WO2016055118A1

WO2016055118A1 - Coherent optical communication transceivers

Info

Publication number: WO2016055118A1
Application number: PCT/EP2014/071708
Authority: WO
Inventors: Bengt-Erik Olsson
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2014-10-09
Filing date: 2014-10-09
Publication date: 2016-04-14

Abstract

A hub transceiver 200 comprising a laser 230 configured to output an optical carrier signal to a modulator 235 and to an up-link receiver 220 over a polarization maintaining, PM, distribution network 240. The modulator 235 arranged to output a down-link signal comprising the optical carrier signal to a branching unit 215 over a first PM transmission line 241. The branching unit 215 configured to forward the down-link signal from the modulator 235 to an I/O port 210 and also to forward a received up-link signal from the I/O port 210 to an up-link receiver 220 over a second PM transmission line 242. The up-link receiver 220 is arranged to receive up-link data in the up-link signal by means of the optical carrier signal.

Description

COHERENT OPTICAL COM M UNICATION TRANSCEIVERS

TECHN ICAL FI ELD

The present disclosure relates to optical communication systems and in particular to hub transceivers arranged for bi-directional communication with one or more client transceivers.

BACKGROUND

Optical communication systems are commonly deployed in a wide variety of applications where information is distributed back and forth between a hub transceiver and one or more client transceivers.

One such application relates to distributed antenna systems (DAS) which are becoming more and more common in modern wireless communication systems. Some such DASs are implemented by feeding antenna elements from a central hub using one fiber per antenna, while other DASs are implemented by using wavelength division multiplexing (WDM) which only requires a single fiber from the central hub with different optical wavelengths to each antenna element.

Figure 1 schematically illustrates the concept of a WDM-based DAS according to prior art. The radio processor is in some implementations realized as a collection of single-antenna radios, where each radio uses a single antenna. I n other implementations the radio processor comprises one or more multi-antenna radios, where each multi-antenna radio uses more than one antenna. The trunk fiber carries signals to all antennas and the individual antenna signals can be decoupled one at a time in drop nodes or all in the same node. I n a WDM-based system, each optical wavelength carries the required signals to one individual antenna and the optical antenna signal is converted to electrical signal at each antenna location. Current solutions have several issues. One such issue relates to the cost of the terminal transceivers at the antennas. Another issue related to distribution systems based on WDM is that client transceivers need to be specifically designed for a given optical carrier wavelength, i.e., the client transceivers are not colorless, which drives cost and system complexity. Yet another problem relates to that current distribution systems for analog signals, such as DAS, require high linearity, i.e., linear response in output signal versus input signal. SUMMARY

An object of the present disclosure is to provide at least hub and client transceiver arrangements, a communication system, and methods which seek to mitigate, alleviate, or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination.

This object is obtained by a hub transceiver for communicating with a client transceiver over a reciprocal bi-directional optical link. The hub transceiver comprises an input/output, I/O, port connectable to the optical link, a branching unit, an up-link receiver, and a down-link transmitter comprising a laser and a modulator. The laser is configured to output an optical carrier signal to the modulator and to the up-link receiver over a polarization maintaining, PM, distribution network. The modulator is arranged to modulate down-link data onto the optical carrier signal and output a down-link signal comprising the optical carrier signal to the branching unit over a first PM transmission line. The branching unit is configured to forward the down-link signal from the modulator to the I/O port and also to forward a received up-link signal from the I/O port to the up-link receiver over a second PM transmission line. The up-link receiver is arranged to receive up-link data in the up-link signal by means of the optical carrier signal.

The disclosed hub transceiver improves performance in terms of system linearity and also significantly simplifies and reduces cost of optical communication systems, such as distributed antenna systems (DAS) and access systems based on passive optical networks (PON), in particular combined with wavelength division multiplexing (WDM). The hub transceiver is according to aspects utilized in a high performance and low cost radio over fiber (RoF) system, and is particularly useful for optical implementation of a front-haul system for mobile data networks. The present concept of transmit laser reuse in the hub transceiver furthermore allows for simpler implementation of the signal processing required in the up-link receiver since requirements on, e.g., carrier recovery and phase noise mitigation are reduced. The current technique also simplifies implementation of coherent up-link receivers, providing superior receiver sensitivity and linearity compared to conventional systems. The above-mentioned object is also obtained by a client transceiver for communicating with a hub transceiver over a reciprocal bi-directional optical link. The client transceiver comprises an input/output, I/O, port connectable to the optical link, a branching unit, a down-link receiver, and an up-link transmitter. The branching unit is arranged to forward a received down-link signal comprising an optical carrier signal from the I/O port to the down-link receiver as well as to the up-link transmitter, and to forward an up-link signal from the up-link transmitter to the I/O port. The down-link receiver is arranged to receive down-link data in the down-link signal. The up-link transmitter comprises a modulator and a Faraday mirror. The modulator is arranged to modulate up-link data onto the optical carrier signal and to output the up-link signal. The Faraday mirror is arranged to convert the polarization of the up-link signal of the up-link transmitter into a polarization orthogonal to that of the received optical carrier signal.

An advantage of the disclosed client transceiver is that it can be realized as a colorless client terminal, i.e., the client transceiver is according to aspects wavelength channel agnostic, which is of importance for low-cost deployment. Furthermore, the client transceiver allows for reuse of the hub transceiver transmitter laser in the hub transceiver up-link receiver, which is enabled by the use of a Faraday mirror in the client transceiver.

