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WLAN 802.11ac Physical layer


This tutorial section on WLAN-11ac basics covers following sub topics:
Main page  frame  PHY layer  MAC layer  data rates  spectral mask  beamforming  radio planning


This page of WLAN 802.11ac tutorial covers 802.11ac physical layer.

802.11ac physical layer

Figure depicts 802.11ac single user transmitter block diagram. As shown in the figure, 11ac physical layer consists of following modules:
• PHY layer pading to the MAC frame.
• Scrambler
• Forward Error Correction
• Stream parser
• Segment parser
• Convolutional code interleaver
• Data mapping
• LDPC tone mapping
• Segment deparsing
• Space time block coding
• pilot carrier insertion and cyclic shift diversity
• spatial mapping
• Inverse Fourier Transform
• Guard insertion and windowing
• preamble construction and frame formation
• ADC and RF Up conversion

As shown in the figure,11ac transmitter can be used for single user as well as multiple users. Input to this physical layer is MAC frame from MAC layer. We will see how the information flows through 802.11ac physical layer chain below.

PHY padding: First of all service field is formed and added at the start of data field. At this module, padding is added to match length of the frame as required to fill complete symbol boundary.

Scrambling and FEC encoding: Scrambling is constructed using shift registers and EX-OR gates which removes long string of 1's . After this the data is passed through convolutional encoder or LDPC encoder or BCC encoder. To derive higher and different data rates puncturing is performed on encoded data.

Stream parser: It operates on encoder output and divides the encoded bits into multiple streams. Output of stream parser is referred as spatial stream which is fed to the interleaver.

Segment parser: The transmissions of bandwidths 160 MHz and two blocks of 80 MHz will get mapped onto two 80 MHz frequency segments before the interleaving is performed. Segment parsing is not done on 20,40 and 80 MHz bandswidths.

Convolutional code interleaver: During transmission data bits get affected consecutively due to radio channel conditions. To disperse the consecutive bits over different subcarriers interleaving is performed. This makes it easier to correct the bit errors.

Data mapping: BPSK,QPSK,16QAM,64 QAM,256QAM are used for data constellation mapping. For BPSK one bit is mapped on carrier,for QPSK 2 bits are mapped and so on.

LDPC tone mapping: They are mapped to OFDM subcarriers seperated by sufficient gap. It functions same as interleaver. For example, in a 40 MHz case, two constellation points are seperated by about 6 subcarriers.

Segment deparsing: For 160 MHz bandwidth case,segment deparser gets two frequency segments together and makes it favourable for transmission.

Space time block coding: This is optional in the implementation. It converts one stream to be fed to multiple antennas to increase the redundancy. This way spatial streams are transformed into space time streams.

pilot carrier insertion and cyclic shift diversity: Pilot subcarriers are inserted and proper symbol is formed as per the table below based on bandwidth of operation. The pilot subcarriers are used for channel estimation and equalization. Each space time streams have been given different phase shifts so that they are easily identified at the receiver.

spatial mapping: Space time streams will be mapped on the transmit chains by the spatial mapper.In beamforming space time stream is shaped in the particular direction to focus the energy.

Inverse Fourier Transform: It converts frequency domain data into time domain data.64,128,256,512 FFT sizes are used for 20,40,80 and 160 MHz bandwidths.

Guard insertion and windowing: Guard is inserted at the start of the symbol and the symbol is windowed.This helps in overcome delay spread and useful for limiting the spectrum.

preamble construction and frame formation: VHT preamble is formed and is appended to the frame. Preamble helps in channel estimation and front end synchronization.

ADC and RF Up conversion: Digital data is converted to analog data and applied for RF frequency up conversion.



Subcarriers in 11ac

11ac
Bandwidth
Subcarrier
range
Pilot
subcarriers
Total subcarriers,
data subcarriers
20 MHz -28 to -1 and
+1 to +28
-21,-7,+7,+21 56 total,
52 data
40 MHz -58 to -2 and
+2 to +58
-53,-25,-11,
+11,+25,+53
114 total,
108 data
80 MHz -122 to -2 and
+2 to +122
-103,-75,-39,-11,
+11,+39,+75,+103
242 total,
234 data
160 MHz -250 to -130,-126 to -6,
+6 to +126, +130 to +250
-231,-203,-167,-139,-117,
-89,-53,-25,+25,+53,+89,
+117,+139,+167,+203,+231
484 total,
468 data

This tutorial also provide link to 802.11ac data rate, beamforming frame format, MAC layer, radio network planning, spectrum mask. Refer links mentioned on the left side panel.

RELATED LINKS

What is wlan?
WLAN standards-11a,11b,11g,11n,11ac
11a WLAN Physical layer
11b WLAN Physical layer
11n WLAN Physical layer
WLAN 802.11-ac
WLAN 802.11-ad
Difference between 11a,11b,11g,11n
Difference between 11-n,11-ac and 11-ad
WLAN router providers
WLAN providers

               

Other Standard Physical Layers

•  Wireless physical layer overview
•  11b physical layer
•  11a physical layer
•  fixed wimax physical layer-OFDM
•  mobile wimax physical layer-OFDMA
•  11n physical layer
•  GSM Physical layer
•  TD-SCDMA Physical layer
•  GPRS physical layer
•  LDACS1 Physical layer
•  10,40,100 Gigabit Ethernet Physical layer
•  Zigbee Physical layer
•  WCDMA Physical layer
•  Bluetooth Physical layer
•  WLAN 802.11ac Physical layer
•  WLAN 802.11ad Physical layer
•  LTE Physical layer


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