Thursday, December 29, 2011

The Next Decade of Bandwidth - Part IV

It won’t be long until wireless broadband has commandeered all the spectrum it can between a few hundred and a few thousand MHz. These are seen by the industry as being the most desirable frequencies. They offer high bandwidth capacity, good penetration into buildings and small antennas. Unfortunately, there are only so many of these channels available. Just like in real estate, when all the best locations are bought up expansion continues into whatever is available.

The future of computing is in the cloud...Going up or down the electromagnetic spectrum in search of more broadband capacity has its challenges. As you go down in frequency, up in wavelength, channels have to be proportionally larger to get the same carrying capacity. A 1 MHz channel at 10 GHz seems small. A 1 MHz channel at 10 MHz eats up a huge chunk of that band. At 1 MHz, there would only be room for a single 1 MHz channel.

One way to deal with this is to divvy up your transmissions into hundreds or thousands of smaller channels. This is the principle behind OFDM or Orthogonal Frequency Division Multiplexing. The system generates an array of carriers spaced out across the available spectrum. Each carrier transmits only a portion of the data. When those pieces are combined at the far end, you have a single continuous transmission again.

OFDM is the system used for BPL or Broadband over Power Line Internet services. It uses the shortwave frequencies, with some carriers omitted to avoid interference with other services. By using many, many small channels instead of one big one, any packet losses are small and easily replaced. It also offers a way to fit a wireless service into a band that doesn’t have big open chunks of spectrum. Just use the channels you can and combine a bunch of them to create a larger bandwidth service.

BPL may not be going anywhere because other wireline and wireless services are more cost effective competitors. WiMAX uses OFDM, as do LTE, WiFi and digital radio broadcasting. Seems like there is no reason a wireless equivalent of BPL couldn’t be deployed to use available lower frequencies that haven’t been previously considered for broadband use.

Going up the spectrum in frequency is also challenging. The higher you go, the more line of sight transmission becomes. The Ku band that broadcast satellites use from 12 to 18 GHz has lots of capacity, but not much ability to penetrate obstacles. Even tree leaves will interrupt service, as will a heavy rain. Wireless point to point transmissions in the Super and Extremely High Frequency bands is via outdoor antennas within line of sight.

That’s the state of the art today, but does it need to be so limited? Perhaps someone will come up with a way to flood areas with many very low power cells to ensure that a mobile antenna can receive a signal no matter where it travels. That same idea is being put to use now with mesh networks consisting of WiFi radios. Each WiFi hotspot communicates with the radios nearby to share traffic, so you don’t need backhaul connections to a central controller.

This idea of mesh networks with low power transmissions and short range coverage has application for even higher parts of the spectrum. Above 300 GHz, electromagnetic waves start to be called infrared light waves. Infrared light is the carrier used in fiber optic cables. You can eliminate the fiber and use infrared beams from point to point, as long as you have a direct line of sight. This is called free space optical transmission. With enough emitters and receivers connected in a mesh network, you can create a very high capacity network with decent coverage.

In Part I, Part II, Part III, and Part IV, we’ve seen how wireless broadband has a near insatiable need for bandwidth and how that may be satisfied by reassignment of desirable channels and more efficient use of underutilized frequencies up and down the electromagnetic spectrum. In the fifth and final part of this series on the next decade of bandwidth, we’ll take a look at wired connections that include both copper and fiber to see what else is in store for business bandwidth.

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