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`A Mobile Base Station Phased Array Antenna
`
`C. Alaki‘a and S.P. Stapleton
`School 0 Engineering Science
`Simon Fraser University
`Burnaby, BC, Canada
`
`ABSTRACT
`
`A mobile communications base station antenna, which
`utilizes a cylindrical array design. is presented. A
`scanning antenna beam can be achieved by arranging a
`number of travelling wave patch antennas in acylindrical
`array configuration. Using a switching matrix, different
`subsets of antenna elements, in the array, can be excited,
`thus producing a narrow steerable beam. Narrow scan-
`ning beams allow these base station antennas to retain
`both the high gain characteristic of directional antennas
`as well as the multidirectional characteristic of omnidi-
`rectional antennas. For a given transmit power such base
`station antennas can broadcast greater distances than
`omnidirectional base station antennas, while still being
`able to communicate with users in a 360' radius (at
`differenttime intervals). An alternative perspective is that
`scanning antennas can be placed in much closerproximity
`than omnidirectional antennas with the same co~channel
`interference. This decrease in cell size means that a larger
`number of mobile users can be concentrated in a smaller
`area without increasing spectrum allocation.
`
`INTRODUCTION
`
`In recent years the need for adaptive antennas in mobile
`communication systems has been steadily growing. High
`system capacity requirements make issues such as spec-
`tral efficiency and frequency reuse extremely important
`in the field of mobile communications. Ctr-channel
`interference limits the capacity achievcable in present day
`systems.
`In cellular systems, frequency reuse is accomplished
`by dividing coverage areas into cells. Frequencies are
`then allocated to each cell in a way that no two adjacent
`cells use the same frequencies. Further frequency reuse
`can be accomplished by dividing each cell into sectors.
`However, sectorization has disadvantages in terms of
`antenna cost and mounting costs. Furthermore,
`the
`hardware associated with sectorization may not be used
`efficiently due to the variable nature of system loading
`and the fixed nature of sector antennas. If base-station
`antennas exist which have variable beam directions and
`widths, then sector sizes can be dynamically adapted to
`accommodate the users in them.
`In mobile communication systems with light or
`moderate loading, issues other than throughput may be of
`prime concem. Some mobile communication systems
`may be allocated only a single channel making sectori-
`zation impossible.
`In this case it may be desireable to
`increase the coverage radius and/or reduce the transmit
`power of the base station. A base station antenna that has
`a high gain and is steerable can be used to accomplish
`these goals. High gain means that transmitted power
`
`levels can be reduced without any degradation ofreceived
`signal strength. This is a great advantage for portable
`units since using 10werportable transmit power translates
`to longer battery life and simpler RF hardware design.
`In digital mobile networks, in Figure 1, this scanning
`beam antenna can be used to realize improved perform-
`ance in throughput.
`Since the gain of this antenna is higher than that for
`an omnidirectional antenna,
`the mobile sites can be
`located further away or alternatively the transmit power
`can be reduced for the same coverage area. The beam
`scans rapidly until it locates a mobile user requesting to
`transmit and then it locks on to that user so data trans-
`mission can occur. Locking can occur by constantly
`menitoring the power level at +/- l beamwidth. At this
`time, mobile stations transmitting from other directions
`are blocked, reducing network collisions. Once the call
`is complete, the beam then continues scanning looking
`for other users.
`Reduced co-channel interference is also an advantage
`obtained. Due to the directionality of the base station
`antenna beam, there is less oo-channel interference power
`received from other the base stations. This means thatbase
`stations can be placed in closer proximity to one another
`resulting in an increase in the number of users on the
`network.
`An antenna which has a shaped beam in the vertical
`plane can also be used to realize advantages over omni-
`directional antennas. Antennas. such as the dipole, have
`omnidirectional antenna patterns which waste power by
`radiating in directions where no users are present. By
`pattern shaping in the elevation plane, (Figure 2), the
`wasted power, being radiated upward and into the ground,
`can bereduccd. Interference into adjacent cells could also
`be reduced The increased gain realized by producing
`antennas with shaped beams would also mean that lower
`power levels could be used to transmit further distances.
