System and method for adaptive measurement collection and handoff queuing in a radio telecommunications network

A system and method for improving handoff performance of a cellular telecommunications network having a serving exchange and a serving cell in which a mobile station is engaged in a call. The system adapts a first timeout period during which signal strength measurements are collected from a plurality of cells that neighbor the serving cell. A variable timing mechanism measures time periods starting when a handoff measurement request is generated. The system then sets the variable timing mechanism to measure a specified time period for each of the plurality of cells, collects the signal strength measurements during the specified time period, and processes the signal strength measurements when the specified time period expires. The system also adapts a second timeout period during which a handoff request from the mobile station is queued while awaiting an available voice channel in a target cell for handoff. The system measures a default value for the second timeout period, changes the default second timeout period to an adapted second timeout period for which there is a maximum probability of handing off a call associated with the handoff request, and determines whether the handoff request should be queued for the default second timeout period or the adapted second timeout period.

BACKGROUND OF THE INVENTION 
1. Technical Field of the Invention 
This invention relates to radio telecommunication systems and, more 
particularly, to a system and method for adapting a time period for the 
collection of signal strength measurements from serving cells and neighbor 
cells prior to and during handoff of a mobile station in layered cell 
structures and across exchange boundaries. 
2. Description of Related Art 
In existing cellular radio telecommunication systems, all base stations 
have signal strength receivers that measure the current signal strength of 
signals on all frequencies utilized in neighboring cells. When a call 
connection between a mobile station and its serving base station 
deteriorates in strength and/or quality, the serving base station requests 
a handoff from the serving mobile switching center (MSC). Before handing 
off the mobile station, the MSC performs a locating function to locate or 
identify available neighboring cells that have adequate signal strength to 
satisfy the handoff request. Signal strength measurements are taken in the 
serving cell in which the mobile station is operating as well as in 
neighbor cells. The measurements are then collected and compared in order 
to determine whether the mobile station should be handed off and if so, to 
which neighbor cell. 
Existing systems have a fixed timeout delay during which the MSC receives 
signal strength measurements. At the conclusion of the timeout delay, the 
MSC compares the measurements that it has received, and makes the handoff 
determination. The existing fixed timeout delay is independent of cell 
configurations even though different cell configurations may affect the 
time required to collect signal strength measurements from all of the 
neighbor cells. For example, if the serving cell is a microcell in a 
layered cell structure, additional time may be required to collect 
measurements from higher layered cells and neighboring microcells. In this 
situation, the fixed timeout delay may not be sufficient to collect all 
the signal strength measurements prior to processing the data, thereby 
excluding the possibility of considering higher layer cells or some 
neighboring cells as handoff candidates. The fixed timeout delay is not 
addressed by any so-called "Fast Handoff Algorithms" proposed within the 
cellular telecommunications industry for microcells. These Fast Handoff 
Algorithms deal with the measurement of signal strengths by a single 
signal strength receiving device as opposed to the collection of 
measurements from a plurality of cells. 
Additionally, for situations in which the neighbor cells include cells 
within the serving cell's exchange as well as outer cells in cooperating 
exchanges, all signal strength measurements from cells within the serving 
cell's exchange are withheld from processing until either (1) measurements 
from cooperating exchanges are received, or (2) timeout from the signaling 
protocol used to communicate with the cooperating exchanges occurs. At 
that point, all received measurements are then processed. By the time 
conditions 1 or 2 occur, measurements from cells within the serving cell's 
exchange may be up to 13 or 14 seconds old, and may no longer be 
representative of signal strengths in those cells. However, the system 
cannot merely ignore the measurements from neighbor outer cells, because 
to do so would preclude inter-exchange handoffs. Therefore, a method is 
needed to balance between waiting for outer cell measurements and 
beginning handoff processing in order to optimize the handoff process. 
