Transmission power control method and apparatus for mobile radio satellite communication system

The invention provides a transmission power control apparatus for a mobile radio satellite communication system which allows frame synchronization maintenance and synchronization acquisition of a receiving station and optimum transmission power control with a simple construction which does not effect complicated control. The transmission power control apparatus includes a transmission power control loop provided in each of a gateway station and a terminal/mobile station for controlling a transmission power of the station. The transmission level during transmission is monitored and compared with a preset transmission level to change over an average time for sampling of the error between the transmission levels to vary the response speed of the transmission power control loop. When the channel for current use is to be changed over from a signalling channel to a communication channel, the transmission power is controlled so that there is no trouble for continuous synchronization of the reception side. But, when the channel is to be changed over from the communication channel to the signalling channel, the transmission power is controlled rapidly to a standard level so that there is no trouble in synchronization acquisition or data transmission.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a transmission power control method and 
apparatus for a mobile radio satellite communication system, and more 
particularly to a transmission power control method and apparatus for a 
mobile radio satellite communication system which controls the 
transmission power based on received control information of a transmission 
power particularly when a channel is switched. 
2. Description of the Related Art 
A satellite communication system which effects channel access of the single 
channel per carrier (SCPC)/frequency division multiple access (FDMA) 
system based on a demand assignment is described first. Referring to FIG. 
3, a satellite communication system of the type mentioned is shown and 
normally includes a network control station 11 which manages and controls 
the communications system, gateway stations 12 each connected to public 
switched telephoney networks (PSTN) or private telephone networks, 
terminals and mobile stations (hereinafter referred to as terminals/mobile 
stations) 13 through which users access the satellite communication 
network, and a communication satellite 14. 
The network control station 11 supervises the entire network and 
successively transmits channel signalling information by time division 
multiplex (TDM) communication using a forward link (also called outbound). 
The TDM communication includes information of an incoming call and a used 
communication channel. Each of the gateway stations 12 and the 
terminals/mobile stations 13 receives information transmitted thereto by 
the time division multiplex communication and effects call request and 
responding to an incoming call based on the received information using a 
return link (inbound). The return link is used for transmission of a call 
request and incoming call responding burst signal transmitted from each of 
the gateway stations 12 and terminals/mobile stations 13 in a 
predetermined time slot in accordance with line information of time 
division multiplex communication in a time division multiple access 
(TDMA). 
If the network control station 11 receives a call request and an incoming 
call response signal transmitted from one of the gateway stations 12 and 
one of the terminals/mobile stations 13 (including an indirect case 
wherein a terminal or a mobile station transmits a call request to a 
gateway station and then the gateway station transmits a channel 
assignment request), then it informs the gateway station 12 and the 
terminal/mobile station 13 of an available communication channel over a 
signalling channel 15. Then, when the gateway station 12 and the 
terminal/mobile station 13 receive the designated communication channel, 
they set the channel so that a communication channel 16 is thereafter used 
in place of the signalling channel 15. Then, through a predetermined 
sequence, the terminal/mobile station 13 is connected to a PSTN or private 
switched network (PSTN)/private switched network 17) through the 
communication satellite 14 and the gateway station 12. 
The signalling channel 15 which communication control information 
therethrough in such a sequence of channel connection operations as 
described above communicates control information normally using a 
prescribed maximum transmission level as a standard level so that call 
connection can be effected stably even in a geographical condition wherein 
the elevation angle is small (which is the most critical communication 
condition in call connection) in order to assure a high degree of 
reliability of the communication system. After changeover to a 
communication channel, the transmission level is adjusted so as to 
minimize interference with another communication channel and allow the 
most efficient utilization of power of the satellite. Further, in a mobile 
terminal which is driven by a battery, power dissipation is minimized by 
such transmission level control to allow utilization for a long time. 
The transmission powers of the gateway station 12 and the terminal/mobile 
station 13 or the communication system described above are controlled each 
by such an apparatus as shown in FIG. 4. Referring to FIG. 4, the 
apparatus shown includes a transmission power controller 1 for controlling 
the transmission power of a modulation signal frequency converted by a 
frequency converter not shown, a power amplifier 2 for power amplifying an 
output of the transmission power controller 1 to a predetermined 
transmission power and outputting the power amplified signal to an antenna 
not shown, a level detector 3 for detecting the output level of the power 
amplifier 2, a comparator 6 for comparing the transmission level detected 
by the level detector 3 with a preset transmission reference level to 
obtain a level error, and an averaging circuit 9 for averaging the level 
error from the comparator 6 and controlling the transmission power 
controller 1 with the averaged level error. 
