A METHOD OF AND APPARATUS FOR COMBINING ELECTRICAL

SIGNALS

INTRODUCTION

The present invention relates to a method of combining a plurality of electrical signals-for-transmission over an optical network. The present invention also relates to a communications apparatus. The present invention also relates to a system for transmitting signals.

Embodiments of the present invention relate to passive optical network (PON) architecture for distributing communication signals to a plurality of end user premises.

BACKGROUND

There is currently much interest in the use of passive optical network (PON) extending fibre optic communication cable to the end user. This is sometimes referred to as "fibre to the home", "fibre to the kerb", or "fibre to the building" architecture. These PON architectures have been demonstrated by using time division multiplexing (TDM), wavelength division multiplexing (WDM), sub-carrier multiplexing (SCM) and hybrid WDM/TDM schemes. Among these architectures, many of them use a time division multiple access (TDMA) scheme for operation, often enabling many users to share an optical channel with bandwidth of typically 10 Gb/s. This type of solution relies on high bandwidth components at the customer's premises and requires ranging to establish signal transmission delays and scheduling on the uplink to avoid interference induced by multiple users simultaneously transmitting in the same optical channel.

A PON is a point-to-multipoint network. A PON may comprise a network architecture comprising fibre to the premises equipment. In such a network,

unpowered optical splitters are used to enable a single optical fibre to serve multiple premises. In some systems, a single optical fibre may serve 32 premises. A PON comprises an optical line terminal (OLT) at the service provider's central office and a number of optical network units (ONUs) near end users. A PON advantageously reduces the number of fibres required when compared with a point-to-point architecture. In some systems, downlink signals (i.e. signals from the central office to the end user premises) are broadcast to each of the premises sharing the same downlink channel. Encryption of both downlink and uplink signals (i.e. signals from an end user premises to the central office) may be used to prevent eavesdropping. In some systems, uplink signals are combined using a multiple access protocol which is typically TDMA. A problem with using TDMA for uplink signals, is that network equipment must provide time slots to end user equipment to allow uplink communication.

A plurality of PON standards have been defined: ITU-T G.983; ITU-T G.984; and IEEE 802.3ah. ITU-T G.983 was defined by the International Telecommunications Union and comprised APON (a synchronous transfer mode (ATM) PON) and BPON (broadband PON). ITU-T G.984 is also known as GPON (gigabit PON). IEEE 802.3ah is also known as EPON (Ethernet PON).

Existing PONs typically take advantage of wavelength division multiplexing (WDM) to use one wavelength for downlink traffic and a different wavelength for uplink traffic. Some WDM based PONs use the same wavelength for both the downlink and uplink traffic.

A PON may comprise a central office node called an optical line terminal (OLT). The PON may further comprise one or more user nodes, which may be called optical network terminals (ONT). The fibres and splitters between the OLT and ONT comprise the optical distribution network (ODN). The OLT provides the interface between the PON and a backbone network. The ONT

provides a link to any apparatus which the end user wishes to connect to the PON. Services suitable for transmission over a PON include: plain old telephone service (POTS); voice over internet protocol (IP); data (e.g. Ethernet); video; and telemetry. A PON is a converged network and is content agnostic. This means that a PON may be used to transmit any service. The signals produced by an appropriate piece of service apparatus may be eonverted and-encapsulated ^~ in-a ^~ single ^" packet type for transmission over the PON.

PONs using TDMA comprise a shared network, in that the OLT sends a single stream of downlink traffic that is seen by all ONTs. Each ONT only reads the content of those packets that are addressed to it. Encryption may be used to prevent unauthorised eavesdropping of downlink traffic. The OLT also communicates with each ONT in order to allocate uplink bandwidth to each node. When an ONT has traffic to send, the OLT assigns a time start in which the ONT can send its packet. Because bandwidth is not explicitly reserved for each ONT but allocated dynamically, a TDMA PON allows statistical multiplexing and oversubscription of both uplink and downlink bandwidth. This gives such a PON yet another advantage over point-to-point networks, in that not only the fibre but also the bandwidth can be shared across a large group of users, without sacrificing security.

A problem with this type of PON architecture is the use of TDMA in the uplink. This requires high bandwidth components at the customer's premises. Further, use of TDMA in the uplink requires ranging to be performed between the end user premises and the local PON node. Furthermore, the use of TDMA requires scheduling of data transmission on the uplink to avoid interference by multiple users simultaneously transmitting on the same optical channel. The use of TDMA in a PON also requires the presence of relatively complex processing equipment to be present in the PON.

Sub-carrier multiplexing overcomes these problems by allocating a portion of bandwidth of the uplink transmission path to an end user without limitation in the time domain.

Sub-carrier multiplexing may be used in a PON to improve the efficiency of use of bandwidth of an optical fibre. SCM is a scheme where multiple signals are multiplexedHn the radio frequency -(RF) domain and transmitted by a single wavelength on an optical fibre. An advantage of SCM in PONs is that microwave-devices are more mature than optical devices such that they exhibit greater stability and greater frequency selectivity than their optical counterparts.

Typically, a PON is arranged having a passive optical splitter on the downlink to distribute a plurality of signals to each end user premises. Some passive optical splitters serve up to 32 end user premises. A plurality of passive optical splitters are connected to a dense wavelength division multiplexing (DWDM) multiplexer/demultiplexer which in turn is connected to a central office. The passive optical splitter in the downlink channel is connected to each end user premises by an optical fibre.

