Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR .ASSIGNING FREQUENCIES IN
CELLULAR COMMUNICATIONS SYSTEMS
FIELD
This invention relates to cellular communications
systems and, more specif=ically, to a method of assigning
frequencies within a cellular communications system.
BACKGROUND
There is a growing demand for wireless communication
services, such as cellular mobile telephone, mobile digital
data cellular networks, mobile personal communications
services, as well as fi~:ed wireless networks. In such
systems, it is essential_ to maximize the use of the
available radio spectrum over a geographic territory.
In order to utilized the available frequencies in an
optimal manner, a large geographic territory is typically
divided into a number of sub-areas called cells. Each cell
typically includes a radio base station and an antenna for
effecting communication; between the base station and the
remote stations, which may be either fixed or mobile.
The available frequencies are allocated to the
individual cells, in order to minimize both adjacent-
channel interference andl co-channel interference. In order
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to further increase the utilization of the available
frequencies, a cell may be subdivided into a number of
sectors, with each sector serviced by an associated
directional antenna. Each sector typically consists of an
integral fraction of 360 degrees, although, in practice,
sectors are not limited to such a precise integer.
In order to provide for extended coverage in large
geographic areas, arrays of cells, known as clusters, are
employed. A cluster consists of a basic cell group.
Clusters are typically aligned such that a symmetrical
pattern is established in two dimensions. Such alignment
is used to ensure that lboth co-channel and adjacent-channel
interference is minimized between clusters.
In any frequency plan, frequency reuse is normally
employed to achieve a system capacity significantly greater
than the total number o:E available channel frequencies. In
frequency reuse, a frequency will be assigned to cells that
are separated sufficiently from each other to achieve
relatively low interference between radio channels in
different cells having l~he same frequency, i.e., low co-
channel interference.
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Existing cellular frequency planning methods and
techniques have been developed for frequency division
duplex, the communications technique most commonly
employed. Newer systems utilize time division duplex,
which has different frequency assignment considerations as
well as different inters°erence mechanisms. It would be
desirable if frequency plans could be developed that would
not only accommodate time division duplex, but would also
be adaptable to frequency division duplex in a cellular
system.
In a fixed wireless cellular system, the subscriber
units are permanently mounted, and may be shared by
multiple subscribers. 'rhi.s technique is used for providing
a communications link to a large building, which might have
a plurality of individual subscribers. Such fixed cellular
systems can also be used in developing countries and in
rural areas where conventional wireline service is either
non-existent or uneconomical.
Frequency reuse patterns have been extensively studied
in the cellular industry. Frequency reuse patterns
minimizing adjacent-channel interference for frequency
division duplex have been proposed for cell clusters of
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greater than nine cells in size. However, claims have been
made in existing studies that adjacent channel interference
may be unavoidable when the number of cells in a cluster is
less than nine. Existing frequency reuse patterns of a
modulo of less than nine cells all suffer from the problem
of adjacent-channel interference.
In U.S. Pat No. 5,549,292 issued to Doner, entitled
"Obtaining Improved Frequency Reuse in wireless
Communication Systems", the cells are sub-divided into six
radial sectors. The frequencies are assigned to the
sectors in such a manner as to enable reuse of each
available frequency in every third cell (i.e. N=3).
However, in such a scheme the start-up cost is high, since
at least two complete sets of transceiver equipment have to
be located in each cell, even in initial low-density
systems.
While an N=3 system can be shown to be highly
efficient, it is not easily adapted to existing low-density
plans. Low-density reuo~e patterns are typically
implemented using a reuse factor of seven, and thus the
reuse patterns do not f:it well into a reuse grid of three
because seven is not divisible by three. Even if the low-
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density reuse pattern is selected to be a multiple of
three, mixing the N=3 cell patterns with such patterns
creates unacceptable interference between homologous cells
at the periphery of the cell patterns of a particular reuse
factor. This is especially true where one attempts to
locate a sectorized cell adjacent to an un-sectorized cell.
A further development in U.S. patent 5,974,323 issued
to Doner, entitled "Frequency Plan for wireless
Communication System that Accommodates Demand Growth to
High Efficiency Reuse Factors" attempts to overcome this
problem by starting with a low-density reuse pattern of 12
cells, with growth to a.n N=3 reuse factor. Such a scheme
overcomes the aforementioned difficulty of attempting to
integrate an N=3 schemes into a reuse pattern which is not
divisible by 3. However, it requires the radio spectrum to
be divided into three groups, which is not always desirable
in some smaller frequency bands. Also, it is specifically
optimized for frequency division duplex systems, and does
not address the interference mechanisms, which arise with
time division duplex systems.
