Ultra-wideband technology (UWB) allows for low-power wireless communication, ideal for wireless sensor nodes. The IEEE 802.15.4 standardization committee who defined the MAC and physical layers adopted by ZigBee recently proposed an alternative physical layer relying on UWB technology.
The advantages of this new UWB air interface are mainly an increased data rate which can be tuned in function of the channel quality, an extended communication range, lower power consumption and the possibility to use the air interface for accurate positioning of the transceivers.
In this article, we discuss some of the key challenges associated to the design of UWB transmitters. We further present the first reported transmitter complying with the new 802.15.4a standard, which has been implemented in standard 90 nm CMOS technology and shows a record low-power consumption of 1 mW for a net data rate of 0.85 Mbps.
UWB for a myriad of applications
Wireless communication based on ultra-wideband (UWB) signals has attracted much attention from the wireless community both from standardization bodies and chip manufacturers. This air interface promises flexibility, robustness and high-precision ranging capabilities.
For example, wireless sensor nodes for medical applications can not be realized using today's low-power radios such as Bluetooth, ZigBeeor by proprietary radios eitherbecause these can't meet the stringent wireless body-area network power requirements.
Typical chipsets for these radios consume in the order of 10 to 100mW for data rates of 100 to 1,000kbps. This leads to a power efficiency of roughly 100 to 1,000mW/Mbps or nJ/bit. For wireless sensor nodes, a radio is needed which is 1 to 2 orders more power efficient. UWB holds this promise.
The Federal Communications Commission (FCC) has authorized UWB communications between 3.1GHz and 10.6GHz. Although the regulations on UWB radiation define a power spectral density (PSD) limit of -41dBm/MHz, there are very few regulations on the definition of the time-domain waveform.
The latter can then be tailored for low hardware complexity as well as low system power consumption. In pulse-based UWB, the transmitter only needs to operate during the pulse transmission, producing a strong duty cycle on the radio and the expensive baseline power consumption is minimized.
Moreover, since most of the complexity of UWB communication is in the receiver, it allows the realization of an ultra-low power, very simple transmitter and shift the complexity as much as possible to the receiver in the master.
However, the impact of the type of UWB signal chosen on the communication performance and on the complexity of the radio implementation must be carefully analyzed. The minimum bandwidth of a UWB signal is usually 500 MHz.
Indeed, various UWB standard proposals have subdivided the entire UWB spectrum in 500MHz sub-bands as a way to mitigate against strong interferers, to improve the multiple access and to compose with the different regulations on UWB emissions worldwide.
Therefore, in order to comply with these regulations and standards, the generated pulses of UWB impulse-radio (UWB-IR) approaches must fulfill stringent spectral masks that can feature such bandwidths. This poses a serious challenge for the pulse generation of UWB-IR transmitters.
UWB design challenges
The challenges associates to the design and implementation of UWB transmitters differ significantly from those encountered in classical low-power radios, typically relying on narrowband air interfaces in the 2.5 GHz ISM band.
Typical design challenges in that narrowband context are related to the accurate control of the generated frequency references used for modulation of the signal, both in phase and amplitude. In contrast, UWB signals are spread over a relatively large bandwidth (500 MHz or more) and the precise control of the carrier is by far less critical than in narrowband radios, which loosens the requirements on the control of the RF frequency reference.
An important challenge in the design of UWB systems stems from the large range of frequencies that must be covered, i.e. from 3GHz to 10 GHz. The different elements of the transmitter must be designed to offer constant performance across a spectrum of 7 GHz.
Moreover, achieving a maximum absolute frequency of 10 GHz with an effective bandwidth of more than 500 MHz with low power consumption is not straightforward, especially when low-cost devices relying on standard CMOS technologies are considered. The wide range of frequencies covered by UWB also poses significant challenges when designing UWB antennas.
UWB signals consist in extremely short pulses (about 2 ns), or groups of adjacent pulses, separated by long silence periods. Accurate control of the pulse amplitude and shape, or RF carrier frequency is not of critical importance in UWB. However, the control of the moment at which UWB pulses are generated is a critical aspect of UWB systems as is the control of the phase of the RF carrier.
Finally, an important advantage of UWB resides in the possibility to significantly reduce the power consumption of the radio front-end by switching off the transmitter during the relatively long silence periods between UWB pulses.
In order to exploit this advantage, the front-end circuits must be designed with well-controlled and relatively fast startup behaviors, such that the stringent timing and phase control requirements can be met.