An advantage of using SCM in the uplink communication path is that processing signals in the microwave domain is much easier, cost effective and efficient. However, when end user premises are separated by some distance and connected to a passive optical splitter by optical fibre, then use of SCM in the uplink transmission channel is problematic. Typically, use of SCM in the transmission path requires a relatively complex and inefficient communication channel in order to allow signals to be sub-carrier multiplexed at a concentrator placed alongside the passive optical splitter. Such a concentrator may require sensitive microwave reception apparatus for receiving microwave signals transmitted either over short distance radio links or over specially laid coaxial cables and additionally appropriate microwave combination and optical modulation equipment in order to prepare a signal

appropriate for transmission back to the central office. Such a system lacks transparency and would prove costly in terms of hardware and signal loss.

It is an object of the present invention to address these problems.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a method of combining a plurality of electrical signals for transmission over an optical network, said method comprising:

a) each of a plurality of light emitters receiving a respective input electrical signal at a different respective centre frequency and emitting a respective light signal in response thereto and indicative thereof, wherein each light emitter emits the respective light signal at a different respective wavelength; and

b) a photoreceptor receiving the light signals and emitting an output electrical signal in response thereto and indicative thereof.

At least one of the input electrical signals may be generated from a baseband signal. A suitable electrical signal may be generated from a baseband signal by mixing the baseband signal with the output of a local oscillator. The local oscillator preferably operates at radio frequency.

At least one of the input electrical signals may be a signal from a wireless access point, such as a wireless access point arranged for communication with a personal computer. At least one of the input electrical signals may conform to one of the IEEE 802.11 standards.

At least one of the input electrical signals may be a signal from a cellular telephone telecommunication base station. At least one of the input electrical signals may be from a cellular telephone telecommunication pico station.

The photoreceptor may be such that its bandwidth is such that the frequency of each of the light signals is in-band, and/or may be such that the beat frequencies of the light signals are out-of-band with respect thereto. Alternatively, or in addition, the light emitters may be such that the light emitted thereby is in-band, and/or may be such that beat frequencies occur out of-band witfrrespect to the bandwidth of the photoreceptorrA filter may be used substantially to prevent the beat frequencies of the light signals from being detected by the photodetector.

The output electrical signal output by the photoreceptor may be input into a transmission light emitter. The transmission light emitter may be a Light Emitting Diode (LED). The transmission light emitter may be a transmission laser. The transmission laser may launch light into an optical fibre. The transmission light emitter may be a coarse wavelength division multiplexing (CWDM) laser. This laser may be uncooled. The transmission laser may be a dense wavelength division multiplexing (DWDM) laser. The output electrical signal may be used to drive a modulator which imposes the output electrical signal onto a continuous wave (CW) wavelength from a distributed source.

A receiving photoreceptor may receive signals transmitted from the transmission light emitter via the optical fibre. A plurality of receivers may be connected to the output of the receiving photoreceptor. The photoreceptor may be a photodiode. The photodiode may be a p-type, intrinsic, n-type (PIN) photodiode.

At least one of the plurality of receivers may generate a baseband signal. A baseband signal may be generated by downconverting a selected frequency of the output of the receiving photoreceptor with the output of a local oscillator. The local oscillator preferably operates at radio frequency.

At least one of the plurality of receivers may be a wireless access point. At least one of the plurality of receivers may conform to one of the IEEE 802.11 standards.

At least one of the plurality of receivers may be a cellular telephone telecommunication base station. At least one of the plurality of receivers may be a cellular telephone telecommunication pico station. ^"

Step (a) of the method may be preceded by the step of receiving a plurality of baseband signals, each in a respective end user equipment module and upconverting each baseband signal onto a respective carrier signal to generate a respective one of the input electrical signals, each carrier signal having a different respective centre frequency.

The method may include the step of receiving at each of a plurality of optical signal concentrators the light signals output by a respective group of end user equipment modules, each concentrator combining the respective light signals from the respective group and outputting a respective combined light signal in response thereto and indicative thereof.

The method may include the further step of receiving at a multiplexer each of the combined light signals and multiplexing these for onward transmission over an optical carrier.

According to a second aspect of the present invention there is provided communications apparatus for combining a plurality of electrical signals for transmission over an optical network, the apparatus having a plurality of light emitters and a photoreceptor, each light receptor arranged to respond to a respective input electrical signal at a different respective centre frequency by emitting a respective light signal indicative thereof and at a different respective wavelength; and the photoreceptor arranged to receive the light

signals and emit an electrical signal indicative thereof and in response thereto.

The communications apparatus may be further arranged to carry out the method of the first aspect. Optional features of the first aspect may also therefore be optional features of this second aspect.

The apparatus may further include a plurality of end user equipment modules, wherein each module includes an upconverter for upconverting a respective baseband signal onto a respective carrier signal, each carrier signal having a different respective centre frequency, thereby generating a respective one of the input electrical signals. Each end user equipment module may further include the light emitters and may be arranged such that each light emitter receives a respective one of the input electrical signals generated thereby. Each end user equipment module may be arranged to receive radio signals and may be arranged to input each radio signal into a respective one of the light emitters.

One, more or all of the light emitters may be a coarse wavelength division multiplexing (CWDM) laser. The light emitter may be an uncooled laser.

The apparatus may include a plurality of optical signal concentrators, each arranged to receive the light signals output by a respective group of end user equipment modules, and each concentrator arranged and operable to combine the respective light signals from the respective group and to output a respective combined light signal in response thereto and indicative thereof. One, more or all of the optical signal concentrators may each include a respective photo diode connected to a respective transmission laser. That transmission laser may be a dense wavelength division multiplexed (DWDM) transmission laser.

The apparatus may further include a multiplexer for receiving each of the combined light signals and multiplexing these for onward transmission over an optical carrier. The multiplexer may be a dense wavelength division multiplexed (DWDM) multiplexer.

