Satellite packet terminal and method for transmitting packet data from the same

To carry out smooth data communication even when a radio communication channel with a satellite is in a fault state, it is judged whether the radio communication channel is in a fault state or not, and packet data transmitted from a data terminal are stored in a prebuffer and are not transmitted to the satellite when it is judged that the radio communication channel is in a fault state, and are transmitted from the prebuffer to the satellite sequentially when it is judged that the radio communication channel is recovered from the fault state.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a satellite packet terminal for the communication 
of packet data through a communication satellite and to a method for 
transmitting packet data from the same in satellite communication. 
2. Description of the Prior Art 
In mobile radio communication, the radio zone may be changed along with the 
movement of a mobile station. When the above communication means is a 
telephone such as a car telephone or ship telephone, even if the zone 
switching time is about 1 second, a hit or interruption in communication 
caused by zone switching does not present a big problem. However, in the 
case of data communication, missing data caused by a hit at the time of 
zone switching are fatal. To cope with this problem, in a conventional 
mobile radio communication system disclosed in Japanese Laid-open Patent 
Application No. Sho 59-212040, transmission data are temporarily stored in 
a buffer when the zone is changed and transmitted after the end of a hit. 
FIG. 4 shows the sequence of a signal flow between a mobile terminal and a 
network in the above conventional mobile radio communication system. When 
a zone switch signal is transmitted from a base station 1 (MBS1) to a 
mobile exchange (AMC) before the processing of zone switching, AMC gives a 
communication interruption signal 100 to the data processor (CPM) of the 
mobile terminal. After zone switching from the base station 1 (MBS1) to a 
base station 2 (MBS2) is completed, the above mobile terminal falls into a 
hit where it cannot communicate with the base stations (1, 2) until a 
communication start signal 101 is given from the AMC to the CPM of the 
above mobile terminal. Then, the above mobile terminal stores data 
generated from its built-in data terminal in its built-in buffer, takes 
out the stored data when it receives the communication start signal 101 
from the AMC and resumes data transmission. 
However, the zone switching system of the mobile communication system which 
has the above zone system and controls zone switching under the directions 
of base stations could not be used in satellite communication which has no 
zone system. For example, when there are buildings, tunnels and trees 
which interrupt communication between a satellite and a mobile station, 
communication between a mobile satellite packet terminal and a satellite 
base station becomes impossible. Even if the satellite packet terminal is 
not a mobile station but a fixed station, communication with a satellite 
base station may become impossible due to temporary jamming caused by an 
airplane or the like. That is, such communication trouble cannot be 
detected by the satellite base station and reported to the satellite 
packet terminal. Therefore, when satellite communication between the 
satellite packet terminal and the satellite base station becomes 
impossible temporarily, confirmation information on the transmission of 
the packet data to the satellite is not provided to the data terminal of 
the satellite packet terminal, and the satellite packet terminal instructs 
the above data terminal to stop the transmission of the subsequent packet 
data in accordance with a packet communication protocol (Receive Not 
Ready). Therefore, smooth data communication is impossible. 
It is an object of the present invention which has been made to solve the 
above problem to provide a satellite packet terminal which enables smooth 
data communication even when a radio communication channel with a 
satellite falls into a fault state and a method for transmitting packet 
data from the same. 
According to a first aspect of the present invention, there is provided a 
method for transmitting packet data from a satellite packet terminal, 
which comprises the steps of judging whether a radio communication channel 
is in a fault state or not based on the reception state of a reception 
signal received from a satellite through the radio communication channel, 
storing packet data to be transmitted to the satellite in storage means 
when it is judged that the radio communication channel is in a fault 
state, and transmitting the packet data stored in the storage means to the 
satellite sequentially when it is judged that the radio communication 
channel is recovered from the fault state. 
According to a second aspect of the present invention, there is provided a 
method for transmitting packet data from a satellite packet terminal, 
which comprises the steps of judging whether a radio communication channel 
will fall into a fault state based on the change rate of the reception 
state of a reception signal received from a satellite through the radio 
communication channel, storing packet data to be transmitted to the 
satellite in storage means when it is judged that the radio communication 
channel will fall into a fault state, and transmitting the packet data 
stored in the storage means to the satellite sequentially when it is 
judged that the radio communication channel is recovered from the fault 
state. 
