Neighbor cell list creation and verification in a telecommunications system

A method and system for neighbor cell list creation and verification in a telecommunications system is provided. Over a period of time measurements are performed on signals transmitted and received on measurement channels of cells neighboring the cell for which a neighbor cell list is being created. The results of the signal measurements are used to create an ordered list of the measurement channels on which the measurements were done. A neighbor cell list containing a desired number of cells is then created by placing a certain number of cells having the most interfered measurement channels in the neighbor cell list. The invention also presents a method and system for verifying an existing neighbor cell list. In the method and system measurements are performed on measurement channels of neighbor cells over a period of time. Periodically the measurement results are checked to determine if cells should be added to or deleted from the existing neighbor cell list.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to mobile telecommunications systems, and more 
particularly, to a method and system for building a neighbor cell 
measurement channel list for mobile station handoff. 
2. Description of the Prior Art 
In a cellular mobile telecommunications system the user of a mobile station 
communicates with the system through a radio interface while moving about 
the geographic coverage area of the system. The radio interface between 
the mobile station and system is implemented by providing base stations 
dispersed throughout the coverage area of the system, each capable of 
radio communication with the mobile stations operating within the system. 
In a typical mobile telecommunications system each base station of the 
system controls communications within a certain geographic coverage area 
ideally represented by a hexagonal shape termed a cell, and a mobile 
station which is located within a particular cell communicates with the 
base station controlling that cell. When a call is initiated by the user 
of a mobile station, or received at the system for a mobile station, the 
call is set up on radio channels assigned to the base station controlling 
the cell in which the mobile station is located. If the mobile station 
moves away from the original cell in which the call was setup and the 
signal strength on the radio channels of the original cell weakens, the 
system will affect transfer of the call to radio channels of a base 
station controlling a neighboring cell into which the mobile station 
moves. As the mobile station user continues to move throughout the system, 
control of the call may be transferred from the neighboring cell to 
another cell. This transfer of the call from cell to cell is termed 
handover or handoff. 
Handoff can only be effective if the call is transferred to radio channels 
that provide adequate signal strength for two way communications. This 
requires sufficient signal strength at both the receiver of the mobile 
station and receiver of the base station to which handoff is made. The 
signals must also be sufficiently strong in relation to any noise or 
interference that is present in the network. For effective handoff it is 
necessary that some sort of signal strength or interference level 
measurement process be used to determine which of the neighboring cells is 
to be selected for handoff. In existing systems the measurement process is 
done by either making measurements at the receivers of neighboring base 
stations on signals transmitted from the mobile station, or by making 
measurements at the receiver of the mobile station on signals transmitted 
from neighboring base stations. The latter method requires that the mobile 
station be a part of the measurement process used to select a cell for 
handoff. 
In analog cellular systems operating according to the EIA/TIA--553 Mobile 
Station--Land Station Compatibility Specification (AMPS) handoff 
measurements are done solely at neighboring base stations. When signal 
measurements made at the base station providing coverage in the current 
cell indicate that the strength of the signal received from a mobile 
station involved in a call has fallen below a certain threshold, the base 
station informs the mobile switching center (MSC) in control of the system 
or system area in which the cell is located. The MSC then initiates the 
handoff measurement process. The MSC orders base stations of neighboring 
cells to monitor the signal transmitted by the mobile station on the 
current radio channel assigned to the call, and measure the strength of 
the signal. After receiving the measurement results from each base station 
that received the measurement order, the MSC will then initiate handoff of 
the call from the current cell to the cell containing the base station 
reporting the highest received signal strength. The MSC uses a "neighbor 
cell list" that is associated with the current cell to determine which 
base stations receive the measurement order from the MSC. The neighbor 
cell list is created and set manually by the system operator and remains 
fixed until the operator later manually modifies the list. This type of 
handoff measurement process requires a large amount of signaling traffic 
between the MSC and the base stations of the cells contained in the 
neighbor cell list. This signaling traffic further consumes many 
processing and signaling link resources. For this reason the number of 
cells included in the neighbor cell list must be limited. The list is 
typically configured to include the cells which immediately border the 
current cell. If hexagonal cell shapes of identical size are used to model 
the system cells' coverage area there will be six bordering cells. 