Condition 1 may cause a handoff failure if one or more of the mobile 
switching centers (MSCs) in the cooperating exchanges is programmed with 
an excessive delay. An excessive delay may allow the mobile station to 
move out of the coverage area of the serving cell, causing the signal 
strength and/or signal quality to deteriorate to the point that the call 
is dropped before the measurements are processed and a handoff 
determination is made. MSCs in cooperating exchanges may be manufactured 
by different manufacturers who design their systems with different delays 
in returning signal strength measurements. Therefore, when the MSC in a 
cooperating exchange is manufactured by a different manufacturer, the 
delay in returning signal strength measurements is not within the control 
of the serving exchange, but nonetheless, may adversely impact its handoff 
performance. 
Condition 2, waiting for timeout from the signaling protocol used to 
communicate with the cooperating exchanges to occur, may also cause 
handoff failures. This timeout delay may allow the mobile station to move 
out of the coverage area of the serving cell before the measurements are 
processed and a handoff determination is made. Some revisions of the IS-41 
intersystem signaling protocol, for example, have had timeout delays as 
long as 15-seconds. This delay in processing the received signal strength 
measurements has particularly adverse effects when the mobile station is 
operating in a microcell and starts to move out of the serving cell. By 
the time the 15-second delay has elapsed, the mobile station may have 
moved out of the serving microcell, causing the signal strength and/or 
signal quality to deteriorate to the point that the call is dropped before 
the measurements are processed and a handoff determination is made. 
Therefore, in existing cellular radio telecommunication systems, the 
deployment of microcells must be restricted to avoid certain 
configurations having a higher probability of dropped calls due to the 
excessive timeout delay. 
Although there are no known prior art teachings of a solution to the 
aforementioned deficiency and shortcoming, U.S. Pat. No. 5,301,356 to 
Bodin et al. (Bodin) discusses subject matter that bears some relation to 
matters discussed herein. Bodin discloses a system and method for ensuring 
that handoff requests take priority over new requests to engage voice 
channels. If no voice channels are available when a handoff request to a 
particular target cell is received, Bodin stores the handoff request in a 
corresponding queue for a predetermined period of time. If a voice channel 
becomes available while the handoff request is stored, the voice channel 
is utilized to satisfy the handoff request. Only if the handoff queue is 
empty are voice channels assigned to new call requests. 
The predetermined time period of Bodin is separate and distinct from the 
timeout delay of the present invention. The predetermined time period of 
Bodin begins after the locating procedure is completed and has located 
neighbor cells which are acceptable as target cells for handoff, and after 
the handoff request is generated and stored in the handoff queue. At the 
conclusion of the time period of Bodin, the handoff request is removed 
from the queue. The purpose of the time period of Bodin is to ensure that 
if no handoff is possible to a particular target cell, each handoff 
request is directed to another target cell which may be able to satisfy 
the handoff request before the call connection deteriorates to the point 
that it is lost. 
The measurement collection timeout delay of the present invention, on the 
other hand, is part of the locating procedure that identifies satisfactory 
target cells for handoff. As noted above, the timeout delay is the length 
of time that the MSC will wait for signal strength measurements to be 
received from serving and neighbor cells before it analyzes the 
measurements received to determine the best target cell. The adaptive 
queuing timeout delay of the present invention adapts the queuing timeout 
delay based upon network topology and the signal strength and quality in 
the target cell for handoff at the time the signal strength measurements 
were collected. This is a capability that is neither taught nor suggested 
by Bodin. Thus, review of the foregoing reference reveals no disclosure or 
suggestion of a system or method such as that described and claimed 
herein. 
In order to overcome the disadvantage of existing solutions, it would be 
advantageous to have a system and method for adapting the measurement 
collection timeout delay to different configurations of serving cells and 
neighbor cells in layered cell structures and across exchange boundaries. 
Such a system and method would adapt the measurement collection timeout 
delay for configurations in which the existing fixed timeout delay, the 
timeout delay of cooperating MSCs, or the timeout delay of intersystem 
signaling protocols increase the possibility of handoff failures. It would 
also be advantageous to have a system and method for adapting the queuing 
timeout delay for different network topologies. The present invention 
provides such a system and method. 
