By Chris Yip
August 2, 2010

Many small to mid-sized businesses have come to distrust Wi-Fi. Spotty coverage, erratic performance and dropped client connections are driving this distrust – and RF interference is the culprit.

A first come, first served technology, 802.11 is a shared medium that uses unlicensed radio frequencies (RF) within the 2.4GHz and 5GHz spectrum. Any noise, obstacles or changes to the environment can cause severe problems to users.

RF interference is generated by a wide range of sources from cordless phones to Bluetooth headsets, microwave ovens to even treadmills. Any device that emits electro-magnetic signals could potentially inhibit the transmission of a Wi-Fi signal.

But what most companies don’t realize is that the single biggest source of Wi-Fi interference is typically their own Wi-Fi network, due to co-channel interference.

While spectrum analysis tools are being integrated into APs to help IT staff visualize and identify Wi-Fi interference, they are rendered useless if nothing can be done to fix the problem.

At the heart of this conundrum is how Wi-Fi APs cope (or don’t cope) with RF interference. Conventional Wi-Fi makes use of omni-directional, dipole antennas. These rubber duck-looking antennas send and receive transmissions equally in all directions. When interference crops up, these systems lower their physical data rate, until errors subside. In other words, they can’t help but listen to everything.

If an AP is sending data to a client in one direction and interference is experienced in another, the AP “hears” this and lowers its data rate until an acceptable level of packet loss is achieved. This is highly inefficient, and all users sharing this AP suffer.

PLEASE DON’T CHANGE THE CHANNEL

A popular approach to dealing with Wi-Fi interference is “channel changing.” This is where a different or “cleaner” channel is automatically selected for the AP when RF noise increases. Within the 2.4GHz frequency however, the most widely-used Wi-Fi band, there are only three non-interfering channels. Even within the 5GHz band, only 11 non-overlapping 40MHz wide channels exist.

Channel changing by an AP requires connected clients to disassociate and re-associate. This causes disruption to voice and video applications and often creates a domino effect as neighboring APs often change their channels to avoid co-channel interference.

These approaches also don’t take into consideration what’s best for the client. Interference is seen from the vantage point of the AP. But what does the client see? Will moving to a cleaner channel really benefit the user’s experience?

Finally, wide-spread channel adjustments throughout a wireless LAN necessitate continuous site surveys and tuning of the network.

MITIGATING INTERFERENCE THROUGH DYNAMIC BEAMFORMING

The holy grail would be to shoot a Wi-Fi signal directly to a user, monitoring that signal to ensure it is delivering the best possible throughput – all while constantly redirecting Wi-Fi transmissions over signal paths that are known to be clean without changing channels.

Recent advances in Wi-Fi technology that marry so-called dynamic beamforming and intelligent antenna arrays come closest to this wireless nirvana.

Dynamic, antenna-based beamforming is a new technique developed to alter the form and direction of RF energy. It focuses Wi-Fi signals only where they are needed while automatically “steering” them around interference as it occurs.

Antenna-based beamforming uses a number of directional antenna elements to create thousands of antenna patterns, or paths, between the AP and client. RF energy is now radiated over the optimal path that yields the highest data rate and lowest packet loss.

Standard Wi-Fi protocol client acknowledgements are used (and monitored) to determine the signal strength, throughput and packet error rate of a select path. This ensures that the Wi-Fi AP knows exactly what the client is experiencing – giving the AP complete control to alter the course of any Wi-Fi packet over a better path if interference starts causing problems.

Another demonstrable benefit of this new technology is mitigating interference. Because Wi-Fi only allows one user to talk at a time, antennas not being used to transmit data to a given client can ignore or reject interference that would normally inhibit Wi-Fi transmissions. This results in significant signal gain, up to -17dB in some cases, from being able to reject interference.

New technologies and techniques to deal with RF interference hold the promise of transforming Wi-Fi from a technology of convenience into the ubiquitous utility that everyone wants.

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