Abstract:
A multi-protocol receiver system for receiving wireless communications. The system comprises programmable stages for controlling each steps of the demodulation of at least one broadband signal to provide the I and Q signals of at least one specific frequency narrow band for further processing. The receiver can be used in a software defined radio system.

Description:
FIELD OF THE INVENTION 
   The present invention relates to wireless receivers. More precisely, this invention describes a multi-protocol receiver for intercepting, monitoring and/or recording conversation over a cellular network. 
   BACKGROUND OF THE INVENTION 
   With the increasing complexity of telecommunication means, criminals are finding new ways to escape from the surveillance of the police or other law enforcement agencies. This can, potentially, become a threat to society. More specifically this problem is increasing with the global development of mobile telecommunication means. 
   The term mobile telecommunication can include technologies ranging from cordless telephones, digital cellular mobile radios, and personal communication systems that are evolving to wireless data and networks. 
   A mobile telecommunication system is usually composed of a base station  13 , connected to the public telephone network via a Mobile Telephone Switching Office (MTSO), and of a group of mobile users  14 . The base station  13  covers a certain geographical area. 
   A communication between a mobile user and a standard public network user is thus performed using the base station  13 . Each communication link uses a particular frequency band known as a voice channel. The uplink transmission refers to the signal sent by a mobile station  14  to a base station  13 , while the downlink transmission refers to a signal sent by the base station  13  to a mobile station  14 . Therefore, a single conversation requires two voice channels. The monitoring of a particular conversation implicitly requires the monitoring of the two voice channels. One prior technique for monitoring cellular telephone conversations involves a simple tuner that scans voice channels. This technique is not efficient when the number of communications to handle is high and when the frequency of the voice channel changes often. 
   A second technique consists of monitoring the two voice channels and the traffic channels of the communication system in order to handle the handover (i.e. when a user, by a physical displacement, changes base stations). Thus it is possible to track a user through a cellular network. 
   Focusing more precisely on wide band receivers, the prior art comprises one type of system which is composed of a group of front end radio frequency demodulation stages which include parallel narrow-band receivers. Usually, the Radio Frequency (RF) band coverage provided by these narrow-band receivers is of adjacent frequency. Each RF stage has its own local oscillator frequency and Intermediate Frequency (IF) stages. Such a system is very costly because only one RF stage is working at a time. The inactive RF stages are superfluous during operations. 
   In another configuration (described in patent U.S. Pat. No. 6,002,924 col 1, line 57 to 67), the choice of the first IF and the first local oscillator, the second IF and the second local oscillator allow the user to access a broad band radio receiver. However, because of inter-modulation and image frequency response, certain frequencies are not available. This element is unacceptable for the purpose of this use. 
   In U.S. Pat. No. 6,002,924, Takano succeeds to create a broadband radio receiver with a continuous spectrum. However, there is still a gap to fill in order to complete the task of intercepting users because for each voice interception, two receivers are needed; this is too costly therefore this is not acceptable. Furthermore, the processing of the RF signal is completed using an analog processing. This analog receiver architecture has the serious drawback to use multiple mixers and filters. Furthermore, the analog receiver architecture is subject to temperature drift and mismatch from part variations. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an efficient architecture that will avoid one or more of the shortcomings of the conventional technology. 
   It is an object of the present invention to provide a scanner for mobile telecommunications. 
   It is an object of the present invention to provide a scanner for mobile communications that will be able to handle simultaneously a consequent number of conversations coming from any part of the available frequency bands. 
   Another object of the present invention is to provide a scanner that will be able to cope with various frequency bands. 
   Another object of the present invention is to provide a scanner that will be able to cope with various telecommunications protocols. 
   Yet another object of the present invention is to provide a receiver that can be software controlled. 
   Yet another object of the present invention is to provide a scanner in which there is a possibility to achieve a protocol upgrade of the system. 
   Yet another object of the present invention is to provide a scanner in which there is a possibility to achieve a frequency band update of the system. 
   Yet another object of the invention is to provide a scanner in which there is a possibility to add other users to the system. 
