Method of controlling power on forward link in a cellular

A method of controlling transmission power of a plurality of base stations associated with a mobile unit in a CDMA (code division multiple access) cellular system, is disclosed. The mobile unit communicates with one base station among the plurality of base stations. According to the present invention, power of each of pilot signals respectively transmitted from the plurality of base stations is measured at the mobile unit. Following this, information about a measured power value of each of the pilot signals is transmitted to the one base station. Thereafter, a first power control coefficient is determined at the one base station. The coefficient is a ratio of total pilot power values of the plurality of base stations, other than the main base station, to a pilot power value of the one base station. Subsequently, the transmission power of each of the plurality of base stations using the first power control coefficient is controlled.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to techniques in transmission power 
control of base stations in a CDMA (code division multiple access) 
cellular system using spread spectrum techniques. More specifically, the 
present invention relates to a power control method on forward links 
(viz., base station to mobile unit links) in a CDMA cellular system in 
order to increase capacity of the overall system. 
2. Description of the Related Art 
As is well known in the art, in a CDMA system, all users transmit 
simultaneously and at the same frequency. The transmitted signals occupy 
the entire system bandwidth, and code sequences, which are orthogonal, are 
used to separate one user from another. That is, each user is assigned a 
unique code sequence. The use of the same frequency in the overall system 
indicates that no "handoff" from one frequency to another is needed as in 
FDMA (frequency division multiple access) and TDMA (time division multiple 
access) systems. This is called a soft handoff that is disclosed in U.S. 
Pat. No. 5,101,501 by way of example. 
In a CDMA system, there is no distinct limit on the number of users. The 
system performance for all users degrades gradually as the number of 
active users increases. More specifically, mobile units in the CDMA system 
transmit independently (viz., asynchronously) from each other. This means 
that their signals arrive randomly at the base station and therefore, the 
crosscorrelation between these randomly arrived signals is not zero and 
thus causes interference. 
The major difficulty with CDMA is a so-called "near-far effect", which 
occurs when a weak signal received at the base station from a distant 
mobile unit is overpowered by a strong signal from a nearby interferer. To 
reduce the near-far effect, power control on reverse links (viz., mobile 
unit to base station links) is necessary. 
Additionally, the system capacity is expanded by power control on the 
forward links (viz., base station to mobile unit links). One example of 
such power control on the forward link is disclosed in Japanese Laid-open 
Patent Application No. 7-38496. According to this conventional technique, 
each of the mobile units in a given cell receives a pilot signal from the 
cell's base station, measures a signal-to-noise (S/N) ratio using the 
pilot signal received, and then informs the base station of the 
measurement results. The base station responds to the measurement results 
and controls the transmission power on the forward link of each mobile 
unit. Thus, the S/N ratios at the mobile units within the cell are 
improved and approach a predetermined level (viz., roughly equalized). As 
a result, a low level of interference is achieved at each mobile unit. 
This conventional technique, however, has suffered from a drawback. That 
is, when a S/N ratio at a given mobile unit is lowered due to increase in 
the number of the active users in the cell, the base station is responsive 
to the reduced S/N ratio and raises the power on the forward link to the 
given mobile unit. This in turn undesirably lowers the S/N ratio at each 
of other mobile units, with the result that the S/N ratio of the first 
base station again is lowered. This cycle is repeated and eventually the 
power of each forward link of many mobile units undesirably is raised to 
the maximum value. 
Further, it takes a relatively long time until the lowering of interference 
is carried out after the measurement of the S/N ratio. Therefore, during 
the long feedback time, the S/N ratio measured has undesirably changed. In 
such a case, a precise control is no longer expected. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method of 
achieving a low level of interference, especially in the vicinity of a 
cell boundary, even if the number of active users increases, whereby it is 
possible to keep constant the system performance for all users. 
