nditions. Coarse control can be achieved by dropping whole bands (or in extreme circumstances whole band groups), but extremely precise shaping is also possible by "nulling out" a certain number of tones within a single band. MIMO Radio As described in the Section "Multipath Fading, p. 114", the multiple paths that a radio signal takes between transmitter and receiver often lead to a degradation of signal strength through multi-path fading. Multi-input multi-output (MIMO) radio takes advantage of this characteristic of RF propagation by sending multiple data streams across multiple transmitters to receiver paths in order to achieve a higher data capacity (Figure 4-37). Mathematical modelling of the propagation paths, using a channel calibration period during each transmitted data packet, allows the different signal paths and data streams to be identified and correctly recombined in the receiver. This technique, space division multiplexing (SDM), is analogous to FDM in the frequency domain but instead of different frequencies carrying data in parallel, here different spatial paths carry data in parallel. Effectively the same bandwidth is being used simultaneously to create multiple communication paths. If these paths are equally strong and can Radio Communication Basics Single input, Single input, single output multiple output Multiple input, Multiple input, single output multiple output Figure 4-37: MIMO Radio Definition be perfectly separated, the overall capacity of the communication channel increases linearly with the number of independent paths used. In a system with M transmitters and N receivers, the number of independent paths is the minimum of M and N. In practice, all paths will not be equally strong or perfectly separated, and performance will be determined by coefficients, known as singular values, which characterise each path between a transmitting and receiving antenna. These singular values are determined by including a short "training period" in the preamble of each transmitted data packet, during 