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Tuesday, May 10, 2011

Multi-antenna Systems (MIMO): How Spatial Multiplexing Works:

MIMO systems are systems with Multiple Element Antennas (MEAs) at both link ends. The MEAs of a MIMO system can be used for four different purposes: (i) beamforming, (ii) diversity, (iii) interference suppression, and (iv) spatial multiplexing (transmission of several data streams in parallel). The first three concepts are the same as for smart antennas. Having multiple antennas at both link ends leads to some interesting new technical possibilities, but does not change the fundamental effects of this approach. Spatial multiplexing, on the other hand, is a new concept, and has thus drawn the greatest attention. It allows direct improvement of capacity by simultaneous transmission of multiple data streams. In this article I provide an insight into how spatial multiplexing works.

Spatial multiplexing uses MEAs at the TX for transmission of parallel data streams (seen in the illustration below). An original high-rate data stream is multiplexed into several parallel streams, each of which is sent from one transmit antenna element. The channel “mixes up” these data streams, so that each of the receive antenna elements sees a combination of them. If the channel is well behaved, the received signals represent linearly independent combinations. In this case, appropriate signal processing at the RX can separate the data streams. A basic condition is that the number of receive antenna elements is at least as large as the number of transmit data streams. It is clear that this approach allows the data rate to be drastically increased – namely, by a factor of min(Nt,Nr).

For the case when the TX knows the channel, we can also develop another intuition (seen in the illustration below). With Nt transmit antennas, we can form Nt different beams. We point all these beams at different Interacting Objects (IOs), and transmit different data streams over them. At the RX, we can use Nr antenna elements to form Nr beams, and also point them at different IOs. If all the beams can be kept orthogonal to each other, there is no interference between the data streams; in other words, we have established parallel channels. The IOs (in combination with the beams pointing in their direction) play the same role as wires in the transmission of multiple data streams on multiple wires.

From this description, we can also immediately derive some important principles: the number of possible data streams is limited by min(Nt,Nr,Ns), where Ns is the number of (significant) IOs. We have already seen above that the number of data streams cannot be larger than the number of transmit antenna elements, and that we need a sufficient number of receive antenna elements (at least as many as data streams) to form the receive beams and, thus, be able to separate the data streams. But it is also very important to notice that the number of IOs poses an upper limit: if two data streams are transmitted to the same IO, then the RX has no possibility of sorting them out by forming different beams.

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