Abstract:
The method and device attenuate an interferer of substantially known spectrum in a communication signal which has been transmitted through a transmission channel and processed through a signal-processing receiver chain. In these method and device, the substantially known spectrum is used to generate, independently of the communication signal, at least one image representative of a replica of the interferer after processing through the signal-processing receiver chain. The generated image is subtracted from the processed communication signal to produce a subtraction signal, and quality-indicative parameters of the processed communication signal and the subtraction signal(s) are calculated. Finally, the quality-indicative parameters are used to select one of the processed communication signal and subtraction signal(s) for further processing through the receiver. When a plurality of images are generated, these images have different features, for instance different amplitudes or phases. At least one image can be feedback controlled in relation to the quality-indicative parameter of the subtraction signal corresponding to this image, in view of improving the quality-indicative parameter of this subtraction signal. This feedback control can also be responsive to the signal selection to improve the quality-indicative parameter of the subtraction signal until the latter signal is selected for further processing through the receiver.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of communications and more specifically to the attenuation of an unwanted interferer signal from a communication signal. 
   2. Brief Description of the Prior Art: 
   In a cellular telephone system, CDMA (Code Division Multiple Access), including IS-95 and WCDMA (Wideband Code Division Multiple Access), offers an inherent protection against narrowband interference. CDMA relies on a despreading operation to minimise the interference effects of undesired signals, including noise. However, this protection has a cost since it often requires a higher transmission power in order to provide the receiver with sufficient gain over the interference, i.e. to provide the required minimum signal-to-noise ratio. This requirement for higher transmission power reduces the range and the capacity of the network. On the uplink side, this also reduces the mobile station battery duration. 
   In an article entitled “CDMA-IC: A NOVEL CODE DIVISION MULTIPLE ACCESS SCHEME BASED ON INTERFERENCE CANCELLATION”, and published by IEEE (Institute of Electrical and Electronics Engineers) in 1992, Paul DENT, Björn GUDMUNDSON and Magnus EWERBRING disclose a method for reducing interference in the CDMA signal of one user caused by the CDMA signals of the other users. For that purpose, the user received at higher signal strength is first identified and despread, and this despread signal is subtracted from the composite signal. Thus, for other users&#39; signals not yet despread, interference is made less significant, and their signal-to-noise ratio is improved. Obviously, this method attenuates only interference related to other CDMA users&#39; signals. 
   In PCT international patent application published under No. WO 99/38270 on Jul. 29, 1999, Terence WIDDOWSON proposes an apparatus for decoding a spread spectrum signal which has been transmitted via a transmission channel. This spread spectrum signal comprises a wanted spread spectrum signal component and an unwanted narrowband signal component. The narrowband signal has been coded using a coding scheme with error correction capabilities. This apparatus includes a narrowband signal substractor in which:
         the composite signal is decoded and corrected according to the narrowband signal coding scheme to provide a decoded version of the narrowband signal;   the amplitude and phase characteristics of the transmission channel are estimated;   the decoded corrected signal is encoded according to the narrowband signal coding scheme to produce an estimated narrowband signal;   the phase and amplitude of the estimated narrowband signal are adjusted according to the estimate amplitude an phase characteristics; and   the adjusted signal is subtracted from the received composite signal to provide an estimate of the spread spectrum signal component.       

   The narrowband coding scheme can also detect uncorrectable errors. Upon detection of an uncorrectable error, the above mentioned subtraction is suppressed and the frequencies of the composite signal corresponding to the narrowband signal component are attenuated. 
   Therefore, the apparatus of PCT international patent application WO 99/38270 requires:
         decoding and correction of the composite signal;   estimation of the amplitude and phase characteristics of the transmission channel;   encoding of the decoded corrected signal according to the narrowband signal coding scheme;
 
to produce an estimated narrowband signal.
       

   Document EP 0 967 734 A2 (Suzuki) published on Dec. 29, 1999 relates to a DS-CDMA multiuser interference canceller for processing a received signal containing CDMA spread signals from a plurality of users. The function of this canceller is to remove, from the signal of each user, signals of other users and thereafter decode the signal of this specific user. This obviously eliminates interference caused by the spread signal of the other users. 
   For that purpose, interference replica signals identical to signal components of the users are generated from a received composite signal and, for every user, the interference replica signals from the other users are subtracted from the received composite signal. 
   When subtraction of the interference replica signals from the composite signal does not improve reception quality, outputting of these interference replica signals is turned off. Reception quality may be monitored through BER measurement. 
   Alternatively, a control process can be used to control the level of the interference replica signals, rather than controlling turning on and off of the interference cancellation process. 
