Abstract:
Antenna systems are used for transmitting common overhead channels (pilot, sync, and paging channels) over a whole sector while transmitting and receiving unique traffic channels on individual beams in the sector. Each beam in the sector is transmitted at a frequency offset from other beams in the sector. The offset frequency is chosen such that the effect of cancellation of the pilot channel caused by the summing of signals from multiple beams is minimized. Alternative, each beam in the sector can have a time dependent phase offset relative to each other to minimize the effect of cancellation of the pilot channel caused by the summing of signals from multiple beams. System capacity is substantially increased since the number of traffic carrying beams per sector is increased without using more pilot channel PN offsets. Beams are fixed and the same antennas are used for the overhead channels as the traffic channels, obviating the need for complex algorithms and calibration procedures.

Description:
[0001]    This invention relates in general to CDMA cellular communication systems and in particular to methods and apparatus for increasing the capacity of such systems.  
         BACKGROUND  
         [0002]    CDMA digital cellular systems are currently in widespread use throughout North America providing telecommunications to mobile users. In order to meet the demand for transmission capacity within an available frequency band allocation, CDMA digital cellular systems divide a geographic area to be covered into a plurality of cell areas. Within each cell is positioned a base station with which a plurality of mobile stations within the cell communicate.  
           [0003]    In general, it is desired to have as few base stations as possible, since base stations are expensive, and require extensive effort in obtaining planning permission, and in some areas, suitable base station sites may not be available. In order to have as few base stations as possible, each base station ideally has as large a capacity as possible in order to service as large a number of mobile stations as possible. The key parameters that determine the capacity of a CDMA digital cellular system are: processing gain, ratio of energy per bit to noise power, voice activity factor, frequency reuse efficiency and the number of sectors in the cell-site antenna system.  
           [0004]    One method of achieving an increase in capacity is to replace a wide beam width antenna with an antenna array that allows the formation of a number of narrower beam widths that cover the area of the original beam. Referring to FIG. 1, a conventional CDMA communication cell  100  is shown comprising 3 adjacent hexagonal sectors, alpha  102 , beta  104  and gamma  106 . Each cell comprises an antenna tower platform  120  located at the intersection of the 3 sectors. The antenna tower platform  120  has 3 sides forming an equal-lateral triangle. Each sector has 3 antennas (only antennas in sector alpha  102  shown) a first antenna  114 , a second antenna  116  and third antenna  112  mounted on a side of the antenna tower platform  120 . Each sector also has 3 beams (only beams in sector alpha  102  shown) a first beam  108 , a second beam  110  and a third beam  112 . The 3 beams  108 ,  110 ,  112  are adjacent with some overlap. The 3 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 (i.e. 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.  
           [0005]    An improvement is to use multiple narrow beams for the traffic channels and transmit the overhead channels (pilot, sync, and paging channels) over the whole sector so that the pilot channel is common to all the narrow beams used by the traffic channels in that sector. This leads to substantial gains in capacity. For example, a change from a system with a single beam per sector to a system with 3 beams per sector with a common pilot channel yields a 200 to 300% increase in capacity. It is therefore desirable that the pilot channel be broadcast over the area covered by the original wide beam. A possible arrangement is to use multiple beams per sector for the traffic channels and transmit the overhead channels over a separate wide beam antenna covering the whole sector. However, this requires the expense of extra hardware as well as the calibration and adjustment needed to match the phase of the pilot channel with the phase of the traffic channels over time and temperature.  
           [0006]    Another possible solution is to use adaptive antenna array techniques to transmit and receive multiple narrow beams for the traffic channels and transmit the overhead channels over the whole sector on the same antenna array. However, this requires complex calibration equipment and algorithms.  
           [0007]    Yet another solution is to use an antenna array that transmits and receives multiple sectors over fixed narrow beams for the traffic channels and transmit the pilot channel on the same fixed narrow beams. However, the problem with this approach is that the strength of the pilot channel signal at any point in the sector is determined by the vector sum of all of the pilot channel signals from each beam. Since the pilot channel signals from each beam are coherent, areas where the vector sum of the pilot channel signals is null or severely degraded will occur. This can result in dropped calls when a mobile station enters one of these areas.  