Also, the possibility to convert wavelength multiplexing in the client domain into multichannel radio frequency (RF) in the hub side is advantageous in that it reduces hardware cost. There is also disclosed herein a communication system comprising a hub transceiver and a client transceiver according to the present teaching.

The proposed communication system enables a cost effective solution for optical information distribution allowing colorless client terminals while at the same time reducing requirements on optical component celerity in the hub transceiver. The system inherently supports multiple radio frequencies (RF), such as multi-band radio standards and orthogonal frequency division multiplexing (OFDM), and is thus suitable for transparent optical distribution of mobile standards such as the long-term evolution (LTE), wide-band code-division multiple access (WCDMA), or global system for mobile communication (GSM) standards. The object is furthermore obtained by a method in a hub transceiver for communicating with a client transceiver over a reciprocal bi-directional optical link. The method comprises generating an optical carrier signal, distributing the optical carrier signal to a modulator and to an up-link receiver over a polarization maintaining, PM, distribution network, and generating, by the modulator, a down-link signal comprising the optical carrier signal. The method also comprises outputting the down-link signal on the optical link to the client transceiver via a first PM transmission line, and receiving an up-link signal on the optical link from the client transceiver via a second PM transmission line, as well as receiving, by the up-link receiver, uplink data in the up-link signal by means of the optical carrier signal. There is also disclosed herein a method in a client transceiver for communicating with a hub transceiver over a reciprocal bi-directional optical link. The method comprises receiving a down-link signal comprising an optical carrier signal from the hub transceiver on the bidirectional optical link, and receiving down-link data in the down-link signal, as well as modulating up-link data onto the optical carrier signal to generate an up-link signal. The method also comprises converting, by a Faraday mirror, a polarization of the up-link signal into a polarization orthogonal to that of the received optical carrier signal, and outputting the polarization converted up-link signal to the hub transceiver on the bi-directional optical link.

In addition to the above methods, there is also provided herein computer programs comprising computer program code which, when executed in a hub transceiver, a client transceiver, or in a communication system, causes the hub transceiver, the client transceiver, or the communication system, respectively, to execute methods according to the present teaching.

The computer programs, the methods, and the communication system, display advantages corresponding to the advantages already described in relation to the hub and client transceivers. BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features, and advantages of the present disclosure will appear from the following detailed description, wherein some aspects of the disclosure will be described in more detail with reference to the accompanying drawings, in which: Figure 1 is a schematic overview of an optical communication system according to prior art.

Figure 2 is a block diagram illustrating a hub transceiver according to some of the aspects presented herein.

Figures 3a and 3b are block diagrams illustrating receiver architectures according to some of the aspects presented herein. Figure 4 is a block diagram illustrating a client transceiver according to some of the aspects presented herein.

Figures 5a and 5b are block diagrams illustrating client transceivers according to some of the aspects presented herein.

Figure 6 is a block diagram illustrating a communication system according to some of the aspects presented herein.

Figures 7a and 7b illustrate up-link and down-link signaling in the frequency domain according to some of the aspects presented herein.

Figure 8 is a block diagram illustrating a communication system according to some of the aspects presented herein. Figure 9 is a flowchart illustrating methods in a hub transceiver according to some of the aspects presented herein.

Figure 10 is a flowchart illustrating methods in a client transceiver according to some of the aspects presented herein. DETAILED DESCRIPTION

The present teaching relates to a physical layer concept for optical communication between a hub transceiver and one or more client transceivers. The disclosed aspects of the physical layer enable a colorless optical network terminal (ONT) with self-coherent detection as well as coherent up-link detection in a hub transceiver. The concept is particularly useful in distributed antenna systems (DAS) for mobile networks and for optical font hauling.

The hub transceivers disclosed herein include a laser that is modulated by a radio frequency signal but with a significant remaining energy in the laser carrier that is coupled into a bidirectional optical link, such as a wavelength division multiplexing (WDM) transmission system, by means of a branching unit. This branching unit is in one implementation realized by a polarization beam splitter (PBS).

At the client transceiver a fraction of the received light is coupled to an optical detector such as a photo detector with subsequent receiver electronics for receiving down-link data. A remaining fraction of incoming light is passed through an up-link optical modulator connected to a Faraday mirror. The electrical input to the up-link optical modulator consists of an RF modulated signal with different frequency, timing, or code compared to the down-link signal coming from the hub-terminal. Thus, this signal can be modulated onto the signal originating from the hub-transmitter and bringing the signal back to the hub-terminal without being interfered by the down-link data. According to some aspects comprising the PBS, due to the Faraday mirror in the client terminal, the light returning into the PBS will exit the PBS in the orthogonal port compared to the transmitter port. The signal coming from the client terminal is then injected into a coherent receiver where it is mixed with a fraction of the light originating from the transmitter laser. The coherent optical to electrical receiver provides improved sensitivity and linearity compared to a conventional photo-detector and is by the present teaching enabled in a single laser system.

Aspects of the present disclosure will now be described more fully with reference to the accompanying drawings. The apparatus, computer program and methods disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the aspects set forth herein. Like numbers in the drawings refer to like elements throughout.

The terminology used herein is for the purpose of describing particular aspects of the disclosure only, and is not intended to limit the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

Figure 1 is a schematic overview of an optical communication system according to prior art. Figure 1 was discussed in the background section and will not be further discussed here.

Figure 2 is a block diagram illustrating a hub transceiver according to some of the aspects presented herein. In particular, Figure 2 illustrates a hub transceiver (HUB-TRX) 200 for communicating with a client transceiver 400 over a reciprocal bi-directional optical link 205.