SUMMARY OF THE INVENTION 
In one aspect, the present invention is a system for adapting a time period 
for collecting signal strength measurements from a plurality of cells that 
neighbor a serving cell in a cellular telecommunications network. The 
system comprises a variable timing mechanism that measures time periods 
starting when a handoff measurement request is generated and means for 
setting the variable timing mechanism to measure a specified time period 
for each of the plurality of cells. The system also includes means for 
collecting signal strength measurements during the specified time period, 
and means for processing the signal strength measurements when the 
specified time period expires. 
The system may also comprise means for determining a layered cell structure 
within the cellular telecommunications network, means for identifying 
microcells and macrocells within the layered cell structure, and means for 
specifying the time period for each of the microcells and macrocells to 
optimize handoff performance in the network. The means for specifying a 
time period for each of the microcells and macrocells may include means 
for assigning a plurality of cell attributes to each of the microcells and 
macrocells in the network, including cell size, position of each microcell 
and macrocell in the layered cell structure, and neighbor cells of each 
microcell and macrocell. The means for specifying a time period also 
includes means for associating a time value with each of the plurality of 
cell attributes, and means for calculating a cumulative time period for 
the specified time period. 
In another aspect, the present invention is a system and method of 
improving handoff performance of a cellular telecommunications network 
having a serving exchange and a serving cell in which a mobile station is 
engaged in a call. The system adapts a first timeout period during which 
signal strength measurements are collected from a plurality of cells that 
neighbor the serving cell. A variable timing mechanism measures time 
periods starting when a handoff measurement request is generated. The 
system then sets the variable timing mechanism to measure a specified time 
period for each of the plurality of cells, collects the signal strength 
measurements during the specified time period, and processes the signal 
strength measurements when the specified time period expires. The system 
also adapts a second timeout period during which a handoff request from 
the mobile station is queued while awaiting an available voice channel in 
a target cell for handoff. The system measures a default value for the 
second timeout period, changes the default second timeout period to an 
adapted second timeout period for which there is a maximum probability of 
handing off a call associated with the handoff request, and determines 
whether the handoff request should be queued for the default second 
timeout period or the adapted second timeout period.

DETAILED DESCRIPTION OF EMBODIMENTS 
FIG. 1 is a block diagram illustrating components of a cellular radio 
telecommunications network 20 associated with the present invention. In 
FIG. 1, an arbitrary geographic area may be divided into a plurality of 
continuous radio coverage areas, or cells C1-C10. Although the network of 
FIG. 1 is illustratively shown to only include 10 cells, it should be 
clearly understood that in practice, the number of cells could be much 
larger. 
Associated with and located within each of the cells C1-C10, 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, receiver, 
signal strength receiver, and a base station controller as are well known 
in the art. In FIG. 1, the base stations B1-B10 are selected to be located 
at the center of each of the cells C1-C10, respectively, and are equipped 
with omni-directional antennas. However, in other configurations of a 
cellular radio network, the base stations B1-B10 may be located near the 
periphery, or otherwise away from the centers of the cells C1-C10 and may 
illuminate the cells C1-C10 with radio signals either omni-directionally 
or directionally. Therefore, the representation of the cellular radio 
network of FIG. 1 is for purposes of illustration only and is not intended 
as a limitation on the possible implementations of a system for providing 
enhanced subscriber services in a mobile radio telecommunications network. 
With continuing reference to FIG. 1, a plurality of mobile stations M1-M10 
will be found within the cells C1-C10. Again, only ten mobile stations are 
shown in FIG. 1, but it should be understood that the actual number of 
mobile stations may be much larger and, in practice, will invariably 
greatly exceed the number of base stations. Moreover, mobile stations 
M1-M10 are illustrated in some of the cells C1-C10. The presence or 
absence of mobile stations in any particular one of the cells C1-C10 
should be understood to depend, in practice on the individual desires of 
subscribers utilizing the mobile stations M1-M10. Subscribers may roam 
from one location in a cell to another, or from one cell to an adjacent 
cell or neighboring cell, and even from one cellular radio network served 
by a mobile switching center (MSC) 21 to another such network all the 
while receiving and placing calls both within the cellular network 20 as 
well as the public switch telecommunication network (PSTN) 22 which is 
connected to the MSC 21. The MSC 21 may also have associated with it a 
home location register (HLR) 23 which may be physically separate or 
connected to the MSC. The HLR 23 serves as a database of subscriber 
information for roaming subscribers. The HLR contains all the mobile 
subscriber data, such as subscriber identity, supplementary services, 
bearer services, and location information necessary to route incoming 
calls. The HLR 23 may be shared by a group of MSC's. Networks employing 
digital services may also include a message center (MC) (not shown) for 
storage and routing of short message service (SMS) messages. 