`
`SYSTEM DESCRIPTION
`
`Cylindrical arrays have been utilized in the past for radar
`and beacon applications [1]. The topology for the
`scanning cylindrical array antenna is shown in Figure 3.
`The beam direction is chosen by applying power to a
`subset of elements on the appropriate side of the array
`(Figure 4). This is done by using an RF switching matrix
`to route power to the desired elements.
`The proposed antenna system has three major sub-
`systems:
`the radiating elements. the beam formrng feed
`and the switching matrix. A block diagram showing the
`major system components is shown in Figure 5. Another
`possible system configuration is shown in Figure 6. By
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`ICWC '92
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`0-7803-0713—2/92 $3.00 © 1992 IEEE
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`5.5
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`TMO1011
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`119
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`combining a number of feed networks into a single
`antenna system, an antenna with multiple independently
`steerable beams is achieved.
`
`TRAVELLING WAVE MICROSTRIP
`PATCH RADIATORS
`
`The radiating elements chosen for the antenna array were
`rectangular microstrip patches [2]. Patches are desirable
`due to their low costand their extremely low profile. They
`are inexpensive to manufacture and due to the use of
`printed circuit techniques1n their production. the design
`1s highly repeatable. Since they are low profile, they also
`can be shaped conformally to cylindrical surfaces. They
`are ideal e emcnts to use in cylindrical arrays since they
`have low back radiation.
`
`There are some problems with patch antennas which
`make their use somewhat less attractive. Power efficiency
`of a patch antenna is usually quite low since the radiation
`resistance is typically on the order of a few hundred ohms.
`Since a patch antenna behaves much like a cavity reso-
`nator. it has a narrow VSWR-bandwidth, typically in the
`order of 2%. Wider VSWR-bandwidths, in the order of
`7%. can be obtained using travelling wave antennas.
`One of the difficulties in making patch antenna arrays
`cost and power efficient is in coming up with suitable feed
`networks for the array. Corporate feeds are both lossy,
`space consuming and complex for shaped patterns. For
`this reason the travelling wave antenna is ideally suited
`for microsttip patch arrays. This type of antenna achieves
`the desired current distribution by varying the width and
`spacing of patches spaced along a microstrip feedline. It
`has been shown in [4] that the desired shaped antenna
`pattern canbc obtained using a travelling wave patch array
`while at the same time maintaining good antenna power
`efficiency.
`
`H-PLANE PATTERNS
`
`The cylindrical shape of this antenna increases the diffi-
`culty in analyzing the far field radiation pattern analyti-
`cally. To compensate for this difficulty. a program was
`written which plots the radiation pattern of a cylindrical
`array of patches with arbitrary element excitation.
`Selection of appropriate array parameters can be
`accomplished by relating the theory used for periodic
`lineararrays to the cylindrical array. The main parameters
`to vary are the array radius, the number of elements. and
`the magnitude and phase of the power radiated by the
`elements.
`The geometry used to analyze a circular array [3] is
`shown in Figure 7. Assuming far-field conditions and
`linear polarization the radiation pattern from the array can
`be expressed by the following summation [3].
`
`Exam=_§=II.SF'.(6.¢—¢.)e
`
`jla linen“. ‘ O.)
`
`(1)
`
`The term SF_(9, ¢ — e.) takes into account the magnitude
`variations of each patch as a function of angle. The term
`I. is a complex number representing the square-mot-
`magnitude and phase, C , of the power to each patch. The
`propagation constant is represented as k.