In operation, upon transmission over the signalling channel 15, the 
transmission level is set to the maximum standard transmission level, and 
the level of a transmission signal detected by the level detector 3 during 
transmission is compared with the preset transmission level value by the 
comparator 6 to obtain a level error. The level error of the detected 
level from the preset transmission reference level is averaged by the 
averaging circuit 9. Then, the transmission power is controlled so that, 
if the detected transmission signal level is higher than the preset 
transmission level value, then the transmission signal level may be 
decreased, but if the detected transmission signal level is lower than the 
preset transmission level, then the decrease of the transmission signal 
level may be decreased. 
A channel is assigned in response to the channel assignment request in the 
signalling channel 15, and communication by the communication channel 16 
is started. After the communication by the communication channel 16 is 
started, the terminal/mobile station 13 and the gateway station 12 
individually measure the reception levels thereof, set optimum 
transmission levels for operation of the system and control the 
transmission powers thereof. Such control is disclosed, for example, in 
Japanese Patent Laid-Open Application No. Heisei 8-237194. The 
transmission power control for the communication channel 16 is performed 
in a similar manner as that for the communication channel 16 except that 
the preset transmission level values set for the comparators are 
different. 
After the communication over the communication channel 16 is completed, 
communication by the signalling channel 15 is entered again. Then, the 
transmission signal level is set to the standard maximum transmission 
level, but is then controlled, upon transmission, to the predetermined 
transmission power by the operation described above. 
While the transmission level is controlled in accordance with the procedure 
described above so that possible interference between communication 
channels may be minimized and the power of the satellite may be utilized 
most efficiently, if the control step of the transmission level becomes 
large, the following trouble occurs. 
FIG. 5 represents transmission powers when a channel to be used is changed 
over between a signalling channel and a communication channel. 
When the transmission level of the self station after communication over 
the communication channel is started is to be decreased to the lowest 
level because the reception level of the other party station is 
sufficiently high, it is varied at a rate which can be followed up 
sufficiently so that synchronization can be held on the reception side 
against a variation in amplitude or phase. When the communication is 
terminated while the transmission power is controlled stably with the 
lowest transmission level and then communication over the signalling 
channel is entered again, the transmission level is set to the standard 
highest transmission level in preparation for next transmission. When a 
control signal burst such as a call request or an incoming call response 
is to be transmitted subsequently over the signalling channel, the top of 
the burst is transmitted with the level controlled to the lowest level 
over the communication channel and the burst is started while the 
transmission level is controlled so as to increase to the highest level. 
Therefore, it sometimes occurs that a carrier power to noise power ratio 
required by the communication system is not obtained and this gives rise 
to a trouble in synchronization acquisition or data transmission. 
Particularly, a top portion of a burst includes a training sequence and/or 
a unique word for carrier synchronization and/or clock synchronization and 
plays an important role for transmission of a data part. 
If, on the contrary, the transmission level control response over the 
signalling channel is made faster so as to allow quick convergence to a 
predetermined transmission level in order to eliminate the problem 
described above, then such a problem may possibly occur that, when the 
transmission level is controlled after reception level information of the 
other party station is received over the communication channel, frame 
synchronization of the other party station cannot be maintained or that 
the transmission power control becomes unstable. Such problems are 
significant particularly where the transmission level control step is 
large. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a transmission power 
control method and apparatus for a mobile radio satellite communication 
system which allows frame synchronization maintenance and synchronization 
acquisition of a receiving station and optimum transmission power control 
with a simple construction which does not effect complicated control. 
In order to attain the object described above, according to an aspect of 
the present invention, there is provided transmission power control method 
for a mobile radio satellite communication system wherein a gateway 
station and a terminal/mobile station effect communication by a demand 
assignment using a signalling channel and a communication channel via a 
communication satellite, comprising the stop of controlling a transmission 
power control loop provided in any of the gateway station and the 
terminal/mobile station for controlling a transmission power of the 
gateway station or terminal/mobile station so that, when a transmission 
channel for current use is to be changed over from the signalling channel 
to the communication channel, a response speed of the transmission power 
control loop is set to a low value, but when the transmission channel for 
current use is to be changed over from the communication channel to the 
signalling channel, the response speed of the transmission power control 
loop is set to a high value. 