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 the MSC 
21. Such calls may be either for voice or data communications. The MSC 21 
is connected by communication links 24 (e.g., cables) to each of the 
illustrative base stations B1-B10 and the PSTN 22 or a similar fixed 
network which may be include an integrated services digital network (ISDN) 
facility (not shown). The relevant connections between the MSC 21 and the 
base stations B1-B10, or between the MSC 21 and the PSTN 22, are not 
completely shown in FIG. 2 but are well known to those of ordinary skill 
in the art. Similarly, it is also known to include more than one mobile 
switching center (MSC) in the cellular radio network and to connect each 
additional MSC to a different group of base stations and to other MSCs via 
cables or radio links. 
Each of the cells C1-C10 is allocated a plurality of voice or speech 
channels and at least one access or control channel, such as a forward 
control channel (FOCC). The control channel is used to control or 
supervise the operation of the mobile station by means of information 
transmitted and received from those units, referred to as messages. 
Control and administration messages within a cellular radio network are 
sent in accordance with industry established air interface standards, such 
as EIA/TIA 553, the standard for analog cellular operations, and/or 
EIA/TIA 627 (formerly IS-54B) and IS-136, the standards for digital 
cellular operations, all of which are hereby incorporated by reference 
herein. Integrated services between different cellular telecommunication 
systems are provided by using the intersystem specification IS-41, which 
is hereby incorporated by reference herein. While these standards govern 
North American operations, similar standards govern other geographic areas 
throughout the world, and are well known to those skilled in the art. 
The information exchanged between base stations and mobile stations via 
messages, may include incoming call signals, outgoing call signals, paging 
signals, paging response signals, location registration signals, voice 
channel assignments, maintenance instructions, SMS messages, and handoff 
instructions as the mobile stations travel out of the radio coverage of 
one cell and into the radio coverage of other cells, as well as other 
additional items of information such as calling party numbers, time 
information, and the like. 
In the busy operating mode, there are two alternative methods of performing 
the locating function and to identify when a handoff should be initiated. 
One method utilizes mobile assisted handoff (MAHO), and one method does 
not utilize MAHO. 
Location Utilizing MAHO 
For those networks that utilize MAHO, the mobile station performs the 
Location function. With MAHO, the mobile station receives on a dedicated 
channel, a neighbor list identifying neighboring cells from which the 
mobile station is to measure the signal strength. The mobile station 
measures the quality of the connection by measuring the bit error rate and 
the received signal strength on its assigned channel. The mobile station 
also measures the signal strength of channels in neighboring cells 
indicated in a Measurement Order from the base station. The Measurement 
Order includes measurement channels in neighboring cells. The channels are 
then ranked according to the signal strength received at the mobile 
station. These signal strength measurements are then utilized to assist 
the network in making a handoff determination and to identify the best 
candidate cell for handoff. 
When a mobile station in idle mode operates on a digital control channel 
(DCC) in a cellular network, the serving MSC transmits the neighbor list 
over the DCC to the mobile station. When the mobile station is in the busy 
mode, the measurement order is broadcast over the digital traffic channel 
to the mobile station at call setup and handoff. The mobile station 
continuously measures between bursts, the received signal strength from 
each of the measurement channels in the cells specified in the neighbor 
list. If, for example, the mobile station is utilizing the first time slot 
for voice communications, it may utilize the second and third time slots 
for obtaining signal strength measurements from neighbor cells. This 
information is then compared to network criteria to make the handoff 
determination and to identify the best candidate cell for handoff. 
When utilizing MAHO, the serving base station receives channel quality 
messages of its neighboring cells from the mobile station and compares the 
channels with each other. The base station considers received signal 
strength and propagation path loss (transmitted power level minus received 
signal strength). Parameters in the base station determine whether a 
request for handoff should be sent to the MSC. 
