U.S. patent application number 09/426782 was filed with the patent office on 2001-11-15 for communications terminal having a receiver and method for removing known interferers from a digitized intermediate frequency signal.
Invention is credited to CELANDER, JAN, LINDQUIST, BJORN, MATTISSON, SVEN.
Application Number | 20010040932 09/426782 |
Document ID | / |
Family ID | 23692180 |
Filed Date | 2001-11-15 |
United States Patent
Application |
20010040932 |
Kind Code |
A1 |
LINDQUIST, BJORN ; et
al. |
November 15, 2001 |
COMMUNICATIONS TERMINAL HAVING A RECEIVER AND METHOD FOR REMOVING
KNOWN INTERFERERS FROM A DIGITIZED INTERMEDIATE FREQUENCY
SIGNAL
Abstract
A communications terminal having a receiver and a method is
provided that substantially removes a known interferer from a
digitally translated intermediate frequency signal. More
specifically, the receiver includes an antenna for receiving a
signal, and at least one combination of a mixer and filter for
translating in the analog domain the signal to the intermediate
frequency signal while maintaining separation from baseband. The
receiver also includes a digitizer for digitally translating the
intermediate frequency signal containing the known interferer from
the analog domain into the digital domain, and an interference
cancellation system for removing the known interferer from the
digitally translated intermediate frequency signal by utilizing
either a DC offset compensator or a correlator compensator.
Inventors: |
LINDQUIST, BJORN; (BJARRED,
SE) ; CELANDER, JAN; (LUND, SE) ; MATTISSON,
SVEN; (BJARRED, SE) |
Correspondence
Address: |
WILLIAM J TUCKER
JENKENS & GILCHRIST PC
3200 FOUNTAIN PLACE
1445 ROSS AVENUE
DALLAS
TX
752022799
|
Family ID: |
23692180 |
Appl. No.: |
09/426782 |
Filed: |
October 22, 1999 |
Current U.S.
Class: |
375/346 ;
455/296 |
Current CPC
Class: |
H04B 15/04 20130101;
H04B 1/0007 20130101 |
Class at
Publication: |
375/346 ;
455/296 |
International
Class: |
H04B 001/10 |
Claims
What is claimed is:
1. A communications terminal having a receiver for substantially
removing a known interferer from an intermediate frequency signal,
said communications terminal comprising: at least one combination
of a mixer and filter for translating in an analog domain a
received signal to the intermediate frequency signal while
maintaining separation from baseband; a digitizer for digitally
translating the intermediate frequency signal containing the known
interferer from said analog domain into a digital domain; an
interference cancellation system, coupled to the digitizer, for
substantially removing the known interferer from the digitally
translated intermediate frequency signal; and a digital
demodulator, coupled to the interference cancellation system, for
demodulating the digitally translated intermediate frequency signal
after removing the known interferer therefrom.
2. The communications terminal of claim 1, wherein said known
interferer further includes a first unmodulated interferer centered
in a middle of a channel used by the receiver, said interference
cancellation system further including: a frequency adjustor for
rotating in digital domain the digitally translated intermediate
frequency signal to baseband; and a DC offset compensator, coupled
to the frequency adjuster, for removing a DC offset signal within
the rotated intermediate frequency signal, said DC offset signal
due to the first unmodulated interferer.
3. The communications terminal of claim 1, wherein said known
interferer further includes a first unmodulated interferer centered
in a middle of a channel used by the receiver, said interference
cancellation system further including: said digitizer for
translating the digitally translated intermediate frequency signal
containing the known interferer down to baseband; and a DC offset
compensator, coupled to the digitizer, for removing a DC offset
signal attributable to the first unmodulated interferer within the
intermediate frequency signal.
4. The communications terminal of claim 2, wherein said first
unmodulated interferer further includes a 72nd harmonic of a 13 MHZ
internal clock, and said channel further includes channel 5 defined
by a Global System for Mobile Communications Standard.
5. The communications terminal of claim 2, wherein said first
unmodulated interferer further includes a 73rd harmonic of a 13 MHZ
internal clock, and said channel further includes channel 70
defined by a Global System for Mobile Communications Standard.
