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Multihop communications, a viable option for providing broader and more efficient coverage, can be categorized as either non-regenerative (Amplify-and-Forward, AF) or regenerative Decode-and-Forward, DF) depending on the relay functionality. In DF systems, each relay decodes its received signal and then re-transmits this decoded version. In AF systems, the relays just amplify and re-transmit their received signal. Furthermore, a system with AF relays can use Channel State Information (CSI)-assisted relays or fixed-gain relays (also known as blind or semi-blind relays).A (CSI)-assisted relay uses instantaneous CSI of the channel between the transmitting terminal and the receiving relay to adjust its gain, whereas a fixed-gain relay just amplifies its received signal by a fixed gain.
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