According to a third aspect of this invention, there is provided an end user equipment module as defined hereinabove.

According to a fourth aspect of this invention, there is provided a plurality of end user equipment modules as defined hereinabove.

At least some embodiments of the present invention allow SCM to be implemented in existing PONs with reduced cost. At least some embodiments of the present invention allow SCM to be implemented in PONs comprising fibre to the customer premises.

At least some embodiments of the present invention provide an optical- electrical-optical conversion of signals at a concentrator. The concentrator may comprise a virtually passive uplink module. The concentrator enables an inexpensive uncooled coarse wavelength division multiplexing laser (CWDM) to be used at the customer's premises.

In at least some embodiments, only one temperature controlled laser is required for the "back haul", which is the transmission of data from a terminal serving a group of users to the central office.

At least some embodiments of the present invention allow for a virtually passive uplink to be used as a means of combining uplinks for multiple users onto a single wavelength. Such a combination of signals enables individual SCM data channels to be combined transparently and with low penalty. Such a combination of signals results in a virtual or an effective point-to-point

network where in each customer premises equipment has a dedicated communication channel both from and to the central office. Such a virtual point-to-point network does not require an optical fibre connection solely dedicated to one customer to run from said customer's premises to the central office.

Atieast ^" Some-embodiments ^" Of ^' thB ^~ pτe ^" seτιt1τi\reτitioTϊ ^" a7e^irtUa1ly^a^ivertrϊis ^~ means that no signal processing occurs within the concentrator mode. Thus, some embodiments of the present invention allow for the use of very inexpensive components in the customer premises.

In at least certain embodiments, a portion of the PON may be replaced by a directly analogous radio-based communication system for maintenance, emergency repair or disaster recovery.

Embodiments of the present invention may provide end users with a far higher uplink bandwidth than is possible with TDMA based PON systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 shows in schematic form the uplink path of a specific communications system, in which two base band signals are transmitted from end users, over a long distance optical transmission line, to a central office;

Figure 2 shows in schematic form the uplink path of a second communications system that is similar to that shown in Figure 1 , but in which more than two base band signals are transmitted over a long distance optical transmission line;

Figure 3 shows in schematic form the downlink path of the communications system shown in Figure 1 or Figure 2, from the central office to the end users;

Figure 4 shows in schematic form the uplink path of a third communications system, which is similar to those shown in Figurei and Figure 2, but which shows in more detail where components of the system are sited; and

Figure 5 shows in more detail an optical signal concentrator of the system shown in Figure 4.

A further embodiment is described in Annex 1 , by way of example and with reference to the following drawings, in which:

Figure 6 shows an experimental setup with λ1 : 1560.61 nm, λ2: 1563.05 nm, λ3:1562. 23 nm.

Figure 7 shows the dependence of Q factor on channel spacing.

Figure 8 shows a plurality of BER measurements.

Figure 9 shows the minimum subcarrier frequency of a first SCM channel.

Figure 10 shows the Q factor of the worst channel versus number of channels.

A still further embodiment is described in Annex 2, by way of example and with reference to the following drawings, in which:

Figure 11 shows a schematic of the uplink of a proposed network.

Figures 12a and 12b show I and Q signals respectively at a subcarrier frequency of 2.5 GHz. Figures 12c and 12d show I and Q signals respectively at a subcarrier frequency of 3.5 GHz after the uplink combiner.

Figures 13a and 13b show I and Q signals respectively at a subcarrier frequency of 2.5 GHz after 25 km SMF transmission. Figures 13c and 13d show I and Q signals at a subcarrier f requency of 3.5 GHz after 25 ^~ krrf SMF transmission.

Figure 14a shows the EVM of a 2.5 GHz SCM channel as a function of 3dB Gaussian filter bandwidth. Figure 14b shows the EVM of the worst channel versus number of channels when an SCM channel is added serially.

SPECIFIC DESCRIPTION OF CERTAIN EMBODIMENTS

Figure 1 shows a first system in which two baseband signals A and B from end users (not shown) are combined and transmitted over a single optical transmission line 111 to a central office (not shown). In this embodiment, the optical transmission line 111 is 25 km of standard single mode fibre (SMF). The optical transmission line 111 constitutes a dense wavelength division multiplexing (DWDM) backhaul to a central office.

The system includes a first upconverter having a first mixer 105 with a first local oscillator 103 connected thereto and arranged to receive baseband signal A at the input to the first mixer 105. The local oscillator 103 is operable to produce a signal at 2.5 GHz. The output of the first mixer 105 is connect to the input of a first laser diode 107, which emits light at a wavelength of 1560.6 nm. The first laser diode is arranged to emit light on the optically sensitive area of a photodiode 109.

In parallel to the first upconverter, there is a second upconverter. The second upconverter includes a second mixer 106 with a second local oscillator 104 connected thereto and arranged to receive baseband signal B at the input to the second mixer 106. The second local oscillator 104 is operable to produce a signal at 6 GHz. The output of the second mixer 105 is connect to the input of a second laser diode 107, which emits light at a wavelength of 1530.3 nm. Tfie second laser diode is Blso arrangetf to emit light on the optically sensitive area of the photodiode 109.

