Zero-forcing relay power allocation for TDBC-based bidirectional relay networks

Although the amplify-and-forward (AF) relaying is an effective and simple technique for cooperative communication networks, it introduces a noise enhancement problem. This problem occurs when the effects of interference and the multipath fades are amplified during the transmission from the source to the relay. AF is usually used with the multiple access broadcast (MABC) protocol which is a two phases transmission (2P) analog network coding (ANC) protocol for the half-duplex (HD) communication mode. However, MABC is not designed to utilize the direct link (DL) signal no matter how strong it is. To utilize the DL signal, a three transmission phases protocol known as time-division broadcast protocol (TDBC) was proposed at which both end nodes transmit their message at consecutive time slots and the relay nodes broadcasts a combined signal at the 3^rd time slot. To deal with the noise enhancement problem and improve the system performance, this paper proposes a zero-forcing based relay power allocation (ZF-RPA) scheme to be used with TDBC protocol, where the noise enhancement is mitigated by introducing adaptive ZF gains at the relay node that inverses the channels effects between sources and relay when the signal is constructed to be transmitted at the third phase of the TDBC. Analytical results of the ZF-RPA scheme is compared to the traditional variable-gain (VG-RPA) scheme under the same nodes power allocation. Simulation results show that the ZF-RPA is superior for all channel qualities in terms of the outage probability compared to the traditional VG-RPA. The total system sum rate comparison reveals that the ZF-RPA scheme could outperform the VG-RPA scheme as long as the channel qualities of the relay-transceiver links are less than certain threshold. This paper also clarifies that it is effective to select either ZF or VG-RPA according to the channel qualities and it can be implemented with VHDL by introducing an adaptive control unit (ACU).