Abstract:
Provided is an apparatus adapted to transmit a first channel on at least two adjacent fixed beams of a plurality of fixed beams defining a coverage area, with each pair of adjacent fixed beams of the plurality of fixed beams partially overlapping and having substantially orthogonal polarizations. The apparatus has a respective transmitter adapted to transmit on each of the plurality of fixed beams a respective unique composite signal, each composite signal containing said first channel and a respective unique traffic channel.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates in general to cellular communication systems and in particular to wireless transmitters, transceivers and methods.  
         BACKGROUND OF THE INVENTION  
         [0002]    In order to satisfy the demand for transmission capacity within an available frequency band allocation, digital cellular systems divide a particular geographic area to be covered into a number of cell areas. A cell consists of a base station from which mobile units within the cell access the cellular system. It is the base station capacity that typically defines the optimal cell coverage area. The capacity of a base station is ideally as large as possible so that each cell can serve as an access point to the cellular system to as many mobile units as possible over a large area.  
           [0003]    One method of achieving an increase in capacity is to replace a wide beamwidth antenna with an antenna array that provides a number of narrower beamwidth beams that cover the area of the original beam. Referring to FIG. 1, a conventional wireless communication cell  100  is shown comprising three adjacent sectors, alpha  102 , beta  104  and gamma  106 . Each cell comprises an antenna tower platform  120  located at the intersection of the three sectors. The antenna tower platform  120  has three sides forming an equallateral triangle. Each sector has three antennas, (only antennas in sector alpha  102  are shown) a first antenna  114 , a second antenna  116 , and a third antenna  112  mounted on a side of the antenna tower platform  120 . The three antennas of each sector produce a corresponding set of three beams (only beams in sector alpha  102  are shown) including a first beam  108 , a second beam  110  and a third beam  112 . The three beams  108 ,  110 ,  112  are adjacent with some overlap. The three sectors alpha  102 , beta  104  and gamma  106  are identical in structure with respect to antennas and beams. The signal for a particular user can then be sent and received only over the beam or beams that are useful for that user. If the pilot channel on each beam is unique (e.g. has a different PN (pseudo-random noise) offset) within each sector then the increase in capacity is limited due to interference between reused pilot channels in different cells.  
           [0004]    An improvement is to use multiple narrow beams for the traffic channels and transmit the overhead channels (pilot, synch, and paging channels) over the whole sector so that the overhead channels are common to all the narrow beams used by the traffic channels in that sector. This leads to substantial gains in capacity. It is therefore desirable that the overhead channels be broadcast over the area covered by the original wide beam.  
           [0005]    Broadcasting the overhead channels over an entire sector can be accomplished by using the original wide beam antenna or by transmitting the overhead channels synchronously using the same multiple narrow beams used to transmit and receive the traffic channels. However, a problem common to both of these arrangements is that both require the expense of extra hardware, complex calibration equipment and algorithms to match the phases of the overhead channels to the phases of the traffic channels.  
           [0006]    Currently, multiple beams of one polarization are used to provide coverage for a single sector, with a second polarization used for diversity purposes. When a full sector transmission is required, as for the overhead channels, transmission of identical signals on all beams simultaneously can create spatial interference nulls at the beam crossover points, assuming that the equipment has not been carefully calibrated (so the relative phases are not controlled). An approach that is proposed in commonly assigned U.S. patent application Ser. No. 09/733,059, entitled “Antenna Systems With Common Overhead For CDMA Base Stations” and filed on Dec. 11, 2000 by McGowan et al, provides a method of phase cycling of the beams to ensure that a spatial null only persists for a short duration of time.  
           [0007]    There is thus a desire to provide an antenna array that uses fixed narrow beams for transmitting and receiving the traffic channels on multiple beams and may broadcast the common pilot channel over all of the sector using the same antenna array. Furthermore, it would be advantageous to provide an antenna system that did not require complex calibration and adjustment to maintain performance over time and temperature.  
         SUMMARY OF THE INVENTION  
         [0008]    Advantageously, embodiments of the invention allow the distinctive interference which would otherwise occur when transmitting the same signal on overlapping beams is avoided through the use of orthogonal polarizations.  
           [0009]    According to one broad aspect, the invention provides an apparatus adapted to transmit a first channel on at least two adjacent fixed beams of a first plurality of fixed beams defining a coverage area, with each pair of adjacent fixed beams of said plurality of fixed beams partially overlapping and having substantially orthogonal polarizations.  
