Guard interval
In telecommunications, guard intervals are used to ensure that distinct transmissions do not interfere with one another, or otherwise cause overlapping transmissions. These transmissions may belong to different users (as in TDMA) or to the same user (as in OFDM). The purpose of the guard interval is to introduce immunity to propagation delays, echoes and reflections, to which digital data is normally very sensitive. Use in digital communications systems In OFDM, 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 interval. Thus, the guard interval protects against data loss within the same timeslot, and protects the following user's timeslot from interference caused by propagation delay. It is a common misconception that TDMA timeslots begin with the guard interval, as with OFDM. However, in specifications for TDMA systems such as GSM, the guard period is defined as being at the end of the timeslot. Longer guard periods allow more distant echoes to be tolerated but reduce channel efficiency. For