Energy dispersal method for TDMA carrier

In a radio communication system using time division multiple access system (TDMA) for access control method, modulation signal sent by a slave station is frequency-dispersed during a TDMA burst period by using a chirp signal which frequency changes continuously on time axis so that signal power density on communication link is decreased and interference signal affected by the slave station to another radio communication system is reduced. According to the present invention, modulation signal transmitted by each slave station is multiplied to a chirp signal which frequency changes continuously on time axis during a TDMA burst period so that carrier frequency which carries the modulation signal changes continuously on time axis to disperse on frequency axis to a predetermined bandwidth thereby decreasing signal power density of transmit signal from the slave station. In a master station, frequency dispersed receive signal from the slave station is multiplied to another chirp signal which has opposite characteristics to that of the slave station during TDMA burst period to obtain reproduced signal. Thus, interference signal by the slave station to another radio communication system is decreased, in a return link from the slave station to said master station.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a frequency dispersion communication 
system for a modulation signal, and is used in all radio communication 
systems including a digital station radio communication system, a digital 
mobile radio communication system, a digital satellite communication 
system, and a digital mobile satellite communication system. 
2. Prior Art 
One of the current commercialized digital satellite communication systems 
using a small earth station is a VSAT (Very Small Aperture Terminal) 
system. Currently, a VSAT system is used in a private network including a 
circuit exchange system and a packet exchange system, but no direct access 
is carried out from a VSAT system to a public network. That corresponds to 
the use of a satellite link instead of a leased line which is used in a 
land communication network. 
The service by a VSAT system is classified into (1) a bidirectional voice 
service and a middle rate data communication service, (2) a bidirectional 
non-voice communication service including a packet communication service, 
and (3) a one way image communication service. The conventional network 
structures of typical VSAT systems are classified into the following four 
services. 
(1) Bidirectional Communication Service Using SCPC/FDMA 
The main communication is voice, FAX, and data communication (around 64 
kbit/s), and a link control procedure is a pre-assignment system, or a 
demand assignment system. As for a network structure, a mesh structure or 
a star network structure is possible. It is practically served in a 
private network between companies for a local/international link. It is 
essentially a leased network service having link exchange function for 
point-to-point communication. 
(2) Bidirectional Service Using TDM/TDMA 
The main communication is packet type data communication which is a 
non-voice communication, and the link control procedure is reservation 
TDMA, or slotted aloha. It has been used in POS data collection and stock 
management, credit card inquiry, satellite LAN, ticket reservation, ATM, 
file transfer between computers. It is essentially a private network 
having packet exchange function for point-to-point service. The 
communication is mainly data communication which is non-voice 
communication. Almost all the VSAT networks in USA are in this category. 
(3) One-way Communication Service 
The communication has the tendency of broadcast communication such as image 
etc., and generally, it is a one-way communication channel from a central 
office to a VSAT. Some practical examples are intracompany message, 
advertisement of products, lecture, seminar, document, and 
distribution/broadcast service of voice and/or image. An SNG service which 
sends image information from a VSAT to a central earth station through 
one-way communication channel is in this category. 
(4) Integrated Communication Service 
It is an integrated service based upon said three services (1) through (3), 
and integrates them. A network in this service is possible to cover low 
rate data communication, voice, FAX, high rate image, and data 
communication. 
As described above, various kind of services using a VSAT are proposed, and 
some of them are commercially available, however, it is merely the use as 
a private network, but a satellite communication for the personal users 
has not been spreaded. One reason why a VSAT has not been used widely is 
that it is difficult to transport, trouble in usage, and mounting, because 
of a large antenna with diameter of 1.2 m in a terminal station. Further, 
cost for communication in a VSAT can not be reduced as compared with that 
of a conventional land circuit. 
In conventional consideration of a VSAT service considering the above 
problems, it has been considered to be an auxiliary service in specific 
area like a far or remote place, where no conventional system is served. 
Therefore, available service has been restricted to limited service like a 
private network, and so, no large amount of users are expected, no 
efficient frequency utilization by demand assignment is expected, and as a 
result, it falls into a vicious cycle of high communication cost for 
communication. 
