Cellular radiotelephone communications system

A cellular radiotelephone communications system comprising a system of cells that are made up of an array of diretional sector antennas. These antennas are centrally located in the cell and radiates into a 60.degree. area of the cell. Each antenna in a cell has a group of frequencies assigned to it that is different than the group of frequencies assigned to the other antennas within that cell. These frequency groups are repeated either 2 or 8 times respectively in a 4 or 16 cell repeat pattern, effectively forming a two cell reuse pattern. The preferred embodiment of this invention is asymmetrically positioning the repeating frequency groups in an alternating fashion so that one row faces in the opposite direction of another row. The asymmetrical positioning of cells is possible only by departing from the prior art, cellular positioning rules. The positioning rules used in this invention create a 4 or 16 cell repeat pattern by locating co-channel cells closer in one direction than another.

FIELD OF THE INVENTION 
This invention relates generally to cellular radiotelephone communications 
systems and in particular to asymmetrical reuse patterns for the cellular 
systems. 
BACKGROUND OF THE INVENTION 
A cell in present cellular radiotelephone communications systems typically 
includes six directional antennas, centrally located in the cell, each 
radiating into a 60.degree. sector of the cell. A plurality of these cells 
is combined to form a cellular radiotelephone communications system. This 
cellular system, covering a metropolitan area, allows mobile traffic to 
communicate on landline telephone networks while moving through the area. 
Communication between the mobile traffic and the celluar system is 
accomplished using either digital or analog transmission techniques. The 
digital method digitizes the information before transmission. The analog 
transmission technique is the prevalent method in use, while digital is 
now being introduced. Signals transmitted by digital transmission can 
tolerate a lower threshold of quality, referred to in the art as the 
Carrier to Interference ratio (C/I), than analog transmitted signals. What 
is lost in C/I performance can be made up through coding gain. 
C/I is a ratio of the signal strength of the received desired carrier to 
the signal strength of the received interfering carriers. A number of 
physical factors can affect C/I in cellular systems: building, geography, 
antenna radiation patterns, mobile traffic transmitting power, and mobile 
traffic location within the cell. 
Due to the low power of the cell's transmitters, the same frequency can be 
reused in other cells, referred to as co-channel cells, in the same 
metropolitan area. There are, however, constraints on the location of the 
co-channel cells. Even though the transmitters are typically low power, 
placing co-channel cells too close may cause interference. Greater 
frequency reuse allows more mobile traffic to use the cellular system. 
The frequency plan in a symmetrical cellular system, in the sense of which 
channels should be assigned to each cell, begins with two integers, i and 
j, that are called shift parameters. The frequency plan is established by 
starting with a reference cell and moving over i cells along the chain of 
cells. After reaching the i.sup.th cell, a counter-clockwise turn of 
60.degree. is made and another move of j cells is made. The j.sup.th cell 
can safely be a co-channel cell. The frequency plan can also be 
established by moving j cells before turning i cells or by turning 
60.degree. clockwise instead of counterclockwise. After all the possible 
co-channel cells of the initial cell are laid out, another reference cell 
is chosen and the procedure repeated. This entire procedure is repeated as 
often as necessary to establish the frequency plan of the entire 
metropolitan cellular system. 
The cells thus established by the above procedure form a reuse pattern of 
i.sup.2 +ij+j.sup.2 cells. The number of cells in this reuse pattern is a 
predominant concern of the cellular industry since this number determines 
how many different channel groups can be formed out of the frequency 
spectrum allocated to cellular radiotelephones. A low number of cells in a 
reuse pattern means more channel groups can be formed and more users 
accommodated. 
Presently, a four cell reuse pattern is one of the densest frequency reuse 
patterns that produces an acceptable C/I for analog systems (U.S. Pat. No. 
4,128,740 to Graziano, assigned to Motorola, describes such a four cell 
reuse pattern). 
Graziano teaches that frequency reuse is a function of antenna beam width 
antennas and the antennas' spatial relationship with one another. In order 
to increase frequency reuse, the antenna beam is narrowed from 120.degree. 
to 60.degree.. Since a 120.degree. antenna beam covers a wider area, it 
will interfere with more co-channel cells than a 60.degree. antenna beam. 
