WO2018133932A1

WO2018133932A1 - Node for a fronthaul network and monitoring of optical trasceivers in fronthaul networks

Info

Publication number: WO2018133932A1
Application number: PCT/EP2017/050999
Authority: WO
Inventors: Roberto Magri; Alberto Deho
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2017-01-18
Filing date: 2017-01-18
Publication date: 2018-07-26

Abstract

A node (30) for use at a main site of a fronthaul network. The node comprises: an optical splitter (32) configured to receive a radio transport optical signal (34) carrying an out-of-band electrical signal comprising an indication of diagnostic monitoring information, DMI, of an optical transceiver, the optical splitter is configured to power split the radio transport optical signal to form a monitoring portion; an optical detector (16) configured to receive the monitoring portion of the radio transport optical signal and configured to convert the monitoring portion of the radio transport optical signal into an electrical signal; diagnostic monitoring interface receiver apparatus (36) configured to extract the out-of-band electrical signal from the electrical signal and configured to obtain the diagnostic monitoring information from the extracted out-of-band electrical signal; and a controller configured to collect the diagnostic monitoring information and configured to transmit the diagnostic monitoring information to a fronthaul network management system.

Description

NODE FOR A FRONTHAUL NETWORK AND MONITORING OF OPTICAL TRASCEIVERS IN FRONTHAUL NETWORKS

Technical Field

The invention relates to a node for use at a main site of a fronthaul network and to an optical transceiver. The invention also relates to methods of receiving and transmitting diagnostic monitoring information in a fronthaul network.

Background

Fronthaul networks are used to transport radio transport signals, such as common public radio interface, CPRI , between remote radio units, RRU, and centralized main units in centralized radio access networks, RAN . A typical topology used in such networks is the 'tree topology' or 'Hub-&-Spoke' which are characterized by a fibre section between a main site and a remote 'hub' or 'splitter' site from which several fibre branches connect to the RRUs.

Since the fronthaul transport network carries the CPRI signals transparently, additional Optical Supervisory services are necessary to provide for operations, administration and management, OA&M, functionality. If fronthaul is implemented with an active solution using transponder units on main and remote sites then the transponder equipment can manage the OA&M and monitoring functions by means of an optical supervisory channel, OSC, or other equivalent supervisory services, and Digital Diagnostic Monitoring Interface, DDMI, information management on the transponders.

DDMI is used to retrieve information stored in the EEPROM memory of a transceiver, giving various information on the transceiver status, such as calibration, alarms, temperature, input and output power, etc, as defined in Storage Networking Industry Association, SNIA, standard SFF-8472 Rev 12.2. This information is useful for monitoring the transport system and performing diagnostic and troubleshooting operations. DDMI in combination with other analogue monitoring features, such as optical channel monitoring, OCM, optical time domain reflectometry, OTDR, and optical link power monitoring, may provide a complete monitoring and diagnostic subsystem. DDMI information can be easily extracted by the fronthaul transponder units and sent to a network management system; at the remote site, DDMI information is locally extracted from transceiver EEPROMs and communicated to the main site by means of an OSC, for example.

In passive fronthaul, the wavelength division multiplexed, WDM, transport network infrastructure consists only of passive optical components such as multiplexer/demultiplexers, splitters and filters. The CPRI signal is transmitted and received by fixed wavelength transceivers equipped directly on the radio systems both at remote sites, RRUs, and main sites, baseband units, BBU. In order to add monitoring and diagnostic functionalities to a passive fronthaul network the concept of 'semi-passive fronthaul' has been introduced in which the passive infrastructure is paired with an active monitoring subsystem at the main site. These active monitoring subsystems typically implement one or more of total uplink power monitoring, OTDR (to check for fibre faults) and OCM of received upstream optical signal powers. These functionalities provide some useful information to monitor fibre integrity and optical signal power levels, but for a complete diagnostic and monitoring picture they should be integrated with DDMI , to provide information on transceiver status. Unfortunately, in this scenario the DDMI information can only be extracted at radio equipment which usually belongs to a separate management system domain and radio systems may not be designed to access, manage and report DDMI information.

Another proposed option is known as 'hybrid fronthaul', reported at page 5 'Active wavelength monitoring' of Transmode/lnfinera 'Application Note Mobile Fronthaul G-1 ' where the remote site is made passive while the main site retains active transponder units to allow for DDMI extraction. The remote site's DDMI information is accessed by exploiting the 'remote DDMI' approach reported in US 2010/0054733 A1 . This technique provides DDMI remote functionalities by adding an out of band , OOB, over-modulated digital tone on the CPRI signal. This additional signal is typically a low rate digital signal (tenths of Kb/s) independent from the CPRI protocol and so can be easily detected by a low rate cheap transceiver. DDMI information is transported remotely via this over-modulated tone on the CPRI signal.

Summary

It is an object to provide an improved node for use at a main site of a fronthaul network. It is a further object to provide an improved optical transceiver. It is a further object to provide an improved method of receiving diagnostic monitoring information in a fronthaul network. It is a further object to provide an improved method of transmitting diagnostic monitoring information in a fronthaul network.

An aspect of the invention provides a node for use at a main site of a fronthaul network. The node comprises an optical splitter, an optical detector, diagnostic monitoring interface receiver apparatus and a controller. The optical splitter is configured to receive a radio transport optical signal that carries an out-of-band electrical signal comprising an indication of diagnostic monitoring information of an optical transceiver. The optical splitter is configured to power split the radio transport optical signal to form a monitoring portion. The optical detector is configured to receive the monitoring portion of the radio transport optical signal and is configured to convert the monitoring portion of the radio transport optical signal into an electrical signal. The diagnostic monitoring interface receiver apparatus is configured to extract the out-of-band electrical signal from the electrical signal and is configured to obtain the diagnostic monitoring information from the extracted out-of-band electrical signal. The controller is configured to collect the diagnostic monitoring information and is configured to transmit the diagnostic monitoring information to a fronthaul network management system. The node may enable a fronthaul network to have a passive infrastructure and provide diagnostic monitoring information to the NMS without requiring use of transponders. The node is able to manage diagnostic monitoring information in the fronthaul transport domain independently from radio systems, decoupling the radio systems from the fronthaul infrastructure from a monitoring and diagnostic point of view. The node may therefore enable a fronthaul network to have its own self-contained monitoring sub-system including transceivers diagnostic monitoring information made available to the network management system , NMS.