   According to one aspect of the invention, there is provided a method for scanning a group of mobile telecommunication users. The method comprising the steps of: collecting a wideband frequency signal using an antenna, performing a first demodulation of the wideband frequency signal to obtain a demodulated wide band signal; digitizing the demodulated signal to obtain a digital demodulated signal; routing the digital demodulated signal to a device which performs on the digital demodulated signal a narrowband extraction, performing the narrow band extraction on the digital demodulated signal and generating an in-phase component and a quadrature component for a selected voice channel. 
   In accordance with another aspect of the present invention, there is provided an apparatus for monitoring the communications of a group of mobile users. The apparatus first comprises at least one demodulation unit, also called RF receiver unit, a number of demodulation units depending on a number of different frequency bands that are targeted to be monitored. Each demodulation unit is composed of two separate circuits that are each dedicated to the scanning of a downlink transmission and an uplink transmission. The apparatus also comprises an Analog to Digital Converter (ADC) whose goal is to convert the analog signal into a digital signal. The apparatus also comprises a Central Processing Unit (CPU), which routes the signal through the Digital Down Converters (DDC) and commands the demodulation unit. The narrow band extraction is performed at this stage. Finite Input Response (FIR) filtering is also performed. The output signal can be decoded by a processing unit in accordance with the protocol that was used to encode the information. 
   According to another aspect of the invention, there is provided a method for tracking a mobile. The method comprising the steps of: requesting from a user at least one wireless conversation to monitor, receiving at least one control channel signals using a control channel receiver, processing said received control channel signals according to at least one of said wireless conversations to monitor to provide at least one voice channel, each one of said voice channels corresponding to one wireless conversations to monitor, storing in a memory a relation between one of said wireless conversations to monitor and one of said voice corresponding voice channel, setting, using pre-recorded data on the type of wireless telecommunication protocol and frequency band coming from a non-volatile memory, the parameters of the filters, the frequency of the down converters, and the paths in the switch in order to track said users. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood by an examination of the following description, together with the accompanying drawings, in which: 
       FIG. 1  shows an overview of a base station and mobile unit wideband scanner; 
       FIG. 2  shows the RF stage and the processing unit, the RF stage comprises four RF units; the system depicted there can process four different full duplex conversations coming from four different frequency bands; 
       FIG. 3  shows the RF unit that can process a full duplex wireless conversation; 
       FIG. 4  shows a DDC unit and a filter stage that can process a full duplex wireless conversation; 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   While the present invention may be provided in various embodiments, there is shown in the drawings and described in the following text a specific preferred embodiment, with the understanding that the present description is only one embodiment and is not limiting of scope of the invention. 
     FIG. 1  presents one embodiment of the invention. The system in the preferred embodiment is a scanner, that is a device which monitors transmission from either a base station  13  or a mobile  14 . Such a device is used typically by police for lawful intercept of cellular telecommunications. An antenna  10  collects the radio signals coming from the base unit  13  and the mobile unit  14 . A demodulation unit  11  handles radio signals. The demodulation unit  11  provides the IF signal to the processing unit  12 . The RF unit  11  imposes limitations on this signal in order to prevent it from being harmful for the following stages. The processing unit  12  is able to select parts of the wideband signal coming from either a base unit  13  or a mobile unit  14 . A management unit  15  controls the processing unit  12  and get signals from the RF unit  11 . In a preferred embodiment, the management unit  15  comprises a computer with a data acquisition hardware and a software. 
     FIG. 2  shows the different parts of the architecture of the system in the preferred embodiment. 
   Each unit  16 , composed of a RF stage  11  and an ADC stage  21 , is capable to collect the base station signal and the mobile station signal of a certain frequency band. In the preferred embodiment, the scanner is able to tap simultaneously four different frequency bands because the device is composed of four units  16 . Each RF unit  11  sends a power information signal  33  to the management unit  15 . The ADC stage  21  is connected to a router  22  that is, in the preferred embodiment, a Field Programmable Gate Array (FPGA). The router  22  is controlled by the management unit  15  using signal  36 . Next to the router unit  22 , a group of DDC units  23  are connected via a bus  28 . Finally a filter stage  24  is connected to each DDC unit  23  using signals  29  and  30 . This filter stage  24  is controlled by the management unit  15  using signal  35 . The filter stage  24  provide signals  31  and  32  to the demodulation units. The demodulation units are not shown in  FIG. 2 , and may comprise standard demodulation circuits for extracting information from the regular bandwidth signal according to the wireless transmission protocol of each regular bandwidth signal, e.g. TDMA, AMPS, GSM. 
