Resumen de Digital design and experimental validation of high-performance real-time OFDM systems
Josep Oriol Font Bach
The goal of this Ph.D. dissertation is to address a number of challenges encountered in the digital baseband design of modern and future wireless communication systems. The fast and continuous evolution of wireless communications has been driven by the ambitious goal of providing ubiquitous services that could guarantee high throughput, reliability of the communication link and satisfy the increasing demand for efficient re-utilization of the heavily populated wireless spectrum. To cope with these ever-growing performance requirements, researchers around the world have introduced sophisticated broadband physical (PHY)-layer communication schemes able to accommodate higher bandwidth, which indicatively include multiple antennas at the transmitter and receiver and are capable of delivering improved spectral efficiency by applying interference management policies. The merging of Multiple Input Multiple Output (MIMO) schemes with the Orthogonal Frequency Division Multiplexing (OFDM) offers a flexible signal processing substrate to implement the PHY-layer of various modern wireless communication systems. This is mainly due to the fact that this technology combination is able to provide increased channel capacity and robustness against multipath fading channels. Additionally, Orthogonal Frequency Division Multiple Access (OFDMA) is augmenting the capacities of the MIMO-OFDMtechnology to serve various mobile subscribers at the same time. A prominent scheme proposed to capitalize the benefits of diversity is the closed-loop MIMO communications, where the receiver is providing information to the transmitter related to the current channel conditions by means of a dedicated feedback channel. In the transmitter, the Channel State Information (CSI) is exploited to adapt at run-time the transmission and, thus, take advantage of the capacities provided by MIMO-OFDM(A). The increased performance and flexible PHY-layer features of communication systems featuring MIMO-OFDM come at a cost of an increased computational load at baseband. Thus, innovating algorithmic, design and implementation solutions are required to provide the required PHY-layer schemes. Indeed, many levels of innovation are required to pass from a high-level model-based description of the system and its embedded algorithms to their digital realization. In fact, innovating digital design techniques aiming at maximizing the parallelization and resource re-utilization of the baseband Digital Signal Processing (DSP) algorithms have to be employed towards this end, in order to efficiently realize advanced PHY-layer schemes based on the MIMO-OFDM technology. The Field-Programmable Gate Array (FPGA) custom-processing devices were selected for realizing the proof-of-concept developments of the thesis. When facing the real-time implementation of custom bit-intensive DSP architectures FPGAs satisfy a trade-off between performance and flexibility, whilst providing the proof-of-concept environment, where innovating Register Transfer Level (RTL)-design techniques can be realistically validated. Moreover, a custom Hardware Description Language (HDL) coding approach was adopted to optimally define the processing demanding RTL designs. An important aspect of the presented PHY-layer prototyping is the utilization of real-life operating conditions, hardware specifications, constraints and mobile channel propagation conditions. This was made feasible by using the GEDOMISR testbed. GEDOMIS is a high capacity signal generation and signal processing platform that enables the end-to-end real-time prototyping of multi-antenna wireless communication systems. In order to support the complex development cycle it was introduced an iterative design, implementation and verification methodology, covering all the required steps from the definition of the system requirements and high-level modelling to the comprehensive evaluation of the resulting prototype based on a realistic hardware-setup. The core contribution of this thesis is the simplification and optimization of a number of DSP operations that form part of certain baseband building blocks, which are encountered in modern OFDM-based communication systems. This not only allowed to meet the stringent real-time performance requirements, but also enabled the intelligent re-utilization, resource sharing or parallelization of the processing and memory resources available at the target FPGA devices. Two representative use cases detail this core contribution and underlined the suitability of the proposed development flow. The first use case is a 2x2 MIMO closed-loop PHY-layer scheme, based on the mobile Worldwide Interoperability for Microwave Access (WiMAX) wireless communication standard and featuring a Transmit Antenna Selection (TAS) mechanism. The second use case implements a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE)-based macrocell/femtocell interference-management scheme.
