Abstract:
A full duplex, broadband system for sparsely populated areas operating in the licensed VHF and UHF range of the electromagnetic spectrum provides service over an area of thousands of square kilometers from the base station.

Description:
RELATED APPLICATIONS 
       [0001]    Applicant claims the priority benefit of provisional application No. 61/592,542 filed on Jan. 30, 2012, incorporated in full by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to systems for providing broadband services using the licensed UHF band portion of the electromagnetic spectrum. 
       BACKGROUND 
       [0003]    Broadband services are ubiquitous. The supplying of such services usually rely on setting up towers, transmitting at low power, and at a high frequency range of over 900 MHz and operating in unlicensed frequencies 
         [0004]    Recently, systems employing the 90-860 MHz range and operating in licensed frequencies have been deployed in Canada to service rural and remote communities. Such systems also require a tower. But the system allows for higher power which results in coverage of 2,000 to 3,000 or more, square kilometers (and provides high downstream data rates throughout the coverage area) compared to the 75-100 square kilometers covered by systems using higher frequencies and lower power levels. 
         [0005]    Systems using the 90-860 MHz frequency range presently are comprised of a broadband antenna on the tower and a low loss cable from the antenna to a duplexer, a down-converter, a power amplifier and a cable modem termination system (CMTS) all located in an enclosure at the foot of the tower. The down-converter resides in the downlink; the power amplifier resides in the uplink. A computer server is located in the enclosure for controlling the system and the typical housekeeping and handshake issues. The duplexer needs to provide over 120 dB of separation between the frequencies in the downlink and uplink and costs about $1,200. An omni-directional pattern is created by using a co-linear antenna or by combining multiple directional sectorial broadband antennas to give any desired pattern with a gain of about 8 dB at a cost of $7,800. The low loss cable connecting the antenna to the duplexer is typically 200 feet long and costs around $1,500. The power amplifier costs about $7,800. 
         [0006]      FIG. 1  is a block diagram of such a prior art system presently in operation in a number of communities in Canada and Alaska. The system comprises a single omni-directional sectorial broadband antenna TR (transmit/receive) on a tower with uplink (cable)  13  and downlink  14  connected between the antenna (via duplexer  12 ) and a server  16  in an enclosure at the base of the tower. Internet and telephone signals are processed via server  16  as indicated in  FIG. 1 . 
         [0007]    The downlink  14  comprises a down-converter  19  and is connected to server  16  via a cable modem termination system (CMTS)  20  as indicated in the figure. The uplink comprises a power amplifier  21  (which is connected to antenna TR. via duplexer  12 ) and CMTS  20  (including an up converter), which connects to server  16  as is also indicated in the FIG. 
         [0008]    Importantly, all system active components are housed in an enclosure at the base of the tower along with a power supply not shown as indicted by broken line  22 . 
         [0009]    The system of  FIG. 1  uses two separate frequencies that are spaced apart as close as  24  MHz and which operate simultaneously. The transmit signal level could be as high as +93 dBmV into the duplexer and the receive signal level could be as small as −35 dBmV. With such a huge difference between the two signals, the duplexer ( 12 ) has to be able to provide a separation of over 120 dB to enable the low receive signal to be satisfactorily decoded. The duplexer is necessarily tunable over the entire band (90-860 MHz) to be shared with digital television broadcasting. The signal to noise ratios of receive signals has to be such as to enable reception of at least QAM 64 signals in the uplink (downstream) direction with signal to noise ratio of greater than 25 dB and handle QAM16 signals with at least a signal to noise ratio of greater than 18 dB in the downlink (upstream) direction. 
         [0010]    The system of  FIG. 1  operates as follows: 
         [0011]    Server  16  outputs the data to the CMTS  20  via Ethernet cables. The CMTS creates a QAM 64 (could also be QAM 128 or QAM 256) modulated signal. The modulated signal is then up converted to the correct downstream channel frequency (example centered on 743 MHz). This is a 6 MHz wide signal from 740-746 MHz. The 743 MHz signal goes into Power Amplifier  21 . From there it is fed into duplexer  12  transmit input. The duplexer ensures that all signals outside the desired range are filtered out. The output of the duplexer is fed into cable  11  (uplink) that is connected to Transmit/Receive Antenna TR. The downstream signal is radiated from the Antenna to client receivers. 
