BATC Forum

First of first DATV emitter from Germany group uses a GMSK modulation instead of QPSK. This modulation can be amplified with class C ampli, thus having more RF output compared to a linear amplifier for QPSK. I didn't find any real comparaison of gain and robustness of the 2 modulations. Some documents describes it theoricaly but not on a real QSO. Has someone made some experiments on that ?

If GMSK have interest, the issue could be to receive it. AGAF had made a FPGA board which convert GMSK to QPSK. However, I don't know if it is still produced. My idea is to use a cheap RTL-SDR usb dongle to make the GMSK demodulation and remodulate it in QPSK localy with the Ugly mode thanks to a raspberry. Of course this could be only achievable on low SR because of the bandwidth of RTL-SDR (2.4MSample/s). A standard QPSK demodulator display then the video coming from the Raspberry Pi.

Do you think this project could be of any interest ?

73's Evariste F5OEO

Hi Evariste F5OEO,

Yes, one of the primary advantages of GMSK modulation is that it can use lower-cost FM RF amplifiers and achieve the same power output level as FM modulation.

It also important to remind readers that GMSK modulation requires an RF bandwidth that is typically twice as wide as other digital modulations used by DATV in order to transmit a given data-bit-rate.
GMSK = 1 data-bit per Symbol transition.
QPSK = 2 data-bits per Symbol transition

The ICOM D-STAR radios for the ham radio FM audience uses GMSK for digital-voice and data transmissions. The main complaint I hear from D-STAR data transmission users is that the data-rate is too small for a given bandwidth.

I would also be very interested in hearing from one of the BATC readers to provide a real comparison of gain and robustness of the 2 modulations (GMSK and QPSK).

73...de Ken W6HHC

If we're making our own SDR based receivers, offset QPSK (OQPSK) would be better for power amplifier linearity. There must be chips to do the demodulation, although they're likely to be very small.

In QPSK, I and Q can change at the same time, causing large amplitude variations. In OQPSK, I and Q change alternately, avoiding large variations.

PI/4 QPSK is also a possibility.

Brian

http://shams.kohat.edu.pk/Courses/Lectu ... pter4f.pdf