Note: Descriptions are shown in the official language in which they were submitted.
W092/11736 1 2~984 Pcr/sE9l/no87o
QUALITY CHECK FOR A MOBIL.E CELLULAR RADIO SYSTEM
FIELD OF THE INV~NTION
The present invention relates generally to cellular radio systems
having channels for transmittinq information between base
s stations and mobile stations. More precisely, the invention
relates to a method for selecting and assigning traffic channels
on the basis of signal strength and signal disturbance.
BACRGROUND OF THI~ INVBNTION
In cellular mobile radio systems, it is fundamental that a mobile
station with an established connection on a radio channel shall
be able to maintain the established connection when moving from
one cell serviced by a base station to another cell serviced by
another base station. It is also highly desirable that the
moblle station with an established connection on a radio channel
shall be able to maintain the established connection when moving
within the same cell and when the channel which is used is
subject to increased interference. The process by which a mobile
station can maintain an established connection when moving in a
cellula~r radio system is generally called a hand off.
In general, radio communication is only possible when the desired
information-carrying radio signals have sufficient signal
strength and are sufficiently strong relative to the noise and
interfering radio signals at the receiver. The minimum strength,
of course, depends on particular features of the system, e.g.,
the ~ind of modulation and the type of receiver. In order to
insure that the established connection may continue on an
intended radio channel between a mobile station and an intended
base station, the handoff process includes measurement of the
parameters of the radio signals at the intended base station
and/or at the mobile station. The first cellular mobile systems
placed in public use were analog systems typically used for
speech and other types of analog information. The systems
'
~ WO92~11736 2 2 0 7 ~ 9 8 ~ PCT~SE91/~870
include multiple radio channels for transmitting analog in-
formation between base and ~obile stations by transmitting analog
modulated radio signals. In general, first cellular mobile radio
systems had relatively large cells, and the signal measurements
in the handoff process in such systems were performed by the base
station. One such system is the Nordic Mobile Telephone System,
NMT 450. Another known cellular mobile radio system is the AMPS
mobile radio system in the United States. An excellent general
description of a mobile cellular radio system can be found in a
publication entitled "CMS 88 Cellular Mobile Telephone System"
published by Ericsson Telecom AB, 1988. The rapidly increasing
usage of these radio systems often cause the cells to be utilized
at maximum capacity.
In a cellular mobile radio system, it is important when setting
up a call or when handing over a call to a new traffic channel to
assign a traffic channel which will not have transmission
problems. In existing systems, poor transmission quality is
prevented by sealing off channels, i.e., not using traffic
channels which have disturbances above a predetermined level.
However, the simplicity of this method decreases the call
handling capacity of the system since all channels which have
disturbances greater than a predetermlned level are sealed off
regardle~ss cf the signal strength between the requesting mobile
station and the base station. In addition, poor transmission
guality can result if the signal of the requesting mobile station
has a relatively low signal strength.
8UMMARY OF THE INVENTION
The present invention overcomes the shortcomings of the prior art
by choosing a traffic channel on which the disturbance is
proportionally less than the signal strength of the requesting
mobile station. In the present invention, the signal strength of
the requesting mobile station is measured when the mobile station
requests a traffic channel between the base station and the
mobile station. A computer then determines whether there are any
available traffic channels in the cellular mobile system. If
207~984
WO92/117~ 3 PCT/SE91/~870
there are no available traffic channels in the cellular system,
the request for a traffic channel by the mobile station is denied
unless the system contains sealed traffic channels. If the
system contains sealed traffic channels, a sealed traffic channel
is temporarily unsealed thereby becoming an available traffic
channel.
In one embodiment of the present invention, a traffic channel is
selected and the disturbance level on the traffic channel is
deter~ined. A comparison is then made between the signal
strength of the requesting mobile station and the disturbance
level on the traffic channel to determine if the disturbance
level on the traffic channel is ~elow a minimum threshold value. r
If the disturbance level on the traffic channel is below the
signal strength of the requesting mobile station minus the
minimum threshold value, then the requesting mobile station is
assigned to the selected traffic channel. However, if the
disturbance level on the selected traffic channel is above the
minimum threshold value, then another traffic channel is
selected. This cycle continues until a traffic channel is found
with a disturbance level below the signal strength of the
requesting mobile station minus the minimum threshold value.
