Optical fibre communications system

An optical fibre communications system includes a head end connected to n customers by an optical fibre network. The head end has a transmitter unit and a receiver unit, and each of the customers has an optical network unit with an optical receiver and a transmitter unit. The head end transmitter and receiver units are connected respectively to the receivers and the transmitter units of the customer optical network units by at least one passive optical network. The head end transmitter unit includes an optical transmitter driven by a sub-carrier multiplexer, the sub-carrier multiplexer having a plurality of input sub-carriers at different frequencies. One of the sub-carriers carries interactive signals provided by a further optical transmitter unit. The remaining sub-carriers carry broadband service signals. Each customer transmitter unit includes an optical transmitter and apparatus for multiplexing interactive signals and control signals to drive the optical transmitter. The head end includes a system control unit for controlling the transmission of broadband services by the head end transmitter unit in dependence upon the control signals received by the head end receiver unit from the customer transmitter units.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an optical fibre communications system, and in 
particular to an optical fibre communications system capable of carrying 
both broadband signals and interactive signals such as telephony and ISDN. 
2. Related Art 
In optical transmission systems, the radiation used is not necessarily in 
the visible region of the electromagnetic spectrum, and so the word 
"optical" and "light" when used in this specification are not to be 
interpreted as implying any limitation to the visible spectrum. For 
example, the wavelengths preferred for transmission through silica optical 
fibres are in the infra red region of the spectrum, because the low loss 
minima of silica fibres occur at about 1.3 and 1.5 microns. 
Optical transmission systems may be utilised to distribute both interactive 
services such as telephony and ISDN, and broadband services such as video 
channels, video telephony and information services such as picture 
videotext. In general, the primary service, at least as presently measured 
in terms of subscriber lines, is telephony. Increasingly, however, there 
is a perceived need for optical transmission systems to be able to carry 
both interactive services and broadband services. 
Various techniques are available for separating different services for 
transmission over the same lines, for example the transmitted signals may 
be time, wavelength or sub-carrier frequency multiplexed. Wavelength 
division multiplexing (WDM), with different services on different 
wavelengths, would require additional optical transmitters and receivers 
to be installed wherever an expansion of services and additional channels 
is required. 
Conventional broadband service provision, for example that provided by the 
cable TV companies, uses amplitude modulated (AM) transmission. 
Unfortunately, AM transmission is not suitable for passive optical 
networks (PONs), due to signal-to-noise-ratio limitations and 
intermodulation distortion imposed by optical transmitters. Although 
lasers are being developed which offer the prospect of meeting the 
required performance in these two areas, the optical budget achievable 
will still be very limited, and optical splitting of the signal will, 
therefore, be minimal. 
SUMMARY OF THE INVENTION 
The present invention provides an optical fibre communications system 
comprising a head end connected to n customers by an optical fibre 
network, the head end comprising a transmitter unit and a receiver unit, 
and each of the customers has an optical network unit comprising an 
optical receiver and a transmitter unit, the head end transmitter and 
receiver units being connected respectively to the receivers and the 
transmitter units of the customer optical network units by at least one 
passive optical network, said at least one passive optical network 
constituting the optical fibre network, wherein the head end transmitter 
unit is constituted by an optical transmitter driven by a sub-carrier 
multiplexer, the sub-carrier multiplexer having a plurality of input 
sub-carriers at different frequencies, one of said sub-carriers carrying 
interactive signals provided by a further optical transmitter unit, and 
the remaining sub-carriers carrying broadband service signals, wherein 
each customer transmitter unit includes an optical transmitter and means 
for multiplexing interactive signals and control signals to drive said 
optical transmitter, and wherein the head end includes a system control 
unit for controlling the transmission of broadband services by the head 
end transmitter unit in dependence upon the control signals received by 
the head end receiver unit from the customer transmitter units. 
Advantageously, the head end optical transmitter is arranged to operate at 
a first predetermined wavelength, and each of the customer transmitters is 
arranged to operate at a second predetermined wavelength. Preferably, the 
first predetermined wavelength lies in the range of from 1500 nm to 1650 
nm, and the second predetermined wavelength lies in the range from 1260 nm 
to 1360 nm. 
