Pulse or digital communications – Receivers – Angle modulation
Patent
1997-12-17
1999-10-26
Chin, Stephen
Pulse or digital communications
Receivers
Angle modulation
H04L 2714
Patent
active
059740949
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an electronic circuit for and a method of decoding a signal. More specifically the invention relates to an electronic circuit for and a method of decoding a signal comprising a data signal at a frequency within a range of discrete frequency slots representing a predetermined range of digital values and a pilot signal. The circuit and method are suitable for use in data transmission systems where data is transmitted at a slow rate as well as in systems where data is transmitted at a high rate.
In paging systems generally data is transmitted at a slow rate. There are problems associated with such low data transmission rates, especially if the receiving pager is moving. FIG. 1 of the accompanying drawings illustrates a situation where a pager (not shown) in a vehicle 1 is moving at velocity v past two buildings 2, 3. At time t=0 the vehicle 1 is in a position where there is a clear line of sight to a source (e.g. a satellite) and signals from the source arrive at the vehicle 1 at an angle of incidence .theta.. Because the vehicle is moving the frequency of the arriving signal is subjected to a doppler shift .DELTA.f=-v/fcos.theta.. At time t=1 the vehicle has moved to a position 1' where the building 2 obscures the source and the signal is reflected from the building 3 before arriving at the vehicle 1'. Thus, at time t=1 the frequency of the arriving signal is subjected to a doppler shift .DELTA.f=v/fcos.theta..
In data transmission systems, digital data is often transmitted using frequency shift keying (FSK) techniques. In m-ary FSK transmission a frequency range is divided into a number m of discrete frequency tones each representing a respective different digital value. Digital data is transmitted by transmitting on a carrier signal a tone for a predetermined period of time at the frequency corresponding to the value of the data. FIG. 2 of the accompanying drawings shows the frequency spectrum of a 32-ary FSK transmission system in which a signal 4 is being transmitted at a frequency corresponding to the data value D.sub.16. The effect of doppler shift on the transmitted signal 4 is shown in FIG. 3 of the accompanying drawings. At time t=0 (when the vehicle is at position 1) the received signal 4' is shifted down to a frequency somewhere between the frequencies corresponding to the data values D.sub.15 and D.sub.16. At time t=1 the received signal 4" is shifted up to a frequency somewhere between the frequencies corresponding to the data values D.sub.16 and D.sub.17.
In densely populated urban areas the problem is made worse by multiple reflections of the signal which results in multipath reception of the signal. This can cause frequency smearing, such as illustrated in FIG. 4 of the accompanying drawings, when the receiver is moving. Multipath reception also causes multiple peaks over a few frequency tones, such as illustrated in FIG. 5 of the accompanying drawings, when the receiver is moving. Multipath effects can be so bad that the amplitude of the signal is reduced to noise levels in which case the signal will be lost.
One method that has been proposed for overcoming the effects of doppler/multipath signal degradation is to apply a differential encoding to the data. That is to say, the present data is transmitted as the difference from the previously transmitted data. The advantage of this method is that all signals are doppler shifted by the same amount and (assuming there is a clear line of sight to the source) the doppler effects are therefore cancelled out. However, the method is not suitable for eliminating multipath effects because it is unable to compensate for errors which are introduced to the received signal each time there is a path change.
Another method proposed for overcoming the aforementioned problems is a technique in which data is transmitted together with a known tone, i.e. a reference frequency, which is transmitted between data from time to time. FIG. 6 of the accompanying drawings illustrates a signal 5 comprising a sequence of data tones 6 to 12. During
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IEEE Transactions On Communication Technology, vol. COM-16, No. 5, Oct. 1968, New York US, pp. 657-668, XP002011456 Ferguson: Communication at low data rates--spectral analysis receiver.
Chan & Couture: "Comparison of two FFT-based demodulation schemes for M-ary FSK", New York, US, pp. 603-607, XP00346661, Oct. 1992.
Vargauzin: Use of fast Fourier transform for realization of programmable digital demodulators, Telecommunications And Radio Engineering, vol. 46, No. 11, New York, US, pp. 80-82, XP00311105, Nov. 1991.
Fines Panagiotis
Wong Siu Wah
Chin Stephen
Inmarsat Limited
Park Albert
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