Here, reciprocal and bi-directional means that the optical link is configured to pass signals in two directions, i.e., up-link an down-link directions, and that any polarization changes affecting a signal on the down-link also affects a signal on the up-link, although in reverse order. In other words, assume that a signal that is transmitted in a first polarization onto the down-link direction arrives at the other end of the optical link in a second polarization, then a signal transmitted on the up-link in the second polarization will arrive on the other end of the optical link in the first polarization.

An advantage of the present technique is that, due to said reciprocity, the actual optical link over which down-link and up-link propagate need not be polarization maintaining.

The hub transceiver 200 shown in Figure 2 comprises an input/output, I/O, port 210 connectable to the optical link 205, a branching unit 215, an up-link receiver 220, and a downlink transmitter 225.

The down-link transmitter comprises a laser 230 and a modulator 235. The laser 230 is configured to output an optical carrier signal to the modulator 235 and to the up-link receiver (UL-RX) 220 over a polarization maintaining, PM, distribution network 240. This means that the distributed optical carrier signal arrives at the modulator 235 and at the up-link receiver with a polarization that is a deterministic function of the polarization of the generated optical carrier signal. Thus, the polarization of the arriving optical carrier signals at the modulator and at the up-link receiver are not necessarily the same, but deterministically adjustable to be the same or at least substantially the same.

The laser 230 illustrated in Figure 2 is according to some aspects a tunable laser. Light from this potentially tunable laser 230 is injected into the modulator 235 that modulates a radio frequency (RF) signal onto the optical carrier from the laser 230 such that the down-link data intended for a client transceiver at an opposite end of the optical link 205 is contained in optical sidebands with a defined frequency separation as given by the RF carrier frequency. The RF signal is illustrated in Figure 2 as down-link (DL) data, while a corresponding signal received from the client transceiver is illustrated in Figure 2 as up-link (UL) data. It is appreciated that the laser 230 which is illustrated in Figure 2 as being comprised in the DL transmitter (DL-TX) is, according to aspects, not comprised in the DL-TX but implemented as a separate entity, or as comprised in the UL-RX, in which case the distribution network 240 is changed accordingly such that the modulator 235 and the up-link receiver receives the optical carrier signal from the laser 235 over the polarization maintaining distribution network 240. The modulator 235 is arranged to modulate down-link data onto the optical carrier signal and output a down-link signal comprising the optical carrier signal to the branching unit 215 over a first PM transmission line 241. The first PM transmission line 241, similar to the PM distribution network 240, is configured to maintain polarization of signals traversing the transmission line. According to some aspects, the down-link signal comprises a modulated optical carrier signal modulated by the down-link data, and the un-modulated optical carrier signal.

The output of the modulator, and the down-link signal, comprises the optical carrier signal as a component. In cases where there is down-link data to modulate onto the optical carrier signal, the down-link signal comprises several components, i.e., at least the modulated down-link data and the optical carrier signal. In cases where there is no down-link data to transmit, then the down-link signal may consist solely of the optical carrier signal. Of course, the down-link signal, according to some aspects, comprises additional components other than the modulated down-link data and the optical carrier signal. As will be further discussed below in connection to Figure 7, the modulator 235, according to some aspects, modulates the down-link data on both sides of the optical carrier frequency or, according to other aspects, at only one side. The modulation of two sidebands can be accomplished by an optical amplitude modulator or optical phase modulator. A single side band can be obtained by using an optical quadrature modulator. Using a quadrature modulator also allows different information at the two sides of the optical carrier, i.e., the laser carrier, which can be useful to add multiple information channels, e.g., for feeding multiple mobile data channels to the client. As already noted, a requirement on the modulator is that it must also provide substantial energy at a continuous wave (CW) wavelength, i.e., it must pass some fraction of the optical carrier un-modulated but possibly translated in frequency such that the output of the modulator comprises the optical carrier signal. This is, according to some aspects, accomplished by biasing the modulator 235 so that some light from the laser passes through the modulator un-modulated. However, this CW part can also be generated by adding a fixed frequency to the input signal to the modulator 235, which allows for optimization of system linearity as well as frequency translation of the modulated RF down-link data by means of the modulator 235.

Thus, according to aspects, the modulator 235 is arranged to be biased to pass a fraction of the optical carrier signal from the laser 230 un-modulated, and to output said fraction of the optical carrier signal as part of the down-link signal. According to other aspects, the modulator 235 is arranged to add a pre-determined fixed radio frequency (RF) component to the modulator input down-link data, as well as to shift a frequency band of the down-link data by means of said pre-determined fixed radio frequency component.

The branching unit 215 is configured to forward the down-link signal from the modulator 235 to the I/O port 210 and also to forward a received up-link signal from the I/O port 210 to the up-link receiver 220 over a second PM transmission line 242. The up-link receiver 220 is arranged to receive up-link data in the up-link signal by means of the optical carrier signal.

The second PM transmission line 242, similar to the PM distribution network 240, is configured to maintain polarization of signals traversing the transmission line. Thus, the up-link receiver 220 uses the optical carrier signal to receive the up-link data. According to some aspects, this usage comprises de-modulating the received up-link signal by the optical carrier signal to generate an electrical signal. According to some other aspects, the usage comprises using the optical carrier signal as a reference signal when receiving the up- link data, i.e., as a phase reference signal, a frequency reference signal, or a timing reference signal.

