Demodulators – Phase shift keying or quadrature amplitude demodulator – Input signal combined with local oscillator or carrier...
Patent
1995-12-13
1997-06-17
Grimm, Siegfried H.
Demodulators
Phase shift keying or quadrature amplitude demodulator
Input signal combined with local oscillator or carrier...
329308, 375326, 375327, 375332, H04L 2722
Patent
active
056401256
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method and to apparatus for demodulating a signal in which a carrier is modulated with M phase states, M being an integer, and generally even, and in particular M=2.sup.m states (m being an integer).
A particularly important, though non-exclusive application of the invention lies in digital transmission systems using quaternary phase shift keying or 8-state phase shift keying (QPSK or 8PSK) or modulation of a microwave carrier (i.e. having a frequency of at least 3 GHz). A particular application that may be mentioned is simultaneous transmission of a plurality of digital television programs in the form of a multiplex in a satellite broadcast channel in the 12 GHz band.
Nevertheless, as can be seen below, the invention applies to demodulating any signal using PSK modulation of a carrier and it is advantageous whenever carrier recovery conditions are unfavorable.
Systems for transmission and broadcasting over a satellite channel often use PSK type modulation (in particular QPSK or 8PSK) with prior encoding for protection against errors that increases the required data rate. In particular, use is made of convolutional error protection codes or of concatenated codes (associating convolutional encoding with Reed-Solomon encoding), thereby eliminating errors almost completely even for Eb/N.sub.0 ratios as small as 2 dB to 4 dB (where Eb is energy conveyed per bit and N.sub.0 is noise level). The efficiency of encoding systems causes these values to correspond to Ec/N.sub.0 ratios that are close to 0 dB (where Ec is energy per bit transmitted after encoding).
The same problem is encountered when using trellis modes of encoding which enable the spectrum efficiency of a non-coded system to be retained by increasing the number of modulation states.
Error protection encoding systems increase the data rate to be transmitted for a given number of useful bits, but complicate carrier recovery when demodulating on reception, and very often it is carrier recovery performance that puts a limit on the overall performance of the system.
SUMMARY OF THE INVENTION
The invention relates more particularly to demodulation methods which include converting the received signal into a baseband signal on two paths in quadrature by means of a signal provided by a local oscillator at the carrier frequency (generally after transposition to an intermediate frequency). The baseband signal is digitized by sampling it at the clock frequency of the modulating signal. Conventionally, the generally transposed carrier is recovered "blind", i.e. without knowing the instants at which the eye diagram is open at its widest. The loss of performance due to this "blind" recovery is not fatal when the transposition local oscillator is sufficiently stable for reliable carrier recovery to be possible with a narrow frequency acquisition range. Those circumstances apply to professional decoders.
The same is not true of consumer receivers in which the transposition local oscillator is liable to drift by several MHz: carrier recovery over such a wide range then becomes difficult and may turn out to be impossible with a signal having a low Ec/N.sub.0, as is frequently the case for signals having highly redundant error protection encoding.
It might be thought that better results would be obtained by simultaneously recovering the carrier and the clock by means of a highly non-linear recovery loop with feedback.
The present invention proposes performing clock recovery prior to carrier recovery so as to enable sampling to be performed at instants when the opening of the eye diagram is optimum, and for this purpose makes use of the fact that the correspondence between the frequency of the local clock and the frequency of the received signal clock is generally very accurate, unlike the correspondence between the transposed carrier frequency and the local clock frequency for transposing to baseband. The reason is that because the local clock operates at fixed frequency, it can be crystal stabilized and
REFERENCES:
patent: 4466108 (1984-08-01), Rhodes
patent: 5036296 (1991-07-01), Yoshida
Grimm Siegfried H.
Matra Communication
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