Location Without Utilizing MAHO 
In cellular networks that do not utilize MAHO to assist in the handoff 
process, signal strength receivers perform the base station's portion of 
the locating function. The signal strength receivers are deployed in base 
stations throughout the network for measuring signal strengths from mobile 
stations in conversation state in neighboring cells. The signal strength 
receiver in a particular base station operates on each frequency band 
operated by that base station and its neighbor cells. The signal strength 
measurements are provided to the MSC which determines the best candidate 
cell for handoff. 
FIG. 2 is an illustrative drawing of a layered cell structure in a cellular 
radio telecommunications network illustrating a situation in which the 
present invention is utilized to select a shortened timeout delay in order 
to effect a successful handoff. A macrocell G overlays microcells A, B, 
and C. A macrocell H overlays microcells D, E, and F. MSC-1 is associated 
with macrocell G and microcells A, B, and C, while MSC-2 is associated 
with macrocell H and microcells D, E, and F. Therefore, an inter-exchange 
boundary exists between macrocells G and H. MSC-1 and MSC-2 communicate 
via a signaling link, and may utilize a standard intersystem 
communications protocol such as IS-41. 
For a mobile station located in microcell C, microcells A, B, and D may be 
considered as neighbor cells. In addition, macrocells G and H may also be 
considered as neighbor cells to microcell C. In such a configuration of 
layered cell structures, MSCs normally prioritize neighbor cells in order 
of preferred handoff. For a mobile station operating in a microcell such 
as microcell C, and moving into microcell B, the MSC-1 prefers to handoff 
the mobile station to microcell B rather than the umbrella macrocell G, 
and will attempt the handoff to microcell B first. Otherwise, the benefit 
of having microcells is reduced. This prioritization, however, imposes 
stricter handoff requirements on the system. In such a configuration of 
serving cell and overlying and inter-exchange neighbor cells, a shorter 
timeout delay than the existing fixed timeout delay may be needed in order 
to assure a successful handoff. 
In a first embodiment of the present invention, the network operator is 
provided with a system for specifying a time value for each cell in the 
network. The system then applies different collection timeouts to 
different cells. The timeout values for each cell may be stored as an 
additional cell attribute in the MSC, or may be added as a command 
parameter in an existing command or a new command setting up a feature. 
The operator weighs known cell attributes such as configuration data 
regarding which cells neighbor a given cell and the given cell's layer in 
the network cell structure in order to specify a time value for the cell. 
If no time value is specifically assigned to a given cell by the operator, 
then a default value is assigned by the system. 
In a second embodiment of the present invention, a lookup table is defined 
within the system to relate combinations of neighbor cell attributes to 
timeout values. Unique timeout values are defined for each neighbor cell 
depending on its cell type (e.g., within the serving exchange, in a 
cooperating exchange, type of signaling protocol utilized, microcell, 
etc.). For a given serving cell, the system correlates the neighbor cells 
to timeout values in the lookup table, and determines the shortest timeout 
value for all the applicable neighbor types. This timeout value is then 
utilized by the system for the time at which signal strength measurements 
are processed. 
The present invention provides an operator with the capability to optimize 
different areas of a particular telecommunication system for the best 
handoff performance. For example, in the layered cell structure of FIG. 2, 
the operator may select shorter timeout delays between microcells even 
though the shorter delay may preclude receiving measurements from neighbor 
outer cells in some cases. Since some percentage of handoffs will be to 
other serving-exchange cells anyway, the operator may find that the 
shorter delay increases the percentage of successful handoffs. The 
operator may also collect statistics relating to the delay in receiving 
measurements from cooperating exchanges and set the timeout delay for 
border cells at a value slightly greater than the time required to receive 
all measurements. This value may be significantly less than waiting the 
full 15 seconds provided by IS-41, and may, therefore, enhance handoff 
performance. 
The system and method of the present invention are applicable to both 
analog cellular telecommunication systems and digital systems in which 
MAHO is not utilized. 
