Patent Publication Number: US-6658263-B1

Title: Wireless system combining arrangement and method thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of wireless communications. 
     2. Description of Related Art 
     Wireless networks typically rely on relatively short-range transmitter/receiver (“transceiver”) base stations, each connected to a switching center, to serve mobile subscriber terminals in small regions (“cells”) of a larger service area. By dividing a service area into small cells with limited-range transceivers, the same frequencies can be reused in different regions of the service area, and mobile terminals which consume relatively little power can be used to communicate with a serving base station. Service providers of such wireless networks incur substantial costs to establish the dense pattern of base stations needed to ensure adequate service, including the cost of buying/leasing the property on which base stations and switching centers are located, the cost of licensing the frequency bandwidth used for air-interface channels, and hardware/software costs associated with each base station, switching center, and landline connections between switching centers and base stations. 
     A significant percentage of the cost for a single base station is the cost of the antenna structure used to transmit/receive radio frequency (RF) signals to/from wireless subscriber terminals. The specific antenna structure used depends on various factors, such as cell radius (e.g., requiring a high-gain antenna structure), whether the cell is sectorized (e.g., a number of directional antennas may be used for a sectorized cell while an omni-directional antenna may be used for a non-sectorized cell), and whether diversity reception is implemented. 
     For many geographic regions, particularly metropolitan regions, consumer demand for wireless services can support several coexisting wireless systems, each allocated a different block of frequency spectrum. Such coexisting wireless systems will typically have independent network infrastructures and use separate antennas which provide mutual isolation. Because each base station must filter out frequencies which are not in their allocated transmit/receive bands and because transmit amplifier specifications set limits on acceptable spurious noise levels, for example to comply with FCC (Federal Communications Commission) regulations, communications from base stations/mobile subscriber terminals of first and second wireless systems will typically not interfere with each other when using separate antennas. 
     In rural regions, and for marginally competitive service providers, infrastructure costs may preclude establishing or expanding wireless network service in a given geographic area because of a limited number of subscribers. To address the substantial costs required to establish a wireless network, and thereby improve a service provider&#39;s ability to establish/expand their network service area, it has been proposed to share antenna structures between multiple service provider base stations, recognizing that base stations of different wireless systems will transmit/receive on different RF frequencies. 
     Despite the filtering circuitry of individual base stations (e.g., using a duplexer arrangement having a first band pass filter which passes frequencies in the transmit band and a second band pass filter which passes frequencies in the receive band) and transmit amplifier specifications which limit acceptable spurious noise levels at frequencies outside the allocated block of spectrum, the frequency bandwidths allocated to different wireless systems may be near enough that the conventionally-implemented filtering performed by each base station will be insufficient to prevent interference between the communication signals of each wireless system in a shared antenna environment. Additionally, the physical connection of transmission lines from multiple base stations at a common connection point will generally cause considerable power loss (“insertion loss”), as much as 50% loss, attributable to the transmit/receive signal of one system feeding into the transmission line of the second system. Such insertion loss will require increased power and/or a higher gain antenna structure to achieve acceptable signal-to-noise characteristics. 
     SUMMARY OF THE INVENTION 
     The present invention is a system and a method for effectively combining communications of the base stations of multiple wireless systems on the same antenna structure. In one embodiment, the present invention is a wireless system combiner which serves as an interface between base stations of first and second wireless systems (“first base station” and “second base station”) and a shared antenna to substantially eliminate spurious noise from the first base station at frequencies allocated to the second base station and further to prevent transmit power from the first base station from feeding into the reception circuitry of the second base station in a shared antenna configuration. 
     The combiner according to one implementation of the present invention includes a first combiner filter connected between a duplexer of the first base station and a common connection point and a second combiner filter connected between a duplexer of the second base station and the common connection point. The first combiner filter in this implementation filters out spurious noise generated by first base station transmitter at frequencies outside the frequency band allocated to the first base station, for example using a high Q value band-pass or band-reject filter. The second combiner filter in this implementation filters out signal power at frequencies outside the second base station receive band to prevent transmit signal power of the first base station from feeding into the second base station&#39;s receiver circuitry, thereby preventing intermodulation. 
     The first and second combiner filters may be implemented as discrete elements from the circuitry of each base station, thereby allowing service providers of each wireless system to design their base station, and in particular base station transmit amplifier and filtering circuitry, without regard to whether the base station will be implemented in a shared antenna environment. Alternatively, the first and second combiner filters may be incorporated in the filtering circuitry of the first and second base stations respectively. 