In an embodiment, the optical splitter is configured to receive a wavelength division multiplexed, WDM, radio transport optical signal comprising a plurality of said radio transport optical signals. The plurality of radio transport optical signals have a plurality of wavelengths and the respective out-of-band electrical signals have respective frequencies. The out-of- band electrical signals are frequency division multiplexed. The diagnostic monitoring interface receiver apparatus is configured to extract the out-of-band electrical signals from the electrical signal and is configured to obtain respective diagnostic monitoring information from the extracted out-of-band electrical signals. This may enable each transceiver connected to the node to use a different out-of-band signal frequency according to its wavelength. Use of frequency division multiplexed, FDM, out-of-band electrical signals enables use of a single optical splitter and a single optical detector to detect the FDM out-of-band multi tone electrical signal.

In an embodiment, the WDM radio transport optical signal comprises uplink radio transport optical signals from transceivers at respective radio antenna modules. This may enable the optical detector to perform optical channel monitoring of the uplink optical power in addition to power splitting the uplink WDM signal to collect diagnostic monitoring information.

In an embodiment, the WDM radio transport optical signal comprises downlink radio transport optical signals from transceivers at respective baseband processing modules. This may relax the receiver sensitivity required at the optical detector since the downlink WDM signal is detected close to the main site and will therefore have a higher optical power than an uplink optical signal arriving at the node.

In an embodiment, the optical splitter is configured to receive a first wavelength division multiplexed, WDM, radio transport optical signal comprising a plurality of said radio transport optical signals from respective optical transceivers at respective radio antenna modules. The optical splitter is additionally configured to receive a second wavelength division multiplexed, WDM, radio transport optical signal comprising a plurality of said radio transport optical signals from respective optical transceivers at respective baseband processing modules. This may enable the node to obtain diagnostic monitoring information both from transceivers at radio antenna modules, at a remote site, and from transceivers at baseband processing modules, at the main site. In an embodiment, the node comprises a plurality of optical splitters and a plurality of optical detectors. The plurality of optical splitters are configured to receive a plurality of radio transport optical signals; each radio transport optical signal carrying a respective said out-of- band electrical signal. The plurality of optical detectors are configured to receive the monitoring portions of the radio transport optical signals and are configured to convert the monitoring portions of the radio transport optical signals into respective electrical signals. The diagnostic monitoring interface receiver apparatus is configured to extract respective out-of- band electrical signals from the electrical signals and is configured to obtain respective diagnostic monitoring information from the extracted out-of-band electrical signals. This may enable the node to obtain diagnostic monitoring information from a plurality of transceivers all using the same out-of-band signal frequency.

In an embodiment, the radio transport optical signals are uplink radio transport optical signals from transceivers at respective radio antenna modules. This may enable the optical detector to perform optical channel monitoring of the uplink optical powers in addition to power splitting the uplink signals to collect diagnostic monitoring information.

In an embodiment, the radio transport optical signals are downlink radio transport optical signals from transceivers at respective baseband processing modules. This may relax the receiver sensitivity required at the optical detectors since the downlink signals are detected close to the main site and will therefore have a higher optical power than uplink optical signals arriving at the node.

In an embodiment, a plurality of radio transport optical signals are received from respective optical transceivers at respective radio antenna modules and a plurality of radio transport optical signals are received from respective optical transceivers at respective baseband processing modules. The out-of-band electrical signals comprise an indication of diagnostic monitoring information of the respective optical transceiver. This may enable the node to obtain diagnostic monitoring information both from transceivers at radio antenna modules, at a remote site, and from transceivers at baseband processing modules, at the main site.

In an embodiment, a said radio transport optical signal is received from one of a pair of optical transceivers, one optical transceiver of the pair being at a radio antenna module and the other optical transceiver of the pair being at a baseband processing module. The out-of- band electrical signal comprises an indication of diagnostic monitoring information of each optical transceiver of the pair. This may enable the node to obtain diagnostic monitoring information from a transceiver at radio antenna modules, at a remote site, and from a corresponding transceiver at baseband processing modules, at the main site from a single out-of-band electrical signal; only one transmission direction need be monitored in order to obtain the diagnostic monitoring information for transceivers at both a remote site and a main site in a fronthaul network. In an embodiment, a said radio antenna module is one of a remote radio unit, RRU, a remote radio head , RRH, and radio equipment, RE, and a said baseband processing module is one of a baseband unit, BBU, and a radio equipment controller, REC. The node may be used in any fronthaul network in which baseband processing is separated from the radio antennas.

In an embodiment, a said radio transport optical signal is one of a common public radio interface signal and a low latency packet interface signal.

In an embodiment, a said out-of-band electrical signal is a digital diagnostic monitoring interface, DDMI , signal carrying DDMI information. The DDMI information may comply with the SNIA standard SFF-8472. The node may enable DDMI information to be provided in a passive fronthaul network.

In an embodiment, the out-of-band electrical signal is an out of band , OOB, over- modulated digital tone carried on the radio transport optical signal according to the remote DDMI method described in US 2010/0054733 A1 . The node may enable remote DDMI information to be provided in a passive fronthaul network.

In an embodiment, the diagnostic monitoring interface receiver apparatus comprises at least one digital diagnostic monitoring interface, DDMI, receiver and the diagnostic monitoring information is DDMI information according to the SN IA standard SFF-8472.

In an embodiment, the at least one DDMI receiver is configured to filter the electrical signal to extract the out-of-band electrical signal.

In an embodiment, where the out-of-band electrical signals are frequency division multiplexed, the at least one DDMI receiver comprises a sweeping digital filter configured to extract each respective out-of-band electrical signal. A single optical detector can detect the out-of-band multi tone electrical signal. Then a sweeping digital filter can scan all the tone frequencies and detect each wavelength DDMI data.

In an embodiment, the fronthaul network is a passive fronthaul network. The node may therefore enable a fronthaul network to have a passive infrastructure and provide diagnostic monitoring information to the NMS without requiring use of transponders.

The controller could be implemented as one or more processors, hardware, processing hardware or circuitry.

Corresponding embodiments are also applicable to the optical transceiver and the methods described below.

