Monday, May 14, 2007

Spread Spectrum

Spread spectrum is a radio technique that continuously alters its transmission pattern either by constantly changing carrier frequencies or by constantly changing the data pattern.

If you asking why, then let me point out that in order to support a high speed or multi-media transmissions you will need a bit of bandwidth. The 2 meter band is only 4 MHz wide, and already well populated. Spread spectrum can share the same frequency band (overlay) with other uses.

Spread Spectrum was invented in 1940. Austrian-born Hedy Lamarr, considered one of the sexiest actresses of her time, was also a co-inventor of frequency hopping.

There are two basic types; Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS).

DSSS is the basis for CDMA cellphones and 802.11 wireless transmission. FHSS is used for bluetooth devices.

It wasn't till 1981 that the FCC authorized spread spectrum on amateur frequencies. In late 1999 the FCC relaxed Amateur Spread Spectrum rules. (Prior only certain spreading codes where allowed)

Presently Part 97.311 of the rules effectively limits amateur spread spectrum to 1 watt. Please support the passage of RM-11325, to have that 1 watt limit lifted. Spread spectrum this day in age, should not be treated as an inferior mode.

It may seem like a moot point to some. For you see 802.11b is direct sequence spread spectrum (DSSS), but it has largely be replaced by 802.11g and 802.11a, both of which use Orthogonal frequency-division multiplexing (OFDM) modulation instead of DSSS. OFDM is technically not spread spectrum and thusly is not encumbered by the 1 watt rule.

Frequency hopping spread spectrum (FHSS) is in my mind the best fit for the present ham band plans. In FHSS the carriers are narrow and "hop" around only staying on a given frequency for less than a second. It's also hops around interference.

DSSS on the other hand is spread over a wide fixed spread of frequencies, usually occupying several megahertz. 900 MHz and above the bands are pretty un occupied so fitting DSSS in isn't really and issue. But as you should see FHSS stands the best chance of working in amateur radio on band 70 centimeters and below, on bands where there is already occupancy.

Data networking aside, there are now some new FHSS voice radios out there for 900 MHz. They are both obviously digital voice and capable of text messaging.

Motorola DTR650
Apparently Motorola has repackaged their off network iDen 900 MHz ISM FHSS walkie talkie feature into a simplex radio. Range is actually 1/2 way decent on them from what I've seen, couple miles line of sight. It has a removable antenna. The price is around $300

TriSquare eXRS Radios
906 to 923.75 MHz, 50 hopping channels, 1 watt power, -124 dBm receive sensitivity, 397 ms. dwell time, 11.5 KHz occupied bandwidth. Integrated antenna, but can be modified with an SMA connector. Nothing to compare with the Motorola DTR radios. Nowhere near the price either. These can be picked up at higher end sporting good stores usually for under $100 a pair.

Brian NB9E writes about the Trisquare radios:

While doing some mountain top maintenance on some of our ham repeaters here in northeastern NV (Elko area) I made contact with my wife (kc1ndy) around 30 miles from Swails Mt in Carlin to Pleasant Valley(near Lamoille). One radio was modified with an SMA connector and an SMA-3 Comet rubber duck which is rated for 2m,440 and 915!!! Impressive range to say the least for 1 watt!!! As far as legalities go the unit operates in the ham band limits and can be re-certified under Part 97 rules. The unit boasts over 1 billion channels actually hopping sequences. It really only hops 50 channels between 906 and 923 about 396 Ms each if I remember correctly. A better choice though might be the Moto DTR-650 which has a removable antenna!!! I've yet to get my hands on a pair of these yet, but they're on my list!!!

For more information see the March 2008 Popular Communications Magazine Tech Showcase article on the TSX300 by Bernard Bates

The TriSquare units use use analog FM instead of digital modulation for the voice, while the Motorola DTR's use digital modulation. There have been reports that an Optoelectronics R-10 can lock right onto the TriSquare FHSS signal and hearthe audio with no problems whatsoever.


bernie said...

The "reports that an Optoelectronics R-10 or Xplorer can lock right onto the TriSquare FHSS signal and hear the audio with no problems whatsoever" are not true. The claimant has some kind of personal axe to grind.

Kevin at Optoelectronics--who was on the design team for both of these test eqpt receivers--has stated unequivocally that neither can reliably follow the 400ms hopping rate of the TriSquare eXRS radios at any distance. I have been able to confirm this because I own both of these Optoelectronics test receivers and several TriSquare TSX300's.

Some very high-end test equipment and surveillance receives can do this, however.

Bill said...

I want to give a comment or review on this radio. I am a ham (amateur) radio operator. A friend had a pair of these that he got from the company as a beta tester. He used them and liked them. Another friend had purchased a pair of them, but did not want them anymore. So, I purchased them from him for $60 for the pair and the drop-in charger. I have had them for about two years and love them! We have used them for family outings and I will use them with my ham friend at hamfests to have private means of communications. I really like that they are truly private communications. I will never go back to the 460Mhz FRS radios. Easy to program and easy to operate. If the ones I have wear out, I will buy them again!