`The phase of the current distribution is chosen to
`compensate for the curvature of the cylinder. Figure 8
`shows the geometries involved in calculating the phase
`distribution. Using simple geometry,
`
`c=a—acos¢,,
`
`(2)
`
`Any phase shift which is constant to all elements can be
`deleted with no change to the pattern. Therefore, the phase
`distribution should be
`
`C= —ka cos ti),l
`
`(3)
`
`A cosine amplitude distribution along the array surface
`was chosen in order to achieve a good tradeoff between
`beamwidth and sidelobe level. The chosen current
`distribution was found to give quite reasonable antenna
`patterns. This current distribution can however, be varied
`to cater to other pattern shapes. By altering the phase
`distribution, wider pattern beamwidtlts can be obtained.
`This characteristic can be used to add another dimension
`of flexibility to the antenna in terms of variable sector
`size. Figure 9 shows different patterns obtained by
`varying the phase distribution.
`The number of elements needed for the array is mainly
`determined by the desired beamwidth and the scanning
`step size. The number of elements may also be adjusted
`to allow for switching matrix optimization. Since the
`radiators used do not produce much radiation past 90
`degrees it is not useful to excite elements that are more
`than 90 degrees from boresight. Otherwise, back lobes
`may increase considerably which will
`interfere with
`adjacent cells. It was foundthat a 32 element cylindrical
`array in which 8 elements are excited at a time will achieve
`a 12 degree scanning step size and a 20 degree beamwidth.
`Calculations show that a narrower beam can be obtained
`by increasing the number of elements which are excited
`in the array (Figure 10).
`In linear arrays, the optimum spacing for maximum
`directivity and no grating lobes is M [3]. Fora cylindrical
`array 8 similar analogy can be made. If the problem is
`looked at geometrically, a circular-sector array in which
`the sector angle15 small can be approximated as a linear
`array.50 if the elements are spaced a distance of m then
`the cylinder radius a, must be chosen so that
`
`2m 1
`77:5
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`‘4)
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`(5)
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`120
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`Calculations show that this radius is indeed close to the
`optimum value. Figure ll shows the antenna pattern for
`Circular arrays with varying values of radius. For an 800
`MHz antenna. (5) gives a radius value of 0.95m. The
`figure also shows thatby making the radius slightly larger,
`the beamwidth becomes narrower at the cost of higher
`side lobes. Making the radius smaller reduces side lobe
`levels at the cost of a wider beamwidth.
`
`CONCLUSIONS
`
`The use of cylindrical array scanning antennas in the
`mobile communication field looks quite promising. The
`antennacan be used to realize advantages such as reduced
`portable transmit power, reduced err-channel interfer-
`ence. hardware savings. low manufacturing costs, low
`installation costs, and increased system capacity. Work is
`proceeding on the fabrication of a subset of the system.
`
`REFERENCES
`
`[l]
`
`J.E. Boyns et-al, "Step-Scanned Circular-Array
`Antenna," IEEE Trans. Antennas and Propagat,
`Vol. AP-l8, No. 5. pp. 590-596, Sept. 1970.
`
`[2] A.G. Demeryd, "Linearly Polarized Microsm'p
`Antennas," IEEE Trans. Antennas and Propagat.
`Vol. AP-24, No. 6, pp. 846-851, Nov. 1976.
`
`[3] C.A.Balat1is, Antenna Theory. New Yorkzl-larper &
`Row. 1982
`
`[4] C. Alakija, A Power Efficient Pattern Synthesis
`Algorithm for Travelling Wave Microstrip Patch
`Antenna Arrays: Application to Mobile Base Station
`Anéennar, Masters Thesis, Simon FraserUniversity,
`19 l.
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`Figure 3: Cylindrical array of travelling wave patch
`antennas.
`
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`Figure 4: Sub element array excitation.
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`Figure 1: Digital communication system with scan-
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`Figure 5: Scanning array system diagram.
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`121
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`thh different phase distributions.
`
`Figure 8: Geometry for calculating phase distribution.
`
`Figure 7; Circular array geometry.
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`m.
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`Figure 9_: Simulated cylindrical array patterns
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`Figure 10: Patterns of a 32 clement array with
`varying number of excited elements.
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`Figure 11: Antenna patterns versus array radius.
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