A Distributed Multilayer Software Architecture for MIMO Testbeds

Abstract

The use of multiple antennas at both transmission and reception, also known as Multiple Input Multiple Output (MIMO) transmission systems, has received a lot of interest from the wireless communications industry during the last years. Communications in wireless channels using MIMO technologies exhibits a superior performance in terms of spectral efficiency, reliability and data rate when compared to conventional single antenna technologies (Foschini & Gans, 1998; Telatar, 1999). Existing and emerging standards for wireless communications such as IEEE 802.11 (WiFI), IEEE 802.16 (WiMAX) and Long Term Evolution (LTE), support multi-antenna transmission in their highest performance profiles. In spite of their potential performance-enhancing capabilities, most of the research on MIMO technologies up to the moment is based on theoretical studies. Typically, the expected gains of MIMO technologies are only shown under ideal conditions since most analysis rely on simulations. Experiments in real-world scenarios by means of hardware implementations are necessary to measure the actual performance of multi-antenna transmission methods. Hardware implementations not only take into account the real multipath propagation in wireless channels but also the implementation impairments so often ignored during the simulations. Hardware implementations can be split into three groups (Rupp et al., 2006). The first one is constituted by demonstrators which are frequently designed having in mind a particular standard or specification. Demonstrators usually exhibit good technical features for real-time implementations but they are extremely expensive and present poor flexibility and modularity. The second group is formed by prototypes of a final product. A prototype is a real-time implementation of a system specifically developed to support an industrial need. Prototypes often constitute a preliminary stage where the system is implemented and debugged and later on implemented as a consumer product. Finally, the third set is formed by testbeds that support real-time transmission capabilities while data is generated and post-processed off-line. In addition, hybrid solutions can be devised. As an example, a testbed can carry out some operations in real-time with the purpose of speeding up the measurement process. Usually, candidate signal processing operations to be implemented in real-time are those that operate at sample level and/or do common tasks for all experiments, i.e. I/Q modulation, up-sampling, pulse-shaped filtering, etc. Throughout this chapter we will focus on testbeds because they use open designs and are more often found in public research centres and academia. Various MIMO testbeds have been reported in the literature (Borkowski et al., 2006; Caban et al., 2006; Fabregas et al., 2006; Haustein et al., 2006; Nieto et al., 2006; Ramirez et al., 2008; 5