Incoherent spatial diversity combining scheme for spread spectrum
There are applications in spread spectrum channels where spatial diversity is required to mitigate fading. The spreading gain in such channels is often limited to reduce the chances of a nearby interferer being in-band. With modest spreading gains the signal bandwidth may be less than the coherence bandwidth. In such cases receiver-based multi-path diversity is not feasible, and spatial diversity is used to combat fading. This study improves and extends the work presented in US patent #6389085 B1, which exploits the structure of spread spectrum signals to implement N th order spatial diversity with simple incoherent radio frequency combining. An improved combining scheme is proposed here together with a special pseudo-random noise (PN) sequence to reduce intra symbol interference. A unique coherent all-digital demodulator is also provided. The signal-to-noise ratio (SNR) of the new system is about 3.5 dB higher than that of the original system described in the patent. The new receiver is comparable in cost to selection diversity, but does not switch antennas and therefore, does not suffer the momentary signal drop-outs. However, the penalty for eliminating the drop outs is a 1.5 dB lower output SNR.
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