     Still further, the first and second combiner filters according to embodiments of the present invention significantly decrease insertion loss (i.e., the power loss resulting when the transmission lines for each base station are connected at a common point between the antenna structure and the individual base stations) by creating very high impedance in the first base station side of the shared antenna configuration for frequencies of the second base station, and vice versa. Insertion loss can be even further reduced by achieving an electrical length of the transmission line between the first/second combiner filter and the common connection point which is tuned to the frequencies allocated for the first/second base stations respectively. As such, transmit/receive signal power for each of the first base station and the second base station will not substantially be lost in the other base station side of the shared antenna configuration. 
     In one exemplary implementation, a base station of a CDMA (Code Division Multiple Access) system, e.g., operating in accordance with the IS-95 A/B CDMA standard, and a base transceiver station of a GSM (Global System for Mobile communication) system are connected to the same antenna structure via a combiner. Base stations for CDMA wireless systems are typically allocated a receive band of 825 MHz-835 MHz and a transmit band of 870 MHz-880 MHz (for “A-Band”) while base stations of GSM wireless systems are typically allocated a receive band of 890 MHz-915 MHz and a transmit band of 935 MHz-960 MHz. Even after each base filters out frequencies which are not in their respective transmit and receive bands, spurious noise from the CDMA base station transmitter will exist at receive frequencies of the GSM base station (e.g., at 890 MHz) due to the performance of the CDMA base station&#39;s transmit amplifier and the roll-off characteristics of filters typically used by a CDMA base station. Furthermore, CDMA base station transmit power in the range of 870 MHz-880 MHz will directly feed into the GSM base station receiver in a shared antenna configuration if not addressed, thereby degrading GSM receive performance. First and second combiner filters according to the present invention address these drawbacks by substantially eliminating spurious noise from the CDMA base station at frequencies allocated to the GSM base station, and preventing transmit power from the CDMA base station from feeding into the reception circuitry of the GSM base station. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects and advantages of the present invention will become apparent upon reading the following detailed description, and upon reference to the drawings in which: 
     FIG. 1 is a general block diagram of shared antenna configuration according to an embodiment of the present invention; 
     FIG. 2 is a block diagram illustrating select elements of first and second base stations and a combiner for the shared antenna configuration of FIG. 1 according to an embodiment of the present invention; 
     FIG. 3A illustrates an exemplary duplexer configuration suitable for use in accordance with principles of the present invention; 
     FIG. 3B illustrates exemplary base station transmit and receive bands for different wireless systems; and 
     FIG. 4 is block diagram illustrating an alternative arrangement to the embodiment illustrated in FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     The following detailed description relates to a system and a method for effectively combining communications for the base stations of multiple wireless systems on the same antenna structure. In one embodiment, the present invention is a wireless system combiner which substantially eliminates spurious noise from a first base station at frequencies allocated to a second base station, and prevents transmit power from the first base station from feeding into the reception circuitry of the second base station in a shared antenna configuration, thereby isolating the communications of each wireless system. Exemplary embodiments of the present invention will be described with reference to the Figures. 
     In FIG. 1, there is shown a general block diagram illustrating a shared antenna configuration  100  according to an embodiment of the present invention. As shown in FIG. 1, the shared antenna configuration  100  includes a base station of a first wireless system  110  (“first base station  110 ”) and a base station of a second wireless system  130  (“second base station  130 ”) which are connected to an antenna  180  via a combiner  150 . As discussed in detail below, the combiner isolates RF communications of the first base station  110  and the second base station  130 . 
     FIG. 2 illustrates select components of the first base station  110 , the second base station  130 , and the combiner  150  according to an embodiment of the present invention. As shown in FIG. 2, the first base station  110  includes transmit circuitry  112 , a transmit amplifier  113 , receive circuitry  114 , and a duplexer  116 . The transmit amplifier  113  and the receive circuitry  114  are each connected to the duplexer  116 . The transmit circuitry  112  receives a plurality of communication inputs Input 1 , . . . , Input M , for example voice traffic received from the Public Switched Telephone Network and/or data traffic received from a frame relay network, via a mobile switching center (not shown), and generates a modulated RF signal, for example using known baseband and RF processing techniques, which is amplified by the transmit amplifier  113  to create an amplified RF transmission signal Tx. The transmit amplifier  113  outputs Tx to the duplexer  116 . 
