Base station selection scheme for CDMA cellular system using perch channel and received SIR

A base station selection scheme for a CDMA cellular system capable of reducing the transmission power of the mobile station and increasing the subscriber capacity, by accounting for the uplink interference power at each base station. An uplink interference level is measured at each base station, while a perch channel is transmitted from each base station to the mobile station, and a perch channel reception level of the perch channel transmitted from each base station is measured at the mobile station. Then, a connection target base station to be connected with the mobile station is selected according to a received SIR (Signal to Interference Ratio) of each base station determined from the perch channel reception level measured at the mobile station and the uplink interference level measured at each base station.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a base station selection scheme for a CDMA 
(Code Division Multiple Access) cellular system, and more particularly, to 
a base station selection scheme in which a perch channel transmitted from 
each base station is received at a mobile station, and a base station is 
selected according to a reception level of the perch channel. 
2. Description of the Background Art 
In the cellular system, a service area is formed by a plurality of small 
zones (cells), and these small zones are covered by a plurality of base 
stations, so that it is possible to achieve effects of a transmission 
power reduction and a subscriber capacity increase. In order to improve 
these effects, it is necessary to connect a mobile station with the 
closest base station. Since the mobile station and the base station are to 
be connected via a radio link, it is preferable to Judge which base 
station is closest to the mobile station according to a propagation loss 
between the mobile station and each base station. 
For example, there is a conventional base station selection scheme for a 
cellular system which uses the perch channel. In this scheme, the perch 
channel transmitted from each base station is received at the mobile 
station, and the closest base station is selected by comparing the perch 
channel reception levels for different base stations. In other words, the 
base station for which the perch channel reception level is the largest is 
Judged as the closest base station. 
In addition, at a time of the communication start and during the 
communication, the perch channel reception levels measured at the mobile 
station are reported to the closest base station or the base station 
currently in communication with the mobile station, and the base station 
selection is carried out at the closest base station or the base station 
currently in communication with the mobile station according to the 
reported perch channel reception levels. 
Now, one radio access scheme that can be used in the cellular system is a 
CDMA scheme, and a use of the base station selection based on the perch 
channel reception level measurement in the CDMA is currently contemplated. 
On the other hand, in the CDMA, the identical frequency is commonly used by 
a plurality of users, so that the signals of the other user cause the 
interference, and the receiving quality is determined by a ratio of the 
reception level and the interference level (SIR: Signal to Interference 
Ratio). Consequently, the transmission power control is indispensable in 
the CDMA, and the transmission power control based on the received SIR has 
been proposed in order to maintain a constant receiving quality (see, T. 
Dohi, et al: "Performance of SIR Based Power Control in the Presence of 
Non-uniform Traffic Distribution", 1995 Fourth IEEE International 
Conference on Universal Personal Communications Record, pp. 334-338, 
November 1995). 
In addition, in the CDMA, it is preferable to suppress the transmission 
power as small as possible in order to reduce the interference as much as 
possible. 
However, when the base station selection based on the perch channel 
reception level is carried out in the CDMA, the base station for which the 
perch channel reception level at that base station is the largest will be 
selected by assuming that the uplink and downlink propagation losses are 
equal, but the interference levels are different for different base 
stations, so that the transmission power of the mobile station may not 
necessarily becomes smallest when the mobile station is connected with the 
base station selected in this manner. 
Thus, in a case of the CDMA, it is not always possible to select the most 
appropriate base station by the conventional base station selection 
scheme, and the base station selection according to the conventional base 
station selection scheme may lead to an increase of the transmission power 
and a degradation of the subscriber capacity. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a base 
station selection scheme for a CDMA cellular system capable of reducing 
the transmission power of the mobile station and increasing the subscriber 
capacity, by accounting for the uplink interference power at each base 
station. 
