna8 and an L-branch RAKE receiver gives rise to diversity of order LM.
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