Digital Dispatch Systems (DDS) 4800 bps QPSK - Taxi MDT Protocol

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inigo88

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Hey everybody,

I figured this was as good a place as any to post this. I've long been interested in this protocol, which runs mobile data terminals in taxi cabs (including alpha-numeric dispatches, text messaging and GPS AVL), and it's fairly common in the 150-152 MHz range around the US. The older style DDS MDTs - still the most common - look like this (newer models feature touchscreens with GPS moving map):

mdt1.jpg


The only other thread I could find on this topic is here:
http://forums.radioreference.com/ge...541-dds-digital-dispatch-system-decoding.html

Here's a sound sample (from the excellent kb9ukd digital modes samples site):
http://www.kb9ukd.com/digital/152.33-taxi.wav

Both the previous thread and the digital modes samples site refer to this mode as using Quadrature Phase-Shift Keying (QPSK) at 4800 bps. The author of the original thread claims he was able to decode it using QPSK64, which sounds suspiciously like 64-QAM to me (commonly used for digital cable TV) shown below:

WTYIgWa.gif


Just for fun I tried running UniScope on the local DDS MDT frequency at 4800 baud QPSK, and was loosely able to get an hourglass pattern:

D6geutN.png


As you can see it was very jittery even with glass and filtering enabled, but it's close enough that I'm reasonably convinced the earlier QPSK 4800 bps description is accurate.

I was also able to find some info on the DDS network infrastructure from their brochure here:
http://www.ddswireless.com/brochures/PathFinder.pdf

Nowadays many MDT users choose to use cellular-based network solutions like CDPD or GPRS, but the option of running the system on a traditional business frequency still exists and is called "Private Mobile Radio." The PMR option is discussed on page 5 of the PDF, and consists of "Redundant Base Controllers" (RBC) at each site and an optional "Multi-Site Multi-Channel Network Controller" (MNC) which can run a multi-site network. (More on the MNC here.) Regarding the RBC's radio modem protocol, it says:

The RBC incorporates advanced Digital Signal Processing (DSP) techniques to achieve superior radio modem performance in the demanding mobile data environment. Our radio modems utilize advanced techniques including Forward Error Correction (FEC), Interleaving and the Digital Sense Multiple Access (DSMA) contention scheme. The modulation used allows speeds up to 9600 bits per second using conventional voice grade radios. The benefit of this advanced and unique technology is the ability to support more vehicles per radio channel.

Thus the newer DDS MDTs can apparently support both 4800 bps and 9600 bps protocols, although I've only ever heard the former. (This can be verified by checking sales brochures for their individual MDTs.)

Just curious if anyone else has ever looked into this or tried decoding it. Or am I the only one crazy enough to care? ;)

Ian Wraith's Taxi MDT Decoder program seemed like a good bet, but it only supports two European standards called Autocab and Auriga which I've never come across in the U.S.
 

SCPD

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Just curious if anyone else has ever looked into this or tried decoding it. Or am I the only one crazy enough to care? ;)
You're not the only crazy around here. :)

I'm a little confused by the modulation description.

Baud is symbol rate, not bit rate. If the signal is indeed 64QAM (or 64QPSK), those are six bit symbols. At 4800 symbols per second, that's 28,800 bits per second.

Just for giggles, what does the waveform look like at 4800 and 9600 baud with a symbol type of "Impulse"? You may need to run the newer Uniscope to get results.
 

inigo88

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Hey Rick, I'd be happy to! Do you have a link to the newer Uniscope by chance?
 

inigo88

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As requested,

DDS in Oscilloscope mode:

chYR6SV.png


DDS in "Impulse" at 4800 bps:

Wo3XKVm.png


DDS in "Impulse" at 9600 bps:

ZK1FgU9.png


The 9600 bps version is definitely 'smoother' (more linear) while the 4800 bps version is kind of like spaghetti. Think it's the 9600 bps variant after all?

Regarding 64QPSK, I'm still skeptical. Maybe 4QPSK haha. DDS has such a simple RTTY-like sound to it that I originally thought it was just an FSK protocol. But there's definitely some lower pitched data bursts underneath, and prior posts indicate it's definitely QPSK.

Edit: For completeness, here it is also at 300 bps:

fJTz23e.png
 
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