802.11--802.11n protocol phy

One . summary

This paper mainly focuses on 802.11n Agreed PHY Layer to make a simple record , Not particularly detailed , But try to PHY Features are listed .

Two . PHY Pattern

802.11n Of phy Defined 3 Middle mode , Respectively Non-HT format、HT-mixed format、HT-greenfield format, their PPDU The format is as shown below ：

1. Non-HT format;

The traditional model , abbreviation NON_HT Pattern , This mode follows 11g The pattern is exactly the same , Details can be found in 《802.11–802.11g agreement 》 The section , This mode must be supported .
　　Note： In terms of composition , This PPDU The format is 11g Under the agreement ERP-OFDM Pattern , How does it relate to 11b Compatible ？ Is this Non-HT PPDU The format of is just an example , It represents the 11g Of 3 Middle mode ？

2. HT-mixed format;

Mixed mode , For forward compatibility 802.11bg Pattern , abbreviation HT_MF Pattern , It mainly consists of the following parts ：
　　　1) L-STF、L-LTF、L-SIG stay 802.11a Introduced in , Details are available 《802.11–802.11a agreement 》 The section ;
　　　2) HT-SIG： Long 2 Symbols （8us）,HT-SIG The contents are as follows ：

　　　HT-SIG In two parts ：HT-SIG1、HT-SIG2, Each leader 24bits, in total 48bits,HT-SIG1 stay HT-SIG2 front , Their composition is shown in the figure below ：

　　　3) HT-STF： Long 1 Symbols （4us）, The function is to re AGC Evaluation settings （automatic gain control estimation）, Use 20MHz Time and L-STF It's exactly the same ,40MHz It's a little different , comparison L-STF,HT-STF Can do more accurate AGC assessment ;
　　　4) HT-LTF： Take up several symbols , It mainly consists of two parts ：HT-DLTFs、HT-ELTFs, be used for MIMO Channel evaluation ;
　　　　a) HT-DLTFs： The quantity may be 1、2、4 individual HT-LTF, For demodulation HT-Data, Is a necessary component ,DLTFs The relationship with the number of space-time streams is as follows ：

　　　　b) HT-ELTFs： The quantity may be 0、1、2、4 individual HT-LTF, For detecting frames （SDU）, It is applicable to situations where the number of transmission links is greater than the number of space-time streams , To evaluate more components of the channel ,ELTFs The relationship with the number of additional transmission links is as follows ：

　　　Note：HT-MF In mode ,L-SIG、HT-SIG There are length、rate etc. ,L-SIG The information in can be used for NAV Set up （ Because it may not be possible to PPDU Successfully resolved MPDU Of duration Field ）, It can also be used for txop The protection of the ;

3. HT-greenfield format（ Optional ）

Green space pattern , abbreviation HT-GF, It mainly consists of the following parts , In order not to increase the difficulty of understanding , The rest can be considered as follows HT-MF The same as in China ：
　　　1) HT-GF-STF： replace HT-MF Medium L-STF, The length is also 2 Symbols （8us）, Use long HT Cyclic shift , You can get MIMO The precise acquisition time and time required by the receiver AGC Set up ;
　　　2) HT-LTF1： replace HT-MF Medium L-LTF, The length is also 2 Symbols （8us）,HT-LTF1 Is the length of the HT-LTF Of 2 times ;
　　HT-GF and HT-MF comparison , Preamble saves 8us, They are as follows ：
　　　1) HT-GF-STF It's done L-STF and HT-STF The job of , Saved 4us;
　　　2) You don't need to follow 802.11bg Mode compatible ,L-SIG Unwanted , save 4us;
　　　3) additional 4us One is missing HT-LTF And provincial , As shown in the figure below ：

3、 ... and . Subcarriers

Simply compare 11n The difference between the subcarrier and the previous protocol , As shown below ：