The nature of the operation of digital cellular communications systems, as 
opposed to analog cellular systems, allows that the handoff measurement 
process be performed at the mobile station. An example of these types of 
systems, are systems operating according to the IS-54B EIA/TIA Cellular 
System Dual-Mode Mobile Station (IS-54B)--Base Station Compatibility 
Standard. In IS-54B systems the time division multiple access (TDMA) 
signal transmission mode is used. In TDMA, communications between a base 
station and a particular mobile station are transmitted on radio channels 
that also may be used for communications between the same base station and 
up to two different mobile stations. The communications are carried out 
through data or digitized voice signals that are transmitted as bursts in 
timeslots that are time multiplexed on the radio channels. Each mobile 
station in communication with a base station is assigned a timeslot on 
both the reverse channel and forward channels. The assigned timeslots are 
unique to each mobile station so communications between different mobiles 
do not interfere with each other. The handoff process in IS-54B is known 
as mobile assisted handoff (MAHO). In MAHO handoff measurement is done at 
the mobile station during the times when the mobile station is neither 
transmitting in the assigned reverse channel timeslot nor receiving in the 
assigned forward channel timeslot. During the times between signal bursts 
in an ongoing call, the mobile station periodically monitors radio 
channels of each base station located in close proximity. The control 
channel of each neighboring base station is typically used as the 
measurement channel. For each ongoing call the measurement channels are 
contained in the neighbor cell list of the cell in which the call is 
proceeding. In addition to measuring the measurement channels of 
neighboring base stations, the mobile station also measures the received 
signal strength on the current channel on which the call is proceeding. 
The mobile station measures the received signal strength on these radio 
channels and transmits the measurement results to the current base 
station. The current base station then forwards these measurement results 
to the MSC. If the received signal strength on the current channel falls 
below the received signal strength on a measurement channel of a 
neighboring cell the MSC initiates handoff to that neighboring cell. 
The analog control channels (ACCH) of the neighboring cells are used as the 
measurement channels for IS-54B MAHO. With the introduction of the new 
IS-136 EIA/TIA Cellular System Dual-Mode Mobile Station--Base Station 
Compatibility Standard (IS-136), which is essentially the IS-54B standard 
with a digital control channel introduced, it is also possible to use a 
digital control channel (DCCH) for MAHO measurements. 
Because MAHO is performed mostly within the mobile station the resources 
for carrying out the process are limited. IS-54B or IS-136 mobile stations 
can perform only fifty measurements per second. Radio conditions such as 
Rayleigh fading, shadowing, etc. are such that it is necessary to average 
measurements in order to provide a reliable signal strength value. 
Therefore it is necessary to limit the number of cells that comprise the 
neighbor cell list for MAHO measurement purposes to much less than fifty 
cells. The IS-54B standard limits the size of the neighbor cell list to 
twelve cells. IS-136 sets a size limit of twenty four. The increase in the 
size of the list in IS-136 over IS-54B has limited effect because the 
limit of fifty measurements per second still holds and any increase in the 
number of cells in the list dilutes signal strength measurement precision 
on any given measurement channel. 
When a system operator creates a neighbor cell list for a cell he attempts 
to ensure that calls in the cell can be handed over to a second cell, no 
matter what type of movement takes place. One of the difficulties in 
creating a neighbor cell list is that the actual coverage area of a cell 
is difficult to predict. The size and shape of a cell's coverage area may 
vary due to various effects. Examples of such effects are, base station 
antennas being located in different positions, or shadowing effects on 
radio coverage caused by obstacles such as buildings. Although the ideal 
representation of the coverage area of a cell may be a hexagonal cell 
having six neighbors of identical shape, the actual coverage areas of 
cells have differing sizes and shapes. The best candidate cell for handoff 
may not always be one of the six bordering cells as depicted in the ideal 
representation when cells within a system are modeled as being of equal 
size. It is possible that the best candidate for handoff would be a cell 
located beyond the six bordering cells. In the ideal representation this 
could be one of the twelve cells adjacent to the ring formed by the six 
bordering cells. Since it is difficult to predict the actual coverage area 
of each individual cell in a system, it would be very difficult to create 
a neighbor cell list for the handoff process in the above situation. 
Without knowing the actual coverage area of the base stations it would be 
necessary to include all eighteen of these cells in the neighbor cell list 
to create the most accurate list for handoff measurements. In EIA/TIA--553 
and IS-54B systems it is not possible to include all of these eighteen 
cells in the neighbor cell list. In IS-136 systems, although the standard 
allows eighteen cells in the neighbor cell list, the number is excessive 
and the precision of signal strength measurements would not be as great as 
it could be. 
It would provide an advantage then in a cellular system to have a method 
and system for creating a neighbor cell list that accounts for the 
differing coverage areas of cells. A method and system of this type would 
allow creation of a neighbor cell list that would contain the best 
possible candidate cells for handoff of a call. The method and system 
would also allow creation of a neighbor cell list of a size that allowed 
accurate handoff signal strength measurements, while still providing the 
best possible candidate cells for a list of that size. Automation of the 
method and system would free the system operator from having to manually 
create a neighbor cell list for a new cell or, from having to manually 
reconfigure the neighbor cell list of affected cells when a new cell is 
added to a system. The present invention provides such a method and 
system. 