In some existing frequency reuse patterns, adjacent
channel frequencies may be deployed in the same cell. This
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deployment can create a problem with the power amplifiers
used in the cell. Power amplifiers operate with the
greatest efficiency close to their saturation region.
However, as the saturation region is approached, a form of
distortion known as third order intermodulation distortion
can negatively affect t:he performance of adjacent channels
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operating in the cell using the power amplifier.
Therefore, frequency reuse methods should be designed to
forbid adjacent channels from occupying the same cell.
The object of this invention is to provide a method of
assigning frequencies, polarizations, and sectorization in
both time division duplex and frequency division duplex
wireless cellular communications systems.
It is a further object of this invention to optimize
the frequency planning process for both frequency division
duplex and time division duplex.
It is still a furtl:~er object of this invention to
provide a method of assigning frequencies that prohibits
adjacent frequency channels from occupying the same cell or
adjacent sectors on adjacent cells, to prevent third order
intermodulation distortion from the power amplifiers. This
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should also allow the power amplifiers to be operated
closer to the saturation region for improved efficiency.
STJMMARY
The method of assigning frequencies starts by dividing
the geographic region for wireless coverage into a number
of clusters. Each cluster is then divided into 16 cells.
The cells are arranged :into 4 columns of 4 cells, with the
cells preferably arranged in a triangular pattern such that
the cluster shape is a :rhomboid.
The available frequencies are divided into two groups,
with adjacent frequencies assigned to separate groups. One
group of frequencies is allocated to the first and third
columns of cells in the cluster, and the second group of
frequencies to the second and fourth columns of cells.
Each cell is then divided into n equal sectors of
360/n degrees, with n being an even integer greater than or
equal to four. Basic examples include 4 sectors of 90
degrees or 8 sectors of 45 degrees. The number of sectors
is restricted to twice the number of frequencies available
in one group or the total number of frequencies available.
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Each sector is then assigned a frequency. The
frequencies available a:re assigned to the sectors in each
cell such that no two adjacent sectors in a cell are
assigned the same frequency. Finally, each frequency
assigned either a horizontal or vertical polarization such
that no two cells in a cluster have an identical
combination of polarizat.ions and frequencies assigned.
Generally, the number of sectors per cell is equal to
the total number of available frequencies. The greater the
number of frequencies and polarizations assigned to a cell,
the greater the traffic capacity that can be handled by
that cell and the corresponding cluster and network. The
number of sectors n is E=_qual to S, where S is equal to the
number of available frequencies. Alternatively, the number
of sectors per cell may be equal to S/2 or S/4, with the
result that only one-half or one-quarter of the available
frequencies are assigned. In this manner, future growth is
accommodated by allowing for the division of the cell into
additional sectors, as opposed to requiring the addition of
new frequencies. For e:~cample, if eight (S=8) total
frequencies are available, the cells may be originally
divided in four (S/2) sectors each, resulting in the
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assignment of four of the available frequencies, or half
the total available in t=he cluster. The cells can later be
divided into eight (S) ;hectors each, allowing for the
assignment of all eight available frequencies and doubling
the traffic-handling abilities of the system. This allows
the system to grow and handle more wireless traffic without
demanding additional frequencies beyond those initially
available.
This invention also permits the power amplifiers used
at cell hubs to be operated more efficiently by restricting
adjacent channels from occupying the same cell.
BRIEF DESCRIPTION OF THF~ DRAWINGS
The invention itse7_f both as to organization and
method of operation, as well as additional objects and
advantages thereof, will_ become readily apparent from the
following detailed description when read in connection with
the accompanying drawings:
Figure 1 illustrates a typical wireless communication
frequency spectrum, con"isting in this case of eight remote
to hub frequency channels and eight hub to remote frequency
channels;
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Figure 2 illustratE~s the assignments of frequency and
polarizations within the basic 16-cell cluster for an
initial deployment;
Figure 3 illustrates the assignments of frequency and
polarizations within the basic 16-cell cluster for a growth
strategy, in which all 8 available frequencies have been
allocated within the ba:~ic 16-cell cluster; and
Figure 4 illustrates a rectangular and a rhomboidal
arrangement of cells.