　　1. Total number of subcarriers ： By the previous 52 Add to 56,40MHz It is 114 individual ;
　　2. Number of data subcarriers ： By the previous 48 Add to 52 individual , To increase the 4 All subcarriers are used for data transmission ,40MHz It is 108 individual , Than two 20MHz There's more 4 individual ;
　　3. Subcarrier bandwidth ：312.5 kHz, Same as before ;
　　4. Subcarrier pilot ： Is still 4 individual , but 40MHz yes 6 individual ;

Four . Time related parameters

A few related parameters are introduced in a simple diagram , Personally, the most important thing is TDFT and TGIS, The others are either unchanged , Or it is a new addition , Understanding is good. ：

IDFT/DFT cycle （Symbol length ） remain unchanged , Are all 3.2us, But new GI（short GI）, Only half of the original （0.4us）, This shows that the effective use time is improved , Before 4us Next 3.2us Is the effective use of time , Now? 3.6us in 3.2us It works , Efficiency has improved by about 10%;
　　 Note：GI The original purpose of the introduction is to solve the problem of transmission delay and multipath collision ,11n take GI Shortened 1 times , It means that in the case of obvious multipath effect ,short GI Not better , It could be worse .

5、 ... and . PPDU Sending rate

1. Non-HT format：L-STF、L-LTF、L-SIG Use 6Mbps Send at the rate of , The data part uses L-SIG Send at the rate specified in , The following is 11a The sending composition of ：

　　2. HT-mixed format：L-STF、L-LTF、L-SIG Use BPSK, r=1/2（ namely 6Mbps）,HT-SIG、HT-STF、HT-LTF Use QBPSK, r=1/2（ namely 6Mbps）, The data section uses HT-SIG Send at the rate specified in ;

　　Note： Working in 40MHz Bandwidth of ,L-STF、L-LTF、L-SIG、HT-SIG It was copied to 40M Other of the band 20MHz part , This has two advantages ：
　　　1) With the traditional 11a/g The device remains compatible , No matter in 40MHz Which one is on 20MHz, Traditional equipment can receive and process L-STF、L-LTF、L-SIG、HT-SIG,20MHz The equipment （non-HT、HT） The device is discovering HT-SIG Medium CBW Set to 1 when （ Indicate the use of 40MHz send out ）, Stop receiving ;
　　　2) about 11n For equipment , Two 20MHz Upper L-STF、L-LTF、L-SIG、HT-SIG It's the same , In this way, additional frequency diversity and processing gain can be obtained , Can improve the understanding of L-SIG and HT-SIG Detection success rate ;

　　3. HT-greenfield format：HT-GF-STF、HT-LTF1、HT-SIG Use BPSK, r=1/2, namely 6Mbps Send at the rate of , The data part uses HT-SIG Send at the rate specified in ;

6、 ... and . MIMO System

802.11a/g Time , Both the sender and the receiver have only one antenna , So it's also called SISO System , As shown below ：

　　 from 802.11n Agreement start , Introduced MIMO System ,MIMO The system is divided into two models
　　1. The sender sends the same signal from each antenna

　　 As shown in the figure above , The data sent by the two antennas of the sender is x, In this system, the sender can use beamforming technology , So as to improve the quality of the link .
　　2. The sender sends different signals from each antenna

　　 As shown in the figure above ,MIMO Exists in the system 2 A spatial flow , It is also called space division multiplexing （SDM）, stay SDM blessings , The data rate can be multiplied , The amount of increase is ：min（tx Number of antennas ,rx Number of antennas , Number of spatial streams ）;
　　3. Cyclic shifts（CSD, Cyclic shift ）
　　 stay HT-MF In mode , How to select which antenna to send L-STF、L-LTF、L-SIG Well ？ If you select a single antenna to send , The sender has 2 When the antenna , The power has decreased 3dBm, The sender has 4 The antenna is , The power has decreased 6dBm, and L-STF be used for AGC Set up , In order to avoid the problem of inconsistent transmission power ,L-STF、L-LTF、L-SIG It needs to be sent from all the antennas , The content transmitted on each antenna is the same , It's easy to inadvertently （ Don't want to ） Resulting in beamforming , This kind of unintentional beamforming has a high probability of causing coverage problems to terminals , And L-STF Large power changes can also lead to AGC Saturation or quantization error occurs in the setting process , therefore , For the correlation between different antenna signals , Introduced CSD Definition , Artificially create multipath , To solve ：
　　　1) The received signal power will not differ too much due to the different location of the receiving end ;
　　　2) It can reduce the peak to average ratio ,OFDM The signal is less likely to be distorted , For RF devices at the receiving end , A larger gain range can be obtained ;
　　 As for how different numbers of antennas need to be shifted , I haven't learned more about , Paste a picture first , The figure below shows L-STF、L-LTF、L-SIG、HT-SIG The cyclic shift value of ：