FIGS. 3A-3B are a flow chart illustrating the steps involved in calculating 
and utilizing a discrete measurement collection timeout period for each 
cell according to the teachings of the present invention. The process 
begins at step 30 and moves to step 31 where it is determined whether or 
not the serving exchange has a layered cell structure. If the serving 
exchange does not have a layered cell structure, the process moves to step 
32 and identifies border cells along inter-exchange handoff borders. At 
step 33, the process assigns attributes to each cell based on each cell's 
position relative to neighbor cells and the exchange border, the number of 
neighbor cells, and the number and position of neighbor outer cells in 
adjacent exchanges. 
The process then moves to step 34 where a time value is associated with 
each of the assigned cell attributes. Then, at step 35, the process 
calculates a discrete time period for each cell based upon the attributes 
of each cell and the associated time values. The process then moves to 
FIG. 3B, step 36 where a discrete measurement collection timeout period is 
specified for each cell and stored in the MSC. At step 37, the measurement 
collection timeout period is started for a particular cell when a handoff 
measurement request is generated. Signal strength measurements are then 
collected at step 38 from neighbor cells and neighbor outer cells during 
the measurement collection timeout period. Finally, at step 39, the signal 
strength measurements are processed to determine the best candidate cell 
for handoff when either the measurement collection timeout period expires, 
or all signal strength measurements are received, whichever occurs first. 
If, however, at step 31 it was determined that the serving exchange has a 
layered cell structure, the process moves to step 41 where microcells and 
macrocells in the layered cell structure are identified. At step 42, the 
process identifies border cells along inter-exchange handoff borders. At 
step 43, the process assigns attributes to each microcell and macrocell 
based on each cell's size, layer, position relative to neighbor cells and 
the exchange border, the number of neighbor cells, and the number and 
position of neighbor outer cells in adjacent exchanges. 
The process then moves to step 44 where a time value is associated with 
each of the assigned cell attributes. Then, at step 45, the process 
calculates a discrete time period for each microcell and macrocell based 
upon the attributes of each cell and the associated time values. The 
process then moves to FIG. 3B, step 36 where the process continues as 
described above. 
The present invention may also be expanded to include an adaptive handoff 
queuing mechanism. This aspect of the invention is applicable to the 
handoff process after an attempt to handoff a mobile station has failed 
because of the lack of an available voice channel (i.e., congestion) in 
every candidate cell. The handoff request is queued while the MSC checks 
for an available voice channel in the first candidate cell only. The 
system queues the request for a maximum predetermined time period, for 
example 5 seconds. The handoff request is removed from the queue at the 
expiration of the 5-second timeout period, or earlier if (1) a voice 
channel becomes available, (2) the parties terminate the call, or (3) a 
new handoff request is received from the mobile station. If a new handoff 
request is received, the process starts over. 
Additionally, after a certain period of time, the list of candidate cells 
may no longer be valid, and new signal strength measurements should be 
taken. This is particularly evident when the mobile station is operating 
in an exchange with a layered cell structure of microcells and macrocells. 
It is a waste of system resources to perform the processing required to 
continue the queue under circumstances in which the target cell may no 
longer be a viable candidate, or the entire list of candidate cells may no 
longer be valid. 
When queuing, there are two competing processes in the MSC. One which is 
trying to get a voice channel from the first candidate cell, and another 
which is obtaining new handoff requests from the mobile station. In 
digital telecommunication systems, the mobile station may send another 
handoff request every second. If a handoff request is queued when another 
request is received, the MSC terminates the queue and cycles through the 
candidate list included in the second handoff request searching for an 
available voice channel. Thus, it is unlikely that a request in a digital 
system will be queued for the full 5 second handoff request queuing 
period. 
In analog systems, handoff requests are less frequent, and it is more 
likely that the request will be queued for the full 5 second handoff 
request queuing period. This longer queuing period increases the 
probability of receiving a handoff request specifying a different target 
cell during the 5-second timeout period, especially in an environment with 
microcells. Because of the smaller cell size, adjacent cell boundaries are 
closer together, and the mobile station is more likely to move out of the 
smaller cells within the 5-second period. 
