Cognitive radio cooperative spectrum sensing
- an overview detailing cognitive radio cooperative spectrum sensing techniques and methodologies.
Cognitive radio cooperative spectrum sensing techniques have been developed considerably as a variety of cognitive radio network systems technologies have increased. While non-cooperative spectrum sensing is applicable for many applications, it has a number of drawbacks for many applications.
In cognitive radio applications where a cognitive radio network is present, cooperative spectrum sensing is not only advantageous, it becomes essential if the network as a whole is to avoid interference with any primary users.
What is cooperative spectrum sensing
Cognitive radio cooperative spectrum sensing occurs when a group or network of cognitive radios share the sense information they gain. This provides a better picture of the spectrum usage over the area where the cognitive radios are located.
There are broadly two approaches to cooperative spectrum sensing:
- Centralised approach: In this approach to cognitive radio cooperative spectrum sensing, there is a master node within the network that collects the sensing information from all the sense nodes or radios within the network. It then analyses the information and determines the frequencies that can and cannot be used.
The cognitive radio central node or controller can also organise the various sensor nodes to undertake different measurements at different times. In this way it is possible to undertake a number of different sense actions at the same time. For example, some nodes may be instructed to detect on channel signal levels, while others may be instructed to measure levels on adjacent channels to determine suitable alternatives in case a channel change is required.
- Distributed approach: Using the distributed approach for cognitive radio cooperative spectrum sensing, no one node takes control. Instead communication exists between the different nodes and they are able to share sense information. However this approach requires for the individual radios to have a much higher level of autonomy, and possibly setting themselves up as an ad-hoc network.
Advantages of cooperative spectrum sensing
While cognitive radio cooperative spectrum sensing is obviously more complicated than a single non-cooperative system, it has many advantages that outweigh the added complexity. Naturally cooperative spectrum sensing is not applicable in all applications, but where it is applicable, considerable improvements in system performance can be gained.
- Hidden node problem is significantly reduced: One of the chief problems with non-cooperative spectrum sensing is that even though the cognitive radio may not be able to detect a primary user transmitter, it may still interfere with receivers who may be able to detect both the primary user and also the cognitive radio system transmissions. By using a cooperative sensing system, it is possible to reduce the possibility of this happening because a greater number of receivers will be able to build up a might more accurate picture of the transmissions in the area.
- Increase in agility: An increase in the number of spectrum sensing nodes by cooperation enables the sensing to be more accurate and better options for channel moves to be processed, thereby providing an increase in agility.
- Reduced false alarms: By having multiple nodes performing the spectrum sensing, channel signal detection is more accurate and this reduces the number of false alarms.
- More accurate signal detection: Cooperative spectrum sensing provides for more accurate signal detection and a greater reliability of the overall system.
There are many advantages to incorporating a cooperative spectrum sensing system within a cognitive radio network wherever possible.
System requirements for CR cooperative spectrum sensing
The use of cognitive radio cooperative spectrum sensing provides many advantages, but to provide this ability there are a number of requirements that need to be provided. While these may be seen as an overhead and in some instances a disadvantage, the advantages often outweigh the disadvantages.
- Control channel: In order for the different elements within the cognitive radio cooperative spectrum sensing network to communicate, a control channel is required. This will take up a proportion of the overall system bandwidth.
- System synchronisation: It is normally necessary to provide synchronisation between all the nodes within the cognitive radio cooperative spectrum sensing network. This is to keep the channel free from transmissions from the cognitive network while sensing is under way. In some instances adaptive scheduling of the sense period may prove beneficial. In this way the dead time arising from sense periods can be minimised within the need to ensuring the sensing is undertaken sufficiently well. Accurate spectrum sensing requires a longer period of time than a rough sense to see if a strong signal has returned. By adapting the sense periods, channel throughput can be maximised, although there is a greater need to maintain synchronisation under these circumstances.
- Suitable geographical spread of cooperating nodes: In order to gain the optimum sensing from the cooperating nodes within the cognitive network, it is necessary to obtain the best geographical spread. In this way the hidden node syndrome can be minimised, and the most accurate spectrum sensing can be gained.
By Ian Poole
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