SUMMARY OF THE INVENTION 
In order to overcome the deficiencies and shortcomings of the prior art, 
the present invention provides a method and system for creating a neighbor 
cell list for a cell within a cellular system. The neighbor cell list 
created according to the teachings of the present invention accounts for 
the fact that coverage areas of cells differ from the ideal coverage area 
that is represented by a hexagonal shaped cell. This neighbor cell list 
may be used during the process of handing off a call from the cell for 
which the list was created to one of its nearby neighbor cells. By using 
the neighbor cell list of the invention during handoff a more accurate and 
efficient handoff than is obtainable with a neighbor cell list created by 
existing methods can be obtained. Existing methods of neighbor cell 
creation do not account for the fact that coverage areas of nearby 
neighbor cells within the cellular system cannot be accurately predicted. 
The method and system utilizes signal measurements performed within the 
coverage area of a cell to create the neighbor cell list for that cell. An 
initial neighbor cell list for the cell for which the neighbor cell list 
is to be created is initially used for handoff measurement orders. The 
initial neighbor cell list comprises one or more of the immediately 
adjacent cells. Over a period of time, as communications take place within 
the system, measurements are performed on signals transmitted and received 
on measurement channels of nearby cells not contained in the initial 
neighbor cell list. The results of the signal measurements are then used 
to create an ordered list of the measurement channels on which the 
measurements were done. Then, a neighbor cell list containing a desired 
number of cells is created by adding a certain number of cells having the 
highest signal strength on their measurement channel to the initial 
neighbor cell list. 
Signal measurements are performed at one or more mobile stations located 
within the coverage area of the cell for which the neighbor cell list is 
being created. The mobile stations measure the signal interference on base 
to mobile (downlink) transmissions on the measurement channels of base 
stations that control the nearby cells. Additional signal measurements are 
also performed at the base station controllPatent Appverage area of the 
cell for which the neighbor cell list is being created. The base station 
measures the signal quality of mobile to base (uplink) transmissions on 
the measurement channels of base stations that control the nearby cells. 
The method and system may also be used for verifying an existing neighbor 
cell list. In this aspect of the invention, signal measurements are 
performed within the coverage area of a cell on measurement channels of 
neighboring cells. The results of the signal measurements are then used to 
reconfigure an existing neighbor cell list to include the best candidate 
cells for handoff.

DETAILED DESCRIPTION 
Referring to FIG. 1, there is shown a portion of a conventional cellular 
radio communication system of the type to which the present invention 
generally pertains. In FIG. 1, an arbitrary geographic area may be divided 
into a plurality of contiguous radio coverage areas, or cells Cell A-Cell 
J. While the system of FIG. 1 is illustratively shown to include only ten 
cells, it should be clearly understood that in practice, the number of 
cells will be much larger. 
Associated with and located within each of Cell A-Cell J is a base station 
designated as a corresponding one of a plurality of base stations B1-B10. 
Each of the base stations B1-B10 includes a transmitter, a receiver, and a 
base station controller as are well known in the art. In FIG. 1, the base 
stations B1-B10 are illustratively located at the center of each of Cell 
A-Cell J, respectively, and are equipped with omni-directional antennas. 
However, in other configurations of the cellular radio system, the base 
stations B1-B10 may be located near the periphery, or otherwise away from 
the center of the Cell A-Cell J and may illuminate Cell A-Cell J with 
radio signals either omni-directionally or directionally. Therefore, the 
representation of the cellular radio system of FIG. 1 is for purposes of 
illustration only and is not intended as a limitation on the possible 
implementations of the cellular radio system within which the present 
invention is implemented. 
With continuing reference to FIG. 1, a plurality of mobile stations M1-M10 
may be found within Cell A-Cell J. Each of the mobile stations M1-M10 
includes a transmitter, a receiver, and a mobile station controller as are 
well known in the art. Again, only ten mobile stations are shown in FIG. 1 
but it should be understood that the actual number of mobile stations will 
be much larger in practice and will invariably greatly exceed the number 
of base stations. Moreover, while none of the mobile stations M1-M10 may 
be found in some of Cell A-Cell J, the presence or absence of the mobile 
stations M1-M10 in any particular one of Cell A-Cell J should be 
understood to depend in practice on the individual desires of the mobile 
stations M1-M10 who may roam from one location in the cell to another or 
from one cell to an adjacent cell or neighboring cell, and even from one 
cellular radio system served by an MSC to another such system. 
Each of the mobile stations M1-M10 is capable of initiating or receiving a 
telephone call through one or more of the base stations B1-B10 and a 
mobile station switching center (MSC). A mobile station switching center 
(MSC) is connected by communication links, e.g., cables, to each of the 
illustrative base stations B1-B10 and to the fixed public switched 
telephone network (PSTN), now shown, or a similar fixed network which may 
include an integrated system digital network (ISDN) facility. The relevant 
connections between the mobile station switching center (MSC) and the base 
stations B1-B10, or between the mobile station switching center (MSC) and 
the PSTN or ISDN, are not completely shown in FIG. 1 but are well known to 
those of ordinary skill in the art. Similarly, it is also known to include 
more than one mobile station switching center in a cellular radio system 
and to connect each additional mobile station switching center to a 
different group of base stations and to other mobile station switching 
centers via cable or radio links. 
