Interference canceling device

Details Interference canceling device Interference canceling device

1999-11-23

2003-03-25

Le, Thanh Cong (Department: 2684)

Telecommunications

Carrier wave repeater or relay system

Transmitter/receiver feedback compensation

C455S278100, C455S307000

Reexamination Certificate

active

06539202

ABSTRACT:

BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to an interference canceling device used in a receiver for mobile communication or the like.
2) Description of Related Art
In mobile communications, both FDD (Frequency Division Duplex) systems and CDMA (Code Division Multiple Access) systems are employed. In the FDD system, it is possible to prevent the effects of interference by selecting frequency channels so as to exclude frequencies in which interference exits. In the CDMA system, however, because this system uses a broad frequency band for signal transmission, there is no means effective other than providing an interference canceling device within a receiver to directly eliminate interference.
FIG. 10
illustrates an example of an interference canceling device, which comprises a canceling circuit
10
provided in a main line
12
. The canceling circuit
10
consists of M notch filters
14
-i (M: a natural number larger than 2; and i: 1, 2, . . . M) connected in cascade.
Each notch filter
14
-i at the i-th stage comprises a local oscillator
16
oscillating at a frequency which differs from the oscillation frequencies of local oscillators in other stages, a mixer (MIX)
18
for mixing a signal on the main line
12
(hereinafter referred to as “main signal”) with the oscillation output from the local oscillator
16
to convert it to a signal having a lower intermediate frequency (IF), an IF notch filter
20
for filtering the IF output from the MIX
18
, and a mixer MIX
22
for mixing the filtered output from the IF notch filter
20
with the oscillation output from the local oscillator
16
to revert it to a signal having the original radio frequency (RF).
The IF notch filter
20
in the notch filter
14
-i is configured to eliminate specific frequency components from a main signal which has been converted into an IF. The frequency to be eliminated is fixed to an identical value for all the IF notch filters at every stage.
Further, the frequency band in which interference is considered to exist has a certain width. Therefore, in order to cover this band, the frequency at which the local oscillator of each stage oscillates is set to differ from one stage to another by an appropriate amount. It is possible to eliminate interference with regard to a frequency band having a certain width, by thus setting the oscillating frequency value of each oscillator in accordance with frequencies considered to include interference, and also with appropriate differences.
However, the device shown in
FIG. 10
has certain drawbacks. Among these, the canceling circuit
10
is provided on the main line
12
. Accordingly, when insertion loss and delay occurring in the canceling circuit
10
is large, a noise figure NF of the main signal output deteriorates and also adaptability over the whole system is lost. This problem is especially notable when the number of stages M of the notch filters is large. Also, in the device shown in
FIG. 10
, down conversion from RF to IF and up conversion from IF to RF are executed so as to facilitate signal processing. At the time of these conversions, noise generated at the local oscillator
16
is added to the main signal, and also distortion introduced at MIXs
18
and
22
is influential in deteriorating the channel quality.
To solve the above problems, it is appropriate to displace a canceling circuit
10
A from the main line
12
, as shown in FIG.
11
. In the canceling circuit
10
A, a signal which has been branched from a main signal by means of a directional coupler
24
is converted into a signal having an IF by the local oscillator
16
and the MIX
18
, and is then filtered by an IF narrow band BPF
20
A to extract frequency components in which interference is considered to exist. The filtered output from the IF narrow band BPF
20
A is further converted into a signal having an RF by the MIX
22
. Of the outputs from the MIX
22
, sum frequency components, namely frequency components corresponding to those of a main signal are recombined, as a cancellation signal, with the main signal by a directional coupler
28
. Further, at the time of this recombining, a control circuit
30
uses a directional coupler
32
to detect amplitude and phase of the main signal or detect whether or not there are signals at specific frequencies. On the basis of the detection result, phase shift and gain in a variable phase shifter
34
and a variable gain amplifier
36
, respectively, are controlled such that at the specific frequencies, namely at frequencies including interference, the cancellation signal has an amplitude which is of the same level as that of the main signal and a phase which is opposite to that of the main signal. In this device, the aforementioned problems, such as quality deterioration of a main signal found in the device of
FIG. 10
, does not occur.
However, in the device shown in
FIG. 11
a drawback remains in that a frequency band for interference elimination is much narrower than that in the device of FIG.
10
. Specifically, the IF narrow band BPF
20
A usually represents band pass characteristics in the vicinity of its pass band center frequency f
0
, as depicted in
FIGS. 12A
to
12
C. In particular, phase characteristics have a certain gradient, as shown in FIG.
12
C. Therefore, even if the aforementioned automatic control is executed in the circuit of
FIG. 11
by the variable phase shifter
34
and the variable gain amplifier
36
, phases of the cancellation signal and of the interference match only at one point in frequency (e.g., at f
0
) as shown in
FIG. 13
, and this leads to deterioration of cancellation characteristics due to phase rotation caused by frequency separation from the frequency f
0
. It is therefore difficult, or even impossible, to eliminate interference spreading over a frequency band having a certain width, for example, a frequency modulated wave, with such “pin-point” cancellation characteristics.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to overcome disadvantageous pin-point cancellation characteristics in the prior art, thereby allowing sufficient cancellation of interference spreading in a frequency band having a certain width. In the present invention, this object is achieved by improving the configuration of a narrow band BPF constituting a canceling circuit.
In the present invention, the narrow band BPF in the canceling circuit is configured such that it represents substantially flat phase characteristics in a specific frequency band having a certain width. A signal which has been branched from a main signal is filtered by this narrow band BPF to generate a cancellation signal in such a manner that a relation of “substantially the same amplitude level and opposite phases” can be established between the main signal and the cancellation signal over the entire range of the above-mentioned specific frequency band. Accordingly, within the above-mentioned specific frequency band, phase rotation of a cancellation signal caused by separation from the specific frequency can be eliminated (or can be suppressed to a level which can be substantially disregarded), thereby solving a problem in the prior art resulting from such phase rotation. In other words, the present invention enables elimination of interference ranging over a frequency band having a certain width, for example, a frequency modulated wave.
A narrow band BPF having flat phase characteristics, which is one feature of the present invention, can be implemented as a filter comprising, for example, a plurality of signal paths having filtering characteristics different from one another; distribution means for distributing signals which have been branched from a main signal to the signal paths; and combining means for combining the outputs from the signal paths to generate a filtered output. For example, a narrow band BPF having a substantially flat phase characteristics can be obtained relatively easily by providing a main signal path having predetermined filtering characteristics and a de

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