An aspect of the invention provides an optical transceiver comprising an optical receiver, out-of-band signal processing apparatus, a controller and an optical transmitter. The optical receiver is configured to receive a first radio transport optical signal carrying a first out- of-band electrical signal comprising an indication of diagnostic monitoring information of a remote optical transceiver. The optical receiver is configured to convert the first radio transport optical signal into a received electrical signal. The out-of-band signal processing apparatus is configured to extract the out-of-band electrical signal from the received electrical signal and is configured to obtain the diagnostic monitoring information of the remote optical transceiver from the extracted out-of-band electrical signal. The controller is configured to receive the diagnostic monitoring information of the remote optical transceiver and is configured to obtain diagnostic monitoring information of the optical transceiver. The optical transmitter is configured to generate a second radio transport optical signal carrying a second out-of-band electrical signal. The out-of-band signal processing apparatus is configured to generate the second out-of-band electrical signal comprising an indication of the diagnostic monitoring information of the remote optical transceiver and an indication of the diagnostic monitoring information of the optical transceiver.

The second out-of-band electrical signal therefore carries both local and remote diagnostic monitoring information. This means that diagnostic monitoring information for transceivers at both ends of an optical link can be obtained by monitoring the radio transport optical signal in only one direction of transmission.

In an embodiment, the controller comprises a storage medium configured with a remote diagnostic monitoring information table and a local diagnostic monitoring information table and wherein the controller is configured to store the diagnostic monitoring information of the remote optical transceiver in the remote diagnostic monitoring information table and is configured to retrieve diagnostic monitoring information of the optical transceiver stored in the local diagnostic monitoring information table. The transceiver therefore doubles the diagnostic monitoring information table as compared to the data table at remote transceiver in the 'remote DDMI' method described in US 2010/0054733 A1 .

An aspect of the invention provides a method of receiving diagnostic monitoring information in a fronthaul network. The method comprises receiving a radio transport optical signal carrying an out-of-band electrical signal comprising an indication of diagnostic monitoring information of an optical transceiver. The radio transport optical signal is power split to form a monitoring portion. The monitoring portion of the radio transport optical signal is converted into an electrical signal. The out-of-band electrical signal is extracted from the electrical signal and the diagnostic monitoring information is obtained from the extracted out- of-band electrical signal. A reporting signal comprising an indication of the diagnostic monitoring information is transmitted to a network management system.

The method may enable a fronthaul network to have a passive infrastructure and provide diagnostic monitoring information to the NMS without requiring use of transponders. The method enables management of diagnostic monitoring information in the fronthaul transport domain independently from radio systems, decoupling the radio systems from the fronthaul infrastructure from a monitoring and diagnostic point of view. The method may therefore enable a fronthaul network to have its own self-contained monitoring sub-system including transceivers diagnostic monitoring information made available to the network management system, NMS. An aspect of the invention provides a method of transmitting diagnostic monitoring information in a fronthaul network. The method is performed at a first optical transceiver in the fronthaul network. The method comprises receiving a first radio transport optical signal carrying a first out-of-band electrical signal comprising an indication of diagnostic monitoring information of a second optical transceiver in the fronthaul network. The first radio transport optical signal is converted into a received electrical signal. The out-of-band electrical signal is extracted from the received electrical signal and the diagnostic monitoring information of the second optical transceiver is obtained from the extracted out-of-band electrical signal. Diagnostic monitoring information of the optical transceiver is also obtained . A second out-of- band electrical signal is generated; the second out-of-band electrical signal comprises an indication of the diagnostic monitoring information of the first optical transceiver and an indication of the diagnostic monitoring information of the second optical transceiver. A second radio transport optical signal is transmitted , the second radio transport optical signal carrying the second out-of-band electrical signal.

In an embodiment, the method further comprises storing the diagnostic monitoring information of the first optical transceiver in a first diagnostic monitoring information table and storing the diagnostic monitoring information of the second optical transceiver in a second diagnostic monitoring information table, and wherein generating a second out-of-band electrical signal comprises retrieving the diagnostic monitoring information of the first optical transceiver from the first diagnostic monitoring information table and retrieving the diagnostic monitoring information of the second optical transceiver from second diagnostic monitoring information table. The method therefore doubles the diagnostic monitoring information table as compared to the data table in the 'remote DDMI' method described in US 2010/0054733 A1 .

In an embodiment, the first optical transceiver and the second optical transceiver independently and asynchronously transmit the second out-of-band electrical signal and the first out-of-band electrical signal respectively. The transmission of the diagnostic monitoring information in each of the diagnostic monitoring information tables is asynchronous and may be performed without any additional signalling between the first and second optical transceivers.

An aspect of the invention provides a computer program, comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out any of the above steps of the method of receiving diagnostic monitoring information in a fronthaul network. An aspect of the invention provides a computer program, comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out any of the above steps of the method of transmitting diagnostic monitoring information in a fronthaul network.

An aspect of the invention provides a carrier containing a computer program as described above, wherein the carrier is one of an electronic signal, optical signal, radio signal, or computer readable storage medium.

An aspect of the invention provides a node for use at a main site of a fronthaul network. The node comprises processing circuitry configured to cause the node to: extract an out-of-band electrical signal from an electrical signal representative of information carried on a radio transport optical signal received at the node; obtain diagnostic monitoring information of an optical transceiver from the extracted out-of-band electrical signal; and transmit the diagnostic monitoring information to a fronthaul network management system.

An aspect of the invention provides an optical transceiver comprising processing circuitry configured to cause the optical transceiver to: extract an out-of-band electrical signal from a received electrical signal representative of information carried on a radio transport optical signal received from a remote optical transceiver; obtain diagnostic monitoring information of the remote optical transceiver from the extracted out-of-band electrical signal; obtain diagnostic monitoring information of the optical transceiver; generate a second out-of- band electrical signal comprising an indication of the diagnostic monitoring information of the remote optical transceiver and an indication of the diagnostic monitoring information of the optical transceiver; and generate a second radio transport optical signal carrying the second out-of-band electrical signal.

References to processors, hardware, processing hardware or circuitry can encompass any kind of logic or analog circuitry, integrated to any degree, and not limited to general purpose processors, digital signal processors, ASICs, FPGAs, discrete components or logic and so on. References to a processor are intended to encompass implementations using multiple processors which may be integrated together, or co-located in the same node or distributed at different locations for example.

Embodiments of the invention will now be described , by way of example only, with reference to the accompanying drawings.

