This chapter introduces the concept of Multiple Input Multiple Output (MIMO) wireless communication system and the necessity to use a testbed to evaluate its performance. A comprehensive review of different types of testbeds available in the literature is presented. Next, the design and development of a 2×2 MIMO testbed which uses in-house built antennas, commercially available RF chips for an RF front end and a Field Programmable Gate Array (FPGA) for based signal processing is described. The operation of the developed testbed is verified using a Channel Emulator. The testing is done for the case of a simple Alamouti QPSK based encoding and decoding scheme of baseband signals.
Evolution Of Smart Antennas And Mimo
In the past two decades, wireless communication systems have grown with an unprecedented speed from radio paging, cordless telephone, and cellular telephony to multimedia platforms offering voice and video streaming, interactive services, and even a global positioning information of the user. One undesired outcome of this expansion is a heavy utilization of the available frequency spectrum. A particular pressure comes from new multimedia applications, which require larger operational bandwidth for their implementations. Conventional coding and modulation techniques that are based on frequency, time or code division have difficulty to provide a suitable solution to this problem. For example, allocating new users or new applications leads to consuming of an additional frequency band in frequency division multiplexing systems. In time division and code division multiplexing systems this results in an increased level of a man-made noise, which in turn adversely affects the signal to noise (SNR) ratio and thus the quality of the received signal.
As the conventional coding and modulation techniques are unable to overcome the problem associated with the limited frequency spectrum, the designers turn to the space/angle domain to improve capacity and reliability of wireless systems. This approach is already in use in many currently available terrestrial and satellite communication systems. The best known example is the cellular telephony. By creating physically separated cells, the same frequency spectrum is re-used only at an expense of small interference between adjacent cells. A further extension of the cellular concept is via the introduction of sectors within cells. This task is realized by antennas at base stations having sectoral radiation patterns instead of omnidirectional ones. More advanced versions of this concept are antennas serving a variable width sector. This is required, to overcome a bottleneck caused by an excessive number of users in a particular time of the day in this sector (Rosol, 1995). Such an intelligent antenna system is an example of the simplest type of smart antenna (Liberti and Rappaport, 1999). A more advanced form, known as an adaptive antenna, can follow individual mobile users by forming narrow beams towards them. At the same time, they can produce nulls or low side lobes towards undesired users. In all of these cases, multiple element antennas (MEAs) are instrumental to divide space into angular slots and adapt their width in time so that the wireless system can serve an increased number of users. It is apparent that such intelligent systems are capable to fight co-channel interference and thus, they are able to improve the quality of a communication link.
All of these benefits are possible if an MEA systems are capable to distinguish between signals coming from desired and undesired directions. Such a task is relatively easy to accomplish under Line of Sight (LOS) signal propagation conditions. As a result, adaptive antennas rely on LOS conditions for their proper operation. In order to achieve this condition in practice, base station antennas are located on elevated platforms so their operation is unaffected by the presence of surrounding objects. Such conditions are usually difficult to fulfill for mobile units. This is because they operate at low heights and therefore their operation is affected by the presence of various scattering, refracting or reflecting objects that surround them.
Meeting LOS conditions becomes more challenging for indoor scenarios, as the signal’s propagation is affected by walls, indoor furniture, equipment and humans. Under such conditions, signals of similar strength arrive from many directions on antennas. As a result, there is not much benefit from forming narrow beams. This is because there are too many directions at which these beams need to be formed.