Channel estimation in a MIMO relay system: Challenges & approaches channel estimation in MIMO relay system: A review

The overall performance of the communication system is highly influenced by the accuracy with which the channel is estimated at the receiver node. It is imperative to design methods that can efficiently provide the channel estimates even in worst case channel scenarios. It becomes extremely challenging to extract reliable channel estimates in the mobile node scenario, wherein the Doppler influences are also participating. Hence, the designed channel estimation methods need to be robust and spectrally efficient, provided that these are not computationally demanding, as well. In this paper, an extensive review of the channel estimation methods for MIMO relay systems that are adopted under varied channel conditions is conducted. We discuss the concept of relaying, the advantage of including relays in MIMO systems and their classification. This is followed by a comparison of these approaches that is made in terms of their spectral efficiency, computational complexity as well as their limitations. Further, we discuss the channel estimation methods specifically designed for the time-varying frequency-selective channels. We emphasize that more work needs to be done for improving the channel estimation methods, especially when the channel is influenced by the Doppler Effect.