Guard Interval Use in Digital Communications Systems
In COFDM, the beginning of each symbol is preceded by a guard interval. As long as the echoes fall within this interval, they will not affect the receiver's ability to safely decode the actual data, as data is only interpreted outside the guard interval.
In TDMA, each user's timeslot ends with a guard period, to avoid data loss and to reduce interference to the following user, caused by propagation delay. Thus a user's timeslot is protected from interference from the preceding user, by the guard interval (guard period) at the end of that preceding user's timeslot. It is a common misconception that each TDMA timeslot begins with a guard interval, however the ITU Technical Specifications (such as GSM 05.05) clearly define the guard period as being at the end of each timeslot, thus providing protection against data loss within that timeslot, and protection against interference to the following timeslot.
Longer guard periods allow more distant echoes to be tolerated. However, longer guard intervals reduce the channel efficiency. For example, in DVB-T, four guard intervals are available (given as fractions of a symbol period):
- 1/32 ; 1/16 ; 1/8 ; 1/4
Hence, 1/32 gives lowest protection and the highest data rate. 1/4 results in the best protection but the lowest data rate.
Radio waves propagate at the speed of light, 3 µs per 1000 meter (5μs/mile). Ideally the guard interval is just longer than the delay spread of the channel.
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