The 700MHz Question: Will the Wireless Spectrum Auction Lead to Innovation or More of the Same?
- Speed and Distance
- License Poker
- I Want to Be Free
- The Final Conditions
Analog television has been around for longer than most InformIT readers. Standards like PAL and NTSC were designed to maintain backwards compatibility with older black and white equipment, and even today it's relatively simple to turn an oscilloscope into a simple monochrome TV receiver.
This is about to end, however. Analog TV has been kept around for a long time based largely on the huge installed base. Gradually, it has begun to be replaced by digital signals, which make more efficient use of the spectrum by heavily compressing the signal.
As with so many other things, one of the primary motivating factors for this migration is money. Since digital TV uses less bandwidth for the same number of channels than analog, this leaves some spare. Since the use of the spectrum in any given country is heavily regulated, this migration gives governments a chance to grab some cash by selling off the rights to use the newly-freed space. In the US, the 700MHz spectrum is beginning this process soon.
Speed and Distance
Picking a good frequency for a particular application is difficult. High frequencies have a greater information carrying capacity. An encoding used to transmit data typically has a fixed number of bits per cycle. On a pure, noiseless, square wave, this would be one; the wave is either up or down, giving a one or zero. More commonly, the amplitude is varied in smaller steps, giving more than two possible symbols per cycle. All other things being equal, however, doubling the frequency (that is, doubling the number of cycles per second), will double the data rate that can be transmitted.
If this were the only factor, higher frequencies would always be better. Unfortunately, there is a conflicting requirement. Higher frequencies mean shorter waves, and shorter waves are much easier to block. During the cold war, submarines typically used 50-100Hz signals to communicate while submerged. These can pass through the Earth without any problems, while the 2.4GHz signal from an 802.11 device has trouble passing from one room to the next.
The 700MHz region was chosen for analog television since it was capable of carrying enough data for analog television while having enough range to cover a country with signal and still be economically feasible. This also makes it attractive for mobile data services, since it requires a fairly low investment to provide good coverage.