On the other hand, a broadcast satellite (BS) antenna of approximate 40 cm 
diameter has been popular with consumers because of easy mounting, 
although it is only for reception. Therefore, an USAT which has a smaller 
antenna than a VSAT has been proposed. An object of an USAT is to provide 
a small size terminal in order to improve easy mounting, transportation, 
and usage. A terminal with an antenna in 40-50 cm of diameter in an USAT 
has a feature of simple set up like a conventional BS antenna, it may be 
used in a small building like a super-market, a bank, a restaurant etc., 
and it may be used by consumers in a final stage. 
In spite of the small size of the terminal equipment, communication 
capacity does not decrease because of the latest improvement of satellite 
power. The manner of service in an USAT is basically the same as those in 
said VSAT. In other words, said three kinds of services and the integrated 
service in a VSAT are available in an USAT. 
Further, the specific use of an USAT is as a portable terminal. The use of 
a portable terminal differs from that of a VSAT which has a semifixed 
terminal, but the service itself provided by an USAT is the same as that 
of a VSAT. A direct access from an USAT terminal to a public network, or 
from a public network to an USAT is not used, but a private network is 
basically used. This differs from a conventional Inmalsat system which has 
a portable terminal coupled with a public network. Although the Inmalsat 
system provides only voice service and low rate data communication because 
of restriction of the allocation of the frequency band, an USAT system 
which uses a fixed communication satellite having a wide frequency band 
may provide high rate data communication, and image communication etc. 
which a conventional mobile satellite communication system can not 
provide. 
In realizing an USAT system, a problem is interference with an adjacent 
satellite system because of a small size antenna in an USAT. No design 
specification in Ku band is established for an antenna less than 1 m (50 
wavelengths) of an aperture diameter. Therefore, when an USAT system is 
realized, sidelobe characteristics required for an USAT antenna based upon 
allowable interference signal by an adjacent satellite must be clearly 
defined, and an antenna which satisfies said characteristics must be used. 
However, even when an antenna which solves an interference problem with an 
adjacent satellite is developed and is used, an interference signal to an 
adjacent satellite would be a big problem because of satellite tracking 
error due to an unpredictable accident in an USAT system which is used by 
personal users. No solution for that problem has been proposed, but an 
antenna which satisfies sidelobe characteristics even considering the 
affect by tracking error must be used. Therefore, an USAT antenna with a 
diameter of 40-60 cm which is considerably smaller than that of a VSAT 
antenna has been impossible in practice. 
SUMMARY OF THE INVENTION 
An object of the present invention is to solve an interference problem to 
an adjacent satellite in an USAT system, and to provide a frequency 
dispersion communication system which makes it possible to use an antenna 
having a diameter of 40-60 cm in an USAT system. 
In order to achieve the object, according to the present modulation signal 
frequency dispersed communication system, a modulation signal sent by each 
USAT station in a group of USAT stations multiplied by a chirp signal 
changes its frequency on a time axis so that a carrier frequency of the 
modulation signal also changes on a time axis to disperse to a 
predetermined bandwidth on a frequency axis. Thus signal power density of 
a transmit signal of the USAT station decreases. A center earth station 
(HUB station) multiplies the frequency dispersed receive signal from the 
USAT station with another chirp signal which has opposite characteristics 
to that of the USAT station during a TDMA burst period, to provide a 
reproduced signal. Thus, in a return link from the USAT station to the HUB 
station, the interference signal, by the USAT station, to another radio 
communication system is decreased. 
A chirp signal changes its frequency for instance continuously on a time 
axis. 
Because of frequency dispersion of a modulation signal from an USAT station 
by using a chirp signal which changes its frequency on a time axis, signal 
power density in a satellite circuit is decreased, and transmit power 
density out of bandwidth of an USAT station decreases. Thus, an 
interference problem to adjacent satellites, due to a pointing error of an 
antenna, is solved. Then, the size of an antenna used in a prior VSAT 
system having diameter 1.2 m is considerably decreased and a small size 
USAT antenna of diameter 40-60 cm is possible, thereby, it may serve many 
uses including personal users. Further, since a TDMA access control 
system, used in a conventional VSAT system, is used in the present 
invention as it is, the cost for construction of the present invention may 
be low.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an embodiment of a circuit diagram of a modulation signal 
frequency dispersed communication system which uses a chirp signal 
according to the present invention. In the figure, an USAT station 1 and a 
HUB station 2 communicates through a satellite 2. The operation is 
described for a return link from the USAT station 1 to the HUB station 2. 