According to a third aspect of the present invention, there is provided a 
satellite packet terminal which comprises a satellite circuit monitoring 
unit for measuring the reception level of a reception signal received from 
a satellite through a radio communication channel, a judging unit for 
judging whether the radio communication channel is in a fault state or not 
based on the measured reception level, and a prebuffer for storing packet 
data transmitted from a data terminal, wherein the packet data transmitted 
from the data terminal housed in the satellite packet terminal are stored 
in the prebuffer when it is judged that the radio communication channel is 
in a fault state and transmitted from the prebuffer to the satellite 
sequentially when it is judged that the radio communication channel is 
recovered from the fault state. 
According to a fourth aspect of the present invention, there is provided a 
satellite packet terminal, wherein the satellite circuit monitoring unit 
calculates the change rate of the reception level of the reception signal, 
the judging unit judges whether the radio communication channel will fall 
into a fault state based on the change rate of the reception level, and 
the packet data transmitted from the data terminal are stored in the 
prebuffer when it is judged that the radio communication channel will fall 
into a fault state and transmitted from the prebuffer to the satellite 
sequentially when it is judged that the radio communication channel is 
recovered from the fault state. 
According to a fifth aspect of the present invention, there is provided a 
satellite packet terminal which comprises a satellite circuit monitoring 
unit for detecting whether a reception signal received from a satellite 
through a radio communication channel is synchronized or not, a judging 
unit for judging whether the radio communication channel is in a fault 
state or not based on the synchronism of the reception signal, and a 
prebuffer for storing packet data transmitted from a data terminal, 
wherein the packet data transmitted from the data terminal housed in the 
satellite packet terminal are stored in the prebuffer when it is judged 
that the radio communication channel is in a fault state and transmitted 
from the prebuffer to the satellite sequentially when it is judged that 
the radio communication channel is recovered from the fault state. 
According to a sixth aspect of the present invention, there is provided a 
satellite packet terminal, wherein the satellite circuit monitoring unit 
calculates the frequency of the asynchronism of the reception signal, the 
judging unit judges whether the radio communication channel will fall into 
a fault state based on the frequency of the asynchronism of the reception 
signal, and the packet data transmitted from the data terminal are stored 
in the prebuffer when it is judged that the radio communication channel 
will fall into a fault state and transmitted from prebuffer to the 
satellite sequentially when it is judged that the radio communication 
channel is recovered from the fault state. 
According to a seventh aspect of the present invention, there is provided a 
satellite packet terminal which comprises a satellite circuit monitoring 
unit for measuring the C/N ratio of a reception signal received from a 
satellite through a radio communication channel, a judging unit for 
judging whether the radio communication channel is in a fault state or not 
based on the measured C/N ratio, and a prebuffer for storing packet data 
transmitted from a data terminal housed in the satellite packet terminal, 
wherein the packet data transmitted from the data terminal are stored in 
the prebuffer when it is judged that the radio communication channel is in 
a fault state and transmitted from the prebuffer to the satellite 
sequentially when it is judged that the radio communication channel is 
recovered from the fault state. 
According to an eighth aspect of the present invention, there is provided a 
satellite packet terminal, wherein the satellite circuit monitoring unit 
calculates the change rate of the C/N ratio of the reception signal, the 
judging unit judges whether the radio communication channel will fall into 
a fault state based on the change rate of the C/N ratio, and the packet 
data transmitted from the data terminal are stored in the prebuffer when 
it is judged that the radio communication channel will fall into a fault 
state and transmitted from the prebuffer to the satellite sequentially 
when it is judged that the radio communication channel is recovered from 
the fault state. 
The above and other objectives, features and advantages of the invention 
will become more apparent from the following description when taken in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the present invention will be described 
hereinunder with reference to the accompanying drawings. 
Embodiment 1 
FIG. 1 shows the configuration of a mobile satellite packet terminal and 
the configuration of a satellite communication network according to 
Embodiment 1 of the present invention. Reference numeral 1 denotes a data 
terminal for producing packet data to be transmitted to a satellite, 2 a 
mobile satellite packet terminal for the communication of the above packet 
data, 3 a communication satellite, 4 a satellite base station, 5 a packet 
data network, and 6 a data terminal of another party. Since the above data 
terminal 1 transmits packet data through the mobile satellite packet 
terminal 2, the data terminal 1 is called data terminal housed in the 
mobile satellite packet terminal 2. 
