How Frequency Diversity of Hopping in GSM
Hopping can lower the influence of Rayleigh fading to signal strength. This influence equals to frequency diversity. One voice code block of GSM is transmitted in 8 bursts separately after channel coding.
Since the Rayleigh fading is based on frequency, in other words, the valley point of signal fading appears in different locations at different frequencies. Therefore, after hopping is used, the modulation frequency of each burst is different.
If frequency selective fading occurs on certain frequency carrier, the period that signal received by mobile phone located at valley point won’t exceed one burst cycle, and only one burst is affected. The variation of signal strength is cut to a level small enough to satisfy the requirement of channel coding and interleave error correction, thus the signal valley point of rayleigh fading can be omitted, and the valley point where rayleigh fading signal level appears will become smooth.
In this way, the slow moving mobile will be in an environment with more even radio wave. For the mobile phone moving quickly, the influence from frequency diversity is the same regardless of its speed.