According to one aspect of the present invention there is provided a method 
of base station selection in a CDMA cellular system formed by a plurality 
of base stations connected with a communication network and at least one 
mobile station, comprising the steps of: measuring an uplink interference 
level at each base station; transmitting a perch channel from each base 
station to the mobile station; measuring a perch channel reception level 
of the perch channel transmitted from each base station at the mobile 
station; and selecting a connection target base station to be connected 
with the mobile station according to a received SIR (Signal to 
Interference Ratio) of each base station determined from the perch channel 
reception level measured at the mobile station and the uplink interference 
level measured at each base station. 
According to another aspect of the present invention there is provided a 
CDMA cellular system with a base station selection function, comprising: a 
plurality of base stations connected with a communication network, each 
base station including: a device for measuring an uplink interference 
level at each base station; and a device for transmitting a perch channel 
from each base station; at least one mobile station, including: a device 
for measuring a perch channel reception level of the perch channel 
transmitted from each base station; and means for selecting a connection 
target base station to be connected with the mobile station according to a 
received SIR of each base station determined from the perch channel 
reception level measured at the mobile station and the uplink interference 
level measured at each base station. 
Other features and advantages of the present invention will become apparent 
from the following description taken in conjunction with the accompanying 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1 to FIG. 10, one embodiment of a base station 
selection scheme for a CDMA cellular system according to the present 
invention will be described in detail. 
FIG. 1 shows an exemplary configuration of a CDMA cellular system to which 
the base station selection scheme of this embodiment is applied. In this 
CDMA cellular system of FIG. 1, base stations 1 to 3 connected with a 
communication network 5 are transmitting perch channels PERCH1 to PERCH3 
and broadcast control channels BCCH1 to BCCH3, respectively. A mobile 
station 4 measures reception levels of the perch channels transmitted from 
the base stations 1 to 3, and selects a connection target base station 
according to the measured reception levels. 
Each of the base station 1 to 3 has a configuration as shown in FIG. 2, 
which comprises an antenna 21 for transmitting and receiving radio signals 
to and from the mobile stations, an antenna duplexer 22 for enabling a use 
of the antenna 21 for both signal transmission and reception, a plurality 
of transmitters 23, 24 and 27a to 27n, a plurality of receivers 25 and 26a 
to 26n, and a control unit 28 for carrying out a control of each element 
of this base station, various data processing, and a control of signal 
transmission and reception for the purpose of communications with the 
mobile stations. 
The transmitter 23 is used for transmitting the perch channel, while the 
transmitter 24 is used for transmitting the broadcast control channel. 
Note here that the perch channel and the broadcast control channel are 
separately provided in this embodiment, but it is also possible to provide 
them commonly. 
The receiver 25 is used for measuring an uplink interference level, while 
the receivers 26a to 26n and the transmitters 27a to 27n are used for 
communications with the mobile stations. Signals received from the mobile 
stations by the receivers 26a to 26n are sent to the communication network 
5, while signals sent from the communication network 5 are transmitted to 
the mobile stations by the transmitters 27a to 27n. 
The control unit 28 produces an interference level information according to 
the interference level measured by the receiver 25, and controls the 
transmitter 24 to transmit the interference level information through the 
broadcast control channel. 
The mobile station 4 has a configuration as shown in FIG. 3, which 
comprises an antenna 31 for transmitting and receiving radio signals to 
and from the base stations, an antenna duplexer 32 for enabling a use of 
the antenna 31 for both signal transmission and reception, receivers 33 
and 34, a transmitter 35, a control unit 36 for carrying out a control of 
each element of this mobile station and various data processing, a 
baseband processing unit 37, and a handset 38. 
The receiver 33 is used for measuring reception levels of the perch 
channels transmitted by the base stations, and for receiving the broadcast 
control channels transmitted by the base stations. The receiver 34 
receives speech data from the base stations, and the received speech data 
are converted into speech signals by the baseband processing unit 37 and 
sent to the handset 38. Also, the speech signals sent from the handset 38 
are converted into speech data by the baseband processing unit 37 and 
transmitted to the base stations by the transmitter 35. 