The mobile satellite packet terminal 2 comprises a terminal-side 
communication protocol terminating unit 21 for terminating a protocol for 
data communication with the data terminal 1, a network-side communication 
protocol terminating unit 22 for terminating a protocol for data 
communication with a network, a satellite circuit monitoring unit 23 for 
monitoring the state of a radio communication channel from the satellite, 
judging unit 24 for judging the state of the radio communication channel 
from the current satellite based on information collected by the satellite 
circuit monitoring unit 23, a prebuffer 25 for temporarily storing packet 
data from the data terminal 1 when there is a fault in the radio 
communication channel from the satellite, a switching unit 26 for 
selecting the storage of packet data processed by the terminal-side 
communication protocol terminating unit 21 in the prebuffer 25 or the 
direct transmission of the packet data to the network-side communication 
protocol terminating unit 22 in accordance with an instruction from the 
judging unit 24, and a prebuffer reading unit 27 for reading packet data 
when there are packet data stored in the prebuffer 25 after the recovery 
of the radio communication channel from the satellite and sending it to 
the network-side communication protocol terminating unit 22. 
A description is subsequently given of the case where the data terminal 1 
and the data terminal 6 communicate with each other through the mobile 
satellite packet terminal 2, the satellite 3, the satellite base station 4 
and the packet data network 5. The satellite circuit monitoring unit 23 of 
the mobile satellite packet terminal 2 measures the reception level of the 
radio communication channel from the satellite and gives information on 
the measured reception level to the judging unit 24. FIG. 2 is a flow 
chart for judging the circuit state by means of the above judging unit 24. 
The judging unit 24 acquires and stores the above information on the 
reception level (step S21), checks the state of the radio communication 
channel from the satellite based on the above information on the reception 
level and judges whether the radio communication channel is in a fault 
state (step S22). When the judging unit 24 judges that the radio 
communication channel from the satellite 3 is in the fault state, it 
instructs the switching unit 26 to store packet data processed by the 
terminal-side communication protocol terminating unit 21 in the prebuffer 
25 without giving it to the network-side communication protocol 
terminating unit 22 (step S23). At this point, the switching unit 26 gives 
a receive ready signal to the terminal-side communication protocol 
terminating unit 21 as it has confirmed the transmission of the packet 
data stored in the prebuffer 25. The terminal-side communication protocol 
terminating unit 21 which has received the signal transmits a receive 
ready packet (RR packet) to the data terminal 1 to urge the transmission 
of the next packet. Accordingly, the data terminal 1 can transmit the next 
packet to the mobile satellite packet terminal 2. 
Meanwhile, when the judging unit 24 judges that the radio communication 
channel from the satellite 3 is not in a fault state in step S22, that is, 
the radio communication channel is normal or recovers from the fault 
state, it judges whether data are stored in the prebuffer 25 (step S24). 
When data are not stored in the prebuffer 25, the judging unit 24 
instructs the switching unit 26 to deliver packet data processed by the 
terminal-side communication protocol terminating unit 21 to the 
network-side communication protocol terminating unit 22 and not to the 
prebuffer 25. When data are stored in the prebuffer 25, the judging unit 
24 instructs the prebuffer reading unit 27 to read packet data stored in 
the prebuffer 25. Thus, the prebuffer reading unit 27 reads the packet 
data from the prebuffer 25 (step S25) and delivers them to the 
network-side communication protocol terminating unit 22 (step S26). The 
network-side communication protocol terminating unit 22 transmits the 
packet data to the radio communication channel for the satellite 3 (step 
S27). The judging unit 24 further judges whether data are still stored in 
the prebuffer 25 (step S28). When the stored packet data still remain in 
the prebuffer 25, the routine returns to the above step S25 to transmit 
the packet data to the radio communication channel in accordance with the 
above-described procedure. The prebuffer reading unit 27 reads all the 
packet data stored in the prebuffer 25, informs the network-side 
communication protocol terminating unit 22 to clear the prebuffer 25 (step 
S29), reports the judging unit 24 of the clearance of the prebuffer 25 and 
notifies the switching unit 26 through the judging unit 24 that all the 
packet data stored in the prebuffer 25 have been transmitted through the 
satellite circuit. The informed switching unit 26 changes the transmission 
channel to deliver the packet data processed by the terminal-side 
communication protocol terminating unit 21 to the network-side 
communication protocol terminating unit 22 and not to the prebuffer 25. 