The longer a handoff request is queued, the higher the probability that a 
voice channel will become available in the first candidate cell. However, 
there is also a higher probability that the mobile station will have 
requested handoff to a different cell by that time. Thus, the longer the 
handoff request is queued, the more likely it is that the mobile station 
will be handed off to a different cell than the one requested in its most 
recent handoff request. 
Since the cell requested in the most recent handoff request is the optimal 
cell at that time, in terms of signal strength and signal quality, there 
is a higher probabillity that the handoff is made to a cell of lesser 
signal strength or quality, and the likelihood of lower voice quality, 
higher interference, and more dropped calls is increased. This is 
particularly true for system implementations that give priority for 
handoff to neighboring microcells rather than umbrella macrocells. 
Reducing the queue time for network topologies where there are microcells 
should increase the percentage of successful handoffs. 
The present invention provides a system and method for adapting the handoff 
queuing time depending on the network topology and, in particular, whether 
or not there are microcells present. While waiting, the system analyzes 
whether it should continue to hold the handoff request in the queue for 
the full five seconds. The system analyzes the likelihood of the target 
cell still being a viable candidate at the expiration of the 5-second 
delay based on signal strength and quality at the time the handoff 
measurements were taken, and the network topology. The queuing time may 
then be lengthened or shortened, based upon the determined likelihood that 
the target cell and handoff candidate list are still valid. 
At the expiration of the adaptive queuing time period, the system may take 
one of several actions. First, the MSC may simply remove the handoff 
request from the queue, thereby saving system resources required to queue 
the handoff request when the likelihood of a successful handoff is low. 
Second, the system may alter the target cell for handoff if the mobile 
station is passing from one microcell to another, and there is an 
overlying umbrella macrocell. Instead of queuing the handoff request which 
is targeted to a congested microcell, the system may redirect the handoff 
request to the umbrella macrocell because the macrocell will most likely 
remain a viable candidate for a longer period of time. Third, the system 
may remove the handoff request from the queue and cycle through the 
candidate list a second time to ascertain whether a voice channel has 
become available in any of the other candidate cells. 
FIG. 4 is a simplified block diagram illustrating the implementation of an 
adaptive measurement collection timer and an adaptive handoff queuing 
mechanism and timer in a mobile switching center (MSC). It is shown in 
FIG. 4 that a mobile station (MS) 41 sends a handoff measurement request 
message to a base station (BS) 42 which forwards the request to the MSC 
43. A central processing unit (CPU) 44 in the MSC controls and coordinates 
the functions performed by the MSC. A measurement collection timer 45 is 
connected to the CPU 44 and identifies to the MSC when the discrete 
timeout period for each cell expires. A handoff queuing timer 46 is also 
connected to the CPU 44 and identifies to the MSC when the standard 
5-second queuing timeout period and adapted timeout periods have expired. 
An adaptive handoff queuing mechanism 47 is connected to the CPU 44 and 
the handoff queuing timer 46, and calculates the adapted timeout period 
for each queued handoff request, depending on the location of the mobile 
station in the layered cell structure of the serving exchange. 
FIGS. 5A-5D are a flow chart illustrating the steps involved in 
implementing and utilizing an adaptive handoff queuing mechanism in 
accordance with the teachings of the present invention. The process begins 
at step 51 where the MSC checks for an available voice channel in the 
cells in the candidate list for handoff. At step 52 it is determined 
whether or not a voice channel is available. If a voice channel is 
available, the process moves to step 53 where the call is assigned to the 
available voice channel. If, however, it is determined at step 52 that a 
voice channel is not available, then the process moves to step 54 where 
the handoff request is queued for the first candidate cell in the 
candidate list for handoff. 
As noted above, the handoff request is removed from the queue at the 
expiration of the queuing timeout period, or earlier if the parties 
terminate the call, if a voice channel becomes available, or if another 
handoff request is received from the mobile station. Therefore, at step 
55, the process determines whether or not the call has been terminated. If 
the call has been terminated, the process moves to step 56 where the 
handoff request is removed from the queue. If the call has not terminated, 
the process moves from step 55 to step 57 where it is determined whether 
or not a voice channel has become available in the first candidate cell. 