According to another aspect of the present invention, there is provided a 
transmission power control apparatus for a mobile radio satellite 
communication system wherein a gateway station and a terminal/mobile 
station effect communication by a demand assignment using a signalling 
channel and a communication channel via a communication satellite, 
comprising a transmission power control loop provided in each of the 
gateway station and the terminal/mobile station for controlling a 
transmission power of the gateway station or terminal/mobile station, and 
control means for controlling the transmission power control loop of each 
of the gateway station and the terminal/mobile station so that, when a 
transmission channel for current use is to be changed over from the 
signalling channel to the communication channel, a response speed of the 
transmission power control loop is set to a low value, but when the 
transmission channel for current use is to be changed over from the 
communication channel to the signalling channel, the response speed of the 
transmission power control loop is set to a high value. 
The transmission power control apparatus may be constructed such that the 
signalling channel is used for communication with the gateway station, the 
terminal/mobile station and a network control station, and the 
transmission power levels of the gateway station and the terminal/mobile 
station are set to the highest level. Further, the transmission power 
control apparatus may be constructed such that the communication channel 
is used for communication between the gateway station and the 
terminal/mobile station, and the transmission power levels of the gateway 
station and the terminal/mobile station are set to an optimum transmission 
level based on a reception level detected by one and transmitted to the 
other of the gateway station and the terminal/mobile station. 
The transmission power control loop may include a transmission power 
controller for receiving an input signal and controlling a transmission 
level for the signal in accordance with a control signal, comparison means 
for detecting a transmission level of an output of the transmission power 
controller and comparing the detected transmission level with a preset 
transmission level to detect an error level of the detected transmission 
level from the preset transmission level, averaging means for controlling 
the response speed of the transmission power control loop based on the 
error level detected by the comparison means, and means for receiving an 
output of the averaging means and a producing a control signal controlling 
the transmission power controller so that the error level may be 
minimized. 
The averaging means may include discrimination means for discriminating a 
polarity of the error level, conversion means for converting the error 
level from an analog signal into a digital signal for a period 
corresponding to a predetermined sample number, selection means for 
selecting the predetermined sampling number from within a plurality of 
average sample numbers based on an output of the discrimination means, and 
control means for averaging a number of error levels successively 
outputted after each fixed interval of time from the comparison means 
which is equal to the predetermined sample number selected by the 
selection means to obtain a digital signal to be used as the control 
signal for the transmission power controller. 
Alternatively the averaging means may include discrimination means for 
discriminating a polarity of the error level, a filter for receiving error 
level, the filter having a cutoff frequency which is variable in response 
to a control signal, selection means for selecting the cutoff frequency of 
the filter based on an output of the discrimination means, and control 
means for controlling the transmission power controller through the filter 
with the cutoff frequency selected by the selection means. 
According to a further aspect of the present invention, there is provided a 
transmission power control apparatus for a mobile radio satellite 
communication system wherein a gateway station and a terminal/mobile 
station effect communication by a demand assignment using a signalling 
channel and a communication channel via a communication satellite, the 
transmission power control apparatus being provided in each of the gateway 
station and the terminal/mobile station, the transmission power control 
apparatus comprising a transmission power controller for receiving an 
input signal and controlling a transmission level for the signal in 
accordance with a control signal, a power amplifier for power amplifying 
an output of the transmission power controller to a predetermined 
transmission level, a level detector for detecting a transmission level of 
an output of the power amplifier, a control circuit for analyzing 
reception information transmitted from the other one of the gateway 
station and the terminal/mobile station to set an optimum transmission 
level, a comparison circuit for comparing the transmission level detected 
by the level detector with the optimum transmission level, a 
discrimination circuit for discriminating a polarity of an output signal 
of the comparison circuit and outputting a selection control signal for an 
average sample number or average time, a selection circuit for selecting 
an average sample number in accordance with the selection signal from the 
discrimination circuit, and an averaging circuit for averaging a number of 
error levels successively outputted after each fixed interval of time from 
the comparison circuit which is equal to the average sample number 
designated by the selection circuit to obtain a digital signal to be used 
as the control signal for the transmission power controller. 