DETAILED DESCRIPTION
A representative frequency spectrum ranging from 38.6
GHz to 40.0 GHz is depicted in Figure 1. This frequency
spectrum is further subdivided according to licensed
frequency usage. In the following description, reference
is made to "hub to remote", and "remote to hub"
transmission. This is used to illustrate the well-known
technique of frequency division duplex. The terminology
and the techniques of this invention apply equally to time
division duplex, in which both "hub to remote" and "remote
to hub" transmissions utilize the same frequency, but at
different time periods.
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The overall frequency spectrum consists of two major
blocks, a remote to hub block 1 and a hub to remote block
2. Further, the remote to hub block 1 is sub-divided into
shared portions 3, and an exclusive portion 4. Similarly,
the hub to remote block 2 is sub-divided into shared
portions 5, and exclusive portion 6. This invention is
concerned with, but not restricted to, the exclusive
portions 4 and 6.
Exclusive portions 4 and 6 are divided into, for
purposes of illustration, two sets of eight frequency
channels each. The frequency channels are numbered from fl
to f8 for the exclusive portion 4 of remote to hub block 1,
and from fly to f8~ for the exclusive portion 6 of hub to
remote block 2. Only the frequency channels fl to f8 in
the exclusive portion 4 of remote to hub block 1 will be
described. The frequency planning methods described apply
equally to the frequency channels fl' to f8' contained in
the exclusive portion 6 of hub to remote block 2.
The frequency plan of this invention is based on an
n=16 repeat pattern and a frequency reuse of N=2. In
addition, for the N=2 frequency reuse pattern, all
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frequencies must be able to be operated on either the
vertical or the horizoni~al polarization.
This frequency plan can be used in an initial low-
density application, as shown in Figure 2, and can be
subsequently expanded to use all allocated frequencies
indicated in the exclus_Lve portion 4 of Figure 1. The
frequency plan utilizin<i all frequency channels fl to f8 of
exclusive portion 4 is depicted in Figure 3.
Referring to Figure: 2, an arbitrary area has been
mapped into a number of r_ells. A specific area of such
cells has been designated as a cluster 11, which consists
of 16 cells, numbered 1 to 16, corresponding to an n=16
repeat pattern. The cluster 11 is designed to be repeated
in two dimensions, by placing adjacent clusters contiguous
to the cluster 11, such that cluster symmetry is
maintained. Lines extending from cluster 11 in Figure 2
show the contiguous boundaries of these adjacent clusters.
The cluster 11 is rhomboidal-shaped arising from triangular
cell spacing as opposed to a more conventional rectangular
or hexagonal shape. The basic unit of each pattern is
shown in more detail in Figure 4, wherein it can be seen
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that closer cell spacing is achieved by triangular cell
spacing, as well as a smaller area of reduced coverage.
In an initial deployment, four of the available
frequencies are used. These are chosen from the eight
available in exclusive portion 4 in Figure l, to be as
widely separated in frequency as possible, to reduce
adjacent channel interference. Each of the cells 1 to 16
is then split into four 90-degree sectors. For purposes of
description, the cells 1. to 16 are assigned to columns and
rows, with column 1 consisting of cells 1 through 4, column
2 consisting of cells 5 through 8, column 3 consisting of
cells 9 through 12, and column 4 consisting of cells 13
through 16. Likewise, row 1 consists of cells 1, 5, 9, 13,
row 2 consists of cells 2, 6, 10, 14, row 3 consists of
cells 3, 7, 11, 15, and row 4 consists of cells 4, 8, 12,
16.
With reference to Figure 2, there are two frequencies,
fl and f5, deployed in column 1. The frequencies fl and f5
are always displaced 90 degrees with respect to one
another. Furthermore, in column 1 the sector locations of
fl and f5 repeat every second cell. In order to further
reduce the co-channel interference caused by sectors that
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carry the same frequency, polarization separation of the
frequency into vertical polarization (V) and horizontal
polarization (H) is intraduced. Each frequency is
identified as fxy, where x is the frequency number and y is
the polarization. This i.s shown in column 1, where
horizontal polarization i.s introduced as f1H in cell 3 and
f5H in cell 2. By using two separate polarizations for the
same frequency, the bore-sight frequency repetition
distance is increased over that found in low N frequency
reuse methods without frequency polarization. All sectors
within a given cell using the same frequency channel are
given the same polarization, to prevent potential problems
from depolarization caused by external effects, such as
rain.
