Understanding UWB Frame Structure: SYNC, SFD, PHR, Payload, and STS

This document describes four different UWB frame structures, outlining the functions of UWB frame fields such as SYNC, SFD, PHR, PHY Payload, and STS.

Introduction

UWB (Ultra Wideband) technology adheres to the IEEE 802.15.4 series of standards. Specifically, IEEE 802.15.4z defines four possible frame structures, utilizing up to 5 different fields, each with specific patterns that serve unique functionalities.

UWB Frame Structures

The figure illustrates four distinct UWB frame structures or configurations.

• Configuration-0: This format lacks the STS field and is designed for legacy BPRF devices. It can also be employed in HRPF mode without requiring additional security.
• Configuration-1 and 2: These configurations alter the position of the STS field within the frame.
• Configuration-3: This configuration omits the data payload section.

Let’s delve into the functions, sizes, and patterns of these UWB frame fields.

SYNC Field

• SYNC stands for Synchronization field. It’s transmitted at the beginning of the UWB frame to synchronize the transmitter and receiver.
• It consists of a predefined pattern of pulses that repeat a specific number of times. These pulses follow a Root Raised Cosine (RRC) shape with a 0.5 roll-off factor, hence it’s also called a “preamble.”
• A reference pulse is used to create a code sequence composed of pulses. Pulses can be positive, negative, or absent (zero).
• The sequence may contain 31, 91, or 127 possible pulses depending on the BPRF or HPRF mode.
• Approximately 32 different code indices are used to distinguish between various code sequences.

SFD Field

• SFD stands for Start of Frame Delimiter. It is located between the SYNC and PHR fields.
• For proper UWB transmitter and receiver operation, both must be configured with the same SFD format.
• It is used for time-stamping, which is useful for ranging.
• The SFD field contains one of three codes: a ternary code with a length of 8, a ternary code with a length of 64, or binary codes with lengths of 4, 8, 16, or 32.

PHR Field

• PHR stands for Physical Layer Header.

• This field provides the receiver with information about the payload transmitted by the transmitter, as well as other useful physical layer details.

  • PHR = {Data rate (0-1 bits), Frame length (2-8 bits), Ranging (bit 9), Reserved (bit 10), Preamble duration (bits 11-12), SECDED (bits 13-18)}

• Data rate indicates the rate of the PHY payload, which can be 850 kbps or 110 kbps.

• Frame length indicates the number of octets in the payload, with a maximum of 127. It’s an unsigned integer value.

• The Ranging field indicates whether the present frame is used for ranging or not. It is set to binary one if the frame is used for ranging; otherwise, it’s set to zero.

• Bit 10 is reserved.

• Preamble bits (11-12) indicate PSR as defined in the IEEE 802.15.4z specification. PSR stands for Preamble Symbol Repetitions, which can contain 16-4096 repetitions.

• SECDED bits (13-18) contain a simple Hamming block code used for the correction of single-bit errors and the detection of two-bit errors at the receiver.

PHY Payload Field

• This field carries the actual data of an application or timestamp information.
• The data is modulated differently based on BPRF and HPRF modes. Both use the same convolution encoder and Reed-Solomon encoding types.
• In BPRF (Base Pulse Repetition Frequency) mode, data is modulated using BPM-BPSK modulation.
• In HPRF (High Pulse Repetition Frequency) mode, data is modulated using BPSK modulation.

STS Field

• STS stands for Scrambled Timestamp Sequence.
• It is used to provide additional integrity and accuracy for ranging measurements.
• The STS field requires additional synchronization between AES-128 keys and counter values.
• In BPRF mode, STS is sent at 64 MHz PRF, whereas in HPRF mode, pulses are repeated at 128 MHz.