Patent Publication Number: US-6223021-B1

Title: Signal filtering in a transceiver for a wireless telephone system

Description:
FIELD OF THE INVENTION 
     The present invention relates to wireless communications systems, and more particularly to a remote transceiver that carries telephony communication signals between wireless telephones and a central transceiver via a broadband distribution network. 
     BACKGROUND OF THE INVENTION 
     The prior art teaches the use of existing cable television network cables to carry telephony signals between a telephone network and remote transceiver sites in defined cells or sectors. The transceivers are used to establish wireless telephony communication links with wireless telephones that are operating within an area covered by each remote transceiver. To increase the number of wireless telephone subscribers that can use the wireless telephone system it has been suggested to decrease the size and operational range of each cell or sector, and to increase the number of cells or sectors required to provide wireless telephone service to a given area. Having cells or sectors of decreased size permits greater reuse of the limited number of frequency channels allocated for wireless telephone service because other cells or sectors located at a closer range can reuse the same frequency channels for additional calls without signal interference. The advantages of reducing cell or sector size to increase the call carrying capacity of the wireless telephone network is offset by the requirement for additional remote transceivers for the additional cells. This offset is minimized by utilizing an existing broadband distribution network to provide the communications path between remote transceivers in each of the cells or sectors and a central transceiver. The base transceiver station acts as the interface between the telephone network and the wireless telephone system, and the central transceiver acts as the wireless telephone system interface with the broadband distribution network. 
     To carry wireless telephony signals over a broadband distribution network, as described above, a predetermined bandwidth on the network is typically allocated for this purpose. However, as required, more bandwidth may be allocated to carry wireless telephony signals. To most efficiently use a given bandwidth to carry wireless telephony signals between wireless telephones and the telephone network, a combination of frequency and time division multiplexing is utilized. This requires base transceiver station equipment that acts as the interface with the telephone network and the wireless telephone system. With the base transceiver station equipment is a central transceiver (RASP), also called a Headend Interface Converter (HIC), that interfaces with the broadband distribution network, and it must function with telephony signals in the wide frequency spectrum of radio frequency signals on the telephone network, and up to 1000 Mhz over the broadband distribution network. This system also requires a plurality of remote transceivers, also called cable microcell integrators (CMI) or Remote Antenna Drivers (RADs), in each of the cells or sectors that can carry many channels of telephony signals between the wireless telephones and the central transceiver via the broadband distribution network, without creating signal interference with the telephony signals in adjacent cells or sectors. In addition, the remote transceivers (RADs) must function with and translate telephony signals in the wide frequency spectrums of up to 1000 Mhz on the broadband distribution network and between 1850-1990 MHz for the radio link between remote transceivers and wireless telephones. 
     SUMMARY OF THE INVENTION 
     Thus, there is a need in the art for remote transceivers that are relatively small and inexpensive, and that can carry a large number of communication signals. 
     The above described need in the wireless telephone system prior art is satisfied by the present invention. A small transceiver is provided which is used in a wireless telephone system. This transceiver carries wireless telephone signals between wireless telephones and the telephone network via a broadband distribution network, such as HFC, fiber optic cable, or coaxial cable, on which the transceivers are hung and to which they are connected, thus eliminating the need for antenna towers. Further, the remotely located transceivers may be interrogated from a central location and control signals may be sent to each remote transceiver to change internal settings of the transceiver. 
    
    
     DESCRIPTION OF THE DRAWING 
     The invention will be better understood upon reading the following Detailed Description in conjunction with the drawing in which: 
     FIG. 1 is a block diagram of a wireless telephony system integrated with a broadband distribution network; 
     FIG. 2 is a simplified block diagram of a remote transceiver used with the wireless telephony system, and having a microprocessor that communicates with a central transceiver via a broadband distribution network to carry telephony signals and control signals between the wireless telephones and the central transceiver; 
     FIG. 3 is a detailed block diagram of the portion of a remote transceiver that transmits to wireless telephones telephony signals received via a broadband distribution network from the central transceiver and telephone network, and showing circuitry used for signal filtering in accordance with the teaching of the present invention; and 
     FIG. 4 is a detailed block diagram of the portion of a remote transceiver that receives telephony signals from wireless telephones and forwards them via the broadband distribution network to the central transceiver and telephone network, and showing circuitry used for signal filtering in accordance with the teaching of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the drawing and the following detailed description, all elements are assigned three digit reference numbers. The first digit of each reference number indicates in which figure of the drawing an element is located. The second and third digits of each reference number indicate specific elements. If the same element appears in more than one figure of the drawing, the second and third digits remain the same and only the first digit changes to indicate the figure of the drawing in which the referenced element is located. As used herein the term “telephony signals” includes voice, data, fax and any other types of signals that are sent over a telephone network now or in the future. Throughout the Figures and the following description, reference is made, for one example, to a combined band pass filter and amplifier  325   a.  There are a number of other such combined band pass filters and amplifiers. They are shown and referenced this way for ease of presentation only. In reality they are each a discrete, separate filter the output of which is input to an amplifier. 
