Method and apparatus for optimizing antenna tilt

In a cellular telecommunications network, uplink interference, as measured by a base station in a target cell, and co-channel interference, as measured by mobile units in a co-channel cell are reduced by increasing the base station antenna tilt angle. However, increasing the base station antenna tilt angle reduces the effective coverage area of the target cell. To obtain an optimum base station antenna tilt angle, interference reduction and target cell coverage area reduction are quantified for each of a number of candidate base station antenna tilt angles. An interference reduction-to-target cell coverage area reduction ratio can then be established for each of the candidate base station antenna tilt angles. The optimum base station antenna tilt angle can then be identified as the one candidate base station antenna tilt angle that reflects the maximum interference reduction-to-target cell coverage area reduction ratio.

BACKGROUND 
The present invention is related to the field of cellular 
telecommunications. More particularly, the present invention involves 
adjusting the tilt angle of a base station antenna in a target cell for 
the purpose of improving the uplink signal quality received by the base 
station and for improving the signal quality received by mobile units 
operating in co-channel cells. 
In a cellular telecommunications system (e.g., a cellular mobile 
telecommunications system) maintaining and/or improving speech quality is 
of great importance. One factor which can significantly and adversely 
affect speech quality is the presence of co-channel interference. 
Co-channel interference occurs when two or more cells (i.e., co-channel 
cells) located adjacent to one another or in relatively close proximity to 
one another reuse (i.e., share) the same frequency or set of frequencies. 
In essence, a signal being transmitted over a reused frequency in one cell 
is perceived as interference in the other cell. 
One way in which co-channel interference can be avoided is to assign a 
group of dedicated frequencies to each cell in the system so that no two 
cells reuse the same frequency. Unfortunately, there are but a limited 
number of frequencies available to cover an ever increasing demand for 
cellular service. Accordingly, assigning a dedicated group of frequencies 
to each cell is generally not a feasible solution to the co-channel 
interference problem. 
Another technique which is often used to minimize, rather than eliminate, 
co-channel interference involves maximizing reuse distance. Reuse distance 
is generally understood to be the distance between two cells (i.e., 
co-channel cells) that reuse the same frequency or set of frequencies. As 
one skilled in the art will readily understand, as reuse distance 
increases, co-channel interference decreases (i.e., signal strength 
diminishes with distance). However, as the demand for cellular service 
increases, while the number of available frequencies remains the same, 
cellular service providers are forced to establish additional cells, which 
generally have smaller coverage areas. This, in turn, tends to decrease 
rather than increase reuse distance. Consequently, techniques that rely on 
increasing reuse distance to counter the effect of co-channel interference 
are also not an overly practical solution. 
Yet another method for reducing co-channel interference involves adjusting 
the orientation or tilt angle of the base station antenna. In general, the 
base station antenna transmits and receives telecommunications signals to 
and from the various mobile units operating within the corresponding cell, 
herein referred to as the target cell. By repositioning the antenna so 
that the antenna beam points further and further below the horizon, the 
energy associated with the antenna beam is, to a greater extent, directed 
into the target cell and away from any adjacent cells or co-channel cells 
in close proximity to the target cell. Consequently, uplink interference 
received by the base station antenna in the target cell is reduced, as is 
co-channel interference received by mobile units operating in co-channel 
cells caused by transmissions emanating from the base station antenna in 
the target cell. 
As with the other above-identified techniques for avoiding or minimizing 
co-channel interference, repositioning the base station antenna to reduce 
co-channel interference is not without trade-offs. The primary tradeoff 
associated with repositioning the base station antenna is best illustrated 
by FIG. 1. In FIG. 1, if the tilt angle 101 is increased, thereby causing 
the peak of the antenna beam 103 to be directed inward away from the 
target cell's boundary 105, the signal strength or carrier-to-interference 
ratio (i.e., C/I) will undesirably decrease for those signals being 
transmitted between the base station and mobile units operating in the 
target cell at or near the target cell boundary 105. Stated differently, 
an increase in the tilt angle 101 of the antenna beam 103 effectively 
reduces the coverage area of the target cell despite the fact that it also 
reduces the level of co-channel interference in the target cell as well as 
in nearby co-channel cells. Accordingly, it is important to determine the 
antenna tilt angle at which co-channel interference has been sufficiently 
reduced, while minimizing the loss in coverage area associated with the 
target cell. 
Despite the fact that both interference reduction and target cell coverage 
area reduction should be taken into consideration when determining an 
optimum antenna tilt angle, there are no known cellular telecommunication 
systems which employ such a technique. For example, U.S. Pat. No. 
