Monday, 13 August 2012

450Mbps Wireless. Spatial Streams and 4x4 MIMO

So I've not posted for a while, which is bad and I will definitely start again soon. But in the mean time I've learnt something very interesting about 802.11n wireless, spatial streams and 450Mbps that I want to jot down.

802.11n radios give 300Mbps with 2x2 antennas, this is 2 transmit and 2 receive. They also utilise 2 spatial streams. The maths behind this is that each channel gives 75Mbps, 2 channels (2 x 20MHz channels = 40MHz) gives 150Mbps. 2 spatial thus gives 300Mbps. Note that each side needs the same setup to achieve these rates, this is critical.

MIMO allows multiple datastreams to be sent simultaneously however there are two sides to this coin. Each MIMO stream can be used to send the same data, thus you have multiple redundant copies of the data and the connection is very reliable, this is Diversity. The other extreme is throughput where each stream sends different data but there is no redundancy in the path so it is potentially faster but also less reliable. Most commonly a balance in the middle is used to give you reliable throughput.

Thus APs can theoretically achieve 450Mbps by using 3x3 radios with 3 spatial streams, the problem is this is a extreme throughput situation and you will likely not get the fastest possible speeds due to distances and errors in the transmission. If one of the streams experiences deep fading (low signal) the transmission fails or slows down to a lower transmission rate.

Cisco has a solution to the above in the 3600 AP. It has 4x4 radios and 3 spatial streams meaning that three can send and receive while the remaining is used for diversity to help achieve the reliable throughput needed. It's worth noting that this is custom silicon as well and only available to Cisco, at the time of writing anyway.

There's a fantastic video from techwize tv called 'fundamentals of spatial streams'. Currently on this page:

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