           [0010]    In some embodiments, the apparatus further comprises a respective transmitter adapted to transmit on each of said first plurality of fixed beams a respective unique composite signal, each composite signal comprising said first channel and a respective at least one unique traffic channel.  
           [0011]    In some embodiments, the apparatus further comprises the apparatus adapted to transmit CDMA signals  
           [0012]    In some embodiments, the apparatus further comprises a respective receiver coupled to receive a respective receive signal over each of said first plurality of fixed beams.  
           [0013]    In some embodiments, the apparatus comprises a dual polarization array adapted to produce all of the beams of the first and second pluralities of beams  
           [0014]    In some embodiments, the apparatus is further adapted to receive over a second plurality of fixed beams comprising a corresponding fixed beam for each fixed beam of said first plurality of fixed beams which is substantially co-extensive with the fixed beam of the first plurality of fixed beams and has a respective polarization which is substantially orthogonal to the polarization of the fixed beam of the first plurality of fixed beams.  
           [0015]    In some embodiments, the respective polarization of each of the first and second plurality of fixed beams is one of two substantially orthogonal polarizations.  
           [0016]    In some embodiments, the respective polarization of each of the first and second plurality of fixed beams is one of two substantially orthogonal polarizations, and each of the beams from both the first and second plurality of fixed beams are preferably transmitted from a single dual polarization antenna array capable of providing two substantially orthogonal polarizations simultaneously.  
           [0017]    In some embodiments, the apparatus further comprises a first antenna array and a second antenna array, the first antenna array being adapted to produce each fixed beam of said first and second plurality of fixed beams having a first of said two substantially orthogonal polarizations and the second antenna array being adapted to produce each fixed beam of said first and second plurality of fixed beams having a second of said two substantially orthogonal polarizations.  
           [0018]    In some embodiments, the apparatus further comprises a first multiple fixed beam former connected to the first antenna array and a second multiple fixed beam former connected to the second antenna array.  
           [0019]    In some embodiments, the apparatus further comprises a fixed beam forming matrix connected to the first antenna array and the second antenna array.  
           [0020]    In some embodiments, the apparatus further comprises a respective receiver coupled to receive for each of said first and second pluralities of fixed beams a respective receive signal over the fixed beam.  
           [0021]    In some embodiments, the apparatus further comprises a respective receiver coupled to receive for each of said first and second pluralities of fixed beams a respective receive signal over the fixed beam.  
           [0022]    In some embodiments, the apparatus further comprises for each pair of fixed beams comprising a fixed beam of said first plurality of the corresponding fixed beam of the second plurality of antennas, a respective combiner adapted to perform diversity combining of the receive signals received over the pair of fixed beams.  
           [0023]    In some embodiments, the apparatus further comprises for each pair of fixed beams comprising a fixed beam of said first plurality of the corresponding fixed beam of the second plurality of antennas, a respective combiner adapted to perform diversity combining of the receive signals received over the pair of fixed beams.  
           [0024]    According to another broad aspect, the invention provides a method which involves transmitting a first channel on at least two adjacent fixed beams of a first plurality of fixed beams defining a coverage area, with each pair of adjacent fixed beams of said plurality of fixed beams partially overlapping and having substantially orthogonal polarization.  
           [0025]    In some embodiments, the method further comprises transmitting on each of said first plurality of fixed beams a respective unique composite signal, each composite signal comprising said first channel and a respective at least one unique traffic channel.  
           [0026]    In some embodiments, the first channel is a CDMA signal.  
           [0027]    In some embodiments, the method further comprises receiving a respective receive signal over each of first said plurality of fixed beams.  
           [0028]    In some embodiments, the method further comprises receiving a respective receive signal over each of a second plurality of fixed beams comprising a corresponding fixed beam for each fixed beam of said first plurality of fixed beams which is substantially co-extensive with the fixed beam of the first plurality of fixed beams, and has a respective polarization which is substantially orthogonal to the polarization of the fixed beam of the first plurality of fixed beams.  
           [0029]    In some embodiments, the respective polarization of each of the first and second plurality of fixed beams is one of two substantially orthogonal polarizations.  
           [0030]    In some embodiments, the method further comprises receiving a respective receive signal over each of said first plurality of fixed beams.  
           [0031]    In some embodiments, the method further comprises performing, for each pair of fixed beams comprising a fixed beam of said first plurality of the corresponding fixed beam of the second plurality of antennas, diversity combining of the receive signals received over the pair of fixed beams.  
