## 3 Physical Layer in WMSN  
The Physical Layer in WMSNs consists of the basic hardware transmission technologies of a network and defines the means of transmitting raw bits, rather than logical data packets, over the wireless link that is connecting network nodes. It is responsible also for frequency selection, modulation and channel encoding. In WMSNs, the physical layer should be designed in a way that it underlies all the higher-layer communications-related functions and meets the specific requirements and characteristics of WMSN. Therefore:  
* The physical layer technology must work in a compatible way with higher layers in the protocol stack to support their application-specific requirements and to meet the design challenges of WMSN. This can be done with higher efficiency if a cross-layer model is used especially between physical layer and MAC layer. * The physical layer should utilize the available bandwidth and data rate in the best possible way, and to be more power efficient. * The physical layer should have a good performance (gain) against noise and interference and provide enough flexibility for both different channel and multiple path selection. * The cost of the radio should be taken into account since it will be deployed in large number of nodes. Physical layer technologies can be classified either into three groups (Narrow band, Spread spectrum, Ultra-Wide band (UWB) technologies) based on the modulation scheme and bandwidth consideration [12], or into different standard protocols (IEEE 802.15.4 ZigBee, IEEE 802.15.1 Bluetooth, IEEE 802.11 WiFi, 802.15.3a UWB). ZigBee [13] is the most common standard radio protocol used in wireless sensor networks because of its lightweight standard and its low-cost and low-power characteristics. ZigBee supports: data rate up to 250 kbps at 2.4 GHz, more than 65,000 nodes, coding efficiency of 76.52 %, and range of 10-100 meters. ZigBee standard is being used by most of WSN devices such as MICA-family, Tmote sky, and imote2. However, ZigBee standard is not suitable for high data rate applications such as multimedia streaming over WMSN and for guaranteeing application-specific QoS. On the other hand, other standards like Bluetooth and WiFi have higher data rate and code efficiency -- as shown in Table 1-- but they consume more energy. Bluetooth has been used in [14, 15] for wireless communication in WMSN, while WiFi has been used with Stargate device in many projects as shown in the Hardware section later on. UWB [16, 17]--with coding efficiency of 97.94%, data rate up to 250 Mbps, and nominal range of 10 meters in addition to its immunity to multipath propagation and precise positioning capabilities--has the potential to enable low power consumption, high data-rate of short range wireless communication and seems to be a promising candidate for the physical layer standard of WMSN. UWB spreads the information over a large bandwidth, about 20% of the center frequency or more than 500 MHz. The physical layer of UWB is implemented by using either impulse radio (IR) of extremely short duration pulses, or multiband orthogonal frequency division multiplexing (MB-OFDM) where hybrid frequency hopping and OFDM are applied. IR-UWB has simpler transmitter and rich resolvable multipath components, but it needs a long channel acquisition time and requires high speed analog-to-digital converters, while MB-OFDM-UWB offers robustness to narrowband interference, spectral flexibility, and efficiency but it needs slightly complex transmitter. The multiple access of IR-UWB can be realized by using direct sequence UWB (DS-UWB), or time hopping UWB (TH-UWB). The low duty cycle of IR-UWB (\(<\)1%), because of the short duration of the pulse, is a key advantage for low power consumption in WMSN, also spreading information over wide bandwidth decreases the power spectral density and in turn reducing the interference with other systems and lowering the probability of interception.