Brief Description of the drawings

Figures 1 to 8 illustrate fronthaul network nodes according to embodiments of the invention;

Figures 9 and 10 illustrate optical transceivers according to embodiments of the invention; Figure 1 1 illustrates a fronthaul network comprising a fronthaul network node according to an embodiment of the invention and optical transceivers according to an embodiment of the invention;

Figure 12 illustrates steps of a method according to an embodiment of the invention of receiving diagnostic monitoring information in a fronthaul network;

Figures 13 and 14 illustrates steps of methods according to embodiments of the invention of transmitting diagnostic monitoring information in a fronthaul network; and

Figure 15 and 16 illustrate signalling at optical transceivers implementing methods according to embodiments of the invention of transmitting diagnostic monitoring information in a fronthaul network.

Detailed description

The same reference numbers will be used for corresponding features in different embodiments.

Referring to Figure 1 , an embodiment of the invention provides a node 10 for use at a main site of a fronthaul network. The node 10 comprises an optical splitter 12, an optical detector 16, diagnostic monitoring interface, DMI, receiver apparatus 18 and a controller 20.

The optical splitter 12 is configured to receive a radio transport optical signal 14 carrying an out-of-band electrical signal comprising an indication of diagnostic monitoring information of an optical transceiver. The optical splitter is configured to power split the received radio transport optical signal to form a monitoring portion.

The optical detector 16 is configured to receive the monitoring portion of the radio transport optical signal and to convert the monitoring portion into an electrical signal. The DMI receiver apparatus 18 is configured to extract the out-of-band electrical signal from the electrical signal. The DMI receiver apparatus is also configured to obtain the diagnostic monitoring information from the extracted out-of-band electrical signal.

The controller 20 is configured to collect the diagnostic monitoring information and to transmit the diagnostic monitoring information 22 to a network management system , NMS, of a fronthaul network.

Figure 2 illustrates a node 30 according to an embodiment of the invention for use at a main site of a fronthaul network. The node 30 comprises an optical splitter 32, an optical detector 16, a frequency division multiplexed, FDM, digital diagnostic monitoring interface, DDMI, receiver 36, a controller (not shown) and an optical multiplexer/demultiplexer, Mux/Demux, 38.

The optical splitter 32 is configured to receive a wavelength division multiplexed,

WDM, radio transport optical signal 34 comprising a plurality of radio transport optical signals from transceivers at radio antenna modules at a remote site, such as remote radio units, RRU or radio equipment, RE. The plurality of radio transport optical signals have a plurality of wavelengths and each of the radio transport optical signals carries a respective out-of-band electrical signal having a respective frequency. The out-of-band electrical signals are DDMI tones and are frequency division multiplexed. The optical splitter 32 is configured to power split the WDM radio transport optical signal to form a monitoring portion 34a, which is delivered to the optical detector 16. The remainder of the WDM radio transport optical signal is delivered to a WDM port 38a on one side of the Mux/Demux 38.

On the other side, the Mux/Demux 38 has a plurality of single-wavelength output ports 38b configured to output radio transport optical signals 40 to transceivers at baseband processing modules, such as baseband units, BBU, or radio equipment controllers, REC, and input ports 38c configured to receive radio transport optical signals 42 from transceivers at baseband processing modules.

The optical detector 16 is a photodetector configured to receive the monitoring portion of the WDM radio transport optical signal 34 and to convert the monitoring portion into an electrical signal. The electrical signal comprises the plurality of FDM out-of-band electrical signals carried by the plurality of radio transport optical signals. The FDM DDMI receiver 36 comprises a sweeping digital filter and a DDMI receiver. The sweeping digital filter is configured to scan all the tone frequencies and extract the DDMI data for each transceiver. The DDMI receiver is configured to obtain the respective diagnostic monitoring information from each of the extracted DDMI signals. The diagnostic monitoring information is DDMI information according to the SNIA standard SFF-8472.The DDMI receiver may be a 'remote- DDMI receiver' as described in US 2010/0054733 A1 .

The controller is configured to collect the diagnostic monitoring information and to transmit the diagnostic monitoring information 22 to a network management system , NMS, of a fronthaul network.

The node 30 of this embodiment, having a single optical splitter, a single photodiode and DDMI receiver, can be realized by exploiting frequency division multiplexing of the DDMI tones. Each transceiver therefore uses a different DDMI tone frequency depending on the wavelength of the radio transport optical signal, for example a CPRI signal or a low latency packet interface signal, that it transmits. In this way a single detector can detect the low frequency, DDMI , multi tone spectrum from the WDM radio transport optical signal 34.

The node 30 of this embodiment is configured to monitor an uplink WDM radio transport optical signal, being transmitted from radio antenna modules at a remote site to baseband processing modules at the main site. This allows the same photodiode also to be used for optical channel monitoring, OCM, of the radio transport optical signals transmitted from the remote site, which provides useful additional monitoring information to perform a wavelength continuity check. Configuring the optical splitter 32 to power split an uplink WDM radio transport optical signal may therefore enable integration of DDMI and OCM.

The node 30 may enable DDMI data to be detected within the fronthaul domain without requiring the use of transponders at remote radio antennas as in the active-fronthaul or hybrid-fronthaul approaches. The node 30 may enable a fronthaul network which may be described as being 'semi- passive' meaning that the network infrastructure remains passive, i.e. only passive optical components are required, but the node 30 provides an active monitoring sub-system which can interface with the NMS.

In an embodiment, the uplink radio transport optical signals carry OOB electrical signals comprising an indication of diagnostic monitoring information of both an optical transceiver at a radio antenna module at the remote site and an optical transceiver at a baseband processing unit at the main site. The uplink radio transport optical signals may be generated by an optical transceiver 200 as described below with reference to Figure 9. The node 30 may therefore be used to obtain DDMI information for transceivers at both the main site and at a remote site

Figure 3 illustrates a node 50 according to an embodiment of the invention for use at a main site of a fronthaul network. The node 50 is similar to the node 30 of the previous embodiment, but in this embodiment the optical splitter 52 is configured to receive a downlink WDM radio transport optical signal 54 comprising a plurality of radio transport optical signals from transceivers at baseband processing modules at the main site, such as baseband units, BBU, or radio equipment controllers, REC.

The optical splitter 32 is configured to power split the downlink WDM radio transport optical signal output from the Mux/Demux 38 to form a monitoring portion 54a, which is delivered to the optical detector 16. The remainder of the downlink WDM radio transport optical signal is delivered to an optical link for transmission to a remote site.

Detecting the downlink WDM signal results in the DDMI receiver sensitivity requirement being relaxed compared to the node 30 of the previous embodiment, because the WDM radio transport signal is power split at the main site, and is therefore close to the transceivers at the baseband processing modules. On the other hand , monitoring the downlink WDM radio transport signal means it is not possible also to perform optical channel monitoring, OCM, of the radio transport optical signals transmitted from the remote site.

