Note: Descriptions are shown in the official language in which they were submitted.
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MEANS OF INCREASING CAPACITY
IN CELLULAR RADIO (MOBILE AND FIXED) SYSTEMS
Field of the Invention
The present invention relates to a method of operating an antenna
arrangement in a cellular communications system and more particularly to a
method of assigning frequencies to multi-beam directional antennas.
~;~Caround to the Invention
l0 In conventional cellular radio systems, geographical areas are divided up
into a plurality of adjoining cells, in which mobile stations within a cell
communicate with a base transceiver station. In general, each mobile (or set
of
mobiles :>haring a multiplexed channel) communicating with a base station in a
cell uses. a different carrier frequency to other mobiles in the cell, to
avoid
interfering with the other mobiles. Thus the number of mobiles which can be
served in a cell is limited by the number of available carrier frequencies.
There is
increased capacity demand for use of cellular radio systems, however the
frequency band within which cellular radio systems operate is limited in
width,
and so to provide increased capacity in the system available carrier
frequencies
2 0 are re-used from cell to cell.
The re-use of frequencies in a locality is restricted by co-frequency
interference between different cells which re-use the same or close
frequencies
and which are geographically close to each other. To obtain maximum capacity
in a system comprising a plurality of cell areas, cellular radio system
designers
aim to re-use as many different carrier frequencies of the set of available
carrier
frequencies as possible in each cell. However there are limits on the re-usage
of
carrier frE;quencies in a cell due to other potentially interfering signals,
particularly
from:
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(1 ) interference between a carrier frequency in a first cell and an identical
frequency re-used in neighboring cells and (2) interference between a carrier
frequency used in a first cell and adjacent carrier frequencies used in
neighboring
cells.
The minimum physical distance between geographic cells which re-use a
same carrier frequency or an adjacent carrier frequency is limited by the
required
quality of signals received at the carrier frequency. One metric used to
describe
the quality of the signal is referred to in the art as the carrier to
interference ratio
(C11 ratio). The C/I ratio is a ratio of signal strength of a received desired
carrier
frequency to a signal strength of received interfering carrier frequencies and
noise. A number of physical factors can affect the C/I performance in cellular
systems, including reflections from buildings, geography, antenna radiation
patterns, mobile station transmitting power, and mobile station locations
within a
cell. In general, calculating the distances between cells which re-use an
interfering carrier frequency is a complex problem, however a general approach
to the calculations may be found in Mobile Cellular Telecommunications Systems
by William Chien-Yeh Lee published by McGraw Hill Book Company, New York
1989.
Taking as an example a Digital Amps TDMA {time division and multiple
access) system having available 12.5 MHZ of frequency spectrum, for example in
the 850 MHZ band, individual carrier frequencies are spaced apart from each
other centered at spaangs of every 30 KHz, giving a total of 416 carrier
2 ~~ frequencies available across the network as a whole. The 416 carrier
frequencies are partitioned so that individual carrier frequencies are re-used
from
cell to cell.
Taking as an example a base station re-use factor n of 7 (n=7), for center-
3 0 excited cells each cell is allocated 416=7=59 carrier frequencies per
cell.
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However, with a base station re-use factor of n=4,
this give 416=4=104 carrier frequencies per cell, resulting
in a higher capacity than for an n=7 re-use factor. At a
base station re-use factor of n=4 cells which re-use a same
carrier frequency (the frequency re-use cells) are closer to
each other than at a base station re-use factor n=7,
resulting in more interference, and a lower C/I ratio in the
base station re-use factor n=4 case than in the base station
re-use factor n=7 case. To implement the lower base station
re-use factor (n=4) frequency, re-use cells must be closer
together than with a higher base station re-use n=7.
However, the distance between the re-use cells must be great
enough so that the carrier to interference ratio is high
enough to allow the cellular radio telecommunications
apparatus to distinguish signals at each re-used carrier
frequency in one cell from the interfering frequencies
present in other cells across the network. The C/I
performance is a limiting factor in implementation of a
lower base station re-use factor.
2 0 STJ1~1ARY OF THE INVENTION
An object of the present invention is to provide
improved carrier to interference ratio for a plurality of
beams which re-use frequencies from beam to beam, and to
provide an acceptably low level of interference overall,
thereby allowing greater re-use of frequencies and providing
a capacity gain for a cellular radio communication system.
