Physical Layer for Wireless Sensor Networks

The main concern of the physical layer is modulation and demodulation of digital data, i.e. transmission and reception of the data. This is done by the transceivers in the sensor nodes. The main functions of physical layer are carrier frequency selection and generation, encryption and decryption, modulation and demodulation, transmission and reception of data.

Wireless sensor network generally work in ISM bands. But many other standards like 802.11b and Bluetooth also use the same band, so all systems, in this band have to be robust against interference form other systems.
The most important parameters which are to be considered while designing Physical layer in wireless sensor networks are
1. Low Power Consumption.
2. Low Transmission and Reception range.
3. Interference from other systems, working in the same band.
4. Low complexity.
5. Low duty cycle, i.e. most of the time sensor nodes are switched off.
6. Low data rates most of the time and high data rate only for a short period of time.
The most challenging aspect in physical layer design for sensor networks is to find, low cost transceivers which consume less power, simple modulation schemes which are robust enough to provide required service.

Generally the transceivers used in sensor network are only 10% efficient. To radiate a power of 1 mw the transceivers will consume at least 10 mw of power. The power consumed for reception is some what similar to power consumed for transmission; some times one of them may be more and one less depending upon design of the transceivers. For Mica motes 21 mw is consumed in transmit mode and 15 mw is consumed in receive mode. The transmission and reception of data are the most power consuming activity in the sensor node. So transmission and reception of data should be kept as less possible. For the WIN nodes, 1500 to 2700 instructions can be executed per transmitted bit, for the MEDUSA II nodes this ratio ranges from 220:1 up to 2900:1, and for the WINS NG nodes, it is around 1400:1. The summary is computation is cheaper than communication.

The power consumed by the transceivers in idle mode will not be significantly less than the power consumed by transceivers in receive or transmit mode. So it is always preferable to put the transceiver in sleep mode rather than in idle mode when not required. But care has to be taken to see to that, the power consumed during start up and time taken to startup the transceiver does not over come the advantage of putting the transceiver in sleep mode. The most commonly used transceiver is CC2420.
The choice of modulation scheme is another important aspect to be considered here. The choice depends upon the complexity that can be supported by the node, as one cannot go for very complex processes in sensor networks. The data rate is another factor to be considered, as this tells about what amount of data can be transmitted or received when the node is on. To save more and more energy the sensor node should be in sleep as much as possible and more the data rate more amount of data can be transmitted and received in small active period. The symbol rate is yet another important factor, as it has be found that more is the symbol rate more is the power consumption. The BER has also to be considered, because to keep BER low the power radiated has to be more.

As an example we shall consider the most commonly used protocol for WSN, IEEE 802.15.4. IEEE 802.15.4 defines wireless personal area network standard for low rate devices, this specifies the standards for Physical and MAC layer. 802.15.4 Works at a data rate of less than 250kbps. It works up to a range of 75m and it supports up to 254 nodes. This protocol is designed specially for networks whose data rate is less, which have energy constraints, and which require good QoS. The network using this protocol can survive from 6 months to 2 years with two AA batteries.

802.15.4 Works in three frequency bands they are 868 MHz (20 kbps data rate), 915MHz (40 kbps data rate) and 2.4GHz (250 kbps data rate). It uses Direct Sequence Spread Spectrum for modulation and uses BPSK for first two bands and 16 bit array QPSK for the last band. The first band has 1 channel in it the second band has 10 channels in it and third band has 16 channels in it. This can be seen from the diagram below




Channels in 802.15.4 physical layer