Radio over fibre signal distribution method and system

Abstract

Signal Distribution Methods and Systems Methods and apparatus for Radio over Fibre (RoF) signal distribution in communication networks are provided herein The method comprises obtaining a radio frequency (RF) carrier tone signal at a Central Radio Unit (CRU), obtaining a data signal at the CRU, transmitting the RF carrier tone signal and the data signal from the CRU to a remote radio unit using an optical fibre, and processing the RF carrier tone signal and the data signal at the remote radio unit to generate a RoF signal.

Description

[0001] SIGNAL DISTRIBUTION METHODS AND SYSTEMS

[0002] TECHNICAL FIELD

[0003] Embodiments of the present disclosure relate to methods and apparatus in communication networks, and in particular methods and apparatus for Radio over Fibre (RoF) signal distribution in communication networks.

[0004] BACKGROUND

[0005] Communication networks such as Sixth Generation (6G), Fifth Generation (5G) and Fourth Generation (4G) New Radio (NR) cellular networks (for example 3rd Generation Partnership Project (3GPP) networks) may distribute signals using wireless communication methods. A communication method that such networks may use to distribute signals is Radio over Fibre (RoF) communication.

[0006] Figure 1 depicts a reference architecture of a RoF system. Such a system may include a Central Radio Unit (CRU) 102 and one or more Antenna Units (AU) 104. The AU 104 may be in communication with the CRU 102 through one or more RoF links. Each AU 104 may have an associated RoF link 10 to the CRU. The system may also include a Baseband Processing Unit (BPU) 106. Accordingly, an amplified replica of the initial signal may be transmitted over the RoF link to the AU 104 such that the amplified replica signal modulates an optical transmitter in each AU 104.

[0007] In a system as shown in Figure 1, the CRU 102 may receive a modulated optical signal. This signal may be received, for example, via an optical fibre. The signal may be transmitted upstream to the CRU 102. The CRU 102 may then photo-detect and digitize the signal, and transmit the digitized signal to the BPU 106.

[0008] As shown in Figure 1A, the CRU 102 and BPU 106 may be integrated into a single unit. For example, the CRU 102 and BPU 106 may be hosted in a single Central Office (CO). The Figure 1A configuration may support centralization and virtualization of network and baseband functions at the same site, which may in turn provide reduced operational costs. However, such a configuration may result in poor latency due to increased transmission distance between the CO and the one or more AUs 104.Alternatively, as shown in Figure 1B, the CRU 102 and BPU 106 may be separate units connected by a digital fronthaul interface. For example, the CRU 102 and BPU 106 may be sited at different network nodes. The Figure 1B configuration may provide improved latency and thus be suitable for time-critical use cases, such as an automated factory based on critical machine type communications (cMTC) with ultra-low latency and jitter. That is, time-critical use cases may require ultra-low latency and jitter for precise operation, high levels of availability, robustness, and / or resilience to attacks. However, such a configuration may result in higher operational costs.

[0009] Figure 2 depicts a reference optical RoF link. The RoF link may comprise a transmitter element (TX) 202 acting as a CRU 102 and a receiver element (RX) 204 acting as a Remote Radio Unit (RRU) 104, connected by an optical link. The optical link may comprise a laser, for example a laser with direct or external modulation. The optical link may further comprise a fibre and a photodiode. The fibre may have a length on the order of kilometres. The fibre may be a Standard Single Mode Fibre (SSMF).

[0010] The TX 202 may further comprise a pre-transmission amplifier (PRE), for example a doublestage amplifier with a variable attenuator between the two stages. The RX 204 may further comprise a post-transmission amplifier (POST), for example a double-stage amplifier with a variable attenuator between the two stages. The PRE and POST amplifiers may be used to compensate for temperature variations during signal transmission.