The photodiode 109 has a 15 GHz bandwidth. The electrical output of photodiode 109 is connected to a transmission laser 110. Transmission laser 110 is operable to emit light having a wavelength of 1566.1 nm. Transmission laser 110 is further arranged to launch light into the optical transmission line 111 , which in this embodiment is a single mode fibre. A PIN (P-type, intrinsic, N-type) photodiode 112 is arranged at the receiving end of the optical transmission line 111 in order to receive the signals transmitted thereon. The PIN diode has a bandwidth of 15 GHz and is operable to amplify signals received over the optical transmission line 111. The output of PIN diode 112 is connected to the input of each of two local receivers, each of which is operable to obtain a respective one of the two baseband signals A, B. A first one of the local receivers has a first receiver mixer 113 and a first receiver local oscillator 115. This local oscillator 115 is arranged to produce a signal at the same frequency as first local oscillator 103 at the end user end of the system, i.e. at 2.45 GHz. Thus, the first receiver mixer 113 is operable to output baseband signal A. Similarly, the second local receiver comprises a second receiver mixer 114 which receives a signal from a second receiver local oscillator 116. This local oscillator is arranged to produce a signal at the same frequency as the second local oscillator 104, i.e. at 6 GHz. Thus, the second receiver mixer 114 is operable to outputs baseband signal B.

The operation of the system shown in Figure 1 will now be described. With continued reference to Figure 1 , baseband signals A and B are upconverted

to different radio frequencies by a local oscillators and mixers 103, 105, 104 and 106. Each baseband signal A and B is upconverted onto a respective RF carrier using a broadband triple balanced mixer to produce a binary phase shift keyed (BPSK) signal, and with a different respective carrier frequency. Each signal is than input into the respective one of the laser diodes 107, 108, and each laser diode 107, 108 responds by outputting light at different wavelength. Each laser diodes 107, 108 independently outputs the respective baseband signal modulated onto the respective carrier wave which in turn is modulated onto the respective light carrier output by the laser diode 107, 108.

As mentioned, the laser diodes 107, 108 operate at different wavelengths. The wavelengths of the laser diodes 107, 108, and the bandwidth of the photodiode 109, are selected such that the beat frequencies produced by the interaction of the light from each laser diode 107, 108 cannot be detected by the photodiode 109. Thus, the 15 GHz bandwidth of the photodiode 109 operates as a filter such that the beat frequencies of laser diodes 107 and 108 are not detected. The electrical signals output by photodiode 109 are sub- carrier modulated (SCM) signals. The SCM signals are centred on the carrier frequencies of 2.45 GHz and 6 GHz. This composite electrical signal is used to drive the transmission laser 110 operating at 1566.1 nm. The transmission laser 110 is used to transmit the composite electrical signal containing the two SCM channels over the 25 km standard single mode fibre 111 , which constitutes the DWDM backhaul to the central office. Thus, a backhaul optical signal is transmitted over the single mode fibre 111. This signal is then detected by the amplified 15 GHz PIN diode 112 at the central office end of the system. The electrical signal output by PIN diode 112 is then split and downconverted at each receiver to give the two baseband signals A, Bs. The resulting baseband signal is low pass filtered.

Figure 2 shows a second system that is a generalised version of the system of Figure 1. The second system is arranged to transmit more than two SCM channels over an optical transmission line 211. Several baseband signals A

to Z are connected to a plurality of upconverters 204. The upconverters are arranged in substantially the same way as those of the first system described with reference to Figure 1. Thus, the upconverters 204 each include a respective local oscillator O ₁ to δ _n , each operable at a different respective frequency i^ to f _n . Thus, the upconverters 204 are arranged to output a plurality of individual sub-carrier modulated signals each centred on a different carrier frequency. The ^" output ^" of eacrr ϋpconverteT 204 ^" is ^~ inpϋtTrifo a respective one of several laser diodes 206. Each of the laser diodes 206 is arranged to output light at a different respective wavelength A ₁ to A _n .

The laser diodes 206 are arranged such that the light output from each is input into a single photodiode 209. The bandwidth of the photodiode 209 is such that the beat frequencies between any of the wavelengths Ai to A _n are not detected by the photodiode 209. Photodiode 209 is operable to produce a composite electrical signal containing each sub-carrier modulated signal which is used to drive a transmission laser 201. The transmission laser 201 is connected to optical transmission line 211 by means of an appropriate launch. Optical transmission line 211 is in turn, at a central office end of the system, connected to a photodiode 212 which is connected to a splitter (not shown) and to several receivers. Each of the receivers comprises a local oscillator and mixer 214 which outputs one of the respective baseband signals A to Z.

The operation of the second system shown in Figure 2 is essentially the same as that of the first system shown in Figure 1. In the second system, each baseband signal 202 is upconverted onto an RF carrier centred on a carrier frequency unique to the group of baseband signals transmitted via optical transmission line 211. This is necessary to allow the receiving apparatus 214 to identify uniquely each received baseband signal 218 by its frequency. Each upconverted baseband signal A to Z is input into a respective one of the laser diodes 206 having a unique wavelength in the group of laser diodes. Each laser diode 206 then transmits a respective light signal photodiode 209. Each laser diode 206 has a unique wavelength Ai to A _n in order to reduce the

effect of interference between the different light signals incident upon the photodiode 209. Beat frequencies are produced by the interaction of the light from different laser diodes, but the wavelengths of the laser diodes 206, and the photodiode 209, are selected such that these beat frequencies cannot be detected by the photodiode 209: the beat frequencies of the laser diodes 206 are out of band of photodiode 209.

The composite electrical signal output by photodiode 207 is input into transmission laser 217. The transmission laser 217 responds by launching light into the optical transmission line 211. At a receiving end of the optical transmission line 211 , a photodiode 212 is arranged to receive the light transmitted by the optical transmission line 211. The Photodiode 212 reproduces an electrical signal which may be split and then downconverted at a plurality of receivers in order to reproduce each of the plurality of baseband signals A to Z.

