UWB Transceiver

Abstract of my Ph. D. Thesis

Ultra-Wideband (UWB) is a promising technology for short-range and low-power indoor data communications. The recent interest in this technology was initiated in February 2002 in the United States. The amendment of the spectrum policies by the Federal Communication Commission (FCC) allowed the use of wideband radio signalling occupying a bandwidth in excess of 500 MHz within the frequency range between 3.1 and 10.6 GHz. This opened the way to two main classes of wireless applications: 1) high-speed wireless aiming at transmission rates above 100 Mb/s for multimedia applications, and 2) low-complexity radio systems with high integration level intended for low-power applications such as wireless sensor networks (WSN). This work focuses on the second class of applications; Ultra-Wideband has been recognized as an interesting candidate for small portable devices transmitting data at faster rates and lower power requirements than the existing short-range wireless Standards such as Bluetooth or ZigBee. Following the United States, the Electronic Communications Committee (ECC) in Europe finalized a decision in February 2007, which clearly states that the 6.5-8 GHz frequency range is the preferred band for long-term operation of FCC-like UWB devices.

The goal of this project was the development of a standalone wireless integrated transceiver using the promising Impulse-Radio Ultra-Wideband (IR-UWB) technology. The benefits and the limitations of this technology were first thoroughly investigated. We especially focused on the specification of a transceiver that could be easily implemented with the minimum loss of performance with respect to an optimum transceiver. Investigation showed that the main limitation comes from the characteristics of the indoor channel. The latter suffers from the multipath effect that induces fading and inter-symbol interferences. In low-complexity transceivers, where no extensive signal processing can be applied, there is an interest to use the multipath rather than to mitigate it. The principle of "energy-collection" is thus applied in the proposed receiver.
The second principle of diversity is based on the use of frequency multiple access (carrier-based IR-UWB). For the class of devices targeted in this work (single antenna), the frequency diversity is also the only diversity strategy for enabling a reliable communication link. If the wireless devices are stationary or seldom moving, time diversity is not a reliable option, while small size rules out spatial diversity (eg. multiple antennas).

The main part of this thesis deals with the development of integrated circuits for an IR-UWB transceiver operating between 3 and 5 GHz. The integrated circuits were all realized using a commercial 0.18-um CMOS technology. The heart of the transceiver is a wideband voltage-controlled oscillator (VCO) that can be both used in closed-loop (PLL) or in free-running mode. The latter solution involves a self-calibration procedure using the PLL and a digital-to-analog conversion. It yields an accuracy of +/-20 MHz, which is sufficient to meet the requirements of our transceiver. Several other solutions have also been investigated to generate a carrier-based IR-UWB signalling, such as a simple switched oscillator or a pulse shaping stage at radio-frequencies. Finally, a single-chip transmitter using BFSK modulation and offering three channels between 3.3 and 4.8 GHz is demonstrated.

The RF front-end receiver features a 3-5 GHz UWB LNA, which feeds quadrature mixers for frequency down-conversion. A variable gain amplifier has also been developed specifically for our application. It provides 60 dB of voltage gain over a bandwidth of 180 MHz. This circuit also features an automatic gain control that compensate transmitter-receiver range variations and multipath fading effects. The back-end section consists of a mixed-mode application-specific integrated circuit (ASIC), whose function is to demodulate the down-converted quadrature signal into a bipolar pulsed signal. The resulting analog signal is integrated over a duration of 20-40 ns and converted into a digital signal by means of a 1-bit A/D conversion (comparator). The digital section of the baseband chip also synchronizes on the incoming pulse stream and provides the corresponding digital data. The baseband processor is also able to provide an estimation of the signal-to-noise ratio without any information from the received signal strength. This enables an assessment of the link quality prior any further processing or data storage.

The entire IR-UWB link exhibits a communication range of 10 m at 5 Mb/s in free-space without any error correction nor any other coding schemes. This corresponds to a receiver sensitivity of -83.7 dBm for a bit error rate of 10^-3. The power consumption of the entire receiver in tracking mode (after synchronization) at 5Mb/s reaches 36 mW.