     In FIG. 1 is shown a simple block diagram of an exemplary broadband distribution network  112  integrated with a wireless telephone system which include a plurality of remote transceivers known as Remote Antenna Drivers (RAD)  118   a-i,  a central transceiver known as Remote Antenna Signal Processor (RASP)  117 , and a Base Transceiver Station (BTS)  115 . There are different types of broadband distribution networks in use. Such networks may utilize coaxial cable, fiber optic cable, microwave links, and a combination of these. In the embodiment of the invention disclosed herein a conventional hybrid fiber coaxial (HFC) cable television signal distribution system is utilized. Electrical power is distributed along broadband distribution network  112  to power line amplifiers (not shown) of the cable television distribution network. This electrical power source, or alternate power sources, are used to provide power to RADs  118   a-i.    
     Integrated with broadband distribution network  112  is a wireless telephony system in which the present invention is utilized. One such wireless telephony system is taught in U.S. patent application Ser. No. 08/695,175, filed Aug. 1, 1996, and entitled “Apparatus And Method For Distributing Wireless Communications Signals To Remote Cellular Antennas”. The telephony system disclosed herein includes a base transceiver station  115  which is connected to a telephone system  116 . Base transceiver station  115  is also connected to a Remote Antenna Signal Processor (RASP)  117  which is the interface to a broadband distribution network  112 . Telephony signals carried between telephone system  116  and wireless telephones  119  are carried via base transceiver station  115 , RASP  117 , broadband network  112 , and RADs  118   a-i.    
     As is known in the prior art, including the above cited prior patent application, one or more frequency bands or channels of the broadband distribution network  112  are reserved to carry telephony signals between telephone system  116  and wireless telephones  119 . Telephony signals originating from telephone system  116  are transmitted by RASP  117  over broadband distribution network  112  in frequency division multiplexing format. Telephony signals originating at wireless telephones  119  are frequency multiplexed together by RADs  118   a-i  and transmitted along with control and gain tones via broadband network  112  to RASP  117 , and thence to base transceiver station  115  and telephone system  116 . 
     In base transceiver station  115  there are a plurality of transceiver modules (not shown), as is known in the wireless telephony art, each of which operates at a single channel frequency at a time, and which can handle a predetermined maximum number of telephone calls from wireless telephones. In the wireless telephone system described and claimed herein, the frequency that the RADs  118  are assigned to operate at must correspond to the operating frequency of the assigned BTS transceiver module. If a particular RAD  118  is re-assigned to function with a different transceiver module within base transceiver station  115 , circuit settings within the particular RAD  118  must be changed to function with the different transceiver module. In the wireless telephony, art transceiver modules in the base transceiver station are also referred to as channel card modules and radio modules. 
     When wireless telephony traffic in a first sector or cell increases to the point where adequate service is not provided to wireless telephone subscribers in the first sector or cell, like during rush hour traffic on a highway, in accordance with the teaching of the present invention the wireless telephone system may be remotely reconfigured by RASP  117  to reassign one or more RADs  118  from one or more nearby sectors or cells, where those RADs  118  have overlapping signal coverage with the first sector or cell, to handle the excess wireless telephony traffic in the first sector cell. 
