Wireless Systems with Quantized Information: Multiuser MIMO and Networked Control Systems

This thesis deals with two multiuser wireless communication systems: i) the multiple-input multiple-output (MIMO) broadcast channel (BC) where a multi-antenna transmitter serves multiple users along spatially multiplexed channels and ii) a networked control system (NCS) where spatially distributed sensors, controllers and actuators exchange information via a digital wireless network in order to estimate or control a dynamical system. In MIMO BC, channel state information at transmitter (CSIT) is essential to achieve spatial multiplexing across users. Special interest is on frequency division duplexing systems where CSIT is provided through limited uplink feedback (FB) from the receivers. Either in case of single antenna or multi-antenna receivers the main contributions are: i) the design of novel linear transceiver strategies that account for limited CSIT, ii) the proposal of channel quantization techniques and FB strategies that exploit spatial and time correlation of the MIMO channel and iii) the derivation of efficient and robust user selection schemes for the maximization of the achievable throughput. In MIMO downlink systems the potential gains of multiuser over more conventional single user transmission strategies are also evaluated in a multi-cell cellular network where coordination among spatially distributed base stations and higher order sectorization are investigated as possible methods to mitigate inter-cell interference. In case of multiuser MIMO orthogonal frequency division multiplexing (OFDM) downlink systems we provide non trivial generalizations of channel quantization strategies proposed for single carrier flat fading systems. Interestingly, concentrating FB bits to characterize only a portion of the available bandwidth at receivers and the possibility of exploiting multiuser diversity can increase significantly the achievable throughput. In NCSs where system measurements come from multiple spatially distributed sensors, the main contribution is the generalization of estimation and control techniques to account for wireless link inefficiencies: i) packet loss, ii) delays and iii) signal quantization. In particular sensors, controller and actuator share a common frequency resource motivating a cross layer optimization of i) signal/measurements quantization processes and ii) network resource allocation. Even with small transmission bandwidth, single-hop communication protocols with binary phase shift keying provide close to optimum performance in applications dealing with state estimation or state control of a stable system. This support the widespread use of low-cost sensors for these applications.