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2.3.2.3 Radio network infrastructure

The radio subsystem is the most important system on a wireless sensor node, since it is the primary
energy consumer among the three highlighted application scenarios. Modern low power, short
range transceivers consume between 15 and 300 milliwatts of power when sending and receiving.
A key hardware observation is that low power radios consume approximately the same amount of
energy when in receive or transmit mode. This energy is consumed if the radio is on, whether or not
it is receiving actual data. The actual power emitted out of the antenna only accounts for a small
fraction of the transceiver's energy consumption. A significant fraction goes to internal operation.
As a result, the overall cost of radio communication can be dominated in some cases by the receiver
power consumption – a metric that is often ignored in wireless studies.

Transmission Range

The transmission range of a wireless system is controlled by several key factors. The most intuitive
factor is that of transmission power. The more energy put into a signal, the farther it should travel.
The relationship between power output and distance travelled is a polynomial with an exponent of
between 3 and 4 (non‐line of sight propagation). So to transmit twice as far through an indoor
environment, 8 to 16 times as much energy must be emitted.

Other factors in determining range include the sensitivity of the receiver, the gain and efficiency of
the antenna and the channel encoding mechanism. In general, wireless sensor network nodes
cannot exploit high gain, directional antennas because they require special alignment and prevent
ad‐hoc network topologies. Omni‐directional antennas are preferred in ad‐hoc networks because
they allow nodes to effectively communicate in all directions.

Both transmission strength and receiver sensitivity are measured in dBm. Typical receiver
sensitivities are between −85 and −110 dBm. Transmission range increases can be achieved by either
increasing sensitivity or by increasing transmission power. When transmitting at 0 dBm, a receiver
sensitivity of ‐85 dBm will result in an outdoor free space range of 25‐50 meters, while a sensitivity
of −110 dBm will result in a range of 100 to 200 meters. The use of a radio with a sensitivity of
−100 dBm instead of a radio with −85 dBm will allow you to decrease the transmission power by a
15
factor of 30 and achieve the same range .

Bit Rate
Unlike many high performance data networks, wireless sensor networks do not require high bit
rates. 10‐100 Kbps of raw network bandwidth is sufficient for many applications. Radio bandwidth
has a more significant impact on node power consumption and its lifetime in case of a battery
powered node. As bit rates increase, transmission times decrease. As the highest instantaneous
energy consumer, it is essential that the radio remain off as much as possible. By increasing the bit
rate without increasing the amount of data being transmitted, the radio duty cycle is decreased.

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15 The dB scale is a logarithmic scale where a 10 dB increase represents a 10x increase in power. The baseline
of 0 dBm represents 1 milliwatt, so 1 watt is 30 dBm.

256 ITU‐T's Technical Reports and Specifications