What’s up with MIMO? – Part Deux
In my first episode on MIMO (What’s up with MIMO), I introduced the concept of multiple antennas and the home audio analogy. In this example, I discussed one particular mode of MIMO that transmits the same stream across all antennas, and makes use of multipath to increase the chance of correctly decoding the received signal – thus lowering the bit error rate. This method exploits what is known as spatial diversity, which is one of the simplest method to achieve MIMO gains. But MIMO is much more than that.There is also another mode of MIMO that is ideal in environments that experience few errors, which is called spatial multiplexing. In this mode, the transmitter would treat each antenna as a “separate channel” – much like you would experience in a 5 channel home audio system. These systems send the audio stream that provides directionality to the whole experience. Of course, in MIMO systems, we don’t care much about directionality, but we certainly care about sending multiple streams to increase the overall effective bandwidth. So for instance, if an AP wanted to send a packet, it could split the packet up into three “chunks”, and transmit each “chunk” across each of its three antennas simultaneously. This would effectively reduce the transmit time – and therefore increase the overall capacity of the network.