Early Digital Comms Concept based upon GMSK - Page 1
Forum section: electronics / rf-microwave

author: mawyatt
author_rank: Super Contributor
timestamp: February 11, 2025, 07:30:29 pm
content: Another thread remind us of this from early days of High Speed Digital MW Comms!!Long ago we were asked to develop a comms system that would work in heavy multi-path environment, this was the main comms system for a major program we had just been awarded. The system had to perform in a high multi-path environment where the present DSSS systems were having lots of problems.We decided on (with theoretical help from of colleague of Dr Andrew Viterbi at JPL) an early rendition of GMSK (Gaussian Minimum Shift Keying) for use in the heavy multi-path environment in the MW area mainly because of the "smooth" transitions in Phase and Frequency with constant waveform envelope.The RF/MW signal generation was decided to be directly created with the modulation in-tack rather adding the modulation later in the RF stages, this should (it did) eliminate any carrier leakage issues which had caused problems with other modulation schemes (things had to be really small and no shielding allowed).A new Bipolar Synthesizer chip (used ECL bipolar process) was being developed at Plessey UK that could be utilized as the core signal generator and we developed a GMSK modulator for that chip which utilized direct injection into the loop filter in an "Around the Loop" Configuration. For this to work and not drive the synthesizer loop filter voltage away from the desired average center (center frequency) we employed a 3/5 coding scheme to keep the bit (+-1) average at ~0. Since major Synthesizer Dynamics are dictated by the loop filter, and the "Modulation Gain" also influenced by the Loop Filter we created an error DAC scaling the Synthesizer Loop Gain to compensate for changes in the Loop Gain at different select center frequencies. The overall "Around the Loop" modulation configuration was arranged to implement a 4th Order Gaussian approximation filter (Gaussian Filters are not realizable with finite # of components, only approximations) for the Phase Transitions. Of course this is the same for the Frequency Transitions since Frequency is the time derivate of Phase and the derivative of a Gaussian is a Gaussian (another way of thinking about this is that the modulation voltage created a Gaussian Frequency Shift in the VCO and the Phase is time integral of Frequency, so also followed a Gaussian response) which is exactly why this modulation scheme was selected!!We'll pause for the Receiver development for the moment, as that's another long thread.On to the testing, we didn't have any high speed BERT equipment to evaluate this new (at the time) modulation scheme, so as usual we developed our own. This setup allowed us to investigate the effects of Multi-Path and other unwanted influences on the GMSK comm system and "tune" the system parameters like TX & RX Filters, BT products and such. We created a very long ECL/CML shift register and some selective feedback paths with ECL/CML Logic to create a PRBS (pseudo-random binary sequence, altho we often incorrectly called this a PR or RN Bit Sequence). This was used as the Data Input to Transmitter System and another path for the Receiver Data comparison. We added a variable delay network and some more ECL/CML logic that drove a high speed external counter, and crammed all this into a small "Bud" box which became our Custom BERT Tester. The in-lab use idea was to delay the TX Data Signal to equal the TX/RX Delay and compare the RX Data with the delayed TX Data and look for errors which were accumulated in the external counter as Bit Errors.During live system testing in a highly congested and disruptive RF environment (both intensional and non-intensional) we were able to outperform the best DSSS System and operated at 2.5X the data thruput of the DSSS system!! Later we were able to purchase an expensive High Speed BERT (believe was Anritsu), however that little ECL/CML based BERT tester box continued to serve well.Anyway, this comm system went on to become an important component in our arsenal and was involved in some very important "activities" way back (these activities are not open for discussion).Best