IEEE 802.22 Spectrum Sensing and Cognitive Network
- an overview of the cognitive radio network and spectrum sensing techniques used within the IEEE 802.22 WRAN standard.
One of the key elements of the 802.22 standard is the fact that it can coexist with other users of the radio spectrum, causing no undue interference. As any 802.22 system is likely to be given access to any spectrum on a secondary basis where no undue interference is caused to the primary user, it is essential that the system is effectively able to adapt itself around the primary users. To achieve this a cognitive radio networking is required to provide the spectrum sensing and adaptation.
802.22 spectrum sensing basics
The 802.22 network is responsible for ensuring that it creates no undue interference to other users of the relevant spectrum. The overall network comprises of the base station, BS, and a number of user equipments known as customer premises equipments, CPE.
In order to effectively provide the level of interference avoidance that is required, 802.22 spectrum sensing is distributed across the network of users. Accordingly, the 802.22 spectrum sensing is undertaken within the Customer Premises Equipments, CPEs.
The CPEs scan the various channels that are open for their use and send back information about signals and strengths on the channels to the base station equipment.
It is BS which makes the decision about which channels are occupied and whether they can be used for the 802.22 transmissions. To make this decision the BS uses the spectrum sensing results as well as geo-location information and other information provided by an entity known as the network manager.
The 802.22 standard takes cogniscence of the fact that there will be three main types of users of the frequencies used by 802.22:
- Analogue television - for North America this based on the NTSC standard, whereas for Europe it is generally PAL. The level of an analogue signal above which the 802.22 system will vacate the channel is -94 dBm measured at the peak of the sync pulse - different levels may be applied for other television systems.
- Digital television - for North America DTV is used, although within Europe DVB-T is most widespread. The level of a DTV digital television signal above which the 802.22 system will vacate the channel is -116 dBm.
- Wireless microphones - these may have a variety of formats as they are not standardised, although typically they use FM and have a bandwidth of around 200 kHz. The level of a wireless microphone signal above which the 802.22 system will vacate the channel is -107 dBm measured in a 200 kHz bandwidth.
In this way the IEEE 802.22 WRAN performs spectrum sensing across the whole network and adjusts itself accordingly. This means that the 802.22 WRAN system is a true cognitive radio network, rather than an individual cognitive radio operating in isolation.
802.22 spectrum measurements
The channel management and spectrum sensing or signal measurements form an important part of the overall 802.22 scheme. The MAC layer within the CPEs carries out many important tasks that enable this to work efficiently and smoothly.
The base station instructs the CPEs to perform periodic measurements in one of two formats;
- In band spectrum sensing: The in-band spectrum sensing applies to the channels that are being currently used by the BS to communicate with the CPEs. In order for this type of sensing to be undertaken it is necessary for the BS to quieten the transmissions on the channel. With a short break of the transmissions, the CPEs can then listen for any other transmissions.
When assessing the presence of other signals on the channel, the CPE is required to look for very low level, the levels required and the accuracy being controlled by the BS. Length of time for the measurement, which channels, length of measurement time, and probability of false alarm are all under the control of the BS. In order to gain the best overall measurement, the BS may instruct different CPEs to make different measurements. The choice of how this is done, is made by the BS and is calculated by the algorithms it contains. By instructing different CPEs to make different measurements and over different lengths of time, the BS can make up an occupancy map for the overall cell.
- Out of band spectrum sensing: The out of band spectrum sensing refers to channels that are not currently being used by the BS to communicate with the CPEs. These measurements are made to locate possible alternative channels, should those in use become occupied. It also ensures that there is a sufficient guard band between the channels in use by the BS and any TVs stations that may be using adjacent channels.
In-band spectrum sensing
The in-band spectrum sensing is carried out on a regular basis. The quiet periods for sensing are built into the transmission timings. There are two types of sensing that are defined:
- Fast sensing: This form of spectrum sensing is accomplished quickly, as the name implies. This form of sensing typically uses a simple energy detection algorithm and is completed within 1 ms. The results of the fast sensing are returned to the BS which then analyses them and determines whether a fine sensing measurement is required.
- Fine sensing: The fine sensing procedure is undertaken if the BS believes there is need for a more accurate measurement. During the fast sensing a more detailed examination is made of the particular channels. This form of spectrum sensing takes around 25 ms. During any fine sensing time, the CPE looks at the signatures of signals that may be the primary user, i.e. television.
It is also possible that adjacent 802.22 networks may cause interference to one another, and the adjacent networks may sense each other. To overcome the possibility of confusion caused by adjacent networks detecting each other algorithms are built into the system to synchronise overlapping cells. This also includes the synchronisation of the quiet periods when the spectrum sensing occurs.
Cognitive radio spectrum sensing
The 802.22 standard is the fist standard to adopt cognitive radio spectrum sensing as a means of gaining greater use of the radio spectrum. By using cognitive radio networking techniques, it is able to sense the environment and adjust the network to accommodate any changes.
By Ian Poole
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