At 09:16 PM 12/16/97 PST8PDT, you wrote:
>Hi Piclisters!
>
>I'm using a TCM3105 single chip modem to send data trhu the cellular network,
>from a PIC16c65 mounted on a car to a PC. The data flow is "half-duplex",
>only from the PIC. This chip uses FSK modulation (V21) and I tested it when
>the transmitter is
>moving and works fine, but is expensive and need a "custom" modem in the PC,
>since
>i cant found a modem card supporting V21.
>Some people tell me that V22 and other modem standards that uses PSK or QAM
>modulation
>techniques are not useful in mobile applications. Is this correct?
>Can I use a V22 chip?
>
>Regards,
>Ernesto Tolocka
>etolocka@onenet.com.ar
>Argentina
>

What data rate do you need? I think that old 1200 baud modems might have
used FSK. Most fast modems will revert down to this rate.

I am not sure about PSK and QAM working well in a mobile environment.
The American Digital Cellular standard uses 4-PSK but most mobile data,
paging services, and I think GSM, all use GMSK (Gaussian minimum shift
keying, I think similar to FSK ) or FSK itself.
I'm sure it would depend on whether this is an urban or rural environment.
You could always do a test and see if low rate PSK or QAM would work well.
I think that some of the low rates on PC modems use gray-encoded PSK (maybe
4800 bits/sec rate). Gray-encoding is a method of optimizing the bit error
rate for a given signal to noise ratio.

Good luck,

Sean


+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
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