Figure 3 shows the downlink path of a PON which may serve as the downlink path in any one of the systems that embody the present invention. A central office 301 is connected by a fibre optic cable to a dense wavelength division multiplexing (DWDM) demultiplexer 302. The DWDM demultiplexer 302 is in turn connected to a several optical fibres, which connect the DWDM demultiplexer 302 to several optical splitters, with each optical fibre being connected to a respective optical splitter. For simplicity of explanation, just one such optical splitter 310 is shown in Figure 3. The optical splitter may be considered a "passive" optical splitter as no signal processing takes place therein. The passive optical splitter 310 is connected and arranged so as to distribute the signal received from the DWDM demultiplexer 302 to several customer premises equipment units 320, via a respective fibre optic cable to each unit 320. Each customer premises equipment unit 320 comprises a photodiode 321 and a downconverter 322 which comprising a mixer and a local oscillator (neither of which are shown).

The operation of the downlink shown in FIGURE 3 will now be described. Central office 301 transmits a dense wavelength division multiplexed (DWDM) signal to DWDM demultiplexer 302. The DWDM signal transmitted by central office 301 comprises a plurality of DWDM channels each operating at a unique wavelength. Signals for each of the customer premises equipment units 320 in a group are encoded, at the central office 301 , onto a DWDM channel at a particular wavelengthr A group ofxustomer premises equipment units 320 is made up of all those customer premises equipment units 320 connected to the same passive optical splitter 310. Each customer premises equipment unit 320 within the group is addressed by a respective RF frequency.

The DWDM signal comprises a wavelength dedicated to each passive optical splitter 310. In other words, the DWDM signal comprises several DWDM channels, one for each passive optical splitter 310. In alternative arrangements, more than one passive optical splitter may receive the same channel from the DWDM demultiplexer 302. However, it is necessary that each customer premises equipment unit 320 is uniquely addressed by a DWDM wavelength and an RF frequency of the SCM signal transmitted thereby. The DWDM demultiplexer 302 demultiplexes the DWDM signal received from the central office. The DWDM demultiplexer 302 outputs one DWDM channel to each passive optical splitter 310. The DWDM channel is identified by a wavelength. Signal degradation may necessitate the use of an optical amplifier at the DWDM demultiplexer 302. Such an optical amplifier may be an erbium doped fibre amplifier (EDFA).

The passive optical splitter 310 receives a single DWDM channel, which, as mentioned, is defined by a wavelength. The passive optical splitter 310 outputs the received DWDM channel to each customer premises equipment 320. The DWDM channel comprises an SCM signal wherein the signal for each customer premises equipment is defined by an RF frequency.

In each customer premises equipment unit 320, the respective photodiode 321 generates an electrical signal from the received DWDM channel. This electrical signal is input into the respective RF receiver 322 of the unit 320 and the associated local oscillator (not shown) decodes the particular RF frequency intended for that particular customer premises equipment unit 320. Thus, it will be appreciated that each customer premises equipment unit 320 has ^{" "} an RF ^~ receiver ^" de ^~ dicated to ^"" decoding from the ^~ sW-caTrier ^" multiplex signal received from the central office 301 the RF frequency intended for that particular unit 320.

In summary, each customer premises equipment unit 320 receives the same DWDM channel or wavelength as each other unit within the same group. This channel is output by the passive optical splitter 310. Each customer premises equipment unit 320 then decodes the RF frequency intended for it.

Figure 4 shows the uplink path of a third system that may form the uplink path of the system described with reference to Figure 3. As can be seen in Figure 4, the third system includes several customer premises equipment units (an exemplary one of which is labelled 420). Each unit 420 includes a mixer 422 with an associated oscillator (not shown), and a laser diode 421. The mixer 422 is arranged to receive as an input a baseband signal A to Z and to upconvert this to a radio frequency electrical signal. The output of the mixer 422 is connected to the input of the laser diode 421. Thus, each of the plurality of customer premises equipment units 420 is operable to output a baseband signal upconverted to a radio frequency and to output this on a light signal of a particular wavelength.

An optical signal concentrator 410 receives all of the output optical signals from the customer premises equipment units 420 by means of an optical fibre. The optical signal concentrator 410 is virtually passive in operation, in that substantially no signal processing is carried out thereby. The optical signal concentrator 410 is arranged to combine each of the received optical signals

and then to output, by means of a transmission laser (not shown in Figure 4), a signal to a DWDM multiplexer 420, to which it is connected by optical fibre. The transmission laser of the optical signal concentrator 410 is a DWDM laser suitable for producing signals for transmission over the DWDM backhaul to central office. The DWDM laser operates within a very limited wavelength variation. In some embodiments, this transmission laser is cooled.

In this third system, there are several optical signal concentrators such as that 410. In similarity with the optical signal concentrator 410, each other concentrator 410 is connected via optical cable to a respective group of customer premises units substantially the same as those 420 already described. Each concentrator also outputs a respective DWDM channel to the DWDM Multiplexer 402. DWDM Multiplexer 402 is arranged to receive via optical fibre, this plurality of DWDM channels. DWDM multiplexer 420 is in turn connected to central office 401 by an optical fibre which, in this embodiment, is in excess of 10 km in length.

The operation of the third system shown in Figure 4 will now be described. In a similar manner to the downlink transmission path described with reference to Figure 3, in the uplink transmission path shown in Figure 4, each customer premises equipment unit 420 is uniquely identified by an RF frequency of a component of the sub-carrier multiplex channel and a wavelength of the DWDM channel transmitted by the optical signal concentrator 410 with which the customer premises equipment is in cooperation. Each customer premises equipment unit 420 operates to upconvert a baseband signal and to transmit the upconverted signal via a laser diode 421. The laser diode 421 is, in this embodiment, an uncooled coarse WDM (CWDM) laser. The output of each laser diode 421 in each customer premises equipment unit 420 is transmitted by optical fibre to the optical signal concentrator 410.