     In FIG. 1 are shown three rows of RADs  118 . Typically a number of RADs  118  are spaced along, and connected to, broadband distribution network  112  to provide overlapping signal transmission and reception coverage for the entire wireless telephone system. Some of the RADs  118  are physically located near the boundary between two or more cells or sectors and, depending on the frequency of operation they are set to, can be used to handle wireless telephony traffic in one or more of the sectors or cells. Let us assume that RADs  118   g,h,i  in the bottom row are physically located along broadband distribution system  112  and are configured to handle wireless telephony traffic in a first sector that includes a highway. During early morning and late afternoon every work day there is rush hour traffic that creates peak wireless telephone traffic that causes unacceptable service delays in the first sector. Let us also assume that the RADs  118   d,e,f  in the middle row in FIG. 1 are configured and located to handle wireless telephone traffic in a second, adjacent sector but they each have an area of signal operation that overlaps the highway in the first sector. 
     One or more of RADs  118   d,e,f  may be dynamically reassigned by RASP  117  to the first sector to handle the increased telephony traffic originating from the highway. In addition, as necessary, additional RASP  117  channels may be assigned, and additional modules in base transceiver station  115  may be assigned to handle the excess wireless telephony traffic from the first sector. To do this RASP  117  sends control signals to the selected remote RADs d,e,f which will cause the frequency at which they operate to be changed to match the frequency of RADS  118   g,h,i  that are normally assigned to handle wireless telephone traffic in the first sector. At the end of the peak traffic period RASP  117  may send control signals to the previously reallocated ones of RADs  118   d,e,f  to change the frequency at which they operate back to their original settings so they are reassigned to handle wireless telephony traffic in the second sector. However, the re-assignment may be permanent depending on traffic patterns encountered. 
     Typically there are a number of RADs  118   a-i  spaced along, and connected to, broadband distribution network  112  to provide overlapping wireless telephony signal transmission and reception coverage for the entire wireless telephone system in a number of cells or sectors. In this arrangement individual RADs  118  may be re-assigned to handle wireless telephony signals from an adjacent cell or sector with which it provides overlapping signal coverage. Each RAD  118   a-i  has antennas  120 ,  121 ,  122  used to transmit to and receive signals from remote wireless telephones  119 . Antenna  120  is used to transmit telephony signals to wireless telephones  119 , while antennas  121  and  122  are used to receive telephony signals from wireless telephones  119 . Antenna  121  is called the primary antenna, and antenna  122  is called the diversity antenna. Antennas  121  and  122  are physically spaced and cooperate to minimize signal fading and thereby provide continuous signal reception from wireless telephones  119 . 
     In FIG. 2 is shown a general block diagram of a Remote Antenna Driver (RAD)  218 . There is a first circuit  208  of RAD  218 , that is shown in detail in FIG. 4, that receives telephony signals originating at telephone system  116  and carried via base transceiver station  115 , RASP  117 , and broadband distribution network  212 , and then re-transmitted via antenna  220  of a RAD  118  to a wireless telephone  119  (not shown). There is also a second circuit  209  of RAD  218 , that is shown in detail in FIG. 3, that receives wireless telephony signals originating from a wireless telephone  119 , and transmits them via broadband network  212  to RASP  117 , and via base transceiver station  115  to telephone system  116 . 
     RAD circuitry  208  and  209  are connected to and controlled by a microprocessor  210 . Frequency multiplexed with the wireless telephony signals carried between RASP  117  and RAD  218  are operational signals of different types that are used for controlling the operation of each RAD  218 . These operations include circuit monitoring, gain control, circuit operation, and setting the frequency of operation, of each RAD  218 . 
     The first of the control operations listed in the previous paragraph is gain control to compensate for losses and gains in a RAD  218  and broadband distribution network  112 . As one part of this gain control operation RASP  117  sends a frequency multiplexed control signal to RAD  218  that is received by microprocessor  210  on leads CTRL from circuit  208 . Responsive thereto microprocessor  210  sends a signal via leads AGC to circuit  209  which causes the output of a gain tone oscillator  342 , with known signal level, to be inserted into the signal path, along with telephony signals, and be returned to RASP  117  via broadband distribution network  112 . The signal level output from gain control oscillator  342  (FIG. 3) is of a low enough amplitude that it does not interfere with telephony signals passing through RAD  218 , but is separated from the telephony signals at RASP  117 . RASP  117  analyzes the amplitude of the gain control oscillator  342  signal received at the RASP, which will reflect gains and losses in RAD  218  and broadband distribution network  212 , as part of a determination whether or not to change gain control attenuators in RAD  218 . 
