Design and Evaluation of OFDM Radio Interfaces for High Mobility Communications

In the last two decades, multicarrier modulations have emerged as a low complexity solution to combat the effects of the multipath in wireless communications. Among them, Orthogonal Frequency Division Multiplexing (OFDM) is possibly the most studied modulation scheme, and has also been widely adopted as the foundation of industry standards such as WiMAX or LTE. However, OFDM is sensitive to time-selective channels, which are featured in mobility scenarios, due to the appearance of Inter-Carrier Interference (ICI). Implementation of hardware equipment for the end user is usually implemented in dedicated chips, but in research environments, more flexible solutions are preferred. One popular approach is the so-called Software Defined Radio (SDR where the signal processing algorithms are implemented in reconfigurable hardware such as Digital Signal Processors (DSPs) and Field Programmable Gate Arrays (FPGAs). The aim of this work is two-fold. On the one hand, to define an architecture for the implementation of real-time OFDM-based physical layers, using as a reference the WiMAX standard, and it is tested on a platform composed by DSPs and FPGAs. On the other hand, to study the effects of the time selectivity on the OFDM signal, defining channel estimation methods aware of the ICI, and its evaluation both in simulation as well as experimental measurements. Two approaches have been followed to assess the behavior of OFDM waveforms under mobility conditions, one based on a real-time channel emulator, and the other on inducing large Doppler spreads in the signal by extending the duration of the OFDM symbols.