This thesis describes a real-time software-defined-radio implementation of the time-slotted round-trip carrier synchronization protocol in two-source and three-source communication systems. The techniques developed in this thesis can be used to synchronize the carriers of two or three single-antenna wireless transmitters with independent local oscillators so that their band-pass transmissions combine constructively at an intended receiver. Synchronization is achieved via the time-slotted transmission of (i) an unmodulated primary beacon from the destination to the sources and (ii) a series of secondary unmodulated beacons between the sources. Explicit channel state information is not exchanged between the sources and/or the destination. When synchronized, the single-antenna sources are able to cooperatively transmit as a distributed beamformer and achieve increased transmission range, reduced transmission energy, and/or increased security. The experimental results in this thesis confirm the theoretical predictions and also provide explicit guidelines for the real-time implementation of a carrier synchronization technique suitable for distributed transmit beamforming.

We describe a set of fundamental contributions to the design, analysis and implementation of distributed MIMO techniques in wireless networks. The main idea behind distributed MIMO is to organize groups of wireless transmitters and receivers into distributed antenna arrays to cooperatively achieve beamforming and spatial multiplexing gains in ad-hoc wireless networks. This technique promises orders-of-magnitude increases in wireless data rates, however it presupposes very stringent timing, carrier frequency and phase synchronization of the RF signals between the cooperating nodes in the array. Specifically in this dissertation, we consider a sub-class of distributed MIMO systems called distributed MISO systems. In other words, we focus on distributed transmit arrays, wherein a group of N transmitters organize themselves into a virtual antenna array (VAA) to talk to a single-antenna receiver. While distributed MIMO involves virtual arrays on both transmit and receive ends, transmit arrays require real-time coordination, and therefore present unique challenges as compared to receive arrays. We explore two specific MISO techniques: i) distributed beamforming and ii) distributed nullforming in this work. Beamforming involves focusing transmitted energy selectively in the direction of an intended receiver, and nullforming involves forming a "null" i.e. having the transmissions of the different array nodes cancel each other completely at a desired location. Beamforming has the potential of substantially increasing the energy efficiency of wireless communications, while nullforming allows multiple nodes to communicate simultaneously over the same frequency band by carefully canceling the resulting interference. Beamforming and nullforming can also be thought of as basic building blocks for more sophisticated MIMO techniques. In this work, we present a set of frequency synchronization and phase control algorithms to establish and maintain a VAA for distributed beamforming and nullforming. For frequency-locking, we propose a novel distributed consensus-based algorithm. For a VAA with two nodes, we show that our algorithm achieves frequency lock globally and exponentially with a residual phase disparity that is either 0 or pi. This is in contrast to PLL-like algorithms that only achieve lock locally. Next, we describe in detail the key ideas behind an implementation of distributed beamforming on a GNU-radio/USRP based software-defined radio (SDR) platform. We introduce a novel DSP-centric Master-Slave (MS) architecture that enables the use of low-rate DSP algorithms for synchronization of high frequency RF signals. We describe the evolution of our implementation from initially using analog signaling with Costas loops/PLLs for frequency offset estimation and compensation, to a digital signaling scheme that uses extended Kalman filters (EKF) to track and compensate for frequency offsets. The EKF-based frequency locking scheme is well-suited for packet wireless networks, e.g., WiFi, ZigBee. We next consider phase control algorithms for forming beams and nulls with a VAA. In our experimental implementation, we have used several variants of classical 1-bit feedback control algorithm during different stages of our work. 1-bit feedback algorithm is an iterative gradient-ascent algorithm which causes the VAA nodes' signals to add constructively at a designated receiver. We present results to demonstrate the gains in the RSS at the receiver due to beamforming in the real-time settings. We also describe a distributed gradient-descent based algorithm that causes VAA nodes to achieve a null at a designated null target. We provide detailed convergence analysis for the proposed null-steering algorithm. This analysis shows that the algorithm always achieves practical null at null-target; moreover, all the spurious stationary points are locally unstable. Finally, we conclude by providing suggestions for future work.

A Distributed Multi-Input Multi-Output (DMIMO) system consists of many transceivers coordinating themselves into a "virtual antenna array" in order to emulate MIMO capabilities. In recent years, the field of research investigating DMIMO Communications has grown substantially. DMIMO systems offer all of the same benefits of standard MIMO systems on a larger scale because arrays are not limited by the physical constraint of placing many antennas on a single transceiver. This additional benefit does come at a cost, however. Since nodes are distributed and run from independent clock signals and with unknown geometry, each one must its own obtain channel state information (CSI) to the target nodes. In existing DMIMO architectures, array nodes depend on feedback from target nodes to properly synchronize. This means that target nodes must be cooperative and are responsible for the overhead calculating and transmitting CSI feedback to each node in the array. Within this work, we develop a set of techniques for Retrodirective Distributed Antenna Arrays. Retrodirective arrays have traditionally been used to direct a beam towards a target node, but the work in this thesis seeks to develop a more generalized definition of retrodirectivity. By our definition, a retrodirective array is one that acquires CSI to one or more intended targets simply by listening to the incoming transmissions of those targets; the array may subsequently use this information to do any number of typical MIMO tasks (i.e., beamforming, nullforming, spatial multiplexing, etc.). We explore two primary techniques: i) distributed beamforming and ii) distributed nullforming. Beamforming involves focusing transmitted power towards a specific target node and nullforming involves directing transmissions of array nodes to cancel one another at a specific target node. We focus on these techniques because they can be thought of as basic building blocks for more sophisticated DMIMO techniques. We first develop the theory for retrodirective arrays. Then, we present an architecture for the implementation of this theory. Specifically, we focus on the pre-synchronization of the array, which involves use of a master/slave architecture and a timeslotted message exchange among the array nodes. Finally, developing algorithms to make these arrays both robust and scalable is the focus of this thesis.