UWB RSF vs RIF: MMS Ranging Fragments in IEEE 802.15.4ab

Introduction : To meet strict regulatory power limits while maximizing range, the IEEE 802.15.4ab standard introduces the Multi-millisecond (MMS) packet structure. There are two distinct types of packet fragments defined in UWB protocol viz. Ranging Sequence Fragment (RSF) and Ranging Integrity Fragment (RIF). While they may look similar on a spectrum analyzer, they serve opposing roles; one for precision, the other for security. This page demystifies these acronyms, explaining how RSF and RIF work together to enable long range and secured distance measurements.

RSF (Ranging Sequence Fragment)

• Purpose: The primary goal of the RSF is to enable very accurate ranging measurements.
• Composition: It consists of a specific repeating code sequence (called MMRS - Multi-millisecond Ranging Segment). It is similar to the preamble used in standard packets.
• Structure: It is constructed using code sequences with good cross-correlation properties (lengths of 128, 91, or 127). These sequences are repeated (32 to 256 times) to allow the receiver to accumulate energy and pinpoint the arrival time precisely.
• Timing Reference (RMARKER): The timestamp reference point is defined as the time of the peak of the first pulse in the first RSF at the local antenna.

RIF (Ranging Integrity Fragment)

• Purpose: The primary goal of the RIF is to secure the ranging estimation (Distance Commitment). It prevents attackers from spoofing the signal to fake the distance.
• Composition: It consists of a Scrambled Timestamp Sequence (STS).
• Structure: Unlike the repetitive RSF, the RIF uses a Deterministic Random Bit Generator (DRBG) to create a non-repeating sequence of pulses. This makes the signal unpredictable to attackers.
• Timing Reference (RMARKER): RMARKERs are defined for each RIF fragment, specifically at the peak of the first pulse and the peak of the last pulse in the fragment.

Comparison Table: RSF vs. RIF

Summary: The dual approach of using RSF for accuracy and RIF for security allows IEEE 802.15.4ab devices to achieve the best of both worlds. In an MMS packet, these fragments can be mixed to achieve the desired balance of accuracy and security.

• RSF-only packets: Used when high accuracy is needed but security is not a concern.
• RIF-only packets: Used when high security is the priority.
• Mixed RSF/RIF packets: The most robust method, using RSF to establish precise timing and RIF to cryptographically verify that the timing hasn’t been tampered with.