Theoretical speed calculation method of WiFi

1. 802.11g Adopted OFDM Can provide 52 Subcarrier channel （ Only one 48 For data transmission ）

2. Adopted by the 64-QAM The coding method can carry the information in each subcarrier channel through one transmission process 6bit Data bits of

3. 64-QAM Encoding each transmission provides 3/4 Bit rate of （ That is, effective data capacity ）

4. The time occupied by a transmission is fixed as 4 Microsecond

According to the above calculation factors ,802.11g The maximum rate that can be provided is calculated as follows ：

（1 second /4 Microsecond ）×（6bit×48×3/4）=54Mbit/s

According to the above calculation method , Let's see 802.11n How to improve the maximum speed through various technologies .

1. First 802.11n stay 11g On the basis of OFDM The modulation mode is optimized , Change the number of subcarrier channels from 52 Up to 56 individual （ Only one 52 For data transmission ）, The maximum rate becomes ：

（1 second /4 Microsecond ）×（6bit×52×3/4）=58.5Mbit/s

1. secondly 802.11n Yes 64-QAM The coding technology is optimized , The code rate provided for each transmission is changed from 3/4 Up to the 5/6, The maximum rate becomes ：

（1 second /4 Microsecond ）×（6bit×52×5/6）=65Mbit/s

1. next 802.11n The working bandwidth ranges from 11g Of 20MHz Turn into 40MHz, such OFDM The number of subcarrier channels that can be provided ranges from 56 One is further promoted to 112 individual , The number of subchannels used to transmit data is 108 individual , The maximum rate becomes ：

（1 second /4 Microsecond ）×（6bit×108×5/6）=135Mbit/s

1. in addition 802.11n As far as conditions permit （ When the multipath effect in the actual environment is small ） take OFDM The protection interval between two transmissions is from 11a/b/g Of 800ns Shorten to 400ns, This will further increase the maximum rate to 150Mbit/s

2. Last , Because of the adoption of MIMO technology , Through spatial reuse technology , stay 1-4 The maximum speed in a space stream environment will be 150Mbit/s Of 1-4 Double growth , namely 2 Space flow reaches 300Mbit/s、3 Space flow reaches 450Mbit/s、4 Space flow reaches 600Mbit/s.

Of course , because 802.11n For downward compatibility 802.11a/b/g, Therefore, the modulation mode that can be supported 、 Encoding mode 、 There are many combinations of bit rate and data bits carried , It is these combinations that form 802.11n Numerous access rate standards . In the following two pictures , The first picture is 802.11n The joint parameters given in the draft and the calculation results of data throughput for one and two spatial streams , For the calculation of three and four spatial flows, the results in the figure below can be used , Get by simple multiplication . The second picture is the code rate corresponding to different coding methods 、 Carry the corresponding relationship of data bits .