In a preferred embodiment, said at least one passive optical network is 
constituted by separate first and second simplex passive optical networks, 
each having an n-way split, the first simplex passive optical network 
connecting the head end transmitter unit to the receivers of the customer 
optical network units, and the second simplex passive optical network 
connecting the head end receiver unit to the transmitter units of the 
customer optical network units. Alternatively, said at least one passive 
optical network may be constituted by a duplex passive optical network 
having an n-way split. In this case, the head end transmitter unit and the 
head end receiver unit are connected to the duplex passive optical network 
via a wave division multiplexer, and the receiver and transmitter unit of 
each customer are connected to the duplex passive optical network via a 
respective wave division multiplexer. 
Preferably, the system further comprises a video server for supplying video 
film signal information to the sub-carriers carrying broadband service 
signals. Conveniently, a respective receiver/tuner unit is associated with 
the receiver of each of the customer optical network units, the 
receiver/tuner units being arranged to convert received video film signal 
information into a form suitable for reception by a television apparatus. 
Advantageously, the interactive signals are digitally phase modulated onto 
said one sub-carrier. Preferably, the broadband service signals are 
frequency modulated onto said remaining sub-carriers. Alternatively, the 
broadband service signals are digitally phase modulated onto said 
remaining sub-carriers. In either case, QPSK may be used for digitally 
phase modulating signals onto said sub-carrier(s).

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
Referring to the drawings, FIG. 1 shows a TPON/BPON optical fibre 
communications system having a head end station including a BPON 
transmitter 1 and a TPON receiver 2. The transmitter 1 is connected to 256 
customer premises (only one of which is shown) via a simplex PON indicated 
generally by the reference numeral 3. Similarly, the TPON receiver 2 is 
connected to the 256 customer premises by a simplex PON indicated 
generally by the reference numeral 4. The BPON transmitter 1 is basically 
a laser provided with control/monitoring circuitry. If necessary, an 
optical amplifier (such as a fibre amplifier) will be provided downstream 
of this laser. A suitable BPON transmitter is that described in the 
article "Broadband systems on passive optical networks" (British Telecom 
Technology Journal, vol 7, no. 2, pages 115-122, April 1989). The TPON 
receiver 2 is a standard TPON optical receiver, for example of the type 
described in the article "The provision of telephony over passive optical 
networks" (British Telecom Technology Journal, vol 7, no. 2, pages 
100-114, April 1989). 
The BPON transmitter 1 is driven by an FM sub-carrier multiplexer 5 which 
has 32 input sub-carriers each of up to 40 Mb/s capacity. This technique 
of sub-carrier multiplexing enables transmission of a multiplex of the 32 
sub-carriers on an optical wavelength, here chosen to be 1550 nm. The 
sub-carriers have frequencies ranging from 950 MHz up to 2 GHz, with the 
carriers being separated by 27 MHz. Each of 31 of the carriers can be 
either frequency modulated or digitally phase modulated. When frequency 
modulated, only a single analogue video channel is conveyed on each of the 
31 carriers. Each carrier can also be digitally phase modulated using 
quadrature phase shift keying (QPSK) of up to a bit rate of 40 Mb/s, and 
so, as each video channel can be compressed to about 2 Mb/s, each of the 
31 carriers can support between 16 and 18 compressed video channels 
whereby the complete multiplex can support 558 channels. The remaining 
sub-carrier carries signals from a TPON head end 6 which provides 
interactive services such as telephony, ISDN, fax etc. The TPON head end 6 
may be a TPON Bit Transport System (BTS) of the type described in the 
above-mentioned article "The provision of telephony over passive optical 
network". 
The TPON head end 6 is connected to the main telephony network as indicated 
by the arrow 7. The entire head end constituted by the TPON transmitter 1, 
the TPON receiver 2, the sub-carrier multiplexer 5 and the TPON head end 6 
are controlled by a control/network management system indicated generally 
be the reference numeral 8. 
Each of the customer premises includes an optical network unit (ONU) 9 
which comprises a BPON receiver 10 and a TPON sub-system unit 11. The 
video channel information output by the receiver 10 is fed to a television 
12 via a control box 13 (similar to the set-top receiver used to tune 
satellite TV signals). The BPON receiver 10 is an avalanche photodiode 
(APD) of the type described in the above-mentioned article "Broadband 
systems on passive optical networks". The TPON sub-system unit 11 is a 
standard BTS network termination unit of the type described in the 
above-mentioned article "The provision of telephony over passive optical 
networks". This unit 11 includes an optical transmitter (such as a 
Fabry-Perot laser) for transmitting video request signals, telephony and 
other interactive service signals at a wavelength of 1300 nm using a time 
division multiple access (TDMA) transmission technique. The unit 11 also 
includes an optical receiver and an RF amplifier circuit for driving the 
control box 13. 
