Optical communications – Hybrid communication system
Reexamination Certificate
2000-05-12
2004-10-19
Chan, Jason (Department: 2633)
Optical communications
Hybrid communication system
C379S056200
Reexamination Certificate
active
06807374
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to collection and delivery of radio frequency signals using an optical transmission system, and particularly to an optical transmission system for mobile communication for transmitting radio signals to a mobile communication unit present in a dead zone at which a radio wave does not easily arrive, such as the inside of a tunnel, inside of a underground street, or inside of a building.
2. Description of the Related Art
A mobile communication system, such as an automobile telephone or portable telephone, makes it possible for a mobile communication unit to perform communication through a public network by radio-connecting to a base station connected to a host network (in the following description, the public network is used as an example) with the mobile communication unit.
As for this type of mobile communication system, preferable radio communication with a base station is realized when a mobile communication unit is present at the outdoors at which a radio wave easily arrives. However, when the mobile communication unit is present in a dead zone at which a radio wave does not easily arrive, such as the inside of an underground street, it is impossible to perform radio communication with the base station.
To solve the above problem, the following improvements have been made: an improvement of setting an antenna to a place where radio communication can be preferably performed with a base station and moreover setting an antenna in a dead zone, cable-connecting a relay amplifier between the both antennas to realize communication between a mobile communication unit present in a dead zone and the base station and an improvement of cable-connecting a radio modem directly connected with a public network with an antenna set in a dead zone to realize communication between a mobile communication unit present in the dead zone and the public network.
Moreover, because it is necessary to efficiently transmit a radio frequency signal in a band of 800 MHz or 1.5 GHz used for portable telephones, a lightweight, flexible, and small-loss optical fiber cable is used for connection between a relay amplifier or radio modem and an antenna instead of a metal cable.
In this case, systems for performing transmission by using the optical fiber cable can be classified into the star type and the multicipital type and the both types of systems are described in, for example, Tsubosaka et al. “Optical Transmitter for PDC” (Matsushita Technical Journal Vol. 44, No. 6 (December, 1998)).
Conventional optical transmission systems using a radio modem are described below by referring to
FIGS. 7 and 8
.
FIG. 7
shows a star-type optical transmission system for mobile communication. This system is provided with a radio modem
3
connected with a public network
1
through an electrical signal transmission cable
2
constituted of a metal cable, a central fixed station
5
connected with the radio modem
3
through an electrical signal transmission cable
4
, and a plurality of terminal fixed stations
7
connected with the central fixed station
5
through an optical transmission cable constituted of a pair of downward optical fiber cables
6
a
and a pair of upward optical fiber cables
6
b.
In the case of this optical transmission system for mobile communication, a mobile communication unit
8
present in a dead zone is connected to the public network
1
through the radio modem
3
by making the terminal fixed station
7
set in the dead zone radio communicate with the mobile communication unit
8
and connecting the terminal fixed station
7
and the central fixed station
5
by an optical transmission cable.
That is, in the case of downward (direction from the network
1
to the mobile communication unit
8
) communication in the optical transmission system for mobile communication, a circuit frequency electrical signal sent from the public network
1
is modulated into a radio frequency electrical signal by a modulating-transmitting section
31
of the radio modem
3
, the radio frequency electrical signal is converted into an analog optical signal by an electrical-optical converter (E/O)
51
of the central fixed station
5
, and moreover the analog optical signal is branched into a plurality of optical signals by a star coupler
52
and transmitted to each terminal fixed station
7
through each downward optical fiber cable
6
a
. Then, the terminal fixed station
7
reconverts the received optical signal to the original radio frequency electrical signal by an optical-electrical converter (O/E)
71
, amplifies the radio frequency electrical signal by an amplifier
72
according to necessity, and radio-transmits the amplified signal to the mobile communication unit
8
from an antenna
73
.
In the case of upward (direction from the mobile communication unit
8
to the network
1
) communication in the optical transmission system for mobile communication, a radio frequency electrical signal radio-transmitted from the mobile communication unit
8
is received by an antenna
73
of the terminal fixed station
7
, the received radio frequency electrical signal is amplified by an amplifier
74
according to necessity, the radio frequency electrical signal is converted into an analog optical signal by an electrical-optical converter (E/O)
75
, and the optical signal is transmitted to the central fixed station
5
through the upward optical fiber cable
6
b
. Symbol
76
in
FIG. 7
denotes an antenna-sharing unit for using the antenna
73
as a transmitting antenna and a receiving antenna. Moreover, the central fixed station
5
reconverts optical signals received from the upward optical fiber cables
6
b
into the original radio frequency electrical signals by optical-electrical converters (O/E)
53
, couples these radio frequency electrical signals by a coupler
54
, and transmits a synthesized signal to the radio modem
3
. The radio modem
3
demodulates the radio frequency electrical signals into electrical signals having a circuit frequency by a receiving-demodulating section
32
and transmits them to the public network
1
.
FIG. 8
shows a multicipital optical transmission system for mobile communication. The system is provided with a radio modem
3
connected with a public network
1
through an electrical signal transmission cable
2
constituted of a metal cable, a central fixed station
5
connected with the radio modem
3
through an electrical signal transmission cable
4
, and a plurality of terminal fixed stations
7
connected to an optical transmission cable constituted of a downward optical fiber cable
6
a
and an upward optical fiber cable
6
b
and connected with the central fixed station
5
.
Also in the case of this optical transmission system for mobile communication, a mobile communication unit
8
present in a dead zone is connected to the public network
1
through the radio modem
3
by making the terminal fixed stations
7
set in the dead zone radio communicate with the mobile communication unit
8
and connecting the terminal fixed stations
7
with the central fixed station
5
by an optical transmission cable.
In the case of downward communication in this multicipital optical transmission system for mobile communication, an analog optical signal modulated by a modulating-transmitting section
31
of the radio modem
3
and converted into the analog optical signal by an electrical-optical converter (E/O)
51
of the central fixed station
5
is transmitted to each terminal fixed station
7
through the single downward optical fiber cable
6
a
without being branched. That is, each terminal fixed station
7
takes out some of optical signals from the optical fiber cable
6
a
by an optical directional coupler
77
, reconverts the optical signals into the original radio frequency electrical signals by an optical-electrical converter (O/E)
71
, and amplifies the radio frequency electrical signals by an amplifier
72
according to necessity, and radio-transmits the amplified signals to the mobile c
Imajo Yoshihiro
Nara Masaru
Chan Jason
Kokusai Electric Co. Ltd.
Leung Christina Y
Wenderoth , Lind & Ponack, L.L.P.
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