6. The communications terminal of claim 2, wherein said frequency
adjustor further includes a selected one of a derotation unit and a
frequency normation unit.
7. The communications terminal of claim 1, wherein said receiver
further includes a selected one of a heterodyne receiver, a
superheterodyne receiver, a double superheterodyne receiver and a
low intermediate frequency receiver.
8. The communications terminal of claim 1, wherein said digitizer
further includes a selected one of an intermediate frequency
sampling receiver and a phase-amplitude digitizer.
9. The communications terminal of claim 1, wherein said known
interferer having a known waveform further includes a modulated
interferer or a second unmodulated interferer off-centered to a
middle of a channel used by the receiver, said interference
cancellation system further including a digital signal processing
means for removing the known interferer from the digitally
translated intermediate frequency signal.
10. A receiver for substantially removing a centered unmodulated
interferer from an intermediate frequency signal, said receiver
comprising: at least one combination of a mixer and filter for
translating in an analog domain a received signal to the
intermediate frequency signal while maintaining separation from
baseband; a digitizer for digitally translating the intermediate
frequency signal containing the centered unmodulated interferer
from said analog domain into a digital domain; an interference
cancellation system, coupled to the digitizer, for substantially
removing a DC offset within the digitally translated intermediate
frequency signal, said DC offset due to the centered unmodulated
interferer being located within a middle of a channel used by the
receiver; and a digital demodulator, coupled to the interference
cancellation system, for demodulating the digitally translated
intermediate frequency signal after removing the DC offset
therefrom.
11. The receiver of claim 10, wherein said interference
cancellation system further includes: a frequency adjustor for
rotating in digital domain the digitally translated intermediate
frequency signal to baseband; a first convertor for converting the
rotated intermediate frequency signal from a log-polar format to a
Cartesian format; a DC offset compensator for removing the DC
offset within the converted intermediate frequency signal; and a
second convertor for converting the intermediate frequency signal
having the removed DC offset from the Cartesian format to the
log-polar format.
12. The receiver of claim 10, further comprising: said digitizer
for translating the digitally translated intermediate frequency
signal containing the centered unmodulated interferer down to
baseband; and said interference cancellation system further
includes a DC offset compensator for removing the DC offset signal
attributable to the centered unmodulated interferer.
13. The receiver of claim 10, wherein said centered unmodulated
interferer further includes a 72nd harmonic of a 13 MHZ internal
clock, and said channel further includes channel 5 defined by a
Global System for Mobile Communications Standard.
14. The receiver of claim 10, wherein said first unmodulated
interferer further includes a 73rd harmonic of a 13 MHZ internal
clock, and said channel further includes channel 70 defined by a
Global System for Mobile Communications Standard.
15. The receiver of claim 11, wherein said frequency adjustor
further includes a selected one of a derotation unit and a
frequency normation unit.
16. The receiver of claim 10, wherein said digitizer further
includes a selected one of an intermediate frequency sampling
receiver and a phase-amplitude digitizer.
17. A receiver for substantially removing an off-centered
unmodulated interferer or a modulated interferer each having a
known waveform from an intermediate frequency signal, said receiver
comprising: at least one combination of a mixer and filter for
translating in an analog domain a received signal to the
intermediate frequency signal while maintaining separation from
baseband; a digitizer for digitally translating the intermediate
frequency signal from said analog domain into a digital domain,
said intermediate frequency signal containing the off-centered
unmodulated interferer or the modulated interferer; an interference
cancellation system, coupled to the digitizer, for substantially
removing the known waveform from the digitally translated
intermediate frequency signal, said known waveform associated with
the off-centered unmodulated interferer or the modulated
interferer; and a digital demodulator, coupled to the interference
cancellation system, for demodulating the digitally translated
intermediate frequency signal after removing the known waveform
therefrom.
18. The receiver of claim 17, wherein said interference
cancellation system further includes: a first convertor for
converting the digitally translated intermediate frequency signal
from a log-polar format to a Cartesian format; a correlator for
correlating the digitally translated intermediate frequency signal
and the known waveform to enable the removal of the known waveform
from the digitally translated intermediate frequency signal; and a
second convertor for converting the digitally translated
intermediate frequency signal having the removed known waveform
from the Cartesian format to the log-polar format.