In the USAT station 1, a phase modulation signal 11, provided by a phase 
modulator 10, is multiplied with a chirp signal 16 provided by a chirp 
signal generator 13, so that the phase modulation signal is converted to a 
frequency dispersed signal 15. 
As shown in FIG. 2, a chirp signal changes its frequency from f.sub.1 to 
f.sub.2 on a time axis with a center frequency f.sub.0 of IF frequency of 
a communication channel during a burst period of a TDMA signal. A 
frequency dispersed signal 15 which is product of a chirp signal 16 and a 
phase modulation signal 11 is frequency-converted to a radio frequency 
signal 18 by an up converter 17, and is transmitted to a communication 
satellite 2 through an antenna 19. It is supposed that a chirp signal 
generator 13 and a phase modulator 10 are subject to burst timing 
synchronization of a TDMA signal, so that a timing of a product of a chirp 
signal 16 and a phase modulation signal 11, and a time length of a product 
for a chirp signal are finely controlled by using a burst timing 
information 12 which is provided by the phase modulator 10. 
In a HUB station 3, a frequency dispersed signal through a communication 
satellite 2 is received through an antenna 20, and is frequency-converted 
to a IF-band frequency dispersed signal by a down converter 22, then, the 
IF-band frequency dispersed signal 23 is multiplied with a chirp signal 28 
which is provided by a chirp signal generator 29 and has opposite 
characteristics to that of the chirp signal 16 in the USAT station 1 on a 
frequency axis. Thus, a phase modulation signal 25 with a narrow bandwidth 
is obtained. The phase modulation signal 25, which is inversely frequency 
dispersed by using the chirp signal 28, is demodulated by a demodulator 26 
so that a demodulated information signal is obtained. A burst timing 
signal 29, which is detected by using a burst signal series for burst 
synchronization, controls the chirp signal generator 27, so that the TDMA 
burst timing synchronization of the chirp signal generator 27 and the 
frequency dispersed phase modulation signal 23 is finely established. 
The above operation is shown by a spectrum on a frequency axis as follows. 
The phase modulation signal spectrum 5, which is generated by the phase 
modulator 10 in the USAT station 1, has a predetermined bandwidth and a 
predetermined power density spectrum around the center frequency f.sub.0 
(4). When the phase modulation signal 11 is multiplied with the chirp 
signal 16, the phase modulation signal spectrum 5, which has a narrow 
band, is converted to a frequency dispersed spectrum 8 in which the 
frequency spectrum spreads from f.sub.1 (6) to f.sub.2 (7) around the 
center frequency f.sub.0 (4) of the IF signal of a channel. The frequency 
dispersed signal 8 is converted to a narrow band phase modulation signal 9 
by a anti-frequency dispersed operation through multiplication with the 
chirp signal 28 which has opposite characteristics to that of the chirp 
signal 16 on the frequency axis, in the HUB station 3. 
FIG. 2 shows an embodiment of the operation of a frequency dispersion 
process in FIG. 1 on a time axis. The symbols (a)-(c) in FIG. 2 correspond 
to the phase modulation signal 11, the chirp signal 16 and the frequency 
dispersed signal 15, respectively, in the USAT station in FIG. 1, and also 
correspond to the frequency dispersed received signal 23, the chirp signal 
28 and the phase modulation received signal 25, respectively, in the HUB 
station in FIG. 1. 