Each MSC may control in a system the administration of communication 
between each of the base stations B1-B10 and the mobile stations M1-M10 in 
communication with it. As a mobile station roams about the system, the 
mobile station registers its location with the system through the base 
stations that control the area in which the mobile station is located. 
When the mobile station telecommunications system receives a call 
addressed to a particular mobile station, a paging message addressed to 
that mobile station is broadcast on control channels of the base stations 
which control the area in which the mobile station is believed to be 
located. Upon receiving the paging message addressed to it, the mobile 
station scans system access channels and sends a page response to the base 
station from which it received the strongest access channel signal. The 
process is then initiated to create the call connection. The MSC controls 
the paging of a mobile station believed to be in the geographic area 
served by its base stations B1-B10 in response to the receipt of a call 
for that mobile station, the assignment of radio channels to a mobile 
station by a base station upon receipt of a page response from the mobile 
station, as well as the handoff communications with a mobile station from 
one base station to another in response to the mobile station traveling 
through the system, from cell to cell, while communication is in progress. 
Each of Cell A-Cell J is allocated a plurality of voice or speech channels 
and at least one control channel, such as an analog control channel (ACCH) 
or digital control channel (DCCH). The control channel is used to control 
or supervise the operation of mobile stations by means of information 
transmitted to and received from those units. Such information may include 
call originations, page signals, page response signals, location 
registration signals and voice channel assignments. 
The present invention involves implementation of a method and system for 
creating an accurate neighbor cell list to be used for handoff in a 
cellular system similar to that shown in FIG. 1. 
In an embodiment of the invention, the method and system is implemented 
into a cellular system like that shown in FIG. 1 that operates according 
to the IS-136 standard. The IS-136 standard is hereby incorporated by 
reference. In this first embodiment of the invention the DCCH channels 
assigned to each cell of the system for control purposes are also used as 
the measurement channels for neighbor cell list purposes. 
Referring now to FIG. 2 therein are shown cells Cell A-Cell J (also shown 
in FIG. 1) with additional neighboring cells Cell K-Cell S, that also 
comprise a portion of the same cellular system. Each of Cell K-Cell S may 
be configured identically to Cell A-Cell J as shown in FIG. 1, with a base 
station (not shown) located in each cell and Cell K-Cell S being 
controlled by one or more MSCs (not shown). In FIG. 2, Cell A is located 
in the center of the collection of Cell B-Cell S. Each of Cell B-Cell S 
has indicated within it an assigned DCCH channel number. For example, Cell 
B is assigned DCCH channel number 63 and Cell E is assigned DCCH channel 
number 42. The DCCH channel number assignments are conventionally fixed 
for an IS-136 cellular system. 
The handoff may be done by the method of mobile assisted handoff (MAHO) 
specified in commonly assigned U.S. Pat. No. 5,200,957 to Dahlin, which is 
hereby incorporated by reference. During the procedure for call setup on 
digital communication channel, the base station informs the mobile station 
of radio channel frequency and also of a time slot that identifies the 
timeslot to be used and digital voice color code (DVCC). During the call 
setup procedure the base station also informs the mobile station of a 
plurality of DCCH channels the signal strength of which are to be measured 
by the mobile for handoff purposes. This plurality of DCCH channels are 
the DCCH channels of cells which comprise the neighbor cell list. As a 
mobile station involved in the ongoing cell moves among Cell A-Cell S of 
FIG. 2, the system will handoff control of call communications from cell 
to cell. Depending upon the movement of the mobile station, as well as 
other circumstances, a new plurality of DCCH channels will be selected and 
the corresponding neighbor cell list transmitted to the mobile station 
from the responsible base station during the course of the connection. 
During the course of the connection the mobile station measures the signal 
strength of signals on the given plurality of DCCH channels. Measurements 
are done during time slots not used by the digital communication channel. 
The mobile station also measures signal strength on the digital 
communication channel used for the established connection and the bit 
error rate on the established connection. The mobile station transmits 
results of its measurements, preferably averaged, frequently to the base 
station, preferably twice a second. 
The base station also measures signal strength on the digital communication 
channel used for the established connection and the bit error rate on the 
established connection. The base station processes and analyzes the 
results of its own measurements and the measurements of the mobile station 
for comparison with handoff criteria. When, according to the results and 
criteria, a handoff is desired, the base station informs the mobile 
switching center indicating at least one target base station assumed 
suitable for taking over the responsibility for the communication with the 
mobile. 
The mobile switching center requests the target base station(s) to measure 
signal strength on a radio channel in the time slot used by the mobile for 
the established connection. The mobile switching center also informs the 
target base station on the digital color code used by the mobile station. 