           [0032]    Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    The invention will now be described in greater detail with reference to the accompanying diagrams, in which:  
         [0034]    [0034]FIG. 1 is a diagram of a conventional wireless communication cell;  
         [0035]    [0035]FIG. 2 is a schematic of wireless transmission system provided by an embodiment of the invention;  
         [0036]    [0036]FIG. 3 is a schematic of a wireless transceiver system provided by an embodiment of the invention; and  
         [0037]    [0037]FIG. 4 is a plot of an example antenna power radiation pattern of the wireless transceiver in FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    In order to transmit unique traffic channels on each beam in a coverage area while simultaneously transmitting common overhead or overhead channels (e.g. pilot, sync, and paging channels) over all of the beams in the coverage area, a wireless transmission system using fixed beams that does not require complex calibration equipment and algorithms is provided. FIG. 2 illustrates a wireless transmission system  400  provided by an embodiment of the invention that may be deployed within a coverage area  60 .  
         [0039]    The wireless transmission system  400  has a first transmission signal chain  401  and a second transmission signal chain  402 . The first transmission signal chain  401  has in series a transmission signal combiner A  410 , a transmitter A  412  and an antenna  500 . Similarly, the second transmission signal chain  402  has a transmission signal combiner B  420 , a transmitter B  422  and an antenna  510 . It would be understood by those skilled in the art that an additional combination of hardware, software and firmware would be required to support the wireless transmission system  400  to make it operable. Illustrated in FIG. 2 are only those components necessary to discuss aspects of the invention.  
         [0040]    The wireless transmission system  400  operates to over the coverage area  60  with two fixed beams  50  and  51  originating from transmission signal chains  401  and  402  respectively. At least one unique traffic channel is transmitted through each transmission signal chain  401  and  402 . Simultaneously, both transmission signal chains  401  and  402  transmit a common overhead channel so that the overhead channel can be received anywhere within the coverage area  60 . Within each transmission signal chain  401  and  402  at least one unique traffic channel is combined with the common overhead channel in the respective transmission signal combiners  410  and  420  before transmission via the fixed beams  50  and  51  respectively.  
         [0041]    Referring to the example illustrated in FIG. 2, the transmission signal chain  401  is used to transmit a first unique traffic channel TRAFFIC A  404  and the common overhead channel BROADCAST  408  and the transmission signal chain  402  is used to transmit a second unique traffic channel TRAFFIC B  406  and the common overhead channel BROADCAST  408 . The fixed beams  50  and  51  are launched from antennas  500  and  510  respectively. It should be stressed that the at least one unique traffic channel transmitted through each transmission signal chain  401  and  402  can only be received in the region of the coverage area  60  that is covered by the fixed beams  50  and  51  respectively. That is, traffic channel TRAFFIC A can only be received in the region of the coverage area covered by fixed beam  50  and the same is true for traffic channel TRAFFIC B and fixed beam  51 . However, since the combination of fixed beams  50  and  51  provide coverage to the entire coverage area  60 , the common overhead channel BROADCAST  408  can be received everywhere within the coverage area  60 .  
         [0042]    In order to avoid any destructive combination of the simultaneous transmissions containing the common overhead channel in an area  65  where the two fixed beams  50  and  51  overlap, the fixed beam  50  is launched with a transmission polarization orthogonal to that of the fixed beam  51 . For example beam  50  could be transmitted with  450  polarization, and beam  51  could be transmitted with −45° polarization. The combination of the received signals from the two beams  50 , 51  in their overlap region  65  will have a variable polarization but will almost never see destructive interference of the magnitude that would substantially lead to the cancellation of the power of the received signal in the overlap region  65 . Polarization mismatch with the mobile antenna may occur, but this is no different to a full sector single polarization transmission system with polarization mixing in the propagation path. In other words, a sector covered by a single wide-beam would also be influenced by polarization mismatch between the base station antenna and the mobile antenna.  
         [0043]    [0043]FIG. 2 provides a single example embodiment of the invention. More generally, an embodiment of the invention provide for a coverage area in which an arbitrary number of fixed beams are employed, each fixed beam having a launch (transmission) polarization that is substantially orthogonal to the adjacent fixed beams. Individual traffic channels are sent on each beam, and a common overhead channel is sent on all beams, or more generally at least two of the beams. The result will be that within the region where the adjacent fixed beams overlap there will be only minimal signal degradation to the common signal channel transmitted on adjacent beams due to destructive combination of the common signal channel received from adjacent beams.  
         [0044]    [0044]FIG. 3 illustrates a more detailed example of a wireless transceiver system  200  that may be deployed within a coverage area according to an embodiment of the invention that operates to provide sector wide coverage for the overhead channels on the downlink and reception diversity on the uplink.  
