Cognitive radio spectrum sensing
- an overview detailing the key techniques, technologies and methods used for spectrum sensing in Cognitive Radio technology.
With Cognitive Radio being used in a number of applications, the area of spectrum sensing has become increasingly important. As Cognitive Radio technology is being used to provide a method of using the spectrum more efficiently, spectrum sensing is key to this application.
The ability of Cognitive Radio systems to access spare sections of the radio spectrum, and to keep monitoring the spectrum to ensure that the Cognitive Radio system does not cause any undue interference relies totally on the spectrum sensing elements of the system.
For the overall system to operate effectively and to provide the required improvement in spectrum efficiency, the Cognitive Radio spectrum sensing system must be able to effectively detect any other transmissions, identify what they are and inform the central processing unit within the Cognitive Radio so that the required action can be taken.
Cognitive Radio Spectrum Sensing basics
In many areas cognitive radio systems coexist with other radio systems, using the same spectrum but without causing undue interference. When sensing the spectrum occupancy, the cognitive radio system must accommodate a variety of considerations:
- Continuous spectrum sensing: It is necessary for the cognitive radio system to continuously sense the spectrum occupancy. Typically a cognitive radio system will utilise the spectrum on a non-interference basis to the primary user. Accordingly it is necessary for the Cognitive radio system to continuously sense the spectrum in case the primary user returns.
- Monitor for alternative empty spectrum: In case the primary user returns to the spectrum being used, the cognitive radio system must have alternative spectrum available to which it can switch should the need arise.
- Monitor type of transmission: It is necessary for the cognitive radio to sense the type of transmission being received. The cognitive radio system should be able to determine the type of transmission used by the primary user so that spurious transmissions and interference are ignored as well as transmissions made by the cognitive radio system itself.
Types of cognitive radio spectrum sensing
There are a number of ways in which cognitive radios are able to perform spectrum sensing. The ways in which cognitive radio spectrum sensing can be performed falls into one of two categories:
- Non-cooperative spectrum sensing: This form of spectrum sensing, occurs when a cognitive radio acts on its own. The cognitive radio will configure itself according to the signals it can detect and the information with which it is pre-loaded.
- Cooperative spectrum sensing : Within a cooperative cognitive radio spectrum sensing system, sensing will be undertaken by a number of different radios within a cognitive radio network. Typically a central station will receive reports of signals from a variety of radios in the network and adjust the overall cognitive radio network to suit.
Cognitive radio cooperation reduces problems of interference where a single cognitive radio cannot hear a primary user because of issues such as shading from the primary user, but a second primary user acting as a receiver may be able to hear both the primary user and the signal from the cognitive radio system.
Cognitive radio spectrum sensing methodologies
There are a number of attributes that must be incorporated into any cognitive radio spectrum sensing scheme. These ensure that the spectrum sensing is undertaken to meet the requirements for the particular applications. The methodology and attributes assigned to the spectrum sensing ensure that the cognitive radio system is able to avoid interference to other users while maintaining its own performance.
- Spectrum sensing bandwidth: There are a number of issues associated with the spectrum sensing bandwidth. The first is effectively the number of channels on which the system will sense whether they are occupied. By sensing channels apart from the one currently in use, the system will be able to build up a picture of alternative channels that can be used should the current one become occupied. Secondly the actual reception bandwidth needs to be determined. A narrow bandwidth will reduce the system noise floor and thereby improve the sensitivity, but it must also have a sufficiently wide bandwidth to detect the likely transmissions on the channel.
- Transmission type sensing: The system must be capable of identifying the transmission of the primary user for the channel. It must also identify transmissions of other units in the same system as itself. It should also be able to identify other types of transmission that may be spurious signals, etc.
- Spectrum sensing accuracy: The cognitive radio spectrum sensing mechanism must be able to detect any other signal levels accurately so that the number of false alarms is minimised.
- Spectrum sensing timing windows: It is necessary that the cognitive radio spectrum sensing methodology allows time slots when it does not transmit to enable the system to detect other signals. These must be accommodated within the frame format for the overall system.
Spectrum sensing instabilities
When developing a methodology it is necessary to ensure that the overall system remains stable. There are instances where levels of occupancy increase where cognitive radio systems will continually move from one channel to another. This considerably reduces the efficiency and at the worst case could almost render the system inoperable.
To illustrate the types of scenario that could be encountered, consider the case where channel occupancy is high and a limited number of channels are allocated or are available. The first cognitive radio system may have settled on a channel, but then detects another user so it moves to the next channel. This second channel may have been in use by another user which detects the new channel occupancy and moves. This could continue until the final user then moves into the first channel and the whole procedure repeats.
While it is possible that events may not occur in exactly this fashion, these types of scenario will occur and the cognitive radio spectrum sensing algorithms must be designed to take account of these forms of scenario, and ensure the optimum usage of the available spectrum.
Also with cognitive radio usage increasing, there will be an increase in signal frequency agility and signals will often appear on new frequencies. Accordingly this must be built into the decision algorithms to ensure that CR systems only move when it is necessary.
Cognitive radio spectrum sensing is one of the key algorithms associated with the whole field of cognitive radio. As experience grows, the cognitive radio spectrum sensing techniques will be refined and they will be designed to accommodate the increasing use of the spectrum as well as any malicious attacks that could be presented to CR systems.
By Ian Poole
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