The target base station(s) tune(s) a receiver to the radio channel 
indicated by the mobile switching center and uses the time slot identifier 
of the indicated time slot for burst synchronization. The target base 
station checks the appearance of the digital verification color code 
indicated by the mobile switching center and measures the signal strength 
of the burst signal provided the digital verification color code is 
correct. The target base station then transmits the results of the signal 
strength measurement to the mobile switching center. The target base 
station also informs the mobile switching center on the result of the 
checking of the appearance of the digital verification color code, i.e., 
whether the digital verification color code appeared in the burst in the 
time slot of the radio channel. 
The mobile switching center determines whether handoff to a target base 
station should be performed taking the results of the signal strength 
measurements of target base(s) into account as well as other 
circumstances, e.g. traffic load. 
The invention herein is used to build a neighbor cell list for use in the 
above described MAHO process within the IS-136 system. Use of the 
invention allows creation of a neighbor cell list that accounts for 
irregular RF effects and radio wave propagation anomalies within the cells 
of the system. 
For example, the situation with Cell A-Cell S of FIG. 2 may be such that RF 
propagation anomalies cause radio propagation islands to form. FIG. 3 
illustrates a radio propagation island within the area covered by Cell A, 
Cell F, Cell E and Cell H of FIG. 2. In FIG. 3 it is shown that, because 
of geographic effects, or otherwise, the base station controlling Cell H, 
of all base stations in the system provides the strongest received and 
transmitted signal strength when communicating with mobile stations 
located in the shaded area. It would be desirable when a mobile station 
involved in a call is located at point 300 in FIG. 3 and moving from Cell 
A into Cell E or Cell F along the shaded area, that control of the cell be 
handed off from the base station of Cell A to the base station of Cell H. 
This is desirable since Cell H provides the best RF propagation. In this 
case, the most efficient neighbor cell list for Cell A must include Cell 
H. 
A system operator who manually sets the neighbor cell list for Cell A 
relying on the model of ideal representation of cell coverage shown in 
FIG. 1 may, if the neighbor cell list is limited to less than 18 cells in 
size, not include Cell H in the manually created neighbor cell list. If 
this manually created neighbor cell list is used to indicate candidate 
handoff cells for handoff measurement purposes when mobile station M1 
moves out of Cell A, call handoff would take place to the base station of 
either Cell E or Cell F. This may not provide as good a communications 
connection as is available with the base station of Cell H. Implementation 
of the invention within the system would solve this problem. 
In the invention, a neighbor cell list for Cell A is created by 
periodically performing signal strength measurements within cell A on the 
DCCHs assigned to cells Cell B-Cell H. Uplink measurements are done at 
base station B1 (shown in FIG. 1) of Cell A and downlink measurements are 
performed by mobile stations located within Cell A and under the control 
of base station Bi at the time of measurement. For example, in the 
situation shown in FIG. 1 mobile stations M3, M4, M6 and M7 would perform 
the downlink signal measurements during a call. The signal strength 
measurements made at the mobile are transmitted to the system via the base 
station. The signal strength measurements are performed periodically over 
a period of time. The results of the signal strength measurements can be 
used to obtain an average signal strength for each of the DCCHs in Cell 
B-Cell S. As an alternative, the signal strength measurements can be used 
to determine the frequency or number of times a signal strength 
measurement above a certain threshold level was obtained on each of the 
DCCHs of Cell B-Cell S. The processing of the signal strength measurements 
is done by the base station controller, or alternatively, the measurement 
results can be sent to the MSC for processing. It will be obvious to a 
practitioner skilled in the art that signal strength measurements may be 
performed by numerous methods. 
The method and system of the invention is implemented into a IS-136 system 
using Adaptive Channel Allocation (ACA) and the neighbor cell list is 
created by utilizing functions of the ACA feature. 
In Adaptive Channel Allocation various measurements of signal quality and 
interference levels of dynamically allocated communications channels are 
performed to build a list of traffic or voice channels that may be 
assigned to a call made from within a cell. The interference levels are 
measured by measuring the signal strengths on channels allocated to 
neighboring cells. Typically, ACA is implemented in systems in which any 
cell may be assigned any dynamically allocated traffic or voice channel of 
the system. The base station controlling a cell and mobile stations within 
the cell's coverage area perform measurements on a set of channels that 
the system operator has assigned to be dynamically allocated for 
communications within the system. The system then builds for each cell a 
table of channels from the least interfered (highest quality) to the most 
interfered (lowest quality). The system then selects a certain number of 
least interfered channels from that list to allocate to communication in 
that cell. Other criteria, such as certain required frequency separation 
between the channels chosen and avoiding certain combinations of channels 
whose frequencies create intermodulation interference, are also considered 
in the selection of channels. Various methods at Adaptive Channel 
Allocation are well known to those having ordinary skill in the art. These 
known Adaptive Channel Allocation methods utilize various criteria for 
selecting channels for allocation. 
