An Overview of the IEEE 802.15.4 HRP UWB Standard
The ultra-wideband (UWB) communications technology development started at the end of the 1990s and got a boost in 2002 when the Federal Communications Commission (FCC) published new regulations for the commercial market. The Institute of Electrical and Electronics Engineers (IEEE) 802.15 working group specifies wireless personal area network (WPAN) standards. Task groups (TG) in IEEE 802.15 leverage UWB technologies for 802.15.3 high-rate and 802.15.4 for low-rate communication applications. Key technologies include multiband orthogonal frequency division multiplexing (MB-OFDM) and pulsed direct sequence UWB (DS-UWB).
Today, these technologies are still widely used in the wireless industry, such as IEEE 802.11ad/ay using MB-OFDM for a multiple gigabit wireless system and IEEE 802.15.4/4z using pulsed DS-UWB for indoor positioning, ranging, and security applications. Among these technologies, the applications and signal characteristics of IEEE 802.15.4 high-rate pulse repetition frequency (HRP) UWB are the most different from other UWB standards. Let’s take a look at the definition of UWB.
What is Ultra-Wideband?
UWB is a radio signal with an instantaneous bandwidth of greater than 500 MHz or a fractional occupied bandwidth (Bf) greater than 0.2. Figure 1 shows the upper frequency fH and lower frequency fL points that are 10 dB below the highest power spectral density of the signal.
Bf = B / fC
Where B = fH - fL and fC = (fH + fL) / 2
Figure 1. Power spectral density
There are two major UWB technologies – multiband orthogonal frequency division multiplexing (MB-OFDM) and direct-sequence ultra-wideband (DS-UWB). In 2006, WiMedia Alliance adopted MB-OFDM to support wireless video after IEEE 802.15 TG3a closed the project. Task group (TG) 4a leveraged DS-UWB for precision ranging and finalized the first standard in 2007. In 2011, IEEE 802.15.4 included TG 4a as one of the physical layers (PHY) options, then completed a revision in 2015 for defining two UWB PHYs – high-rate pulse frequency (HRP) coming from TG4a and low-rate pulse (LRP) coming from TG4f known as radio-frequency identification (RFID).
The IEEE 802.15.4z amendment put forth in 2019 enhances the ultra-wideband physical layer with additional coding and preamble options, improvements to existing modulations to increase the integrity and accuracy of the ranging measurements, and additional element definitions to facilitate ranging information exchange. The amendment also enhances the media access control (MAC) to support the control for time-of-flight ranging procedures and exchange ranging-related information between the participating devices.
Frequency band, channel number, and bandwidth
The HRP UWB physical layer uses an impulse radio signaling scheme with band-limited pulses. It defines operating frequencies in three different bands and 16 channel numbers. The sub-GHz band consists of a single channel, the low band consists of four channels, and the high band consists of 11 channels. Table 1 illustrates the UWB band groups, channel assignments, center frequencies, and bandwidths.
The HRP UWB PHY uses a combination of burst position modulation (BPM) and binary phase-shift keying (BPSK) to modulate symbols. Each symbol is composed of an active burst of UWB pulses, and the variable-length bursts support various data rates. IEEE 802.15.4 standard defines a reference UWB pulse as a root-raised-cosine pulse with a roll-off factor of β = 0.5. A transmitted pulse shape refers to the reference UWB pulse.
Figure 2 illustrates the symbol structure of HRP UWB according to IEEE 802.15.4-2020. The entire symbol period (Tdsym) consists of two BPM intervals (TBPM). In the BPM-BPSK modulation scheme, each symbol can carry two bits of information. The one bit determines the position of a burst of pulses (first or second BPM interval), while an additional bit will modulate the phase (polarity) of this same burst. A guard interval limits the amount of inter-symbol interference caused by multipath effects.
Figure 2. HRP UWB PHY symbol structure
HRP UWB devices communicate using a packet format and a PHY protocol data unit (PPDU) frame. Figure 3 shows it is composed of three parts.
- Synchronization header (SHR): The SHR splits into two parts, the synchronization (SYNC) field and at the start of frame delimiter (SFD) with variable lengths. The SYNC filed (preamble) consists of a predetermined sequence of pulses. The preamble symbol repetitions (PSR) define the number of repeated sequences, ranging from 16 to 4,096 repetitions.
- Physical layer header (PHR): The PHR contains information about the data to be received, including the length of the data and the data rate used to transmit the data.
- Payload (data field): The physical layer payload is actual data, power switching service data unit (PSDU).
Figure 3. HRP UWB PHY frame structure
The 802.15.4z-2020 amendment introduces optional modes for various mean pulse repetition frequencies (PRFs) to reduce the on-air time for higher density and lower power operation. This standard includes base pulse repetition frequency (BPRF) and higher pulse repetition frequency (HPRF) modes.
An HRP-based enhanced-ranging capable device (HRP-ERDEV) incorporates these modes. The IEEE 802.15.4-2015 standard defines the original PHY mode as non-ERDEV. The mean PRF parameter is the average PRF during the PSDU portion of a PHY frame and depends on the value of hot bursts, which is the number of burst positions containing an active burst. Table 2 illustrates the mean PRF for different HRP UWB modes.
The HRP-ERDEV frame includes a ciphered sequence, denoted as the scrambled timestamp sequence (STS), to increase the integrity and accuracy of ranging measurements. The STS consists of sequences of pseudo-randomized pulses generated from Advanced Encryption Standard (AES) 128 bits. Both a transmitter and receiver parties know the keys so that the receiver can correctly receive the data. It is secure against both accidental interference and intentional malicious attack. Figure 4 illustrates the STS packet configurations that indicate the STS position in an HRP-ERDEV frame.
Figure 4. HRP-ERDEV frame structures
The channel coding process consists of several steps, as shown in Figure 5. The payload data is Reed-Solomon encoding. PHR uses a simple Hamming block code that enables the correction of a single error and detects two errors (single-error correct, double-error detect; SECDED) at the receiver. The next step performs further convolutional coding for the PHR and the payload data.
Figure 5. HRP UWB PHY encoding process
The PSDU’s actual data rate depends on the number of burst positions containing an active burst, chips per burst, and coding rate (Viterbi rate). Table 3 shows the PHR and PSDU data rate for different modes.
Are You Prepared for IEEE 802.15.4 HRP UWB?
The IEEE 802.15.4 standard defines the physical layer (PHY) and medium access control (MAC) sublayer for HRP UWB. It is quickly gaining market adoption by enabling new applications such as real-time spatial context to mobile devices, advanced ranging, and location-based services as well as seamless and secure point-to-point (peer-to-peer) services. Demand for UWB technology, driven primarily by smartphones, industrial internet of things (IIoT), and automobiles, create market opportunities in real-time location systems and secure communication applications.
With high precision and ranging capabilities, HRP UWB uses ultra-wide bandwidth and shaped pulses. HRP UWB devices need to meet RF test requirements specified in IEEE 802.15.4 and regulatory compliance tests to ensure interoperability and performance.