         [0045]    The system  200  of FIG. 3 has a full sector splitter  290  connected to receive a common overhead signal  280  which might for example be some combination of pilot and control information. The common overhead signal is input to each of three beam front end modules, namely beam A front end module  210 , beam B front end module  220 , and beam C front end module  230 . The beam front end modules are detailed below.  
         [0046]    Each beam front end module is also connected to receive a respective transmit traffic signal. Thus, beam A front end module receives at input port  202  Tx traffic A, beam B front end module receives at input port  204  Tx traffic B, and beam C front end module  230  receives at input port  208  Tx traffic C. Each beam front end module also outputs a respective receive traffic signal. Thus, beam A front end module outputs at output port  203  Rx traffic A, beam B front end module outputs at output port  206  Rx traffic B, and beam C front end module  230  outputs at output port  209  Rx traffic C.  
         [0047]    Each of the beam front end modules is connected bi-directionally to a respective input beam-port of each of two multiple beam formers  240 , 250 . More specifically, beam A front end module  210  output  215  is connected bi-directionally to beam-port  241  of the first multiple beam former  240 , and beam A front end module  210  output  214  is connected bi-directionally to beam-port  251  of the second multiple beam former  240 . Similarly, beam B front end module  220  output  225  is connected bi-directionally to beam-port  242  of the first multiple beam former  240 , and beam B front end module  220  output  224  is connected bi-directionally to beam-port  252  of the second multiple beam former  240 . Finally, beam C front end module  230  output  235  is i connected bi-directionally to beam-port  243  of the first multiple beam former  240 , and beam C front end module  230  output  234  is connected bi-directionally to beam-port  253  of the second multiple beam former  240 .  
         [0048]    Each of the multiple beam formers  240 ,  250  is connected to a respective antenna array  260 ,  270  through respective sets of antenna ports  249  and  259 . The first antenna array  260  operates to provide a coverage area  115  with a first set of three fixed beams  108   a ,  110   a ,  112   a  at +45° polarization. Similarly, the second antenna array  270  operates to provide the same coverage area  115  with a second set of three fixed beams  108   b , 110   b , 112   a  at −45° polarization which are each substantially coextensive with corresponding beams of the first set of three fixed beams. More generally, any orthogonal polarizations may be employed.  
         [0049]    The details of the beam A front end module  210  will now be described by way of example, the other two beam front end modules being the same. Beam A front end module  210  has a Tx combiner  291  which operates to combine the common overhead signal and the Tx traffic A signal and output this to an input  211  of a transceiver module  199  which connects to a transmitter component  30  within the transceiver module  199 . The transmitter component  30  is connected through a duplexer  32  in the forward direction to beam-port  241  of the first multiple beam former  240 . In the reverse direction, beam-port  241  of the first multiple beam former  240  is connected through the duplexer  32  to a receiver component  30  in the transceiver module  199  an output  212  of which is connected to an Rx combiner  292 . The duplexer  32  operates to select a transmission signal band or receive signal band for the appropriate routing of Tx and Rx signals through the transceiver module  199 . The transceiver module  199  also has a diversity receiver component  33  which connects the first beam-port  251  of the second multiple beam former to the Rx combiner  292 .  
         [0050]    Beam B front end module  220  is connected in the same manner, excepting that its diversity receive signals will be received from beam-ports  242  and  252  of the first and second multiple beam formers  240 , and its transmit signals will be output to beam-port  252  of the second multiple beam former. Similarly, Beam C front end module  230  is connected in the same manner, excepting that its diversity receive signals will be received from beam-ports  243  and  253  of the first and second multiple beam formers  240 ,  250  and its transmit signals will be output to beam-port  242  of the first multiple beam former. It can be seen in the static configuration of FIG. 3 that in fact, beam-ports  251 ,  242  and  253  do not need to be bi-directional since these are only used for receive signals.  
         [0051]    Although the present embodiment has been described as having two antenna arrays providing two sets of co-extensive fixed-beams such that each set of fixed beams has a substantially orthogonal polarization to the other set of fixed-beams, in another embodiment the two sets of fixed-beams are provided by a single dual polarization antenna array capable of providing two sets of co-extensive fixed beams that are substantially orthogonal in terms of their respective polarizations.  
         [0052]    In operation, in the forward direction, the common overhead signal  280  is sent to each of the three beam front end modules  210 ,  220 ,  230  and is transmitted on beam  108   a ,  110   b  and  112   a . Adjacent beams of this set have orthogonal polarization so that destructive interference is avoided. Tx traffic A is transmitted only on beam  108   a . Tx traffic B is transmitted only on beam  110   b , and Tx traffic C is transmitted only on beam  112   a.    
