Target uniformity in 11n antenna choice, Cisco says

* Antennas tips for 802.11n

Some time ago, a reader pinged me with a question about antenna use in 802.11n networks. A Cisco AP 1250 Draft N customer, the reader had been advised to use a single type of antenna consistently across his network. He wanted to know why, if there are three antennas at each end of a given 11n connection, he shouldn’t mix up the antennas he deploys to optimize performance.

The reader’s thinking was that using different antenna types (omnidirectional, omni with high gain and directional, for example) would allow the multiple input, multiple output (MIMO) capabilities of an AP’s antennas to communicate using the antenna with the strongest reception during a given transmission. He also wondered about positioning the three antennas per AP a few feet apart. If there were an obstruction in the transmission path, he reasoned, one of the three antennas would likely have a “better” signal.

Generally, this line of thinking would better match the design concepts behind earlier iterations of 802.11, in which multipath interference was the monkey on WLAN administrators’ backs. Traditional a/b/g Wi-Fi networks use two antennas at each end of the communications for diversity; in other words, to help relieve multipath’s effects.

By contrast, multipath is one of the biggest contributors to 802.11n’s improved throughput. 802.11n is founded on the design premise of sending one signal out of two, three or four antennas at the same time in parallel. Simplistically, obstructions cause the signals to rebound and increase signal-to-noise ratio at the receiving end for better throughput or greater coverage.

Frederick Niehaus, a technical marketing engineer at Cisco, further explains that companies should shoot for performance uniformity across an RF cell. So in any given cell, it’s best to keep the antennas the same, he says, and, generally, operating at the same power output. He likened antenna behavior to light bulbs. If you are the “client,” you are more likely to see more consistent “coverage” (light) from wherever you are in a room of 10 100-watt bulbs than in a room with five 100-watt, five 50-watt and 10 25-watt bulbs.

In addition to better coverage predictability, Niehaus pointed out that mixed antennas “can wreck location” applications. This is because one antenna will detect more powerful signals than the next, and location measurements based in part on received signal strength are more likely to be off.

Niehaus added that signal strength differences can also be problematic for voice. Differing power levels tends to make clients want to hop from AP to AP – wherever the signal is strongest – which can cause connection instability and degrade voice performance.

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