Since a coherent up-link receiver needs a local oscillator (LO) signal having a fixed polarization state or the same polarization state as the input signal in order to recover the input signal into an electrical counterpart and this LO is here taken from the transmitter laser 230 over the polarization maintaining (PM) distribution network 240, i.e., the LO is here the optical carrier signal from the laser 230. By utilizing PM fiber, or other means of optical transmission, between the laser 230, branching unit 215, and up-link receiver 220, equal polarization states can be ensured between the LO and the input signal to the up-link receiver 220.

Thus, according to aspects, the up-link receiver 220 is configured to align polarization states of the optical carrier signal and the up-link signal, and to receive the up-link data by combining polarization aligned optical carrier signal and up-link signal.

The branching unit 215 is, according to some aspects, a polarization beam-splitter, PBS, configured to forward optical signals having a first polarization from the modulator 235 to the I/O port 210, and to forward optical signals having a second polarization, orthogonal to the first polarization, from the I/O port to the up-link receiver 220.

The PBS requires a defined polarization state at the input in order to deliver the signal to the appropriate output port. Thus transmission components 240, 241, 242 between laser 230, modulator 235 and branching unit 215, and between the up-link receiver 220 and the branching unit 215 are polarization maintaining (PM). The signal ejected from the branching unit 215 is injected to a bi-directional optical link 205, which according to aspects comprises a WDM transmission system configured for providing a transparent path to the client transceiver.

It is appreciated that even if WDM links are repeatedly mentioned in the present discussion, WDM links are in no way assumed. The present technique offers the same advantages also for single wavelength links but with the requirement of the laser to be suitable for coherent reception in terms of wavelength stability.

An up-link signal returning from the client transceiver propagates back through the optical link 205 to the branching unit 215 in the hub terminal. Due to a Faraday mirror comprised in the client transceiver discussed below in connection to Figure 4 the polarization state of the light returning on the up-link will be orthogonal to the light transmitted on the down-link. In case the branching unit 215 is implemented by a PBS, the up-link light will exit from an orthogonal terminal of the PBS compared to the terminal connected to the modulator 235, and thus be injected into the up-link receiver 220. According to some other aspects, the branching unit 215 is a power splitter configured to forward optical signals from the modulator 235 to the I/O port 210, and to forward optical signals from the I/O port to the up-link receiver 220.

According to such aspects using a branching unit 215 implemented by a power splitter, the PM property of transmission lines is not necessary in order to obtain full functionality of the communication system.

The up-link receiver can be accommodated in a number of ways depending of the spectrum configuration at the input. There exist two general types of coherent receivers, homodyne and heterodyne receivers with implementations discussed below in connection to Figure 3.

Figures 3a and 3b are block diagrams illustrating receiver architectures according to some of the aspects presented herein.

According to some aspects, the up-link receiver 320a is a homodyne receiver comprising a 90 degree hybrid unit 310, wherein the polarization aligned optical carrier signal and up-link signal are arranged to be combined in the 90 degree hybrid unit 310.

According to other aspects, the up-link receiver 320b is a heterodyne receiver comprising a photo-detector 360, wherein the polarization aligned optical carrier signal and up-link signal are arranged to be combined in the photo-detector 360.

According to further aspects, not shown in Figure 3, the up-link receiver is a homodyne or heterodyne receiver comprising a polarization diversity photo-detector, wherein the polarization aligned optical carrier signal is split in to two orthogonal polarization states arranged to be combined with the up-link signal.

In a homodyne coherent receiver, shown in Figure 3a as Homodyne Rx, the LO is combined with the data input by means of a 90° phase shift between the LO and the signal between the two photo-detectors (PD), i.e., the outputs from the two detectors constitute a complex entity describing the full optical field. In order to obtain maximum mixing in the PDs, the LO and the input signal must have the same polarization state which is accomplished by the present disclosure. By detecting the quadrature components of the input signal it is possible to recover the input signal on both side of the LO frequency, i.e., both positive and negative frequencies relative to the LO.

Contrary to the homodyne receiver, the heterodyne coherent receiver, shown in Figure 3b as Heterodyne Rx, only requires one PD and does not require a 90° hybrid at the input and is thus much simpler than the homodyne receiver. The drawback is that signals present on the left and right side of the LO will be added at the electrical output and thus we must require the two side bands to be the same signals or the signal at a certain frequency is present only at either side of the LO. Here we also note that the electrical processor shown in Figures 3a and 3b is according to aspects a digital or an analog processor, or in the simplest case non-existent, i.e. the output from the PD is used as is without processing.

Since the signal from the client transceiver originates from the same laser as the LO, frequency and phase mismatch are small giving very low phase noise and frequency error in the detected signal after the coherent receiver. In reality there will be a significant time delay between the signal travelling to the client and back compared to the LO and this relative delay may add phase and frequency error to the detected signal depending on the coherence length of the laser used. However, this can to some extent be circumvented by adding some delay to the light path between the laser and the coherent receiver e.g. by adding some length of optical fiber.

Figure 4 is a block diagram illustrating a client transceiver according to some of the aspects presented herein. In particular, there is illustrated a client transceiver (CLIENT TRX) for communicating with a hub transceiver 200 over a reciprocal bi-directional optical link 205. The client transceiver 400 comprises an input/output, I/O, port 455 connectable to the optical link 205, a branching unit 460, a down-link receiver (DL-RX) 465, and an up-link transmitter 470. The branching unit 460 is arranged to forward a received down-link signal comprising an optical carrier signal from the I/O port 455 to the down-link receiver 465 as well as to the up- link transmitter (UL-TX) 470, and to forward an up-link signal from the up-link transmitter 470 to the I/O port 455. The down-link receiver 465 is arranged to receive down-link data in the down-link signal. The up-link transmitter 470 comprises a modulator 475 and a Faraday mirror (FM) 480. The modulator 475 is arranged to modulate up-link data onto the optical carrier signal and output the up-link signal. The Faraday mirror 480 is arranged to convert the polarization of the up-link signal of the up-link transmitter 470 into a polarization orthogonal to that of the received optical carrier signal.

