tailieunhanh - Satellite Communicationsever increasing widespread Part 3

Tham khảo tài liệu 'satellite communicationsever increasing widespread part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Cooperative Strategies for Satellite Access 61 3. Overview on Cooperative Communications Some years ago a new class of techniques called cooperative communications has been proposed as a valuable alternative to the spatial diversity techniques which require the deployment of additional antennas in order to mitigate the fading effects. Cooperative communications are based on the concept that a group of mobile terminals can share their single antennas in order to generate a virtual multiple antenna obtaining the same effects than a MIMO system Nosratinia et al. 2004 Ribeiro Giannakis 2006 . This approach can be seen as a new form of spatial diversity in which however the diversity gain can be achieved through the cooperation of different users opportunely grouped in clusters which can assume the double role of active user . the user which transmits its own information data and cooperator . the user which helps the active user in its transmission Sendonaris et al. 2003a b . The key concept is that each user sees an independent fading process and that spatial diversity can be generated by transmitting each user s data through different paths as shown in Fig. 1. Independent fading paths ACTIVE USER Fig. 1. Example of cooperative communications COOPERATOR An effective way to mitigate fading is to supply the receiver with multiple replicas of the same information-bearing signal transmitted over independent channels. Because of this independence the probability that all the considered signals are simultaneously vanishing due to fading is considerably reduced. If p 0 p 1 is the probability that any signal is faded below a threshold value the probability that all L independent fading channels containing the same signal are faded below the threshold value is given by L ptot n p pL 1 i i and therefore it is lower than p Lee Chugg 2006 . The cooperative approach turns to be useful for mobile terminals which because of their size constraints cannot support multiple .