   Again, processing of the received composite signal is required to obtain the interference replica signals. Also, adjustment of the level of the interference replica signal is made according to a predetermined pattern. More specifically, according to the teaching of this document, the level of the interference replical signal is increased with time so that the interference cancellation process is less susceptible to the effect of the delay in a control process. 
   OBJECTS OF THE INVENTION 
   An object of the present invention is to use the substantially known spectrum of an interferer to generate an image of this interferer independently of a received communication signal. 
   Another object of the present invention is to use a plurality of images having different features and a selection of one of these images which best attenuates the interferer. 
   A third object of the present invention is to use a feedback controlled image of the interferer to optimize attenuation of the interferer. 
   SUMMARY OF THE INVENTION 
   More specifically, in accordance with the present invention, there is provided a method for attenuating an interferer of substantially known spectrum in a communication signal which has been transmitted through a transmission channel and processed through a signal-processing receiver chain. This method comprises generating, from the substantially known spectrum and independently of the communication signal, at least one image representative of a replica of the interferer after processing through the signal-processing receiver chain. The image is subtracted from the processed communication signal to produce a subtraction signal, a quality-indicative parameter of the processed communication signal is calculated, and the quality-indicative parameter for the subtraction signal is computed. Finally, the method selects, in relation to the quality-indicative parameters, one of the processed communication signal and subtraction signal for further processing through the receiver. 
   According to preferred embodiments of the above method:
         the interferer is a narrowband AMPS interferer, the communication signal is a CDMA spread spectrum signal, the signal-processing receiver chain incorporates a despreading scheme whose function is to convert the spread spectrum signal to a despread signal, and the replica is a silent replica of the interferer including no voice of data component; and   the quality-indicative parameters are BER measurements of the despread signal and subtraction signal.       

   The present invention also relates to a method for attenuating an interferer from a communication signal which has been transmitted through a transmission channel and processed through a signal-processing receiver chain, wherein, a plurality of images of the interferer having different features are generated, for each interferer image the interferer image is subtracted from the processed communication signal to produce a corresponding subtraction signal, a quality-indicative parameter of the processed communication signal is calculated, and the quality-indicative parameter is calculated for every subtraction signal. The method finally comprises selecting, in relation to the quality-indicative parameters, one of the processed communication signal and subtraction signals for further processing through the receiver. 
   Preferably, the features of the images comprises amplitudes of these images. 
   The present invention is further concerned with a method for attenuating an interferer from a communication signal which has been transmitted through a transmission channel and processed through a signal-processing receiver chain, comprising generating an image of the interferer, subtracting the interferer image from the processed communication signal to produce a subtraction signal, calculating a quality-indicative parameter of the processed communication signal, computing the quality-indicative parameter for the subtraction signal, and selecting, in relation to the quality-indicative parameters, one of the processed communication signal and subtraction signal for further processing through the receiver. At least one feature of the image of the interferer is feedback controlled in relation to the quality-indicative parameter of the subtraction signal in view of improving this quality-indicative parameter of the subtraction signal. 
   Preferably, this feedback control takes into consideration the selection to improve the quality-indicative parameter of the subtraction signal until the subtraction signal is selected for further processing through the receiver. 
   The present invention also provides corresponding devices for implementing the above defined interferer attenuating methods. 
   Finally, the present invention relates to a method of generating an image of an interferer of substantially known spectrum for use in attenuating this interferer in a communication signal which has been transmitted through a transmission channel and processed through a signal-processing receiver chain. This method comprises generating a replica of the interferer from the substantially known spectrum and independently of the communication signal, and processing the interferer replica through transfer functions reproducing the signal-processing receiver chain in view of producing the interferer image. 
   Preferably, a gain is applied to the processed interferer replica. Also, both generating of an interferer replica and processing of the interferer replica can be conducted through a digital implementation such as software, firmware (e.g. PROM (Programmable Read-Only Memory)), ASIC (Application Specific Integrated Circuit). 
   Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the appended drawings: 
       FIG. 1  is a schematic flow chart showing the general principle of operation of a CDMA communication scheme in a cellular telephone network; 
       FIG. 2  is a graph illustrating the spectrum of an IS-95 or WCDMA wideband signal interfered by a narrowband AMPS interferer; 
       FIG. 3  is a graph illustrating the IS-95 or WCDMA wideband signal and the narrowband AMPS interferer of  FIG. 2  after processing through a CDMA despreading scheme; 
       FIG. 4  is a schematic block diagram of a generator for producing an image of the narrowband AMPS interferer of  FIG. 2 ; 
       FIG. 5  is a schematic block diagram of a first preferred embodiment of a device for attenuating the AMPS interferer from the IS-95 or WCDMA communication signal following despreading thereof; and 
       FIG. 6  is a schematic block diagram of a second preferred embodiment of a device for attenuating the AMPS interferer from the IS-95 or WCDMA communication signal following despreading thereof. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Although the preferred embodiment will be described with reference to CDMA communication in a cellular telephone system, it should be kept in mind that the present invention is not limited to this particular application. 
   As illustrated in  FIG. 1 , a CDMA transmitter  1  receives a baseband signal  10  containing, for example, “voice” or other signal of interest. The CDMA transmitter  1  then conducts a convolution  11  of this baseband signal  10  with a CDMA spreading code PN code1  identified by the reference  12  in  FIG. 1 . As well known to those of ordinary skill in the art, the convolution  10  with the spreading code PN code1    12  spreads the spectrum of the baseband signal  10  to produce a wideband communication signal  13  (spread spectrum signal) subsequently transmitted over a transmission channel  14 . In the case of a cellular telephone system, the transmission channel  14  uses air as transmission medium. 
   During transmission over the transmission channel  14 , the wideband communication signal  13  can accumulate noise of all types. The wideband signal  13  can also be subjected to narrowband interference such as an AMPS (Advanced Mobile Phone System) signal. Finally, other CDMA wideband signals (each resulting from convolution of a baseband signal with another CDMA code PN codex ) transmitted through a common transmission medium, such as air, can add to the wideband signal  13 . Just a word to recall that narrowband AMPS can have a high power and coexist with CDMA in the North American frequency spectrum. 
   A CDMA receiver  2 , for example located in a base or mobile station of a cellular telephone system, accordingly receives a wideband signal  15  charged with noise of many types, and eventually narrowband AMPS interference and interference from other CDMA wideband signals. 
   The receiver  2  comprises receive and IF (Intermediate Frequency) filters  16  conventionally used to isolate as much as possible the original wideband signal  13  from the received, adjacent spectrum interference (not shown). The signal from the filters  16  is subjected to a convolution  17  with the original CDMA spreading code PN code1    12  to despread the spectrum of the wideband signal  15  (despread signal) and thereby recover a baseband signal  18  incorporating the original baseband signal  10  contaminated with the above mentioned noise, narrowband interference and interference from other CDMA wideband signals. 
   CDMA transmission of signals through the transmission channel  14  is otherwise well known to those of ordinary skill in the art and accordingly, will not be further described in the present specification. 
   The preferred embodiment of the present invention is concerned with handling of a narrowband AMPS interferer in a CDMA transmission system. 
   Referring to  FIG. 2 , which illustrates an example of a CDMA wideband signal  15  containing a narrowband interferer  20 . In the illustrated example, the CDMA wideband signal  15  comprises a spread spectrum IS-95 or WCDMA signal  13  and a narrowband AMPS interferer  20 . 
   As illustrated in  FIG. 3 , processing of the CDMA wideband signal  15  through the filters  16  and convolution  17  thereof with the CDMA spreading code  12  despreads the spectrum of the CDMA wideband signal  13  to regenerate the baseband signal  10 , including voice or other signal of interest, and spreads the spectrum of the AMPS narrowband interferer  20 . In this way, the baseband signal  10  has a higher signal level  30  while the interferer  20  has a lower interferer signal level  31 . Therefore, the power level of the baseband signal  10  of  FIG. 3  is higher than the power level of the interferer  20 . 
   Of course, an established, minimun signal-to-noise ratio  32  ( FIG. 3 ) is required to ensure adequate communication. When the AMPS interferer  20  has too high a power level, the transmitter such as a mobile station of a cellular telephone system has to increase its transmission power to ensure a sufficient signal-to-noise ratio. This reduces the battery duration of the mobile station, this reduces the range of transmission since the power output of the mobile station is limited, and this obviously reduces the overall capacity of the cellular telephone system. 
   The present invention makes use of the knowledge of the interferer to cancel at least some components of this interferer. In the presently described preferred embodiment, a CDMA wideband signal is believed to be interfered by an AMPS signal. 
   For that purpose, an image of the AMPS interferer is first generated. Since the spectrum of the AMPS interferer is substantially known, this can be made through an image generator as illustrated in  FIG. 4  independently the wideband signals  13  and/or  15 . 