19. A method for substantially removing a known interferer from an
intermediate frequency signal, said method comprising the steps of:
receiving a signal; translating in an analog domain the signal to
the intermediate frequency signal while maintaining separation from
baseband; digitally translating the intermediate frequency signal
containing the known interferer from said analog domain into a
digital domain; removing the known interferer from the digitally
translated intermediate frequency signal; and demodulating the
digitally translated intermediate frequency signal after removing
the known interferer therefrom.
20. The method of claim 19, wherein the the known interferer
further includes a first unmodulated interfering signal centered in
a middle of a channel used by the receiver, said step of removing
the known interferer further including the steps of: translating
the digitally translated intermediate frequency signal containing
the known interferer down to baseband; and removing a DC offset
signal attributable to the first unmodulated interfering signal
within the intermediate frequency signal.
21. The method of claim 19, wherein the known interferer further
includes a first unmodulated interfering signal centered in a
middle of a channel used by the receiver, said step of removing the
known interferer further including the steps of: rotating in said
digital domain the digitally translated intermediate frequency
signal to baseband; and removing a DC offset within the
intermediate frequency signal, said DC offset due to the first
unmodulated interferer and removed by using a DC offset
compensator.
22. The method of claim 21, wherein said first unmodulated
interferer further includes a 72nd harmonic of a 13 MHZ internal
clock, and said channel further includes channel 5 defined by a
Global System for Mobile Communications Standard.
23. The method of claim 21, wherein said first unmodulated
interferer further includes a 73rd harmonic of a 13 MHZ internal
clock, and said channel further includes channel 70 defined by a
Global System for Mobile Communications Standard.
24. The method of claim 19, wherein said receiver further includes
a selected one of a heterodyne receiver, a superheterodyne
receiver, a double superheterodyne receiver and a low intermediate
frequency receiver.
25. The method of claim 19, wherein said known interferer having a
known waveform further includes a modulated interferer or a second
unmodulated interferer off-centered to a middle of a channel used
by the receiver, said step of removing the known interferer further
includes the step of correlating the digitally translated
intermediate frequency signal and the known waveform to enable the
removal of the known waveform from the digitally translated
intermediate frequency signal, said known waveform is associated
with the second unmodulated interfering signal or the modulated
signal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field of the Invention
[0002] The present invention generally relates to the wireless
telecommunications field and, in particular, to a communications
terminal having a receiver and method for substantially cancelling
known interferers from a digitally translated intermediate
frequency (IF) signal.
[0003] 2. Description of Related Art
[0004] A mobile phone incorporates many components including a
receiver that operates to demodulate signals received from a
transmitter by removing a carrier signal and outputting a desired
signal. Receivers commonly used today include homodyne receivers,
heterodyne receivers, superheterodyne receivers and double
superheterodyne receivers.
[0005] Of course, the above-mentioned receivers are sensitive to
interference from external jamming equipment and internal
equipment. For example, the mobile phone used in accordance with
the Global System for Mobile Communications (GSM) Standard has an
interference problem attributable to a 13 MHZ internal clock which
generates 72nd and 73rd harmonics that interfere with a desired
signal on channels 5 and 70, respectively.
[0006] Currently, the interference caused by the clock harmonics
may be addressed by paying particular attention to the shielding
and decoupling within the mobile phone which can be a very
expensive and complicated task. Another technique used today to
cancel the known interferers (e.g., 72nd and 73rd harmonics) caused
by the 13 MHZ internal clock, is to in the receiver translate in
the analog domain the signals received from the antenna down to
baseband so that the clock harmonics are translated to a direct
current (DC) offset voltage which is then removed by analog or
digital DC-offset cancellation techniques. Unfortunately, the
receivers that translate the received signals down to baseband in
the analog domain must also remove a lot of extraneous DC offset
voltages that are created by mixers used to produce the analog
baseband signals or by analog circuitry used to process the analog
baseband signals. Extraneous DC offset voltages may also be due to
component mismatches and carrier leakage. Consequently, the
extraneous DC offset voltages must be removed since they can be
larger than the desired signal. The extraneous DC offsets is one
reason why digitizing an IF signal is more attractive than
digitizing baseband signals.
