High-wireless act: Can high-frequency WiFi be practical?
- By Greg Crowe
- Oct 16, 2012
This article has been updated to correct an inaccurate reference to the 60 GHz band as 60 MHz.
As agencies continue to build out wireless networks and extend the use of mobile devices, a new spectrum specification, which will add a lot of speed and capacity, could become a key factor in how well those networks function.
In a recent article explaining the wireless spectrum allocation situation, we touched on the 60 GHz band that the Wireless Gigabit Alliance (WiGig) proposes to use for the next generation of wireless networking, IEEE 802.11ac.
But although the bandwidths are quite roomy up there in the 60 GHz band — more than 50 times as wide as in the current Institute of Electrical and Electronics Engineers (IEEE) 802.11n specification, enough to allow streaming of uncompressed video — there is an innate hurdle to working with higher frequencies: signal propagation loss over distance.
Since the air is made up of randomly-arranged molecules of matter, any waveform signal sent through them has a chance of being bounced off of whatever it runs into. Higher-frequency waves are more susceptible to signal loss than lower-frequency ones because of this.
A good example exists in everyday nature. Light on the blue end of the visible spectrum has a higher frequency than on the red end. When the sun’s light hits the atmosphere, the blue light scatters more, so the sky looks blue. When the sun is low on the horizon, its light has to go through even more atmosphere to reach you, making the sun look more reddish. So now if your kids ask why the sky is blue, you’ll know what to tell them.
WiGig proposes several ways to combat this innate problem. For one, the alliance is continuing to refine the multiple-input multiple-output (MIMO) antenna configuration that was first implemented in 802.11n. With MIMO, several antennae are all talking to each other to determine the best path to the other station. The 802.11ac standard doubled the MIMO streams used in ‘n’ from 4 to 8, so what WiGig is working on will likely have at least that many.
But the area that will likely have the greatest impact on the practical range of WiGig-designed devices is in the precoding stage of the MIMO process. This is when the device will use what is called “beamforming” to focus the signal. This process, an example of which is illustrated in the accompanying graphic by Stephane Dedieu, uses the multiple antennae to combine into a phased array. The signal produced will experience constructive interference in one direction and destructive in other directions.
So the signal will go farther in the desired direction. This type of transmission is also sometimes called “unidirectional,” meaning that a signal with beamforming will go much farther than it would without it. How far will depend upon the technological improvement that happen between now and when WiGig and the IEEE come out with the new specification based on WiGig’s current research.