UWB Communications Technology . Introduction

The ultra-wideband (UWB) communications technology development started at the end of the 1990s and received a significant 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 plays a crucial role in specifying wireless personal area network (WPAN) standards. Task groups (TG) within IEEE 802.15 leverage UWB technologies for high-rate and low-rate communication applications. Two key UWB technologies are 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. For example, IEEE 802.11ad/ay utilizes MB-OFDM for multiple gigabit wireless systems, while IEEE 802.15./4z employs pulsed DS-UWB for indoor positioning, ranging, and security applications. Among these technologies, the applications and signal characteristics of IEEE 802.15 high-rate pulse repetition frequency (HRP) UWB stand out as the most different from other UWB standards. Let's delve deeper into the definition of UWB.

UWB refers to a radio signal with an instantaneous bandwidth greater than 500 MHz or a fractional occupied bandwidth (Bf) greater than a certain threshold. Figure 1 illustrates the upper frequency (fH) and lower frequency (fL) points that are 10 dB below the highest power spectral density of the UWB signal. The formula for calculating Bf is as follows: Bf = (fH - fL) / fC, where fC = (fH + fL) / 2.

There exist two major UWB technologies, namely multiband orthogonal frequency division multiplexing (MB-OFDM) and direct-sequence ultra-wideband (DS-UWB). In 2006, the WiMedia Alliance adopted MB-OFDM to support wireless video after the closure of IEEE 802.15 TG3a project. Task Group 4a (TG4a) leveraged DS-UWB for precision ranging and finalized the first standard in 2007. In 2011, IEEE 802.15 included TG4a as one of the physical layer (PHY) options and completed a revision in 2015 to define two UWB PHYs: high-rate pulse frequency (HRP) from TG4a and low-rate pulse (LRP) from TG4f, known as radio-frequency identification (RFID).

The IEEE 802.15.4z amendment, introduced in 2019, enhances the ultra-wideband physical layer by incorporating additional coding and preamble options, improving existing modulations to increase the integrity and accuracy of ranging measurements, and defining additional elements to facilitate ranging information exchange. The amendment also enhances the media access control (MAC) to support time-of-flight ranging procedures and the exchange of ranging-related information among participating devices.

The HRP UWB physical layer utilizes an impulse radio signaling scheme with band-limited pulses. It defines operating frequencies in three different bands, each consisting of various channel numbers. The sub-GHz band has a single channel, the low band consists of four channels, and the high band is composed of 11 channels. Table 1 illustrates the UWB band groups, channel assignments, center frequencies, and bandwidths.

The HRP UWB PHY employs 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. The IEEE 802.15 standard defines a reference UWB pulse as a root-raised-cosine pulse with a specific roll-off factor. The transmitted pulse shape is based on this reference UWB pulse.

Figure 2 illustrates the symbol structure of HRP UWB according to IEEE 802.15-2020. 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. One bit determines the position of the burst of pulses within the two BPM intervals, while the additional bit modulates the phase (polarity) of the burst. A guard interval is included to limit the amount of inter-symbol interference caused by multipath effects.

HRP UWB devices communicate using a packet format and a PHY protocol data unit (PPDU) frame. Figure 3 shows that the PPDU frame is composed of three parts:

1. Synchronization header (SHR): The SHR is split into two parts the synchronization (SYNC) field at the start and the frame delimiter (SFD) with variable lengths. The SYNC field, or preamble, consists of a predetermined sequence of pulses. The number of preamble symbol repetitions (PSR) defines the number of repeated sequences, ranging from 16 to 4096 repetitions.
2. Physical layer header (PHR): The PHR contains information about the received data's length and the data rate used for transmission.
3. Payload (data field): The physical layer payload contains the actual data, referred to as the power switching service data unit (PSDU).