FIG. 3 is a flow chart for processing packet data from the data terminal 1. 
Packet data from the data terminal 1 are first received by the 
terminal-side communication protocol terminating unit 21 (step S31). The 
switching unit 26 judges whether packet data are stored in the prebuffer 
25 (step S32). When data are stored in the prebuffer 25, the received data 
are stored in the prebuffer 25 from the terminal-side communication 
protocol terminating unit 21 through the switching unit 26 (step S33). At 
this point, the terminal-side communication protocol terminating unit 21 
transmits a receive ready packet to the data terminal 1 in accordance with 
a protocol. On the other hand, when data are not stored in the prebuffer 
25, the transmission channel is changed by the switching unit 26, and the 
received data are transmitted to the network-side communication protocol 
terminating unit 22 from the terminal-side communication protocol 
terminating unit 21 through the switching unit 26 (step S35), processed by 
the network-side communication protocol terminating unit 22 and 
transmitted to the satellite circuit (step S36). 
Judgment on whether the radio communication channel is in a fault state or 
not based on the reception level is carried out as follows. The reported 
reception level is stored each time it is received, and it is judged that 
a fault occurs when the reported reception level falls below a preset 
reference reception level an "n" number of times continuously while it is 
judged that the circuit is recovered when the reported reception level 
exceeds the above reference reception level an "m" number of times 
continuously. The value of the above reference reception level and the 
numbers "n" and "m" are suitably set according to the mobile terminal 
system or the like. 
In this Embodiment 1, the judging unit 24 of the mobile satellite packet 
terminal 2 judges whether the radio communication channel is in a fault 
state or not based on the reception level of the radio communication 
channel from the satellite 2 measured by the satellite circuit monitoring 
unit 23. When it is judged that the radio communication channel is in the 
fault state, packet data transmitted from the data terminal 1 are stored 
in the prebuffer 25 without being transmitted to the satellite circuit. 
When it is judged that the radio communication channel is recovered from 
the fault state, the data stored in the prebuffer 25 are transmitted 
sequentially. Therefore, satellite communication can be carried out with 
certainty without missing packet data to be transmitted. Further, since 
the transmission of the next packet data is urged by giving a receive 
ready signal to the data terminal 1 from the mobile satellite packet 
terminal 2 when a fault in the radio communication channel occurs, the 
transmission of the packet data to the mobile satellite packet terminal 2 
from the data terminal 1 can be carried out smoothly. 
Embodiment 2 
In the above Embodiment 1, it is judged whether the radio communication 
channel is in a fault state or not based on the reception level of the 
radio communication channel from the satellite 3 measured by the satellite 
circuit monitoring unit 23. In Embodiment 2 of the present invention, the 
satellite circuit monitoring unit 23 calculates the reception level and 
the change rate of the reception level and informs the judging unit 24 of 
these. The judging unit 24 judges that the possibility that the radio 
communication channel will fall into a fault state is large when the 
change rate meets a predetermined condition (fault prediction judgment), 
stores packet data processed by the terminal-side communication protocol 
terminating unit 21 in the prebuffer 25 and gives a receive ready signal 
to the data terminal 1 from the mobile satellite packet terminal 2 to urge 
the transmission of the next packet data, thereby making it possible to 
prevent missing packet data to be transmitted and transmit packet data to 
the mobile satellite packet terminal 2 from the data terminal 2 smoothly. 
Recovery from a fault state is judged based on the reception level as in 
the above Embodiment 1, and when the radio communication channel is 
recovered from the fault state, the data stored in the prebuffer are 
transmitted sequentially. 
The above predetermined condition is suitably set according to a mobile 
terminal system or the like. For example, the condition is that the change 
rate of reception level is calculated by dividing a difference between the 
reception level of a p-th time and the reception level of a (p+1)-th time 
by the reference reception level and the absolute value of the negative 
change rate is larger than a predetermined value, or that the change rate 
is continued to be negative a predetermined number of times. 
