Abstract:
A programmable band-pass filter in a radio-frequency receiver. The band-pass filter has a bandwidth substantially covering a channel bandwidth. Once the appropriate channel in use is determined, the frequency band of the band-width filter is set to correspond to it.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to telecommunication systems and, particularly, to a system that is more robust against in-band interferers.  
           [0003]    2. Description of the Related Art  
           [0004]    In conventional radio-frequency (RF) receivers, a band-pass filter is used to filter out-of-band interferers. In many cordless and/or wireless systems, the available bandwidth is subdivided into smaller channels. However, the band-pass filter typically has a bandwidth covering the entire available bandwidth.  
           [0005]    Such a receiver is illustrated more particularly in FIG. 1. The system  100  includes an antenna  101 , a band-pass filter  102 , a mixer  104 , a voltage-controlled oscillator  106 , a band-pass filter  108 , a demodulator  110 , a low-pass filter  112 , and a synchronization block  114 .  
           [0006]    A modulated signal is received at the antenna  101  and is band-pass filtered by the band-pass filter  102 . The band-pass filter  102  reduces the receiving signal bandwidth to the bandwidth that covers all the used channels. By doing so, the band-pass filter  102  filters out the out-of-band interference. The signal output from the band-pass filter  102  is mixed in the mixer  104  with a lower constant frequency signal which may be generated, as shown, by the voltage controlled oscillator  106 . The modulated receive signal is thus transferred down to a lower frequency, typically referred to as the Intermediate Frequency (IF). The band-pass filter  108  is provided behind the mixer  104  because the output of the mixer  104  is two down-converted modulated receive signals on two different frequencies, only one of which can be used in the demodulator  110 . Thus, only one of the down-converted IF signals is passed through the band-pass filter  108  to the demodulator  112 . The demodulator  110  converts the frequency-modulated signal into a baseband signal, which is low-pass filtered using the low-pass filter  112 . Finally, the sync block  114  synchronizes to the low-pass filtered signal. For example, the synchronization block may detect one or more synchronization words.  
           [0007]    As can be appreciated, when the signal bandwidth is less than the available system bandwidth, the band-pass filter  102  fails to filter out the “out of channel” interferers. These are then mixed and can negatively impact system robustness, which results in a higher bit error rate and voice quality degradation.  
         SUMMARY OF THE INVENTION  
         [0008]    These and other problems in the prior art are overcome in large part by a system and method according to the present invention. A programmable band-pass filter is provided in a radio-frequency receiver. The band-pass filter has a bandwidth substantially covering only a channel bandwidth. Once the appropriate channel in use is determined, the frequency band of the band-width filter is set to correspond to it.  
           [0009]    The receiver may be used in direct sequence or frequency hopping spread spectrum cordless telephone systems. One such frequency hopping system includes a base station and one or more handsets. The handsets lock onto the base station to synchronize with the frequency hopping scheme. The receivers in the base station and the handsets set their band-pass filter bandwidths according to the frequency of the frequency hopping scheme.  
           [0010]    According to one implementation of the invention, a channel selector of a base station selects a frequency according to a frequency hopping scheme. The channel selector provides the frequency to the base station&#39;s receiver for the channel band-pass filter. The base station provides control data to the handsets&#39; channel selectors, which lock to the frequencies being hopped-to by the base station. The handsets&#39; channel selectors then provide this information to the bandpass filters of their receivers, which then filter the band of the frequency chosen.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    A better understanding of the invention is obtained when the following detailed description is considered in conjunction with the following drawings in which:  
         [0012]    [0012]FIG. 1 is a diagram illustrating a radiofrequency receiver according to the prior art;  
         [0013]    [0013]FIG. 2 is a block diagram of an exemplary radiofrequency receiver according to an implementation of the present invention;  
         [0014]    [0014]FIG. 3 is a diagram illustrating exemplary badnwidth selection;  
         [0015]    [0015]FIG. 4 is a diagram of an exemplary frequency hopping cordless telephone system according to an implementation of the invention;  
         [0016]    [0016]FIG. 5 is a diagram of exemplary frame frequencies for a frequency hopping cordless telephone system according to an implementation of the invention;  
         [0017]    [0017]FIG. 6 is a diagram illustrating exemplary subdividing a frequency band according to an implementation of the invention; and  
         [0018]    [0018]FIG. 7 is a flowchart illustrating operation of an implementation of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    FIGS.  2 - 7  illustrate a telecommunications system including telecommunications devices according to an implementation of the present invention.  
         [0020]    [0020]FIG. 2 is a diagram of an exemplary receiver in accordance with an implementation of the present invention. The receiver  200  includes an antenna  201 , a band-pass filter  202 , a mixer  204 , a voltage-controlled oscillator  206 , a band-pass filter  208 , a demodulator  210 , a low-pass filter  212 , and a synchronization block  214 . As will be explained in greater detail below, both the VCO  206  and the band-pass filter  202  receive frequency select inputs  216 , defining the frequency and frequency band that is to be used. The band-pass filter  202  then adjusts the band for band pass filtering accordingly.  
