A GI is a period of time between symbol transmission that allows reflections (from multipath) from the previous data transmission to settle before transmitting a new symbol. The signal content inside the GI, called Inter-Symbol Interference (ISI), is rejected by receivers. Clause 17 and 19 OFDM use 800ns GIs. This value was chosen because the maximum multipath echo time is typically considered to be 800ns in an indoor environment. Outdoor GIs are typically higher.
The time used to transmit a symbol is comprised of two parts: Fast Fourier Transform (FFT) time and Guard Interval (GI) time. FFT is a signal processing technique, thus FFT time is the time during transmission or reception when signal processing is accomplished. The 802.11a/g OFDM symbol rate is 250 kHz, corresponding to a symbol period of 4 µs. This means the GI is 0.8 µs and the FFT is 3.2 µs in a clause 17 or 19 OFDM symbol.
The GI should be 2-4 times higher than the delay spread, so in most environments, delay spread should not exceed 200ns. Most offices and homes have a delay spread of approximately 50–100 ns. Choosing a GI that is too short means that ISI will increase and throughput will decrease. Choosing a GI too long means decreased overhead due to unnecessary idle time on the wireless medium. Choosing a proper GI time period is crucial to optimizing throughput.
There are 77 Modulation and Coding Schemes (MCSs) specified in the current IEEE P802.11n draft, with eight of them being mandatory for 802.11n compliance. The highest data rate of 600 Mbps is achievable with MCS 31 using 64-QAM modulation in a 40-MHz channel, four spatial streams, and operating with a short guard interval (GI) of 400 ns. Choosing a 400 ns GI adds approximately 11% to the achievable data rate over the 800 ns GI.