According to one aspect of the present invention
there is provided in a cellular radio communications system
having a plurality of base stations each for communicating
over at least one corresponding tri-cellular region using a
plurality of directional beams, the tri-cellular region
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having three corner excited cells, a method of configuring
the plurality of directional beams comprising: arranging
each of the base stations at the center of the corresponding
at least one tri-cellular region; arranging the plurality of
beams across each of the cells such that a pair of beams
which reuse a like carrier frequency as each other are
disaligned with respect to each other; selecting at least
one of the beams reusing the like carrier frequency; and
restricting usage of the like carrier frequency on the at
least one selected beam.
Preferably the step of the method according to
claim 1, wherein said step of arranging the plurality of
beams comprises: at a first of the base stations, forming a
first set of beams in a first tri-cellular region; at a
second of the base stations, forming a second set of beams
in a second tri-cellular region; wherein at least one beam
of the first set is directed in a substantially same
direction to and reuses a first carrier frequency as at
least one beam of the second set; and at least one remaining
beam of the first set reuses a second carrier frequency as
at least one remaining beam of the second set, the remaining
beam of the first set being disaligned away from the
remaining beam of the second set.
According to a second aspect of the present
invention there is provided a cellular radio system
comprising a plurality of base stations each for
communicating over at least one corresponding tri-cellular
region using a plurality of directional beams, the at least
one tri-cellular region having three corner excited cells,
the base stations being arranged at the center of the
corresponding at least one tri-cellular region, and the
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plurality of base stations operating to: arrange the
plurality of beams across each of the cells such that a pair
of beams which reuse a like carrier frequency as each other
are disaligned with respect to each other; select at least
5 one of the beams reusing the like carrier frequency; and
restrict usage of the like carrier frequency on the at least
one selected beam.
The use of a plurality of directional beams in a
single cell of a tri-cellular arrangement may enable
improved carrier to interference ratio, and allow tighter
frequency re-use in a cellular radio system. Preferably
each of the set of beams comprises four directional beams.
Preferably each of the set of directional beams comprises
two inner beams and two outer beams.
Another broad aspect of the invention provides a
cellular radio communications system comprising: a
plurality of base stations each for communicating over at
least one corresponding tri-cellular region using a
corresponding plurality of directional beams, the tri-
cellular region having three corner excited cells; a common
pool of carrier frequencies for the directional beams such
that like carrier frequencies are reused as between
different ones of the base stations, wherein usage of at
least one of the like carrier frequencies is restricted and
wherein beams from different ones of the base stations
having the like carrier frequencies are disaligned with
respect to each other.
Preferably the beams of the first set extend along
directions diverging within an angle of 60° from a main
direction of the first cell; the beams of the second set
extend along directions diverging within an angle of 60°
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from a main direction of the second cell; and the main
directions of the first and second cells being substantially
coincident with each other.
According to a fourth aspect of the present
invention there is provided a method for improving the
carrier to interference ratio of a cellular radio
communications system, comprising: arranging a plurality of
base stations each for communicating over a corresponding
tri-cellular region through the use of a plurality of
directional beams, the tri-cellular region having three
corner excited cells; arranging the plurality of beams
across each of the cells such that a pair of beams which
reuse a like carrier frequency are disaligned with each
other; selecting at least one of the beams reusing a like
carrier frequency; and restricting usage of the like carrier
frequency on the at least one selected beam.
Preferably the first set of base stations are
arranged substantially along a first line and the second set
of base stations are arranged substantially along a second
line. Preferably the first line is substantially parallel
to the second line.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention and to
show how the same may be carried into effect, there will now
be described by way of example only, specific embodiments,
methods and processes according to the present invention
with reference to the accompanying drawings in which:
Fig. 1 illustrates an arrangement of center
excited tri-sectorized hexagonal cells in which each sector
is served by a plurality of directional beams;
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Fig. 2 illustrates a prior art tri-cellular
arrangement wherein each of three sectors of tri-cellular
area are served by a separate beam;
Fig. 3 illustrates beam coverage of one sector of
a tri-cellular area of the prior art arrangement of Fig. 2,
showing a -3dB contour and a -lOdB contour, illustrating
coverage of the cell from a beam originating at a corner of
the sector;
Fig. 4 illustrates a directional beam layout
having a frequency re-use between cells for edge excited
sectors with four beams per sector in a tri-cellular
arrangement;
Fig. 5 illustrates a carrier to noise and
interference ratio graph corresponding to the layout in
Fig. 9;
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Fig. 6 illustrates a directional beam layout for edge excited sectors having
frequency re-use between sectors with four beams per sector in a tri-cellular
arrangement according to a specific method of the invention herein; and
Fig. 7 illustrates a carrier to noise and interference ratio graph
corresponding to the layout in Fig. 6.
failed Description of the Best Mode for Carr~ringi Out the Invention
There will now be described by way of example the best mode
1 o contemplated by the inventors for carrying out the invention. In the
following
description numerous specific details are set forth in order to provide a
thorough
understanding of the present invention. It will be apparent however, to one
skilled in the art, that the present invention may be practiced without using
these
specific cletails. In other instances, well known methods and structures have
not
been described in detail so as not to unnecessarily obscure the present
invention.