         [0012]    The same antenna (TR) also receives the client transmitter signals. The received signal travels down cable  11  to duplexer  12 . The receive signal is present on the duplexer  12  receive signal side only. The duplexer ensures that the transmit signal is not present on the receive side of the duplexer. The duplexer receive signal side is connected to down-converter  19 . The receive signal in the UHF band (470 to 860 MHz) is down converted to 5-60 MHz and fed into the CMTS. The CMTS decodes the signal and provides it to server  16  via the Ethernet connection between the two devices. 
         [0013]    The following is a list of commercial components suitable for use in the prior art system of  FIG. 1 . 
         [0014]    Antenna  11 —Power antenna model SVP600-360 RRBS Base Station OMNI Panel Antenna 
         [0015]    Duplexer  12 —Com-Tech Model MX5C 
         [0016]    Server  16 —HP PROLIANT DL-380 Server 
         [0017]    Down-converter  19 —Vecima Model MSDC 1000 series 
         [0018]    CMTS  20 —Arris C3 CMTS 
         [0019]    Power Amplifier  21 —Technalogix TAUD-40 
       SUMMARY 
       [0020]    In accordance with the present invention, a full duplex system, also operating in the 90 -860 MHz range, provides significantly improved signal strength, greater range and reduced costs. The invention is based on the recognition that, by splitting the system and by connecting the down and uplink paths to separate, dedicated, narrow band antennas, the duplexer is no longer needed and is replaced with two low cost narrow band filters. The downlink (upstream) filter illustratively has a minimum power rating (i.e. less than 1 watt). The receive cable can be a low cost coaxial cable with attenuation of greater than about 3 dB/100 ft and its loss characteristic has no bearing on receive signal to noise ratio. The cost of the system is reduced by over 30% and the performance of the receive signal is improved by more than 8 dB which allows for a significant improvement in range. It is considered that receive cable attenuation up to about 5 dB/100 ft would allow still lower cost cable and unaffected performance. Therefore, an attenuation range from about 3 dB/100 ft to 5 dB/100 ft is considered to provide the benefits of the reduced cost of cable without impact on performance. Moreover attenuation greater than about 5 dB/100 ft is considered acceptable while an upper limit such as over 15 dB/100 ft may be encountered if s/n degradation prevents delivery of acceptable service. The use of two relatively low cost narrow band antennas instead of one broadband antenna also allows for a significant reduction in cost. The use of two spaced-apart antennas also allows for significant advantage by locating system active components on the tower near the antennas rather than in an enclosure at the tower base. The narrow band transmit antenna (such as Kenbotong Model TQJ-600.011) ensures, that there is only limited spurious noise received in the narrow band receive antenna (also Kenbotong Model TQJ-600II). Using spaced apart narrow band antennas for transmit and for receive, located in the null pattern of each other, ensures that the receive antenna receives only a very limited amount of the transmit signal and there is minimal interference from the transmit signal entering the receive antenna. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a block diagram of a prior art, full duplex, rural broadband system; 
           [0022]      FIGS. 2-4  are block diagrams of a full duplex, rural broadband system showing the locations of system components in accordance with the principles of this invention. 
           [0023]      FIGS. 5   a  and  5   b  show the overlap and non-overlap of frequencies in the antenna passband. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The embodiments of  FIGS. 2 ,  3  and  4  illustrate systems, in accordance with the present invention, employing spaced-apart narrow band transmit and receive antennas connected to a server in an enclosure at the base of the tower, defining an uplink and a downlink there between respectively. The sequence of figures illustrates the location of active system components on the tower rather than in the enclosure at the base of the tower. In this context, narrow band is used to the describe the condition whereby the passband of one or both antennas is such that they do not overlap, and thus there is a high rejection of signal from one antenna to the other.  FIGS. 5   a  and  5   b  show the overlap and non-overlap in the antenna passband. 
         [0025]      FIG. 2  illustrates the use of two narrow band, transmit and receive antennas designated Tx and Rx, respectively, rather than the single broadband antenna TR employed in the prior art system of  FIG. 1 . With specific reference to  FIG. 2 , spaced apart transmit antenna Tx and receive antenna Rx, on a tower (not shown) are connected to a server  23  defining an uplink  33  and a downlink  34  there between respectively. The uplink, in this embodiment comprises CMTS  35  and power amp  36  as is the case in the prior art system of  FIG. 1 . Filter  37  is added as will become clear hereinafter. 