[0007] Accordingly, there is a need for a communications terminal
having a receiver and method that effectively removes known in-band
interferers from received IF signals that are initially separated
from baseband signals in analog domain and later digitized in the
digital domain, so as to avoid the undesirable extraneous DC offset
voltages associated with traditional communications terminals. This
need and other needs are satisfied by the communications terminal
and method of the present invention.
BRIEF DESCRIPTION OF THE INVENTION
[0008] The present invention is a communications terminal having a
receiver and method that substantially removes a known interferer
from a received IF signal that is digitized. More specifically, the
receiver includes an antenna for receiving a signal, and at least
one combination of a mixer and filter for translating the signal in
the analog domain to an IF signal while maintaining separation from
the baseband. The receiver also includes a digitizer for digitally
translating the IF signal containing the known interferer from the
analog domain into the digital domain, and an interference
cancellation system for removing the known interferer from the
digitally translated IF signal by utilizing some signal processing
means.
[0009] In accordance with the present invention, there is provided
a method and receiver using a digital DC offset compensator to
remove from a digitized IF signal a known interferer including a
centered unmodulated interferer caused by the 72nd and 73rd
harmonics of a 13 MHZ internal clock.
[0010] Also in accordance with the present invention, there is
provided a method and receiver using a digital signal processing
block to remove from a digitized IF signal a known interferer
including an off-centered unmodulated interferer or a modulated
interferer each having a known waveform.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] A more complete understanding of the method and apparatus of
the present invention may be had by reference to the following
detailed description when taken in conjunction with the
accompanying drawings wherein:
[0012] FIG. 1 is a block diagram of a communications terminal
having a double superheterodyne receiver in accordance with the
present invention;
[0013] FIG. 2 is a block diagram illustrating in greater detail a
first embodiment of an interference cancellation system
incorporated within the double superheterodyne receiver shown in
FIG. 1;
[0014] FIG. 3 is a graph indicating the improved performance of the
communications terminal after removing a known interferer such as a
centered unmodulated interferer using the first embodiment of the
interference cancellation system shown in FIG. 2;
[0015] FIG. 4 is a block diagram illustrating in greater detail a
second embodiment of the interference cancellation system
incorporated within the double superheterodyne receiver shown in
FIG. 1;
[0016] FIG. 5 is a block diagram illustrating in greater detail an
exemplary correlator compensator shown in FIG. 4; and
[0017] FIG. 6 is a block diagram of a communications terminal
having a low intermediate frequency receiver in accordance with the
present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
[0018] Referring to the Drawings, wherein like numerals represent
like parts throughout FIGS. 1-6, there are disclosed two
embodiments of an exemplary communications terminal 100 in
accordance with the present invention.
[0019] Although the communications terminal 100 will be described
with reference to a double superheterodyne receiver 110 (FIGS. 1-5)
and a low intermediate frequency receiver 610 (FIG. 6), it should
be understood that the present invention can be used with other
types of receivers including, for example, heterodyne and
superheterodyne receivers that do not translate the IF signals down
to baseband in the analog domain. Accordingly, the communication
terminal 100 and double superheterodyne receiver 110 described
should not be construed in such a limited manner.
[0020] Referring to FIG. 1, there is illustrated the basic
components associated with the communications terminal 100 and
double superheterodyne receiver 110 of the present invention.
Basically, the communications terminal 100 utilizes an interference
cancellation system 130 within the double superheterodyne receiver
110 to effectively cancel known interferers from a digitized IF
signal by removing a DC offset voltage caused by a centered
unmodulated interferer (first embodiment see FIGS. 2 and 3) or by
removing a known waveform of an off-centered unmodulated interferer
or a modulated interferer (second embodiment see FIG. 4). In other
words, the communications terminal 100 effectively removes known
interferers from IF signals that are initially separated from
baseband in analog domain and later digitized in digital domain so
as to avoid the undesirable extraneous DC offset voltages
associated with traditional communications terminals that translate
the IF signals to baseband in the analog domain.
