Code designs for cooperative communication

The implementation of two cooperative protocols decode-and-forward (DF) and estimate-and-forward (EF) has been discussed. The starting point is an information theoretic random coding scheme that motivates a practical code construction. The relay in a DF protocol first decodes information received during the source broadcast, re-encodes it and then forwards it to the destination. In EF relaying, the relay estimates and compresses its received signals prior to channel coding and retransmission. The coding schemes used are for time-variant Gaussian channels. They can be observed in systems where the channel state, consisting of all channel gains, is known at all nodes, whether transmitting, receiving or relaying. Meanwhile, studies have shown that relaying can mimic multiple-antenna systems even when the cooperating terminals can individually only support a single antenna. It is possible to send multiple copies of the same signal with multiple terminals to improve reliability. Another way is to transmit several parallel data streams to increase rate, which is called the multiplexing gain. The key to building viable cooperative systems is to construct efficient protocols for initiating cooperation, synchronization and channel estimation, and to selectively choose network topologies and relay protocols where the cooperative gain is high enough to justify the additional overhead.