In an embodiment, the downlink radio transport optical signals carry OOB electrical signals comprising an indication of diagnostic monitoring information of both an optical transceiver at a baseband processing unit at the main site and an optical transceiver at a radio antenna module at a remote site. The downlink radio transport optical signals may be generated by an optical transceiver 200 as described below with reference to Figure 9. The node 50 may therefore be used to obtain DDMI information for transceivers at both the main site and at a remote site

Figure 4 illustrates a node 60 according to an embodiment of the invention for use at a main site of a fronthaul network. The node 60 is similar to the nodes 30, 50 of the previous embodiments, but in this embodiment the optical splitter 62 is configured to receive both an uplink WDM radio transport optical signal 34 and a downlink WDM radio transport optical signal 54. The optical splitter 62 is configured to power split both WDM radio transport optical signals to form respective monitoring portions 34a, 54a, which are delivered to the optical detector 16. The node 60 may therefore be used to obtain DDMI information for transceivers at both the main site and at a remote site, plus uplink OCM if required.

Figure 5 illustrates a node 70 according to an embodiment of the invention for use at a main site of a fronthaul network. The node 70 comprises a plurality of optical splitters 72, a plurality of optical detectors, a plurality of DDMI receivers 76 and an optical multiplexer/demultiplexer, Mux/Demux, 38, as described above.

The optical splitters 72 are each coupled to a respective single-wavelength output port 38b of the Mux/Demux 38 and each optical splitter 72 is configured to receive a respective uplink radio transport optical signal 78, carrying a respective DDMI out-of-band electrical signal. Each optical splitter 72 is configured to power split the respective radio transport optical signal to form a respective monitoring portion 78a, which is delivered to the respective optical detector 16. The remainder of the radio transport optical signal is transmitted to a respective baseband processing unit.

The optical detectors 16 are configured to receive the monitoring portions of the radio transport optical signals and are configured to convert the monitoring portions of the radio transport optical signals into respective electrical signals. The DDMI receivers 76 are configured to receive respective electrical signals and are configured to extract respective out- of-band electrical signals from the electrical signals. The DDMI receivers 76 are also configured to obtain respective diagnostic monitoring information from the extracted out-of- band electrical signals, the diagnostic monitoring information is DDMI information according to the SNIA standard SFF-8472. The DDMI receivers may be 'remote-DDMI receivers' as described in US 2010/0054733 A1 .

The node 70 of this embodiment is configured to monitor uplink radio transport optical signals 78, being transmitted from radio antenna modules at a remote site to baseband processing modules at the main site. This allows the same photodiode 16 to be used for both DDMI and optical channel monitoring, OCM, of an uplink radio transport optical signal.

The node 70 may be used where a plurality of transceivers use the same DDMI tone frequency, independent of the wavelength of the radio transport optical signal, for example a CPRI signal or a low latency packet interface signal, that each transmits.

In an embodiment, the uplink radio transport optical signals carry DDMI signals comprising an indication of DDMI information of both an optical transceiver at a radio antenna module at the remote site and an optical transceiver at a baseband processing unit at the main site. The uplink radio transport optical signals may be generated by an optical transceiver 200 as described below with reference to Figure 9. The node 70 may therefore be used to obtain DDMI information for transceivers at both the main site and at a remote site

Figure 6 illustrates a node 90 according to an embodiment of the invention for use at a main site of a fronthaul network. The node 90 is similar to the node 70 of the previous embodiment, but in this embodiment the optical splitters 92 are each coupled to a respective single-wavelength input port 38c of the Mux Demux 38 and each optical splitter 92 is configured to receive a respective downlink radio transport optical signal 80, carrying a respective DDMI out-of-band electrical signal. Each optical splitter 92 is configured to power split the respective radio transport optical signal to form a respective monitoring portion 80a, which is delivered to the respective optical detector 16. The remainder of each radio transport optical signals is delivered to the respective input port of the Mux/Demux 38.

In an embodiment, the downlink radio transport optical signals carry DDMI signals comprising an indication of DDMI information of both an optical transceiver at a baseband processing unit at the main site and an optical transceiver at a radio antenna module at a remote site. The downlink radio transport optical signals may be generated by an optical transceiver 200 as described below with reference to Figure 9. The node 90 may therefore be used to obtain DDMI information for transceivers at both the main site and at a remote site

Figure 7 illustrates a node 100 according to an embodiment of the invention for use at a main site of a fronthaul network. The node 100 is similar to the nodes 70, 90 of the two previous embodiments, but in this embodiment respective optical splitters 72, 92 are coupled to each output port 38b and each input port 38c of the Mux/Demux. The node 100 may therefore be used to obtain DDMI information for transceivers at both the main site and at a remote site, plus uplink OCM if required.

Figure 8 illustrates, in terms of a number of functional units, the components of a node 1 10 according to an embodiment of the invention for use at a main site of a fronthaul network. Processing circuitry 120 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product, e.g . in the form of a storage medium 160. The processing circuitry 120 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

Particularly, the processing circuitry 120 is configured to cause the node 1 10 to perform a set of operations, or steps, 400-410, as disclosed below in Figure 12. For example, the storage medium 160 may store the set of operations, and the processing circuitry 120 may be configured to retrieve the set of operations from the storage medium 160 to cause the node 1 10 to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus, the processing circuitry 120 is thereby arranged to execute methods as herein disclosed.

The storage medium 160 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory.

The node 1 10 may further comprise a communications interface 140 for communications at least with a network management system, NMS. As such the communications interface 140 may comprise one or more transmitters and receivers, comprising analogue and digital components and a suitable number of antennas for wireless communications and ports for wireline communications.

The processing circuitry 120 controls the general operation of the node 1 10 e.g. by sending data and control signals to the communications interface 140 and the storage medium 160, by receiving data and reports from the communications interface 140, and by retrieving data and instructions from the storage medium 160. Other components, as well as the related functionality, of the node 1 10 are omitted in order not to obscure the concepts presented herein.

Referring to Figure 9, an embodiment of the invention provides an optical transceiver

200 comprising an optical receiver, Rx, 202, out-of-band, OOB, signal processing apparatus 206, a controller 208 and an optical transmitter, Tx, 210.

