Note: Descriptions are shown in the official language in which they were submitted.
CA 02256293 1998-12-17
FREQUENCY REUSE IN MILLIMETRIC WAVE POINT-TO-MULTI-POINT
RADIO SYSTEM
FIELD OF THE INVENTION
This invention relates to a frequency plan for point-to-minti-point radio
systems and is particularly concerned with frequency reuse.
BACKGROUND OF THE INVENTION
Frequency planning is a method of optimizing spectrum usage, enhancing
channel capacity and reducing interference. Frequency planning involves
channel
numbering, channel grouping into subsets, cell planning and channel
assignment. A
principle aim of frequency planning is ensuring adequate channel isolation to
avoid
energy spillover between channels so that the adjacent channel interference is
reduced
to a minimum. In the wireless mobile environment, frequency planning is
required to
provide adequate repeat distance in order to ensure co-channel interference is
acceptable while maintaining a high channel capacity. On the other hand, in a
multi-
point radio system, fixed remote stations make use of highly directional
antennas
2o pointing at a cell site. A minti-point radio system basically consists of
cell sites and
remote stations for one way or two-way communications. The horizontal area to
be
covered is divided into one or more cells wherein each cell is divided into
equal
sectors. Each cell site makes use of sector beams to provide radio coverage in
each
sector.
In order to increase capacity, same frequency carrier is reused in more that
one
sector. I~owever, the problem associated with using the same frequency carrier
in
more than one sector of a cell is that if the same frequency carrier is reused
in adjacent
sectors, a remote station located near a sector boundary, i.e. between a first
and second
3o sector of a cell, will see nearly equal strength of the two signals on the
same carrier
frequency thereby providing an unacceptable signal to noise ratio.
CA 02256293 1998-12-17
Frequency reuse in point-to-minti-point radio systems have in the past mainly
dealt with the reuse of frequencies across different cell sites.
Unfortunately, the
problem associated with this approach is that at VHF and microwave
frequencies,
scattering is too high to permit the reuse in the same cell.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved frequency plan
for a point-to-minti-point radio system.
A further object of the present invention is to provide a method of reusing
frequencies across sectors in the same cell site efficiently.
In accordance with the present invention there is provided a method of
assigning frequencies in a cell site in a point-to-mufti-point radio system
comprising
assigning a first set of frequencies from a plurality of frequencies available
at said cell
to a first sector of said cell, assigning a second set of frequencies from
said plurality of
frequencies to sectors adj acent to said first sector, the frequencies in said
second set
being different than the frequencies in said first set, and reusing said first
set of
frequencies in non-adj acent sector to alternate from one sector to the next
between
said first and second set of frequencies, wherein said first and second set of
frequencies comprise n frequencies wherein n = z /2, and z = number of
frequencies
available at that cell.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be further understood from the following
description with reference to the drawings in which:
FIG. 1 illustrates a known cell site configuration with six sectors;
CA 02256293 1998-12-17
3
FIG. 2a illustrates a frequency plan in accordance with one embodiment of the
present invention; and
FIG. 2b illustrates a further frequency plan in accordance with a further
embodiment of the present invention.
DETAILED DESCRIPTION
to
Referring now to FIG. 1, we have shown a typical cell site configuration for
use in a point-to-mufti-point radio system. The cell site 10 can be divided up
into a
number of sectors 11. The cell site may have a radius of several kilometers to
provide
point-to-mufti-point communication between a site transceiver centrally
located
thereof 12 and remote stations 13 and 14. Remote stations 13 and 14 make use
of
highly directional antennas 15 and 16, respectively, pointing at the cell site
antenna 17
albeit within the confines of the sector 18.
Generally, the horizontal area to be covered is divided into one or more cells
2o wherein each cell is divided into equal sectors. In FIG. 1, the cell shown
is divided
into six sectors. The cell site makes use of a sector beam 19 to illuminate
the sector
and in particular provide coverage where remote stations 13 and 14 are
located. In
order to prevent adjacent channel interference, different carrier frequencies
are used in
each of the sectors as indicated earlier, however, this results in the
decrease in a
capacity of the system.
With the frequency reuse plan of the present invention, the capacity of a cell
site is increased by reusing the same carrier frequency in alternate sectors
as shown in
FIG. 2a. That is, adjacent sectors make use of different carrier frequencies,
thereby
3o enabling the increase in capacity of the cell site.
CA 02256293 1998-12-17
4
As illustrated in FIG. 2b, a system wherein four channels per cell sites are
assigned, each sector is allocated two channels. For example, sector 21 in a
four
channel system would be allocated channels 1 and 3 and sector 22 would be
allocated
channels 2 and 4. Channels 1 and 3 would not be allocated again unless
separated by
a sector making use of a different set of carrier frequencies or channels.
This way,
sectors making use of the same channels or carrier frequency such as sector 22
and 23
are provided with 120 degrees of angular isolation 24 between the reuse of the
same
frequency. If the cell site is allocated eight channels or carrier
frequencies, each sector
can make use of four channels in a frequency plan as illustrated in FIG. 2b.
Therefore,
1 o each sector is allocated a set of frequencies comprised of n channels
wherein n = z/2
and z = number of frequencies in channels available at that cell. Frequencies
that are
reused in alternate sectors are provided with an angular isolation of ~6 = 2 x
360°/x
x > 4 wherein x is an even number of sectors in the cell.
If additional cell sites are required, sector frequencies are located to
prevent a
sector of a first cell site from using the same channel or carrier frequency
as a sector
of an adjacent cell site.
Links between cell sites or inter-cell point-to-point links is achieved by
2o providing an inter-cell beam aligned with edges of sector beams illustrated
by lines 25
and 26 of FIG. 2b where sector antenna gain is lowest and thereby reduces
interference. This method provides three times the system capacity compared to
the
prior art wherein all the frequencies are used once in the whole cell.