Thus, a signal received on the I/O port 455 of the client transceiver 400 is split into two paths 456, 457, as indicated in Figure 4. The first output signal is received by the down-link receiver 465, which recovers down-link data transmitted from the hub transceiver. According to some aspects, this down-link receiver 465 converts the received optical signal to an electrical signal by means of a single photo-detector which then outputs the same down-link data signal as was injected to the modulator 235 at the hub transceiver 200. Here we note that if the data signal is present at both side of the CW data with equal frequency distance to a CW light, the down-link data signal will be more distorted by chromatic dispersion (CD) in the transmission fiber 205 compared to if only one sideband was present at each side with equal separation from the CW light, i.e., from the optical carrier signal. The detection process in the down-link receiver is sometimes called self-heterodyne reception since the incoming data signal is mixed with a signal that also came from the transmitter.

The signal in the lower branch 457 of the branching unit 460 is injected into an optical modulator 475 that is connected to a Faraday mirror 480 which alters the polarization of the light.

Figures 5a and 5b are block diagrams illustrating client transceivers according to some of the aspects presented herein. According to aspects, the branching unit 560a is a power splitter configured to split a signal received from the I/O port 455 into first and second parts, and to forward the first part to the down-link receiver 465, and to forward the second part to the up-link transmitter 570a. The power splitter is further configured to combine signals received from the down-link receiver 465 and from the up-link transmitter 570a into a combined signal, and to output the combined signal to the I/O port 455.

In Figure 5a, aspects where the Faraday mirror (FM) 580a reflects the signal back but with a 90° polarization rotation into the optical modulator 575a are illustrated. Preferably the FM should be in close vicinity of the modulator since the electrical signal injected into the modulator will be modulated twice onto the optical signal, the distance between modulator and mirror is in Figure 5a illustrated as distance D.

Thus, according to some aspects, the modulator 575a and the Faraday mirror 580a are arranged at a distance D from each other, the distance D being determined as a fraction of an information-symbol duration of the up-link data or based on a bandwidth of a frequency band comprising the up-link data. The referred to information-symbol duration is according to some aspects defined as the inverse of a frequency bandwidth of the UL data.

Here we note that the use of a FM also allows the use of a polarization dependent modulator, i.e., the amount of modulation is dependent on the polarization state of the incoming optical signal. Since most low-cost optical modulators are heavily polarization dependent this feature enables the use of such modulator in the colorless client terminal.

Consequently, according to aspects, the modulator 575a is arranged to receive the optical carrier signal, and to modulate up-link data onto the optical carrier signal and to output a modulated optical carrier signal to the Faraday mirror 580a. The Faraday mirror 580a is arranged to reflect the modulated optical carrier signal back to the modulator unit 575a with a polarization orthogonal to that of the received optical carrier signal. The modulator unit 575a is also arranged to modulate the up-link data again onto the reflected modulated optical carrier signal and to output an up-link signal to the I/O port 455 via the branching unit 460a having polarization orthogonal to that of the received optical carrier signal. The implementation of the client transceiver 550a in Figure 5a is simple and robust but a signal reflected in the FM and returned to the optical link 205 suffers the splitting loss two times, which may limit the transmission reach.

In Figure 5b, aspects of the client transceiver 550b where the signal is passed through an optical circulator 561 followed by a power splitter 562 are illustrated. The signal in the lower branch 563 of the splitter 562 now passes through a Faraday mirror 580b, here implementing a Faraday rotator, followed by an optical modulator 575b and finally returned into the optical link 205 by means of the circulator 561. The advantage is that a higher power level can be obtained in the up-link since the splitter loss only occurs once. Potential issues with aspects illustrated in Figure 5b are that the optical path between the circulator ports must be stable and correctly designed in order to ensure correct relation between input and output polarization state. However, from a cost perspective it is a nyway preferred to build the optics in the client terminal using integrated photonics technology.

In other words, according to some aspects, the branching unit 560b comprises a circulator 561 connected to a power splitter 562. The circulator 561 is arranged to forward the down-link signal from the I/O port 455 to the power splitter 562. The power splitter 562 is arranged to forward the down-link signal to the down-link receiver 465 and to the up-link transmitter 570b. The circulator 561 is also arranged to forward the up-link signal from the up-link transmitter 570b to the I/O port 455. According to aspects, the Faraday mirror 580b is arranged to receive the down-link signal comprising the optical carrier signal from the power splitter 562. The Faraday mirror 580b is arranged to reflect the optical carrier signal to the modulator unit 575a with polarization orthogonal to that of the received optical carrier signal. The modulator unit 475b is arranged to modulate up-link data onto the reflected optical carrier signal and to output an up-link signal to the I/O port 455, via the circulator unit 461, having polarization orthogonal to that of the received optical carrier signal.

An advantage of the disclosed client transceiver is that it can be realized as a colorless client terminal, i.e., the client transceiver is according to aspects wavelength channel agnostic, which is of importance for low-cost deployment. Thus, according to some aspects, the client transceiver is a colorless optical network terminal, ONT, in that the client transceiver constitutes a collection of wavelength independent hardware components.