According to a still further aspect of the present invention, there is 
provided a transmission power control apparatus for a mobile radio 
satellite communication system wherein a gateway station and a 
terminal/mobile station effect communication by a demand assignment using 
a signalling channel and a communication channel via a communication 
satellite, the transmission power control apparatus being provided in each 
of a transmission section of the gateway station and the terminal/mobile 
station, the transmission power control apparatus comprising a 
transmission power controller for receiving an input signal and 
controlling a transmission level for the signal in accordance with a 
control signal, a power amplifier for power amplifying an output of the 
transmission power controller to a predetermined transmission level, a 
level detector for detecting a transmission level of an output of the 
power amplifier, a control circuit for analyzing reception information 
transmitted from the other one of the gateway station and the 
terminal/mobile station to set an optimum transmission level, a comparison 
circuit for comparing the transmission level detected by the level 
detector with the optimum transmission level, a discrimination circuit for 
discriminating a polarity of an output signal of the comparison circuit 
and outputting a selection signal for selecting a filter cutoff frequency, 
a selection circuit for selecting a filter cutoff frequency in response to 
the selection signal from the discrimination circuit, and an averaging 
circuit including a filter of a variable cutoff frequency for averaging a 
number of error levels successively outputted after each fixed interval of 
time from the comparison circuit which corresponds to the cutoff frequency 
designated by the selection circuit to obtain a digital signal to be used 
an the control signal for the transmission power controller. 
With the transmission power control method and apparatus described above, 
the transmission level during transmission is monitored and the monitored 
transmission level is compared with a preset transmission level to change 
over the average time for sampling of the error between the transmission 
levels to vary the response speed with a simple construction which does 
not require complicated control. Consequently, when the channel for 
current use is to be changed over from a signalling channel to a 
communication channel, the transmission power is controlled so that there 
is no trouble for synchronization maintenance of the reception side. On 
the other hand, when the channel for current use is to be changed over 
from the communication channel to the signalling channel, the transmission 
power is controlled rapidly to a standard level so that there is no 
trouble in synchronization acquisition or data transmission. 
The above and other objects, features and advantages of the present 
invention will become apparent from the following description and the 
appended claims, taken in conjunction with the accompanying drawings in 
which like parts or elements are determined by like reference symbols.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows in block diagram a transmission power control apparatus for a 
mobile radio satellite communication system to which the present invention 
is applied. The transmission power control apparatus is incorporated in 
each of a gateway station and a terminal/mobile station such as the 
gateway stations 12 and the terminal/mobile stations 13 described 
hereinabove with reference to FIG. 3. 
Referring to FIG. 1, the transmission power control apparatus is 
incorporated in a transmission section of each of gateway stations and 
terminals/mobile stations of a satellite communication system which 
effects call connection of the single channel per carrier (SCPC) or 
frequency division multiple access (FDMA) system based on a demand 
assignment, and includes a transmission power controller 1 for controlling 
the transmission level of a modulation signal frequency converted by a 
frequency converter (not shown), a power amplifier 2 for power amplifying 
an output of the transmission power controller 1 to a predetermined 
transmission level and outputting the power amplified signal to an antenna 
not shown, a level detector 3 for detecting the output level of the power 
amplifier 2, a control circuit 4 for analyzing reception information 
transmitted to the self station from the other party station and setting 
an optimum transmission level, a transmission level setting circuit 5 for 
outputting a reference signal of the transmission level set by the control 
circuit 4, a comparator 6 for comparing the transmission level detected by 
the level detector 3 with the reference signal of the preset transmission 
level, a discrimination circuit 7 for discriminating the polarity of a 
signal outputted from the comparator 6 and outputting a selection control 
signal of an average sample number (average time), a selection circuit 8 
for selecting an average sample number in response to the selection 
control signal from the discrimination circuit 7, and an averaging circuit 
9 for averaging a number of error levels successively outputted after each 
fixed interval of time from the comparator 6 designated by the selection 
circuit 8 to obtain a digital signal to be used as a control signal for 
the transmission power controller 1. 
The averaging circuit 9 may be formed from an analog to digital (A/D) 
converter which receives an error component (analog signal) outputted from 
the comparator 6 and converts the error component into a digital signal 
with a predetermined sampling number. 
In this instance, the predetermined sample number can be selected from 
between two average sample numbers L1 and L2 in response to an output of 
the discrimination circuit 7, and a digital signal produced based on the 
selected average sample number is used to control the transmission level 
of a modulation signal mentioned above by the transmission power 
controller 1. 
Operation of the transmission power control apparatus having the 
construction described above is described below. 