The optical receiver 202 is configured to receive a first radio transport optical signal 204 carrying a first out-of-band electrical signal, which comprises an indication of diagnostic monitoring information of a remote optical transceiver. The optical receiver is configured to convert the first radio transport optical signal into a received electrical signal. The out-of-band signal processing apparatus 206 is configured to extract the out-of-band electrical signal from the received electrical signal. The out-of-band signal processing apparatus is also configured to obtain the diagnostic monitoring information of the remote optical transceiver from the extracted out-of-band electrical signal.

The controller 208 is configured to receive the diagnostic monitoring information of the remote optical transceiver and is configured to obtain diagnostic monitoring information of the optical transceiver. The out-of-band signal processing apparatus is configured to generate a second out-of-band electrical signal comprising an indication of the diagnostic monitoring information of the remote optical transceiver and an indication of the diagnostic monitoring information of the optical transceiver.

The optical transmitter 210 is configured to generate a second radio transport optical signal 212 carrying the second out-of-band electrical signal.

The optical transceiver 200 may be provided at a radio antenna module at a remote site of a fronthaul network, in which case the remote optical transceiver is an optical transceiver at a baseband processing module at a main site of the fronthaul network, or the optical transceiver 200 may be provided at a baseband processing module at a main site of a fronthaul network, in which case the remote optical transceiver is an optical transceiver at a radio antenna module at a remote site of the fronthaul network. The optical transceiver 200 works in the same manner irrespective of whether it is located at a remote site or a main site of a fronthaul network; both local and remote diagnostic monitoring information will be carried by the second out-of-band electrical signal.

In an embodiment, the controller 208 comprises a storage medium configured with a remote diagnostic monitoring information table and a local diagnostic monitoring information table. The controller is configured to store the diagnostic monitoring information of the remote optical transceiver in the remote diagnostic monitoring information table and is configured to retrieve diagnostic monitoring information of the optical transceiver stored in the local diagnostic monitoring information table.

In an embodiment, the diagnostic monitoring information is DDMI information, according to SNIA standard SFF-8472.

In an embodiment, the out-of-band signal processing apparatus is DDMI signal processing apparatus configured to obtain DDMI information of the remote optical transceiver from the extracted out-of-band electrical signal.

In an embodiment, the controller 208 comprises a storage medium configured with a

DDMI data table comprising a remote part and a local part. The controller is configured to store DDMI data of the remote optical transceiver in the remote part of the DDMI table and is configured to retrieve DDMI data of the optical transceiver stored in the local part of the DDMI table. This embodiment may therefore provide an enhanced remote-DDMI mechanism as compared to US 2010/0054733 A1 , doubling the DDMI data table in order to enable the DDMI signal to carry both remote and local DDMI data.

In an embodiment, the radio transport optical signal is a CPRI signal or a low latency packet interface signal.

Figure 10 illustrates, in terms of a number of functional units, the components of an optical transceiver 210 according to an embodiment of the invention. Processing circuitry 220 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product, e.g. in the form of a storage medium 260. The processing circuitry 220 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

Particularly, the processing circuitry 220 is configured to cause the optical transceiver 210 to perform a set of operations, or steps, 500-526, as disclosed below in Figures 13 and 14. For example, the storage medium 260 may store the set of operations, and the processing circuitry 220 may be configured to retrieve the set of operations from the storage medium 260 to cause the optical transceiver 210 to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus, the processing circuitry 220 is thereby arranged to execute methods as herein disclosed .

The storage medium 260 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory.

The optical transceiver 210 may further comprise a communications interface 240 for communications at least with an optical transmitter and an optical receiver. As such the communications interface 240 may comprise one or more transmitters and receivers, comprising analogue and digital components and a suitable number of antennas for wireless communications and ports for wireline communications.

The processing circuitry 220 controls the general operation of the optical transceiver 210 e.g. by sending data and control signals to the communications interface 240 and the storage medium 260, by receiving data and reports from the communications interface 240, and by retrieving data and instructions from the storage medium 260. Other components, as well as the related functionality, of the optical transceiver 210 are omitted in order not to obscure the concepts presented herein.

Figure 1 1 illustrates a fronthaul network 300 comprising a main site, a plurality of remote sites (only one is shown for simplicity), an NMS 310, optical links 350 and an optical splitter 320.

The main site comprises a node 10, 30, 50, 60, 70, 90, 100, 1 10 according to any of the above embodiments and a plurality of baseband processing modules, BBU, 340. The remote site comprises a Mux/Demux 322 and a plurality of radio antenna modules, RRU, 330. Each BBU 340 and RRU 330 comprises a transceiver, TRx, 200, 210 according to any of the above embodiments. The remote sites are connected to the node via optical links 350 and the optical splitter 320. In the illustrated fronthaul network 300 the optical links 350 may be single- fibre optical links or double-fibre optical links; where double-fibre optical links are used the fronthaul network 300 may comprises two nodes, one for each of the uplink and downlink directions or the node may comprise two Mux/Demux 38, one operating as a Demux for uplink signals and the other operating as a Mux for downlink signals.

Where the main site comprises a node 30, 50, 60, 70, 90 as illustrated in Figures 3 to 7, uplink CPRI signals may be transported between RRU and BBU through a semi-passive front-haul infrastructure. Transceivers 200 are equipped directly on radio systems and may be configured to support the 'remote-DDMI' feature described in US 2010/0054733 A1 . The passive mux/demux 38 at the main site includes an active monitoring subsystem where DDMI data are extracted: this is accomplished by splitting part of the uplink signals and detecting the remote DDMI modulated tone inserted at the RRU transceivers. The uplink remote DDMI covers both the main site and remote site transceivers thanks to the modified remote DDMI scheme described herein. The DDMI signals may also be extracted splitting the downlink CPRI signals (from BBU to RRU).

Detecting the uplink signal (RRU to BBU) allows for exploiting the same photodiode to monitor both the remote wavelength optical power (OCM) and the DDMI data. On the other hand , it requires higher sensitivity at the DDMI receiver. Detecting the downlink signal (BBU to RRU) allows the DDMI receiver sensitivity requirement to be relaxed due to the fact that the signal is received close to the main site. On the other hand , it is not possible to detect the CPRI signals powers, which is a useful additional monitoring information to perform wavelength continuity checks. Providing the fronthaul network 300 with a node 30, 50, 60, 70, 90 as illustrated in Figures 3 to 7 enables remote DDMI data to be non-intrusively intercepted before it reaches the transceiver and made available to the fronthaul domain NMS. The photodiodes at the same time can provide the usual OCM information as in the prior art. The term 'semi-passive' may be used here to describe this fronthaul network 300, meaning that the fronthaul network infrastructure remains passive but an active monitoring subsystem , i.e. the node 30, 50, 60, 70, 90, interfaced with NMS is added. The node 30, 50, 60, 70, 90 enables the 'remote-DDMI' technology to be used, as in the hybrid fronthaul scenario, but extracts the DDMI signal via a semi-passive node at the main site. The node 30, 50, 60, 70, 90 enables DDMI data to be access inside the fronthaul domain without using transponders at the RRU and BBU, as would be in the prior art active- and hybrid-fronthaul scenarios.