Furthermore, the client transceiver allows for reuse of the hub transceiver transmitter laser in the hub transceiver up-link receiver, which is enabled by the use of a Faraday mirror in the client transceiver.

Also, the possibility to convert wavelength multiplexing in the client domain into multichannel radio frequency (RF) in the hub side is advantageous in that it reduces hardware cost.

The electrical input up-link data signal to the optical modulator 475 is according to aspects one or more data channels modulated onto electrical carriers. According to some aspects, these are located at different modulation frequencies than in the down link but arranged not to be distorted by interference from the down link channels, or contained in different time slots, or spread by different spreading codes. In the case of mobile data channels this is usually not a big difficulty since the down-link and up-link channels have different carrier frequencies but still in close vicinity.

Thus, according to some aspects, the modulated up-link data is modulated onto the optical carrier in an up-link frequency band separated from a down-link frequency band of the modulated down-link data.

According to some other aspects, the modulated up-link data is modulated onto the optical carrier in one or more up-link time slots separated from one or more down-link time slots of the modulated down-link data.

According to further aspects, the modulated up-link data is spread over frequency by one or more up-link spreading codes prior to being modulated onto the optical carrier, the up-link spreading codes being different from one or more down-link spreading codes of the modulated down-link data.

Figure 6 is a block diagram illustrating a communication system 600 according to some of the aspects presented herein. The communication system 600 comprises a hub transceiver 200 connected to a client transceiver according to the above discussion via the reciprocal bidirectional optical link 205. Figure 7 illustrates up-link and down-link signaling in the frequency domain according to some of the aspects presented herein.

Figure 7 exemplifies two schematic frequency plans with down-link and up-link interference indicated by capital I in Figures 7a and 7b. Figure 7a shows an example where a single input optical modulator is used, i.e. a double side band signal is generated. The separation between the laser frequency fL and the two symmetric signal spectrums around the laser frequency is determined by the RF carrier frequency of the input signal. In order for the up-link data modulation process to work, sufficient power must be present in the CW carrier, here generated by the transmitter laser in the hub terminal. When the client terminal modulates another RF frequency onto the incoming signal, the desired spectrum components are denoted 'main up-link'. However, the up-link data is also modulated onto the down-link data spectrum components and thus mixing products between up-link and down-link data are obtain as marked in Figure 7a. However, in a frequency separated system, as long as the interference components do not overlap with the desired up-link data spectrum, the system interference will be low and in most mobile system the down-link frequency is different from up-link frequency anyway. Figure 7b exemplifies the spectrum when an optical quadrature modulator is used in the hub-terminal. Now the down-link data is only present on one side if the laser frequency and when the client modulates up-link data onto such spectrum the right hand side spectrum is clean and therefore interference components only occur on the left side of the laser frequency. If a homodyne coherent receiver is used in the hub-terminal the frequencies on the right side of the laser carrier can be isolated and no subsequent filtering is needed, i.e. the left hand spectrum is discarded in the coherent receiver.

Since optical systems often offer very large bandwidths, often many GHz and currently up to 100 GHz, many antenna nodes (client terminals) can be supported by a single hub-terminal as shown in Figure 8 discussed below. The idea here is that the optical transmission link has a frequency plan that not necessarily matches the frequencies of the radio system. In some cases all antennas will operate on the same frequencies and then each antenna will be addressed by a unique set of frequencies (up-link and down-link) in the optical link. In order to support this, frequency conversion modules 876, 877 in the client-terminal is required. If necessary frequency conversion can of course also be implemented in the hub-terminal. Figure 8 is a block diagram illustrating a communication system according to some of the aspects presented herein. In particular, Figure 8 shows a communication system 800 such as the communication system illustrated in Figure 6, wherein the modulator 835 comprised in the hub transceiver 200 is arranged to be fed by a plurality of multiplexed radio frequency down-link channels, and wherein the up-link receiver 820 is arranged to receive a plurality of up-link radio frequency channels. The reciprocal bi-directional optical link 805 further comprises a splitter 806 configured to split the down-link signal between a plurality of client transceivers 400, 550a, 550b according to the above discussion. Each of the client transceivers is configured to modulate up-link data onto a respective up-link signal at a pre-determined frequency shift arranged to be applied by a down-link frequency conversion unit. The downlink receiver comprised in the client transceiver comprises an up-link frequency conversion unit arranged to select a pre-determined frequency band of the down-link signal.

Figure 9 is a flowchart illustrating methods in a hub transceiver according to some of the aspects presented herein, at least some of which methods are implementable by the hub transceivers, the client transceivers, and the communication systems discussed above.

Figure 9 illustrates a method in a hub transceiver 200 for communicating with a client transceiver 400 over a reciprocal bi-directional optical link 205. The method comprises generating an optical carrier signal, and distributing SH3 the optical carrier signal to a modulator 235 and to an up-link receiver 220 of the hub transceiver 400 over a polarization maintaining, PM, distribution network 240. The method also comprises generating SH5, by the modulator 235, a down-link signal comprising the optical carrier signal, outputting SH7 the down-link signal to the client transceiver 400 on the bi-directional optical link 205 via a first PM transmission line 241, and receiving SH9 an up-link signal from the client transceiver 400 on the bi-directional optical link 205 via a second PM transmission line 242, as well as receiving SH11, by the up-link receiver 220, up-link data in the up-link signal by means of the optical carrier signal.