   The antenna  10  receives the radio signal. Typically, the signal power ranges between −110 dBm and −14 dBm. 
   In the case of a cellular system, the frequency band is located between 824 MHz and 894 MHz. The uplink band is located between 824 MHz and 849 MHz, while the frequency band located between 869 MHz and 894 MHz is dedicated for downlink communications. 
   In the case of a Personal Communication Service (PCS) system, the frequency band is located between 1850 MHz and 1990 MHz. The uplink frequency band is located between 1850 MHz and 1910 MHz, while the downlink frequency band ranges from 1930 MHz and 1990 MHz. The downlink frequency band is shared into two frequency bands: the low downlink frequency band which is located between 1930 MHz and 1960 MHz, and the high downlink frequency band which is located between 1960 MHz and 1990 MHz. The uplink frequency band is shared into two frequency bands: the low uplink frequency band which is located between 1850 MHz and 1880 MHz, and the high uplink frequency band which is located between 1880 MHz and 1910 MHz. 
   The RF unit  11 , described in  FIG. 3 , filters and amplifies the incoming signal  20 , collected by the antenna  10 . More precisely, a duplexer divides the signal  20  into two different signals  80  and  81 , corresponding to the downlink and the uplink transmission. Thus, these separated signals  80  and  81  can be sent to two different parts: a mobile RF stage (which comprises elements:  61 ,  62 ,  63 ,  64 ,  65 ,  66 ,  67 ,  68 ,  69 ) and a base RF stage (which comprises elements:  51 ,  52 ,  53 ,  54 ,  55 ,  56 ,  57 ,  58 ,  59 ). These two different stages allow the scanning of either the radio signals coming from the mobile unit  14  or the radio signals coming from the base unit  13 . 
   In both cases, the signal collected by the antenna  10  is filtered by filter. In the case of a base station signal, the filter is filter  51  which will only select frequency signals located between 869 and 894 MHz in the case of a cellular system. In the case of a PCS system, filter  51  only selects frequency signals located between either 1930 MHz and 1960 MHz (low part of the base station signal) or 1960 MHz and 1990 MHz (high part of the base station signal). 
   In the case of a mobile station signal, the filter  61  only selects frequency signals located between 824 MHz and 849 MHz in the case of a cellular system. In the case of a PCS system, filter  61  only selects frequency signals located between either 1850 MHz and 1880 MHz (low part of the mobile signal) or 1880 MHz and 1910 MHz (high part of the mobile signal). 
   In the case of a base station signal, a RF amplifier  52  amplifies signal  82  coming from the frequency filter  51 . In the case of a mobile signal, the RF amplifier  62  amplifies signal  92  coming from the frequency filter  61 . 
   In the preferred embodiment, and in the case of a cellular system, the local oscillator  53  comprises an ultra low noise TCXO local oscillator followed with a signal amplifier, a frequency multiplier, by a factor of 2, a signal amplifier, frequency multiplier, by a factor of 2, a signal amplifier, a frequency multiplier, by a factor of 2, an amplifier, an harmonic filter and an amplifier. The harmonic filter is a bandpass filter that will reject non desired harmonics. 
   In the case of the cellular system, the frequency of the ultra low noise TCXO local oscillator is 117 MHz. Thus, the oscillator  53  generates a signal  84  with a frequency of 936 MHz. In the case of the cellular system, the local oscillator  53  calibrated at 936 MHz, is connected to a mixer  54 . Such a system will translate the signal from a central frequency of 881.5 MHz down to a central frequency of 54.5 MHz, that has been chosen in order to be compatible with the central frequency of the following filter  55 . The bandwidth will remain 25 MHz. 