4,249,181 ("Lee") describes a system wherein the level of co-channel 
interference in a co-channel cell is reduced by tilting the antenna beam 
downward by a predetermined amount. More specifically, the antenna beam is 
redirected such that the "notch" in the antenna pattern between the main 
lobe and the first side lobe is generally pointing in the direction of the 
neighboring or co-channel cell. While this technique reduces co-channel 
interference in the target and co-channel cells, there is no guarantee 
that the signal quality for all mobile units in the target cell as a whole 
has been improved because in redirecting the antenna beam, the coverage 
area associated with the target cell may have been substantially reduced, 
effectively leaving mobile units operating at the boundary of the target 
cell without service. 
In another example, U.S. Pat. No. 5,093,923 ("Leslie") describes adjusting 
antenna tilt angle to reduce interference. More particularly, Leslie is 
concerned with adjusting the orientation of an antenna associated with a 
cellular repeater or booster relative to the donor cell's antenna. 
However, the orientation of the repeater or booster antenna that is used 
for transmitting and receiving signals with mobile units does not change. 
Therefore, the region covered by the repeater and the boundary of the 
donor cell (i.e., target cell) are unaffected by the orientation of the 
antenna. Accordingly, Leslie does not take into consideration target cell 
coverage area in determining the most appropriate antenna tilt angle. 
As explained, neither of the two existing designs for reorienting antenna 
tilt angle, nor any other known designs, take target cell coverage 
reduction into consideration. Nevertheless, target cell coverage reduction 
is an important consideration in determining optimal antenna tilt angle. 
Accordingly, it would highly desirable to provide a technique to reduce 
co-channel interference by optimizing antenna tilt angle that takes into 
consideration both co-channel interference reduction as well as target 
cell coverage area reduction. 
SUMMARY 
It is an object of the present invention to optimally adjust the 
orientation of a base station antenna. 
It is another object of the present invention to adjust the tilt angle of a 
base station antenna to reduce co-channel interference. 
It is yet another object of the present invention to adjust the tilt angle 
of a base station antenna to reduce co-channel interference and to 
minimize any reduction in the coverage area of the target cell. 
In accordance with one aspect of the present invention, the foregoing and 
other objects are achieved by a method and/or an apparatus for optimizing 
a target cell base station antenna tilt angle. The method and/or apparatus 
involves determining an interference reduction measure and determining a 
target cell coverage area reduction measure for each of a plurality of 
candidate base station antenna tilt angles. The optimum base station 
antenna tilt angle is then identified, from amongst the plurality of base 
station antenna tilt angles, as a function of interference reduction and 
target cell coverage area reduction. The base station antenna can then be 
repositioned in accordance with the optimum base station antenna tilt 
angle. 
In accordance with another aspect of the present invention, the foregoing 
and other objects are achieved by a method for reducing co-channel 
interference by optimizing base station antenna tilt angle in a target 
cell. The method positions the base station antenna at each of a plurality 
of candidate antenna tilt angles. During a test interval for each of these 
candidate antenna tilt angles, uplink interference levels in the target 
cell are periodically measured, and an overall interference level for each 
of the candidate base station antenna tilt angles is determined as a 
function of the periodically measured uplink interference levels. Then, 
based on the overall interference level of each candidate base station 
antenna tilt angle and an interference level of a reference antenna tilt 
angle, an interference reduction measure is established for each tilt 
angle. The method and/or apparatus also involves determining a target cell 
coverage area for each of the plurality of candidate base station antenna 
tilt angles. Then a target cell coverage area reduction measure for each 
of the plurality of candidate base station antenna tilt angles is 
established , based on the target cell coverage area of each candidate 
base station antenna tilt angles. An optimum base station antenna tilt 
angle is then identified as a function of interference reduction and 
target cell coverage area reduction, and base station antenna tilt angle 
is optimized by repositioning the base station antenna in the target cell 
according to that one candidate base station antenna tilt angle.

DETAILED DESCRIPTION 
FIG. 2 illustrates an exemplary cellular telecommunications network 200 
comprising the cells C1-C10. FIG. 2 also illustrates that each cell C1-C10 
contains at least one base station, for example, base stations B1-B10. 
