Method and system for demodulating GMSK signals in a cellular digital packet data system
AssigneeHughes Aircraft Company
InventorYoungky Kim
Life of this patent
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Abstract
A decision feedback multiple symbol differential demodulator for Gaussian filtered Minimum Shift Keying (GMSK) has a performance very close to theoretical limit even under a fading channel. The demodulator implements a non-coherent, maximum likelihood (ML) search over multiple bit-time intervals. The past bit decisions are fed back to the ML decision logic. The improved performance of the demodulator is achieved by using old decisions in the ML decision logic.
Claims22
A decision feedback multiple symbol differential demodulator for Gaussian filtered Minimum Shift Keying (GMSK) comprising:
- a matched filter used to filter Minimum Shift Keying (MSK) signals, the matched filter receiving samples of a GMSK signal, {z(n)} and filtering the GMSK signal as an MSK signal;
- a non-coherent correlator connected to receive an output of the matched filter and computing a correlation, C.sub.D, based on (2D-1) past bit decisions }d(k-1), d(k-2), . . . , d(k-2D+1)}, d(k).epsilon.{-1,1};
- and
- maximum likelihood decision logic connected to receive correlation outputs and selecting a bit d(k) as an output of the differential demodulator and feeding back the selected bit d(k) to the non-coherent correlator.
A method of demodulating Gaussian filtered Minimum Shift Keying (GMSK) signals comprising the steps of:
- filtering a sampled received GMSK signal, {z(n)} as a Minimum Shift Keying (MSK) signal, in a matched filter used to filter MSK signals;
- computing a correlation C.sub.D, of an output of the matched filter based on (2D-1) past bit decision {d(k-1), d(k-2), . . . , d(k-2D+1)}, d(k).epsilon.}-1,1};
- and
- selecting a bit d(k) from correlation outputs, C.sub.D, using maximum likelihood decision logic connected to receive correlation outputs, C, and feeding back the selected bit to perform the step of computing the correlation outputs, C.sub.D.
A device for demodulating Gaussian filtered Minimum Shift Keying (GMSK) signals comprising:
- a matched filter for receiving samples of a GMSK signal and filtering the GMSK signal as a Minimum Shift Keying (MSK) signal;
- a correlator connected to receive an output of the matched filter and computing a correlation based on at least one previously selected bit;
- and
- maximum likelihood decision logic connected to receive correlation outputs, select a selected bit, and feed back the selected bit to the correlator.
A device for processing a received signal, said device comprising:
- an amplifier for amplifying the received signal;
- a down converter connected to said amplifier for recovering a Gaussian filtered Minimum Shift Keying (GMSK) signal from said received signal;
- a matched filter used to filter Minimum Shift Keying (MSK) signals, said filter connected to receive said GMSK signal and filtering said GMSK signal as an MSK signal;
- a correlator connected to receive an output of the matched filter and computing correlation outputs based on past bit decisions;
- and
- maximum likelihood decision logic connected to receive said correlation outputs, said decision logic for selecting a selected bit, feeding back the selected bit to the correlator, and including said selected bit as a decision output.
A method of demodulating Gaussian filtered Minimum Shift Keying (GMSK) signals comprising the steps of:
- receiving a GMSK signal;
- filtering said GMSK signal in a matched filter used to filter Minimum Shift Keying (MSK) signals;
- computing a correlation output from said filtered signal;
- selecting a selected bit using maximum likelihood decision logic connected to receive said correlation output;
- feeding back the selected bit to perform the step of computing said correlation output;
- and
- including said selected bit in a decision output.
Dependencies are inferred from the claim text.