           [0008]    There is thus an advantage to provide an antenna array that uses fixed narrow beams for transmitting and receiving the traffic channels on multiple beams and can 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  
         [0009]    The invention may be summarized according to a first broad aspect as an antenna system having multiple antennas defining a respective plurality of fixed beams that together cover a sector and are connected to a beam-forming matrix. Transceivers are connected respectively to the beam-forming matrix to drive the plurality of antennas, with signals comprising common overhead channels. The signals may be IS-95, IS-2000 or any other similar CDMA communications standard designed for terrestrial cellular communications. In accordance with this first broad aspect the transceivers provide transmit frequencies that are slightly offset from one another. The offsets are chosen such that undesirable effects of the signal cancellation are reduced. More particularly, the offsets are chosen such that the overall system performance is optimized.  
           [0010]    The invention may be summarized according to a second broad aspect as an antenna system having multiple antennas defining a respective plurality of fixed beams that together cover a sector and are connected to a beam-forming matrix. The transceivers are connected respectively to the beam-forming matrix to drive the plurality of antennas, with signals comprising common overhead channels. The signals may be IS-95, IS-2000 or any other similar CDMA communications standard designed for terrestrial cellular communications. In accordance with this second broad aspect the transceivers provide transmit phases that have time dependent offsets with respect to one another. The offsets are chosen such that undesirable effects of the signal cancellation are reduced. More particularly, the offsets are chosen such that the overall system performance is optimized.  
           [0011]    The invention may be summarized according to a third broad aspect as an antenna system having a digital beam former connected to a plurality of transceivers and a plurality of antennas defining a respective plurality of fixed beams that together cover a sector. The transceivers are connected to the plurality of antennas to drive them with signals comprising common overhead channels. The signals may be IS-95, IS-2000 or any other similar CDMA communications standard designed for terrestrial cellular communications. In accordance with this third broad aspect the transceivers provide transmit frequencies that are slightly offset from one another. The offsets are chosen such that undesirable effects of the signal cancellation are reduced. More particularly, the offsets are chosen such that the overall system performance is optimized.  
           [0012]    The invention may be summarized according to a fourth broad aspect as an antenna system having a digital beam former connected to a plurality of transceivers and a plurality of antennas defining a respective plurality of fixed beams that together cover a sector. The transceivers are connected to the plurality of antennas to drive them with signals comprising common overhead channels. The signals may be IS-95, IS-2000 or any other similar CDMA communications standard designed for terrestrial cellular communications. In accordance with this fourth broad aspect the transceivers provide transmit phases that have time dependent offsets with respect to one another. The offsets are chosen such that undesirable effects of the signal cancellation are reduced. More particularly, the offsets are chosen such that the overall system performance is optimized.  
           [0013]    Advantageously, the ability to use a plurality of fixed beams with common overhead channels results in a significant increase in system capacity.  
           [0014]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of the specific embodiments of the invention in conjunction with the accompanying figures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a diagram of a conventional tri-cellular CDMA communication cell modified to show 3 narrows beams in place of the normal single wide beam per sector;  
         [0016]    [0016]FIG. 2A is a diagram of an antenna system of sector alpha of the CDMA communication cell of FIG. 1;  
         [0017]    [0017]FIG. 2B is a diagram of an alternative antenna system of sector alpha of the CDMA communication cell of FIG. 1;  
         [0018]    [0018]FIG. 3 is a diagram showing a transceiver of FIG. 2A and 2B in greater detail.  
         [0019]    [0019]FIGS. 4A and 4B are diagrams showing the vector addition of signals from the first beam and the second beam. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    In order to transmit and receive unique traffic channels on each beam in a sector while transmitting common overhead channels (pilot, sync, and paging channels) over all of the beams in the sector an antenna system using fixed narrow beams that does not require complex calibration equipment and algorithms is provided.  