   In the case of a PCS low-band base station signal, the frequency of the ultra low noise TCXO local oscillator is 118.125 MHz. The ultra low noise TCXO local oscillator is followed by a signal amplifier, a frequency multiplier, by a factor of 2, a signal amplifier, a frequency multiplier, by a factor of 2, a signal amplifier, frequency multiplier, by a factor of 2, a signal amplifier, a frequency multiplier, by a factor of 2, an amplifier, an harmonic filter and an amplifier. Thus, the oscillator  53  generates a signal  84  with a frequency of 1890 MHz. In the case of a PCS low-band base station signal, the local oscillator  53  calibrated at 1890 MHz is connected to a mixer  54 . Such a system will translate the signal from a central frequency of 1945 MHz down to a central frequency of 55 MHz, that has been chosen in order to be compatible with the central frequency of the following filter  55 . The bandwidth will remain 30 MHz. 
   In the case of a PCS high-band base station signal, the frequency of the ultra low noise TCXO local oscillator is 120 MHz. The ultra low noise TCXO local oscillator is followed by a signal amplifier, a frequency multiplier, by a factor of 2, a signal amplifier, a frequency multiplier, by a factor of 2, a signal amplifier, a frequency multiplier, by a factor of 2, a signal amplifier, a frequency multiplier, by a factor of 2, an amplifier, an harmonic filter and an amplifier. Thus, the oscillator  53  generates a signal  84  with a frequency of 1920 MHz. In the case of a PCS high-band base station signal, the local oscillator  53  calibrated at 1920 MHz is connected to a mixer  54 . Such a system will translate the signal from a central frequency of 1975 MHz down to a central frequency of 55 MHz, that has been chosen in order to be compatible with the central frequency of the following filter  55 . The bandwidth will remain 30 MHz. 
   A local oscillator  63 , in the case of a mobile signal, is connected to a mixer  64 . In the preferred embodiment, and in the case of the cellular system, the local oscillator  63  comprises an ultra low noise TCXO local oscillator followed with a signal amplifier, a frequency multiplier, by a factor of 2, a signal amplifier, a frequency multiplier, by a factor of 2, a signal amplifier, a frequency multiplier, by a factor of 2, an amplifier, an harmonic filter and an amplifier. The harmonic filter is a bandpass filter that will reject non desired harmonics. 
   In the case of the cellular system, the frequency of the ultra low noise TCXO local oscillator is 97.75 MHz. Thus, the oscillator  63  generates a signal  94  with a frequency of 782 MHz. In the case of a mobile cellular frequency band, the local oscillator  63  calibrated at 782 MHz is connected to a mixer  64 . Such a system will translate the signal from a central frequency of 836.5 MHz down to a central frequency of 54.5 MHz, that has been chosen in order to be compatible with the central frequency of the following filter  65 . The bandwidth will remain 25 MHz. 
   In the case of a PCS low-band mobile station signal, the frequency of the ultra low noise TCXO local oscillator is 120 MHz. The ultra low noise TCXO local oscillator is followed by a signal amplifier, a frequency multiplier, by a factor of 2, a signal amplifier, a frequency multiplier, by a factor of 2, a signal amplifier, a frequency multiplier, by a factor of 2, a signal amplifier, a frequency multiplier, by a factor of 2, an amplifier, an harmonic filter and an amplifier. Thus, the oscillator  63  generates a signal  94  with a frequency of 1920 MHz. 
   In the case of a PCS low-band mobile station signal, the local oscillator  63  calibrated at 1920 MHz is connected to a mixer  64 . Such a system will translate the signal from a central frequency of 1865 MHz down to a central frequency of 55 MHz, that has been chosen in order to be compatible with the central frequency of the following filter  65 . The bandwidth will remain 30 MHz. 
