Wireless sensor networks are typically comprised of large numbers (e.g., hundreds to thousands) of nodes that collectively perform tasks such as environment monitoring, motion detection, surveillance, and so on. Each node is a low power device, typically battery-operated, that is equipped with one or more sensors and a low power radio. The network is often expected to operate for a number of years without refreshing the batteries in the nodes. In typical operation, the nodes periodically sense their environment, and communicate with their peers using predefined transmission protocols. In order to achieve long battery life, the node hardware and transmission protocols must be designed to ensure extremely low average current draw and conform to an energy budget that gives low time rates of power consumption per node.
The communication links between wireless sensor nodes in a network typically utilize one or more unlicensed frequency bands (channels), or frequency bands specifically designated for the particular network application. In general, any number of sensor networks may operate simultaneously, which is possible primarily because each node usually communicates over short-range links and the principle of spatial reuse allows many adjoining networks to operate with tolerable interference from one another. Nevertheless, accommodating for the effects of interference is a critical issue with regard to the proper time and energy-efficient operation of the network.
In a static topology formation, it has been observed that certain wireless channels have better signal-to-interference-noise (SINR) than others, and are thus more tolerant to sudden interference effects. Moreover, certain wireless channels that may not be significantly affected by interference operate on better signal strength reception than others, and are therefore more resistant to fading than others. In most network applications, there may be several options of links (routes) available to the nodes for topology formation. Networks can often be dynamically reconfigured to ensure that the links comprising the node routes are more resistant to interference and/or fading, thus yielding a more efficient topology formation in the long run as fewer packets are lost and fewer retransmissions are required. The choice of a bad link can drain the energy of a node substantially by requiring useless data transmission cycles. Therefore, it is of great importance to assess the SINR of the channels in a sensor network to ensure the utilization of superior links between nodes to maximize the power efficiency of the network.
Present methods of link assessment typically sample each channel of the network in the absence of a signal to determine the ambient noise present in the channel. However, a drawback to this scheme is that while it indicates the interferer noise power present in a frequency band, it does not provide any real information about the usability of a link since this depends on the actual signal-to-interference-noise experienced by the receiver. The actual quality of a link is a function of both the relative placement of transmitter and receiver (the spatial model), as well as the communication frequency. If a localized interferer exists in a certain part of the network, then this information must be encapsulated in the determination of link quality. The consideration of both frequency and spatial quality is especially important for scheduling transmission-reception along different links while choosing from all available frequencies for each link, in order to maximize the simultaneous communication at a given time-slot. Therefore, present methods of link assessment are disadvantageous because they do not consider the link to be a function of the frequency, but only a parameter that encapsulates the received signal-to-noise ratio that occurs due to spatial placement of the receiver with respect to the transmitter.
An additional disadvantage of present link assessment methods involves the time required to complete one cycle of assessment for all links in the network on a given frequency. Present methods typically assess all frequencies for all links in a given network, therefore, the total time required for this operation can be quite significant. The per-cycle time (Tcycle), is the time required to give reasonably stable time averages of link quality based on Packet Success Rate (PSR) on each link. If there are n adjacent frequency channels available for use in the unlicensed band, then the total time taken for evaluating the link quality of all links for all frequencies is Ttotal=n*Tcycle. A linear increase on the total time with number of frequency channels is redundant and possibly unacceptable since the initialization steps of node discovery and link assessment must generally form a negligible part of the total energy budget, and must also be carried out within reasonable time periods.