Additionally, the transceivers 200 enable the fronthaul network 300 to access DDMI data from both the main and remote sites' transceivers using an enhanced remote DDMI mechanism which doubles the DDMI data table in order to carry both remote and local DDMI data. Each transceiver writes its local DDMI in a 'local' part of the table and copies received DDMI data in a 'remote' part of the table, and transmits both in the DDMI signal. It is therefore possible to access both local and remote DDMI data by monitoring one transmission direction only, i.e. uplink or downlink.

Figure 12 illustrates steps of a method 400 according to an embodiment of the invention of receiving diagnostic monitoring information in a fronthaul network.

The method comprises steps of:

receiving 402 a radio transport optical signal carrying an out-of-band electrical signal comprising an indication of diagnostic monitoring information of an optical transceiver;

power splitting 404 the radio transport optical signal to form a monitoring portion; converting 406 the monitoring portion of the radio transport optical signal into an electrical signal;

extracting 408 the out-of-band electrical signal from the electrical signal and obtaining the diagnostic monitoring information from the extracted out-of-band electrical signal; and transmitting 410 a reporting signal comprising an indication of the diagnostic monitoring information to a network management system.

The method 400 may be applied at a node 10, 30, 50, 60, 70, 90, 100, 1 10 according to any of the above embodiments.

Figure 13 illustrates steps of a method 500 according to an embodiment of the invention of transmitting diagnostic monitoring information in a fronthaul network.

The method is performed at a first optical transceiver in the fronthaul network and comprises steps of:

receiving 502 a first radio transport optical signal carrying a first out-of-band electrical signal comprising an indication of diagnostic monitoring information of a second optical transceiver in the fronthaul network; converting 504 the first radio transport optical signal into a received electrical signal; extracting 506 the out-of-band electrical signal from the received electrical signal and obtaining the diagnostic monitoring information of the second optical transceiver from the extracted out-of-band electrical signal;

obtaining 508 diagnostic monitoring information of the optical transceiver;

generating 510 a second out-of-band electrical signal comprising an indication of the diagnostic monitoring information of the first optical transceiver and an indication of the diagnostic monitoring information of the second optical transceiver; and

transmitting 512 a second radio transport optical signal carrying the second out-of- band electrical signal.

The method 500 may be performed at a transceiver 200, 210 as described above.

Figure 14 illustrates steps of a method 510 according to an embodiment of the invention of transmitting diagnostic monitoring information in a fronthaul network.

The method 510 is similar to the method 500 of the previous embodiment with the addition of steps of storing 522 the diagnostic monitoring information of the first optical transceiver in a first diagnostic monitoring information table and storing the diagnostic monitoring information of the second optical transceiver in a second diagnostic monitoring information table. In this embodiment, the diagnostic monitoring information of the first optical transceiver is retrieved 524 from the first diagnostic monitoring information table and the diagnostic monitoring information of the second optical transceiver is retrieved 524 from second diagnostic monitoring information table. The second out-of-band electrical signal is then generated 526 comprising the diagnostic monitoring information retrieved from both the first diagnostic monitoring information table and the second diagnostic monitoring information table, i.e. comprising an indication of the diagnostic monitoring information of both the first and second optical transceivers.

In an embodiment, transmission of the second out-of-band electrical signal by the first optical transceiver is independent and asynchronous to transmission of the first out-of-band electrical signal by the second optical transceiver.

Figure 15 and 16 illustrate signalling at optical transceiver 200 as described above implementing the methods 400, 510 of transmitting and receiving diagnostic monitoring information in a fronthaul network described above, with reference to the fronthaul network 300 illustrated in Figure 1 1 .

Each transceiver 200, 210 fills its local DDMI table (in grey in the picture below) with local DDMI data and fills its remote DDMI table (in white) with received DDMI data. In this way, the DDMI signal, the out-of-band electrical signal, transmitted by each transceiver carries both local and remote DDMI data that can be extracted by monitoring only one direction of propagation in the optical link 350.

The DDMI data transmission is asynchronous and doesn't require any additional signalling between the transceivers at the main site and at the remote site. A possible frame flow and data loading implementation, transmission and reception is shown in Figures 15 and 16. The numbered arrows in Figure 16 show one communication cycle:

1 ) At the main site a new data frame is loaded with local DDMI, M_n, and previously received remote DDMI, R_n-i

2) The frame is transmitted using the 'remote-DDMI' method reported in US 2010/0054733 A1

3) At the remote site the received DDMI M_n are loaded in the next frame

4) Local DDMI data R_n+i at the remote site are also loaded in the frame to be transmitted

5) The frame is transmitted to the main site containing a snapshot of both the remote-site's DDMI, R_n+i , and the main-site's DDMI, M_n. This frame is detected in the semi- passive monitoring subsystem node 10, 30, 50, 60, 70, 90, 100, 1 10 which transmit both DDMI data to the fronthaul NMS 310.

By buffering the received data the local and remote DDMI can be synchronized, but this is not strictly necessary since monitoring is a slow process and data belonging to two adjacent time frames are still valid information.

The frame transmission may be performed in two ways: the frames are transmitted continuously, reading data from the EEPROM; or the frames to be transmitted await the arrival of remote DDMI frames.

The actual refresh rate of the DDMI data at the node is determined by the 'remote DDMI' transmission speed rather than by the local EEPROM update rate by the transceiver controller, which is typically faster. To increase the refresh speed (if necessary) a smaller table of only the most relevant data may be transmitted.