     As part of monitoring circuit gain levels, as requested by a control signal received from RASP  117 , microprocessor  210  receives information from RAD circuits  208  and  209  on leads MON indicating the gain level of signals only within these circuits, and reports this information to RASP  117  as described in the last paragraph. Using this circuit gain level information, and the gain tone information described in the previous paragraph, RASP  117  can determine gains and losses introduced in broadband distribution network  212 . 
     Responsive to the gain level information described in the previous two paragraphs RASP  117  can send other control signals back to RAD  218  which are received by microprocessor  210  on leads CTRL. Microprocessor  210  uses the control information received from RASP  117  to send signals via leads AGC to RAD circuits  208  and  209  which results in adjustments being made to attenuators to adjust the signal gain levels in these circuits. 
     RAD  218  receives an interrogation control signal, as previously described, which cause microprocessor  210  to send back information about RAD circuit  208  (FIG. 4) and circuit  209  (FIG.  3 ). This information indicates the settings of attenuator pads, the temperature at which each RAD  218  is operating, and the frequency of local oscillators within RAD circuits  208  and  209 . 
     Microprocessor  210  may receive other control signals from RASP  117  and respond thereto to change the frequency of some of the local oscillators within RAD circuitry  208  and  209  to change the frequency on which telephony signals and control signals are carried over broadband distribution network  112  to and from RASP  117 . In this manner the sector which each RAD  118  is assigned to may be changed to handle peak traffic loads and for other reasons. 
     In FIG. 3 is shown a detailed block diagram of circuit  309  within Remote Antenna Driver (RAD)  118  that carries telephony signals from a wireless telephone  119 , via broadband communications network  112 , to central transceiver RASP  117 . This is the circuit shown as RAD circuit  209  in FIG.  2 . 
     Briefly, primary receive antenna  321  is connected to a first portion of the circuitry in FIG. 3, and that circuitry is identical to a second portion of the circuitry that is connected to diversity receive antenna  322 . The telephony signals received by both antennas  321  and  322  from a wireless telephone  119  (not shown in FIG. 3) are initially processed in parallel, then the two signals are frequency multiplexed together and are both returned via broadband distribution network  112  (shown in FIG. 1) to remote RASP  117  and base transceiver station  115  (FIG. 1) to be processed. 
     Built into RAD circuitry  309  in FIG. 3 is circuitry which is enabled by microprocessor  210  in FIG. 2, responsive to a control signal received from remote RASP  117 , to provide gain control for the telephony signal as it appears at the input of RASP  117 . Further, RASP  117  can send other frequency multiplexed control signals to each RAD  118  which microprocessor  210  responds to and changes the frequency at which RAD  118  transmits and receives telephony signals over broadband distribution network  112  to and from RASP  117 , and can also change the frequency at which each RAD  118  communicates with wireless telephones. 
     Telephony signals from a wireless telephone  119  (not shown in FIG. 3) operating in a sector assigned to one or more RAD  118 s are received by primary receive antenna  321 . These signals are input to an isolator  323   a  which isolates antenna  321  from RAD circuit  309 . The telephony signal is then input to directional coupler  324   a  that has a second signal input thereto from power divider  343  which is used for the aforementioned gain control purposes. 
     The telephony signal (modulated RF carrier) received from remote wireless telephone  119 , and the gain tone, low level signal, are applied via directional coupler  324   a  to a combined band pass filter and amplifier  325   a.  The signals are amplified and extraneous signals are filtered from the received telephony signal by bandpass filter  325   a.  The operation just described also applies to isolator  323   b,  coupler  324   b  and bandpass filter and amplifier  325   b.    
     The amplified and filtered telephony signal is then input to mixer  326   a  which is used along with SAW filter  329   a  primarily to assist in filtering of the spread spectrum, digital telephony signal in accordance with the teaching of present invention. Mixer  326   a  also has input thereto a signal from local oscillator  327 . This signal from local oscillator  327  is input to power divider  328  which applies the signal to both mixers  326   a  and  326   b  while providing isolation between these two mixers. 
     The frequency of local oscillator  327  is digitally controlled and is determined by a binary control word applied to its control input  327   a  from microprocessor  210  (FIG.  2 ), responsive to control signals received from RASP  117 . Similarly, control signals from remote RASP  117  causes microprocessor  210  to set the frequency of digitally controlled local oscillators  333   a  and  333   b.    