   It is usually correct to assume that speech is absent from an AMPS signal 60% of the time. Therefore, an AMPS replica  40  ( FIG. 4 ) wanted to have as much as possible the same spectrum as the AMPS interferer  20  is generated by an AMPS replica sub-generator  41  in the CDMA receiver  2 , assuming that the AMPS signal comes from a silent user. As indicated in the foregoing description, this is a true assumption for 60% of the time. This locally generated silent AMPS replica  40  will become an “image” of the AMPS interferer. 
   Still referring to  FIG. 4 , the silent AMPS replica  40  is processed through the same transfer functions as the transfer functions imposed by the receiver to the wideband signal  15  including the AMPS interferer  20 . 
   More specifically, the silent AMPS replica  40  is filtered by receive and IF filters  42  similar to the receive and IF filters  16  of the CDMA receiver  2  to produce a filtered silent AMPS signal  43 . Then, convolution  44  of the filtered AMPS signal  43  with the same CDMA code PN code1    12  is performed to spread the spectrum of the filtered silent AMPS signal  43  and produce a spread spectrum silent AMPS signal  45 . Finally, a gain (for example amplifier  46 ), adjustable or not, is applied to the spread spectrum silent AMPS signal  45  to produce the image  47  of the AMPS interferer  20 . 
   Production of the image  47  does not need to be implemented as illustrated in  FIG. 4 . Of course, a digital implementation of the image generator of  FIG. 4  can be built through software, firmware (e.g. PROM (Programmable Read-Only Memory)), ASIC (Application Specific Integrated Circuit) implementing the AMPS modulation and the necessary transfer functions. 
     FIG. 5  illustrates a first implementation of a method and device for attenuating, that is removing at least some components of the AMPS interferer  20  from the baseband signal  18  ( FIG. 1 ). 
   The baseband signal  18  is first produced by processing the wideband signal  15  of  FIG. 1  through the CDMA receiver chain  50 , including the filters  16  and convolution  17  of the CDMA receiver  2  ( FIG. 1 ). 
   Bit error analysis is performed in a BER (Bit Error Rate) measurement module  51  on the baseband signal  18  and the resulting BER ratio is applied to a corresponding input of a selector  52 . Bit error rate analysis measures the ratio of the amount of erroneous bits of a message to the total number of bits of the message received. Also, the BER ratio provides an estimation of the signal-to-noise ratio. Accordingly, the BER ratio constitutes a quality-representative parameter of the signal on which bit error analysis is performed. 
   At least one image generator  53  uses the scheme of  FIG. 4  or a corresponding digital implementation such as software, firmware (e.g. PROM (Programmable Read-Only Memory)), ASIC (Application Specific Integrated Circuit) to generate an image  54  of the AMPS interferer  20 . A subtractor  55  subtracts the image  54  from the baseband signal  18  to produce an image-removed baseband signal  56 . BER measurement module  57  then conducts a bit error analysis on the image-removed baseband signal  56  and the resulting ratio is applied to a corresponding input of the selector  52 . 
   If the BER ratio from module  51  is lower than the BER ratio from module  57 , this means that the image  54  from generator  53  is not a good representation of the AMPS interferer  20 . In that case, the image-removed baseband signal  56  is ignored and baseband signal  18  is selected by the selector  52  as the signal for further processing through the CDMA receiver  2 . 
   If the BER ratio from module  51  is larger than the BER ratio from module  57 , this means that at least a portion of the AMPS interferer  20  has been successfully cancelled from the baseband signal  18 , and that the quality of the communication has been improved. When the BER ratio is sufficiently low and, therefore, the signal-to-noise ratio is sufficiently high, the power of transmission can be reduced, hence increasing the capacity and range of the cellular telephone system and the mobile station battery duration. 
   Many images of the AMPS interferer  20  can be generated through a series of image generators  1 ,  2 , . . . , X. As an example, parallel images can be produced for:
         various amplitudes of the AMPS interferer;   various phases of the AMPS interferer;   replacing the silent speech by AMPS signals representing background noise or actual voice (inasmuch as there is a possibility to identify known patterns that are worthwhile to generate in this context);   etc.       

   For each image, a subtractor such as  55  subtracts the image from the baseband signal  18  to produce an image-removed baseband signal. A BER measurement module such as  57  then conducts a bit error analysis on the image-removed baseband signal and the resulting ratio is applied to a corresponding input of the selector  52 . The selector  52  selects the signal having the lowest BER ratio amongst the baseband signal  18  and the image-removed baseband signals as the output signal  58  for further processing through the CDMA receiver  2  ( FIG. 1 ). 