         [0021]    To this end, FIG. 2A shows a conventional antenna system  200  that is within sector alpha  112 . The sectors beta  104  and gamma  106  have identical antenna systems. The antenna system  200  defines a first beam  108 , a second beam  110  and a third beam  112 . The three beams  108 ,  110 ,  112  are radiation/reception patterns formed by a first antenna  114 , a second antenna  116  and a third antenna  118  respectively. The three antennas  114 ,  116 ,  118  are connected to a beam-forming matrix  240  that may be, for example, a Butler matrix. The beam-forming matrix  240  comprises three bi-directional ports: a first port  242 , a second port  244  and a third port  246 . The input signals of the first port  242 , the second port  244  and the third port  246  are transmitted on the first beam  108 , the second beam  110  and the third beam  112  respectively. The signals received on the first beam  108 , the second beam  110  and the third beam  112  are the outputs of the first port  242 , the second port  244  and the third port  246  respectively. The antenna system  200  also comprises a first transceiver  220 , a second transceiver  222  and third transceiver  224 . The first transceiver  220  has an input  226 , an output  228  and a bi-directional port  252 . The second transceiver  222  has an input  230 , an output  232  and a bi-directional port  254 . The third transceiver  224  has an input  234 , an output  236  and a bi-directional port  256 . The first port  242 , second port  244  and third port  246  of the beam-forming matrix  240  are connected to bi-directional port  252  of the first transceiver  220 , bi-directional port  254  of the second transceiver  222  and bi-directional port  256  of the third transceiver  224  respectively.  
         [0022]    [0022]FIG. 2B shows another conventional antenna system  202  that may be deployed within sector alpha  112 . The antenna system  202  defines a first beam  108 , a second beam  110  and a third beam  112 . The three beams  108 ,  110 ,  112  are radiation/reception patterns formed by a first antenna  114 , a second antenna  116  and a third antenna  118  respectively. The antenna system  202  also comprises a first transceiver  220 , a second transceiver  222  and third transceiver  224 . The first transceiver  220  has an input  226 , an output  228  and a bi-directional port  252 . The second transceiver  222  has an input  230 , an output  232  and a bi-directional port  254 . The third transceiver  224  has an input  234 , an output  236  and a bi-directional port  256 . The three antennas  114 ,  116 ,  118  are connected to the three respective bi-directional ports  252 ,  254 ,  256  of the transceivers  220 ,  222 ,  224 . The antenna system  202  also comprises a digital beam former  260  that has a first input  262 , a first output  268 , a second input  266 , a second output  268 , a third input  270 , a third output  272 , a first beam output  274 , first beam input  276 , a second beam output  278 , a second beam input  280 , a third beam output  282  and a third beam input  284 . The first beam output  274  and input  276  of the digital beam former  260  are connected the input  226  and output  228  of the first transceiver  220  respectively. The second beam output  278  and input  280  of the digital beam former  260  are connected the input  230  and output  232  of the second transceiver  222  respectively. The third beam output  282  and input  284  of the digital beam former  260  are connected the input  234  and output  236  of the third transceiver  220  respectively.  
         [0023]    Although three antennas forming three beams per sector are used in this example of the preferred embodiment, any number of antennas and beams per sector greater than one may be used while remaining within the scope of the invention.  