In operation, the optical signal concentrator 410 is virtually passive in that no signal processing is performed at the optical signal concentrator 410, the

optical signal concentrator merely combines signals in the optical domain. The optical signal concentrator 410 is also virtually passive in that it must be powered in order to operate the DWDM transmission laser for producing a DWDM channel suitable for transmission back to the central office 401 via DWDM multiplexer 402. Again, and in summary, in the uplink path signals from each customer premises equipment unit 420 in a group (i.e. those that are-connected to the same-optical signal concentratof 410) are received on a single DWDM wavelength at the central office 401. Each optical signal concentrator 410 outputs a respective DWDM channel, at a particular wavelength. The DWDM multiplexer 402 operates to combine a plurality of DWDM channels received from each optical signal concentrator 410 and outputs a DWDM signal comprising a plurality of wavelengths to the central office 401 by means of an optical fibre.

Figure 5 shows a detailed view of the optical signal concentrator 410 of the third system shown in, and described with reference to, Figure 4. With reference to Figure 5, optical signal concentrator 510 includes a photodiode 512 and a transmission laser 514. The photodiode 512 receives an optical signal from each customer premises equipment unit 520 connected to the optical signal concentrator 510. Each of those optical signals comprises a signal on a different RF frequency transmitted by light having a different wavelength. The photodiode 512 combines these signals to produce a composite electrical signal comprising a sub-carrier multiplex of each signal from the customer premises equipment units 420. The composite electrical signal is input into the transmission laser 514 which transmits a composite light signal, in this embodiment, by means of optical fibre.

In the embodiments described above, an optical fibre may be either a single mode fibre or a multi-mode fibre.

In the embodiments described above, the transmission of baseband signals has been described. In alternative embodiments, at least one of the

baseband signals upconverted to a radio frequency may be replaced by a radio signal. Such a radio signal may be the output from a wireless access point, such as a wireless access point in a wireless local area network (WLAN) to which a personal computer has access. Such a radio signal may be the output from a cellular telephone telecommunications base station.

In the embodiments ^* described above, the ^" photodiodes 109, ^~ 209^and 512 operate as filters in order to eliminate beat frequencies created by the interaction of light of different wavelengths incident upon the photodiodes. In an alternative embodiment, an additional filter is provided in order perform or augment this function.

In the above described embodiments, SCM modulation has been described using a BPSK modulation format. However, in alternative embodiments of the present invention any modulation format may be used. In particular, the modulation format used may comprise one of the following: binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16 quadrature amplitude modulation (16 QAM), and 64 quadrature amplitude modulation (64 QAM). Similarly, the SCM modulation may be used to transmit other radio or wireless standards such as 2G, 3G, IEEE 802.11.

An example of a further system that embodies the present invention and that employs QPSK is described in "Spectrally efficient 10 x 1 Gb/s QPSK Multi- User Optical Network Architecture" by J. Y. Ha, A. Wonfor, R. V. Penty, I. H. White and P. Ghiggino, incorporated herein by reference. This document describes a two channel example using different wavelengths to those given above with reference to Figure 1.

An example of a still further system that embodies the present invention and that employs QAM is described in "Highly Spectral Efficiency Multi-User

Optical Network Architecture using 1Gb/s 16QAM Subcarrier Multiplexing" by

J. Y. Ha, A. Wonfor, R. V. Penty, I. H. White and P. Ghiggino, incorporated

herein by reference. This document describes a two channel example using different wavelengths to those given above with reference to Figure 1.

Embodiments of the present invention have been described with reference to the examples illustrated. It will be appreciated that variations and modifications may be made to the examples described within the scope of the present invention as defined ^~ in ^~ the ^~ appended ^~ craims:

Further embodiments are described in Annex 1 and Annex 2 that are annexed hereto.

Annex 1

The dramatic growth of bandwidth requirements for residential and business services has led to the development of a wide range of passive optical networks (PONs) with a series of standards having been defined allowing bandwidths to users of 10 - 100 Mb/s. With the growth of broadband networks and bandwidth hungry applications such as HDTV, interest has grown in POM architectures which provide larger user bandwidths (for example up to 1 Gb/s) and yet use low cost technology. As a result, leveraging recent advances in high speed RF technologies, subcarrier multiplexing (SCM) has been proposed as an alternative to TDM and CDMA technologies for PON applications. The proponents of SCM have cited advantages of flexible protocol transmission (where different channels can therefore be used for different services), relaxed dynamic specifications for user components and good fibre transmission performance. However, significant disadvantages have been acknowledged including in particular the generation of optical beat interference between uplink sources. As a solution to this problem, we have previously demonstrated a WDM-SCM multi-user optical network architecture incorporating a novel uplink combiner which allows the simple combination of SCM channels and no optical beat interference from multiple users. Here, we showed that excellent uplink transmission performance could potentially be achieved.

Here we demonstrate the feasibility of a 10 x 1 -Gb/s quadrature phase shift keying (QPSK) WDM-SCM multi-user optical network architecture. An experimental proof-of-principle demonstration of the uplink is demonstrated for a 2-user system by measuring the bit error rate obtained for each of the two SCM channels in the uplink. The improved spectral efficiency for the single wavelength 10 channel case is validated theoretically using a time domain laser model.