   In the case of a PCS high-band mobile station signal, the frequency of the ultra low noise TCXO local oscillator is 121.875 MHz. The ultra low noise TCXO local oscillator is followed by a signal amplifier, a frequency multiplier, by a factor of 2, a signal amplifier, a frequency multiplier, by a factor of 2, a signal amplifier, a frequency multiplier, by a factor of 2, a signal amplifier, a frequency multiplier, by a factor of 2, an amplifier, an harmonic filter and an amplifier. Thus, the oscillator  63  generates a signal  94  with a frequency of 1950 MHz. In the case of a PCS high-band mobile station signal, the local oscillator  63  calibrated at 1950 MHz is connected to a mixer  64 . Such a system will translate the signal from a central frequency of 1895 MHz down to a central frequency of 55 MHz, that has been chosen in order to be compatible with the central frequency of the following filter  65 . The bandwidth will remain 30 MHz. 
   In all cases, and in the preferred embodiment, the multiplication of the signals provided by the mixer is completed with a high S/N ratio. 
   In the case of a base cellular signal, a filter  55 , with a central frequency of 54.5 MHz only selects the desirable bandwidth, i.e. 25 MHz, of the incoming signal  85 . In the case of a PCS base signal, the filter  55 , with a central frequency of 55 MHz, will only select the desirable bandwidth, i.e. 30 MHz, of the incoming signal  85 . 
   In the case of a mobile cellular signal, a filter  65 , with a central frequency of 54.5 MHz only selects the desirable bandwidth, i.e. 25 MHz, of the incoming signal  95   
   In the case of a PCS mobile signal, the filter  65 , with a central frequency of 55 MHz, will only select the desirable bandwidth, i.e. 30 MHz, of the incoming signal  95 . 
   In the case of a base signal, an IF amplifier  58  boosts the selected signal  86  after filter  55 . In the case of a mobile signal, an IF amplifier  68  boosts the selected signal  96  after filter  65 . 
   In both cases, the amplification of the signal can be chosen between 14 dB and 56 dB by a gain controller  60 . The amplification is performed with a high S/N ratio. The objective of the RF stage  11  is to get an output power of 0 dBm at the end of the RF stage  11 . 
   In the case of a base signal, amplifier  58  is connected to a limitation circuit  56  via signal  87 . The limitation circuit  56 , which is composed of diodes, protects the end of the RF stage  11  from voltage peaks that would be harmful to the following stages. 
   A filter  57  is connected to the limitation circuit  56  using signal  88 . The filter  57  is composed of resistances and capacitors. The filter  57  is connected to a detection circuit  59  which converts the output signal  89  into a DC value comprised between 0 and 2.5 V, and proportionate to the output power of signal  89 . The DC value is then used as a feedback reference in order to control the IF amplifiers  58  and to provide an output power signal closed to 0 dBm. The gain controller  60 , that collects the feedback signal  91  comprises, in the preferred embodiment, a PIC 16C74 processor. In the preferred embodiment, the gain controller  60  adjusts the gain of amplifiers  58  and  68  each 46 msec. The gain controller  60  also comprises, in the preferred embodiment, an EEPROM which stores the relation between voltage signal and power in dBm. The output power computed by the gain controller  60  using signal  91  and  101  is transmitted to the management unit  15 . In the preferred embodiment, the transmission is completed using a RS232 interface. 
   In the case of a mobile signal, the amplifier  68  is connected to a limitation circuit  66  via signal  97 . The limitation circuit  66 , which is composed of diodes, protects the end of the RF stage  11  from voltage peaks that would be harmful to the following stages. 
   A filter  67  is connected to the limitation circuit  66  using signal  98 . The filter  67  is composed of resistances and capacitors. The filter  67  is connected to a detection circuit  69  which converts the output signal  99  into a DC value comprised between 0 and 2.5 V, and proportionate to the output power. The DC value is then used as a feedback reference in order to control the IF amplifiers  68 , using the gain controller  60 , and to provide an output power signal closed to 0 dBm. 
