Precoder Detection for Cooperative Decode-and-Forward Relaying in OFDMA Systems

We consider an LTE network where a secondary user acts as a relay, transmitting data to the primary user using a decode-and-forward mechanism, transparent to the base-station (eNodeB). Clearly, the relay can decode symbols more reliably if the employed precoder matrix indicators (PMIs) are known. However, for closed loop spatial multiplexing (CLSM) transmit mode, this information is not always embedded in the downlink signal, leading to a need for effective methods to determine the PMI. In this paper, we consider 2x2 MIMO and 4x4 MIMO downlink channels corresponding to CLSM and propose two techniques to estimate the PMI at the secondary user using a hypothesis testing framework. We evaluate their performance via simulations for various ITU channel models over a range of signal-to-noise ratio and for different channel quality indicators. We compare them to the case when the true PMI is known at the relay and show that the performance of the proposed schemes are within 2 dB at 10 percent block error rate for almost all the cases considered. Furthermore, the proposed algorithms add minimal computational overhead over the existent receiver structure.