Precoding for coordinated multi-point transmission [CoMP] for LTE-A

Abstract

The last 60 years has witnessed the creation of extraordinary communicaions technologies, including radar, televsion, the digital telephone network, optical communications, the internet and celluar mobile telephony. These efforts reflect a never-ending drive to build new generations of technology upon ealier ones. One of the growing technology is Coordinated Multi-Point (CoMP) transmission which is targeted for Long Term Evolution Advanced (LTE-A). It is designed to reduce cell-edge interference, improve cell-edge spectrum efficiency and enlarge effective cell-edge coverage by means of multi-point coordination including coordination between Remote Radio Units (RRUs) within an eNodeB, coordination between an eNodeB and its relays, and coordination between eNodeBs. Its aim is to enhance the overall system performance, utilize the resources more effectively and improve the end user service quality. One of the key parameters for LTE as a whole, and in particular 4G LTE Advanced is the high data rates that are achievable. These data rates are relatively easy to maintain close to the base station, but as distances increase, they become more difficult to maintain. Obviously the cell edges are the most challenging. Not only is the signal lower in strength because of the distance from the base station (eNB), but also interference levels from neighboring eNBs are likely to be higher as the user equipment (UE) will be closer to them. For CoMP, the cooperating entities (eNodeBs, RRUs, relays etc) need to know the full downlink channels prior to the transmission. The acquisition of channel state information at the transmitter (CSIT) is challenging in the frequency division duplex (FDD) systems as the uplink and downlink are not reciprocal. UEs need to estimate the channels to the transmitters and then feedback this information to the transmitters. Need of dedicated feedback channels severely affect the system capacity thereby undermining the gains of CoMP. The purpose of this research is to look at the required feedback in CoMP systems and design an efficient precoding scheme based on minimium feedback. The main focus will be made on two important characteristics of the LTE precoders, i.e., their low resolution and their use of equal gain transmission (EGT) for incorporating in CoMP technology in LTE-A.