To reduce the on-air time for higher density and lower power operation, the IEEE 802.15.4z-2020 amendment introduces optional modes with various mean pulse repetition frequencies (PRFs). These modes include the base pulse repetition frequency (BPRF) and higher pulse repetition frequency (HPRF). An HRP-based enhanced-ranging capable device (HRP-ERDEV) incorporates these modes. The original PHY mode defined in IEEE 802.15-2015 is non-ERDEV. The mean PRF parameter represents the average PRF during the PSDU portion of the PHY frame and depends on the value of hot bursts, which refers to 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 known as the scrambled timestamp sequence (STS), which enhances the integrity and accuracy of ranging measurements. The STS consists of sequences of pseudo-randomized pulses generated from an Advanced Encryption Standard (AES) with 128 bits. Both the transmitter and receiver parties possess the keys required for correct data reception. This encryption ensures security against accidental interference and intentional malicious attacks. Figure 4 illustrates the STS packet configurations indicating the STS position in an HRP-ERDEV frame.

The channel coding process consists of several steps, as shown in Figure 5. The payload data undergoes Reed-Solomon encoding. The PHR utilizes a simple Hamming block code that enables the correction of single errors and detects two errors (single-error correct, double-error detect; SECDED) at the receiver. The next step involves further convolutional coding for the PHR and the payload data.

The actual data rate of the PSDU depends on the number of burst positions containing an active burst, the chips per burst, and the coding rate (Viterbi rate). Table 3 shows the PHR and PSDU data rates for different modes.

The IEEE 802.15 standard defines both the physical layer (PHY) and medium access control (MAC) sublayer for HRP UWB. This technology is gaining rapid market adoption by enabling new applications such as real-time spatial context for mobile devices, advanced ranging, location-based services, and seamless and secure point-to-point (peer-to-peer) services. The demand for UWB technology, driven primarily by smartphones, the industrial internet of things (IIoT), and automobiles, creates market opportunities for real-time location systems and secure communication applications.

With its high precision and ranging capabilities, HRP UWB utilizes ultra-wide bandwidth and shaped pulses. HRP UWB devices must meet RF test requirements specified in IEEE 802.15 and comply with regulations to ensure interoperability and performance.

Q/A

1. What technologies are used in UWB communications?

UWB communications use two major technologies: multiband orthogonal frequency division multiplexing (MB-OFDM) and direct-sequence ultra-wideband (DS-UWB).

2. What boost did UWB technology receive in 2002?

In 2002, UWB technology received a boost when the Federal Communications Commission (FCC) published new regulations for the commercial market.

3. What is the definition of UWB?

UWB refers to a radio signal with an instantaneous bandwidth greater than 500 MHz or a fractional occupied bandwidth higher than a certain threshold.

4. What are the different UWB bands and their characteristics?

The UWB bands include the sub-GHz band, low band, and high band, each with specific channel assignments, center frequencies, and bandwidths.

5. What modulation techniques are used in HRP UWB?

HRP UWB utilizes burst position modulation (BPM) and binary phase-shift keying (BPSK) to modulate symbols.

6. What are the components of an HRP UWB packet?

An HRP UWB packet comprises a synchronization header (SHR), physical layer header (PHR), and payload (data field).

7. What enhancements does the IEEE 802.15.4z amendment provide for UWB?

The IEEE 802.15.4z amendment introduces additional coding and preamble options, improves modulations, and facilitates ranging information exchange.

8. What encryption is used in HRP-ERDEV frames?

HRP-ERDEV frames incorporate a scrambled timestamp sequence (STS) generated from an Advanced Encryption Standard (AES) with 128 bits.

9. What is the mean PRF parameter in HRP UWB?

The mean PRF parameter represents the average pulse repetition frequency (PRF) during the PSDU portion of the PHY frame and depends on the value of hot bursts.

10. What market opportunities are created by UWB technology?

UWB technology creates market opportunities in real-time location systems and secure communication applications driven by smartphones, the industrial internet of things (IIoT), and automobiles.

TAGS: UWB communications, IEEE 802.15, MB-OFDM, DS-UWB, HRP UWB, PHY, MAC, wireless technology, wireless networks.