WLAN 802.11ad Physical Layer Overview

This page provides an overview of the WLAN 802.11ad physical layer. It covers different 802.11ad physical layer configurations, including Control PHY, Single Carrier PHY, OFDM PHY, and Low Power Single Carrier PHY. All physical layer versions share a similar packet structure, containing preamble, header, payload, and optional TRN fields.

General Packet Structure

All 802.11ad physical layer versions share a common packet structure. This structure is composed of the following:

• Preamble: Consists of a Short Training Field (STF) and a Channel Estimation Field (CEF). The preamble aids in Automatic Gain Control (AGC) and frequency offset synchronization. It also helps in identifying the type of PHY used.
• Header: Contains critical information like Modulation and Coding Scheme (MCS), the length of the data payload, and a checksum. The header structure varies depending on the PHY type (OFDM, SC, or Control).
• Payload: The actual data being transmitted. The MCS from the header informs the receiver how to decode this payload.
• TRN (Training) Field: An optional field containing beamforming-related information.

Preamble Details

The preamble is constructed using Golay Sequences. These sequences consist of bipolar symbols (+1 or -1). Different preamble types utilize basic building blocks such as Golay sequences like Ga 128 and Gb 128. The receiver utilizes the CE field for channel estimation and correction.

Header Details

The header carries vital information, with the most important being the MCS (Modulation and Coding Scheme). The MCS informs the receiver of the modulation and coding scheme used for the payload, enabling it to decode the data correctly.

OFDM PHY Packet Structure

Figure 1 illustrates the OFDM PHY packet structure. The header is always QPSK modulated.

Figure 1: WLAN 11ad OFDM PHY frame

The receiver first decodes the header after front-end synchronization using the known preamble sequence. Based on the decoded MCS, the OFDM variant receiver modules are configured, and the payload is subsequently decoded.

WLAN 802.11ad OFDM Physical Layer Transmitter

The OFDM transmitter modules perform several functions to prepare data for transmission.

Figure 1: OFDM-802.11ad PHY Tx

• Scrambler: The binary information from the MAC layer is passed to the scrambler, which uses the polynomial $X^7 + X^4 + 1$. This process eliminates long strings of ones and zeros, improving adjacent channel rejection and facilitating time synchronization at the receiver by ensuring frequent transitions between 1 and 0.

• LDPC Encoder: LDPC (Low-Density Parity Check) encoding is a forward error correction technique common to all PHY versions (Control, SC, and OFDM). It uses a codeword length of 672 bits and various code rates (1/2, 5/8, 3/4, or 13/16) to carry different numbers of payload bits.

  LDPC Code Rate	Code Word Size	Number of Data Bits
  1/2	672	336
  5/8	672	420
  3/4	672	504
  13/16	672	546

• 3X Repetition: The header data is repeated three times to enhance error correction, crucial for reliable communication in challenging channel conditions.

• Carrier Mapping/Modulation: Converts bits into complex symbols using modulation schemes like SQPSK, QPSK, 16-QAM, and 64-QAM, based on the MCS.

Figure: IFFT symbol structure

• Symbol Formation: Forms OFDM symbols, using 16 pilot subcarriers, 1 DC carrier, 336 data subcarriers, and 159 guard subcarriers as per the IFFT structure.

• IFFT: Performs a 512-point IFFT to convert frequency domain data symbols into time domain data symbols.

• Cyclic Prefix Insertion: Adds a guard interval by inserting the last few samples of the OFDM symbol at the beginning, resulting in about 25% overhead. The cyclic prefix is equal to 1/4 of the symbol duration.

• Packet Formation: Constructs the 11ad packet consisting of preamble, header, and payload.

• Windowing: Smoothens the transition between adjacent fields, except for SC modulated packet fields and the preamble. A sample transfer function is:

  $W_T(t) = \sin^2(\pi/2(1/2 + t/T_R))$ for $-T_R/2 < t \le T_R/2$

  $W_T(t) = 1$ for $-T_R/2 < t \le T - T_R/2$

  $W_T(t) = \sin^2(\pi/2(1/2 - ((t-T)/T_R)))$ for $T - T_R/2 < t \le T + T_R/2$

• RF Upconversion: Converts the baseband data to RF modulated data for over-the-air transmission, supporting various RF frequencies, including 2.4 GHz, 5 GHz, and 60 GHz.

WLAN 802.11ad OFDM Physical Layer Receiver

The OFDM receiver reverses the transmitter’s operations to recover the original data.