A narrower antenna beam, as illustrated in FIG. 1, reduces the area 
covered by the antenna's radiation pattern. By reducing the beam width and 
spatially arranging antennas, while remaining cognizant of the power 
directivity of each and the cumulative power of co-channel interferers, 
allows greater frequency reuse. 
The frequency reuse pattern described in Graziano is a symmetrical reuse 
pattern. The symmetrical pattern is obtained using the cellular layout 
procedure described above. In this configuration, each co-channel cell is 
substantially equidistant from the other co-channel cells. With a 
symmetrical configuration, cell layout is limited by the frequency reuse 
equation in the number of different reuse configurations possible. 
There exists a need, therefore, to decrease the number of cells in a 
cellular reuse pattern thereby increasing the number of times a frequency 
can be reused. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a cellular 
radiotelephone communications system including a plurality of cells that 
are made up of an array of directional sector antennas. These antennas are 
centrally located in the cell and each radiates into a 60.degree. area of 
the cell. Each antenna in a cell has a group of frequencies assigned to it 
that is different than the group of frequencies assigned to the other 
antennas within that cell. These frequency groups can be repeated either 2 
or 8 times respectively in a 4 or 16 cell repeat pattern, effectively 
forming a two cell reuse pattern. This invention recognizes increased 
frequency reuse through a pattern that is asymmetrical. In other words, 
the frequencies repeat closer in one direction than another. 
One embodiment of this invention is asymmetrically positioning the 
repeating frequency groups so that they are radiating in the same 
direction. The preferred embodiment is asymmetrically positioning the 
repeating frequency groups in an alternating fashion so that they radiate 
toward one another. 
While this invention may produce a lower C/I than the symmetrical 
positioning of cells, the lower C/I can be tolerated in the digital 
cellular environment. Digital cellular can use coding gain to compensate 
for the lower C/I.

BEST MODE FOR CARRYING OUT THE INVENTION 
The asymmetrical positioning of cells is possible only by departing from 
the prior art, cellular positioning rules. The positioning rules used in 
this invention create a 4 or 16 cell repeat pattern by locating co-channel 
cells closer in one direction than another. 
The asymmetrical configuration yields a lower C/I due to the proximity of 
the co-channel cells. In a cellular system using analog transmission 
techniques, the lower C/I might cause a low quality signal to the mobile 
traffic. A digital cellular system, however, through the use of coding 
gain, has the ability to tolerate the lower quality signals without 
degradation of the service provided the cellular mobile traffic. The 
coding gain makes up for what was lost in C/I. 
FIG. 2 illustrates one embodiment of this invention. The cells in this 
configuration are laid out in a two cell reuse pattern where the repeating 
channel groups (201) face the same direction. 
FIG. 3 illustrates the preferred embodiment of this invention. The cells 
are also laid out in a two cell reuse pattern (301). However, the 
repeating channel groups of one row of co-channel cells (302) face in the 
opposite direction of the repeating channel groups of the adjacent row of 
co-channel cells (303). 
In both configurations, the co-channel cells are laid out in an 
asymmetrical pattern; in this case, co-channel cells are separated by one 
cell in one direction and are adjacent in the other direction. While these 
are the embodiments illustrated, other asymmetrical configurations may be 
possible. 
The asymmetrical cell configuration is made possible only by departing from 
the prior art method for positioning cells for frequency reuse. This 
invention provides a repeat pattern of either 4 or 16 cells, depending on 
whether the repeating channel groups face the same direction, with the 
frequencies reused 2 or 8 times respectively. This, in effect, produces a 
2 cell reuse pattern. In other words, while 4 or 16 cells are required 
before a cell's frequency pattern repeats, within that 4 or 16 cell 
pattern, a frequency is reused 2 or 8 times. This concept is illustrated 
in FIGS. 4 and 5. 
FIG. 4, a frequency plan of an alternate embodiment, shows a 4 cell repeat 
pattern with each frequency used 2 times. A frequency is reused every 
other cell, producing a 2 cell repeat pattern. 