[0021] The communication terminal 100 can be any communication
device that communicates over a wireless communication link such as
a cordless or cellular mobile phone, two way radio, MODEM
(modulator-demodulator), radio, base station, or the like. The
communications terminal 100 incorporates the exemplary double
superheterodyne receiver 110 which includes an antenna 112 that
receives a signal on one of a plurality of up-link channels and
outputs the signal to a radio frequency (RF) filter 114. The RF
filter 114 permits the received signal located within a
predetermined pass band (e.g., 925 MHZ to 960 MHZ for the EGSM
standard) to pass through an amplifier 116 and into a first mixer
118, while attenuating the received signal located outside the pass
band.
[0022] The first mixer 118 steps down or translates in the analog
domain the frequency of the received signal passed by the RF filter
114 to a predetermined first IF signal which is separated from
baseband and is inputted to a first IF filter 120. A first
oscillator 121 connected to the first mixer 118 enables the
frequency translation of the signal to the first IF signal.
[0023] The first IF signal is filtered by the first IF filter 120
and output to a second mixer 122 that combines the first IF signal
with a signal from a second oscillator 123. The second mixer 122
using the signal from the second oscillator 123 operates to step
down or translate in the analog domain the frequency of the first
IF signal to a predetermined second IF signal. The second IF signal
is separated from baseband and is input to a second IF (IF) filter
124. It should be understood that the bandwidth of the second IF
filter 124 is preferably equal to the bandwidth of one channel
(e.g., channel 5 or channel 70 in GSM) and each channel in the
frequency band passed by the second IF filter 124 has a unique
center or middle frequency that contains most, if not all, of the
information of the signal received by the antenna 112.
[0024] The second IF signal is input to a limiter 126 operable to
prevent an amplitude of the second IF signal from exceeding a
predetermined level and operable to preserve the shape of the
second IF signal at amplitudes less than the predetermined level.
The limiter 126 outputs the second IF signal to a digitizer
128.
[0025] The digitizer 128 operates to digitize or digitally
translate in the digital domain the second IF signal containing a
know interferer (described below) to another frequency which could
be a new IF or baseband signal. The digitizer 128 can be referred
to as a phase-amplitude digitizer or an IF sampling receiver. It
should be understood that the digitizer 128 should have enough
resolution in the analog-to-digital (A/D) conversion to handle the
known interferers, for instance a log amplitude A/D converter has a
limit as to what magnitude of the unmodulated co-channel
interferers can be tolerated because the resolution of the desired
signal is reduced with larger interferers.
[0026] The digitizer 128 outputs the digitally translated second IF
signal to the interference cancellation system 130 which
effectively cancels the known interferers included within the
digitally translated second IF signal. The known interferers are
classified as a centered unmodulated interferer described in
greater detail with respect to the first embodiment of the
interference cancellation system 130 (see FIGS. 2 and 3), or
classified as a modulated interferer or an off-centered unmodulated
interferer described in greater detail with respect the second
embodiment of the interference cancellation system (see FIG. 3).
After removing the known interferers from the digitized second IF
signal, a digital demodulator 132 coupled to the interference
cancellation system 130 operates to demodulate the remaining second
IF signal.
[0027] Referring to FIG. 2, there is illustrated in greater detail
the first embodiment of the interference cancellation system 130.
In the first embodiment, the digitizer 128 operates to digitally
translate in the digital domain the second IF signal to a baseband
signal so that the known interferer or centered unmodulated
interferer can be transformed to a DC offset voltage which is
removed by the interference cancellation system 130. More
specifically, the second IF signal which is not at baseband in the
analog domain is digitally translated in the digital domain by the
digitizer 128 to a baseband signal so that the only DC offset
voltage created is the DC offset voltage associated with the
centered unmodulated interferer. Also, any extraneous DC offset
voltages due to component mismatches and carrier leakage (for
example) are not created, in accordance with the present invention,
because the second IF signal sampled is "DC free" since it is
higher than the baseband frequency before entering the digitizer
128.