Transmission data to be transmitted first undergo coding processing such as 
error correction, differential operation and scrambling and are then 
digitally modulated, whereafter they are converted into a signal of a 
radio frequency by a frequency converter not shown. The transmission level 
of the signal is adjusted by the transmission power controller 1, and 
then, the signal is power amplified to a predetermined level by the power 
amplifier 2. The thus power amplified signal is sent out from the antenna 
(not shown) through a duplexer (not shown). Further, the transmission 
level of the signal being transmitted is monitored by the level detector 
3. 
The level with which the signal is to be transmitted is set to the 
transmission level setting circuit 5 by the control circuit 4, and tho 
transmission level setting circuit 5 produces a reference signal 
corresponding to one of the transmission levels. 
The transmission level P detected by the level detector 3 and the reference 
signal R of the thus set transmission level are compared with each other 
by the comparator 6 to detect an error (P-R) of the transmission level P 
from the transmission level R. The error is averaged by the averaging 
circuit 9 while the polarity of the error (P-R) is discriminated by the 
discrimination circuit 7, and the average sample number of the averaging 
circuit 9 is controlled based on a result of the discrimination. Here, the 
two average sample numbers L1 and L2 are prepared in advance by the 
selection circuit 8, and one of the average sample numbers L1 and L2 is 
selectively determined based on a result of the discrimination of the 
discrimination circuit 7. The transmission power controller 1 is 
controlled with a result of the averaging of the error with the selected 
average sample number. Consequently, a transmission power control loop is 
formed from the elements 1, 2, 3, 6, 7, 8 and 9 described above. 
If the average sample number of the averaging circuit 9 is set to a 
comparatively high value, then the response speed is comparatively low, 
but the control system operates stably, but if the comparatively small 
average sample number is selected, then the comparatively high response 
speed is obtained and convergence to the preset level is reached 
comparatively quickly. 
If the detected transmission level P is lower than the preset transmission 
level R as seen from Table 1 below, that is, when P-R.ltoreq.0, the error 
is averaged with the comparatively large average sample number L1, but if 
the detected transmission level P is not higher than the preset 
transmission level R, that is, when P R&gt;0, the error is averaged with the 
comparatively small average sample number L2 (.gtoreq.L1). 
TABLE 1 
______________________________________ 
Control Logic in Average Sample Number Selection 
Condition Used average sample number 
______________________________________ 
Detection level P &lt; 
L1 
Preset transmission level R 
Detection level P .gtoreq. 
L2 
Preset transmission level R 
______________________________________ 
L1 .gtoreq. L2 
By the control in accordance with the logic, the transmission channel is 
changed over from the signalling channel 15 to the communication channel 
16 such that, when the transmission level is to be changed over from the 
maximum transmission level which is a standard level to the designated low 
power level, until after the transmission level is stabilized at the 
preset level to effect stabilized transmission, the transmission level 
remains higher than the preset level, and consequently, the error from the 
preset level is averaged with the comparatively large average sample 
number L1. The average sample number L1 is set so that synchronization 
maintenance on the reception side can be performed without any trouble 
with the response speed then. 
Then, when the transmission channel is changed over from the communication 
channel to the signalling channel and the transmission level is to be 
changed over from the low power level to the maximum transmission level 
which is a standard level, since the transmission level remains lower than 
the preset level until after it converges to the maximum transmission 
level, the error from the preset level is averaged with the comparatively 
low average sample number L2. The convergence to the preset transmission 
level is accelerated with the comparatively low average sample number L2 
so that the transmission power is controlled to allow transmission with a 
predetermined level so that call connection can be performed stably even 
in a communication condition which is most severe for call connection. 
The averaging circuit 9 described hereinabove need not be formed from an 
analog to digital converter, but may be formed from some other element 
having an equivalent function. For example, the averaging circuit 9 may be 
formed from a low-pass filter whose cutoff frequency can be controlled 
from the outside. In this instance, the output of the discrimination 
circuit 7 is inputted to the selection circuit 8, by which the cutoff 
frequency of the filter is controlled. 
In particular, transmission power control is performed by control of the 
selection circuit 8 such that, where P.quadrature.R, the cutoff frequency 
is set to a comparatively low value to make the response speed low, but 
where P&lt;R, the cutoff frequency is set to a comparatively high value to 
make the response speed high. By the transmission power control, the 
problem upon changing over from a control circuit to a communication 
channel can be prevented. 
While a preferred embodiment of the present invention has been described 
using specific terms, such description is for illustrative purpose only, 
and it is to be understood that changes and variations may be made without 
departing from the spirit and scope of the following claims.