One aspect of the present invention resides in a method of controlling 
transmission power of a plurality of base stations ass with a mobile unit 
in a CDMA (code division multiple access) cellular system, the mobile unit 
communicating with one base station among the plurality of base stations, 
the method comprising the steps of (a) measuring, at the mobile unit power 
of each of pilot signals respectively transmitted from the plurality of 
base stations; (b) advising the one base station of information about a 
measured power value of each of the pilot signals; (c) determining, at the 
one base station, a first power control coefficient which is a ratio of 
total pilot power values of the plurality of base stations, other than the 
main base station, to a pilot power value of the one base station; and (d) 
controlling the transmission power of each of the plurality of base 
stations using the first power control coefficient

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, there are shown only three cells 10, 12, and 14 which 
respectively include base stations BS1, BS2, and BS3. Further, as shown, 
another three base stations BS4-BS6 are respectively assigned to the other 
three cells (not shown). As is well known in the art, all the base 
stations in the system, including BS1-BS6, are coupled to a MTSO (mobile 
telephone switching office) 16, which supervises the overall operation of 
the system and which is in turn coupled to a public switched telephone 
network. Still further, two mobile units 18 and 20 are shown in FIG. 1. 
The mobile unit 18 is located in the vicinity of the boundary between the 
cells 10 and 12 and simultaneously communicates with two base stations BS1 
and BS2 in order to attain the above mentioned soft handoff. However, it 
is to be noted that the mobile unit 18 in fact establishes a speech 
channel with either BS1 or BS2. It is assumed that the other mobile unit 
20 is not located in the vicinity of a cell boundary and thus keeps 
communication only with the base station BS1. 
The present invention is not directly concerned with a handoff operation 
but directed to effectively achieve a low level of interference in the 
vicinity of a cell boundary. Therefore, the system capacity can markedly 
be increased (viz., the number of active users can be increased without 
inducing degradation of signal quality). 
Each of the base stations in the system constantly transmits a pilot signal 
the transmission power of which may vary depending on the cell size. 
However, in the instant disclosure, it is assumed that each base station 
radiates the corresponding pilot signal with a predetermined (constant) 
power for the sake of simplifying the description. Each pilot signal is 
assigned a unique code and thus, it is possible for the mobile unit to 
discriminate which base station generates the pilot signal. 
On the other hand, each mobile unit is provided with a device for measuring 
the strength of each of the pilot signals arriving at the mobile unit. 
More specifically, the mobile unit selectively acquires a predetermined 
number of pilot signals using codes which are applied thereto from a 
currently communicating base station. 
FIG. 2 is a diagram showing a pilot signal acquiring (or measuring) frame 
which consists of six time slots 1-6 in this instance. Each mobile unit 
acquires one pilot signal during one time slot and thus, is able to 
cyclically receive a total of six different pilot signals on a 
frame-by-frame basis in this particular case. The mobile unit typically 
measures the power (viz., signal strength) of one pilot signal during one 
time slot. If more than six pilot signals should be received at the mobile 
unit, the frame length can be expanded to meet the requirement. The 
instantaneous power of the pilot signal typically varies drastically and 
thus, it is a current practice to average the power over a sufficiently 
long time. Throughout the instant disclosure, the power of a pilot signal 
means an average value. 
It is assumed that a mobile unit has already established a speech channel 
with a given base station (sometimes referred to as a current base 
station). In this case, the mobile unit receives, from the current base 
station, information indicating a set of neighboring base stations. Based 
on this information, the mobile unit measures the power of each of the 
pilot signals transmitted from the neighboring base stations in addition 
to the power of the pilot signal from the current base station. 
A first embodiment of the present invention will be described with 
reference to FIGS. 3, 4A and 4B. 