         [0021]    A modulated signal is received at the antenna  201  and is band-pass filtered by the band-pass filter  202 . The band-pass filter  202  reduces the receiving signal bandwidth to the bandwidth that covers substantially only the channel currently in use, responsive to the frequency select signal. By doing so, the band-pass filter  202  filters out the out-of-band interference. The frequency select signal is provided from a processor  216  which determines the channel in use, by any of a variety of methods.  
         [0022]    The signal output from the band-pass filter  202  is mixed in the mixer  204  with a lower constant frequency signal which may be generated, as shown, by the voltage controlled oscillator  206 . The modulated receive signal is thus transferred down to the Intermediate Frequency (IF). Again, a second band-pass filter  208  is provided behind the mixer  204  because the output of the mixer  204  is two down-converted modulated receive signals on two different frequencies, only one of which can be used in the demodulator  210 . Thus, only one of the down-converted IF signals is passed through the bandpass filter  208  to the demodulator  210 . The demodulator  210  converts the frequency-modulated signal into a baseband signal, which is low-pass filtered using the low-pass filter  212 . Finally, the sync block  214  synchronizes to the low-pass filtered signal.  
         [0023]    Operation of the programmable band-pass filter is illustrated more particularly in FIG. 3. Shown are an available frequency band fb, subdivided into a plurality of channels fc 1 -fc 7 . It is noted that, in practice, a substantially larger number of channels are available. Thus, FIG. 3 is exemplary only. As will be described in greater detail below, communication occurs over one of the channels fc 1 -fc 7 . As described above, the bandwidth of the band-pass filter  208  (FIG. 2) is set to one of the channel bandwidths fc 1 -fc 7 , once it is determined which channel is in use.  
         [0024]    The receiver  200  of FIG. 2 is suitable for use in any radio-frequency system. FIG. 4 is a block diagram of one implementation of an exemplary radio-frequency system according to the present invention. In particular, the system may be implemented as a frequency hopping cordless telephone system, indicated generally as  10 . System  10  includes one or more base stations  12 , each of which can also be referred to as a fixed part (FP). Each base station  12  can support communication with a plurality of handsets  14  and handsets  16  using radio frequencies. The interface between base station  12  and handsets  14  and  16  can be referred to as the air interface. Handsets  14  and handsets  16  can also be referred to as portable parts (PP). The base station  12  and the handsets  14  and  16  further include receivers  200  according to the present invention as well as channel selectors  40 ,  41 , as will be described in greater detail below. An exemplary system suitable for use with a receiver according to the present invention is the Gigaset system, available from Siemens Corp.  
         [0025]    In operation, base station  12  can support a defined total number of handsets  14  and  16 . For example, in one implementation, base station  12  can support a total of eight handsets, either idle locked or active locked. Of the total number of handsets, a given number “M” can be active locked handsets  16 . For example, base station  12  could support up to four active locked handsets  16  from the eight total handsets. Of the remaining handsets, base station  12  can support a given number “N” of idle locked handsets  14 . For example, “N” can be less than or equal to the difference between the total number of supported handsets (e.g., 8) and the number “M” of active locked handsets  16  (e.g., 0-4). Idle locked handsets  14  are handsets that are currently inactive but are in contact with and in sync with base station  12 .  
         [0026]    Base station  12  can communicate with handsets  14  and handsets  16  using a time division multiplexed (TDM) frame-based communication protocol. For example, each frame can be ten milliseconds (10 ms) in duration and can include transmit and receive channels for communication and control data. One protocol used with digital cordless telephone systems is the Digital Enhanced Cordless Telecommunications (DECT) protocol, which is the pan-European standard for digital cordless systems and supports up to six locked handsets  16  (i.e., M=6). There are, of course, other protocols used for communicating across the air interface between base station  12  and handsets  14  and handsets  16 . For example, the DECT protocol can be modified to support up to four locked handsets  16  (i.e., M=4), each with enhanced communication features due to higher data rates.  
         [0027]    In the implementation of FIG. 4, system  10  uses an ISM band of radio frequencies for supporting communication between base station  12  and handsets  14  and  16 . For example, system  10  can use the ISM band extending from 2.4 GHz to 2.4835 GHz. An advantage of using the ISM band is that it is unlicensed and does not require a license fee for use. However, in order to operate within FCC or other government regulations, system  10  implements a frequency hopping scheme. This allows system  10  to support robust cordless communications in the ISM band while operating within regulation guidelines. Under the frequency hopping scheme, base station  12  and handsets  14  and  16  move in the time domain from frequency to frequency. Because of the changing frequency, handsets are initially in an unlocked state when entering an area serviced by base station  12 . Unlocked handsets can then “listen” at a specific radio frequency to attempt to lock on to base station  12 . When base station  12  hops to that frequency specific frequency, unlocked handsets can identify and receive control data transmitted by base station  12 . This allows unlocked handsets to lock with base station  12  and sync with the frequency hopping scheme. As will be described in greater detail below, the control data used to synchronize to the frequency are also used to synchronize the band-pass filter.  
