80211 (B/A/N/AC) Rate Introduction

The reference book is the "802.11 wireless network authoritative guide", which can be downloaded online to
Some basic knowledge of WiFi can refer to this series of posts http://support.huawei.com/ecommunity/bbs/10219401.html

1. First generation WiFi standard 80211 (80211b) (2.4G)

Completed in 1997, the transfer rate supports 1Mbps and 2Mbps. Support the use of FHSS (frequency hopping) and dsss (direct sequence spread spectrum), the use of different technology devices are incompatible, the following only explain DSSS technology.

The DSSS technology divides the 2.4G band into 14 channels (different country-supported channels, depending on the country channel table. xlsx included in the URL above)

The first defined 1Mbps and 2Mnps dsss belong to the 80211 protocol, and subsequent 80211b defines a physical layer of up to 5.5 and 11Mbps, so the usual assertion is that the 80211b rate includes 1, 2, 5.5, 11Mbps

The operation of DSSS is to deal with the data stream by means of the frequency code (chipping sequence, or the chip sequence).

Generation transmission of data is 1, then use the frequency code "01001000111" instead of 1, the frequency code itself is sufficient redundancy check, improve anti-jamming ability.

1. Mbps Rate Transmission

In the use of RF carrier signal transmission, using the Dbpsk method, DBPSK (differential binary phase shift keying) with two carriers of the digital signal 0 and 1 respectively, two carrier phase difference 180 °, a wave called the reference wave
(Reference wave), the corresponding encoding is 0. A wave with an offset of half a period, the corresponding encoding is 1. When the symbol is 0, the waveform is like the last symbol. If the symbol is 1, it will be offset by half a period.

is the waveform at transfer 1001101.

When the 1Mbps transmission of data as required by the Protocol, the radio waves are actually transmitting 11M bits per second (need to be performed first), then the frequency of the carrier is 2.4G Hz, then the duration of each carrier hold is (1/11M)/(1/2.4G) = 218. (I don't know if it's necessary to consider the ACK response and the time of the competition)
(How does this correspond to the bandwidth of the channel, it is unclear)

1. 2Mbps Transmission

Using the DQPSK (differential quadrature phase shift keying), a total of 4 phase carrier, each carrier difference 45 °, so that the information carried in the RF signal than DBPSK added a cup, each carrier represents two bit of data.

The same transmission 01001101, DQPSK only half the time (the retention period of each carrier is constant)

1. 5.5Mbps and 11Mbps

In DQPSK based on the use of CCK code, CCK coding there are two ways to compress the RF signal 8bit data to 2bit or 4bit (CCK uses a complex mathematical conversion function, you can use a number of 8-bit sequence, in each encoded word encoded 4 or 8 bits)

2.80211A Rate (5G)

At first, 802.11A was designed specifically for the United States, using 5-GHZ countries to avoid the information infrastructure band. 802.11A is based on orthogonal crossover multi-work (orthogonal frequency division multiplexing, short-OFDM). The OFDM device will cut a wide channel (frequency channel) into several sub-channels (subchannel). Each sub-channel is used to transmit data. All of these "slower" sub-channels are then combined into "faster" channels in a multi-worker way. So the rate will be faster than a simple channel.

The bandwidth of the working channel in 802.11A is MHZ,OFDM the physical layer to differentiate the spectrum into the operating channel. Each bandwidth 20-mhz channel consists of 52 subcarrier. The sub-carrier asks each other a 0.3125MHz channel number from 26 to 26, of which 4 sub-carriers act as pilot carrier, subcarrier-21,-7, 7, and 21 are designated as guided waves, and No. 0 is not used. (Remove the interval between the subcarrier, the width of each channel is only 0.072Mhz, really so narrow??? ）

802.11A uses "quadrature amplitude modulation" (quadrature amplitude modulation, or QAM) technology to transmit data on each sub-channel. QAM is the encoding of data on a single carrier wave, but the carrier is composed of two signals, the same phase (in-phase) and the lower orthogonal: (quadrature), which are simultaneously modulated for both signals. That is to adjust the carrier waveform according to the size of the input signal? The main carrier is the same as the letter number, abbreviated to I. The quadrature signal lags behind One-fourth Cycles, abbreviated to Q. Quadrature is to ensure that two signals (I and Q) are transmitted simultaneously without interference.