[0028] The centered unmodulated interferer associated with the
first embodiment is an unmodulated interferer centered in a middle
of a channel used by the communications terminal 100. For example,
the known centered unmodulated interferers can be caused by the
72nd and 73rd harmonics of the 13 MHZ internal reference clock and,
without employing the techniques of the present invention, the 72nd
and 73rd harmonics would interfere with the desired signal on
channels 5 and 70, respectively, used by the traditional GSM mobile
phone.
[0029] In accordance with the first embodiment, the interference
cancellation system 130 includes a frequency adjustor 140 operable
to phase rotate or "add a ramp"to the digitized phase samples in
the digitally translated second IF signal so as to be converted to
baseband. The phase rotation can be required because in GSM the
sampling frequency is 13/48 MHZ and the second IF signal is at 6
MHz. The frequency adjustor 140 may be referred to as a derotation
unit or a frequency normation unit.
[0030] The frequency adjustor 140 can be coupled to a first
convertor 142 that converts the rotated second IF signal from
log-polar format to a Cartesian (I and Q) format. Thereafter, a DC
offset compensator 144 operates to remove the DC offset voltages
within the converted second IF signal, where the DC offset voltage
is due to the known centered unmodulated interferer.
[0031] The DC offset compensator 144 can be coupled to a second
convertor 146 that converts the second IF signal having the removed
DC offset voltage from the Cartesian format to a log-polar format.
It should be understood that the removal of the DC offset voltage
is independent of the type of detection or A/D conversion made and
that the second IF signal could be detected in log-polar format or
Cartesian format while the same interference cancellation scheme is
applied.
[0032] It should also be understood that the frequency adjustor 140
does not necessarily have to rotate the digitally translated
intermediate signal to baseband. For instance, the digitizer 128
can automatically move the digitally translated intermediate signal
down to baseband if the digitizer sub-samples the analog IF signal
on a sub-harmonic of the IF signal.
[0033] Referring to FIG. 3, there is a graph indicating the
improved performance of the GSM communications terminal 100 after
removing the centered unmodulated interferer using the first
embodiment of the interference cancellation system 130. The
traditional GSM communications terminal (e.g., GSM mobile phone)
would only tolerate an interferer that is approximately 5 dB below
a desired signal. However, when a centered unmodulated interferer
is approximately 20 dB stronger than the wanted signal it would
adversely effect the sensitivity of the traditional receiver by a
50% bit error rate (BER) (see alphanumeric "a"). In contrast, by
using the interference cancellation system 130 of the present
invention, the same 20 dB strong-centered unmodulated interferer
would provide only a 1 dB desensitization of the double
superheterodyne receiver 110 (see alphanumeric "b"). Therefore, at
least a 20 dB improvement with regard to interference by the
centered unmodulated interferers (e.g., 72nd and 73rd harmonics of
13 MH internal clock 123) can be expected which would substantially
improve the performance on certain channels (e.g., channels 5 and
70) used by the communications terminal 100.
[0034] Referring to FIG. 4, there is shown in greater detail the
second embodiment of the interference cancellation system 130 being
illustrated with prime referenced numbers. In the second
embodiment, the digitizer 128 operates to digitally translate the
second IF signal such that the known interferers having a known
waveform (including the modulated interferer or the off-centered
unmodulated interferer) can be removed by a correlation process
within the interference cancellation system 130'. The off-centered
unmodulated interferer is an unmodulated interferer off-centered in
a middle of a channel used by the communications terminal 100.
[0035] The interference cancellation system 130' may include a
first convertor 140' that converts the digitally translated second
IF signal containing the off-centered unmodulated interferer or
modulated interferer from log-polar format to Cartesian (I and Q)
format.
[0036] The first convertor 140' can be coupled to a correlator
compensator 142' that functions to correlate the digitally
translated second IF signal and the known waveform of the
interferer to enable the removal of the known waveform provided a
correlation factor is above a predetermined threshold. Again, the
off-centered unmodulated interferer or the modulated interferer
each have a known waveform. Thereafter, the correlator 142' can be
coupled to a second convertor 144' that converts the remaining
second IF signal from Cartesian format to log-polar format. The
correlator compensator 142' can also be applied directly on the
digitized phase and amplitude samples. Thus, blocks 140' and 144'
may not be needed.