In a second embodiment of the present invention, the disturbance
level is measured for all available traffic channels. The
traffic channels are then ordered according to their disturbance
level. The siqnal strength of the requesting mobile station is
then compared with the disturbance levels of the traffic channels
to determine if at least one traffic channel has a disturbance
level below a minimum threshold value. The minimum threshold
value can fluctuate depending upon the traffic activity in the
cellular mobile system. If at least one ~raffic channel has a
disturbance level below the signal strength of the requesting
mobile station minus the minimum threshold value, the requesting
mobile station is assigned to the traffic channel which has a
disturbance level closest to the signal strength of the reques-
ting mo~ile station minus the minimum threshold value or any
other predetermined order, e.g., the furthest away from the
2075984
W092/117~ 4 PCT/SE91/~870
minimum threshold. However, if all the available traffic
channels have disturbance levels greater than the signal strength
of the requesting mobile station minus the minimum threshold
value, then the computer measures the disturbance levels on all
the available traffic channels again and continues to do so until
a traffic channel is found to ~e compatible with the signal
strength of the requesting mobile station wherein the signal
strength is continuously monitored by the system.
BRIEF DESCRIPTION OF TH~ DRA~ING~
Figure l illustrates a portion of a cellular mobile radio system
having cells, a mobile switching center, base stations and mobile
stations.
Figure 2 illustrates a flow chart of a subroutine used by
computer in one embodiment of the present invention.
Figure 3 illustrates a flow chart of a subroutine utilized by a
computer for another embodi~ent of the present invention.
D~TA~LED D~CRIPTION
Figure l illustrates ten cells Cl-ClO in a cellular mobile radio
system. Normally, a cellular mobile radio system according to
the present invention would be implemented with more than ten
cells. However, for the purposes of simplicity, the present
invention can be explained using the simplified representations
illustrated in Figure l.
For each cell Cl-ClO, there is a base station Bl-BlO, with the
same reference number as the corresponding cell. Figure l
illustrates the base stations as situated in the vicinity of the
cell center and having omni-directional antennas. The cells Cl-
ClO are, therefore, schematically represented as hexagons. The
base stations of adjacent cells may, however, be located in the
vicinity of cell borders and have directional antennas as is well
known to those skilled in the art.
: ,. . :-
: ~:
.. : ~ . . . . ..
,, ,
~'O 736 5 207~98~
92/11 PCT/SE91/00870
Figure l also illustrates nine mobile stations Ml-M9, movable
within a cell and from one cell to another. In a typical
cellular radio system there would normally be more than nine
cellular mobile stations. In fact, there are typically many
times the number of mobile stations as there are base stations.
However, for the purpose of explaining the invention, the reduced
number of mobile stations is sufficient.
Also illustrated in Figure l is a mobile switching center MSC.
The mobile switching center MSC illustrated in Figure l is
connected to all ten base stations Bl-BlO by cables. The mobile
switching center (MSC) is also connected to cables to a fixed
public switching telephone network or similar fixed network. All
cables from the mobile switching center MSC to the base station
Bl-BlO and cables to the fixed network are not illustrated.
In additi;on to the mobile switching center MSC illustrated, there
may be another mobile switching center connected by cables to
base stations other than those illustrated in Figure l. Instead
of cables, other means, for example, fixed radio links may be
used for connecting base stations Bl-BlO to the mobile switching
center. The mobile switching center MSC, the base stations Bl-
BlO, and the mobile stations Ml-M9 are all computer controlled.
The cellular mobile radio system illustrated in Figure l includes
a plurality of radio channels for communication. Such systems
can be designed for either analog information on digital
information and the present invention is applicable to either
analog or digital systems, but in the following description an
analog system is assumed. Some of the radio channels are used
for control channels, and each base station Bl-BlO has at least
one control channel. Normally a control channel is not used for
the transfer of speech information. Control channels are
typically used for monitoring and controlling mobile stations
during the set up of the connection and during the registration
of a mobile (i.e., when the mobile reports to the land system in
which it is located).