For example, H. Eriksson, "Capacity Improvement by Adaptive Channel 
Allocation", IEEE Global Telecomm. Conf., pp. 1355-1359, Nov. 28-Dec. 1, 
1988, illustrates the capacity gains associated with a cellular radio 
system where all of the channels are a common resource shared by all base 
stations. In the above-referenced report, the mobile measures the signal 
quality of the downlink, and channels are assigned on the basis of 
selecting the channel with the highest carrier to interference ratio (C/I 
level). 
It is preferable to implement ACA schemes in two parts: a "slow" part, and 
a "fast" part. The "slow" part determines, for each cell, a set of 
channels to be used based on interference and traffic fluctuations that 
occur over a relatively long period of time (e.g., 20-30 busy hours, which 
could take several weeks to occur). This eliminates the frequency planning 
problem, and may also adapt to average traffic loads in the system. The 
"fast" part is concerned with selecting at any given moment, from the 
slowly determined set of channels, the "best" channel for each connection, 
based on short term interference measurements. Implementation of both the 
"slow" and the "fast" parts of an ACA scheme may be distributed in the 
system, so that each base station determines its portion of the frequency 
plan as well as channel assignments based on local observations within the 
cell. 
One reason for splitting an ACA scheme into two parts (i.e., "fast" and 
"slow") is because of the use of auto-tuned combiners that are 
mechanically tuned, by means of small motors, to desired frequency ranges. 
Tuning is an automatic, but slow, operation that cannot be performed when 
a call arrives at the cell. 
In the invention the neighbor cell list creation process for a particular 
cell in the IS-136 system is implemented by including the DCCH channels of 
neighboring cells in the list of channels to be measured for ACA within 
that cell. The ACA process used in this first embodiment utilizes the 
measurement process of the MAHO process described above to perform channel 
measurements at the mobile station. The ACA measurements at the mobile 
station are performed by placing a different channel from the ACA list in 
the list of channels included with the MAHO measurement order transmitted 
at each call setup. 
Base station ACA measurements are made at the base station of each cell of 
the system using the same ACA list. In the invention the DCCH channels of 
neighboring cells are also added to the base station ACA list. The base 
station then performs periodic measurements on the channels in the ACA 
list. 
To create a neighbor cell list for a particular cell, the process of the 
invention adds the extra channel or channels from the ACA list to the 
channels of cells contained in an initial neighbor cell list used for MAHO 
measurements. The initial neighbor cell list for a cell consists of the 
immediate neighbors of that cell as determined by the ideal hexagonal 
representation of cell shape. For example, the initial neighbor cell list 
for Cell A of FIG. 2 would consist of Cell B-Cell G. At each call setup 
within a cell for which a neighbor cell list is to be created, the extra 
ACA channel is included in the MAHO measurement order. 
Continuing using Cell A as an example, as a call setup is made in Cell A, a 
mobile station will receive a MAHO measurement list including the DCCH 
channels of Cell B-Cell G, and an additional channel taken from the ACA 
list. The additional channel could include one of the channels to be 
dynamically allocated within the system or, a DCCH channel of Cell H-Cell 
S. Each time a new call setup occurs, a different channel from the ACA 
list is used. The ACA measurements including the DCCH channels are 
collected by the system over a relatively long period of time preferably 
20-30 busy hours, which could take several weeks to occur. An ordered 
interference level table is then created within the base station 
controller or MSC from these ACA measurements. 
From the standpoint of Cell A the DCCH channel frequencies of Cell B-Cell G 
(and certain other cells of Cell H-Cell S) will exhibit a lot of 
interference (strong signal strength) compared to other DCCH channel 
frequencies of the system since these cells are located close to Cell A. 
Base station B1 will measure an amount of interference on the DCCH uplink 
frequencies of Cell B -Cell G comparatively larger than the amount of 
interference measured on the DCCH channels of other cells since there are 
mobile stations continuously registering and making call accesses on these 
channels. B1 will also measure much interference on the DCCH uplink 
frequency of Cell H since the coverage area of Cell H overlaps the 
coverage areas of Cell E and Cell F. The mobile stations located within 
Cell A which collectively measure on all DCCH downlink frequencies will 
measure an amount of interference on the DCCHs of Cell B-Cell G, and Cell 
H, larger than that measured on the DCCH channels of other cells since the 
neighboring base stations are continuously transmitting on these 
frequencies. 