When the amplitude variation of two orthogonal wave I and Q is 2 4 8 o'clock, the modulation mode of QPSK 16QAM 64QAM can be formed respectively. BPSK can be used as a special orthogonal wave, only I-wave without Q-wave. When the amplitude changes more and more, the lower the anti-jamming ability, the equipment section flexibly chooses the different amplitude modulation through the environment monitoring. The BPSK and QPSK here are only modulated in the same way as in 80211b, in fact the channel width and other physical parameters are completely different.

Note: Why there is no modulation of amplitude change of 6, guess is 6 and 8 modulation, the demand for hardware is the same (or higher, because 6 is not the power of 2, demodulation trouble), so there is no 6 modulation mode

In order to ensure the accuracy of data transmission, the need to add error correction code, according to the environmental interference, the addition of the error correction code is more and less, 80211a specified 3 kinds of Correction code added (valid data) encoding rate of 1/2 2/3 3/4.
The 802.11A standard stipulates that 6, 12, and Mbps are necessary (required) data rates. 802.11 MAC requires a positive response to each frame. When transmitting a positive response, the data rates that are supported by the connected workstations must be used. Most devices transmit a positive response message at a data rate of up to Mbps, which not only meets the requirements above, but also minimizes the burden on the network (overhead). Transmission rates for different encoding and encoding rates

Note: The last 72Mbps rate does not include an error correction code. 48 sub-channel information is combined to count as a single symbol.

There is also a doubt that the 80211a in the BPSK and 80211b BPSK, the same modulation method, The former bandwidth of each sub-channel is only 0.072MHz, the rate of 0.25Mbps, the latter bandwidth 20MHz, the rate of only 1mbps,80211a in each sub-channel and what kind of technology to take is unknown.

Channel assignment for 80211A
According to the following formula, you can assign a number to a channel in the 5-ghz band and 5 MHz between channels:
Center frequency (MHz) = 5,000+5 x n;n = 0,1,2, ... 199
Center frequency (MHz) = 5,000–5* (256-n), n=240,241,.. 255

Each 802.11a channel with a bandwidth of 20-mhz occupies four channels. The channel center frequency is as follows

3 80211g,80211a application in 2.4G Band (2.4G)

80211G Technology (OFDM and QAM) in 80211a is applied to 2.4g, while the original 80211b on 2.4G is compatible with the highest rate and 80211a of 54Mbps.

4 80211n (2.4g/5g)

The 80211N protocol also stipulates the implementation of the 2.4g and 5g, in order to increase the rate, 80211n used a new addition of several other technologies (80211a in the same way to follow the encoding):
1 MIMO (Multi-input multi-output)
An antenna of an AP (router) and an aerial of STA (mobile phone) can form a spatial stream, and when the AP and STA have n antennas at the same time, n space streams can be composed, and the transfer rate is n times of a single spatial stream. Up to 4 of the paper specified in 80211n
Note that when the STA has only one antenna, even if the AP has 4 antennas, it can only form 1 spatial streams
When I look at the wiki, I see a 3x2:2 mimo saying, and I don't quite understand the definition, but I'll add it later.
2 Wider bandwidth
The previous protocol specifies that bandwidth can be as wide as 40Mhz and 20Mhz two in 20mhz,80211n. The bandwidth becomes larger, and the sub-carriers of OFDM become more. Note that it is not necessary to combine two contiguous channels into a 40M bandwidth.
3 Short GI (Protection time)
GI (Guard Interval) is only two streams (symbols (the smallest unit of data sent at once)) between the intervals. When there are multiple spatial flows, there is interference between the spatial flows, and the STA takes a while to combine the data from multiple spatial streams into a single stream, thus requiring an additional guard time to prevent the next stream from interfering with the previous
Two types of GI for 800ns and 400ns are specified in the 80211n

The following table is from Wikipedia (China has recently been able to wiki), the first column MCS (modulation and Coding Scheme) value different modulation encoding method, You can see that the maximum rate of 80211n is 600Mbps (both 2.4G and 5G), and the maximum rate for a single antenna (most phones) is 150Mhz.

5 80211ac (5G)

The 80211AC works only in the 5G band. The following new technologies are used on a 80211n basis:
1 larger bandwidths of 80MHz and 160Mhz,
2 higher MIMO values, 4 in 80211n, 11AC increased to 8
3 Up to 254QAM

For MIMO values of 1 o'clock, the rate value of 11AC, when the space flow is 8 o'clock, the maximum rate is 866*8=6928mbps

80211 (B/A/N/AC) Rate introduction