The control unit carries out the base station selection according to the 
perch channel reception levels measured by the receiver 33 and the 
interference level information received through the broadcast control 
channels, while controlling the transmitters 35, the receivers 33 and 34, 
the baseband processing unit 37 and the handset 38 for the purpose of 
communication with the base stations. 
Now, the first scheme for the mobile station 4 to select the connection 
target base station in the CDMA cellular system of FIG. 1 according to the 
present invention will be described with references to FIG. 4 and FIG. 5. 
In this first scheme, the uplink interference level measured by the 
receiver 25 of the base station is converted into the interference level 
information by the control unit 28 of the base station according to a 
conversion table shown in FIG. 4, and the converted interference level 
information is transmitted by the transmitter 24. For example, as shown in 
FIG. 5, when the interference level measurement result at the base station 
1 is 30 dB.mu., the interference level measurement result at the base 
station 2 is 50 dB.mu., and the interference level measurement result at 
the base station 3 is 40 dB.mu., the interference level information 
transmitted from the base stations 1 to 3 are "00110010" through BCCH1, 
"01000110" through BCCH2, and "00111100" through BCCH3. 
The mobile station 4 receives these interference level information, 
corrects the measured perch channel reception levels according to these 
interference level information, and selects the connection target base 
station according to the corrected perch channel reception levels. For 
example, suppose that the perch channel reception level for PERCH1 is 50 
dB.mu., the perch channel reception level for PERCH2 is 60 dB.mu., and the 
perch channel reception level for PERCH3 is 30 dB.mu., as indicated in 
FIG. 5. In this case, if the base station selection is made without the 
level correction, the base station 2 that has the largest perch channel 
reception level would be selected. However, when the mobile station 4 
transmits signals at the same transmission power for all the base 
stations, the received SIR at the base station 1 is (.alpha.+50-30) dB, 
the received SIR at the base station 2 is (.alpha.+60-50) dB, and the 
received SIR at the base station 3 is (.alpha.+30-40) dB, where .alpha. is 
an offset determined by the transmission power of the mobile station 4, so 
that the received SIR is actually the largest for the base station 1. 
Consequently, the transmission power of the mobile station 4 can be made 
smaller by connecting the mobile station 4 with the base station 1 rather 
than the base station 2, but this optimal base station selection cannot be 
realized by the base station selection without the level correction. 
For this reason, the following level correction is carried out at the 
mobile station 4 in this first scheme. 
(1) Corrected reception level of PERCH1!=Uncorrected reception level of 
PERCH1!-interference level of the base station 1!; 
(2) Corrected reception level of PERCH2!=Uncorrected reception level of 
PERCH2!-interference level of the base station 2!; and 
(3) Corrected reception level of PERCH3!=Uncorrected reception level of 
PERCH3!-interference level of the base station 3!. 
As a result of this level correction, the corrected reception level of 
PERCH1 becomes 20 dB.mu., the corrected reception level of PERCH2 becomes 
10 dB.mu., and the corrected reception level of PERCH3 becomes -10 dB.mu., 
so that by selecting the base station with the largest corrected perch 
channel reception level as the connection target base station, it is 
possible to select the optimal base station 1 which has the largest 
received SIR. 
Next, the second scheme for the mobile station 4 to select the connection 
target base station in the CDMA cellular system of FIG. 1 according to the 
present invention will be described with references to FIG. 6 and FIG. 7. 
In the first scheme described above, there is a need for the mobile station 
4 to receive the broadcast control channels transmitted by its surrounding 
base stations in order to obtain the interference level information of the 
surrounding base stations. In contrast, in this second scheme, each base 
station obtains the information level information of its surrounding base 
stations through the control unit 28 which is connected with the control 
units 28 of the surrounding base stations by a wire or radio control link 
40 as shown in FIG. 6, and transmits the interference level information of 
this base station as well as the interference level information of its 
surrounding base stations through the broadcast control channel. 
Consequently, the mobile station 4 can obtain the interference level 
information of its surrounding base stations by simply receiving the 
broadcast control channel of the nearest base station alone. 
