Enabling massive distributed MIMO for small cell networks

According to the Cisco Visual Networking Index, the demand for wireless data traffic is expected to increase at an exponential rate for the foreseeable future. This demand is largely driven by the success of smartphones and their de facto killer‐app, mobile video. However, current generation mobile networks are ill‐equipped to meet this demand in very dense user environments such as airports, campuses, conference halls, and stadiums. This problem is fundamentally limited by the quantity of available wireless spectrum, and the efficiency with which this spectrum is used. A novel network architecture known as large‐scale distributed Multiuser MIMO has the potential to solve this impending spectrum crunch. ❧ Large‐scale distributed MU-MIMO unifies two recent trends in wireless research: "massive MIMO" and "small cells." It consists of several Access Points (APs) connected to a central server via a wired backhaul network and which act as a large distributed antenna system. This dissertation presents scalable solutions to the two primary implementation challenges of Distributed MIMO: AP synchronization and uplink/downlink reciprocity calibration. AP synchronization refers to the act of forcing each AP's inexpensive crystal oscillator to operate on the same frequency, as well as the act of forcing each AP to transmit at the same time. Reciprocity calibration refers to the ability to infer downlink (AP to user) channel conditions based on uplink (user to AP) transmissions. The focus of this work is on the downlink, which is both more demanding in terms of traffic and more challenging in terms of implementation than the uplink. All of the proposed synchronization and calibration protocols utilize over‐the‐air signaling, which allows for reduced hardware requirements and for algorithms which scale well with the size of the network. The proposed schemes can be applied to networks formed by a large number of APs, each of which is driven by an inexpensive 802.11-grade clock and has a standard RF front‐end, not explicitly designed to be reciprocal. ❧ In addition to novel synchronization and calibration algorithms, this work presents a number of system‐level optimizations which are needed in practical networks. Experimental and simulation‐based results indicate that a realistic distributed MIMO system is capable of delivering the data rates required by next generation networks.