         [0026]    Downlink  34  includes a filter  39 , low noise amplifier  40  and down-converter  41 . An additional filter  47  is added to remove any spurious ingress picked up in cable  34 . The CMTS also is in the downlink. The downlink includes a power extractor  45  located on the tower near antenna Rx and is connected via a coaxial cable to a power inserter  46  located in the enclosure at the tower base. The enclosure is indicated by broken line  48  in the figure. The two antennas typically are located one above the other on the tower separated by several feet. If side mounted, the antennas are typically secured at least several wavelengths from the tower. 
         [0027]    The filter  39  and low noise amplifier (LNA)  40  (such as AI.NOO60-33.006 low noise RF amplifier 500-700 MHz, 30 dB gain, 0.6 dB NF, 12 VDC Power, SMA female connectors) are located at the base of the receive antenna. The use of two narrow band antennas and the location of the filter and LNA at the Rx antenna provide such a significant increase in gain that it allowed the use of a significantly smaller cable at much lower cost. 
         [0028]    Filters  37  and  39  (in the uplink and in the downlink respectively) need to be 6 MHz band pass filters with different center frequencies. The center frequencies typically are at least 24 MHz apart, illustratively 743 and 713 MHz. The power extractor and power inserter illustrate one method of providing DC power to the low noise amplifier up on the tower. 
         [0029]      FIG. 3  illustrates a modification of the system of  FIG. 2  where the down-convertor is moved from the enclosure at the base of the tower to near antenna Rx. This relocation of the down-converter allows the use of a low cost receiving cable because it now carries signals in the 5-60 MHz range and not signals in the VHF-UHF frequency range and has much lower signal loss in the cable. 
         [0030]      FIG. 4  illustrates the system, in accordance with the principles of this invention in which the CMTS ( 35 ) the power amplifier (PA)  36  and filter  37  are mounted on the tower near the base of the antenna TX. In this embodiment, all active components (except server  23 ) are located near the respective antennas on the tower. 
         [0031]    In the operation of the system of  FIG. 4 , server  23  (located at the base of the tower) sends data via a Cat 5 e cable to the (outdoor) CMTS  35  located next to the transmit antenna. The CMTS creates a modulated QAM signal that is up-converted to the correct transmit frequency (i.e. 740-746 MHz). The signal is then fed into the (outdoor) power amplifier  36  also located close to the transmit antenna. There is a short cable from the power amplifier that is connected to the Filter  37  that eliminates signal outside the transmit band. The filter output is connected directly to the transmit antenna. The loss of signal in the Cat 5 e cable is low. Cable  13  is expensive and losses (i.e. −3db) in the cable required the use of a bigger power amplifier than was otherwise needed. 
         [0032]    The receive signal is received in receive antenna Rx. There is no physical connection between the transmit antenna and the receive antenna. The receive signal is fed into a filter ( 39 ) close to the receive antenna. The filter eliminates all signal outside the receive band. Not having the high power transmit signal electrically connected to the receive antenna ensures that there is little transmit signal on the output of the receive filter. The output of the receive filter is then fed into a low noise amplifier (LNA)  40 . The output of the LNA goes into down-converter  41  that converts the UHF frequency band (470-860 MHz) into the 5-60 MHz band that is fed into the CMTS. 
         [0033]    Like numbers are used in  FIGS. 2 ,  3  and  4  to simplify a comparison between the figures in viewing the relocation of active components from the tower base to the antennas. 
         [0034]    The location of active components on a tower is antithetical to industry practice because of difficulty of servicing the equipment, the necessity of supplying power to the components, and the increased exposure to the elements increasing the necessity for servicing. In spite of such disincentive, active components have been located on a tower only in high frequency applications (over 900 MHz) and employing a single broadband antenna where loss of signal in the cables is a huge disadvantage. But that loss diminishes with lower frequencies and higher quality cable and is negligible at the frequency range herein. 
         [0035]    Split systems also have been employed in the prior art. But such systems use broadband antennas and do not use narrow band antennas, which are required in accordance with the principles of this invention. 
         [0036]    Table 1 is a cable attenuation chart showing approximate attenuation in dB for 100 feet of cable and includes the specification and costs for 1 ⅝″ coax cable (Heliax) used in the prior art system of  FIG. 1  and the RG 6  cable of  FIG. 3  and the CAT 5 e cable of  FIG. 4 . 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Cable Attenuation Chart 
               