If a voice channel has become available, the handoff request is removed 
from the queue, and the call is assigned to the available voice channel. 
If, however, a voice channel has not become available, the process moves 
to FIG. 5B, step 61. 
At step 61, it is determined whether or not a predetermined default queuing 
timeout period has expired. A default setting such as, for example 
5-seconds, may be set for the queuing timeout period. If the period has 
expired, the handoff request is removed from the queue at step 62. If the 
period has not expired, the process moves from step 62 to step 63 where it 
is determined whether or not a subsequent handoff request has been 
received from the mobile station. If another handoff request has been 
received, the process removes the handoff request from the queue at 64, 
and then returns to step 51 (FIG. 5A) where the handoff process starts 
over by checking for an available voice channel in all of the cells in the 
candidate list for handoff included in the subsequent handoff request. 
If, however, at step 63 it is determined that another handoff request has 
not been received, then the process moves to step 65 where it is 
determined whether or not the serving exchange has a layered cell 
structure for which the predetermined default queuing timeout period is 
not optimal. If the serving exchange does not have a layered cell 
structure for which the predetermined default queuing timeout period is 
not optimal, the process continues to queue the handoff request at 66 and 
then returns to step 55 (FIG. 5A). Thereafter, if the call is not 
terminated, a voice channel does not become available, or another handoff 
request is not received before the expiration of the default queuing 
timeout period, the request remains queued until the default period 
expires. 
If, however, it is determined at step 65 that the serving exchange does 
have a layered cell structure for which the default queuing timeout period 
is not optimal, then the process moves to step 67 where the queuing 
timeout period is adapted (shortened or lengthened) for the mobile 
station's location in the layered cell structure. Thereafter, if the call 
is not terminated, a voice channel does not become available, or another 
handoff request is not received before the expiration of the default 
queuing timeout period, the request remains queued until the adapted 
timeout period expires. 
Therefore, following step 67, the process moves to FIG. 5C, step 71 where 
it is determined whether or not the call has been terminated. If the call 
has been terminated, the process moves to step 72 where the handoff 
request is removed from the queue. If the call has not terminated, the 
process moves from step 71 to step 73 where it is determined whether or 
not a voice channel has become available in the first candidate cell. If a 
voice channel has become available, the handoff request is removed from 
the queue at step 74, and the call is assigned to the available voice 
channel. If, however, a voice channel has not become available, the 
process moves to step 75 where it is determined whether or not a 
subsequent handoff request has been received from the mobile station. If 
another handoff request has been received, the process removes the handoff 
request from the queue at 76, and then returns to step 51 (FIG. 5A) where 
the handoff process starts over by checking for an available voice channel 
in all of the cells in the candidate list for handoff included in the 
subsequent handoff request. 
If, however, at step 75 it is determined that another handoff request has 
not been received, then the process moves to step 77 where it is 
determined whether or not the adapted timeout period has expired. If the 
adapted timeout period has not expired, the process returns to step 71 and 
continues to queue the handoff request. If it is determined that the 
adapted timeout period has expired, the process moves from step 77 to FIG. 
5D, step 81 and takes one of three alternative steps. First, the process 
may merely remove the handoff request from the queue at step 82. Second, 
if the mobile station is in a microcell having an umbrella macrocell, and 
the target cell for handoff is another microcell, the process may redirect 
the target cell to the umbrella macrocell at step 83. There is a greater 
likelihood that the umbrella macrocell will still be a viable handoff 
target at the expiration of the queuing timeout period. Finally, the 
process may move to step 84 where the process re-checks for an available 
voice channel in all the cells in the candidate list for handoff. At step 
85, it is determined whether or not there is an available voice channel in 
any of the cells in the candidate list for handoff. If there is an 
available voice channel, the call is assigned to the available channel at 
step 86. If, however, there is not an available channel, the process moves 
to step 87 where the handoff request is removed from the queue. 
It is thus believed that the operation and construction of the present 
invention will be apparent from the foregoing description. While the 
method, apparatus and system shown and described has been characterized as 
being preferred, it will be readily apparent that various changes and 
modifications could be made therein without departing from the spirit and 
scope of the invention as defined in the following claims.