## 3 MAC Protocols for CRBANs
### Cognitive Radio for Medical Body Area Networks Using Ultra-Wideband  
Medical body area networks (MBANs) have been introduced to unlicensed frequency bands, where mutual interference between electronic devices is high. CR technology can potentially alleviate such problems in MBANs and increase the efficiency of spectrum management. In [20], CR features based on ultra-wideband (UWB) technology for MBANs were proposed by considering both the PHY and MAC layers. A WBAN comprises multiple sensor nodes capable of sampling, processing and communicating biomedical information [21]. Cognitive capabilities are implemented in the body network controller (BNC) using impulse radio (IR) and multiband orthogonal frequency-division multiplexing (MB-OFDM). Impulse radio ultra-wideband (IR-UWB) technology was introduced as a solution in IEEE 802.15.6 and is currently being developed by the IEEE 802.15.6 Task Group 6 [22]. The advantages of IR-UWB technologies are their low power, low complexity and low cost with highly reliable wireless communication. The main objective of the standard is to specify a MAC sublayer that can support several PHY layers, including UWB. IR-UWB provides high QoS, and frequency modulation UWB (FM-UWB) is optimized for low power consumption and reliable communication. The European Computer Manufacturers Association's ECMA-368 specification is a high-rate UWB PHY and MAC wireless standard, uses MB-OFDM and divides the spectrum into 14 bands of 528 MHz each [23]. OFDM is based on a frequency diversity transmission scheme that distributes modulated data across closely and mutually overlapping space. An OFDM signal is generated in the frequency domain by using inverse fast Fourier transform (IFFT) to create a time domain multiplexed signal. UWB signals have an inherent noise-like behavior that makes UWB difficult to detect and requires a complex encryption algorithm. In MB-OFDM, symbols are interleaved over multiple sub-bands across time and frequency, as shown in Figure 6. By interleaving the OFDM symbols across sub-bands, multiband UWB can maintain the power level associated with a single-band OFDM. The characteristics of UWB technology can be exploited to turn a BNC into a cognitive radio controller (CRC) that controls the transmission parameter of CR clients. The communication links between the sensors and the BNC should be implemented using IR-UWB, which is specified by IEEE 802.15.6. The architecture for CRC consists of two transceivers: an IR-UWB transceiver with on-off keying (OOK) and an MB-OFDM UWB transceiver. The division of the 3.1-10.6 GHz UWB spectrum into 14 sub-bands of 528 MHz each is adopted. The lower UWB frequency band (3.1-4.8 GHz) is enclosed with three sub-band frequencies at 3232, 3960 and 4488 MHz. The three sub-band frequencies are preferred for first-tier communication, because of their better propagation characteristics. The MB-OFDM transceiver also protects medical devices and sensors from UWB interference by using frequency domain spectrum shaping. UWB facilitates a high processing gain and allows coexistence with a narrow wide-band system through spectrum shaping. Spectrum shaping can be achieved by adopting time hopping codes using a pulse-position modulation scheme or the pulse shape at a transmitter. The MB-OFDM UWB system provides clear advantages over IR-UWB in terms of sensing using power spectral density (PSD). A sub-band or channel ranking scheme can be implemented using a combination of two methods: using the sub-band's PSD information to identify available channels and using bit error rate (BER) information obtained from active channels to yield channel quality [24]. To avoid interferences and collisions between signals, wireless devices need to coordinate access time periods with their neighbors. This type of coordination is governed by the MAC sublayer. The IEEE 802.15.6 standard is used for the intra-WBAN tier, whereas inter-tier access is provided by ECMA-368 [23]. IEEE 802.15.6 supports the beacon and non-beacon modes of operations.