Page 226 - Antennas for Base Stations in Wireless Communications
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Antenna Issues and Technologies for Enhancing System Capacity        199

                      −60

                      −70
                    Mutual coupring [dB]  −80



                      −90

                     −100

                     −110                                       4λ
                        2.1  2.12  2.14  2.16  2.18  2.2
                               Frequency [GHz]
                  Figure 5.25  Mutual coupling characteristics of booster antennas



                  antenna—for instance, a cosecant beam pattern—is preferable because
                  the shaped-beam pattern in a service area is capable of providing a
                  sufficiently constant received power level for mobile users and uniform
                  receiving power levels in spite of the cell edge while imparting only a
                                                                     23
                  slight interference power level to other adjacent cells.  The cosecant
                  beam pattern can be achieved when the elements of the array are fed
                  with linearly varied amplitudes and phases.
                    In the design of cosecant beam patterns, a downward tilting beam is
                  designed to direct sidelobes to the horizon for preventing radiated power
                  to other cells and receiving the signals with constant power levels in
                  its own cell, as indicated in Figure 5.26. The beamtilting angle can be
                  remotely controlled by using an electrical phase-tilt control box from
                  a base station. Figure 5.27 shows a base station antenna array with a
                  remote electric phase-tilt control box. In order to form a complete cosecant
                  beam pattern with fine tilting angle q t , many elements and phase shifters



                              Horizontal line  Tilting angle q  t
                                      Main beam

                    BS’s Antennas                                     BS’s Antennas
                                                    Interference






                  Figure 5.26  Radiation pattern of array antenna
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