How to mitigate 802.11n interference with PC peripherals

While working group 11 of the IEEE LAN/WAN Standards Committee still is working towards ratification of the standard with leading solution suppliers, 802.11n enabled products have been gaining popularity among consumers due to its improved performances comparing with widely adopted 802.11b/g based applications.

Many consumer electronics OEMs have started releasing 802.11n enabled products. Apple's Apple TV, for example, is based on an IEEE 802.11n draft that allows the user to unleash its capability through software configuration.

In January 2007, IEEE approved 802.11n Draft 2.0, and the Wi-Fi Alliance started certifying products using a pre-standard version in June 2007. It is foreseeable that 802.11n technology may eventually gain more market shares and replace 802.11b/g with improved bandwidth and range.

What is not as well known is the potential problems that 802.11n can cause both for 802.11a/b/g networks and wireless PC peripheral devices that operate in the same frequency spectrum.

Today's 802.11 Pre-N consumer electronics products typically operate in the 2.4 GHz frequency band for backward compatibility reasons. 802.11n has a theoretical maximum data rate of 540 Mbit per second (200 Mbit/s typical), and a range of up to 50 meters.

An 802.11n signal takes a much wider bandwidth (40 MHz) to achieve the higher data throughput, and it also requires higher output power to achieve the longer transmitting range.

The approach used to achieve the higher data rate is to employ multiple antennas at the transmitting and receiving applications. This is referred to as multiple-input-multiple-output (MIMO) system.

As such, a MIMO system introduces a bigger impact on interference to other consumer electronics devices. The impact comes from two challenges. The first challenge is increased bandwidth causing increased sideband effects that reduce the Signal to Noise Ratio (SNR).

Second, the wider bandwidth also means a reduced number of "clean" channels for other devices to operate in the same 2.4 GHz frequency band, which is known as a co-location issue.

These interference issues are commonly found in existing products designed with 2.4 GHz based wireless technology standards, such as Bluetooth (802.15.1) and Zigbee (802.15.4).

In addition, these wireless technology standards are not ideal for wireless human interface applications both from cost and battery life perspectives. PC peripheral OEMs who recognize these issues are adopting proprietary 2.4 GHz wireless solutions.

These vendors design their products to address specific application concerns of lower BOM costs, longer transmitting range and battery life. The main differentiating factor between these products that ultimately determines the system reliability and quality is their RF interference immunity performance.

It is a challenge to product managers to select a proprietary 2.4 GHz solution for their PC peripheral applications that can co-exist with other 2.4 GHz based electronics devices and still perform well.

In addition to RF interference signals from Bluetooth based devices, cordless phones, microwave ovens, and existing 802.11b/g Wi-Fi networks, the upcoming 802.11n technology will pose a "killer" interference threat to proprietary 2.4 GHz wireless PC peripheral applications. A good 2.4 GHz radio product will need to address both issues in order to provide a reliable RF link for the PC peripheral systems.

In order to overcome the first challenge, the 2.4 GHz PC Peripheral device may simply stay farther away from the interference source. However, sometimes this is not an option because the wireless receiving dongle is connected to the USB host, which is likely to be in close proximity to the 802.11n transmitter of a desktop or notebook computer.

Reducing the power output of the 802.11n network is an alternative solution, but the data throughput will be reduced, which defeats the purpose of using 802.11n technology.

Therefore, the PC peripheral device needs to employ a radio that can transmit at a higher power output greater than 0 dBm and receive at a higher sensitivity level. These capabilities should be incorporated in the radio hardware. In addition, the radio also needs to be able to retry transmission automatically and quickly, if the first attempt of the transmission fails.