Time diversity techniques

Time diversity techniques aim to spread the data out over time so that a noise
burst affects fewer bits. Time diversity can be quite effective in a region of slow fading.
If a mobile unit is moving slowly, it may remain in a region of a high level of fading
for a relatively long interval. The result will be a long burst of errors even though
the local mean signal level is much higher than the interference. Even powerful
error correction codes may be unable to cope with an extended error burst. If digital
data is transmitted in a time division multiplex (TDM) structure, in which multiple
users share the same physical channel by the use of time slots (see Figure 2.13b),
then block interleaving can be used to provide time diversity. Figure 5.16a, based on
one in [JONE93], illustrates the concept. Note that the same number of bits are still
affected by the noise surge, but they are spread out over a number of logical channels.
If each channel is protected by forward error correction, the error-correcting

code may be able to cope with the fewer number of bits that are in error in a particular
logical channel. IfTDM is not used, time diversity can still be applied by viewing
the stream of bits from the source as a sequence of blocks and then shuffling the
blocks. In Figure 5.16b, blocks are shuffled in groups of four. Again, the same number
of bits is in error, but the error correcting code is applied to sets of bits that are
spread out in time. Even greater diversity is achieved by combiningTDM interleaving
with block shuffling.
The tradeoff with time diversity is delay. The greater the degree of interleaving
and shuffling used, the longer the delay in reconstructing the original bit sequence
at the receiver.