Figure 1 shows a schematic of uplink of the proposed network. Multiple SCM channels are generated at each user. These are transmitted over short optical links to a "virtually passive" uplink combiner where the optical signals are combined, detected by a broadband photodetector and then retransmitted using a dense wavelength division multiplexing (DWDM) laser with an external modulator. Thus the SCM channels are multiplexed onto the same WDM wavelength. Fora ^~ proof of pπnciple ωφeriment two SCM-QPSK channels were transmitted over the network. In order to generate the required 1 Gb/s QPSK signals experimentally, two 500 Mb/s "data" and "not data" pseudorandom outputs from an Anritsu MP1763C 12.5GHz pattern generator are de-correlated by propagation along different lengths of cable. Each output is used as the in-phase (I) and quadrature (Q) component for the signal respectively, being up-converted onto each orthogonal RF carrier using a broad-band triple balanced mixer and then superimposed to form a QPSK data signal. The two QPSK channels used in the network are then set with different carrier frequencies. Each QPSK SCM signals then drives uplink optical source, with different wavelengths, namelyl 560.61 nm and 1563.05 nm. The two optical signals are combined and detected using a 15 GHz bandwidth photodiode. The resulting composite electrical signal is used to re- modulate a third optical source operating at1562.23 nm which is used to transmit the resulting optical signal containing two SCM channels over 25 km of standard SMF, representing the DWDM back-haul to the central office. The back hauled optical signal is then detected with an amplified 15 GHz PIN diode. The electrical signal is then split and down-converted with each of the original local oscillators. The resulting base band signal is low pass filtered and detected with an oscilloscope and an error detector.

Figure 6 shows a schematic of uplink of the proposed network. Multiple SCM channels are generated at each user. These are transmitted over short optical links to a "virtually passive" uplink combiner where the optical signals are combined, detected by a broadband photo detector and then retransmitted using a dense wavelength division multiplexing (DWDM) laser with an

external modulator. Thus the SCM channels are multiplexed onto the same WDM wavelength. For a proof of principle experiment two SCM-QPSK channels were transmitted over the network. In order to generate the required 1 Gb/s QPSK signals experimentally, two 500 Mb/s "data" and "not data" pseudorandom outputs from an Anritsu MP1763C 12.5GHz pattern generator are de-correlated by propagation along different lengths of cable. Each output is used as thein-phase respectively, being up-converted onto each orthogonal RF carrier using a broad-band triple balanced mixer and then superimposed to form a QPSK data signal. The two QPSK channels used in the network are then set with different carrier frequencies. Each QPSK SCM signals then drives uplink optical source, with different wavelengths, namely 1560.61 nm and 1563.05 nm. The two optical signals are combined and detected using a 15 GHz bandwidth photodiode. The resulting composite electrical signal is used to re- modulate a third optical source operating at 1562.23 nm which is used to transmit the resulting optical signal containing two SCM channels over 25 km of standard SMF, representing the DWDM back-haul to the central office. The back hauled optical signal is then detected with an amplified 15 GHz PIN diode. The electrical signal is then split and down-converted with each of the original local oscillators. The resulting base band signal is low pass filtered and detected with an oscilloscope and an error detector.

In order to investigate the feasibility of the 10x1 -Gb/s QPSK WDM-SCM PON, detailed measurements have been made of the quality factor of the down- mixed data as a function of modulation channel spacing or carrier frequency separation (Fig. 7). It is found that the quality factor of the SCM channel at a subcarrier frequency of 2.7GHz increases and then stabilises with increasing channel spacing owing to reduced inter-channel crosstalk. The quality factor of both I & Q signals is 9.45 and 9.56 dB respectively when the spacing between two SCM channels is 1 GHz. The error performance of the uplink is evaluated by measuring the bit error rates obtained for each of the two SCM channels in the uplink as shown in Fig. 8. The left two BER curves show back

to back operation with similar sensitivities for each channel and with no error floor. The right two BER curves show that there is only a 1dB penalty at a BER of 10-9 for virtually passive multiplexing and transmission over 25 km, representing the DWDM back haul to the central office. In order to evaluate the possibility of implementing a 10 x 1-Gb/s WDM-SCM optical network, a computer simulation using time domain laser model is performed. In order to find a first subcarrier frequency of the SCM channels, the quality factor of a SCM channel after the uplink combiner as a function of subcarrier frequency is calculated as shown in Fig.9. The result represents that the quality factor is degraded sharply as the subcarrier frequency closes to 0 GHz. In this simulation, the optimised 3dB Gaussian filter bandwidth is 0.3 GHz. Fig. 10 represents the feasibility of 10 x 1-Gb/s QPSK SCM transmission spaced at 1 GHz in our proposed WDM-SCM optical network. The subcarrier frequency is chosen over the 1.35 GHz to 10.35 GHz subcarrier frequency range. Here, as the number of channels increases, the quality factor is calculated for the worst case channel positions. The result shows that the Q factor deteriorates with increasing channel number. This is mainly because the harmonics of the SCM channels caused by the laser nonlinearity increase. However the quality factor of the worst channel is over 8.6 dB even though 10 SCM channels are simulated. This result brings out the potential for multichannel WDM-SCM transmission to achieve a 10-Gb/s aggregate data rate with a high 1 Gb/s single user data rate.

We propose a spectrally efficient 10 x 1 Gb/s QPSK multi-user optical network architecture based on subcarrier multiplexing for use in PONs. A proof of principle demonstration with 2 uplink channels with 1 GHz RF carrier separation achieves 25 km transmission with a power penalty of 1dB. Simulation results indicate that each optical channel is capable of supporting 10 full 1 Gb/s dedicated links and so serving 400 users simultaneously when using 40 WDM channels.