　　HT-LTF Circularly shift the flow in different spaces according to the above figure, as shown in the following figure ：

7、 ... and . STBC And LDPC

1. STBC It is called control block coding （space-time-block-coding）, yes 802.11n An optional feature of the protocol , Send multiple copies of the same data through multiple antennas , Even if some data is lost due to interference or attenuation , Reorganize different copies into one piece of data , So as to improve the reliability of data transmission , Data transmission over longer distances can be realized ;
　　2. LDPC Codes are called low density parity check codes （Low-density Parity-check）, It's also 802.11n An optional feature of the protocol , Its main advantages are ：
　　　1) LDPC Code has good anti-aging property , Under the premise of not affecting the system of Xi'an energy , Leave out the original 802.11 Interleaving module in the standard , This reduces the system delay ;
　　　2) Improve coding gain , Receiver in low SNR environment , It can still achieve a relatively low bit error rate , Thus, the sender （ such as AP） Coverage of ;
　　　3) comparison Turbo Code decoding algorithm ,LDPC The decoding operation of the code is low , And the hardware is easy to implement , It is very suitable for large capacity communication applications ;
　　Note： Some netizens have made analysis and comparison ,5.8G Frequency band use MT7612E chip , Turn on LDPC After function , Throughput can be improved 80Mbps;

8、 ... and . Channel binding

This feature is also an improvement to greatly improve throughput , Detailed see 《802.11–802.11n Channel binding of the protocol 》 One article ;

Nine . PHY Characteristic parameters

As shown in the following two figures ：

Ten . Channel evaluation CCA

20MHz and 40MHz Upper cca Different , Describe them separately ：
　　1. 20MHz Next , Following conditions CCA It is required to indicate that the channel is busy
　　　1) Signal detection ： Detected. OFDM Preamble And the signal strength is greater than -82dBm;
　　　2) Power detection ： Missed OFDM Preamble But the detected energy is greater than -62dBm;
　　　3) HT-GF testing ： I won't support it HT-GF Mode site , Signal strength greater than or equal to... Is detected -72dBm Of PPDU;
　　　Note： I have a little doubt , Feel No 3 Point and No. 2 What's the difference , It feels like energy detection ？
　　2. 40MHz Under the CCA
　　　1) Use 40MHz Bandwidth sites , Must be in the Lord 、 On the secondary channel CCA, Both are free , The channel is considered idle , Otherwise, as long as there is a busy , Think the channel is busy ;
　　　2) Only power detection is performed on the secondary channel ;
　　　3) When the secondary channel is idle , Main channel detection and 20MHz Exactly the same ;
　　　4) 40MHz Power detection ： Main channel 、 All secondary channels have detected 40MHz Signal time , The power is greater than or equal to -59dBm, Think the channel is busy ;
　　　5) 40MHz HT-GF testing ： I won't support it HT-GF Mode site , detected 40MHz The upper signal strength is greater than or equal to -69dBm Of PPDU when , It is required to indicate that the channel is busy ;

11、 ... and . Reference material

1. 《IEEE 802.11n-2009》;
　　2. 《IEEE 802.11-2016》;
　　3. 《 Next generation wireless LAN –802.11n Throughput 、 Robustness and reliability 》;
　　4. https://ww2.mathworks.cn/help/wlan/ref/wlanlstf.html#buzr5pu-1;
　　5. https://zhuanlan.zhihu.com/p/105614896;
　　6. 《IEEE802.11 OFDM Automatic gain control system for wireless area network 》;