In this specification, the term sector is used to denote an azimuth angular
range of view from a base station of nominally 120° or less over which
the radio
base station produces beam coverage. A sector may typically subtend an angle
2 0 of view (azimuth angle) of nominally 120° in an arrangement of
three sectors per
base station as illustrated in Figs. 1 and 6 herein. In an arrangement of six
sectors per base station (a hex-sectored arrangement), each sector may subtend
a nominal angle of 60° from a base station. In a conventional tri-
sectored center-
excited hexagonal cell arrangement a sector may comprise a nominal
quadrilatE:ral area, which is edge or corner excited. Three such sectors form
a
nominally hexagonal cell. In a conventional tri-cellular arrangement, a sector
comprises a nominally hexagonal area excited from one edge or comer. Three
such sectors surround a base station to provide a tri-cellular area. In each
case,
the sector is edge excited and extends radially outwards from the base
station.
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A capacity increase over a prior art center-excited tri-sectorized cellular
arrangement having three 120° sectors each served by a separate
120° azimuth
beam can be achieved by the use of a plurality of directional beams in each
sector, as shown in Fig. 1 herein.
Referring to Fig. 1 there is shown an example of a geographical area served
by a cellular radio network covering a plurality of nominally hexagonal center
excited cells each cell divided into three 120° sectors, each sector
served by four
directional beams. First, second and third hexagonal cells 100, 101, 102
l0 respectively re-use a common set of frequencies, and are spaced apart from
each other by intermediate cells 103 - 105 which use different frequencies to
the
first to third cells and which are non-interfering with the common frequencies
used in the first to third cells. Each of the first second and third cells is
tri
sectorize~d into three 120° sectors, wherein four directional beams per
sector are
provided.
Using a plurality of directional beams it is possible to increase the capacity
of a cellullar radio system by reducing interference from neighboring beams
which
re-use common frequencies. Using the example of a base station re-use factor
2 0 n=7, where each cell is allocated 416=7=59 carrier frequencies per cell,
where
the cell is tri-sectored, each sector having four directional beams, 19
carrier
frequencies can be allocated to each set of four beams 106-109 in a sector as
shown in Fig. 1. Most of these frequencies can be allocated to traffic, but
some
are reserved for control purposes.
A base station re-use factor of n=4, gives 416=4=104 carrier frequencies
per cell. In a tri-sectored hexagonal cell, each sector having a multibeam
arrangement of four beams per sector, 34 carrier frequencies can be allocated
per sector, ie across four directional beams, in a 120° sector.
Improvement in the
CII ratio obtained by the use of directional beams within a sector enables the
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implementation of a lower base station re-use factor n than would otherwise be
available with a tri-sectorized center excited cell arrangement.
Considering now a conventional tri-cellular arrangement (otherwise known
as edge excited or comer excited cells) as shown in Fig. 2 herein, a base
station
serves three nominally hexagonal sectors comprising a tri-cellular region,
from a
position at a center of the tri-cellular region where the three sectors meet
each
other. Conventionally, network planners use nominally hexagonal areas which
are loosely termed "cells" to plan terrestrial cellular network coverage.
Hence the
1 o arrangement of three nominally hexagonal edge-excited sectors surrounding
a
base station has become known as a tri-cellular arrangement. In this
specification terminology used for each edge-excited area surrounding a base
station is maintained as a sector, in accordance with the description
hereinabove
irrespective of whether the edge excited area is nominally hexagonal or
quadrilatE~ral or any other shape. The tri-cellular arrangement has an
inherent
advantage in terms of C/I performance compared to an equivalent conventional
center excited tri-sectorized hexagonal cell of comparable area, due to the
narrower beam which can be used in each sector of the tri-cellular arrangement
as compared to each sector of the known center excited tri-sectorized center
2 o excited arrangement .