WO92/117~ 6 2 0 7 5 9 8 ~ PCT/SEg1t00870
Each cell is always equipped with a signal strength receiver
which consists of a receiver and a control unit. The signal
strength receiver is typically the same design as the receiver
used for each traffic channel. The signal strength receiver in
each cell performs cyclical measurements, sampling the radio
frequencies received from the mobile stations. All the system
frequencies may be sampled but only the traffic channel frequency
allocated to mobile stations in the neighboring cells are of
interest for handoff. The information about which channel should
be taken under consideration, during the above mentioned sampling
is originally received from the MSC. The measurement results are
updated in the control unit as a mean value after cyclical
sampling. In this way, each cell knows what the transmission
parameters within a mobile station currently using the neighbor's
traffic channel would be if the cell in question would have to
take over the transmission. If a handoff has been requested by
a cell, the MSC will ask the neighboring cell to send the
measurement results of the signal strengths from the mobile
station.
Referring now to Figure 2, a simplified flow chart illustrates
the subroutine of a computer located at either the MSC or a base
station, for one embodiment of the present invention. The
computer supervises the measurement of the signal strength from
a mobile station that is requesting access to the base station.
The signal can represent a new call or a preexisting call needinq
a handoff. The signal strength of the mobile requesting access
to the base station may need to be adjusted for expected changes
in the received signal strength on the traffic channel. For
instance, the power level used by the mobile station on a control
channel may be different than the power level the mobile station
is expected to use on a traffic channel. For example, a mobile
station may contact the base station using a control channel
which has a power level 3 corresponding to -6dBW in an AMPS
system. The traffic channels in the cell may howevPr allow a
power level of 2 which corresponds to -2dBW. As a result, the
signal strength of the requesting mobile station should be
increased by 4dB. In another example, a mobile station may have
.
.. ~ . ~.
: :,. .
~.
- 207~8~
WO92/117~ 7 PCT/SE91/~870
a very high signal strength. After the call set-up or the
handoff, the Mobile Station Power Regulation Control in the
candidate traffic channel may reduce the output power of the
mobile station, hence reducing the received signal strength.
The computer then causes the traffic channels in the system to be
scanned in order to determine whether there are any traffic
channels available to handle the requesting mobile station in
step 103. An available traffic channel is any unsealed channel
which is not in use in a cellular system which seals channels
with disturbance levels over a predetermined level or any channel
which is not already in use in a cellular system which does not
seal channels with disturbance levels over a predetermined level.
If there are no available traffic channels in the system, the
computer checks to see if there are any sealed channels in step
105. If there are no sealed channels, the computer then denies
access to the requesting mobile station in step 107 and the
request is terminated. However, if the computer determines that
there are channels in the system which have been sealed, the
computer selects a sealed traffic channel and unseals the traffic
channel thereby ma~ing it available to handle incoming calls and
handoffs in step 111. The computer selects a sealed traffic
channel by selecting the traffic channel which has been idling
the longest, the traffic channel which has the lowest disturbance
level or at random.
After the computer determines that at least one traffic channel
is available for handling incoming calls in step 103, the
computer selects one of the available traffic channels in step
113. The computer can select an available traffic channel either
at random or by selecting the channel that has been idling the
longest. The computer then supervises the measurement of the
strength of any disturbance on the selected traffic channel in
step 115. The disturbance level is preferably measured by
sampling the traffic channel for 300 ms and calculating an
average disturbance level.
.. .
. . ~
~, . .
,: :,: -
.- . . . .
.~
.
WO92~117~ 2 ~ 7 ~ 9 8 ~ PCT/SE91/~870
The strength of the disturbance on the selected traffic channel
is then compared with the signal strength of the requesting
mobile station in step 117. If in step 119 the strength of the
disturbance level is less than the signal strength of the mobile
S station requesting access by at least a predetermined minimum
threshold value, then the computer assigns the reguesting mobile
station to the traffic channel, thereby completing the request in
step 123. If the computer determines in step 119 that the
selected traffic channel has a disturbance level greater than the
signal strength of the requesting mobile station minus the
minimum threshold value, the computer returns to step 103 wherein
the computer checks to see if there are any available traffic
channels. This cycle continues until a traffic channel is found
which is compatible with the requesting mobile station or the
requesting mobile station is terminated.
