## 3 IEEE 802.15.6: The Key-Enabling Standard for WBANs
### IEEE 802.15.6 Physical Layer  
The graphical summary of the spectrum allocation of all available frequencies for WBANs applications is shown in Figure 6. The IEEE 802.15.6 standard has proposed three different alternatives for the PHY layer based on the specific applications and their requirements, including Human Body Communications (HBC), Narrowband (NB) PHY and Ultra wideband (UWB) PHY, which are as explained below:  
* _Human Body Communications (HBC) PHY_: IEEE 802.15.6 introduced a new communication medium (HBC) for the frequency range of 5-50 MHz for implanted devices and their interaction with coordinating device. Its primary technology is electric field coupling which includes capacitive coupling and galvanic coupling. The HBC channel of the capacitive coupling type is developed based on the near electric field around the human body, which is induced by a transmitter terminal, and a receiver terminal is used to detect the weak coupling changes of the near electric field along the body channel. Research has shown that the data rate of capacitive coupling is up to 2 Mb/s. Two bands of operation cantered at 16 MHz and 27 MHz with a bandwidth of 4 MHz are proposed in the standard. * _Narrow Band (NB) PHY_: In NB communication multiple nodes are on the body to communicate with each other and normally it is not used for implanted devices due to relatively high power consumption. A compliant device shall be able to support transmission and reception in one of the following frequency band:
* 402-405 MHz: Medical Implant Communication System (MICS) band, it is widely accepted although the possible bandwidth is limited;
* 420-450 MHz: Wireless Medical Telemetry System (WMTS) band (available in Japan);
* 863-870 MHz: WMTS band (available in Europe);
* 902-928 MHz: ISM band, it is available for use without a license in North America, Australia and New Zealand;
* 950-956 MHz: Available in Japan;
* 2360-2400 MHz: This frequency band is proposed by the standardization group to be adopted in BAN applications;
* 2400-2483.5 MHz: ISM band, it is available worldwide, but there could be issues of coexistence with other IEEE standards that use the same band;
* _Ultra Wide Band (UWB) PHY_: In UWB the frequency range varies between 3.1 GHz and 10.6 GHz. It provides high performance, robustness, low complexity and ultra-low power operations for WBANs. It is available worldwide while some spectral masks are defined differently in different countries or regions. A typical WBAN is usually composed of a few body sensor nodes, even though up to 256 nodes are supported by the IEEE 802.15.6 standard. A relatively wide range of data rates can be employed; varying from 1 kbps to 1 Mbps, which is one of the most important features while considering the wide range of target WBAN applications. The Physical-Layer Protocol Data Unit (PPDU) represents the information that is sent through the propagation medium to the receiver device. It is composed of the physical layer convergence protocol (PLCP) preamble, physical layer convergence protocol (PLCP) header, and physical layer service data unit (PSDU), as illustrated in Figure 7, and is briefly explained below:  
* _PLCP Preamble_: The purpose of the preamble is to aid the receiver in packet detection, timing synchronization and carrier-offset recovery. Two unique preambles are defined in order to mitigate false alarms due to other networks operating on adjacent channels. The preamble is transmitted at the symbol rate for the desired band of operation and will be encoded using the same modulation parameters as defined for different physical types. More details on the PLCP Preamble can be found in section 8.2 of the standard [9]. * _PLCP Header_: It is added to convey information about the PHY and MAC parameters that are needed at the receiver side in order to decode the PSDU. Details on the different parts composing the PLCP Header and how to properly set the bits of each field can be found in section 8.3 of the standard [9].