[0011] The signal used to transmit from the TX 202 to the RX 204 may be either a digital signal or an analog signal. For configurations using an analog signal, the signal may undergo mixing with a radio frequency (RF) signal or RF carrier for transmission across the optical link. For configurations using a digital signal, the TX 202 may further comprise a Digital-to-Analog Converter (DAC) which will convert the signal to an analog signal before mixing with the RF carrier.

[0012] Both analog and digital RoF configurations require generating an RF carrier at the AU, which may introduce undesirable costs or phase noise. Common methods of generating a high frequency RF carrier include the use of synthesizers which produce a desired frequency by multiplying a reference frequency from a local oscillator. However, this method may introduce unwanted Phase Noise (PN) at higher frequencies. One countermeasure that may be used to handle PN at higher carrier frequencies is to increase the spacing between Orthogonal Frequency-Division Multiplexing (OFDM) subcarriers. For example, in 5G communication networks the subcarrier spacing may be flexible and may range from 15 to 240 kHz, wherelarge OFDM spacing correspond to higher RF carrier frequencies. However, such countermeasures may result in less efficient subcarrier usage.SUMMARY

[0013] It is an object of the present disclosure to provide methods and apparatus for RoF signal distribution in communication networks with improved efficiency. Embodiments of the disclosure aim to provide apparatuses and methods that alleviate some or all of the problems identified.

[0014] A first embodiment of the present disclosure provides a Radio over Fibre (RoF) signal distribution method. The method comprises obtaining a radio frequency (RF) carrier tone signal at a Central Radio Unit (CRU), obtaining a data signal at the CRU, transmitting the RF carrier tone signal and the data signal from the CRU to a remote radio unit using an optical fibre, and processing the RF carrier tone signal and the data signal at the remote radio unit to generate a Radio over Fibre (RoF) signal.

[0015] A second embodiment of the present disclosure provides a system for Radio over Fibre (RoF) signal distribution. The system comprises a Central Radio Unit (CRU) and a remote radio unit. The CRU is configured to obtain a radio frequency (RF) carrier tone signal, obtain a data signal, and transmit the RF carrier tone signal and the data signal to the remote radio unit using an optical fibre. The remote radio unit is configured to receive the RF carrier tone signal and the data signal using the optical fibre and process the RF carrier tone signal and the data signal to generate a Radio over Fibre (RoF) signal.

[0016] Further embodiments provide methods, communication devices, and networks as discussed herein.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] For a better understanding of the present disclosure, and to show how it may be put into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0018] Figure 1A and Figure 1B (collectively referred to as Figure 1) are diagrams of a reference RoF architecture;

[0019] Figure 2 is a diagram of a reference optical RoF link;

[0020] Figure 3 is a flowchart of a method for a RoF communication network, in accordance with embodiments;

[0021] Figure 4 is a schematic diagram of a communications network, in accordance with embodiments;

[0022] Figure 5 is a schematic diagram of a Central Radio Unit, in accordance with embodiments;

[0023] Figure 6 is a schematic diagram of a Remote Radio Unit, in accordance with embodiments;

[0024] Figure 7A and Figure 7B (collectively referred to as Figure 7) are diagrams of a further communications network, in accordance with embodiments;

[0025] Figure 8 is a diagram of a first embodiment of the present disclosure; and

[0026] Figure 9 is a diagram of a second embodiment of the present disclosure.DETAILED DESCRIPTION

[0027] For the purpose of explanation, details are set forth in the following description in order to provide a thorough understanding of the embodiments disclosed. It will be apparent, however, to those skilled in the art that the embodiments may be implemented without these specific details or with an equivalent arrangement.

[0028] The following sets forth specific details, such as particular embodiments for purposes of explanation and not limitation. It will be appreciated by one skilled in the art that other embodiments may be employed apart from these specific details. In some instances, detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as to not obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or more nodes using hardware circuitry (e.g., analog and / or discrete logic gates interconnected to perform a specialized function, ASICs, PLAs, etc.) and / or using software programs and data in conjunction with one or more digital microprocessors or general purpose computers that are specially adapted to carry out the processing disclosed herein, based on the execution of such programs. Nodes that communicate using the air interface also have suitable radio communications circuitry. Moreover, the technology may additionally be considered to be embodied entirely within any form of computer-readable memory, such as solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein.

