New multiplexing technologies are also emerging. MIMO is an SDM technology that increases transmission capacity by using spatially separated transmission antennas together with spatially separated receiving antennas. Currently used in 802.11n wireless technology, MIMO works by splitting a source signal into several lower-rate components and transmitting those components in the same frequency channel from spatially separated antennas, as shown in Figure 1. The transmitters radiate into the broadcast area on the same frequency, and multiple antennas are used to receive the transmitted signals, which arrive from different directions. (Note that the input/output terminology refers to the “air” medium, and not the source or destination equipment.)
MIMO technology is based on an earlier concept called Distributed-Transmit-Directional-Receive (DTDR). In some respects, the Single-frequency Network (SFN) and distributed transmission modes of digital terrestrial television share similarities with MIMO, the key difference being that multiple receive antennas are not used with digital transmission systems such as DVB and ATSC.
MIMO is not diversity reception, which uses multiple receiving antennas to combat multipath, but only one transmit antenna. Nor is it a directional array, which uses multiple transmitting antennas to generate a directional transmission pattern. And is not a combination of both.
One intriguing aspect of MIMO technology is the ability to operate in a closed-loop fashion, i.e., with feedback. When one-to-one communications are used (such as for unicast and peer-to-peer links) and a return channel is available, receivers can send, back to the source, channel information regarding scattering, fading and power decay with distance. This information can then be used by an intelligent transmitter to change coding parameters that can maximize the use of the channel, and minimize the received error rate.
Combined with large-area self-configuring networks, emerging multiplexing technologies are hoped to offer significant gains in bandwidth efficiency — a national resource that is in ever-increasing demand.
—Aldo Cugnini is a consultant in the digital television industry.