One such clever idea is to squat on any channel not currently being used. This is the principle behind “white space” transmissions. White space is a term often used in printing. Everywhere on the page not covered by ink is white space, the color of the unprinted paper. Pages with text spread out and empty areas have lots of white space.You can carry that analogy over to the TV band. Before the days of cable and satellite, you know that there were only a few channels you could pick up with an outdoor antenna. Stations were never assigned channels right next to each other. That’s because early receivers didn’t have tuners that could separate adjacent channels well enough to keep the signals from interfering. The FCC made sure to put at least one blank channel in between each broadcaster in a particular area. Those blank channels were assigned to other cites too far away to interfere. In any given area, there is lots of white space where there are no assigned transmitters in the TV band.
White space transmitters take advantage of this situation by using those unused channels to transmit fairly weak signals that won’t interfere with TV reception. Wireless microphones were one of the first applications to take advantage of this available spectrum, hiding between stations before the switch to DTV. Since the switchover, the idea of using the white spaces in the remaining VHF and UHF TV bands has been blessed by the FCC, although with restrictions. In order to prevent chaos, any device operating unlicensed in these bands must take action to prevent interference to licensed operations. That means consulting a database of assignments maintained by the FCC and listening for transmissions on a channel to ensure it is unused before sending out any signals.
The idea of listening before transmitting is reminiscent of early Ethernet networks that used collision domains before everything evolved to switching. It’s the same as intersections where there are no traffic lights. Each user is responsible to make sure there is no traffic in the way before crossing. As long as everyone follows those rules, there is an amazing amount of spectrum still available that can be used on a shared basis.
Another technique being used to squeeze more performance out of existing bandwidth is being deployed by the cellular carriers themselves. It’s based upon the fact that all cellular towers aren’t used equally. Some are grossly overloaded and others lightly accessed. Those areas of high congestion need more towers, but there aren’t always more licensed frequencies available. Relieving the congestion involves breaking up the existing cells into many smaller cells using lower power transmissions. You don’t need a high power signal when you are standing right next to the cell tower.
These small cellular stations are called microcells, picocells and femtocells. A microcell covers an area about a mile wide, a picocell is 600 feet wide and a femtocell is a 30 foot cell. These small cells can cover a business district, a campus or even a single building to fill in dead spots or congestion in cellular coverage. They can be connected to the rest of the cellular phone system by traditional T1 line, Ethernet over Copper or even a broadband Internet connection.
You might have noticed that picocells have a similar coverage to WiFi hotspots. With the move to 4G for data and even VoIP voice, WiFi may turn into an alternative to the cellular phone networks. WiFi is built into nearly every Internet connected device already. WiFi routers have proliferated in both homes and offices. Many restaurants, hotels, auto service shops, retailers and other business locations already have free WiFi hotspots available. What’s missing is universal coverage and roaming. With cellular, you are free to move about while maintaining your connection. The system takes care of handing off the connection from cell to cell. With WiFi, you need to log into each hotspot individually. A similar system for WiFi plus higher power transmissions and more hotspots could make this a serious candidate for universal wireless broadband, especially free access.
In Part I, Part II, and Part III, we’ve seen how the coming juggernaut of wireless broadband Internet is sucking up all the available spectrum it can find and how more efficient techniques are being developed to make better use of existing spectrum. In Part IV, we’ll have a look at how technological improvements can open up additional bandwidth to satisfy our insatiable broadband appetite.
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