Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1997-05-13
2001-01-16
Ghebretinsae, Temesghen (Department: 2734)
Pulse or digital communications
Receivers
Angle modulation
C375S340000, C455S229000
Reexamination Certificate
active
06175600
ABSTRACT:
FIELD OF THE INVENTION
The field of the invention is that of digital communications, in particular by satellite, and the invention relates more particularly to an apparatus for detecting the presence (or absence) of a carrier wave carrying a signal received by a receiver, said carrier wave being modulated by a digital signal at a clock rate, also called a symbol clock.
The invention applies to any type of digital modulation, for example to phase shift modulation (M-PSK for M-state phase shift keying) or more generally to amplitude and phase modulation (M-APSK, for M-state amplitude and phase shift keying), and modulation of the kind used in particular in time division multiple access (TDMA), in frequency division multiple access (FDMA), or in single carrier per change (SCPC) telecommunications.
The invention can apply to detecting a carrier (e.g. of the 2-PSK or the 4-PSK type) before demodulating the received signal, such demodulation consisting of synchronization and decision-making processes that enable the digital signal to be restored, in the context of satellite transmission.
BACKGROUND OF THE INVENTION
In this context, three major problems arise:
the bandwidth of the carrier can be less than the frequency uncertainties of the satellite link. These uncertainties can originate in various ways: low relative stability of the local oscillators in the frequency transposition equipment, the presence of a Doppler effect, satellite drift, etc. Consequently, it must be possible to accommodate a large frequency difference between the received carrier and the local demodulation oscillator;
the carrier may be absent or present (transmission in SCPC mode); and
carrier detection must take place quickly (particularly with DTMA transmission) and reliably, in spite of a bit energy
oise (Eb/No) ratio close to 2 dB without information coding.
More precisely, the detection system of the invention must be capable of being integrated in a receiver that is entirely digital, with detection taking place prior to demodulation, and in spite of unfavorable noise conditions (Eb/No=2 dB). Also, detection must be capable of being performed quickly, with a maximum duration in the vicinity of 100 symbol times.
A system is known for detecting the presence or the absence of a carrier wave based on establishing a phase histogram of the sampled signal. Detecting non-uniformity in the histogram indicates the presence of a carrier wave.
FIG. 1
shows the constellation of a 4-PSK type signal as received, together with the observed phases &thgr;
i
. The axes representing the carriers in phase quadrature are referenced P and Q. On each new received symbol (assuming that the clock rate has already been recovered), the difference &Dgr;&thgr;
i
=(&thgr;
i
-&thgr;
i−1
) modulo ±&pgr;/4 is calculated. On the basis of the observed phase differences &Dgr;&thgr;
i
, a histogram of phase differences is constructed as shown in FIG.
2
. In this case, the histogram has four classes (4-PSK) and is obtained by incrementing the class corresponding to &Dgr;&thgr; and by decrementing by the same amount the class &Dgr;&thgr;
k-N
where N is the analysis window expressed as a number of symbols. In the presence of a signal that is actually being received, as opposed to in the presence of noise, the accumulated phase differences &Dgr;&thgr;
i
give rise to a difference D (where D=&Sgr;&Dgr;&thgr;
max
-&Sgr;&Dgr;&thgr;
min
) greater than a predefined threshold value.
The drawback of that method is that in the presence of noise or a high level of frequency drift, phase differences are observed to become more uniform and the threshold value is not reached, in spite of a carrier being present. If the threshold value is made smaller, than the false alarm rate becomes too great. Consequently, that method cannot be applied to signals suffering from large amounts of frequency drift between the carrier frequency and the demodulation frequency, or to signals suffering from a large amount of noise.
In addition, that method assumes that the clock has already been acquired, and that is expensive in time.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the present invention is to remedy those drawbacks.
More precisely, one of the objects of the invention is to provide a digital system for detecting the presence or the absence of a signal carrier wave that is present at a given clock frequency, which system combines speed, reliability, operation at a poor Ed/No ratio (close to 2 dB), and is relatively insensitive to drift in the frequency of the carrier wave.
Another object is to provide such a system in which detection reliability depends little on the Eb/No ratio.
A further object is to simplify implementation of such a detection system by combining it with a timing estimator, e.g. of the Oerder and Meyr type.
These objects, and others which appear below, are achieved by a system for detecting the presence of a carrier wave in a digital signal at a given clock frequency, the system comprising means that, at each symbol time, provide at least two baseband samples of the digital signal, and the system comprising:
a timing estimator responding to the baseband samples to provide an error signal corresponding to the phase error between the clock frequency that is to be recovered and the local clock frequency; and
detection means generating a detection signal indicating that the carrier wave has been detected, whenever a level representative of the variance of the error signals reaches a threshold value.
Observing a level representative of the variance in the error signal makes it possible to avoid being affected by variations in the error signal over time.
Advantageously, the detection means include a variance calculator for calculating the variance of the error signal.
Preferably, the system of the invention includes means for removing &pgr;/2 phase ambiguity between the timing estimator and the detection means.
Advantageously, the timing estimator is of the Oerder and Meyr type.
The invention also provides a digital signal receiver including such a detection system.
REFERENCES:
patent: 3746993 (1973-07-01), Ahmed et al.
patent: 4068174 (1978-01-01), Svelund
patent: 4338574 (1982-07-01), Fujita et al.
patent: 4554508 (1985-11-01), Haque
patent: 5638406 (1997-06-01), Sogabe
patent: 07240765 (1995-07-01), None
F. Classen et al, “An All Feedforward Synchronization Unit for Digital Radio”,Personal Communication Freedom Through Wireless Technology,NJ, May 18-20, 1993, Conf. 43, 18, Institute of Electrical and Electronics Engineers, pp. 738-741.
N. A. D'Andrea et al, “Comparison of Carrier Recovery Methods for Narrow-Band Polyphase Shift Keyed Signals”, IEEE Global Telecommunications Conference & Exhibition, CA, Nov. 18-Dec. 1, 1988, vol. 3 of 3, No. 1988, Institute of Electrical & Electronics Engineers, pp. 1474-1478.
M. Oerder et al, Digital Filter and Square Time Recovery.
Ferrad Abdelkrim
Guillemain Christian
Alcatel Telspace
Ghebretinsae Temesghen
Sughrue Mion Zinn Macpeak & Seas, PLLC
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