Reff:rring to Fig. 3 herein, one sector of a three sector tri-cellular
arrangement is illustrated in which one hexagonal sector 300 is covered by a
beam having a coverage pattern having a 60° angular beamwidth 301 at -3
dB
2 5 gain. Such a beamwidth may give adequate performance for coverage of a
120°
angle azimuth 302 at the corner of the sector where the base station is
situated,
as the coverage pattern of the beam falls away in power in such a way that at
a -
10 dB contour 303 of the beam, nearest corners 304, 305 of the nominally
hexagonal sector which are adjacent to the corner occupied by the base station
3 o are relatively close to the base station compared to oppositely facing
corners
306, 307 and are within the -10 dB contour of the beam with the result that
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acceptak~le power levels are available for communicating with mobile stations
at
the nearest corners 304, 305. Thus, the known tri-cellular arrangement has an
inherent advantage over the known center excited tri-sectorized hexagonal cell
arrangement in terms of the narrower beam which can be used to cover an
equivalent area. The reduced power received on the downlink by mobile stations
located at the nearest corners to the base station corner is compensated by
the
fact that these corners are closer to the base station corner than other parts
of
the tri-cellular sector. Typically a -3 dB azimuth beamwidth of 60° to
70° is
acceptable for coverage of the tri-cellular sector.
Referring to Fig. 4 of the accompanying drawings there is illustrated a
cellular radio system serving a geographical area divided into a plurality of
adjoining hexagonal edge-excited sectors 40 of substantially equal area to
each
other in a tri-cellular configuration in which a plurality of base stations B
are each
surrounded by a corresponding respective set of three hexagonal sectors, which
they serve. Each base station has one or more directional beam antennas 45.
Each base station supports coverage of its three surrounding sectors
comprising
a tri-cellular region. Tri-cellular regions are shown enclosed by a thickened
line in
Fig. 4.
A pllurality of frequency re-use base stations B which use a common set of
frequencies are arranged in a plurality of substantially straight lines which
are
approximately parallel to each other, the base stations of a line being spaced
approximately equidistantly from each other along the line. Base stations of
one
2 5 line are positioned off-set to base stations of a neighboring line. Each
tri-cellular
area comprises three nominally hexagonal sector areas. Each sector area is
served by a plurality of substantially radially extending beams extending
outwardly from the base station and covering the area of the sector. The
plurality
of beams extend either side of a main length of the sector, the main length
3o extending between a corner of the hexagonal sector at which the base
station is
situated, and a furthermost comer of the sector opposite the corner at which
the
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base station is located. Each beam is of relatively narrow beamwidth,
typically of
the order 45° to 50° azimuth at the -3dB gain contour.
For ease of description, hereinafter a method corresponding to one sector
of a tri-cellular region, the tri-cellular regions supported by two base
stations
which are spaced apart from each other and re-use a common set of carrier
frequencies will be described. It will be understood that coverage of all
sectors in
the cellular radio system requires duplication of the method described
hereinafter.
In Fig. 4 a first set of directional beams has been labeled 41A, 42A, 43A and
44A
for one of the sectors covered by first frequency re-use base station 45A and
a
second set of directional beams has been labeled 41 B, 42B, 43B and 44B for
one of the sectors covered by second frequency re-use base station 45B. When
referring to Fig. 4 herein, a beam referred to by a number 41 shall represent
beam 41 A, 41 B or any other beam of equivalent re-used carrier frequency and
substantially similar direction transmitted by any other frequency re-use base
station 45 in Fig. 4. Likewise beams referred to by a number 42, 43 or 44
shall
represent beams of identical re-used carrier frequency and substantially
similar
direction of any frequency re-use base station 45 in Fig. 4. All other sectors
in
Fig. 4 have a corresponding pattern of four beams 41 to 44 which use other
2 0 frequencies but these are not illustrated for clarity.
In the arrangement of beams shown in Fig. 4, outer beam 41A supported by
first basE: station 45A re-uses the same carrier frequency as outer beam 41 B
supported by second base station 45B. Likewise all inner beams 42, have the
2 5 same carrier frequency as each other, and similarly inner beams 43 re-use
another same carrier frequency, and outer beams 44 re-use a further same
carrier frequency, as between the first and second base stations 45A, 45B in
Fig.
4.
30 ThE; sector served by first base station 45A containing first set of
directional
beams 41A-4.4A uses a same set of frequencies as second set of beams 41B-
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44B of second base station 45B serving the second tri-cellular area.
