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Tham khảo tài liệu 'vehicular technologies increasing connectivity part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Cognitive Radio Communications for Vehicular Technology - Wavelet Applications 231 reliable sensing of the wireless environment. Secondary users might experience losses in the signal which can result in an incorrect judgment of the wireless environment which can in turn cause interference at the licensed primary user by the secondary transmission. Furthermore the issues with signal quality are aggravated when secondary users rapidly change location as it is the case for specific vehicular technology applications. Briefly as shown in figure 5 unreliable results can be produced based on the following phenomena - Multipath a sensor CR1 under multipath receiving conditions features short term Rayleigh fading. The fluctuations of the power level may cause unreliable detection. - Shadowing a sensor CR2 may move behind an obstacle exhibiting lognormal long term fading. Its covered position may create disturbance for a PRx in its proximity hidden terminal problem . - Distance-dependent path loss a sensor CR3 lies outside the primary transmission range. It receives a low power level due to the distance but its transmission can produce interference to the primary receiver which is inside the primary range. Primary Transmitter PTx a Cognitive Radio CR Solid obstacle CR3 Fig. 5. Layout of a network with moving terminals This arises the necessity for the cognitive radio to be highly robust to channel impairments and also to be able to detect extremely low power signals. These stringent requirements pose a lot of challenges for the deployment of CR networks. Channel impairments and low power detection problems in CR can be alleviated if multiple CR users cooperate in sensing the channel. Thanayankizil Kailas 2008 suggest different cooperative topologies that can be broadly classified into three regimes according to their level of cooperation Decentralized Uncoordinated Techniques the cognitive users in the network don t have any kind of cooperation which means that each CR user