Referring now to FIG. 4A, therein is shown a flow diagram illustrating 
measurement steps performed by each mobile station during each call within 
a particular cell according to the first embodiment of the invention. At 
step 602 the mobile station receives the MAHO list from the MSC via the 
base station. The MAHO list includes the extra channel for ACA 
measurements during call setup. The ACA measurement process is modified to 
include the DCCH channel numbers of neighboring cells so that a neighbor 
cell list can be created according to the invention. Next, at step 604 the 
mobile station measures the interference level (signal strength) on the 
downlink of each channel in the measurement list during the MAHO 
measurement process. Next, at step 606 the mobile station transmits the 
measurement results to the MSC via the base station controlling the cell. 
The process then moves to step 608 where it is determined if the call is 
over. If the call is over the process ends. If, however the call is 
ongoing the process returns to step 604 and, after an appropriate delay, 
repeats the interference level measurements. The process of FIG. 4A is 
repeated for every call set up within a cell of the system. 
Referring now to FIG. 4B, therein are shown the steps performed within the 
base station according to the first embodiment of the invention. The 
process begins at step 610 as the base station receives the ACA 
measurement list from the MSC. The ACA measurement list for the base 
station contains all the ACA channels as well as the DCCH channels of 
neighboring cells. Next, the process moves to step 612 as the base station 
waits for a measurement timer signal. The measurement timer signal is 
generated by the base station controller depending upon the desired ACA 
measurement period. Upon receiving a measurement timer signal at step 614, 
the process moves to step 616 where the base station measures uplink 
interference (signal strength) on each measurement channel in the 
measurement list. Next at step 618 the base station transfers the results 
to the MSC. From step 618 the process then returns to step 612. This 
process is repeated periodically according to the measurement timer 
signal. 
Referring now to FIG. 4C, therein are shown the steps performed within the 
MSC according to the first embodiment of the invention. At step 619 the 
measurement process begins as the MSC sends the base station and mobile 
station measurement lists to the base station. The mobile station 
measurement list will then be further transmitted to the mobile station 
from the base station. From step 619 the process then moves to the wait 
state of step 620. At step 622 the MSC receives an input. The input is 
either a set of measurement results from a mobile station or base station 
or, a neighbor cell list signal. The measurement results will be received 
over a period of time, whenever the mobile station transmits results to 
the system or the base station transfers measurement results to the MSC. 
The neighbor cell list signal is received from a system timer and 
indicates that it is time to average the interference measurements. Next, 
at step 622 it is determined what type of input was received. If 
measurement results were received the process moves to step 634 where the 
results are stored. From step 634 the process returns to step 620. If, 
however, a neighbor cell list signal was received, the process moves to 
step 626 where the stored measurement results are averaged to create an 
average interference level for each measurement channel. Next, at step 628 
the MSC controller creates an ordered list of all measurement channels for 
which measurement results were received. 
Referring now to FIG. 5, there is shown an example of a table of Channel 
Numbers built from measurements performed by modifying Adaptive Channel 
Allocation for Cell A according to the teachings of the present invention. 
The table in FIG. 5 shows Channel Numbers used within Cell A-Cell S of 
FIG. 2 ranked from least interfered (lowest received average signal 
strength) to most interfered (highest received average signal strength), 
as measured at the base station B1 and at mobile stations located within 
Cell A during ACA measurement times. In FIG. 5, the DCCH channels of Cell 
B-Cell G and of Cell H are located at or near the bottom of the table. The 
DCCH channels of these cells are therefore among the most interfered 
channels as measured within the coverage area of Cell A. 
From step 626 the process moves to step 628, where the MSC controller 
determines the N most interfered DCCH channels in the ordered measurement 
list. 
In order to create the neighbor cell list for any cell, a number (N) of the 
most interfered DCCH channels are determined from the table of FIG. 5. The 
N most interfered DCCH channels are the N channels having the N highest 
received signal strengths. 
Next, at step 632 a neighbor cell list is created. Continuing with the 
example of Cell A, the N most interfered DCCH channels, if not already 
contained in the list, are added to the initial neighbor cell list 
containing the DCCH channels of Cell B-Cell G. If the DCCH channels of any 
of cells Cell B-Cell G are not in the group of N cells they may be removed 
from the initial list. This same result may be accomplished by simply 
replacing the initial neighbor cell list with the N most interfered DCCH 
channels. 
As an alternative, the neighbor cell list for Cell A may be created by 
taking the DCCH channels from the table of FIG. 5 that have an 
interference level above a certain threshold, rather than taking a set 
number of N channels. The threshold may be set to create a neighbor cell 
list within a desired size limit. 
For the system of FIGS. 1-4, if the ACA measurements are collected over a 
statistically valid time period, Cell H will be among the cells added to 
the initial neighbor cell list. As an alternative, a certain number of the 
cells Cell B-Cell S having the most interfered DCCH channels could be 
placed in the neighbor cell list for Cell A, without using interference 
threshold criteria. 
If a mobile station happens to be located at location 300 in FIG. 3 and 
moving into the coverage area of Cell H, the MAHO process will result in a 
call handoff to Cell H as long as the neighbor cell list created is of a 
size N.gtoreq.7. 
