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UWB Frame Structure | SYNC SFD PHR PHY Payload STS

This page describes four different UWB frame structures.It mentions functions of UWB frame fields viz. SYNC, SFD, PHR, PHY Payload and STS.

Introduction: UWB (Ultra Wideband) technology follows IEEE 802.15.4 series of standards. IEEE 802.15.4z defines four possible frame structures which use upto 5 different fiels as shown. Each of these fields have specific patterns are used for specific functionalities.

UWB Frame Structures

Four different UWB frame structures or configurations are shown in the figure.
Configuration-0 : It is the format which does not contain STS field. It is used for legacy BPRF devices. It can also be used for HRPF mode without additional security.
Configuration-1 and 2 : It changes position of the STS field within the frame as shown.
Configuration-3 : It does not contain data payload part.
Let us understand functions, sizes and patterns of these UWB frame fields.

SYNC field

• SYNC stands for Synchronization field. It is transmitted at the beginning of the UWB frame for synchronization between transmitter and receiver.
• It contains predefined pattern of pulses which repeat specific number of times. Pulses follow RRC (Root Raised Cosine) shape with 0.5 roll off factor. Hence it is also called as "preamble".
• Reference pulse is used to form code sequence with sequence of pulses. Pulses can be positive pulse, negative pulse or no pulse (i.e. zero). Sequence contains either 31, 91 or 127 possible pulses based on BPRF or HPRF mode. Code index of about 32 different codes are used to differentiate various code sequences.

SFD field

• SFD stands for Start of Frame delimiter. It is positioned between SYNC field and PHR field.
• In order for UWB transmitter and receiver to work correctly both transmitter and receiver should have be configured with same SFD format. It is used for time-stamping which is useful for ranging.
• SFD field contains one of the three codes viz. ternary code with length 8, ternary code with length 64 or binary codes with 4/8/16/32 length.

PHR field

• PHR stands for Physical Layer Header. This field provides information to receiver about payload transmitted by the transmitter and other useful physical layer information.
➨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 rate of PHY payload. It can be 850 kbps or 110 kbps.
• Frame length indicates number of octets in payload with max. of 127. It is unsigned integer value.
• Ranging field indicates whether the present frame is used as ranging or not. It is set to binary one if frame is used as ranging otherwise set to zero.
• Bit 10 is reserved.
• Preamble bits (11-12) indicates PSR as defined in IEEE 802.15.4z specification. PSR stands for Preamble Symbol Repetitions which can contain 16-4096 repetitions.
• SECDED bits (13-18) contain simple hamming block code which is used for Correction of single bit error and detection of two bit errors at the receiver.

PHY Payload field

• This field carry actual data of an application or timestamp information.
• The data is modulated in different ways as per BPRF and HPRF modes. Both uses 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 as well as accuracy for ranging measurement.
• STS field requires additional synchronization between AES-128 keys and counter values.
• In BPRF mode, STS is sent at 64 MHz PRF where as in HPRF mode, pulses are repeated at 128 MHz.

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