There is also disclosed herein a computer program comprising computer program code which, when executed in a hub transceiver 200, causes the hub transceiver to execute a method according to the above. Figure 10 is a flowchart illustrating methods in a client transceiver according to some of the aspects presented herein.

Figure 10 illustrates a method in a client transceiver 400 for communicating with a hub transceiver 200 over a reciprocal bi-directional optical link 205 comprising receiving SCI a down-link signal comprising an optical carrier signal from the hub transceiver 200 on the bidirectional optical link 205, receiving SC3 down-link data in the down-link signal, modulating SC5 up-link data onto the optical carrier signal to generate an up-link signal, and converting SC7, by a Faraday mirror 480, a polarization of the up-link signal into a polarization orthogonal to that of the received optical carrier signal, as well as outputting SC9 the polarization converted up-link signal to the hub transceiver 200 on the bi-directional optical link 205.

There is also disclosed herein a computer program comprising computer program code which, when executed in a client transceiver 400, causes the client transceiver to execute a method according to the above.

The various aspects of the methods described herein are described in the general context of method steps or processes, which may be implemented in one aspect by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers in networked environments. A computer-readable medium may include removable and non-removable storage devices including, but not limited to, Read Only Memory (ROM), Random Access Memory (RAM), compact discs (CDs), digital versatile discs (DVD), etc. Generally, program modules may include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. Computer-executable instructions, associated data structures, and program modules represent examples of program code for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps or processes.

Claims

1. A hub transceiver (200) for communicating with a client transceiver (400) over a reciprocal bi-directional optical link (205), the hub transceiver (200) comprising an input/output, I/O, port (210) connectable to the optical link (205), a branching unit (215), an up-link receiver (220), and a down-link transmitter (225) comprising a laser (230) and a modulator (235), the laser (230) being configured to output an optical carrier signal to the modulator (235) and to the up-link receiver (220) over a polarization maintaining, PM, distribution network (240), the modulator (235) being arranged to modulate downlink data onto the optical carrier signal and output a down-link signal comprising the optical carrier signal to the branching unit (215) over a first PM transmission line (241), the branching unit (215) being configured to forward the down-link signal from the modulator (235) to the I/O port (210) and also to forward a received up-link signal from the I/O port (210) to the up-link receiver (220) over a second PM transmission line (242), the up-link receiver (220) being arranged to receive up-link data in the up-link signal by means of the optical carrier signal.

2. The hub transceiver (200) according to claim 1, wherein the down-link signal comprises a modulated optical carrier signal modulated by the down-link data, and the un-modulated optical carrier signal.

3. The hub transceiver (200) according to claim 1 or 2, wherein the branching unit (215) is a polarization beam-splitter, PBS, configured to forward optical signals having a first polarization from the modulator (235) to the I/O port (210), and to forward optical signals having a second polarization, orthogonal to the first polarization, from the I/O port to the up-link receiver (220).

4. The hub transceiver (200) according to claim 1 or 2, wherein the branching unit (215) is a power splitter configured to forward optical signals from the modulator (235) to the I/O port (210), and to forward optical signals from the I/O port to the up-link receiver (220).

5. The hub transceiver (200) according to any of claims 1-4, wherein the up-link receiver (220) is configured to align polarization states of the optical carrier signal and the up-link signal, and to receive the up-link data by combining polarization aligned optical carrier signal and up-link signal.

The hub transceiver (200) according to claim 5, wherein the up-link receiver (320a) is a homodyne receiver comprising a 90 degree hybrid unit (310), wherein the polarization aligned optical carrier signal and up-link signal are arranged to be combined in the 90 degree hybrid unit (310).

The hub transceiver (200) according to claim 5, wherein the up-link receiver (320b) is a heterodyne receiver comprising a photo-detector (360), wherein the polarization aligned optical carrier signal and up-link signal are arranged to be combined in the photo- detector (360).

The hub transceiver (200) according to any of claims 1-7, wherein the modulator (235) is arranged to be biased to pass a fraction of the optical carrier signal from the laser (230) un-modulated, and to output said fraction of the optical carrier signal as part of the down-link signal.

The hub transceiver (200) according to any of claims 1-7, wherein the modulator (235) is arranged to add a pre-determined fixed radio frequency component to the modulator input down-link data, as well as to shift a frequency band of the down-link data by means of said pre-determined fixed radio frequency component.

A client transceiver (400) for communicating with a hub transceiver (200) over a reciprocal bi-directional optical link (205), the client transceiver (400) comprising an input/output, I/O, port (455) connectable to the optical link (205), a branching unit (460), a down-link receiver (465), and an up-link transmitter (470), the branching unit (460) being arranged to forward a received down-link signal comprising an optical carrier signal from the I/O port (455) to the down-link receiver (465) as well as to the up-link transmitter (470), and to forward an up-link signal from the up-link transmitter (470) to the I/O port (455), the down-link receiver (465) being arranged to receive down-link data in the down-link signal, the up-link transmitter (470) comprising a modulator (475) and a Faraday mirror (480), the modulator (475) being arranged to modulate up-link data onto the optical carrier signal and output the up-link signal, the Faraday mirror (480) being arranged to convert the polarization of the up-link signal of the up-link transmitter (470) into a polarization orthogonal to that of the received optical carrier signal.

The client transceiver (550a) according to claim 10, wherein the branching unit (560a) is a power splitter configured to split a signal received from the I/O port (455) into first and second parts, and to forward the first part to the down-link receiver (465), and to forward the second part to the up-link transmitter (570a), the power splitter further being configured to combine signals received from the down-link receiver (465) and from the up-link transmitter (570a) into a combined signal, and to output the combined signal to the I/O port (455).

