Spread spectrum is well suited for wireless communication systems because of its ``built-in'' frequency diversity. As discussed before, in cellular systems the delay spread measures several microseconds and, hence, the coherence bandwidth of the channel is smaller than 1 MHz. Spreading rates can be chosen to exceed the coherence bandwidth such that the channel becomes frequency selective, i.e., different spectral components are affect in unequally by the channel and only parts of the signal are affected by fades. Expressing the same observation in time domain terms, multi-path components are resolvable at a resolution equal to the chip period and can be combined coherently for example by means of a RAKE receiver [Proakis, 1989]. An estimate of the channel impulse response is required for the coherent combination of multi-path components. This estimate can be gained from a training sequence or by means of a so-called ``pilot'' signal.
Even for cordless telephone systems, operating in environments with sub-microsecond delay spread and corresponding coherence bandwidths of a few MHz, the spreading rate can be chosen large enough to facilitate multi-path diversity. If the combination of multi-path components described above is deemed to complex a simpler, but less powerful, form of diversity can be used which decorrelates only the strongest received multi-path component and relies on the suppression of other path components by the matched filter.
