Thursday, October 1, 2009

The range of D-Star




Several Digital Voice operators have reported hands down, absolutely no question, tested it for months, the D-Star is usable for reliable (non-garbled communications) for mobiles about 10% or less of the coverage area of the Analog FM and for base stations about 80%. This is largely due to the picket fencing of a mobile signal, multipath for non line of sight, etc. On a high peak in the desert with line of sight, one can imagine them being comparable, but not in the real world of trees and buildings.

Yes the R2D2, digital garble on P25 and D-Star transmissions is due to multipath. Experience is that 2 meters is more prone to R2D2 issues than 70cm Partially because 70 centimeters penetrates buildings better. So yes analog copies with some conditions better than D-STAR. Mobile, multipath for one.

Here is a sample for those curious but not actively experimenting: http://www.ussc.com/~uvhfs/dstar_test_excerpt_2a_20080116.mp3

Digital voice should in theory get out further as a narrow band mode advantage. Some hams have noticed that their radios specifications it takes a stronger signal to decode.

The Icom repeaters have poor receive sensitivity and poor adjacent channel
rejection compared to Motorola/Kenwood repeaters. The spec's on the RP-2000V are
12dB sinad at .45uv and only 65db isolation from adjacent channels.

The D-Star user radios spec. worse sensitivity in DV mode than FM.... However in theory with proper implementation:

The DV Advantage Per Mark, N5RFX:

The DV signal has a steady noise level to –119 dBm and drops off at –120 dBm. The analog FM signal SINAD begins to drop at –102 dBm. Between –102 and –119 dBm DV has a SINAD advantage over analog FM. The advantage occurs over a 17 to 18 dB range. When noise free signals are desirable, DStar digital voice can meet this requirement with a 17dB to 18dB increase in the range that noise free operation can occur. For weak signal work, the analog FM signal will prevail.

CONCLUSION

Trading 2 dB of sensitivity for a 17dB increase in nearly noise free reception is an advantage of DStar over analog FM. When weak signal reception is necessary, the analog signal will provide better performance.


D-STAR isn't perfect, but it's here and it's being used, so we should familiarize ourselves with it. Besides, it's fun to play with a new mode. :)

One method to combat multipath it is found in the IC-2820H mobile, with its diversity receive. Another possible combatant might be to put circular polarization at the repeater site.

Reference: http://www.qsl.net/n9zia/cir_pol_rpt.html

There is an article on circular polarization antennas in August 2007 QST. It might be interesting to do some research in this area. Bases could use horizontally polarized antennas to cut down interference from man made sources and be 20 db down from adjacent vertically polarized FM repeaters and mobiles. Using slot antenna designs horizontal might also be practical on 23-cm mobiles. (John, K7VE mentioned this back in 2007.)

I still haven't found any reports of any D-Star repeaters trying circular polarization at the repeater site. But I would be curious to read about it.

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