For example, in this second scheme, the base station 1 transmits the 
broadcast control information as shown in FIG. 7, which indicates the 
interference level information of each base station in correspondence to a 
base station ID of each base station, for this base station 1 as well as 
for its surrounding base stations 2 and 3. 
In this second scheme, the procedure for the level correction according to 
the obtained interference level information and the base station selection 
after the level correction is the same as in the first scheme described 
above. 
Next, the third scheme for the mobile station 4 to select the connection 
target base station in the CDMA cellular system of FIG. 1 according to the 
present invention will be described with references to FIG. 8. 
In this third scheme, the mobile station 4 can select the optimal base 
station without using the interference level information. Namely, in this 
third scheme, the control unit 28 of the base station specifies the perch 
channel transmission power to be used by the transmitter 23 in 
correspondence to the uplink interference level measurement result, 
according to a correspondence table shown in FIG. 8 which is stored in a 
table form at a memory within the control unit 28. As a result, each base 
station reduces the perch channel transmission power as much as the uplink 
interference level at each base station, so that the perch channel 
reception levels measured at the mobile station 4 reflect the interference 
levels at the base stations. 
Consequently, in this third scheme, by simply selecting the base station 
with the largest perch channel reception level as the connection target 
base station at the mobile station 4, it is possible to select the optimal 
base station which has the largest received SIR, without requiring the 
level correction. 
Next, the fourth scheme to select the connection target base station in the 
CDMA cellular system of FIG. 1 according to the present invention will be 
described with references to FIG. 9 and FIG. 10. 
In this fourth scheme, the level correction for the perch channel reception 
level is carried out at the base station side. 
Namely, in this fourth scheme, at the mobile station 4, the perch channel 
reception level measured by the receiver 33 is converted into a 
corresponding perch channel reception level information by the control 
unit 36, according to a conversion table shown in FIG. 9 which is stored 
in a memory (not shown) provided within the mobile station 4, and the 
converted perch channel reception level information is transmitted as a 
control signal to one base station by the transmitter 35. Here, this one 
base station is chosen by the mobile station 4 according to the measured 
perch channel reception levels, as a base station for which the measured 
perch channel reception level is largest. For example, in an exemplary 
case of FIG. 5 where the uplink interference levels and the perch channel 
reception levels are as described above, the mobile station 4 reports to 
the base station 2 the perch channel reception level report information as 
shown in FIG. 10, which indicates the perch channel reception level 
information of each base station in correspondence to a base station ID of 
each base station, for this base station 2 as well as for its surrounding 
base stations 1 and 3. 
Then, at the base station 2, the level correction for the perch channel 
reception levels reported from the mobile station 4 is carried out 
according to the uplink interference level (50 dB.mu.) measured at this 
base station 2 as well as the uplink interference level (30 dB.mu.) of the 
base station 1 and the uplink interference level (40 dB.mu.) of the base 
station 3 which are obtained through the control unit 28 in a manner 
similar to that indicated in FIG. 6 described above, and the base station 
to be connected with the mobile station 4 is selected. More specifically, 
the level correction is carried out in this fourth scheme as follows. 
(1) Corrected reception level of PERCH1!=Uncorrected reception level of 
PERCH1!-interference level of the base station 1!; 
(2) Corrected reception level of PERCH2!=Uncorrected reception level of 
PERCH2!-interference level of the base station 2!; and 
(3) Corrected reception level of PERCH3!=Uncorrected reception level of 
PERCH3!-interference level of the base station 3!. 
As a result of this level correction, the corrected reception level of 
PERCH1 becomes 20 dB.mu., the corrected reception level of PERCH2 becomes 
10 dB.mu., and the corrected reception level of PERCH3 becomes -10 dB.mu., 
so that by selecting the base station with the largest corrected perch 
channel reception level as the connection target base station, it is 
possible to select the optimal base station 1 which has the largest 
received SIR, without requiring the change of the perch channel 
transmission power and the interference level information transmission 
through the broadcast control channel. 