In FIG. 3, at step 22, the mobile unit checks to determine if the current 
base station should be changed (viz., handoff). The instruction of 
changing the current base station (denoted by BS.sub.0) is advised from 
the current base station itself. If the current base station should be 
changed, the routine goes to step 24 whereat a new base station is advised 
together with a new set of neighboring base stations BSi (i=1, 2, . . . 
n)(n is five in the case shown in FIG. 1 for example). On the other hand, 
if the answer is negative at step 22, the routine proceeds to step 26. At 
this step 26, the power of each of the pilot signals on the forward link 
(viz., inbound link or base station to mobile unit link) in connection 
with the base stations BS.sub.0 and BSi are measured. Following this, at 
step 28, each of the measured pilot signal's power values is compared with 
a predetermined value (T1) so as to select the values exceeding T1. The 
power values thus selected are denoted by B.sub.0 and Bi (i=1, 2, . . . , 
m (m.ltoreq.n)) wherein B.sub.0 is the power value of BS.sub.0 and Bi are 
power values of BSi. Thereafter, at step 30, the power values B.sub.0 and 
Bi are transmitted to the current base station BS.sub.0. 
FIGS. 4A and 4B show the steps which are implemented at the current base 
station. At step 32, the base station receives the power values B.sub.0 
and Bi from the mobile unit. Thereafter, at steps 34 and 36, a check is 
made to determine if the current base station should be changed based on 
the power values B.sub.0 and Bi received at step 32. If a change of the 
base station is to be implemented, the data indicating the new base 
station (denoted by BS'.sub.0) is stored in the current base station. If a 
change of the current base station is not required, the routine directly 
goes to step 38 at which a handoff indicator Gh is calculated as follows. 
In this case, Bi are rewritten by Qi 
EQU Gh=(Q.sub.1 +Q.sub.2 +. . . +Q.sub.m)/B.sub.0 (1) 
Following this, at step 40, the power values B.sub.0 and Qi, exceeding a 
second predetermined value (T2), are selected. The selected power values 
are denoted by B.sub.0 and Qi (i=1, 2, . . . , k (k.ltoreq.m). It is to be 
noted that the value B.sub.0 is selected in that this value is the largest 
one. Following this, a power control coefficient R is calculated as 
follows at step 42. 
EQU R=(Q.sub.1 +Q.sub.2 +. . . +Q.sub.K)Gh.multidot.B.sub.0 (2) 
Therefore, R can be rewritten using equation (1) as follows. 
EQU R=(Q.sub.1 +Q.sub.2 +. . . +Q.sub.K)/(Q.sub.1 +Q.sub.2 +. . . +Q.sub.m) (3) 
Thereafter, the routine goes to the steps of FIG. 4B wherein if the current 
base station should not be changed (determined at step 43a) the routine 
goes through steps 43b and 44 to step 32 (FIG. 4A). On the other hand, if 
the current station should be changed, the routine goes through steps 46, 
48 and 50 and is terminated. More specifically, as shown in FIG. 4B, at 
step 43b, the base station advises the MTSO of the power control 
coefficient R, and at step 44, the base station changes the transmission 
power thereof to R.P.sub.0 (P.sub.0 is a reference transmission power). On 
the other hand, if an answer is positive at step 43a, the routine goes to 
step 46 at which the base station advises the MTSO of the new station. 
Thereafter, at step 48, the base station receives a new set of neighboring 
base stations associated with the new base station. Subsequently, at step 
50, the base station advises the mobile unit of the new base station and 
the new neighboring base stations. 
A second embodiment of the present invention will be described with 
reference to FIGS. 5A and 5B. 
As shown in FIG. 5A, steps 32' to 40' are identical to step 32 to 40 and 
hence further descriptions thereof are omitted for brevity. The second 
embodiment features that the power control coefficient R is derived using 
total transmission power values (Pi) of the base stations and the 
corresponding power values Qi. In FIG. 5A, Pmax indicates the maximum 
allowable power value of each base station. More specifically, at step 52, 
the base station receives, from the MTSO, a total transmission power value 
of each of the base stations associated with Qi (the total transmission 
power values are denoted by Pi). Thereafter, the routine goes to step 54 
where the power control coefficient R is calculated as follows: 
R=((P.sub.1 .multidot.Q.sub.1 +P.sub.2 .multidot.Q.sub.2 +. . . +P.sub.k 
Q.sub.k)/Gh.multidot.Pmax.multidot.B.sub.0) On the other hand, the power 
control coefficient R should be in a range between previously determined 
minimum and maximum values (Rmin and Rmax). The manner of defining the 
coefficient R between Rmin and Rmax is shown in FIG. 5B. As shown in FIG. 