     Transmit amplifiers typically must comply with performance specifications, e.g., as regulated by the FCC, to limit the amount of spurious noise output by the base station amplifier over a range of non-allocated frequencies, such as over a 30 kHz non-allocated band. For example, if the transmit power for the first base station is 20 Watts (i.e., 43 dBm), the performance specifications of the transmit amplifier may require a maximum of −60 dB for spurious noise emissions at frequencies just outside the base station&#39;s allocated transmit band (measured over a 30 kHz band). 
     The receive circuitry  114  receives an RF reception signal Rx from the duplexer  116  and recovers traffic/control information from Rx, for example using well known techniques, and outputs a plurality of traffic signals Output 1 , . . . , Output N  to the mobile switching center (not shown). The second base station  130  similarly includes transmit circuitry  132 , a transmit amplifier  133 , receive circuitry  134 , and a duplexer  136 , and operates in a manner discussed above regarding the first base station  110 . 
     The combiner  150  includes a first combiner filter  154  which is connected between the duplexer  116  of the first base station  110  and a common connection point  156 , and a second combiner filter  152  which is connected between the duplexer  136  of the second base station  130  and the common connection point  156 . The common connection point  156  is connected to the antenna  180 . The operation of the first combiner filter  154  and the second combiner filter  152  will be discussed in detail below. 
     FIG. 3A illustrates a typical duplexer configuration which is suitable for implementing the duplexer  116  of the first base station  110  and the duplexer  136  of the second base station  130 . As illustrated in FIG. 3A, the duplexer  116  includes a base station transmit band pass filter (BPF BT)  116   a  which receives Tx from the transmit amplifier  113 , filters out frequencies in Tx which are above and below the base station transmit band boundaries, and outputs the result to the first combiner filter  154  of the combiner  150 . The duplexer  116  further includes a base station receive band pass filter (BPF BR)  116   b  which receives RF signals from the first combiner filter  154  of the combiner  150 , filters out frequencies above and below the base station receive band boundaries, and outputs the resulting signal Rx to the receive circuitry  114 . The duplexer  136  of the second base station  130  may likewise have the configuration shown in FIG. 3A but will have different pass-bands for BPF BT and BPF BR. 
     FIG. 3B illustrates exemplary band pass filtering effects of the duplexer  116  of the first base station  110  and the duplexer  136  of the second base station  130 . The example of FIG. 3B assumes for illustration purposes that the first base station  110  belongs to a CDMA wireless system allocated a receive band of 825 MHz-835 MHz and a transmit band of 870 MHz-880 MHz (“A-Band”), and that the second base station  130  belongs to a GSM wireless system allocated a receive band of 890 MHz-915 MHz and a transmit band of 935 MHz-960 MHz. It should be recognized that the principles of the present invention are not solely applicable to a shared antenna configuration for CDMA and GSM base stations, which are instead specifically discussed for illustrative purposes. 
     In FIG. 3B, the lower and upper boundaries of the CDMA base station receive band are labeled BRL CDMA  and BRH CDMA  respectively, the lower and upper boundaries of the CDMA base station transmit band are labeled BTL CDMA  and BTH CDMA  respectively, the lower and upper boundaries to of the GSM base station receive band are labeled BRL GSM  and BRH GSM  respectively, and the lower and upper boundaries of the GSM base station transmit band are labeled BTL CSM  and BTH GSM  respectively. As seen from the example of FIG. 3B, the filters of the duplexer arrangement in a base station exhibit roll-off effects at frequencies which are just above and below the upper and lower band boundaries. Although such roll-off effects at the CDMA receive band and the GSM transmit band boundaries are not detrimental in this example, the proximity of BTH CDMA  and BRL RSM  will cause interference between the first and second base stations because of the performance of the first base station&#39;s transmit amplifier  113 , which will create spurious noise at lower receive frequencies of the GSM base station, and the relatively gradual roll-off characteristics of the filtering performed by the duplexer  116  of the first base station  110  and the duplexer  136  of the second base station  130 . 
     As applied to a configuration in which the first base station  110  is a CDMA base station and the second base station  130  is a GSM base station, the combiner  150  serves the following two purposes:(1) eliminating spurious noise from the first base station  110  at GSM receive frequencies (i.e., between 890 MHz to 915 MHz); and (2) preventing CDMA transmit power of the first base station  110  (i.e., between 870 MHz to 880 MHz) from feeding into the GSM receiver of the second base station  130  so as to prevent intermodulation between GSM receive signals and CDMA transmit signals. 
