# FM-UWB: Towards a Robust, Low-Power Radio for Body Area Networks
## 1 Introduction  
The recent advances in circuit design, combined with technology scaling, enabled highly miniaturized, ultra-low power radios that have been the key driving force behind the development of Wireless Body Area Networks (WBANs). These highly localized wireless networks find use in various medical and consumer applications [1]. In order to allow for the unobtrusive use, the size and the battery life of WBAN nodes are critical issues. The need for frequent replacement is undesirable and in the case of implanted nodes unacceptable, which sets the limit to the power consumption of the devices used in sensor nodes, in particular the radios, as they are often identified as the dominant source of power consumption. With the increasing popularity of WBAN, the number of applications continues to grow, as well as the number of connected devices. Furthermore, some applications require that a large number of devices coexist in a small area. One example is the _WiseSkin_ concept (Figure 1), where sensor nodes embedded in a kind of artificial skin are placed on top of a prosthetic arm. The nodes communicate wirelessly between each other and transmit the information from sensors to the central processing unit that drives the actuators and provides a sense of touch to the patient [2]. The _WiseSkin_ application is one example of a High-Density Wireless Sensor Network (HD-WSN). To deal with a large number of nodes operating in a small area, new protocols are needed, and a communication system that could support Frequency Division Multiple Access (FDMA) and multi-user communication ishighly desirable. The short distance between adjacent nodes can be exploited to save power as it loosens the sensitivity constraints. The designers are now faced with more and more challenges in an attempt to implement miniature radios that will not only consume very little power but also be capable of providing robust communication while dealing with the increasing number of interferers in the crowded spectrum of the future. The two Ultra-Wideband (UWB) modulations, the Impulse Radio (IR-UWB) and the Frequency Modulated UWB (FM-UWB) have interesting properties with regards to these requirements. First of all, since the spectrum is spread over a very wide band, the two UWB modulations are inherently more robust against frequency selective fading [3]. A significant attenuation at a specific frequency might prevent communication with a narrow-band radio, whereas it will only have a minor impact on a UWB radio. Second, FM-UWB radios are inherently resilient to narrow-band interference, achieving good performance without external components such as high-Q Surface Accoustic Wave (SAW) filters, and thus providing more potential for miniaturization. Large transmit spectrum also means that there is no need for precise frequency synthesis and no need for a Phase Locked Loop (PLL). This could allow for saving some power as the costly frequency dividers are not used, but also brings advantages in duty cycled networks where the transceiver is turned on and off periodically. Since a transceiver cannot turn on instantly, some energy will be wasted during the wake-up time. The wake-up time is usually limited by the wake-up time of the PLL due to its limited bandwidth and thus removing the PLL would mean reducing this energy overhead. The main downside of the UWB approach is that the large bandwidth requires more complex circuits that usually require more power; however, with all the benefits in mind, it remains an attractive choice for a WBAN. The IEEE 802.15.6 standard was tailored to the needs of the WBAN [4]. It includes both the wideband and the narrow-band communication. Two UWB modulation schemes that became part of this standard are the Impulse Radio (IR-UWB) and Frequency Modulated UWB (FM-UWB). The IR-UWB uses narrow pulses that can be modulated using on-off (OOK), position (PPM), phase (PSK) or frequency (FSK) modulation (although only OOKs and PSKs are supported by the standard).