The client transceiver (550a) according to claim 11, wherein the modulator (575a) is arranged to receive the optical carrier signal, and to modulate up-link data onto the optical carrier signal and to output a modulated optical carrier signal to the Faraday mirror (580a), the Faraday mirror (580a) being arranged to reflect the modulated optical carrier signal back to the modulator unit (575a) with a polarization orthogonal to that of the received optical carrier signal, the modulator unit (575a) further being arranged to modulate the up-link data again onto the reflected modulated optical carrier signal and to output an up-link signal to the I/O port (455) via the branching unit (460a) having polarization orthogonal to that of the received optical carrier signal.

The client transceiver (550a) according to claim 12, wherein the modulator (575a) and the Faraday mirror (580a) are arranged at a distance D from each other, the distance D being determined as a fraction of an information symbol duration of the up-link data or based on a bandwidth of a frequency band comprising the up-link data.

The client transceiver (550b) according to claim 10, wherein the branching unit (560b) comprises a circulator (561) connected to a power splitter (562), the circulator (561) being arranged to forward the down-link signal from the I/O port (455) to the power splitter (562), the power splitter (562) being arranged to forward the down-link signal to the down-link receiver (465) and to the up-link transmitter (570b), the circulator (561) further being arranged to forward the up-link signal from the up-link transmitter (570b) to the I/O port (455).

15. The client transceiver (550b) according to claim 14, the Faraday mirror (580b) being arranged to receive the down-link signal comprising the optical carrier signal from the power splitter (562), the Faraday mirror (580b) further being arranged to reflect the optical carrier signal to the modulator unit (575a) with polarization orthogonal to that of the received optical carrier signal, the modulator unit (475b) being arranged to modulate up-link data onto the reflected optical carrier signal and to output an up-link signal to the I/O port (455), via the circulator unit (461), having polarization orthogonal to that of the received optical carrier signal.

16. The client transceiver (400, 550a, 550b) according to any of claims 10-15, wherein the client transceiver is a colorless optical network terminal, ONT, in that the client transceiver constitutes a collection of wavelength independent hardware components.

17. The client transceiver (400, 550a, 550b) according to any of claims 10-16, wherein the modulated up-link data is modulated onto the optical carrier in an up-link frequency band separated from a down-link frequency band of the modulated down-link data.

18. The client transceiver (400, 550a, 550b) according to any of claims 10-16, wherein the modulated up-link data is modulated onto the optical carrier in one or more up-link time slots separated from one or more down-link time slots of the modulated down-link data.

19. The client transceiver (400, 550a, 550b) according to any of claims 10-16, wherein the modulated up-link data is spread over frequency by one or more up-link spreading codes prior to being modulated onto the optical carrier, the up-link spreading codes being different from one or more down-link spreading codes of the modulated down-link data.

20. A communication system (600) comprising a hub transceiver (200) according to any of claims 1-9 connected to a client transceiver (400, 550a, 550b) according to any of claims 10-19 via a reciprocal bi-directional optical link (205).

21. The communication system (800) according to claim 20, wherein the modulator (835) comprised in the hub transceiver (200) is arranged to be fed by a plurality of multiplexed radio frequency down-link channels, and wherein the up-link receiver (820) is arranged to receive a plurality of up-link radio frequency channels, the reciprocal bi-directional optical link (805) further comprising a splitter (806) configured to split the down-link signal between a plurality of client transceivers (400, 550a, 550b) according to any of claims 9-18, each of the client transceivers being configured to modulate up-link data onto a respective up-link signal at a pre-determined frequency shift arranged to be applied by a down-link frequency conversion unit (876), the down-link receiver (865) comprised in the client transceiver comprising an up-link frequency conversion unit (877) arranged to select a pre-determined frequency band of the down-link signal.

A method in a hub transceiver (200) for communicating with a client transceiver (400) over a reciprocal bi-directional optical link (205), comprising;

- generating (SHI) an optical carrier signal;

- distributing (SH3) the optical carrier signal to a modulator (235) and to an uplink receiver (220) over a polarization maintaining, PM, distribution network (240);

- generating (SH5), by the modulator (235), a down-link signal comprising the optical carrier signal;

- outputting (SH7) the down-link signal on the optical link (205) to the client transceiver (400) via a first PM transmission line (241);

- receiving (SH9) an up-link signal on the optical link (205) from the client transceiver (400) via a second PM transmission line (242); and

- receiving (SH11), by the up-link receiver (220), up-link data in the up-link signal by means of the optical carrier signal.

A computer program comprising computer program code which, when executed in a hub transceiver (200), causes the hub transceiver to execute a method according to claim 22.

A method in a client transceiver (400) for communicating with a hub transceiver (200) over a reciprocal bi-directional optical link (205) comprising;

- receiving (SCI) a down-link signal comprising an optical carrier signal from the hub transceiver (200) on the bi-directional optical link (205);

- receiving (SC3) down-link data in the down-link signal; - modulating (SC5) up-link data onto the optical carrier signal to generate an uplink signal;

- converting (SC7), by a Faraday mirror (480), a polarization of the up-link signal into a polarization orthogonal to that of the received optical carrier signal; and

- outputting (SC9) the polarization converted up-link signal to the hub transceiver (200) on the bi-directional optical link (205).

A computer program comprising computer program code which, when executed in a client transceiver (400), causes the client transceiver to execute a method according to claim 24.