     The operation of mixer  326   a  results in multiple frequencies being output from the mixer as is known in the art, but due to the frequency of oscillator  327 , most of the signals present at the input of RAD circuit  309  from antenna  321  are shifted far outside the band of frequencies which can pass through SAW filter  329 . Only the desired signals can pass through SAW filter  329 . This frequency shift also helps to prevent leak through of unwanted signals present at the front end of circuit  309  because they are blocked by narrow bandpass filter  325  which is passing signals of a frequency far from the signals applied to SAW filter  329   a.  Due to the sharp filtering action of SAW filter  329   a,  even spurious signals close to the desired telephony and control tone signals are removed. The same filtering operation applies to mixer  326   b  and SAW filter  329   b.    
     The filtered telephony signal is then amplified by amplifier  329   a  and input to step attenuator  330   a  which is used to adjust the gain level of the signal in one-half dB steps. The amount of attenuation provided by step attenuator  330   a  is controlled by a binary word at its control input  331  a from microprocessor  210 . The control of step attenuators  330   a,    330   b,  and  336  is all accomplished responsive to control signals from RASP  117  as part of the gain control operation that assures that the signal level of telephony signals appearing at the input to RASP  117  from all RADs  118   a-i  are within an acceptable range. Attenuator  330   b  in the parallel channel handling the telephony signals from diversity antenna  322  performs the same function. 
     The telephony signal that is output from step attenuator  330   a  is input to mixer  332   a  along with a fixed frequency signal from local oscillator  333   a.  Mixer  332   a  is used to shift the frequency of the telephony and gain tone signals to the frequency required to apply the signals to broadband distribution network  112 . This same operation applies to the telephony and gain tone signals output from mixer  332   b.    
     The frequency of oscillators  333   a  and  333   b  is determined by binary words applied to their control input  333   c.  A control signal is sent from RASP  117  which causes microprocessor  210  to set the frequency of local oscillators  333   a  and  333   b.  The frequency of the telephony signal output from step attenuator  330   a  is the same as the frequency of the telephony signal output from step attenuator  330   b.  However, the frequency of local oscillator  333   a  is different from the frequency of local oscillator  333   b.  The result is that the carrier frequency of the telephony and gain tone signals output from mixer  332   a  is different than the carrier frequency of the telephony and gain tone signals output from mixer  332   b.  This is done so that both primary antenna  321  and diversity antenna  322  signals are both sent to RASP  117  and base transceiver station  115  for processing. However, all carrier frequencies are within the assigned wireless telephony channel on broadband distribution network  112 . 
     The telephony signals received by primary antenna  321  and diversity antenna  322  are frequency multiplexed together and sent via broadband network  112  to RASP  117 . To accomplish this, combiner  334  is utilized. Combiner  334  has the telephony and gain tone signals output from both mixers  332   a  and  332   b  input thereto. As described in the previous paragraph these two telephony signals modulate carriers that are at different frequencies, but both frequencies are in an assigned channel of broadband distribution network  112 . Combiner  334  combines the two sets of signals so they are all frequency multiplexed together. 
     The combined signal is input to bandpass filter and amplifier  335  which removes spurious frequencies created by the mixing action in circuits  332   a  and  332   b,  and amplifies the signals that pass through the filter. The combined and filtered telephony and gain tone signals are input to step attenuator  336  to adjust the gain level of signals. Similar to the operation of the previously described step attenuators, this digitally controlled attenuator is set responsive to control signals received from remote RASP  117  as part of the gain control operation. 
     The frequency multiplexed telephony and gain tone signals output from step attenuator  336  are input to signal combiner  337  which has a second input from control signal oscillator  338 . The frequency of control signal oscillator  338  is set responsive to a binary signal on its control leads  338   a  from microprocessor  210 . RASP  117  is the origin from which the control signal is received to set the frequency of control signal oscillator  338 . The frequency chosen is different than the frequencies used for the telephony signals received via the primary and the diversity antennas and for the gain tone signal. 
     Responsive to different control signals received from RASP  117 , microprocessor  210  (FIG. 2) sends signals on control inputs  338   a.  These microprocessor  210  signals cause control signal oscillator  338  to produce an information signal. The information signal indicates various information about RAD  218 , but particularly including the settings of step attenuators  330   a,    330   b  and  336 , to RASP  117  as part of the novel gain control operation. RASP  117  uses this information to keep an updated status regarding each of the RADs  118   a-i.    