The TPON sub-system unit 11 includes a demodulator 14 for providing TDM 
TPON signals to the TPON sub-system unit 11. The TDM signals are then 
demultiplexed to drive customer premises equipment (CPE) such as a 
telephone 15. Outgoing (upstream) telephony from the telephone 15 is 
multiplexed by the TPON sub-system unit 11 for TDMA transmission via the 
PON 4 to the TPON receiver 2 at the head end. Upstream video request 
signalling from the customer is passed from the control box 13 to the TPON 
sub-system unit 11, and then back to the head end via the PON 4 and the 
TPON receiver 2. The control box 13 can be controlled via an infra red 
remote control unit 16. 
In use, the head end broadcasts all the information on all 32 sub-carriers 
to all of the associated 256 customers via the PON 3. As mentioned above, 
31 of the sub-carriers carry video channels, telephony and other 
interactive services being provided on the remaining sub-carrier dedicated 
for such services. 
The video channels carried by the remaining 31 sub-carriers are provided by 
a video server 17. The arrangement is such that the network can transmit 
up to 560 video channels simultaneously (assuming each video channel is 
compressed to 2 Mb/s, and each sub-carrier carries 18 video channels). If 
a given subscriber wants to view a particular video film, the necessary 
request is sent to the head end by that customer's control box 13 
(possibly using the remote control unit 16 ), the associated TPON 
sub-system unit 11 and the PON 4. The signalling information is received 
by the TPON receiver 2, and is passed on to the video server 17 via the 
TPON head end 6 and the control system 8. The video server 17 then 
transmits the requested video film on to a spare video channel for 
transmission to that customer together with a control signal which enables 
that customer's control box 13 to receive that video channel. 
FIG. 2 shows a modified form of the system shown in FIG. 1, in which the 
two simplex PONs 3 and 4 are replaced by a duplex PON 3'. Accordingly, 
like reference numerals will be used for like parts, and only the 
modifications will be described in detail. In this embodiment, all the 
downstream signalling takes place in exactly the same manner as for the 
embodiment of FIG. 1, that is to say the head end broadcasts all the 
information on all 32 sub-carriers to all of the associated 256 customers 
via the BPON transmitter 1 and the PON 3'. Upstream signalling is, 
however, also carried on the PON 3', with the TPON sub-system unit 11 of 
each customer being connected to its associated fibre via a WDM 18. 
Similarly, the TPON receiver 2 is connected to the PON 3' via a WDM 19 
upstream of the first splitting point. As before, downstream signalling is 
at 1550 nm, and upstream signalling at 1300 nm. 
It will be apparent that the system described above has a number of 
advantages compared with known broadband transmission systems. In 
particular, the use of FM sub-carrier multiplexing has the following 
advantages, namely: 
1. optical constraints which are similar to those for digital (baseband) 
modulation; 
2. low cost lasers can be used, as linearity requirements are less 
stringent; 
3. a high PON split is possible without optical amplification; 
4. the frequency band is compatible with the satellite TV receiver band of 
950 MHz to 2000 MHz; 
5. the equipment used is compatible with terrestrial radio systems; and 
6. very small, low power optical receivers can be used. 
The compatibility with low-cost satellite receivers already in use is 
particularly important. Thus, the customer ONU 9 could directly replace a 
satellite dish, in which case it would be connected directly to the 
set-top receiver (control box 12), and could be powered by that receiver 
from the supply intended for the low noise block (LNB). 
Each of the systems described above is, therefore, capable of delivering 
advanced interactive broadband services, such as video on demand direct to 
the home on fibre. The FM-BPON approach described is capable of a high 
level optical split, and the integration of the TPON technology with this 
approach provides a powerful interactive capability. 
It will be apparent that the system described above could be used to carry 
other broadband services than video on demand. For example, each of the 
TPON/BPON systems described could be modified to provide broadcast TV, 
satellite TV from a central dish, and innovative new dial-up services 
applications such as telepresence, broadband videoconferencing, virtual 
reality and video games.