As described, according to the first scheme described above, the mobile 
station corrects the perch channel reception level of each base station 
according to the uplink interference level information of each base 
station, and selects the base station according to the corrected perch 
channel reception levels, so that it is possible to select the optimal 
base station which can minimize the mobile station transmission power. 
Consequently, it is possible to reduce the power consumption and realize a 
longer communication possible period and/or a reduced size of the mobile 
station, while reducing the interference and increasing the subscriber 
capacity in the system. 
Also, according to the second scheme described above, each base station 
obtains the uplink interference levels of the surrounding base stations 
and transmits the interference level information for this base station as 
well as for the surrounding base stations through the broadcast control 
channel, while the mobile station corrects the perch channel reception 
level of each base station according to the received interference level 
information and selects the base station according to the corrected perch 
channel reception levels, so that it is also possible to select the 
optimal base station which can minimize the mobile station transmission 
power. Consequently, it is possible to reduce the power consumption and 
realize a longer communication possible period and/or a reduced size of 
the mobile station, while reducing the interference and increasing the 
subscriber capacity in the system. In addition, it is sufficient for the 
mobile station to receive the broadcast control information from one base 
station alone, so that it is also possible to reduce the processing at the 
mobile station. 
Also, according to the third scheme described above, each base station 
measures the uplink interference level and changes the perch channel 
transmission power according to the measured interference level, and the 
mobile station selects the base station according to the perch channel 
reception levels resulting from the changed perch channel transmission 
powers, so that it is also possible to select the optimal base station 
which can minimize the mobile station transmission power. Consequently, it 
is possible to reduce the power consumption and realize a longer 
communication possible period and/or a reduced size of the mobile station, 
while reducing the interference and increasing the subscriber capacity in 
the system. In addition, there is no need for each base station to 
transmit the interference level information through the broadcast control 
channel, and there is no need for the mobile station to carry out the 
level correction processing according to the interference levels, so that 
it is also possible to reduce the processing at the mobile station and the 
base station. 
Also, according to the fourth scheme described above, each base station 
measures the uplink interference level and obtains the uplink interference 
levels of the surrounding base stations, corrects the perch channel 
reception levels reported from the mobile station according to the 
interference levels, and selects the base station according to the 
corrected perch channel reception levels, so that it is also possible to 
select the optimal base station which can minimize the mobile station 
transmission power. Consequently, it is possible to reduce the power 
consumption and realize a longer communication possible period and/or a 
reduced size of the mobile station, while reducing the interference and 
increasing the subscriber capacity in the system. In addition, there is no 
need for each base station to change the perch channel transmission power 
and to transmit the interference level information through the broadcast 
control channel, so that it is also possible to reduce the processing at 
the base station. 
Thus, according to the present invention, the base station to be connected 
with the mobile station is selected according to the received SIR of each 
base station by accounting for the uplink interference level at each base 
station, so that it is also possible to select the optimal base station 
which can minimize the mobile station transmission power. Consequently, it 
is possible to reduce the power consumption and realize a longer 
communication possible period and/or a reduced size of the mobile station, 
while reducing the interference and increasing the subscriber capacity in 
the system. 
It is to be noted that, in the above description for the first to third 
schemes of the connection target base station selection, the selection of 
the connection target base station is to be carried out at the mobile 
station, but it is also possible to modify the present invention in such a 
manner that the mobile station only selects candidate connection target 
base stations according to the scheme of the present invention as 
described above and then notifies the selected candidate connection target 
base stations to the base stations so that the final connection target 
base station selection can be made at the base station side. This 
modification can be used for a case of setting up a radio channel between 
the base station and the mobile station at a beginning of a communication 
where the final selection of the connection target base station also 
depends on the available idle communication channels which are managed at 
the base station side. 
Note here that, in a case of the fourth scheme described above, the 
selection of the connection target base station should be carried out at 
the base station side, as should be apparent from the above description. 
It is also to be noted that, besides those already mentioned above, many 
modifications and variations of the above embodiments may be made without 
departing from the novel and advantageous features of the present 
invention. Accordingly, all such modifications and variations are intended 
to be included within the scope of the appended claims.