Guard interval
Encyclopedia
In telecommunications, guard intervals are used to ensure that distinct transmissions do not interfere with one another. 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.
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):
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, per 1000 meter (5μs/mile). Ideally the guard interval is just longer than the delay spread of the channel.
The shorter guard interval results in a higher packet error rate when the delay spread of the channel exceed the guard interval and/or if timing synchronization between the transmitter and receiver is not precise. A scheme could be developed to work out whether a short guard interval would be of benefit a particular link. To reduce complexity, manufacturers typically only implement a short guard interval as a final rate adaptation step when the device is running at its highest data rate.
Time division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
) or to the same user (as in OFDM
Orthogonal frequency-division multiplexing
Orthogonal frequency-division multiplexing is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio...
).
The purpose of the guard interval is to introduce immunity to propagation delays, echoes and reflections, to which digital data is normally very sensitive.
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
Time division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
, 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
DVB-T
DVB-T is an abbreviation for Digital Video Broadcasting — Terrestrial; it is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television that was first published in 1997 and first broadcast in the UK in 1998...
, 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, per 1000 meter (5μs/mile). Ideally the guard interval is just longer than the delay spread of the channel.
802.11 Guard Interval
The standard symbol guard interval used in 802.11 OFDM is 0.8μs. To increase data rate, 802.11n added optional support for a 0.4μs guard interval. This provides an 11% increase in data rate.The shorter guard interval results in a higher packet error rate when the delay spread of the channel exceed the guard interval and/or if timing synchronization between the transmitter and receiver is not precise. A scheme could be developed to work out whether a short guard interval would be of benefit a particular link. To reduce complexity, manufacturers typically only implement a short guard interval as a final rate adaptation step when the device is running at its highest data rate.
Teleport's
In case of transmitting CHANNEL (TV) the GI will be maxium of 120KHz or minium 50KHz if in case of less bandwidth.External links
- Technical Standard GSM 05.05 Radio Transmission and Reception. Contains descriptions and diagrams of the GSM use of TDMA timeslots, bursts, and guard periods.