802.11 Physical Layer

IEEE 802.11 Wi-Fi 通訊協定摘要

802.11 Physical Layer

802.11 splits the PHY into 2 generic components:

1. The Physical Layer Convergence Protocol (PLCP) to map the MAC frames onto the medium.
  The PLCP straddles the boundary of the MAC and physical layers.
  The PLCP adds the preamble to help synchronize incoming transmissions.
  The preamble depends on the modulation method.
  Each physical layer has its own PLCP to add specific framing parameters.

   The detail diagram of OFDM framing;

2. The Physical Medium Dependent (PMD) to transmit frames
  The PMD is responsible for modulating and transmitting any bits it receives from the PLCP into the air using antenna.

Physical layers are defined for different radio technology:
* Frequency-hopping (FH)
* Direct-sequence spread spectrum (DSSS)
* Infrared light (IR)
* Orthogonal Frequency Division Multiplexing (OFDM): 802.11a
* High-rate Direct Sequence (HR/DS) : 802.11b
* Extended Rate PHY (ERP): 802.11g
   802.11g is really several physical layer specification in one:
   * ERP-DSSS and ERP-CCK: Compatible with DSS( 1 Mbps and 2 Mbps) and 802.11b enhancement ( 5.5 Mbps and 11 Mbps )
   * ERP-OFDM: It is the major mode  and essentially running 802.11a in 2.4 GHz. It supports speeds as 802.11a: 6, 9, 12, 18, 24, 36 and 54 Mbps.
   * ERP-PBCC: An optional extension to the PBCC in 802.11b. It is not widely used.
   * DSSS-OFDM: This is optional and not widely used.
  802.11b chipsets have no way of making sense of 802.11g transmissions.
The 1st solution is to require that 802.11g stations transmit a a rate supported by all stations in BSS.
The 2nd solution is to avoid interference between 802.11g and 802.11b networks. To avoid interference during the transmission of the OFDM frame and its ACK, a slower frame is sent to update the NAV. Protection is controlled through ERP information element in Beacon frames.
* High-throughput (HT) / MIMO: 802.11 n


Single-Input/Single-Output (SISO) system: In the transmission link, the transmitter uses one antenna and the receiver uses one antenna. Only one antenna is active for any given data frame. SISO devices with antenna diversity would choose the antenna that received the strongest signal.
Multiple-Input/Multiple-Output (MIMO) system: All of the antennas are active simultaneously. Each antenna receives its own data stream and each antenna transmits its own data stream. 802.11n supports the use up to 4 spatial streams.

The Shannon–Hartley theorem says that the limit of reliable information rate (data rate exclusive of error-correcting codes) of a channel depends on bandwidth and signal-to-noise ratio according to:

where

I is the information rate in bits per second excluding error-correcting codes;

B is the bandwidth of the channel in hertz;

S is the total signal power (equivalent to the carrier power C); and

N is the total noise power in the bandwidth.

dB = 10 x log (power-out/power in)

the power x 2 --> 3 dB increase

the power x 1.25 --> 1 dB increase

dBm stands for dB above 1 mW.

Radio Frequency Front End Module

Front end modules are ultra-small built-in modules integrated with various functional components used in wireless front end circuits, such as LTE, Wi-Fi, Bluetooth, and GPS.
射頻前端(RF front-end)是無線電接收器電路中的通用術語，描述的是在電線以及中頻(intermediate frequency)第一階段的電路。這指的是用於處理接收原無線電頻率(radio frequency)訊號的接收器所有相關組件，並隨後轉換至較低的中頻。
對微波以及衛星接收器而言，一般稱之為低噪音模組(LNB)或低噪音變頻器(LND)，通常設置於天線上，如此一來能夠讓來自antenna(天線)的訊號傳輸到transiever(接收器)。
Acting as an interface between the antenna and RF transceiver, the FEM integrates the RF components required for analog performance, such as multiple RF switches (used in both Tx and Rx paths), Low Noise Amplifiers (LNAs), Power Amplifiers (PAs), and antenna tuners.

802.11n

T x R : S

The maximum data rate is determined by the number of spatial stream S.
Each spatial stream must have its own radio chain on both the sending and receiving side.
T >= S
R >= S

802.11n devices can negotiate the number of streams used, and devices will find the highest common speed they can use.

Space-Time Block Coding (STBC): It requires 2 radio chains to transmit a single spatial stream.
Maximal Ratio Combining (MRC): It uses all radio chains to process a frame. It takes the strongest components of the received signal from each antenna.

The Modulation and Coding Set (MCS) number is a value that describes:
* the number of spatial streams
* modulation method
* error-correcting code
802.11n supports equal modulation and unequal modulation.
in equal modulation, all spatial streams are transmitted in the same manner.
Most products only support equal modulation modes, which are the first 32 MCS values.

802.11n specifies 2 forward-error correction (FEC) codes : convolutional code and the optional support for Low-Density parity Check(LDPC).



802.11n defines the PLCP which supports 3 modes;

* Non-HT mode. This is required to inter-operate with 802.11 a/b/g devices. It is also called legacy mode. The frame format is exactly the same as 802.11 a or 802.11g.
* HT mixed mode (HT-MM). The PLCP header is compatible with 802.11 a/g PLCP headers even the high-speed 802.11n body cannot be decoded by 802.11 a/g devices.
* HT-Greenfield (HT-GF) mode. The PLCP header is slightly shorter than the mixed mode header and can bed used for 802.11n devices.

All 11n devices must support both the non-HT mode and the mixed mode.

The "SIG" field describes the data rate and length. The signal field is transmitted at a standard rate (MCS 0).

Modulation and Coding Scheme ( 7 bits ): one of 76 values.
Channel bandwidth (1 bit ): 0 for 20 MHz and 1 for 40 MHz.
HT length ( 16 bits ): the length of PSDU in bytes.
Sounding ( 1 bit ): 0 for non-beamforming and 1 for beamforming.
Aggregation ( 1 bit ):  1 when the payload is an aggregated frame.
STBC ( 1 bit ): Space-Time Block coding allows multiple radio chains to be used to increase the sensitivity to a single data stream.
Forward Error Correction coding ( 1 bit ): 0 for convolutional code and 1 for LDPC.
Short guard interval ( 1 bit ): 0 for the long guard interval in use.

802.11n has a number of options that together determine the data rate: modulation, coding and spatial streams.

Each combination is given an MCS number.

The short guard interval will add 11% data rate.
The most used spped are the following:

• 150 Mbps: 2 spatial streams + 20 MHz channels + short guard interval ar MCS15
• 300 Mbps: 2 spatial streams + 40 MHz channels + short guard interval ar MCS15
• 450 Mbps: 3 spatial streams + 40 MHz channels + short guard interval ar MCS15
• 600 Mbps: 4 spatial streams + 40 MHz channels + short guard interval ar MCS13

Beamforming

The traditional antennas transmit signal to all directions equally, even though the receiver is only in one direction of those directions.
Beamforming enables an antenna array to focus the energy in the direction of a client device. The power is focused on the direction of the receiver so that the signal to noise ration is increased, higher SNR enables the use of more aggressive coding for higher speed.
With beamforming, the downstream link from AP to clients gets more power. However, the AP cannot set up the antenna with sensitivity pointing at the client because it it may be serving multiple attached client devices. As a result, beamforming generally leads to asymmetric links.