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Wednesday, June 29, 2011

TV White Spaces: Spectrum Sensing


One of the important components of the IEEE 802.22 document to achieve the required cognitive capability is related to spectrum measurements. The spectrum measurement in 802.22 is primarily based on transmitter detection. In order to check the presence of primary signals, 802.22 devices need to be able to detect signals at very low Signal-to-Noise Ratio (SNR) levels. Since the detection is done at low SNR, it is assumed that the detection of TV signals is done in a non-coherent manner, which means that no synchronization is needed.

The required accuracy of the spectrum sensing, the frequency band and time period, is determined in a centralized way by the BS. Using the local measurements, the BS can establish a spectrum occupancy map. The BS does not require the same sensing accuracy of each CPE, and algorithms to optimize or distribute the sensing load across CPEs can be used. To optimize the sensing, 802.22 devices are supposed to be equipped with a dedicated omni-directional antenna for sensing. This is in addition to a directional antenna which is used for data transmission in the target direction, minimizing the interference area. To be able to optimize the sensing accuracy of the omnidirectional antenna, it would most likely have to be mounted outdoors [34]. 802.22 devices can be instructed to perform in-band or out-of-band sensing, where a band denotes the TV band currently used by the cell. For in-band sensing, the 802.22 communication needs to be temporarily halted, in order not to interfere with the sensing. There clearly is a trade-off between speed at which a primary TV signal can be detected, and the efficiency or throughput achieved by the 802.22 cell. To avoid too frequent long connectivity halts, a two-phase sensing mechanism is used as shown below.
Fast sensing, i.e. based on a simple and fast sensing technique, is performed more frequently. After one (or more) fast sensing periods, the BS can decide whether to perform a fine sensing. This fine sensing takes more time but should in fact only be carried out if the fast sensing results are not sufficient to draw conclusions. Given the fact that TV signals do not come on the air frequently, this two-phase sensing method proves highly effective. If multiple 802.22 cells operate in the same area, it is required that their sensing strategy is synchronized (i.e., they should halt communication when other cells sense). Since coexistence among different 802.22 cells is an important issue, such synchronization is embedded in the 802.22 standard. Contrary to the TV signals detection, sensing of wireless microphone transmissions is much harder as these transmit at a much lower power and occupy much lower bandwidths. Therefore, in addition to transmitter detection, a second sensing option is enabled in the 802.22 standards. This second option relies on the transmission of beacons by the microphones themselves or a special device carried by microphone operators. This primary network information monitoring is embedded in the 802.22 MAC. The exact sensing algorithms to be used during the fine and fast sensing periods are not standardized, but an extensive list is provided as annex to the standard.


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