Generally, the base stations communicate directly with the various mobile 
units M1-M10. In the Advanced Mobile Phone System (AMPS), a mobile 
switching center (MSC) 220 is usually connected to several base stations, 
as illustrated. The MSC provides a number of functions including, but not 
necessarily limited to, frequency allocation and transmitter power level 
control. In the Groupe Special Mobile (GSM) system employed in Europe, 
these functions are accomplished by a base station controller (BSC) rather 
than a MSC, as is well known in the art. It will be understood from the 
description herein below that the present invention is primarily 
implemented in software, and in a preferred embodiment of the present 
invention, that software would be stored in and executed by either the MSC 
or the BSC. 
In a typical frequency allocation plan, two or more cells in the cellular 
network 200 reuse (i.e., share) the same frequency or set of frequencies. 
As explained above, reusing frequencies often gives rise to co-channel 
interference. Unlike prior designs, the present invention addresses the 
problem of co-channel interference by establishing an optimal base station 
antenna tilt angle for each base station antenna as a function of both 
interference reduction and target cell coverage reduction, wherein 
interference and target cell coverage are measured during a set period of 
time for each of a number of candidate antenna tilt angles. 
In accordance with one aspect of the present invention, interference 
reduction is quantified by measuring uplink interference for one or more 
frequency channels at the base station receiver in the target cell. The 
uplink interference measurements are then transmitted from the base 
station in the target cell to the MSC/BSC. It will be understood that in a 
typical cellular system, such as AMPS or GSM, the base station receivers 
are already configured to measure and then forward uplink interference 
measurement reports to the MSC/BSC. 
As one skilled in the art will readily appreciate, the uplink interference 
measurements will vary over time. Accordingly, the present invention 
includes an interference measurement filter. In a preferred embodiment, 
this filter is implemented in software, and it is stored in and executed 
by the MSC/BSC. In general, the interference measurement filter generates 
an overall interference measurement for each of the candidate base station 
antenna tilt angles based on the uplink interference measurements the 
MSC/BSC receives from the base station. 
Of course, any number of different interference filters are possible. In a 
first exemplary embodiment, the interference measurement filter may 
continually average the uplink interference measurements which the MSC/BSC 
periodically receives from the base station. At the end of the measurement 
period for each candidate antenna tilt angle, the average interference 
measurement represents the overall interference measurement for that 
antenna tilt angle. 
In an alternative embodiment, the interference measurement filter in the 
MSC/BSC may take each uplink interference measurement and derive a 90 
percent cumulative probability value for each candidate antenna tilt 
angle. The 90 percent cumulative probability value is then used as the 
overall interference measurement for the candidate antenna tilt angle. The 
90 percent cumulative probability value is the interference measurement 
wherein 90 percent of all interference measurements are less than the 90 
percent cumulative probability value and 10 percent of all interference 
measurements are greater than the 90 percent cumulative probability value. 
As one skilled in the art will readily appreciate, a relatively large 
overall interference measurement for a given antenna tilt angle, as 
illustrated in FIG. 3B for example, may indicate a need to increase the 
antenna tilt angle. Increasing the antenna tilt angle as illustrated in 
FIG. 3A, is likely to have the effect of reducing uplink interference at 
the target cell base station and co-channel interference received by 
mobile units operating in co-channel cells. 
While increasing the antenna tilt angle in the target cell tends to reduce 
co-channel interference, increasing the tilt angle also reduces the 
coverage area of the target cell. This is illustrated in FIGS. 4A and 4B. 
For example, as the peak of the antenna beam 405 is directed further and 
further inward toward the center of the target cell 410, the signal 
strength associated with signals between the base station in the target 
cell and mobile units located at or near the border of the target cell, 
such as mobile unit 415, will diminish. Consequently, the effective 
coverage area of the target cell is also diminished. Therefore, in order 
to truly determine the optimum base station antenna tilt angle, it is 
imperative to take into consideration both interference reduction and 
target cell coverage reduction. 
In accordance with another aspect of the present invention, target cell 
coverage area for a given antenna tilt angle may be indirectly measured as 
a function of the signal strength associated with a neighboring cell as 
observed by mobile units operating in the target cell. Referring once 
again to FIGS. 4A and 4B, as the peak of the antenna beam 405 is 
redirected inward toward the center of the target cell 410, the coverage 
area associated with the target cell decreases, while the distance between 
the signal source in the neighboring cell and mobile units operating in 
the target cell increases on average. Accordingly, the signal strength 
associated with the neighboring cell decreases, as measured by the mobile 
units in the target cell. As one skilled in the art will readily 
appreciate, this decrease in signal strength associated with the 
neighboring cell as measured by the mobile units in the target cell 
provides an indirect measure of target cell coverage area reduction. 
