Mobile Satcomms Voice Terminal

This project involved design and construction of proof-of-concept prototypes of both the mobile and earth station units. The voice codecs, antenna system, HPA, LNA and duplexer were provided by the client.

This project was unusual for us: we prefer to build even one-off systems to reasonable production standards, however in this case the client's budget was limited. Out of necessity most of the system was hand-wired, and a simple Vero case was used.

The system allowed direct-dial calls to and from the public telephone network. It was successfully used for demonstrations to obtain approval for further development, and for initial trials. This was the first stage of development of a system that became a global mobile satellite system.

The project was completed about fifteen years ago, hence the rather primitive component technologies!

Mobile Terminal

L-band and PSU unit

This image shows the mobile subunit containing the power supply, HPA, LNA and duplexer. The slots are for ventilation. The array of LEDs monitor various system functions: power supply status, antenna reverse power, HPA temperature etc.

Audio, modem, codec and IF unit

This image shows the IF/baseband subsystem containing the mobile handset interface, codec, modem, control, and IF circuitry.

Access doors open

With the access doors open, the receive IF and transmit IF modules are visible, and to the right the following cards:

Carrier recovery: BPSK was used in both directions. The board includes an analogue costas loop demodulator with controllable loop bandwidth. The loop bandwidth is adjusted by the frame synch card according to the system state (search, dropout, re-acquire etc).
Clock recovery / bit decision.
Frame synch / data multiplexer. Uses a "unique word" for frame synch, and allows digitised voice and control signals to be combined in the same channel. The receiver functions are not trivial for a satcomms system, as the system must work with high bit error rates and deal gracefully with short dropouts, and resynchronisation after longer dropouts. The digital circuitry was implemented in a Xilinx programmable gate array.
Codec: From our point of view a "black box" developed by a separate team.
Handset Interface: The system was used with a standard DTMF phone. The handset interface therefore required a DTMF decoder and ring signal generator in addition to the usual audio functions.
Controller: This was an off-the-shelf 80188 controller card that communicated with the other cards via an STE bus. The programming language Modula-2 was used.

Earth Station Equipment

The earth station equipment was very similar to the mobile IF/baseband unit, but it had different IFs and included a PSTN interface.

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