               
                 Approximate Attenuation in dB for 100 feet of cable 
               
             
          
           
               
                   
                   
                 Diam- 
                 60 
                 500 
                 700 
                 900 
                 2.4 
                 5.8 
                 Approx. 
               
               
                 # 
                 Cable 
                 eter 
                 MHz 
                 MHz 
                 MHz 
                 MHz 
                 GHz 
                 GHz 
                 Cost 
               
               
                   
               
             
          
           
               
                 1  
                 RG-59 
                 0.195″ 
                 1.9 
                 5.5 
                 6.6 
                 7.7 
                 13.1 
                 18.0 
                   $15.00 
               
               
                 2 
                 RG-6  
                 0.220″ 
                 1.55 
                 4.51 
                 5.6 
                 6.2 
                 10.45 
                   
                   $20.00 
               
               
                 3 
                 LDF-5 
                 0.875″ 
                   
                 0.87 
                 1.03 
                 1.60 
                 2.08 
                 3.65 
                   $750.00 
               
               
                 4 
                 Heliax 
                 1.625″ 
                   
                 0.47 
                 0.56 
                 0.65 
                 1.14 
                 2.50 
                 $1,500.00 
               
               
                   
               
             
          
         
       
     
         [0037]    An embodiment of the invention is based on the realization that by using two narrow band filters and antennas and by placing active components on the tower rather than in an enclosure at the tower base is surprisingly beneficial. The benefits are realized in spite of over 10 dB loss in signal strength due to use of the high loss, low cost cable (such as RG 6  or similar cable) to the components, the exposure to the elements, the limited access and the necessity and expense of supplying power to the components. The benefit is due to the fact that by amplifying the receiving signal at the antenna, the signal to noise ratio is captured at its best and the signal strength is so significantly increased that any noise picked up by the unbalanced co-axial cable becomes negligible in comparison. The additional benefits are provided by using the narrow band antenna, which provides higher gain than a comparable size broadband antenna and rejects signals outside the narrow band thereby providing a much cleaner signal into the filter at the base of the Rx antenna. The Rx antenna receive band operates to exclude the Tx signal band to ensure that it provides as much isolation from the Tx signal as possible. 
       SUMMARY OF CHANGES IN THE TECHNOLOGY AND BENEFITS 
       [0038]    1) Replace the broadband antenna with two narrow band antennas. The first benefit is that an at least 2 dB improvement in gain due to the narrow band antenna having a higher gain than is available with a broadband antenna. The second benefit is that the cost of a narrow band antenna is much less (i.e.20% of the cost of broadband omni antenna or 2% of the cost of sectorial antenna. Overall cost reduction of over 60%. The third benefit is that the narrow band receive antenna has a much better signal to noise ratio than a corresponding broadband antenna (i.e. 2 dB improvement) due to the narrow band antenna rejecting all out-of-band signals. 
         [0039]    2) Removing the duplexer and using two antennas and moving the filter and low noise amplifier ( FIG. 2 ) to the top of the tower gives the following benefits:
       a) Much higher signal input into the filter and low noise amplifier since the loss of signal that would have been experienced in the down-cable is now gone (i.e. 0.5 to 1 dB improvement due to loss in the cable and 1 dB improvement due to initial threshold signal level of low noise amplifier being reached earlier.)   b) Much less interference signal from the transmit antenna signal into the receive antenna occurs since the transmit signal is attenuated by both the filter and the out-of-band rejection of the receive antenna, and by placement of the antennas in the null of each other&#39;s radiation pattern, thus adding much less noise to the receive signal. Overall, there is a 3 dB improvement in the signal to noise ratio.   c) Use of a very low cost cable to transport the signal from the top of the tower to the bottom since the signal to noise ratio is already capped at the top and losses in the cable will not change the signal to noise ratio appreciably due to the length of the cable. The cost of low loss, high quality cable is $1,500 while the cost of the high loss cable is $20. In addition, each connector on the larger, high quality cable is $147 per connector, whereas each connector for the smaller, high loss cable is $0.50 per connector. This is a huge cost saving. The high signal loss in the low cost cable does not in any way affect receive signal to noise ratio. The loss in the cable is compensated by using a higher gain low noise amplifier. The cost of the low noise amplifier is the same regardless of the gain of the amplifier. A low noise amplifier is used with a very low noise figure to get the best signal to noise ratio possible.   d) Cost of two filters is more than 50% less than the duplexer cost since the individual filters do not have to meet the higher specifications demanded for the duplexer. The power rating of the receive filter can now be much lower since there is no high power signal entering the filter as was the case with the duplexer. The duplexer has to have an attenuation characteristic which remains high and stays high for a huge segment of the out-of-band spectrum whereas the filter can have out-of-band attenuation characteristics that are more relaxed since the level of the transmit signal entering the receive filter is already low. Overall, much lower cost filters and lower specification filters can be utilized and still have the same benefits.       
 
         [0044]    Total signal quality benefit is 2 dB from the higher gain antenna, +1 dB due to no loss in the cable, +1.5 dB (due to the higher input signal into the LNA and it is working earlier), +3 dB (due to the reduction in noise since the duplexer could not reduce the Tx signal as well), +0.7 dB improvement due to the additional filter after the LNA and cable, according to the above. In field trials there is about 10 dB improvement, which is higher than 8 dB computed above. 
         [0045]    The embodiments described herein are merely illustrative of the principles of this invention. It should be apparent to those skilled in the art that various modifications, adaptations and alternatives may be made within the spirit and scope of the invention as claimed. For example, although the server herein is shown as located in an enclosure at the base of the tower, as it becomes practical to integrate the server, it too can be located on the tower. Also, the cable modem termination system (CMTS) although shown, illustratively, in the uplink and located near the transmit antenna, could also be located near the receive antenna. The CMTS is located near the transmit antenna to be close to the power amplifier.