## 4 WBANs: The Key M2M Area Network Technology for mHealth Applications
### PHY Technologies for WBANs
#### 4.3.2 UWB Technologies  
UWB transceiver technologies, mainly used for on-body communications, are characterized by very low radiated power (_i.e._, \(-41.3\) dBm/MHz), low implementation complexity, and harmonic coexistence with other existing wireless technologies. Hence, there is an increasing interest in the use of UWB transceivers in biomedical scenarios. Based on the method employed to form the carrier signal, the UWB technologies are classified into three groups: (i) Impulse Radio UWB (IR-UWB); (ii) Frequency Modulated UWB (FM-UWB);and (iii) Multi-Band Orthogonal Frequency Division Multiplexing UWB (MB-OFDM UWB). A brief description and related work on each technology are presented in continuation. IR-UWB:IR-UWB employs an ultra-short time pulse (in the order of ns) and, therefore, occupies a wide band in the frequency domain. IR-UWB signals are characterized by: (i) very wide bandwidth that can exceed 1 GHz; (ii) very low power spectral density; and (iii) a relatively simple and inexpensive architecture for both transmitter and receiver. A simple transceiver structure for IR-UWB signals is shown in Figure 2.  
IR-UWB can support many of the well known modulation schemes that are usually employed in NB systems [55], resulting to a wide range of achieved data rates. In order to demodulate IR-UWB signals, coherent, differentially coherent, and non-coherent detection schemes can be applied. A performance study of an IR-UWB system for WBANs is presented in [56], where the impact of two different modulation techniques on the Bit Error Rate (BER) of different on-body radio links is analyzed. The authors in [57] have studied different (coherent and non-coherent) UWB receivers, implemented according to the IEEE 802.15.4a requirements in different hospital environments, showing that both receivers yield equivalent performance in short-distance LOS links. The authors in [58] have investigated the problem of pulse distortion due to the interaction between UWB electromagnetic pulses and the human body. By studying the performance of various detection algorithms in the presence of pulse distortion and receiver noise, it has been shown that non-coherent receivers, such as energy detectors and Transmitted-Reference (TR) receivers (In TR systems, two  
Figure 2: Impulse Radio Ultra-Wideband (IR-UWB) Transceiver. signals--a reference and a data signal--are transmitted per bit. The information is contained in the sign difference of the transmitted signals. ), are suitable for WBANs, due to the fact that they do not require any explicit channel estimation (thus consuming less power and reducing complexity). In [59], two non-coherent modulation/detection pairs for IR-UWB systems with and without chirp pulse compression have been investigated. The authors strongly recommend the usage of chirp pulse compression in medical IR-UWB WBAN devices that are expected to use low data rates. Non-coherent detectors for Pulse Position Modulation (PPM) and TR Pulse Amplitude Modulation (TR-PAM) are proposed in [60]. The authors verify via simulations that the knowledge of the average power delay profile at the receiver can substantially improve the system performance. FM-UWB:FM-UWB is a low-complexity, constant envelope, low data rate UWB communication technology with good interference robustness at very low data rates and lower power consumption with respect to IR-UWB. FM-UWB is typically an analog implementation of a spread-spectrum system using double FM: digital Frequency Shift Keying (FSK), followed by high modulation index analog FM to create a constant envelope UWB signal. A typical FM-UWB transceiver architecture is presented in Figure 3. The data signal modulates a low-frequency subcarrier using NB FSK techniques. Then, the subcarrier signal modulates the RF oscillator using analog FM with high modulation index. The receiver demodulates the FM-UWB signal without requiring local oscillator and carrier synchronization (as in the case of IR-UWB), which makes the device both simpler and cost-efficient. Figure 3: FM-UWB Transceiver.