     For illustration purposes, it can be assumed that the transmit power of the first base station  110  is 20 W (i.e., 43 dBm), the performance specifications of the transmit amplifier  113  of the first base station require −60 dB/30 kHz (i.e., spurious noise measured over a 30 kHz band) at the frequency of 890 MHz, and the duplexer  116  of the first base station  110  achieves 76 dB of rejection at 890 MHz. Therefore, in accordance with these exemplary characteristics, the spurious noise from the first base station  110  at 890 MHz is −93 dBm/30 KHz (i.e., 43 dBm −60 dB −76 dB). If the first base station and the second base stations were to use separate antennas, such a level of spurious noise would be insignificant because the separate antennas would provide approximately 50 dB additional isolation. The inventors of this application have found, however, that the spurious noise from the first base station  110  will interfere with the second base station  130  in a CDMA/GSM shared antenna configuration unless otherwise addressed. 
     In an exemplary implementation of the present invention for the CDMA/GSM combining environment described above, the first combiner filter  154  is a band-pass filter characterized by a passband of 825 MHz-880 MHz and steep roll-off characteristics, e.g., a multi-section resonant filter having a Q value of approximately 2000 to provide approximately 40 dB additional attenuation at 890 MHz, thereby effectively preventing spurious noise from the duplexer  116  of the first base station  110  from interfering with receive frequencies of the second wireless system  130  (i.e., 890 MHz to 915 MHz). The first combiner filter  154  may also be a band-reject filter (or “notch” filter) which rejects possibly interfering frequencies, such as in the range of 890 MHz-915 MHz. 
     The inventors of this application have also found that, in a CDMA/GSM shared antenna configuration, transit power from the CDMA base station is likely to feed into the GSM base station&#39;s receive circuitry from the common connection point, thereby causing intermodulation with GSM receive signals which will affect receiver performance unless otherwise addressed. More specifically, assuming for illustrative purposes that CDMA transmit power at frequencies between 870 MHz-880 MHz should be below −50 dBm at the input of the receive circuitry  134  of the second base station  134 , the nominal CDMA transmit power (at 870 MHz to 880 MHz) at the output of the transmit amplifier  113  of the first base station  110  is 43 dBm, and the duplexer  136  of the second base station  130  achieves 20 dB of rejection at 880 MHz, then an additional 73 dB of rejection is needed at 880 MHz to prevent intermodulation. In an exemplary implementation of the present invention for the CDMA/GSM combining environment described above, the second combiner filter  152  is implemented as a band-pass filter characterized by a passband of 890 MHz-960 MHz and steep roll-off characteristics, e.g., a multi-section resonant filter having a Q value of approximately 2000 to provide approximately 73 dB attenuation at 880 MHz. Like the first combiner filter  154 , the second combiner filter  152  can be implemented as a band-reject filter which rejects possibly interfering frequencies, such as in the band of 870 MHz-880 MHz. 
     In addition to serving the above-described purposes of (1) eliminating spurious noise from the first base station  110  at receive frequencies of the second base station  130 , and (2) preventing transmit power from the first base station from feeding into the receive circuitry  134  of the second base station  130 , an advantage of the combiner  150  according to the present invention, when the combiner is implemented as a discrete element from the circuitry of the first base station  110  and the second base station  130 , is that service providers do not have to modify base station circuit design, and in particular transmit amplifier and filtering circuitry, when the base station is implemented in a shared antenna environment. It should be recognized, however, that the first and second combiner filters may be realized by modifying the filtering circuitry of the first base station  110  and the second base station  130  to achieve the functions described above. 
     As an additional advantage, the combiner structure according to embodiments of the present invention significantly decreases insertion loss (i.e., the power loss resulting when the transmission lines for each base station are connected at a common point between the individual base stations and the antenna structure). More specifically, for the exemplary implementation shown in FIG. 2 in which the first combiner filter  154  is connected to the duplexer  116  of the first base station  110  and the second combiner filter  152  is connected to the duplexer  136  of the second base station  136 , the impedance looking into second base station side of the shared antenna configuration from the common connection point  156  is very high for transmit (and receive) frequencies of the first base station  110  due to the presence of the second combiner filter  152 . If the transmit signal (and receive signal) of the first base station  110  sees such high impedance looking into the second base station side  130  of the shared antenna configuration from the common connection point  156 , the transmit signal (and receive signal) of the first base station  110  will enter/be received from the antenna  180  with very low loss. 
