Link adaptation for cooperative communications with repetition coding

The employment of relay-nodes (RN) to assist the base station-user terminal (BS-UT) transmissions in wireless systems could lead to significantly increased performance when compared to the direct BS-UT transmissions, provided that a PER smaller than a target-PER should be ensured. Nevertheless, the performance of the cooperative BS-RN-UT link is depending on the coded-cooperation algorithm employed and on the quality of the three channels involved; moreover it requires additional time-frequency resources. Therefore, the metric used to assess the improvement brought by cooperative transmissions should be the average number of correctly received information bits on QAM symbols. This paper studies the efficiency provided by a cooperative BS-RN-UT link that employs the repetition coding algorithm, with respect to the SNR values of the constituent channels. The performance is evaluated within a generic transmission structure and compared to the spectral efficiency of the non-cooperative BS-UT link, establishing the SNR domains when the cooperative approach would perform better than the non-cooperative one. The employment of these results in the link adaptation and relay-activation processes of a relay-enhanced transmission are also discussed.