[0037] The correlator compensator 142' (e.g., digital signal
processor) could, for example, include a sliding correlator 146'
that finds the timing and amplitude of the known interferer using a
delay block 148' and multiplier 150' (see FIG. 5). The correlator
compensator 142' further includes a subtractor block 152' that
subtracts the found interference from the received signal that has
been delayed by delay block 154'.
[0038] For example, assuming the desired signal has a frequency of
936.2 MHZ and the off-centered unmodulated interferer is 936 MHZ
(e.g., 72*13 MHZ), then if the second IF signal is digitally
translated down to baseband, the interferer is known sinusoidal at
200 KHz. The correlator 142' using a 200 KHz signal estimates a
multiplying constant and a timing for the 200 KHz known interferer
to be removed provided a correlation factor is greater than a
predetermined threshold. The multiplying constant could be
estimated by correlation estimates or Viterbi estimates. It should
be understood that the waveform or amplitude of the 200 KHz
interferer is known so that it can be recognized and removed.
[0039] The present invention can also address the situation where
two known interferers including the centered unmodulated interferer
(e.g., 936 MHZ) and the off-centered unmodulated interferer (e.g.,
936.2 MHZ) are digitally translated to get both a DC offset voltage
removed by the DC offset compensator 144 of the first embodiment
and a known waveform removed by the correlator 142' of the second
embodiment.
[0040] Referring to FIG. 6, there is illustrated the basic
components associated with the communications terminal 100 and the
low-IF receiver 610 of the present invention. Basically, the low-IF
receiver 610 includes an antenna 602 for receiving a signal from a
transmitter 604. The received signal is filtered by a band pass
filter (BPF) 606 designed to pass a desired frequency band (e.g.,
925 MHz to 960 MHz for the EGSM standard) from the received signal.
The filtered signal is amplified in a low noise amplifier (LNA) 608
and mixed in mixers 614a and 614b. More specifically, each mixer
614a and 614b steps down or translates, in analog domain, the
frequency of the amplified signal to a predetermined low-IF
frequency separated from baseband. The IF frequency could be half
the channel bandwidth or have a higher frequency. The mixers 614a
and 614b translate the amplified signal to an Inphase (I) component
and a Quadrature (Q) component both of which are in quadrature with
one another by using a local oscillator (LO) 616. The two low-IF
frequency signals (I and Q components) are then respectively
filtered by low pass or bandpass filters 618a and 618b, and then
digitized by analog-to-digital convertors (A/Ds) 620a and 620b.
Thereafter, the digitized low-IF frequency signals are input to the
interference cancellation system 130 which effectively cancels the
known interferers included within the digitized low-IF frequency
signals. The interference cancellation system 130 effectively
removes the interfering signal using a frequency adjustor and DC
offset compensator (see FIG. 2), or using a correlator (see FIGS.
4-5) as described above in the earlier examples. After removing the
known interferers from the digitized low-IF frequency signals, a
digital demodulator 622 coupled to the interference cancellation
system 130 operates to demodulate the remaining low-IF signals.
[0041] From the foregoing, it can be readily appreciated by those
skilled in the art that the present invention provides a method and
receiver using a DC offset compensator to remove from a digitized
IF signal a known interferer such as a centered unmodulated
interferer caused by, for example, the 72nd or 73rd harmonics of a
13 MHZ internal clock. Also the receiver and method as disclosed
can operate to remove from a digitized IF signal a known interferer
with a known waveform such as an off-centered unmodulated
interferer or a modulated interferer.
[0042] Although two embodiments of the present invention have been
illustrated in the accompanying Drawings and described in the
foregoing Detailed Description, it will be understood that the
invention is not limited to the embodiments disclosed, but is
capable of numerous rearrangements, modifications and substitutions
without departing from the spirit of the invention as set forth and
defined by the following claims.
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