     The output from combiner  337  now has five signals frequency multiplexed together to be returned via broadband network  112  to RASP  117 . The signals are the telephony signal received by primary antenna  321 , the telephony signal received by diversity antenna  322 , the gain tone signal output from gain tone oscillator  342  as applied to both primary and diversity paths, and the system information signal output from control signal oscillator  338 . This frequency multiplexed signal output from combiner  337  is input to band pass filter and amplifier  339  to remove any extraneous signals and amplify the desired signals before they are input to broadband distribution network  112  and sent to RASP  117 . 
     Transformer and coupler  340  is used to couple the frequency multiplexed signals described in the previous paragraphs to broadband distribution network  112 . The transformer is an impedance matching transformer having 50 ohm primary and 75 ohm secondary windings. When broadband distribution network  112  uses coaxial cable, the secondary winding of transformer  340  is wired in series with the center conductor of the video distribution coaxial cable. As previously described, RAD  118  hangs from the coaxial cabling of the broadband distribution network  112  to which it is connected. In other applications, such as with fiber optic cable, other well known frequency conversion and signal coupling techniques are used. 
     A small portion of the frequency multiplexed signals passing through transformer and coupler  340  is coupled to Built In Test (BIT) and power monitor  341 . Monitor  341  samples the signal level of the combined signals that are being input to broadband distribution network  112  and reports this information to RASP  117  via control signal oscillator  338  which has been previously described. If the output signal level is too high and the level cannot be corrected, microprocessor  210  will shut down RAD  118  and report this to RASP  117 . 
     In FIG. 4 is shown a detailed block diagram of circuit  408  in RAD  118  that carries telephony signals originating at RASP  117  via broadband distribution network  112  and circuit  408  to wireless telephones  119  (not shown). As previously described, RAD  118  hangs from and is connected to broadband distribution network  112 . Transformer  442  is an impedance matching transformer having 75 ohm primary and 50 ohm secondary windings. When broadband distribution network  112  is coaxial cable, the primary winding of transformer  442  is wired in series with the center conductor of the coaxial cable. Transformer  442  is used to connect frequency multiplexed telephony and control signals carried on broadband distribution network  112  to the input of RAD circuit  408 . Only the RADs  118 , the receive frequency which has been tuned to the particular frequency of telephony and control signals on broadband distribution network  112  can actually receive and forward the telephony signals to a wireless telephone  119 . 
     All RADs  118  assigned to a sector receive control signals directed toward any one of the RADs in the sector. However, each RAD  118  has a unique address that prefixes each control signal and is used by the RAD  118  to accept only control signals directed specifically to it by RASP  117 . 
     The frequency multiplexed telephony and control signals received by RAD circuit  408  from broadband distribution network  112  are input to band pass filter and amplifier  443 . The combination of mixers  444  and  447 , and filters  443 ,  446  and  451  are primarily used to provide filtering of the digital, spread spectrum telephony signal in accordance with the teaching of present invention. 
     Filter  443  passes all possible frequency multiplexed telephony and control signals that are carried on broadband distribution network  112 , and excludes most other unwanted signals carried on broadband distribution network  112 . Circuit  443  also amplifies the signals that pass through the filter. 
     The signals output from filter  443  are input to mixer  444  along with a signal from local oscillator  445 . Alike the local oscillators shown in FIG.  2  and described with reference to that Figure, the frequency of local oscillator  445  is digitally controlled at its input  445   a  by microprocessor  210  in FIG. 2 responsive to control signals received from RASP  117 . 
     The operation of mixer  444  results in multiple frequencies being output from the mixer as is known in the art and unwanted frequencies are blocked by band pass filter  446  which passes only desired signals. The selected set of telephony and control signals are now input to mixer  447 . Alike other local oscillators in FIGS. 3 and 4, oscillator  449  is digitally controlled at its control input  449   a  by microprocessor  210  responsive to control signals received from RASP  117 . In a manner well-known in the art, mixer  447  combines the signals input to it and provides a number of output signals at different frequencies. All these frequencies are input to an attenuator  450  which is used to adjust the gain level of the signals. Attenuator  450  is part of the gain control system and is digitally controlled at its input  450   a  in ½ dB steps by microprocessor  210 , responsive to control signals received from RASP  117 , alike the digitally controlled attenuator  336  in FIG.  3 . 