In FIG. 2(a), the phase modulation signal 41 appears during the whole burst 
period T.sub.b, and a guard time T.sub.g (42), for a timing control error, 
is inserted between adjacent burst signals. The length of a burst period 
of a TDMA signal is defined as T.sub.b +T.sub.g. The phase modulation 
signal 41 is multiplied with the chirp signal 46 in FIG. 2(b), which has 
its frequency change continuously from f.sub.1 (44) to f.sub.2 (45) with a 
center frequency f.sub.0 (43), then, the product of the multiplication is 
a frequency dispersed signal 49 in FIG. 2(c). It should be noted that the 
multiplication of the phase modulation signal 41 with the chirp signal 46 
is carried out only during the signal transmission period T.sub.b (40), 
and the frequency f.sub.2 (47) of the chirp signal is reset to f.sub.1 
(44) during the guard time (47). 
In FIG. 2(d), the frequency dispersed signal 50 received in the HUB station 
is divided into a TDMA burst signal with the time length T.sub.g (48) 
based upon a TDMA burst timing information (not shown). Since a guard time 
T.sub.g (42) is set between each burst signals, the frequency dispersed 
signal (50) having a time length T.sub.b (40) is always within the burst 
length T.sub.s (48) of a TDMA signal. 
The frequency dispersed signal 50 received in the HUB station is, then, 
multiplied with the chirp signal 54 of FIG. 2(e) which has opposite 
characteristics on a frequency axis to that of FIG. 2(b), and the phase 
modulation signal in FIG. 2(f) is obtained. The instantaneous frequency of 
the chirp signal 54 changes continuously on a time axis from f.sub.2 '(52) 
to f.sub.1 '(53) during a TDMA burst period T.sub.s (48) from top to burst 
end. Although the chirp signal 46 in FIG. 2(b) changes continuously on a 
time axis between instantaneous frequencies f.sub.1 (44)-f.sub.2 (45), 
since the HUB station takes timing control so that a burst timing control 
error is absorbed in the TDMA guard time T.sub.g (55), the frequency band 
f.sub.1 (44)-f.sub.2 (45) is always included in the frequency band f.sub.1 
'(52)-f.sub.2 '(53). 
If a TDMA burst timing control has an error, a phase modulation signal 56 
after anti-dispersion has some frequency offset, however, the affection by 
the offset may be simply removed by using a conventional frequency control 
circuit (AFC). 
Although the above embodiment is directed to a digital communication system 
for a fixed type station using a satellite, the present invention is 
applicable to a mobile communication system using a satellite, merely by 
replacing a USAT station with a mobile station, and that system is obvious 
to those skilled in the art based upon the embodiment described. 
The present invention is applicable to a land mobile communication system 
by substituting a mobile station in a land mobile communication system 
with the USAT station, and a base station and a network control station in 
the land mobile communication system with the HUB station. Further, the 
present invention is applicable to any kind of radio communication system 
including a land communication system using a fixed type radio terminal. 
Although a phase modulation technique which is usually used a in satellite 
communication system is described in the embodiment, the present invention 
is applicable to any modulation and/or demodulation system. 
Further, although a frequency of a chirp signal which changes continuously 
on a time axis in each TDMA burst period, is linearly changed in the 
embodiment, the frequency change ratio is not restricted to a linear 
function, but a wavelet function, a rectangular function, and other 
functions are possible. 
The present invention has at least the following effects. 
(1) As signal power density in a return link from a slave station to a 
master station is decreased, an interference signal power to another radio 
communication system is reduced. 
(2) Since signal power density in a return link from a slave station to a 
master station is decreased, and since an interference signal power to 
another radio communication system is reduced, it is possible to use a 
small simple antenna which has a broad beam pattern and is simple in 
design, and therefore, cost for an antenna system in a slave station is 
reduced. 
(3) A network control system in a conventional radio communication system 
is used in the present invention. So, the cost for constructing a system 
of the present invention is low. 
(4) As for a spurious impulse signal on a frequency axis generated in a 
transmitter in a slave station, the spectrum of the spurious signal is 
dispersed by anti-dispersion in a receiver in a master station. Therefore, 
the affect on a desired modulation signal by the spurious signal is 
reduced. 
From the foregoing it has now been apparent that a new and improved energy 
dispersal method for a TDMA carrier has been found. It should be 
appreciated of course that the embodiments disclosed are merely 
illustrative and are not intended to limit the scope of the invention. 
Reference should be made to the appended claims, therefore, for indicating 
the scope of the invention.