         [0028]    [0028]FIG. 5 is a block diagram of one embodiment of frame frequencies for a frequency hopping cordless telephone system. As shown, a frame structure, indicated generally at  20 , comprises a plurality of frames  22  each having a frame length  24 . Each frame  22  follows immediately after the previous frame  22  in the time domain. In the embodiment of FIG. 4, a different frequency (F 1 , F 2 , F 3  . . . F N , F N+1 , . . .) is associated with each frame  22  and is used during that frame  22  for communication across the air interface between base station  12  and handsets  14  and  16 . This change from frequency to frequency is handled by the frequency hopping scheme implemented by base station  12  and handsets  14  and  16 . During the duration of a given frame  22 , base station  12  and handsets  14  and  16  communicate using the selected frequency for that frame  22 . When the next frame  22  begins, base station  12  and handsets  14  and  16  communicate using a new selected frequency. In one embodiment, frame length  24  is ten milliseconds. Thus, the frequency being used changes every ten milliseconds, as does the frequency band of the band-pass filter  202 .  
         [0029]    [0029]FIG. 6 is a diagram of one embodiment of subdividing the ISM band for a frequency hopping cordless telephone system. The ISM band used in this embodiment extends from 2.4 GHz to 2.4835 GHz. As mentioned, the FCC defines requirements for use of frequencies within the ISM band. For example, within a 30 second period, the regulations limit the maximum length of time that a system can use one frequency to 0.4 seconds. Thus, the total available frequencies needs to include seventy-five or more frequencies. In the embodiment of FIG. 6, this range is divided into twelve subsets  30 , and each subset  30  is divided into eight channels  32 . Each channel  32  is then associated with one of ninety-six frequencies  34  defined within and equally subdividing the ISM band. Frequencies  34  then provide a set of frequencies from which the frequency hopping scheme can select for each frame  22 .  
         [0030]    The frequency hopping scheme, in addition to selecting frequencies, also needs to implement a scheme for avoiding bad frequencies. For example, a PCS microwave tower may interfere with frequencies in the ISM band in a particular region. Thus, cordless telephone system  10  would not want to use those frequencies. One way to avoid such bad frequencies is to block their selection. By dividing the ISM band into ninety-six frequencies, the embodiment of FIG. 6 provides sufficient frequencies to allow bad frequencies to be blocked while keeping the number of available frequencies above the seventy-five frequency threshold. For example, there is freedom to avoid using the frequencies within two subsets  30  without dropping below the seventy-five frequency threshold.  
         [0031]    The channel selector  40  of the base station  10  may implement a frequency hopping selection system that selects a frequency for communication between a base station  12  and one or more handsets  16 . A similar selection system  41  resides in handset  16 . Selection systems  40  and  41  must be operable to select the same frequency for a given frame, such that base station  12  and handset  16  can continue communication while hopping frequencies. The channel selectors  40 ,  41  may implement any of a variety of frequency hopping schemes. One such scheme is described in U.S. patent application Ser. No. 09/113,539, filed Jul. 10, 1998, titled “Method and System for Table Implemented Frequency Selection in a Frequency Hopping Cordless Telephone System,” which is hereby incorporated by reference in its entirety as if fully set forth herein.  
         [0032]    According to one implementation of the invention, the channel selector  40  selects a frequency according to a frequency hopping scheme. The channel selector  40  provides the frequency to the receiver  200  for the bandpass filter  202 . The base station provides control data to the handsets&#39; channel selectors  41 , which lock to the frequencies being hopped-to by the base station  10 . The channel selectors  41  then provide this information to the bandpass filters of their receivers  200 , which then filter the band of the frequency chosen. The channel selectors  40 ,  41  may be implemented as one or more processors or integrated circuits  216  (FIG. 2).  
         [0033]    This is illustrated more particularly with reference to the flowchart of FIG. 7. In a step  702 , the base station  10 &#39;s channel selector  40  implements a frequency hopping scheme, and selects frequencies for use during the communication. For example, the channel selector  40  may first select particular frequency subsets (FIG. 4) and then individual channels in each subset. In a step  704 , the base station provides the sequence, or the particular frequency, as a frequency select signal to the band pass filter  202 . The band pass filter  202  then selects that channel for filtering, in a step  706 . In a step  708 , the base station provides control signaling to the handset(s), which then lock to the frequency hopping scheme, in a step  710 . For example, the handset may listen on a particular frequency. When the base station transmits data on that frequency, the handset detects it and can lock to the frequency hopping scheme. In a step  712 , the handset&#39;s channel selector  41  provides the frequency (or hop scheme) to the bandpass filer of its receiver. In a step  714 , the band pass filter&#39;s band is adjusted to the currently selected channel.  
         [0034]    The invention described in the above detailed description is not intended to be limited to the specific form set forth herein, but is intended to cover such alternatives, modifications and equivalents as can reasonably be included within the spirit and scope of the appended claims.