ABSTRACT
In this chapter, we consider channel state information (CSI) feedback over dedicated limited capacity feedback links, as applied in 3G and 4G cellular networks. Channel state information at the transmitter (CSIT) is useful for achieving the highest performance in multiple antenna wireless communications. Single-user multiple-input multiple-output (MIMO) systems employ CSI to match the spatial signature of the transmit signal to the channel, enabling reliable communication and highest transmission rates over multiple parallel data streams [1,2,3,4 and 5]. In multiuser MIMO broadcast channels, CSIT is of even higher importance as it allows to spatially separate multiple users and to control the interference caused by parallel transmissions to multiple users [6,7]. Also, most spatial interference coordination and alignment schemes, which are intended to avoid or even exploit the interference caused by parallel transmissions from multiple base stations, rely on accurate CSI at the transmitters to perform reliably [8,9,10,11 and 12]. Accurate CSIT is therefore an important enabler, facilitating the transition to dense heterogeneous 5G cellular networks. In time division duplex systems, CSIT can be made available by inserting pilot symbols into the uplink transmission and applying well-known channel estimation algorithms at the base stations to determine the CSI; see, for example, [13] and references therein. The majority of long-term evolution (LTE) network deployments, however, is frequency division duplex (FDD) based. In such FDD systems, obtaining CSIT is possible only through dedicated feedback from user equipment (UE). In this chapter, we introduce and review efficient CSI feedback algorithms that minimize the feedback burden by exploiting time, space, and frequency correlations of the wireless channel.