5B, at step 56, a check is made to determine if R&gt;Rmax. If the answer at 
step 56 is affirmative, the routine goes to step 58 where R is replaced 
with Rmax, after which the routine proceeds to the flow chart of FIG. 4B. 
On the contrary, if the answer at step 56 is negative, the routine goes to 
step 60 where a further check is made to determine if R&lt;Rmin. If the 
answer at step 60 is affirmative, the routine goes to step 62 where R is 
replaced with Rmin, after which the routine proceeds to the flow chart of 
FIG. 4B. On the contrary, if the answer at step 60 is negative, the 
routine directly goes to the flow chart of FIG. 4B. After implementing 
either step 62 or step 58, the routine goes to the program which is 
exactly identical to that shown in FIG. 4B. 
A third embodiment of the present invention will be described with 
reference to FIGS. 6. 
As shown in FIG. 6, steps 32' to 38' are identical to step 32 to 38 and 
hence further descriptions thereof are omitted for brevity. The third 
embodiment features that the number of pilot signals (m in this case) is 
checked for whether or not the number exceeds the previously determined 
maximum number of pilot signals (Nmax). If m&gt;Nmax at Step 70, steps 72 and 
74 are implemented and the routine proceeds to step 76. Otherwise, the 
routine implements steps 78 and 80 and then goes to step 76. After 
carrying out step 76, the routine goes to the program which is exactly 
identical to that shown in FIG. 4B. 
A fourth embodiment of the present invention will be described with 
reference to FIGS. 7A, 7B and 8. This embodiment is to carry out, at the 
mobile unit, steps which are executed in the current base station in the 
first embodiment Therefore, the burden on the base station can be reduced. 
As shown in FIG. 7A, steps 22' to 26' are identical to step 22 to 26 of 
FIG. 3, while as shown in FIG. 7B, steps 34' to 42' are identical to steps 
34 to 42 shown in FIG. 4A. At step 90 (FIG. 7B), if the current base 
station should be changed, data indicating the new base station (depicted 
by BS.sub.0 ' is informed to the current base station together with the 
power control coefficient R. Otherwise, only the coefficient R is 
transmitted to the current base station BS.sub.0. After step 90, the 
routine returns to step 22' of FIG. 7A in order to repeat the operations. 
On the other hand, as shown in FIG. 8, at step 92, the current base 
station receives the information (viz., BS.sub.0 ' (if any) and R) which 
the mobile unit transmitted at step 90. Following this, steps 40' to 50' 
are implemented which are respectively identical to steps 40 to 50 of FIG. 
4B. 
A fifth embodiment of the present invention will be described with 
reference to FIGS. 9A and 9B The instant embodiment features a calculated 
power control coefficient (denoted by R' in step 42') which is checked to 
determine if R' is within a predetermined range where the current base 
station should not be changed. For this purpose, the power control 
coefficient R is initialized at step 100 (viz., R is set to one (1)). The 
following steps 22' to 40' are exactly identical to steps 22 to 40 shown 
in FIGS. 7A and 7B step 42' of FIG. 9B is similar to the counterpart of 
FIG. 7B. At step 102, a check is made to determine if the current base 
station should be changed. If the answer is negative at this step, the 
routine goes to step 104 at which the calculated power control coefficient 
R' is checked if R' is within the predetermined range as mentioned above. 
If the answer at step 104 is NO, the calculated coefficient R' is adopted 
and then advised to the base station BS.sub.0 at steps 106 and 108. On the 
other hand, if the answer at step 104 is YES, the routine proceeds to step 
22' of FIG. 9A. 
It will be understood that the above disclosure is representative of five 
possible embodiments of the present invention and that the concept on 
which the invention is based is not specifically limited thereto.