802.11速率计算

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.

Guard Interval (TGI) Time Calculation

The following table shows calculated Guard Interval values for a HIPERLAN/2 OFDM signal:

	Guard Interval	T(FFT)	T(GI)
	1/4	3.2 µs	0.8 µs
	1/8	3.2 µs	0.4 µs
	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).

MCS index	Spatial streams	Modulation type	Coding rate	Data rate (Mbit/s)
20 MHz channel	40 MHz channel
800 ns GI	400 ns GI	800 ns GI	400 ns GI
0	1	BPSK	1/2	6.5	7.2	13.5	15
1	1	QPSK	1/2	13	14.4	27	30
2	1	QPSK	3/4	19.5	21.7	40.5	45
3	1	16-QAM	1/2	26	28.9	54	60
4	1	16-QAM	3/4	39	43.3	81	90
5	1	64-QAM	2/3	52	57.8	108	120
6	1	64-QAM	3/4	58.5	65	121.5	135
7	1	64-QAM	5/6	65	72.2	135	150
8	2	BPSK	1/2	13	14.4	27	30
9	2	QPSK	1/2	26	28.9	54	60
10	2	QPSK	3/4	39	43.3	81	90
11	2	16-QAM	1/2	52	57.8	108	120
12	2	16-QAM	3/4	78	86.7	162	180
13	2	64-QAM	2/3	104	115.6	216	240
14	2	64-QAM	3/4	117	130	243	270
15	2	64-QAM	5/6	130	144.4	270	300
16	3	BPSK	1/2	19.5	21.7	40.5	45
17	3	QPSK	1/2	39	43.3	81	90
18	3	QPSK	3/4	58.5	65	121.5	135
19	3	16-QAM	1/2	78	86.7	162	180
20	3	16-QAM	3/4	117	130	243	270
21	3	64-QAM	2/3	156	173.3	324	360
22	3	64-QAM	3/4	175.5	195	364.5	405
23	3	64-QAM	5/6	195	216.7	405	450
24	4	BPSK	1/2	26	28.8	54	60
25	4	QPSK	1/2	52	57.6	108	120
26	4	QPSK	3/4	78	86.8	162	180
27	4	16-QAM	1/2	104	115.6	216	240
28	4	16-QAM	3/4	156	173.2	324	360
29	4	64-QAM	2/3	208	231.2	432	480
30	4	64-QAM	3/4	234	260	486	540
31	4	64-QAM	5/6	260	288.8	540	600
32	1	BPSK	1/2	N/A	N/A	6.5	7.2

802.11N/AC速度计算

物理层	带宽（数据子载波频率的个数）	×	空间流个数	×	每个子载波的数据比特	÷	每个正交频分多路复用符号的时间	=	物理层数据速率(bps)
11n或11ac	56(20MHz)	1到4个	最多5/6×log2(64)=5	3.6us(短保护间隔) 4us(长保护间隔)
108(40MHz)
11ac	234(80MHz)	5到8个	最多5/6×log2(256)≈6.67
2×234(160MHz)

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 Mbp/s