Similarly,
other surrounding frequency re-use base stations 45C, 45D, 45E, 45F, 45G,
each serving a corresponding respective tri-cellular area, re-use the same
frequencies as first base station 45A, allocating those re-use frequencies to
corresponding respective third to seventh beam sets 41 C-41 G, 42C-42G, 43C-
43G, 44C-44G as shown in Fig. 4. Each frequency re-use sector contains a set
of directional beams 41-44. In each case, the directional beams extend
radially
about the corresponding respective base station, and either side of a main
length
of the corresponding respective sector served by the beam set. Each sector
1 o containing a beam set re-using a same set of frequencies has a main length
extendin~~ in a same direction to each other sector re-using the same
frequency
set. Each beam of first beam set 41A-44A extends in a respective general
direction which is the same as a corresponding respective beam 41 B -44B of a
corresponding sector comprising second tri-cellular area supported by second
first tier r~e-use base station 45B.
The plurality of frequency re-use base stations 45 are arranged in such a
way that for each sector of the tri-cellular area supported by the
corresponding
respective re-use base station 45, beams 41, 44 at an outer edge of each
2 o individual sector of the tri-cellular area extend along a line of sight
pointing
midway between corresponding respective outermost frequency beams 41, 44 of
neighboriing first tier re-use base stations. For example, outer beam 41A
extends
along the line of sight pointing to an area midway between corresponding
respective outer beams 41 B, 41 C re-using a same frequency as 41A. Because
beams 41A-C are directional, the likelihood of interference between these
frequency re-use beams is reduced.
Referring to Fig. 5 herein, there is illustrated carrier to interference ratio
graphs G~rresponding to four beams of one sector of the layout shown in Fig.
4.
3o Graph line 51 shows a plot of carrier to interference level in decibels on
a vertical
axis, against beam width on a horizontal axis for outer beam 41 in Fig. 4 over
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beamwidlths in the range 20° to 50°. Likewise graph lines 52, 53
and 54 in Fig. 5
correspond to inner beams 42, 43 and outer beam 44 in Fig. 4 respectively.
As can be seen from graph lines 51 and 54 in Fig. 5 the outer two beams 41
and 44 of a sector in Fig. 4 have a relatively higher carrier to interference
performance compared to inner beams 42, 43. Innermost beams 42A, 43A of the
first base station 45A extend in a direction which points towards the
corresponding respective inner beams 42B, 43B of adjacent first tier re-use
sector of first tier re-use base station, second base station 45B. Areas
covered
l0 by inner ibeams 42B, 43B receive interference from corresponding inner
beams of
adjacent first tier frequency re-use base station 42A, 43B respectively. The
beams 42B and 43B in Fig. 4 experience reduced carrier to interference
performance due to the interference which results from beams 42A and 43A
transmitted by antenna 45A having the same carrier frequencies and being
directed iin substantially the same direction.
Fig. 6 herein illustrates a directional beam layout in a sector of a tri-
cellular
radio sy~;tem with identical apparatus components to those shown in Fig. 4 but
employing a specific method of arranging frequency re-use beams which is
2 0 subject c>f the present invention. For ease of description hereinafter a
method
corresponding to one sector of a tri-cellular region supported by a base
station
will be described. It will be understood that coverage of all three sectors
supported by a base station requires duplication of the method described
hereinafter. For this section of the description a beam referred to by a
number
2 5 61 shall represent first outer beam 61 A, 61 B or any other beam of
substantially
similar direction supported by any base station in Fig. 6 which re-uses a
common
set of carrier frequencies. Likewise beams referred to by a number 62, 63
shall
represent inner beams of substantially similar direction supported by any
frequency re-use base station 65 in Fig. 6 and beams referred to by number 64
3 o shall represent second outer beams of any frequency re-use base station
65.
First outE;r beam 61 has a same carrier frequency for all base stations 65 in
Fig.
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6. Second outer beam 64 also has a same carrier frequency for all base
stations
65 in Fig. 6. However the carrier frequencies of inner two beams 62 and 63
have
been exchanged for each other as between first and second base stations 65A
and 65B so that inner beam 62A of first frequency re-use sector served by
first
base station 65A sector has the same carrier frequency as opposite inner beam
63B of second frequency re-use sector of the second first tier frequency re-
use
base station 65B, and inner beam 63A of the first frequency re-use sector has
the
same carrier frequency as opposite inner beam 62B of the second frequency re-
use sector. The pattern of alternating the carrier frequencies of the two
inner
beams transmitted by base stations 65A and 65B is repeated throughout the
layout of frequency re-use base stations so that the inner two beams of all
adjacent base stations have alternated carrier frequencies in order to
minimize
overall interference.