Again with reference to Figure 2, the other two chosen
frequencies f2 and f6 in the four-frequency plan are
introduced in column 2. The sectors and polarizations used
for f2 and f6 in column 2 are the same as those used for fl
and f5 in column 1, respectively i.e. where f1H was used in
column 1, f2H is used is column 2, similarly, where flv was
used in column 1, f2V is used in column 2, etc.
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Column 3 utilizes t:he same sector assignment for
frequencies as used in column 1, however, all polarizations
used in column 3 are them opposite of those used in the same
row in column 1. For e~s:ample, where column 1 uses fxV,
column 3 uses fxH, and v~rhere column 1 uses fxH, column 3
uses fxV. The opposite polarization of the same frequency
in different columns further reduces co-channel
interference.
Similarly, column 4 utilizes the same sector and
frequency assignments as. used in column 2 and all
polarizations used in column 4 are the opposite of those
used in the same row column 2.
Referring to Figure: 2, each cell uses a unique
combination of palarizations and frequencies such that no
two cells are identical.
The basic plan described in the preceding can be
expanded to permit progressive growth to enable maximum use
to be made of the available frequency spectrum. The
polarized frequencies have been arranged within the sectors
of the cells such that no two cells have an identical
combination of polarizations and frequencies within their
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sectors. Figure 2 is based on using the four frequencies
f1, f2, f5 and f6. Howcwer, there are eight total
frequencies available, iEl to f8. Further subdividing the
existing four 90-degree sectors into eight 45-degree
sectors allows use of all eight frequencies. Figure 3
illustrates the method of this invention wherein all eight
frequencies are utilized.
Figure 3 retains the basic symmetry of Figure 2,
l0 except that where originally one sector and one polarized
frequency were used in 1?figure 2, now two sectors and two
polarized frequencies occupy the same location in Figure 3.
This is most readily de:~cribed in the following manner.
Each sector originally occupied by fl in Figure 2 is
now sub-divided into two sub-sectors in Figure 3, one
occupied by fl and the other by f3. The polarization of
the sub-sector frequencies is the same as those of the
sector frequencies in the original sector. For example,
the lower-left sector oi= cell 1 in Figure 2 is occupied by
flV. In Figure 3, the .Lower-left sector of cell 1 has been
sub-divided in two sub-sectors, one occupied by flV and the
other occupied by f3V. The sub-division method described
above is applied to each sector of each cell, with f2
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repl aced by f 2 and f 4 , iE 5 by f 5 and f 7 , and f 6 by f 6 and
f8.
The expanded frequency plan described in Figure 3 may
be applied to each cluster 11 as needed to accommodate
increased wireless traff=ic. As the frequencies added to
the sector assignments are not present in the original low-
traffic frequency plan in Figure 2, there is no additional
co-channel or adjacent channel interference along the
cluster boundaries between a low-traffic cluster and a
high-traffic cluster.
The embodiments described in the preceding use a
specific sectorization a.nd available frequency group to
illustrate the process used in this invention. In this
description, a maximum of eight frequency channels and
eight sectors per cell were employed. This technique can
be extended to any number of frequencies and any degree of
sectorization. One such. extension would be to 16
frequencies and 16 sectors. However, it should be noted
that the number of frequencies used in a cluster is always
equal to the number of sectors per cell.
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Further, by reference to Figures 2 and 3, it can be
seen that adjacent channel. frequencies never occupy the
same cell, thus permitting power amplifiers to be operated
closer to saturation, resulting in greater efficiency.
Power amplifiers operate most efficiently when operated
close to the saturation region. However, as the saturation
region is approached, a form of distortion known as third
order intermodulation d_Lstortion can affect the performance
of adjacent channels. =Cn the prior art, it was common to
deploy adjacent channel; in the same cell. The method
described herein permits more efficient operation of power
amplifiers by prohibiting adjacent channels from occupying
either the same cell or adjacent sectors in adjacent cells.
This method also prohibits identical frequencies with
opposite polarizations f=rom occupying the same cell, or
adjacent sectors within a cell. This advancement over the
prior art eliminates the potential deterioration in
performance that occurs when depolarization, caused by
rain, reduces the co-cha.nnel separation.
Accordingly, while this invention has been described
with reference to illustrative embodiments, this
description is not intended to be construed in a limiting
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sense. Various modifications of the illustrative
embodiments, as well as other embodiments of the invention,
will be apparent to persons skilled in the art upon
reference to this descr_Lption. It is therefore
contemplated that the appended claims will cover any such
modifications or embodiments as fall within the scope of
the invention.
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