         [0053]    In the reverse direction, signals received on coextensive fixed beams  108   a  and  108   b , are combined in the diversity combiner  292  of the beam A front end module  210  and output as Rx traffic A.  
         [0054]    Similarly, signals received on coextensive fixed beams  110   a  and  110   b  are combined in the diversity combiner (not shown) of the beam B front end module  220  and output as Rx traffic channel B.  
         [0055]    Finally, signals received on coextensive beams  112   a  and  112   b  are combined in the diversity combiner (not shown) of the beam C front end module  230  and output as Rx traffic channel C.  
         [0056]    It is noted that Rx traffic channel A may contain signal content from mobile units in the area of beams  108   a ,  108   b , but may also contain signal content from mobile units, either in the area of beams  110   a ,  110   b  where they overlap with beams  108   a ,  108   b  or in areas where obstructions result in multipath, and a similar situation exists for the other received traffic signals. Upstream processing (not shown) may be provided to resolve these signals if necessary.  
         [0057]    The multiple beam formers operate to simultaneously direct a Tx signal received into one of its beam-ports to one of the three fixed beams provided by the antenna array. The fixed beam selected is dependent upon which beam-port the Tx signal is received into. For example, the multiple beam former  240  will direct the Tx signal received into beam-port  241  onto fixed beam  108   a  by way of amplitude and phase shaping, while simultaneously directing the Tx signal received into beam-port  243  onto fixed beam  112   a . Similarly, multiple beam former  250  will direct a Tx signal received into beam-port  252  onto fixed beam  110   b . Beamports  241 ,  252  and  243  are also able to send Rx signals in the reverse direction after these Rx signals have been coupled from fixed beams  108   a ,  110   b  and  112   a  respectively. As mentioned above, beam-ports  251 ,  242  and  253  are only used for receive signals and thus only receive Rx signals coupled from fixed beams  108   b ,  110   a  and  112   b  respectively.  
         [0058]    Additionally, information originally scheduled to by transmitted on Tx traffic A could be re-routed by backend electronics (not shown) onto Tx traffic B (or Tx traffic C) if the mobile receiver has moved into the coverage area of a different beam. Similar re-routing could be done for Tx traffic B and Tx traffic C.  
         [0059]    Although two antenna arrays forming three beams per polarization per sector are used in this example of the preferred embodiment, any number of beams and antenna arrays per sector greater than one may be used while remaining within the scope of the invention.  
         [0060]    The received signal strength of a pilot channel (or any other overhead channel, e.g., a control channel) at any point in the coverage area is determined by the vector sum of all pilot channel signals received from each beam. Both the wireless transmission system  400  and the wireless transceiver system  200  provide systems in which adjacent beams are preferably of alternating polarizations. A coverage area, such as a sector of a cell, covered by adjacent narrow beams with alternating polarizations results in a combined radiation pattern in the coverage area that is substantially a constant amplitude but has an undetermined polarization. In other words, alternating polarization combines beams with orthogonal polarizations, having unknown relative phase, that in turn produce a combined radiation pattern having a substantially constant amplitude across the sector; however, the polarization of the combined radiation pattern is variable. The relative phases are unknown since no effort has been made to calibrate the internal connections of the components that comprise the wireless transceiver system  200  and the wireless transmission system  400 . The lack of calibration results from internal signal paths that have uncontrolled phase delays. However, this is the situation the invention is intended to operate within since calibration is a lengthy and costly installation feature in a wireless system, and avoiding it would be desirable. The invention would of course still work if such calibration efforts were made.  
         [0061]    [0061]FIG. 4 shows a simulated radiation pattern for the wireless transceiver system  200  illustrated in FIG. 3 for the three adjacent beams  108   a ,  110   b  and  112   a . Considering the first beam  108   a  and second beam  110   b  having +45 degree polarization and −45 degree polarization respectively, the effect of varying the relative phase is that the resultant polarization varies anywhere from being completely in phase to completely anti-phase at the low cross over  320 . Thus, there is always power at the crossover angle, but the polarization is uncontrolled. This is not a problem since it is the power of the received signal that is important not its received polarization. A similar situation exists at the low cross over  321  for the beams  110   b  and  112   a.    
         [0062]    While the preferred embodiment of the present invention has been described and illustrated, it will be apparent to persons skilled in the art that numerous modifications and variations are possible.