Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1998-03-12
2001-06-12
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Angle modulation
C375S152000, C375S286000, C375S340000
Reexamination Certificate
active
06246728
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to suppressor apparatus designed to protect radio receivers, and in particular direct conversion type radio receivers, against interfering DC component shifts that can affect the received signals, and to radio sets including such apparatus.
The invention is more particularly applicable to receivers used in digital cellular radio networks, and in which reception can be disturbed by DC component shifts caused by interference in bursts. The invention also relates to the suppression method implemented.
BACKGROUND OF THE INVENTION
It is possible to simplify radio receivers, and in particular radio receivers equipping radio-communications terminals in mobile radiotelephone networks, by not using an intermediate frequency in the receivers and by going directly from the received radio-frequency signal to a baseband signal. It is then possible to omit the elements of the receiver that involve using an intermediate frequency.
One solution for avoiding use of an intermediate frequency is mentioned in the preamble of Document EP-A-0 474 615. That solution consists of retrieving the wanted signal contained in the received signal whose carrier is a signal of the f=Acos&ohgr;t+&phgr; (type, by using the results of multiplying the received signal by a local cos&ohgr;t signal, and the results of multiplying said received signal by a local sin&ohgr;t signal, where &ohgr; has the same value for the received signal and for both local signals.
The results of those two multiplications correspond respectively to an in-phase signal element I and to a quadrature signal element Q, each of which comprises difference components that are in the vicinity of zero frequency, and sum components that are in the vicinity of twice the frequency of the carrier of the received signal. A low-pass filter makes it possible to retain only the difference components, also referred to as “zero frequency” components, of the I and Q signal elements to obtain the wanted portion of the received signal.
One of the problems that arises with receivers using that technique is that interfering transmissions can create DC component shifts in baseband that degrade the wanted signal obtained at the outlet of a receiver, which wanted signal can be received at a power level that is much lower than the power of the interfering DC component.
That can apply to digital cellular radio networks of the GSM type, when a noise source disturbs reception in a terminal by adding a DC component shift or offset to the wanted signal that the terminal is to receive, in particular if the noise-source operates in bursts, i.e. it transmits on a carrier that it time-shares, as is conventional in time-division multiple access mobile radio networks. In which case, the DC component shift does not necessarily disturb the entire received signal, and it acts differently on the in-phase component I and on the quadrature component Q of the received signal.
To demodulate the received signal correctly, it is necessary to take that problem into account, and in particular to determine the characteristics of the interfering shifts affecting each of the components of the received signal, so as to enable said components to be corrected before they are used.
OBJECTS AND SUMMARY OF THE INVENTION
The invention therefore provides apparatus for suppressing interfering DC component shifts affecting reception of a transmitted radio-frequency signal that is time-division multiplexed in the form of a succession of signal elements referred to as “bursts”, each of which is made up of the same number “m” of samples.
According to a characteristic of the invention as shown in
FIGS. 1 and 1A
, the apparatus comprises means
10
for normalizing the power of the successively received bursts relative to the power of the resulting wanted signal, means
8
,
8
a
for determining the instantaneous value of the DC component for burst segments comprising the same integer number m
of samples in the same burst, and mean values per segment that are obtained cumulatively on the basis of successive bursts, means
8
b
for determining value and position characteristics for the DC component shift for each received burst, by computing on the basis of the mean values and instantaneous values per segment, each shift being assumed to have at least approximately the same time position relative to the burst that it degrades as the following shift relative to the following burst, and corrector means
9
for subtracting the shift characteristics determined for each burst from the baseband signal obtained after normalization.
According to a characteristic of the invention, the apparatus includes means
8
c
for determining the instantaneous DC component value for the in-phase component I and the quadrature component Q at zero frequency from a received radio signal, and means
8
d
for determining a cumulative mean value per component segment of a received burst, firstly for the in-phase component and secondly for the quadrature component, by summing the instantaneous value obtained for said segment, as weighted by a measurement contribution factor, with the cumulative value obtained for the preceding received burst for the same signal, as weighted by a forgetting factor, the sum of the forgetting factor and the measurement contribution factor being equal to one.
According to a characteristic of the invention, the apparatus includes means for determining a DC component shift position that is accurate to within one bit for each received burst on the basis of the DC component values before and after a transition caused by a DC component shift, in the in-phase component or in the quadrature component of a burst, which values have been determined for the segment to which the minimum error for said burst corresponds, and on the basis of an intermediate value corresponding to the DC component shift determined for the “n” bits of the minimum error segment.
According to a characteristic of the invention, the apparatus includes corrector means taking into account the differences between the DC component values “dch” & “dcb”, and between the DC component values “dcqh” & “dcqb”, as determined for a burst for suppressing the interfering shift DC components that affect the in-phase component and the quadrature component of said burst, on the basis of the shift position, to within one bit, as determined for said burst.
The invention also provides a radio receiver of the direct conversion type delivering two baseband components, namely an in-phase component I and a quadrature component Q, on the basis of a received radio-frequency signal, the receiver being more particularly designed to receive a transmitted signal that is time-division multiplexed in the form of a succession of signal elements referred to as “bursts”, each of which is made up of the same number “m” of samples.
According to a characteristic of the invention, the receiver includes apparatus as defined above for suppressing interfering DC component shifts, which apparatus contained at least in part in a digital signal processor and downstream from a radio converter stage delivering the in-phase component I and the quadrature component Q of a baseband signal on the basis of a radio-frequency signal received by the receiver.
The invention further provides a method of suppressing interfering DC component shifts affecting reception of a transmitted radio-frequency signal that is time-division multiplexed in the form of a succession of signal elements referred to as “bursts”, each of which is made up of the same number “m” of samples.
According to a characteristic of the invention, the method comprises:
at least one normalization step in which the power of the successively received bursts is normalized relative to the power of the resulting wanted signal;
a step in which the instantaneous value of the DC component is determined for burst segments that comprise the same integer number m
of samples in the same burst, and mean values are determined per segment that are obtained cumulatively on the basis
Galligo Michel
Vogel Hélène
Alcatel
Chin Stephen
Liu Shuwang
Sughrue Mion Zinn Macpeak & Seas, PLLC
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