In the arrangement of Fig. 6 herein, first base station 65A communicates
with first sector area served by first set of beams 61A-64A and second
frequency
re-use base station 65B communicates with second sector area served by
second set of frequency re-use beams 61B-64B. Outer beams 61A, 64A of the
first beam set are directed in a same direction as corresponding respective
outer
2 o beams 6'I , 64 of the plurality of other beam sets (second to seventh beam
sets
61-64 corresponding to second to seventh frequency re-use base stations 65B-
65G). BE:cause of the layout of the base stations, arranged substantially
along
straight lines parallel to each other where frequency re-use base stations are
spaced substantially equidistantly from each other along each line, the outer
beams 6"~I, 64 of a sector of a tri-cellular area extend along a line of sight
which
points towards an area between nearest adjacent corresponding respective
frequency re-use beams 61, 64 of adjacent frequency re-use base stations, and
interference between outer frequency re-use beams 61, 64 of adjacent frequency
re-use sectors is relatively low.
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First frequency re-use inner beams 62, of each frequency re-use sector
along a Mine of base stations, for example a first line comprising fourth base
station 65D, first base station 65A and seventh base station 65G are all
directed
in a same direction and use a same frequency. However, corresponding first
frequency re-use inner beams of an adjacent parallel line of frequency re-use
base stations, for example comprising second base station 65B and third base
station 65C use a different frequency ie the frequency used by second inner
beams 63 of the frequency re-use base stations along the first line comprising
fourth base stations 65D, first base station 65A and seventh base station 65G.
In
1 o the tri-cellular areas corresponding to the base stations along the second
line, the
frequencies of the inner two beams 62, 63 are reversed as compared to the
corresponding respective beams of tri-cellular areas served by base stations
along adjacent parallel first line of base stations comprising fourth, first
and
seventh base stations 65D, 65A, 65G.
In other words, examining the relationship between frequency re-use at first
base station 65A and second base station 65B, first base station 65A
communicates with a first sector area of a tri-cellular area using a first set
of
beams, second frequency re-use base station 65B communicates with second
2 o sector of second tri-cellular area using a second set of beams, at least
one beam
of the first; set being directed in a substantially same direction as a
corresponding
beam of the second set, and at least one remaining beam of the first set which
re-uses a second same frequency as a beam of the second set, being directed
away from that beam. Outer beams 61A, 64A of the first set of beams have a
2 5 same direction as corresponding respective outer beams 61 B, 64B of the
second
set of beams, corresponding respective beams of each set pointing in the same
direction as each other and using the same frequency as each other. Inner
beams 62.A, 63A of first beam set and inner beams 62B, 63B of second beam set
re-use thE; same two frequencies as each other, however first inner beam 62A
of
3 0 the first set having a common re-used carrier frequency with second,
opposite
inner beam 63B of the second set are directed in different directions to each
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other, and second, opposite inner beam 63A of the first set having a same
common carrier frequency as first inner beam 62B of the second set also are
directed in different directions to each other.
The first set of beams 61A - 64A extending from the first base station 65A
are arranged in a first pattern, extending radially from the first base
station,
whereas the second set of beams 61 B-64B extend in a second pattern
substantially radially outwardly from the second base station 64B, the first
and
second sets of beams re-using a common set of carrier frequencies, the carrier
frequencies being assigned to the first set of beams 61A, 64A in a different
order
as compared with their assignment to the second set of beams 61 B-64B.
Fig. 7 herein illustrates carrier to interference ratio graphs corresponding
to
four beams transmitted by a base station 65 in the beam layout shown in Fig.
6.
Graph line 71 shows a carrier to interference level in decibels on a vertical
axis
plotted against beam width for beam 61 in Fig. 6 over beamwidths in the range
20° to 50". Likewise graph lines 72, 73 and 74 correspond to beams 62,
63 and
64 in Fig. 6 respectively.
2 0 As c;an be seen from graph lines 71 and 72 in Fig. 7 outer two beams 61
and 64 in Fig. 6 achieve a relatively higher carrier to interference
performance for
beamwidths in the range 20° to 50°. An improvement in carrier
frequency to
interference performance resulting from alternating the re-used carrier
frequencies between inner beams 62 and 63 in Fig. 4 is seen for both inner
beams represented by graph lines 72 and 73, as compared to the arrangement of
Fig. 4 herein. For graph line 72 (representing beam 62 in Fig. 6) the carrier
to
interference performance is improved significantly. For graph line 73
(representing beam 63 in Fig. 6) the carrier to interference performance is
also
improved.
