ABSTRACT
Multiple-input multiple-output (MIMO) systems have been used for many years in the field of wireless communications [1,2]. The huge increases in coverage and capacity offered by MIMO technology are the key benefits of using multiple sensors at the transmitter (Tx) and receiver (Rx). For a single-user transmission, and under the assumption that the channel information is perfectly known at both the Tx and Rx, it has been demonstrated that the capacity increases linearly with the number of antennas. However, in more realistic wireless scenarios, the capacity of a MIMO system depends on a number of practical considerations including channel estimation in a time-varying environment, spatial correlation induced by the sensors and the value of the signal-to-noise ratio (SNR) available at the Rx [3]. Recently, MIMO technology has been applied in the context of power line communications (PLCs), with the aim of offering higher channel capacity and therefore larger system coverage, by including the use of the protective earth (PE) wire in addition to the line (L) and neutral (N) wires [4–8] (see Chapter 1). This new application for MIMO technology offers different characteristics as compared to wireless communications, which can in turn effect the capacity gain achieved. On the one hand, the number of input and output ports of a PLC channel is much more constrained than for a radio channel. Due to Kirchhoff’s law, only two differential input ports can be used simultaneously, in the three possible combinations (L-N, N-PE and PE-L). At the Rx, three different signals can be monitored, either on a wire-to-wire basis or using differential reception between two wires. In addition, the 232 common-mode (CM) signal generated by asymmetries in the transmission medium can be measured at the Rx, which provides a fourth output of the MIMO system. As a result, a MIMO PLC transmission up to a 2 × 4 configuration is implementable. More details about coupling for MIMO signal injection and reception are given in Chapter 1. SNR values observed in typical PLC scenarios can be much higher than in the case of a classical wireless communication. This high SNR condition is beneficial for MIMO transmission, as it ensures a high capacity gain with respect to single-input single-output (SISO) transmission, even if the channel presents a high degree of spatial correlation.
