LiFi MAC Layer Basics

This page describes the basics of the LiFi MAC layer, including the LiFi PHY frame structure and the LiFi MAC frame structure.

The LiFi standard employs the following MAC modes for the transmission of both data and management frames:

• Single mode: In this mode, each frame carries only one PPDU (PLCP Protocol Data Unit). It is typically used for short data communications such as acknowledgements (ACKs), association requests, and beacon transmissions.

• Packed mode: In this mode, a single frame carries multiple PPDUs destined for the same destination. This improves the efficiency of the MAC layer by eliminating repetitive PHY and MAC headers for data going to a single receiver.

• Burst mode: In this mode, after the first frame, subsequent frames use a PHY preamble with reduced length. Moreover, it uses RIFS (Reduced Interframe Space) instead of SIFS (Short Interframe Space). This increases efficiency and improves overall throughput.

• Dimmed OOK mode: This mode is used for data transfer in dimming applications, where the light intensity is adjusted.

The figure above illustrates the different LiFi frame modes.

A LiFi system utilizes a PHY frame consisting of a preamble, a PHY header, and a PSDU (PHY Service Data Unit), as shown in the figure below.

LiFi PHY Layer Frame Structure

Fig-1: LiFi PHY Frame (PPDU structure)

Multiple octets are transmitted with the least significant octet first, and within each octet, the least significant bit is transmitted first.

• Preamble: The preamble is used by the transceiver to perform optical clock synchronization.

  • Preamble = { FLP (64 to 16384 bits), TDP(15 bits each), ~TDP, TDP, ~TDP }
  • FLP (Fast Locking Pattern) consists of alternating ones and zeros.
  • Four TDPs (Topology Dependent Patterns) distinguish the different PHY modes being used.
  • Every alternate TDP pattern is inverted to achieve DC balance in the LiFi Physical layer.

• PHY Header: The PHY header is transmitted using OOK (On-Off Keying) modulation. It carries information about:

  • Burst mode
  • Channel number
  • MCS ID (Modulation and Coding Scheme ID), which defines the PHY type (PHY-I, II, or III) and the data rate.
  • PSDU length, etc.

• HCS (Header Check Sequence): This is a 16-bit field used to protect the PHY header in the LiFi Physical layer. There are optional fields in the frame used in PHY-I mode at a clock rate of 200 KHz.

• PSDU (PHY Service Data Unit) Field: This field has variable length and carries the actual data of the PHY frame. PSDU content can be represented as:

  • PSDU: { MHR, MAC Payload i.e. MSDU, FCS, Tail Bits}. This structure is described in the MAC frame section below.

LiFi MAC Layer Frame Structure

Fig-2: LiFi MAC Frame (MSDU structure)

The LiFi MAC frame consists of an MHR (MAC Header), an MSDU (MAC Service Data Unit), and an MFR (MAC Footer).

The Frame Control field, carried within the MHR, is 2 octets in size. It is composed of:

• Frame type
• Addressing fields
• Control flags

The MAC frame type is 3 bits long and is used to differentiate between different frame types, such as Beacon, Data, Acknowledgment, Command, and CVD (Channel Validation Data) frames.

Source and destination addresses can be either 16 bits or 64 bits in length.

Note: The information provided on this page is derived from the IEEE 802.15.7-2011 draft version. Please read latest standard specifications published by IEEE for any updates.