By implementing the present method and system within a cellular system it 
would not be necessary to include all of the eighteen cells in FIG. 2, 
Cell B-Cell S, that are closely proximated to Cell A in the neighbor cell 
list to overcome problems caused by RF propagation effects such as that 
illustrated in FIG. 3. The size of the neighbor cell list could be set to 
a value of N less than eighteen by using an appropriate interference 
threshold when choosing DCCH channels for the list, or by placing a set 
number of most interfered DCCH channels in the list. If the system 
operator desired to have high signal strength measurement precision for 
handoff, the threshold could be set high or the set number of DCCH 
channels chosen could be minimized to account for only the strongest RF 
propagations effects such as that shown in FIG. 3. The nearer the value of 
N to eighteen the greater the number of RF propagation effects accounted 
for. 
Once a neighbor cell list has been created, the method and system of the 
present invention may be used to periodically verify the neighbor cell 
list. The verification is done by continuing to make measurements with the 
ACA measurement list modified as before but also now including the DCCH 
channels of all neighbor cells. 
Again, using Cell A as an example, a neighbor cell list created by the 
method and system of the present invention is verified by continuing to 
use the DCCH channels of Cell B-Cell S in the ACA measurement list. 
Measurements on DCCH channels contained in the ACA measurement list are 
made as before. After measurements have been made over a relatively long 
period of time as before (20-30 busy hours, or several weeks) . The 
existing neighbor cell list is compared with the list of cells having a 
DCCH channel interference level above a certain threshold. If a cell in 
the existing neighbor cell list is not contained in the list of cells 
having an interference level above the certain threshold, that cell is 
removed from the neighbor cell list. If a cell having an interference 
level above the certain threshold is not contained in the existing 
neighbor cell list, that cell is added to the neighbor cell list. In the 
alternative, the N most interfered DCCH channels, where N equals the 
number of DCCH channels in the neighbor cell list, could be compared to 
the neighbor cell list. 
The method and system is also useful when a system operator installs a new 
cell site within a cellular system. In this case relatively short term 
measurements according to the invention could be used to initially set a 
neighbor cell list for the new cell. The neighbor cell list of cells 
surrounding the new cell could also be set by short term measurements. Use 
of short term measurements would allow the system operator to get the cell 
installed and operating quickly. Longer term measurements could then be 
performed as described above to verify the neighbor cell list created for 
the new cell and cells that surround it. 
The number of DCCH channels on which signal strength is measured may be 
much larger than the actual neighbor cell list. The only limitation on 
this DCCH channel list is that it is preferable that no two cells on the 
list have the same measurement channel number. It will be obvious to those 
skilled in the art that there are clear advantages to ensuring that the 
measurement channel number frequencies are not repeated, if only for the 
purpose of MAHO. 
While the invention has been described as implemented into the IS-136 
system, it will be obvious to one skilled in the art that the invention 
has equal applicability to the IS-54B, the EIA/TIA-553, or similar 
systems. In IS-54B the invention would operate similarly to that disclosed 
for an IS-136 system, with the exception that the analog control channel 
(ACCH) would be used in place of the DCCH channel. In EIA/TIA-553 the 
measurements would be made only at the base station of the pertinent cell, 
since analog mobiles are not capable of performing MAHO. 
It would also be obvious to one skilled in the art that other methods may 
be used to perform the downlink measurements at the mobile station. For 
example, the mobile assisted channel allocation (MACA) of IS-136 may be 
used to measure DCCH channel strength in an IS-136 system. MACA is an 
IS-136 option in which the system instructs idle mobile stations to make 
signal strength measurements when idle and report the measurements to the 
system upon a call or registration access. 
The above described embodiments of the invention are also suited for 
implementation into systems that include cells such as cells Cell A-Cell S 
of FIGS. 1-3, that are divided into a number of smaller microcells. For 
example, if a number of microcells share the coverage area of Cell A, 
handoffs between Cell A and these microcells would be frequent. Also, if 
other neighboring cells contained microcells, handoffs between Cell A and 
these neighboring microcells may also be frequent. It would be useful in 
this example to define some of these microcells as neighbor cells to Cell 
A. By including the DCCHs of these microcells among the DCCHs to be 
measured in the invention, any microcells belonging in the neighbor cell 
list will be included. 
As can be seen from the above description, the method and system of the 
invention allows creation of a neighbor cell list which takes into account 
variations in the size and shape of the coverage area of cells within a 
cellular system. The invention also allows the neighborhood cell list to 
be verified periodically to account for long term effects on the size and 
shape of the coverage areas of the cells in the list. 
It is believed that the operation and construction of the present invention 
will be apparent from the foregoing description and, while the invention 
shown and described herein has been characterized as particular 
embodiments, changes and modifications may be made therein without 
departing from the spirit and scope of the invention as defined in the 
following claims.