In a preferred embodiment of the present invention, it is possible to take 
advantage of the fact that in a typical cellular system, such as AMPS and 
GSM, mobile units are designed to measure the signal strength associated 
with the target cell as well as any number of neighboring cells. The 
signal strength measurements are then transmitted to the target cell base 
station, which in turn, forwards the signal strength measurements to the 
MSC/BSC for the purpose of determining whether a mobile assisted handover 
(MAHO) is warranted. As is well understood in the art, MAHO is a procedure 
whereby the control over a mobile unit may be passed from the base station 
in a target cell to the base station in a neighboring cell when the signal 
strength associated with the neighboring cell exceeds that of the target 
cell. In most instances, the MAHO procedure is transparent to the 
subscriber of the mobile unit. 
In accordance with an alternative embodiment of the present invention, 
target cell coverage area reduction may be indirectly measured as a 
function of the signal strength associated with a neighboring cell, as 
measured by each mobile unit that undergoes a MAHO from the target cell to 
the neighboring cell during the measurement period for a given candidate 
antenna tilt angle. As with the interference measurements, the signal 
strength measurements taken over the measurement period for a given 
candidate antenna tilt angle may be averaged to provide a single overall 
target cell coverage area measurement. 
In a second alternative embodiment, traffic load in the target cell and the 
neighbor cell can be monitored and used to indirectly measure target cell 
coverage area reduction. For example, if the target cell coverage area is 
reduced as a result of redirecting the peak of the antenna beam 405 inward 
toward the center of the target cell 410, as illustrated in FIGS. 4A and 
4B, the traffic load in the target cell will naturally decrease while the 
traffic load in the neighboring cell will naturally increase, as mobile 
units operating just within the original border of the target cell are 
likely to be handed-over to the neighboring cell's base station in 
accordance with the MAHO procedure described above. In contrast, as the 
effective coverage area of the target cell increases, as a result of 
redirecting the peak of the antenna beam outward toward the neighboring 
cell, the traffic load in the target cell will naturally increase while 
the traffic load in the neighboring cell will decrease. Again, traffic 
load can be used as an indirect measure of target cell coverage area 
reduction. 
After quantifying both an overall interference level and the target cell 
coverage area at a given base station antenna tilt angle, the antenna is 
repositioned at the next candidate antenna tilt angle. As is well known in 
the art, the antenna can be physically repositioned electrically or 
electro-mechanically without directly involving additional personnel. This 
process is repeated until overall interference level and target cell 
coverage are quantified for each and every candidate antenna tilt angle. 
TABLE I 
______________________________________ 
Signal Signal 
Strength 
Strength 
Measurement 
Uplink of of 
Angle Period Interference 
Neighbor A 
Neighbor B 
______________________________________ 
0.degree. 
1 -93 dBm -96 dBm -82 dBm 
2.degree. 
2 -94 dBm -97 dBm -84 dBm 
4.degree. 
3 -100 dBm -97 dBm -83 dBm 
6.degree. 
4 -101 dBm -99 dBm -90 dBm 
8.degree. 
5 -102 dBm -102 dBm 
-92 dBm 
______________________________________ 
Table I provides exemplary interference and signal strength measures for 
each of a number of candidate antenna tilt angles, wherein the signal 
strength measures serve as an indirect measure of target cell coverage 
area as described above. In accordance with a preferred embodiment of the 
present invention, the uplink interference measurement and the signal 
strength measurements corresponding to an antenna tilt angle of 0.degree. 
are used as reference measurements. The remaining interference and signal 
strength measurements shown in Table I (i.e., the measurements associated 
with antenna tilt angles 2.degree., 4.degree., 6.degree. and 8.degree.) 
can then be used to determine the change in interference level (i.e., 
interference reduction) and the change in target cell coverage area (i.e., 
target cell coverage area reduction) for each candidate antenna tilt 
angle. However, it will be understood that interference and signal 
strength measurements other than those associated with the 0.degree. 
antenna tilt angle may be used for reference purposes. 
TABLE II 
______________________________________ 
Change in Signal 
Change in Signal 
Interference 
Strength of Strength of 
Angle Change, R Neighbor A, S.sub.A 
Neighbor B, S.sub.B 
______________________________________ 
0.degree. 
0 0 0 
2.degree. 
+1 1 2 
4.degree. 
+7 1 1 
6.degree. 
+8 3 8 
8.degree. 