   CDMA signals comprise frames each having a duration of a few milliseconds. Normally, bit error analysis will be conducted through the modules such as  51  and  57  every frame. For example, since the AMPS interferer  20  carries voice and voice activity can change at any time, this is worthwhile repeating the above described process every CDMA frame. 
   The AMPS interferer cancelling process can be greatly improved by means of a feedback loop between the BER measurement module and the image generator.  FIG. 6  illustrates such an implementation, to remove components of the AMPS interferer  20  from the baseband signal  18  ( FIG. 1 ). 
   Again, the baseband signal  18  is first produced by processing the wideband signal  15  of  FIG. 1  through the CDMA receiver chain  50 , including the filters  16  and convolution  17  (see the CDMA receiver  2  of  FIG. 1 ). 
   Bit error analysis is performed in BER (Bit Error Rate) measurement module  60  on the baseband signal  18  and the resulting BER ratio is applied to a corresponding input of a selector  61 . 
   At least one image generator  62  uses the scheme of  FIG. 4  or a corresponding digital implementation such as software, firmware (e.g. PROM (Programmable Read-Only Memory)), ASIC (Application Specific Integrated Circuit) to generate an image  63  of the AMPS interferer  20 . A subtractor  64  subtracts the image  63  from the baseband signal  18  to produce an image-removed baseband signal  65 . BER measurement module  66  then conducts a bit error analysis on the image-removed baseband signal  65 , and the resulting BER ratio is applied to a corresponding input of the selector  61 . 
   Again, if the BER ratio from module  60  is lower than the BER ratio from module  66 , this means that the image  63  from the generator  62  is not a good representation of the AMPS interferer  20 . In that case, the image-removed baseband signal  65  is ignored and baseband signal  18  is selected by the selector  61  as the signal  68  for further processing through the CDMA receiver  2  of  FIG. 1 . 
   If the BER ratio from module  60  is larger than the BER ratio from module  66 , this means that at least a portion of the AMPS interferer  20  has been successfully cancelled from the baseband signal, and that the quality of the communication has been improved. In this case, the image-remove baseband signal  65  is selected by the selector  61  as the signal  68  for further processing through the CDMA receiver. As indicated earlier in this description, when the BER ratio is sufficiently low and, therefore, the signal-to-noise ratio is sufficiently high, the power of transmission can be reduced, hence increasing the capacity and range of the cellular telephone system and the mobile station battery duration. 
   In the implementation of  FIG. 6 , a feedback loop  67  is connected between the output of the BER measurement module  66  and the image generator  62  and eventually between the output of the selector  61  and the image generator  62  (see  69 ). The feedback loop  67  (and eventually  69 ) will provide the image generator  62  with an error signal to enable this generator  62  to vary as required the amplitude or phase (or other characteristics such as the frequency content) of the image  63  in order to closely track the characteristics of the real AMPS interferer  20 . Obviously, the amplitude or phase (or other parameter) of the image  63  can be varied by the generator  62  until the BER comparison in selector  61  falls in favour of the image-removed baseband signal  65 . This is likely to improve quality of the communication. 
   Obviously, feedback control of the generator  62  through the feedback loop  67  (and eventually  69 ) can be implemented through feedback control algorithms. Given the above information, it is believed to be within the capacity of one of ordinary skill in the art to build such an algorithm capable of fulfilling the required conditions of operation. 
   It should be pointed out that it is possible to combine:
         many image generators and corresponding BER measurement modules as illustrated in  FIG. 5 ; with   at least one image generator and BER measurement module with feedback loop as illustrated in  FIG. 6 .       

   Those of ordinary skill in the art will appreciate that the present invention also applies to interferers, narrowband or not, other than AMPS, for example GSM, CDMA, TDMA or any other type of radio signal (for example harmonics from another frequency bandwidth used for another telecommunication service, etc.) and having characteristics that are sufficiently known to enable development and production of corresponding images. 
   In the case of digital wireless technologies used for data communication, especially in circuit switched mode, the present invention would apply quite well when there is no actual data being sent. In the case where encryption is used, for silent voice or silent data, trial and error or full decoding will be required to find images that are worthwhile. 
   Also, just a word to mention that application is not limited to CDMA communication systems and to voice signals and data. With communication systems other than CDMA, processing of the signal in the receiver chain can involve operations completely different from the above described despreading scheme. 
   Finally, the present invention could be adapted to communication systems other than a cellular telephone system, for example a satellite data communication system. 
   Although the present invention has been described hereinabove by way of a preferred embodiment thereof, this embodiment can be modified at will, within the scope of the appended claims, without departing from the spirit and nature of the subject invention.