         [0024]    The transceivers  220 ,  222 ,  224  of FIGS. 2A and 2B are identical in design and are described in greater detail with respect to FIG. 3. For ease of description the transceiver shown in FIG. 3 is given the reference number  300 . Transceiver  300  has its input  302  connected to an input of a modulator  306 . The modulator  306  has an output  308  that is connected to a first input  310  of an up-converter  312 . The up-converter  312  also has a second input  314  and an output  316 . The second input  314  of the up-converter  312  is connected to an oscillator  318  that may be, for example, a digital frequency synthesizer. The output  316  of the up-converter  312  is connected to an input  344  of a duplexor  340  having a bi-directional port  342  connected to the bi-directional port  320  of the transceiver  300 . The transceiver  300  also has an output  322  connected to an output  324  of a demodulator  326 . The demodulator also has an input  328  that is connected to an output  330  of a down-converter  332 . The down-converter  332  also has a first input  334  and a second input  336 . The first input  334  of the down-converter  332  is connected to an oscillator  338  and the second input  336  of the down-converter  332  is connected to an output  346  of the duplexor  340 . The up-conversion stage of the transceiver  300 , comprising the up-converter  312  and oscillator  318 , are shown as a single stage for convenience. In reality the up-conversion may be done in a plurality of stages. Similarly, the down-conversion stage of the transceiver  300 , comprising the down-converter  332  and oscillator  338 , are shown as a single stage for convenience. In reality the down-conversion may be done in a plurality of stages.  
         [0025]    Referring to FIG. 2A, the signals on input  226 , input  230  and input  234  of transceiver  220 , transceiver  222  and transceiver  224  respectively are digital baseband signals that are transmitted on the first beam  108 , the second beam  110  and the third beam  112  respectively. The signals on output  228 , output  232  and output  236  of transceiver  220 , transceiver  222  and transceiver  224  respectively are digital baseband signals that are received on the first beam  108 , the second beam  110  and the third beam  112  respectively.  
         [0026]    The digital baseband signals on input  226 , input  230  and input  234  of transceiver  220 , transceiver  222  and transceiver  224  respectively may be any CDMA standard digital data stream adapted to be received by a plurality of mobile stations (not shown) within the area covered by the first beam  108 , the second beam  110  or the third beam  112 .  
         [0027]    Similarly, referring to FIG. 2B, the signals on input  262 , input  266  and input  270  of the digital beam former  260  are digital baseband signals that are transmitted on the first beam  108 , the second beam  110  and the third beam  112  respectively. The signals on output  264 , output  268  and output  272  of the digital beam former  260  are digital baseband signals that are received on the first beam  108 , the second beam  110  and the third beam  112  respectively.  
         [0028]    The digital baseband signals on input  262 , input  266  and input  270  of the digital beam former  260  may be any CDMA standard digital data stream adapted to be received by a plurality of mobile stations (not shown) within the area covered by the first beam  108 , the second beam  110  or the third beam  112 .  
         [0029]    The frequency of the oscillator  318  in transceiver  222  is chosen such that the frequency of the output  316  of the up-converter  312  in the transceiver  222  is a standard IS-95 base station transmit frequency, f c . The frequency of the oscillator  318  in the transceiver  220  is chosen such that the frequency of the output  316  of the up-converter  312  in the transceiver  220  is f c  plus an offset frequency, f o . The frequency of the oscillator  318  in the transceiver  224  is chosen such that the frequency of the output  316  of the up-converter  312  in the transceiver  224  is f c  minus the offset frequency, f o . For example, if f c =1940 MHz and f o =40 Hz, then the frequency output of the up-converter  312  in the transceiver  222  equal to 1940 MHz, the frequency output of the up-converter  312  in the transceiver  220  is equal to 1940.00004 MHz and the frequency output of the up-converter  312  in the transceiver  224  is 1939.99996 MHz.  
         [0030]    The signal strength of the pilot channel at any point in the sector is determined by the vector sum of all of the pilot channel signals from each beam. For example, referring to FIG. 4A, the signal at a point from the second beam  110  is represented by vector  402 . The signal at the same point from the first beam  108  is represent by vector  404 . Since the frequency of the signal transmitted on the first beam  108  is offset by f o  from the frequency of the second beam  110 , the vector  404  rotates with respect to vector  402  and hence, the magnitude of resultant vector  406  will fluctuate with a 1/f o  time period. FIG. 4B shows a plot of the magnitude  408  of the result vector  406  versus time  410 . Due to the rotation of vector  404  a minimum  416  value occurs every 1/f o    414 . In an IS-95 forward channel, the frame rate f f  is 50 frames per second or a period of 20 ms. Also, each IS-95 frame is repeated once. Therefore the offset frequency f o  is chosen such that 1/f o    414  is not a multiple of 1/f f    412 . This will prevent a minimum  416  from occurring at the same point in two consecutive frames thus significantly reducing the error rate.  
