By Petter Mhyre, Product Marketing Manager, Nordic Semiconductor
Ultra wideband (UWB) is as old as radio communication itself. The father of long-distance radio communication, Guglielmo Marconi, employed a form of UWB communication when he used spark gap radio transmitters to send 850 kilohertz Morse code signals during his experiments with transatlantic communications in 1901 and 1902. However, in these very early years of RF technology the benefits of broadcasting across a very large bandwidth were not appreciated and UWB languished for decades.
Serious work on UWB technology commenced in the late 1960s through the work of engineers such as Henning Harmuth at Catholic University of America, Gerald Ross and Kenneth Robbins at Sperry Rand Corporation, Paul van Etten at the USAF’s Rome Air Development Center and in Russia. Harmuth’s documents, published between 1969 and 1984, detailed the basic design for UWB transmitters and receivers. Independently between 1972 and 1987, Ross and Robbins’ patents pioneered the use of UWB signals in a number of application areas, including military communications and radar.
The technology remained confined to military use for decades, albeit without actually being called “ultra wideband”. That term was coined in 1989 by the U.S. Defense Advanced Research Projects Agency (DARPA). The agency went on to define the technology as a system with a “fractional bandwidth greater than 25 percent” where fractional bandwidth is the ratio of signal bandwidth over the centre frequency.
Outside the military, UWB started to emerge following a 2002 decision by the Federal Communication Commission (FCC) to allow unlicensed use of UWB systems in applications such as data communication solutions. However, the FCC somewhat hamstrung the technology by setting strict rules on the allowed frequencies and power limits to ensure the use of UWB in unlicensed spectrum wouldn’t interfere with existing commercial and military RF systems.
UWB transmits information by generating radio pulses at very short and precise time intervals across a large bandwidth and carries information by using pulse-position or -time modulation. Because each pulse occupies the entire UWB bandwidth, its low spectral density allows the signals to share spectrum with other RF protocols without risk of interference.
Engineers can trade-off pulse length against bandwidth and signal to noise (S/N) ratio. Shorter pulses increase the data throughput but also increase both bandwidth requirement (which is typically restricted by regulations) and noise. These trade-off can be eased by increasing pulse frequency to increase throughput and by boosting transmitter power to ease noise challenges.
The IEEE802.15.4 standard (which defines the physical (PHY) and media access control (MAC) layers for low data-rate wireless personal area networks (PANs)) includes a specification for UWB. The standard continues to evolve, but under the most recent version, compliant UWB PHYs must support three independent bands of operation: A sub-gigahertz band (channel 0, 249.6 to 749.6 MHz) a low band (split into channels 1 to 4, 3.1 to 4.8 GHz) and a high band (split into channels 5 to 15, 5.8 to 10.6 GHz). Some channels support over one gigahertz of bandwidth (for example, channel 15 offers 1.35 GHz). Each channel supports four data rates, 110 and 850 kilobits per second, plus 6.8 and 27.24 megabits per second.
A key advantage of UWB is that the low power spectral density provides immunity to the multipath fading and interference that can prove troublesome to narrowband short-range wireless technologies. However, one drawback is that the typical gigahertz operating frequencies are absorbed by walls and other obstacles limiting operation to line-of-sight applications.
Before SARS-CoV-2, the coronavirus behind the COVID-19 pandemic that still grips the world, real time location services (RTLS) were proving slow to emerge. But the pandemic has spurred innovation particularly in contact tracing solutions - a key weapon in the battle against the virus which relies on knowing whether a person has been close to another who is subsequently shown to be infectious.
Today, most commercial products rely on Bluetooth Low Energy (Bluetooth LE) technology’s Received Signal Strength Indication (RSSI) to estimate the distance from one person to another. RSSI estimates the distance between two transceivers by measuring how much the power of the signal has diminished since it left the transmitter. In perfect conditions the technique works well, but throw in a few walls, ceilings and furniture and variable effects such as multipath fading significantly reduce the measurement’s precision.
UWB overcomes this drawback by ignoring signal strength and instead measuring the distance between two UWB radios by timing how long it takes for a radio pulse to reach the receiver and return. By factoring in the receiver latency and the speed of light, an accurate distance estimate can be calculated. The target’s position is found by measuring the Angle of Arrival (AoA) of the incoming pulses (by using multiple antennas). Combining distance and direction data then enables the system to determine precisely where, in three dimensions, the transmitter is located.
UWB does lack many of the inherent advantages of a mature narrowband short-range wireless technology like Bluetooth LE. Bluetooth LE features lower power consumption, wider industry support and smartphone interoperability. Now, the second generation of social distancing products are cleverly combining the strengths of both Bluetooth LE and UWB. Using the Bluetooth LE radio to approximate the target object’s position—a process which requires a relatively large amount of RF activity—and then switching to the UWB radio for the shorter precision location operation, the on-air time of the higher power UWB radio is minimised to extend battery life. Another advantage of the Bluetooth LE/UWB combination is that it allows for RSSI to be used as a fallback technique should a non-UWB target device be encountered.
Until wide ranging vaccination programs bring the SARS-CoV-2 virus to heel, Bluetooth Low Energy/UWB-powered products are likely to play an increasing role in managing the pandemic. That will further raise UWB technology’s profile in the public’s perception and provide a healthy market for chip makers. The technology’s emergence as a consumer solution received an additional boost when Apple incorporated its own UWB chip in the iPhone 11 to “bring spatial awareness” to the smartphones and encourage new consumer applications, beyond PC peripherals and contact tracing, that would benefit from precision location technology.
The real-time location system (RTLS) sector is set for rapid growth—with analyst Allied Market Research forecasting a 30.2 percent CAGR between 2019 and 2026—helping to underpin UWB technology’s prospects.