     Likewise, the impedance looking into first base station  110  side of the shared antenna configuration from the common connection point  156  is very high for receive (and transmit) frequencies of the second base station  130  due to the presence of the first combiner filter  154 . If the receive signal (and transmit signal) of the second base station  130  sees such high impedance looking into the first base station  110  side of the shared antenna configuration from the common connection point  156 , the receive signal (and the transmit signal) of the first second base station  110  will enter/be received from the antenna  180  with very low loss. 
     Insertion loss can be further reduced by implementing a tuned transmission configuration as discussed below. As illustrated in FIG. 2, the first combiner filter  154  is connected to the common connection point  156  via a transmission line l 1 , e.g., a coaxial cable, and the second combiner filter  152  is connected to the common connection point  156  via a transmission line  12 . The impedance looking from the common connection point  156  into the path of l 1 , Z in (l 1 ), can be expressed as: 
     
       
           Z   in  ( l   1 )=− j Z   0  cot( BL   1 )  (1) 
       
     
     where Z 0  is characteristic impedance of the transmission line, e.g., approximately 50 Ω for coaxial cable, L 1  is the length for the transmission line l 1 , and B is wave number (i.e., 2Π/λ, and thus frequency dependent). Equation (1) is derived by recognizing that Z in  (l 1 ) can be expressed as:                  Z   in          (   l1   )       =       Z   o     ·       (         Z   load        cos                   (   BL1   )       +     j                   Z   o          Sin        (   BL1   )           )             Z   o          cos        (   BL1   )         +     j                   Z   load        Sin                   (   BL1   )         )                 (   2   )                         
     In equation (2), Z load  can be represented by the impedance of the first combiner filter  154 . Because Z load  is extremely high at the frequencies allocated to the second base station relative to Z 0 , the Z 0  terms in the numerator and denominator of Equation (2) can be disregarded, leaving:                  Z   in          (   l1   )       ≈       Z   o     ·         Z   load        cos                   (   BL1   )         j                   Z   load        Sin                   (   BL1   )                   (   3   )                         
     Equation (3) is merely a different expression of Equation (1), and shows that Z in  (l 1 ) will be maximized when BL 1  , “electrical length,” is approximately equal to 180°. For l 1 , λ may be represented as the wavelength at approximately the center frequency of the pass-band for the first combiner filter  154  (e.g., 850 MHz for the CDMA/GSM example described above). 
     Therefore, a length L 1  for transmission line l 1  may be selected which results in an electrical length of approximately 180° for a nominal frequency of 850 MHz to further reduce insertion loss (i.e., achieving a tuned transmission configuration). 
     These same principles apply to  12 , such that Z in (l 2 ) will be maximized for frequencies allocated to first base station  110  when the electrical length for l 2  is approximately equal 180°. For l 2 , A may be represented as the wavelength at approximately the center frequency of the pass band of the second combiner filter  152  (e.g., 935 MHz for the CDMA/GSM example described above). 
     FIG. 4 illustrates an alternative arrangement to the embodiment illustrated in FIG.  2 . As shown in FIG. 4, the first base station  110  of this alternative embodiment includes a pair of simplexers, transmit simplexer  118  and receive simplexer  119 , instead of a duplexer for filtering out frequency components which are not in the base station transmit and base station receive bands respectively. Accordingly, the first combiner filter  154  in this alternative embodiment includes a transmit combiner filter  154   a  which removes spurious noise resulting from the transmission path of the first base station  110 . For the combined CDMA/GSM example discussed above, the transmit combiner filter  154   a  may be a band-pass filter having a pass band of 870 MHz-880 MHz to provide approximately 40 dB additional attenuation at 890 MHz. The transmit combiner filter  154   a  may also be realized as a band-reject filter, which for the CDMA/GSM combining example described above rejects frequencies between 890 MHz and 915 MHz. Although the second base station  130  and the second combiner filter  152  in the alternative embodiment illustrated in FIG. 4 are the same as FIG. 2, the second base station  130  may likewise be implemented using paired simplexers instead of duplexer  136 . Still further, although the transmit combiner filter  154   a  and the second combiner filter  152  illustrated in FIG. 4 are shown as separate elements from the filtering circuitry of the first base station  110  and the second base station  130 , it should be realized that the transmit simplexer  118  of the first base station  110  and the duplexer  136  of the second base station  130  may be modified to achieve the results discussed above. 
     It should be apparent to this skill in the art that various modifications and applications of this invention are contemplated which may be realized without departing from the spirit and scope of the present invention.