MIMO: Wireless Communications

Multiple input multiple output (MIMO) systems in wireless communications refer to any wireless communication system where at both sides of the communication path more than one antenna is used. Systems utilizing multiple transmit and multiple receive antennas are commonly known as multiple input multiple output (MIMO) systems. This wireless networking technology greatly improves both the range and the capacity of a wireless communication system. MIMO systems pose new challenges for digital signal processing given that the processing algorithms are becoming more complex with multiple antennas at both ends of the communication channel. Overviews of MIMO systems can be, e.g., found in Refs. 1–3. MIMO systems constructively explore multi-path propagation using different transmission paths to the receiver. These paths can be exploited to provide redundancy of transmitted data, thus improving the reliability of transmission (diversity gain) or increasing the number of simultaneously transmitted data streams and increasing the data rate of the system (multiplexing gain). The multiple spatial signatures can also be used for combating interference in the system (interferences reduction). A general model of a MIMO system is shown in Fig. 1. A seminal information theory paper by Foschini and Gans of Lucent Technologies shows that the capacity of these systems can increase linearly with the number of transmit antennas as long as the number of receive antennas is greater than or equal to the number of transmit antennas. As an increase in capacity means capability of faster communication, this unmatched capacity improvement over regular one-antenna systems has fueled a huge interest in MIMO techniques, thus leading to the development of many forms of MIMO systems. Traditional wireless communication systems with one transmit and one receive antenna are denoted as single input single output (SISO) systems, whereas systems with one transmit and multiple receive antennas are denoted as single input multiple output (SIMO) systems, and systems with multiple transmit and one receive antenna are called multiple input single output (MISO) systems. Conventional smart antenna systems have only a transmit side or only a receive side equipped with multiple antennas, so they fall into one of last two categories. Usually, the base station has the antenna array, as there is enough space and since it is cheaper to install multiple antennas at base stations than to install them in every mobile station. Strictly speaking, only systems with multiple antennas at both ends can be classified as MIMO systems. Although it may sometimes be noted that SIMO and MISO systems are referred as MIMO systems. In the terminology of smart antennas, SIMO and MISO systems are also called antenna arrays. CAPACITY OF MIMO SYSTEMS From the mathematical point of view, the MIMO communication is performed through a matrix and not just a vector channel, so it is possible to transmit multiple parallel signal streams simultaneously in the same frequency band and thus increase spectral efficiency. This technique is called spatial multiplexing and is shown in Fig. 2. The data stream is encoded with vector encoder and Encyclopedia of Wireless and Mobile Communications DOI: 10.1081/E-EWMC-120043484 Copyright a 2008 by Taylor & Francis. All rights reserved. 604 M eia– M oile C om m transmitted concurrently by M transmitters. The MIMO radio channel introduces distortion to the signal. The receiver has N antennas. Each antenna receives the signals from all M transmit antennas, and consequently the received signals exhibit inter-channel interference. The received signals are down converted to the base band and sampled once per symbol interval. The MIMO processing unit estimates the transmitted data streams from the sampled base-band signals. The vector decoder is a parallel-toserial converter, which combines the parallel input data streams to one output data stream. In a system with M transmit and N receive antennas, there exist M*N sub-channels between transmitter and receiver. In general, each sub-channel exhibits a selective fading and consequently it is modeled as a linear discrete time finite impulse response (FIR) filter with complex coefficients. In the case of flat fading, the signal in each sub-channel is only attenuated and phase shifted due to different propagation times between each receive and transmit antenna. The sub-channel is reduced to one tap FIR filter, i.e., one complex coefficient. When the channel is constant during the whole time slot, the channel is quasistatic. For capacity investigation, we have assumed that the radio channel is quasi-static and fading flat. In the aforementioned case, the received signal on the j-th receive antenna can be expressed as: