## Coping with multipath fading

### Spectral spreading

#### Two alternative methods exist for coping with both flat and frequency selective fading both involve increasing the bandwidth required to send the data. A technique known as direct sequence spread spectrum uses a wideband data sequence to mix with a narrowband data signal and thence spread the energy well outside the coherence bandwidth for the channel. A small proportion of the spread signal energy will be lost in the frequency selective fades, but the majority will pass with little distortion through the channel. By de-spreading the signal in the receiver, a reasonable copy of the original transmitted signal can be obtained. Data coding and channel equalization are often employed in addition to the spreading to improve the integrity of the channel. |

| #### Instead of spreading the data signal instantaneously (and thinly) over a wide frequency range, an equally effective method is to rapidly change the position of the narrow data signal within a much wider bandwidth. This frequency hopping approach means that for some of the time the signal will fall within a selective fade, but for most of the time, it will be passed within a non-fading portion of the channel. The result, as for direct sequence spreading, is that most of the data signal most of the time reaches the receiver with little distortion, and with extra coding a high integrity communications link can be established.

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