     The gain adjusted signal output from attenuator  450  is input to SAW filter and amplifier  451 . Due to the sharp filtering action of SAW filter  451 , even spurious signals close to the desired telephony and control tone signals are removed. Control signals frequency multiplexed with the telephony signal do not pass through SAW filter  451 . Instead, the control signals are input to mixer  448  as is described further in this specification. 
     The telephony signals passed through SAW filter  451  are input to digitally controlled attenuator  452  to adjust the gain level of the signal before it is input to mixer  453  along with the output of digitally controlled local oscillator  454 . Attenuator  452  is part of the gain control system and is digitally controlled at its control input  452   a  in 2 dB steps by microprocessor  210 , responsive to control signals received from RASP  117 . 
     The amplitude adjusted telephony signal output from attenuator  452  is input to mixer  453  along with a signal from digitally controlled  454   a  oscillator  454 . Oscillator  454  is controlled by microprocessor  210 , responsive to control signals received from RASP  117 , in the same manner as local oscillators  445  and  449 . Mixer  453  combines the two signals in a manner well-known in the art to produce several output signals, one of which is the telephony signal now having the desired carrier frequency for transmission of the communications signal to a remote wireless telephone  119 . The signals output from mixer  453  are input to band pass filter and amplifier  455 . Band pass filter  455  passes only the desired carrier frequency. The signal is also amplified before being input to signal divider  456 . 
     A portion of the telephony signal input to divider  456  is divided and input to bit and power monitor  457 , while the remainder of the signal is input to band pass filter and amplifier  458 . Bandpass filter  458  assures that there are no extraneous signals combined with the desired telephony signal, amplifies same, and applies it to power amplifier  459 . Power amplifier  459  amplifies the telephony signal and couples it to transmit antenna  420 . The signal is transmitted within the physical area of the cell or sector covered by this RAD  118  and is received by a remote wireless telephone  119  which is in the area covered by this RAD  118 . 
     The telephony signal input to bandpass filter  458  is sampled by divider  456  and the sample is input to BIT and Power Monitor  457 . The level of signal is reported by microprocessor  210  to RASP  117 . In addition, the output of power amplifier  459  is also sampled and input to BIT and Power Monitor  457 . A signal level measurement is used in concert with attenuators  450  and  452 , as commanded by RASP  117 , to adjust the power level of the telephony signal to be applied to transmit antenna  420 . If the signal level output from power amplifier  459  is too high microprocessor will shut down this RAD  118 . 
     A portion of the signal output from bandpass filter and amplifier  446 , and still including any control signals, is input to mixer  448  along with a signal from local oscillator  460 . The output of mixer  448  is input to reference channel oscillator  462  and forward control channel circuit  461 . Circuit  461  accepts only control signals sent from RASP  117  and sends them to microprocessor  210 . Control signals have a prefix RAD address as part of the control signals and each RAD  118  has a unique address. Therefore, microprocessor  210  in each RAD  118  can recognize and accept only control signals directed to it. 
     When a RAD  118  receives control signals directed to it, microprocessor  210  responds thereto to perform the action required by RASP  117 . The control signal may ask for the settings of the local oscillators and attenuators, and this information is returned to RASP  117  using control signal oscillator  338  as previously described. The control signal from RASP  117  may also indicate revised settings for local oscillators and attenuators. Microprocessor  210  makes the required changes and then sends a confirmation signal back to RASP  117  indicating that the requested changes have been made. As part of the gain control operation the control signal from RASP  117  may also request information concerning the outputs from bit and power monitors  341  and  457 , and request that the output from gain tone oscillator  342  be added to the telephony communications signals. Responsive to any of these control signals, microprocessor  210  performs the requests. 
     Reference channel oscillator  462  processes the output of mixer  448  to generate a phase lock loop reference signal that is used to provide a master frequency to all local oscillators within all RAD  118 s to match their frequency of operation with RASP  117 . 
     While what has been described hereinabove is the preferred embodiment of RAD  118 , it can be understood that numerous changes may be made to the signal filtering disclosed herein by those skilled in the art without departing from the scope of the invention.