FIG. 5, a frequency plan of the preferred embodiment, shows a 16 cell 
repeat pattern with each frequency used 8 times. As in the alternate 
embodiment, a frequency is reused every other cell, producing a 2 cell 
reuse pattern. In both embodiments, a cell's frequency layout is not 
repeated in the pattern. 
In any frequency layout scheme using this invention, the frequencies should 
be configured so as to minimize adjacent channel interference. This is 
accomplished by establishing a lattice connecting the centers of the cells 
in the system. This 3 axis lattice is illustrated in FIG. 4. The 12 
frequency groups are divided into 3 sets of 4 frequency groups each, each 
set positioned along a different axis. The 2 odd frequency groups from the 
first set are established along the first member (407) of the first axis. 
The first group of frequencies (403) points in the opposite direction from 
the third group (404). The next parallel member (408) of the lattice 
contains the even group of frequencies from the first set. This odd/even 
pattern is repeated throughout the system of cells to reduce the incidence 
of adjacent frequency groups. 
The second axis of the lattice contains the second set of frequencies. The 
first member of the lattice (405) on this axis contains the odd frequency 
groups of the set, the identical frequency groups facing the same 
direction. The even frequency groups are positioned on the next parallel 
member (406) of this axis. Again, the identical frequencies face the same 
direction. This odd/even pattern is also repeated throughout the system. 
The frequency groups on the third axis (409) are positioned in the same 
manner as the first two. The third axis (409) contains the last set of 
frequency groups. Each identical frequency group faces the same direction 
with the pattern being repeated throughout the system. 
The frequency positioning concept is illustrated in FIG. 4. Frequency 
groups 1 and 3 are positioned along the first member of the first axis. 
Groups 2 and 4 are positioned along the next parallel member of the first 
axis. The second axis contains groups 5 and 7 along the first member of 
this axis and groups 6 and 8 along the next parallel member. The third 
axis contains groups 9 through 12 positioned in an identical manner to the 
first two axis. The odd/even frequency positioning with similar frequency 
groups facing the same direction is repeated throughout the system. 
FIG. 5 illustrates a frequency placement for the preferred embodiment of 
this invention. This method of frequency group placement is similar to the 
alternate embodiment placement method, however, in this method, the 
frequency groups are flipped in alternating cells. In other words, 
frequency group 1 in the first cell faces the opposite direction from 
frequency group 1 in the second cell. 
The frequency configurations in FIGS. 4 and 5 each result in having 
occurrences of adjacent frequencies (401 and 501). This condition could 
result in the frequencies interfering with each other. To alleviate this 
possible problem, those frequencies can be used as signalling channels in 
other sectors of that cell. Otherwise, one or the other of the adjacent 
frequencies can simply not be used. 
Capacity in the preferred embodiment can be increased by using sector 
sharing, as illustrated in FIG. 6. In sector sharing, frequencies or 
frequency groups are shared between at least 2 sectors within a cell. The 
frequencies are allocated to the sector requiring more frequencies on a 
demand basis. If the mobile traffic in a sector is beyond the capacity for 
the number of frequencies in that sector, unused frequencies can be 
borrowed from lower traffic sectors. 
Using asymmetrical frequency configuration, this invention creates higher 
frequency reuse and, therefore, increased mobile traffic capacity in 
cellular radiotelephone systems. In order to accomplish this, an entirely 
new procedure for allocating cellular frequencies was used. The prior art 
method of symmetrical frequency layout could not provide a dense enough 
reuse of frequencies for future expansion of the cellular system. This 
invention provides an asymmetrical layout which allows a frequency to be 
reused in every other cell, producing a 2 cell reuse pattern and therefore 
higher frequency reuse in a metropolitan area. 
Those skilled in the art will recognize that various modifications and 
changes could be made to the invention without departing from the spirit 
and scope thereof. It should therefore be understood that the claims are 
not to be considered as being limited to the precise embodiments set forth 
in the absence of specific limitations directed to such embodiments.