Annex 2

The dramatic growth of bandwidth requirements for residential and business services has led to the development of a wide range of passive optical networks (PONs) with a series of standards having been defined allowing bandwidths to users of 10 - 100 Mb/s. Leveraging recent advances in high speediRF technologiesrsubcarher multiplexing (SCM) has been proposed as an alternative to time division multiplexing and code division multiplexing access technologies for PON applications. Despite the advantages of SCM technology, future applications of SCM may be limited by the spectral inefficiency of certain SCM modulation formats. Thus, SCM high speed optical networks with good spectral efficiency have been of much interest. SCM- based optical networks using a spectrally efficient digital modulation scheme, such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM), allow low speed ingress data streams which leads to low speed and thus inexpensive electronic components. In order to meet the requirements of future high performance PON services, we have previously demonstrated a wavelength division multiplexed subcarrier multiplexed (WDM-SCM) multi-user optical network architecture using an uplink combiner which removes optical beat noise between optical sources. This enables a number of customers each to use an individual SCM channel, all of which are combined on a single optical wavelength for back-haul to the central office. 40 such DWDM wavelengths can be combined on a single fibre enabling more than 400 customers each with a 1-Gb/s connection on a single PON. We have shown excellent uplink transmission performance with a 1-Gb/s QPSK modulation scheme but with restricted spectral efficiency.

In this work, we carry out a proof of principle demonstration of a 2 x 1-Gb/s 16 QAM WDM-SCM multi-user optical uplink to extend the performance of the network demonstrated in exhibiting high spectral efficiency. The improved spectral efficiency for the single wavelength 20 channel case is validated theoretically using a time domain laser model, showing the scalability of this

architecture.

Fig. 11 shows a schematic of the uplink of the proposed network. An SCM channel is generated by each user. These are transmitted over short coarse wavelength division multiplexing (CWDM) optical links to a "virtually passive" uplink combiner where the optical signals are combined, detected by a broadband-photodetectorand ^" then ^~ retransmitted using aπdense wavelength ^{^} division multiplexing (DWDM) laser with an external modulator. Thus the SCM channels are multiplexed onto a single wavelength for back-haul to the central office. For a proof of principle experiment, we demonstrate the performance of a single DWDM uplink with two SCM-1 Gb/s 16QAM channels. At first, in order to generate a 1-Gb/s 4 level data stream, a 250 Mb/s data channel is attenuated by 6 dB, whilst the not-data channel is delayed by an integer no of bit periods. The two channels are then combined and divided in a 4 GHz resistive splitter/combiner. One output is used as the in-phase (I) and the other delayed and used as the quadrature (Q) component for the signal respectively, being up-converted onto each orthogonal RF carrier using a mixer and then superimposed to form a 16QAM data signal. The two 16QAM channels used in the network are then set with different carrier frequencies. Each 16QAM SCM signal then drives two uplink optical sources, each at a different wavelength, namely 1560.61 nm and 1563.05 nm. The two optical signals are combined and detected using a 15 GHz bandwidth photodiode. The resulting composite electrical signal is used to re-modulate a third optical source operating at 1562.23 nm which is used to transmit the resulting optical signal containing two SCM channels over 25 km of standard single mode fibre (SMF), representing the DWDM back-haul to the central office. The back haul optical signal is then detected with an amplified 15 GHz PIN diode. The electrical signal is then split and down-converted with each of the original local oscillators. The resulting base band signal is low pass filtered and detected with an oscilloscope.

In order to demonstrate the 16QAM optical network, eye diagrams of two

SCM channels at a subcarrier of 2.5 and 3.5 GHz are measured respectively as shown in Fig. 12 and Fig. 13. I and Q signals of the two SCM channels have a similar quality factor for each eye. The quality factor after 25 km SMF transmission is slightly degraded due to transmission loss. Each quality factor of the top, middle and bottom eye is over 3.7 (~10 ⁴ BER) which may be improved by better VSWR matching of RF components and compensated by using forward ^~ error coτre ^~ ction ^~ techniques producing a SER of better than 10 ¹⁵ . In order to evaluate the potential for implementing a 20 x 1-Gb/s 16QAM optical network, a computer simulation using a time domain laser model is performed. This simulation includes the performance of the RF components as expected in an integrated system, rather than the discrete components used in our proof of principle demonstrator. Fig. 14a shows the effect of channel spacing with optimal filtering between SCM channels on EVM. In this calculation, the EVM of a 2.5 GHz SCM channel is measured as a function of channel spacing. Inter-symbol interference (ISI) of a single SCM channel increases with decreasing filter bandwidth, whilst crosstalk is minimised at the optimum optical filter bandwidth. Here, the optimal filter bandwidth is determined to be 0.1625 GHz when the channel spacing is 0.5 GHz. Fig. 14b demonstrates the feasibility of 20 x 1 -Gb/s 16QAM SCM transmission spaced at 0.5 GHz in our proposed optical network. The subcarrier frequency is chosen over the 1.0 GHz to 10.5 GHz subcarrier frequency range. Here, as the number of SCM channels increases whilst a SCM channel is added serially, the EVM is calculated for the worst case channel. The result shows that the EVM deteriorates with increasing channel number. This is mainly because the harmonics of the SCM channels caused by the laser nonlinearity increase. Increasing modulation index (M) of Mach-Zehnder (MZ) modulator in the uplink combiner also induces nonlinearity of modulation. However the EVM of the worst channel is less than 7 % even though 20 SCM channels are simulated (M=0.4). This result shows the potential for multi-channel 1-Gb/s 16QAM transmission to achieve a 20-Gb/s aggregate data rate with high spectral efficiency of 2-bits/s/Hz.

We demonstrate a spectrally efficient 2 x 1-Gb/s 16QAM multi-user uplink architecture based on subcarrier multiplexing for use in PONs. Simulation results indicate that each optical channel is capable of supporting 20 x 1-Gb/s dedicated links and so serving 800 users simultaneously when using 40 DWDM channels from the uplink combiner to the central office.