         [0031]    Since the magnitude of the resultant vector  406  fluctuates with a 1/f o  time period, f o  is chosen by empirical methods such that the overall system performance is optimized. The optimum value of f o , for each base station, is influenced by environmental factors, the maximum velocity of the mobile stations, the frequency band and the over-the-air interface. Typically f o  is greater than 30 Hz and less than 120 Hz for a IS-95 CDMA communication system. Other over-the-air interface standards may have optimum performance at different values of f o .  
         [0032]    The frequencies of oscillator  338  in transceiver  220 , oscillator  338  in transceiver  222  and oscillator  338  in transceiver  224  are identical and chosen such that IS-95 signals at standard frequencies are down-converted and demodulated.  
         [0033]    The traffic channels on each beam are unique and uncorrelated so that no cancellation of the traffic channels occurs.  
         [0034]    In an alternative embodiment, the waveform of the oscillator  318  in transceiver  222  is chosen such that the waveform of the output  316  of the up-converter  312  in the transceiver  222  is a standard IS-95 base station transmit frequency, f c . The waveform of the oscillator  318  in the transceiver  220  is chosen such that the waveform of the output  316  of the up-converter  312  in the transceiver  220  is f c  with a time dependent phase offset within a range of −180° to 180°. The waveform of the oscillator  318  in the transceiver  224  is chosen such that the waveform of the output  316  of the up-converter  312  in the transceiver  224  is f c  with time dependent phase offset within a range of −180° to 180°. The waveform of the output  316  of the up-converter  312  in the transceiver  222  is the reference for 0° phase. The time dependent phase offset within may be sinusoidal, random or any other pattern that results in the phases of the output of oscillator  318  in the transceiver  220 , the output of oscillator  318  in the transceiver  222  and the output of oscillator  318  in the transceiver  224  being incoherent. Hence, the phases of the first beam  108 , the second beam  110  and the third beam  112  are incoherent.  
         [0035]    In the preferred embodiment the signals on input  226 , input  230  and input  234  of transceiver  220 , transceiver  222  and transceiver  224  respectively have identical IS-95 overhead channels (pilot, synchronization and paging channels) and unique IS-95 traffic channels corresponding to mobile station(s) (not shown) that are transmitting/receiving on the first beam  108 , the second beam  110  and the third beam  112  respectively. Mobile stations that move from beam to beam or are in an area of overlapping beams are handled by IS-95 handoff procedures.  
         [0036]    In an alternative embodiment the signals on input  226 , input  230  and input  234  of transceiver  220 , transceiver  222  and transceiver  224  respectively have identical IS-2000 overhead channels and unique IS-2000 traffic channels corresponding to mobile station(s) (not shown) that are transmitting/receiving on the first beam  108 , the second beam  110  and the third beam  112  respectively. Mobile stations that move from beam to beam or are in an area of overlapping beams are handled by IS-2000 handoff procedures.  
         [0037]    It should be noted that while an embodiment of the invention using a Butler matrix  240 , as shown in FIG. 2A, does not require a calibration scheme to compensate for differential phases between the transceivers, an embodiment using a digital beam former  260 , as shown in FIG. 2B, does require a calibration scheme to compensate for differential phases between the transceivers.  
         [0038]    Advantageously, the invention may be used with antenna systems employing diversity schemes, such as space diversity or polarization diversity. In all diversity schemes all overlapping beams should have offset frequencies or time dependent phase offsets.  
         [0039]    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. The scope of the invention, therefore, is only to be limited by the claims appended hereto.