   Referring to  FIG. 2 , an ADC stage  21  follows the RF stage  11 . The ADC stage  21  converts signals  89  and  99 . The conversion is performed at a rate of 75 MHz, thus it more than satisfies Nyquist sampling theorem for the 25 MHz signal. The quantization is 10 bits. It is important to notice that the device used for the analog to digital conversion has a high Signal/noise (S/N) ratio in the preferred embodiment. After the conversion, the information becomes a flow of digital information. A router  22 , that is, in the preferred embodiment a FPGA, will then route this flow of information through the DDC  23 . The router acts as a switch which performs path selection between multiple parts. The choice of a FPGA is justified by the fact that it can be soft-configured very easily, and by the fact that it can handle very large amount of data (it has a large bandwidth). The router  22  is controlled by the management unit  15 . The router  22  creates a data bus  28  that is connected to the DDC  23 . Each DDC  23  is then able to collect data coming from a particular unit  16 . The DDC  23  is a tuneable down converter adapted to numerical signals. 
   Each DDC  23  downconverts the signal  28  with a local oscillator and an in-phase and quadrature downconverter. The management unit  15  controls the DDC  23  and allows each DDC  23  to select a specific conversation. 
     FIG. 4  describes more precisely the operations completed by the DDC  23 . 
   In the preferred embodiment, the DDC  23  is an Intersil HSP50016. The goal of the DDC  23  is to extract a narrow frequency band of interest from a wideband input signal, convert that band to a baseband and output it in either a quadrature or a real form. In the present invention, the goal of each DDC is to select a voice channel coming either from the base station  13  or from the mobile unit  14  via the router  22 , in order to intercept it. 
   The narrow band extraction is performed by down converting and centering the band of interest. The DDC  23  has an input data stream of 16 bits in width and 75 MSPS in data rate. As the ADC  21  performs the conversion with a quantization of only 10 bits, the six (6) Less Significant Bit (LSB) are grounded in order to maintain a good accuracy. The conversion is done by multiplying the input data  28  with a quadrature sinusoid generated by a complex sinusoid generator  110 . In order to get the in-phase component (designated as I)  29  of the quadrature sinusoid, the signal  28  is multiplied by a cosine signal  130  in the mixer  111 . In order to get the quadrature component (designated as Q)  30 , the signal  28  is multiplied by a sine signal  135  in the mixer  120 . The frequency of the complex sinusoid generator  110  of the DDC  23  can be selected by the management unit  15  in order to select a specific voice signal. 
   A quadrature lowpass filter  114  is applied to the output of the mixer  111 . Another quadrature lowpass filter  123  of the same type of the above is connected to the output of the mixer  120 . In the preferred embodiment, filtering chain  114  consists of a cascaded High Decimation Filter (HDF)  112  and a low pass FIR filter  113 . The filtering chain  123 , in a preferred embodiment, consists of a cascaded HDF  121  and a low pass FIR filter  122 . The combined response of the two stages filters results in a −3 dB to −102 dB shape factor. Each filtering chain ( 114  and  123 ) is controlled by the management unit  15  according to the voice channel selected. The decimation factor is 4×78 in the case of AMPS. The decimation factor is 4×77 in the case of DAMPS. 
   Each DDC  23  provides the quadrature signal (Q)  30  and the in-phase component (I) signal  29  of a specific voice channel to tap. The output frequency of signal I  29  and signal Q  30  is 240.384 kHz in the case of AMPS. In the case of DAMPS, the output frequency of signal I  29  and signal Q  30  is 243.506 kHz. 
   The signals  29  and  30  are filtered through a FIR  24 . In the preferred embodiment, FIR  24  is a HSP43124SC-33 which is programmable. The FIR filter  24  is controlled by the management unit  15  which provides the coefficients for the right protocol. 
   In the case of AMPS and in the preferred embodiment, the algorithm of Parks-McClellan is used to compute the coefficients of the order  233  FIR  24 . The algorithm of Parks-McClellan is also used to compute the coefficients of the order  233  FIR  24  in the case of DAMPS. In the case of GSM and CDMA, a similar approach is used to compute the coefficients of the FIR  24 . In the preferred embodiment, the low pass frequency of the filter FIR  24  is 12.4 kHz; and the attenuation is 80 dB at 17.6 kHz in the case of AMPS and DAMPS. 
   The signals  31  and  32  are available for demodulation according to the protocol used for the transmission.