Digital spread spectrum (DSS) technology has its roots back in World War II and
has evolved into the technology of choice for the cordless phone (as well as many other sophisticated communications systems, especially in the military). Cordless phones are now truly "ready for prime-time" thanks to affordable DSS technology.
Spread-spectrum technology can be implemented in several different ways, but the most common and easiest to understand involves frequency-hopping. This technique was co-invented
by actress Hedy Lamarr
during World War II, to help direct American torpedoes and prevent them from
getting jammed and sent off-course. Lamarr and her arranger invented the frequency-hopping
concept. Since Lamarr didn't use her stage name on the patent,
it took years for the story of patent 2,292,387 to surface.
It never made her any money.
The World War II electronics were primitive. Hedy's system used a mechanical switching system,
similar to a player piano roll, to shift control frequencies faster than the enemy could follow. The concept was taken up by
engineers in 1957 and became the basic tool for secure military communications.
It was installed on the ships sent to blockade Cuba in 1962, about three years after the
patent expired. [See this fascinating story. See also Invention & Technology magazine]
The basic idea behind frequency hopping is simple -- instead of transmitting on one frequency, a spread spectrum system switches rapidly from one frequency to the next. The choice of the next frequency is random, so it is nearly impossible for someone to eavesdrop or jam the signal. The challenge is to keep both the transmitter and receiver synchronized, but with accurate clocks and pseudo-random number generators, this is straightforward. Spread-spectrum has been recently combined with digital technology for spy-proof and noise-resistant battlefield communications. In civilian life you see it most often in cordless phones and wireless local area networks.
In non-military applications (like a cordless phone used in the home), the security is nice but the larger problem is limited frequency spectrum. It is getting harder and harder to find clear channels on the crowded airwaves. Spread spectrum gives engineers a way to fit cordless phones into existing spectrum without jamming the devices already using it. Assume a phone is transmitting at 1 watt, but is hopping between dozens or hundreds of channels very rapidly. Other devices don't "see" the phone because it is transmitting for only a fraction of a second on any channel. Therefore, the average perceived power on any given channel is extremely low, and other devices using that channel don't even notice it. The phone creates the equivalent of a low-power noise pattern across all of the channels it uses. Other devices deal with noise already, so the phone is essentially invisible to devices using specific channels.
A cordless DSS telephone provides:
- Longest range - Due to higher transmitter power (1 watt) allowed by the FCC,
you can expect seven to eight times the range of an earlier .001 watt cordless
43-49 MHz telephone, and about three or four times the range of a analog or digital
900 MHz cordless telephone. The 900 MHz phones all have a better range due to the
wavelengths being shorter -- shorter waves are easier to bounce off objects.
However, only the DSS versions of the 900 MHz cordless telephones can use the full one watt of transmitting power.
The normal digital and analog versions of 900 MHz have a lower power limitation.
- Best sound quality - Since the spread spectrum signal is uniformly spread over a wide
range of frequencies for transmission. the signal avoids interference
and noise from other signals. Older 43-49 MHz cordless phones had only 25 crowded channels
and are very susceptible to interference. 900 MHz analog phones still suffer from the interference
problems of any analog cordless telephone, and can be heard on inexpensive radio scanners
or your neighbor's same-brand cordless telephone. A spread spectrum phone is much less susceptible
to signal fading. This makes the reception of a spread spectrum-based cordless phone much
less sensitive to the location and pointing direction of the handset than that of a conventional
lower-power analog cordless telephone.
- Highest security - Due to digital transmissions and constantly changing frequency
channels in use, only the matching receiver has a copy of the pre-assigned spreading code.
Millions of scrambling codes are available and are selected automatically when the headset
is lifted from the cradle. Common radio scanners cannot hear a DSS cordless telephone conversation.
Some useful links: