802.11ac 传输速率计算方式
Guard Interval
The Guard Interval is the ratio of the Cyclic Prefix "CP" time to the inverse FFT time "T(IFFT)." The guard interval is used to eliminate inter-symbol and inter-carrier interference. A copy of the last guard interval T(GI) of the useful symbol period "T(IFFT)", termed Cyclic Prefix "CP", is used to collect multipath, while maintaining the orthogonality of the subcarriers. Each symbol is transmitted for a slightly longer time, extended symbol time T(s), than the active (or useful) symbol time T(IFFT). The extra time is the guard interval.
1/8: Sets the Guard Interval to 1/8 (see Guard Interval Time Calculation below)
1/4: Sets the Guard Interval to 1/4 (see Guard Interval Time Calculation below)
Other: Enables you to enter Guard Interval values between 0 to 1.
The Guard Interval time period T(GI) is specified as a fraction (percentage) of the inverse FFT time period T(IFFT). For 802.11a, the only selection is a Guard Interval of 1/4 (1/8 is greyed). For HIPERLAN/2, both 1/4 and 1/8 are selections. The Other selection allows the input of a non-standard Guard Interval value between 0 and 1.
where:
T(FFT)= FFT time period for the OFDM signal
T(GI) = Guard Interval time period = Guard Interval ´ T(FFT)
可以看到802.11A/G/N的GI 可以分为1/4或者1/8；一个OFDM 符号的时间TS=T(IFFT)+T(GI) ,所以有3.6us(短保护间隔，1/8)，4us(长保护间隔，1/4)。下图Data rate中的800ns GI或者400ns GI值对应上面的
T(GI).
802.11N/AC速度计算
11N 有效子载波数
HT20M 子载波数量 56 ，其中52 个用于传输，4个用于引导帧
HT40M 子载波数量 112 ，其中108 个用于传输，6个用于引导帧，64-QAM 编码率5/6，数据比特率64=2^6,所以是6bit 。GI 时间是上面的TS 时间，3.6/4us
所以11n 的速度=（108*4*(5/6)*6）/(3.6) = 600 Mbps
11ac 速率=（234*1*6.67）
/3.6=433 Mbps