Data precoded relay-assisted scheme for cellular systems

We propose a data precoded relay-assisted scheme, for a system cooperating with L relays, each one equipped with a single antenna. The use of relays is of significant interest to allow radio access in situations where either the direct path is not available or has poor quality. However because of the half duplex constraint at the relays, transmission of a data rate that would be possible using a modulation technique with m bits per symbol in the case a continuous link was available from the BS to the UT, requires the use of a constellation with 2m bits per symbol. This implies a penalty in the power efficiency. In this communication we propose a simple precoding scheme that exploits the relation between QPSK and 4L-QAM, by alternately transmitting through the L relays, keeping the diversity of order L. Numerical results are presented for the case where 2 relay nodes are used, for realistic scenarios with LTE parameters and using turbo coding, showing significant performance improvements relatively to the case of a relay based distributed Alamouti employing 16-QAM.