+9 6 10 
______________________________________ 
Table II contains exemplary values representing the changes in interference 
R and the changes in signal strength S for each candidate antenna tilt 
angle based on the interference and signal strength measures depicted in 
Table I. The present invention can then determine which of the candidate 
antenna tilt angles is optimal by maximizing an interference 
reduction-to-target cell coverage area reduction ratio according to the 
following relationship: 
EQU R/[1+.sub.i=1.sup.N .SIGMA..vertline.S.sub.i .vertline.] 
wherein N represents the number of neighboring cells for which signal 
strength measurements are taken. In the present example, there are two 
such neighboring cells A and B (i.e., N=2). 
TABLE III 
______________________________________ 
Angle The Ratio: R/(S.sub.A + S.sub.B) 
______________________________________ 
0.degree. -- 
2.degree. 0.25 
4.degree. 2.33 
6.degree. 0.67 
8.degree. 0.53 
______________________________________ 
Table III presents values representing the interference reduction-to-target 
cell coverage area reduction ratio for each of the candidate antenna tilt 
angles, based on the values depicted in Table II. In the present example, 
the maximum interference reduction-to-target cell coverage reduction ratio 
of 2.33 corresponds to an antenna tilt angle of 4.degree.. Accordingly, 
the optimal antenna tilt angle is 4.degree.. 
It should be noted that the interference reduction-to-target cell coverage 
area reduction ratio can be determined for each candidate antenna tilt 
angle based on changes in traffic load rather than changes in signal 
strength, as explained above. Furthermore, upon determining the optimal 
antenna tilt angle, the present invention may cause the MSC/BSC to 
transmit control signals to the target cell base station, causing the base 
station to automatically reposition the base station antenna to reflect 
the optimal antenna tilt angle. As stated above, this may, in turn, be 
accomplished electrically or electro-mechanically through a servo 
mechanism. 
FIG. 5 is a flow diagram illustrating a technique 500 for establishing an 
optimum base station antenna tilt angle in accordance with a preferred 
embodiment of the present invention. As shown in block 505, the base 
station antenna in the target cell is repositioned to a candidate antenna 
tilt angle, in response to an antenna control signal generated by the 
MSC/BSC. Once the antenna is repositioned, the MSC/BSC continuously 
monitors and filters uplink interference as measured by the base station 
receiver, as shown in block 510. By filtering the uplink interference 
measurements, the MSC/BSC ultimately derives an overall interference 
measure for the target cell with respect to the candidate antenna tilt 
angle, as shown in block 515. Similarly, the MSC/BSC continuously monitors 
and filters the signal strength measurements transmitted to the base 
station by mobile units operating in the target cell or mobile units that 
undergo MAHO during the measurement period, as shown in block 520. The 
MSC/BSC eventually establishes an overall signal strength measure for each 
neighboring cell as shown in block 525. Based on the overall interference 
measure and the signal strength measures, the MSC/BSC computes the change 
in interference R and each signal strength measure S.sub.i for each 
neighboring cell, as shown in block 530. Of course, the change in 
interference R and the change in signal strength S.sub.i for the reference 
antenna tilt angle (e.g., an antenna tilt angle of 0.degree.) will be 
zero. Upon determining the change in interference R and the change in 
signal strength S.sub.i for each neighboring cell, an interference 
reduction-to-target cell coverage area reduction ratio is derived, as 
shown in block 535. 
The above-identified technique 500 is then repeated for any number of 
candidate antenna tilt angles as illustrated by the "YES" path out of 
decision block 540. After all candidate antenna tilt angles have been 
tested in accordance with the "NO" path out of decision block 540, the 
tilt angle associated with the maximum interference reduction-to-target 
cell coverage area reduction ratio is identified, as shown in block 545; 
this angle represents the optimum antenna tilt angle. The MSC/BSC can then 
transmit an antenna position control signal to reposition the base station 
antenna at the optimum antenna tilt angle, in accordance with block 550. 
As stated above, the antenna may be electrically or electro-mechanically 
repositioned. 
It will be understood by those skilled in the art that a base station may 
employ different antennas for receiving signals and transmitting signals. 
In accordance with another aspect of the present invention, both the 
receiver antenna and the transmitter antenna are positioned at the same 
candidate antenna tilt angles during each measurement period. 
The present invention has been described with reference to several 
exemplary embodiments. However, it will be readily apparent to those 
skilled in the art that it is possible to embody the invention in specific 
forms other than those of the exemplary embodiments described above. This 
may be done without departing from the spirit of the invention. These 
exemplary embodiments are merely illustrative and should not be considered 
restrictive in any way. The scope of the invention is given by the 
appended claims, rather than the preceding description, and all variations 
and equivalents which fall within the range of the claims are intended to 
be embraced therein.