In this paper, we deal with two mode switching strategies for capacity-optimum transmission over a cooperative relay network. Both of them switch the transmission mode back and forth between a cooperative mode and a non-cooperative mode based on their corresponding spectral efficiencies. We use the distributed Alamouti code [1] for the cooperative mode transmission. The first strategy uses the same frame structure in both the cooperative and non-cooperative modes. In the cooperative mode, a source node  S  transmits two signals to a relay node  R  and a destination node  D  over two consecutive time slots during the 1 st  phase. During the 2 nd  phase,  S  and  R  transmit again cooperatively the same set of the above two signals to  D  over two consecutive time slots according to the Alamouti encoding rule, and then  D  detects jointly the two signals received over two consecutive phases. Sometimes a spectral efficiency of the  S-D  link might be higher than that of the  S-R  link. In such a case, cooperative transmission might not be better any longer and non-cooperative transmission in which  D  does not take any relayed replicas of the two signals would be recommended. We also introduce the 2 nd  disruptive strategy that changes disruptively the frame structure between the cooperative and non-cooperative modes in order to utilize best the radio resource in the non-cooperative mode. In the non-cooperative mode, one signal for every slot is transmitted only through the  S-D  link with no interruption for relaying. Numerical and simulation results show that the second one can provide almost twice the spectral efficiency than the first one does, though it may need a little forward signaling that  S  notifies the mode change status to  D.  The performance of the second strategy gets improved more significantly as the signal-to-noise ratio (SNR) of the  S-R  link is getting low and/or the SNR of the  S -D  link is getting high.
['Min Lee', 'Seong Keun Oh']
Capacity-optimum mode switching strategies over a cooperative relay network