In
telecommunications
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
, 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
In telecommunications, orthogonal frequency-division multiplexing (OFDM) is a type of digital transmission and a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital commun ...
).
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
In telecommunications, orthogonal frequency-division multiplexing (OFDM) is a type of digital transmission and a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital commun ...
, 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 Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation ( 2G) digital cellular networks used by mobile devices such ...
, 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 example, in
DVB-T
DVB-T, short for Digital Video Broadcasting – Terrestrial, 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 Singapore in Febr ...
, guard intervals are available as 1/32, 1/16, 1/8 or 1/4 of a symbol period. The shortest interval (1/32) provides the lowest protection and the highest data rate; the longest interval (1/4) provides the highest protection but the lowest data rate. Ideally, the guard interval is set to just above the
delay spread In telecommunications, the delay spread is a measure of the multipath richness of a communications channel.
In general, it can be interpreted as the difference between the time of arrival of the earliest significant multipath component (typically ...
of the channel.
802.11 guard interval
The standard symbol guard interval used in
IEEE 802.11
IEEE 802.11 is part of the IEEE 802 set of local area network (LAN) technical standards, and specifies the set of media access control (MAC) and physical layer (PHY) protocols for implementing wireless local area network (WLAN) computer commun ...
OFDM is . To increase data rate,
802.11n
IEEE 802.11n-2009 or 802.11n is a wireless-networking standard that uses multiple antennas to increase data rates. The Wi-Fi Alliance has also retroactively labelled the technology for the standard as Wi-Fi 4. It standardized support for multipl ...
added optional support for a guard interval. This provides an 11% increase in data rate. To increase coverage area,
IEEE 802.11ax
IEEE 802.11ax, officially marketed by the Wi-Fi Alliance as (2.4 GHz and 5 GHz) and (6 GHz), is an IEEE standard for wireless local-area networks (WLANs) and the successor of 802.11ac. It is also known as ''High Efficiency'' , for ...
(Wi-Fi 6) provides optional support for , , and guard intervals.{{Citation needed, date=October 2020
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 to 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.
[Perahia and Stacey, ''Next Generation Wireless LANs'', Cambridge University Press, 2008]
See also
*
Interpacket gap
In computer networking, the interpacket gap (IPG), also known as interframe spacing, or interframe gap (IFG), is a pause which may be required between network packets or network frames. Depending on the physical layer protocol or encoding used, ...
*
Cyclic prefix
In telecommunications, the term cyclic prefix refers to the prefixing of a symbol, with a repetition of the end. The receiver is typically configured to discard the cyclic prefix samples, but the cyclic prefix serves two purposes:
* It provides a ...
*
Intersymbol interference
In telecommunication, intersymbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have a similar effect as noise, thus making ...
References
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.
Guard interval and ISI-free OFDM transmission Online experiment illustrates ISI-free OFDM transmission if guard time is longer or equal to the channel's maximum delay spread.
Multiplexing