Figure 2: OFDM-11ad PHY Rx

• Down Conversion and De-windowing: The received packet is down-converted, and de-windowing is applied.
• Front-End Synchronization: Coarse time offset estimation and correction identifies the start of the frame using threshold detection, differentiating between noise and the actual WLAN 11ad packet. Cyclic prefix removal follows.
• Fine Time Offset Estimation and Correction: Performed to the level of 1 sample resolution.
• Frequency Offset Estimation and Correction: Compensates for frequency errors.
• Channel Estimation: Complex channel response coefficients are determined in the frequency domain using preamble sequences (STF or CEF).
• Channel Equalization: Uses estimated channel response coefficients for each of the symbols of the OFDM packet to compensate for channel impairments.
• Phase De-rotation: Corrects phase rotation using pilot subcarriers embedded in the OFDM symbol.
• Symbol De-formation: Removes pilots, DC, and guard subcarriers from each OFDM symbol.
• Demapping: Based on the MCS, data subcarriers are demapped using QPSK, 16QAM, or 64QAM.
• LDPC Decoding: Header data undergoes de-repetition (3X) before LDPC decoding.
• Descrambling: Recovers the MAC data by reversing the scrambling process.

WLAN 802.11ad Control Physical Layer Transmitter

The Control PHY is designed for control signaling and beamforming training.

Figure 3: Control 802.11ad PHY Tx

• Control PHY Preamble: Consists of STF and CEF with the following structure:

  • STF = {48 times Gb 128, once -Gb 128, once -Ga 128} = 50 * 128 = 6400 Tc in length

  • CEF = {Gu 512, Gv 512, -Gb 128} = 9 * 128 = 1152 Tc in length

    Where:

    • Gu 512 = {-Gb 128, -Ga 128, +Gb 128, -Ga 128}
    • Gv 512 = {-Gb 128, +Ga 128, -Gb 128, -Ga 128}

• Control PHY Header: A 40-bit header with the following fields:

  • Control packet header = {Reserved-1 bit, Scrambler initialization-4 bits, Length of data field-10 bits, Packet type-1 bit (mentions beamforming training field is for transmitter or receiver), Training length-5 bits (specifies if beamforming training field is used and its length), Turnaround-1 bit, Reserved-2 bits, HCS-16 bits}

• Scrambler: Same as in OFDM PHY.

• LDPC Encoder: Uses shortened 3/4 LDPC resulting in 1/2 or less code rates. Employs 672 bits of codeword length.

• Differential Encoder:

• Modulation: Uses $\pi/2$ BPSK modulation to avoid zero crossing in the I/Q diagram. Only MCS0 is supported.

• 32x Spreading: Performs 32 times spreading with Ga 32 and $\pi/2$ rotation.

• Spectrum Shaping: Not defined in the standard.

• Upconversion: Converts baseband data to RF, as described earlier.

Figure: WLAN 11ad control PHY frame

• Packet Formation: Follows the 802.11ad control frame structure.

WLAN 802.11ad Single Carrier Physical Layer Transmitter

The Single Carrier (SC) PHY supports data rates ranging from 385 Mbps to 4.620 Mbps, depending on the MCS.

Figure 4: SC PHY Tx

The basic packet structure is the same in SC and Low Power SC PHY versions, but 16QAM is not used in Low Power SC mode to conserve energy.

• Transmitter Structure: The SC PHY transmitter includes a scrambler, LDPC Encoder, repetition block (2X), CP insertion, and modulation.

• Modulation: Modulation schemes include BPSK, QPSK, or 16QAM.

  Component	Description
  Scrambler	Randomizes the data stream
  LDPC Encoder	Applies forward error correction
  Repetition Block	Increases redundancy for better reliability
  CP Insertion	Adds a cyclic prefix to mitigate inter-symbol interference
  Modulation	Maps the data onto a carrier signal (BPSK, QPSK, 16QAM)

• SC PHY Preamble:

  • STF = {16 times Ga 128, -Ga 128}
  • CEF = {Gu 512, Gv 512, -Gb 128}

• SC PHY Header: 64 bits in length. SC header= { {7 bits(scrambler init.), 5 bits(MCS), 18 bits(data Length), 1 bit(add.PPDU), 1 bit(packet type), 5 bits(training length), 1 bit(aggregation), 1 bit(beam tracking request), 4 bits(last RSSI), 1 bit(turnaround), 4 bits(reserved), 16 bits(HCS) }

Figure: WLAN 11ad SC PHY frame

WLAN 802.11ad Low Power Single Carrier Physical Layer Transmitter

The Low Power SC PHY is designed for energy-efficient communication.

Figure 5: Low Power SC 802.11ad PHY Tx

• Transmitter Structure: Includes a scrambler, RS Encoder (224,208), Block Encoder, Block interleaver, CP insertion, and modulation types ($\pi/2$ -BPSK, $\pi/2$ -QPSK).

Figure: WLAN 11ad low power SC PHY frame

• Packet Formation: The packet is formed according to the packet structure.
• Up Conversion: Based on the RF frequency required, necessary up conversion is carried out.