NETWORK MIMO FOR DOWNLINK IN-BAND RELAY TRANSMISSIONS WITH RELAYING PHASES OF FIXED DURATION

ABSTRACT 1 A half-duplex relay station (RS)-based cellular system deployment is considered, where multiple base stations (BS) cooperate in the BS-RS in-band transmission for the downlink. The duration of the relay-receive and the relay-transmit phases are fixed beforehand, so that the interference induced by other cells is stationary during a transmission interval. With the optimization of the precoders and powers allocated to the wireless backhaul (relay-receive phase) and to the RS-MS access (relay-transmit phase), it is possible to exploit the benefits of network-MIMO (N-MIMO Index terms-Network-MIMO, Relay transmissions, QoS INTRODUCTION With the advent of new sophisticated terminals and bandwidthdemanding services, system designers are pushed towards the challenge of enhancing system spectral efficiency and providing homogeneous coverage for wireless networks. Next generation standards are already considering that conventional paradigms need to be rethought. In this respect, mature enabling technologies (like MIMO) are considered an integral part of the system, while other (like RS-based deployments and coordinated BS transmissions, or Network-MIMO) are part of ambitious study items. Leveraging on the advantages offered by the joint use of all these techniques is a challenge faced by IEEE 802.16m [1] and LTE-A. While implementation details of full-duplex RS are under investigation, relay-based enhancements in standards consider halfduplex relay operation, which incur a rate penalty as they require at least two timeslots to relay a message from source to destination It has been observed that N-MIMO based on zero-forcing (BD-ZF) performs closely to dirty-paper coding An additional way to improve the efficiency of relay transmissions is by optimizing the duration of the relay-receive and the relay-transmit phases SYSTEM ASSUMPTIONS Our system definition is based on the following practical assumptions: 1. The number of antennas at BS, RS and MS is n B , n R and n M respectively, so MIMO performance gains are captured