Effect of Interleaving
In GSM the speech frame is transmitted over eight consecutive bursts. The fast fading causes bursty bit errors that degrade the efficiency of the convolutional coding. The interleaving is designed to spread these errors over longer time. However, the decoding performance is not significantly improved if consecutive bursts are exposed to the similar radio channel.
If the mobile moves fast enough, the fading of successive bursts is uncorrelated due to spatial movement. Frequency hopping causes consecutive bursts to be transmitted on different frequencies. If these frequencies have sufficient separation the fading of successive bursts is uncorrelated.
Since the interleaving depth is eight, the frequency diversity gain of cyclic hopping doesn’t significantly improve if more than eight frequencies are used in a hopping sequence.
In data calls, the interleaving length is 19. Therefore, the gain for data calls compared to speech calls might be bigger when more than 8 frequencies are used in a hopping sequence.
The fast fading is a significant problem especially in the downlink direction since the mobiles do not employ antenna diversity, which is commonly used in base stations. Fluctuations of the received signal strength are especially harmful for the slow moving mobiles because they tend to stay in a fading dip much longer than the faster moving mobiles. Frequency hopping causes the consecutive bursts to be transmitted on different frequencies. If the separation between these frequencies is sufficient, the fading characteristics of these frequencies are different.
For the fast moving mobiles, the consecutive bursts have different fading characteristics even without frequency hopping, because the spatial movement between the consecutive bursts is significant and the locations of the fading dips are relatively constant in most environments. Thus the frequency diversity gain for the fast moving mobiles is not significant.