Claims

1. A node for use at a main site of a fronthaul network, the node comprising:

an optical splitter configured to receive a radio transport optical signal and configured to power split the radio transport optical signal to form a monitoring portion, wherein the radio transport optical signal carries an out-of-band electrical signal comprising an indication of diagnostic monitoring information of an optical transceiver;

an optical detector configured to receive the monitoring portion of the radio transport optical signal and configured to convert the monitoring portion of the radio transport optical signal into an electrical signal;

diagnostic monitoring interface receiver apparatus configured to extract the out-of- band electrical signal from the electrical signal and configured to obtain the diagnostic monitoring information from the extracted out-of-band electrical signal; and a controller configured to collect the diagnostic monitoring information and configured to transmit the diagnostic monitoring information to a fronthaul network management system .

2. A node as claimed in claim 1 , wherein:

the optical splitter is configured to receive a wavelength division multiplexed, WDM, radio transport optical signal comprising a plurality of said radio transport optical signals having a plurality of wavelengths and the respective out-of-band electrical signals have respective frequencies, and wherein the out-of-band electrical signals are frequency division multiplexed; and

the diagnostic monitoring interface receiver apparatus is configured to extract the out- of-band electrical signals from the electrical signal and is configured to obtain respective diagnostic monitoring information from the extracted out-of-band electrical signals

3. A node as claimed in claim 1 , comprising:

a plurality of optical splitters configured to receive a plurality of radio transport optical signals each carrying a respective said out-of-band electrical signal; and

a plurality of optical detectors configured to receive the monitoring portions of the radio transport optical signals and configured to convert the monitoring portions of the radio transport optical signals into respective electrical signals,

and wherein the diagnostic monitoring interface receiver apparatus is configured to extract respective out-of-band electrical signals from the electrical signals and is configured to obtain respective diagnostic monitoring information from the extracted out-of-band electrical signals.

4. A node as claimed in any preceding claim, wherein a said radio transport optical signal is received from one of a pair of optical transceivers, one optical transceiver of the pair being at a radio antenna module and the other optical transceiver of the pair being at a baseband processing module, and wherein the out-of-band electrical signal comprises an indication of diagnostic monitoring information of each optical transceiver of the pair.

A node as claimed in claim 2, wherein the optical splitter is configured to receive a first wavelength division multiplexed, WDM, radio transport optical signal comprising a plurality of said radio transport optical signals from respective optical transceivers at respective radio antenna modules and is configured to receive a second wavelength division multiplexed, WDM, radio transport optical signal comprising a plurality of said radio transport optical signals from respective optical transceivers at respective baseband processing modules.

A node as claimed in claim 3, wherein a plurality of radio transport optical signals are received from respective optical transceivers at respective radio antenna modules and a plurality of radio transport optical signals are received from respective optical transceivers at respective baseband processing modules, and wherein the out-of- band electrical signals comprise an indication of diagnostic monitoring information of the respective optical transceiver.

A node as claimed in any preceding claim, wherein a said radio transport optical signal is one of a common public radio interface signal and a low latency packet interface signal.

A node as claimed in any preceding claim, wherein a said out-of-band electrical signal is a digital diagnostic monitoring interface signal.

An optical transceiver comprising:

an optical receiver configured to receive a first radio transport optical signal carrying a first out-of-band electrical signal comprising an indication of diagnostic monitoring information of a remote optical transceiver and configured to convert the first radio transport optical signal into a received electrical signal;

out-of-band signal processing apparatus configured to extract the out-of-band electrical signal from the received electrical signal and configured to obtain the diagnostic monitoring information of the remote optical transceiver from the extracted out-of-band electrical signal;

a controller configured to receive the diagnostic monitoring information of the remote optical transceiver and configured to obtain diagnostic monitoring information of the optical transceiver; and

an optical transmitter configured to generate a second radio transport optical signal carrying a second out-of-band electrical signal,

wherein the out-of-band signal processing apparatus is configured to generate the second out-of-band electrical signal comprising an indication of the diagnostic monitoring information of the remote optical transceiver and an indication of the diagnostic monitoring information of the optical transceiver.

10. An optical transceiver as claimed in claim 9, wherein the controller comprises a storage medium configured with a remote diagnostic monitoring information table and a local diagnostic monitoring information table and wherein the controller is configured to store the diagnostic monitoring information of the remote optical transceiver in the remote diagnostic monitoring information table and is configured to retrieve diagnostic monitoring information of the optical transceiver stored in the local diagnostic monitoring information table.

11. A method of receiving diagnostic monitoring information in a fronthaul network, the method comprising:

receiving a radio transport optical signal carrying an out-of-band electrical signal comprising an indication of diagnostic monitoring information of an optical transceiver; power splitting the radio transport optical signal to form a monitoring portion;

converting the monitoring portion of the radio transport optical signal into an electrical signal;

extracting the out-of-band electrical signal from the electrical signal and obtaining the diagnostic monitoring information from the extracted out-of-band electrical signal; and transmitting a reporting signal comprising an indication of the diagnostic monitoring information to a network management system.

12. A method of transmitting diagnostic monitoring information in a fronthaul network, the method comprising, at a first optical transceiver in the fronthaul network:

receiving a first radio transport optical signal carrying a first out-of-band electrical signal comprising an indication of diagnostic monitoring information of a second optical transceiver in the fronthaul network;

converting the first radio transport optical signal into a received electrical signal; extracting the out-of-band electrical signal from the received electrical signal and obtaining the diagnostic monitoring information of the second optical transceiver from the extracted out-of-band electrical signal;

obtaining diagnostic monitoring information of the optical transceiver;

generating a second out-of-band electrical signal comprising an indication of the diagnostic monitoring information of the first optical transceiver and an indication of the diagnostic monitoring information of the second optical transceiver; and transmitting a second radio transport optical signal carrying the second out-of-band electrical signal.

13. A method as claimed in claim 12, further comprising storing the diagnostic monitoring information of the first optical transceiver in a first diagnostic monitoring information table and storing the diagnostic monitoring information of the second optical transceiver in a second diagnostic monitoring information table, and wherein generating a second out-of-band electrical signal comprises retrieving the diagnostic monitoring information of the first optical transceiver from the first diagnostic monitoring information table and retrieving the diagnostic monitoring information of the second optical transceiver from second diagnostic monitoring information table.

14. A method as claimed in claim 12 or claim 13, wherein the first optical transceiver and the second optical transceiver independently and asynchronously transmit the second out-of-band electrical signal and the first out-of-band electrical signal respectively.

15. A computer program, comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to any one of claims 1 1 to 13.

16. A carrier containing the computer program of the previous claim, wherein the carrier is one of an electronic signal, optical signal, radio signal, or computer readable storage medium.