Further, recovery from a fault state may be judged based on both of the 
reception level and the change rate of reception level. For example, it 
may be judged that the radio communication channel is recovered from a 
fault state when the reported reception level exceeds a predetermined 
reference reception level an "m" number of times continuously and the 
absolute value of the positive change rate of reception level exceeds a 
predetermined value. In this case, judgment on recovery becomes more 
accurate and missing packet data to be transmitted can be prevented 
without fail. 
Embodiment 3 
In the above Embodiment 1, it is judged whether the radio communication 
channel is in a fault state or not based on the reception level of the 
radio communication channel from the satellite 3 measured by the satellite 
circuit monitoring unit 23. It may be judged whether the radio 
communication channel is in a fault state or not based on the synchronism 
of the reception state of the radio communication channel detected by the 
satellite circuit monitoring unit 23. That is, in Embodiment 3 of the 
present invention, the judging unit 24 judges whether a UW (unique word: 
synchronism word) added to a reception signal can be detected by the 
satellite circuit monitoring unit 23 and judges that the radio 
communication channel is in a fault state when the UW cannot be detected. 
Alternatively, the satellite circuit monitoring unit 23 detects the above 
synchronism a predetermined number of times, calculates the frequency of 
reception asynchronism and informs the judging 24 of the frequency. The 
judging unit 24 may judge that the radio communication channel is in a 
fault state when the above frequency exceeds a predetermined value. 
Embodiment 4 
In the above Embodiment 1, it is judged whether the radio communication 
channel is in a fault state or not based on the reception level of the 
radio communication channel from the satellite 3 measured by the satellite 
circuit monitoring unit 23. It may be judged whether the radio 
communication channel is in a fault state or not based on the C/N ratio of 
the radio communication channel measured by the satellite circuit 
monitoring unit 23. That is, in Embodiment 4 of the present invention, the 
satellite circuit monitoring unit 23 counts how often the phase of a 
detected reception signal shifts from a preset phase within a 
predetermined time duration, calculates the C/N ratio of the reception 
signal from the counted value and informs the judging unit 24 of the 
calculated C/N ratio. The judging unit 24 judges that the radio 
communication channel is in a fault state when the C/N ratio falls below a 
predetermined value. 
In the above Embodiment 4, the change rate of the C/N ratio of the 
reception signal is calculated, it is judged that the possibility that the 
radio communication channel is in a fault state is large when the absolute 
value of the negative change rate exceeds a predetermined value, and 
packet data processed by the terminal-side communication protocol 
terminating unit 21 are stored in the prebuffer 25, thereby making it 
possible to prevent missing packet data to be transmitted without fail. 
In the above Embodiments 1, 3 and 4, it is judged whether the radio 
communication channel is in a fault state or not by detecting the 
reception level, the synchronism of the reception signal and the C/N 
ratio, respectively. When judgment on whether the radio communication 
channel is in a fault state is carried out by combining two or all of the 
above detection values, judgment accuracy can be further improved. 
Further, it may be judged whether the radio communication channel is in a 
fault state or whether it is recovered from the fault state by combining 
arbitrarily the detected reception level, the detected synchronism of the 
reception signal, the detected C/N ratio, the detected change rate of the 
reception level, the detected change rate of the frequency of the 
synchronism of the reception signal and the detected change rate of the 
C/N ratio. 
While satellite communication between mobile stations has been described in 
each of the above Embodiments, the present invention can be applied to 
communication between a fixed station and a mobile station or between 
fixed stations. 
As described above, the method for transmitting packet data from a 
satellite packet terminal according to the first aspect of the present 
invention comprises the steps of judging whether a radio communication 
channel is in a fault state or not based on the reception state of a 
reception signal received from a satellite through the radio communication 
channel, storing packet data to be transmitted to the satellite in storage 
means when it is judged that the radio communication channel is in a fault 
state, and transmitting the packet data stored in the storage means to the 
satellite sequentially when it is judged that the radio communication 
channel is recovered from the fault state. Therefore, there are no missing 
packet data to be transmitted and satellite communication can be effected 
without fail. 