Referring now to Figure 3, a simplified flow chart illustrates
the subroutine of a computer, located at the MSC or a base
station, for another embodiment of the present invention.
The computer first supervises the measurement of the signal
strength of a mobile station requesting access to a traffic
channel in step 151. The signal strength of the mobile reques-
ting access to the base station may need to be adjusted for
expected changes in the received signal strength on the traffic
channel. For instance, the power level used by the mobile
station on a control channel may be different than the power
level the mobile station is expected to use on a traffic channel.
For example, a mobile station may contact the base station using
a control channel which has a power level 3 corresponding to -
6dBW in an AMPS system. The traffic channels in the cell may
however allow a power level of 2 which corresponds to -2dBW. As
a result, the signal strength of the requesting mobile station
should be increased by 4dB. In another example, a mobile station
may have a very high signal strength. After the call set-up or
the handoff, the Mobile Station Power Regulation Control in the
candidate traffic channel may reduce the output power of the
mobile station, hence reducing the received signal strength.
,
. . .
207~984
WO92/11736 9 PCT/SE91/~870
The computer then determines whether there are any available
traffic channels in the system in step 153. An available traffic
channel is any unsealed channel which is not in use in a cellular
system which seals channels with disturbance levels over a
predetermined level or any channel which is not already in use in
a cellular system which does not seal channels with disturbance
levels over a predetermined level. If the computer determines
that there are no available traffic channels in the system at the
time of the request, the computer determines whether there are
any sealed channels in the system in step 155. If there are no
sealed channels in the system, the computer denies access to the
mobile station requesting access to a traffic channel in step
157. However, if the computer determines that there are sealed
channels in step 155, the computer selects a sealed traffic
channel and unseals the traffic channel in step 161 thereby
making the channel available to handle incoming calls and
handoffs. The computer selects a sealed traffic channel by
selecting the traffic channel which has been idling the longest,
the traffic channel which has the lowest disturbance level or at
random.
After the computer determines that at least one traffic channel
is available in step 153, the computer measures the strength of
the disturbance levels on all of the available traffic channels
in the system in step 163. The disturbance levels on each of the
traffic channels are then arranged according to their disturbance
levels from lowest to highest in step 165.
In step 167, the computer monitors the amount of traffic activity
occurring in the system at the time the request for a traffic
channel is made. The computer in step 169 assigns a minimum
threshold value for the difference between the signal strength of
the requesting mobile station and the disturbance level on the
selected traffic channel depending upon the amount of traffic
activity in the system. The level of traffic activity is
determined as a function of the total number of channels in the
cell versus the number of channels already in use. For example,
if traffic activity in the system is normal when a mobile station
, :
: ~ .., .. ~.......
WO92/117~ 2 0 7 ~ 9 8 ~ PCT/SEgl/00870
reguests access to a traffic channel, the computer will set the
minimum threshold value at substantially 18 decibels. However,
when the traffic activity in the system is relatively low, the
minimum threshold value can be set at substantially 20 decibels
while when the traffic activity in the system is relatively high
the minimum threshold value can be set at substantially 16
decibels. In step 171, the computer then compares the signal
strength of the requesting mobile station with the disturbance
levels of all the traffic channels. In step 171, the computer
then assigns the mobile station to a traffic channel which has a
disturbance level equal to or less than the signal strength of
the mobile station minus the minimum threshold value. However,
if all the measured traffic channels have disturbance levels
greater than the signal strength of the mobile station minus the
minimum threshold value, the program returns to step 153 to
determine whether there.are any new traffic channels available.
~his cycle continues until a traffic channel is found which is
compatible with the requesting mobile station or the request is
terminated.
While the invention has been described in its preferred
embodiments, it is to be understood that the words that have been
used are words of description rather than of limitation and that
changes within the purview of the amended claims may be made
without departing from the true scope and spirit of the invention
in:its broader aspects.
: :'
,