The method for transmitting packet data from a satellite packet terminal 
according to the second aspect of the present invention comprises the 
steps of judging whether a radio communication channel will fall into a 
fault state based on the change rate of the reception state of a reception 
signal received from a satellite through the radio communication channel, 
storing packet data to be transmitted to the satellite in storage means 
when it is judged that the radio communication channel will fall into a 
fault state, and transmitting the packet data stored in the storage means 
to the satellite sequentially when it is judged that the radio 
communication channel is recovered from the fault state. Therefore, 
missing packet data to be transmitted can be prevented without fail. 
The satellite packet terminal according to the third aspect of the present 
invention comprises a satellite circuit monitoring unit for measuring the 
reception level of a reception signal received from a satellite through a 
radio communication channel, a judging unit for judging whether the radio 
communication channel is in a fault state or not based on the measured 
reception level, and a prebuffer for storing packet data transmitted from 
a data terminal, wherein the packet data transmitted from the data 
terminal are stored in the prebuffer when it is judged that the radio 
communication channel is in a fault state to secure the packet data to be 
transmitted to the satellite and transmitted from the prebuffer to the 
satellite sequentially when it is judged that the radio communication 
channel is recovered from the fault state. Therefore, all the packet data 
can be transmitted to the satellite without an omission. 
The satellite packet terminal according to the fourth aspect of the present 
invention calculates the change rate of the reception level of a reception 
signal, judges whether the radio communication channel will fall into a 
fault state based on the calculated change rate of the reception level, 
stores packet data transmitted from the data terminal in the prebuffer 
when it judges that the radio communication channel will fall into a fault 
state, and transmits the packet data stored in the prebuffer to the 
satellite sequentially when it judges that the radio communication channel 
is recovered from the fault state. Therefore, missing packet data to be 
transmitted can be prevented without fail. 
The satellite packet terminal according to the fifth aspect of the present 
invention comprises a satellite circuit monitoring unit for detecting 
whether a reception signal received from a satellite through a radio 
communication channel is synchronized or not, a judging unit for judging 
whether the radio communication channel is in a fault state or not based 
on the synchronism of the reception signal, and a prebuffer for storing 
packet data transmitted from a data terminal, wherein the packet data 
transmitted from the data terminal are stored in the prebuffer when it is 
judged that the radio communication channel is in a fault state to secure 
the packet data to be transmitted to the satellite and transmitted from 
the prebuffer to the satellite sequentially when it is judged that the 
radio communication channel is recovered from the fault state. Therefore, 
all the packet data can be transmitted without an omission. 
The satellite packet terminal according to the sixth aspect of the present 
invention calculates the frequency of the asynchronism of a reception 
signal, judges whether the radio communication channel will fall into a 
fault state based on the frequency of the asynchronism of the reception 
signal, stores packet data transmitted from the data terminal in the 
prebuffer when it judges that the radio communication channel will fall 
into a fault state, and transmits the packet data stored in the prebuffer 
to the satellite sequentially when it judges that the radio communication 
channel is recovered from the fault state. Therefore, missing packet data 
to be transmitted can be prevented without fail. 
The satellite packet terminal according to the seventh aspect of the 
present invention comprises a satellite circuit monitoring unit for 
measuring the C/N ratio of a reception signal received from a satellite 
through a radio communication channel, judging unit for judging whether 
the radio communication channel is in a fault state or not based on the 
measured C/N ratio, and a prebuffer for storing packet data transmitted 
from a data terminal housed in the satellite packet terminal, wherein the 
packet data transmitted from the data terminal are stored in the prebuffer 
when it is judged that the radio communication channel is in a fault state 
and transmitted from the prebuffer to the satellite sequentially when it 
is judged that the radio communication channel is recovered from the fault 
state. Therefore, all the packet data can be transmitted without an 
omission. 
The satellite packet terminal according to the eighth aspect of the present 
invention calculates the change rate of the C/N ratio of a reception 
signal, judges whether the radio communication channel will fall into a 
fault state based on the change rate of the C/N ratio, stores packet data 
transmitted from the data terminal in the prebuffer when it judges that 
the radio communication channel will fall into a fault state and transmits 
the packet data stored in the prebuffer to the satellite sequentially when 
it judges that the radio communication channel is recovered from the fault 
state. Therefore, missing packet data to be transmitted can be prevented 
without fail.