[0029] Hardware implementation may include or encompass, without limitation, digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analog) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and / or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions.

[0030] In terms of computer implementation, a computer is generally understood to comprise one or more processors, one or more processing modules or one or more controllers, and the terms computer, processor, processing module and controller may be employed interchangeably. When provided by a computer, processor, or controller, the functions may be provided by a single dedicated computer or processor or controller, by a single shared computer or processor or controller, or by a plurality of individual computers or processors or controllers, some of which may be shared or distributed. Moreover, the term “processor” or “controller” also refersto other hardware capable of performing such functions and / or executing software, such as the example hardware recited above.

[0031] Figure 3 is a flowchart of a method for a RoF communication network, in accordance with embodiments. As shown in Figure 3, the method may comprise obtaining a radio frequency (RF) carrier tone signal (Step S302). The RF carrier tone signal may be obtained, for example, at a Central Radio Unit (CRU). In specific embodiments, the method may comprise generating the RF carrier tone signal at the CRU. That is, in specific embodiments the CRU may be further configured to generate the RF carrier tone signal. Accordingly, the CRU may comprise a synthesizer or RF generator configured to generate the RF carrier tone signal. In specific embodiments, the synthesizer may be an optoelectrical Frequency Synthesizer. Alternatively or additionally, the synthesizer may be one or more of: a mode locked laser, an optical oscillator, a non linear harmonic generator comprising one or more optical filters, and / or an optical microresonator.

[0032] As shown in Figure 3, the method may further comprise obtaining a data signal (Step S304). In embodiments, the data signal may be obtained at the CRU. The method may further comprise transmitting the RF carrier tone signal and the data signal (Step S306). For example, the RF carrier tone signal and the data signal may be transmitted from the CRU to a remote radio unit. In embodiments, the RF carrier tone signal and the data signal may be transmitted using an optical fibre. Finally, the method may comprise processing the RF carrier tone signal and the data signal (Step S308). In embodiments, the RF carrier tone signal and the data signal may be processed to generate a Radio over Fibre (RoF) signal. Alternatively or additionally, the RF carrier tone signal and the data signal may be processed at the remote radio unit.

[0033] The method of Figure 3 may be performed by any suitable apparatus, for example a communications network. An example of a suitable network is depicted in Figure 4. As depicted in Figure 4, the communications network 400 performing the methods of embodiments may comprise a Central Radio Unit (CRU) 402. The communication network 400 may further comprise a Remote Radio Unit (RRU) 404. Additionally, the communication network 400 may comprise a Baseband Processing Unit (BPU) 406.

[0034] Accordingly, in present embodiments a system 400 for RoF signal distribution may comprise a CRU 402 and a RRU 404. The CRU 402 may be configured to obtain a radio frequency (RF) carrier tone signal, obtain a data signal, and transmit the RF carrier tone signal and the data signal to the remote radio unit using an optical fibre. The RRU 404 may be configured toreceive the RF carrier tone signal and the data signal using the optical fibre and process the RF carrier tone signal and the data signal to generate a Radio over Fibre (RoF) signal.

[0035] An example of a CRU 402 suitable for use in the communications network 400 is depicted in Figure 5. As shown in Figure 5, the CRU 502 may comprise a processor or processing circuitry 502A, interfaces 502B, and a memory or non-transitory machine-readable medium 502C storing a computer program or instructions 502D. The steps of the method of present embodiments, for example the relevant steps as depicted in Figure 3, may be performed in accordance with the computer program 502D stored on the memory 502C, and may be executed by the processor 502A in conjunction with one or more interfaces 502B.

[0036] An example of a RRU 404 suitable for use in the communications network 400 is depicted in Figure 6. As shown in Figure 6, the RRU 604 may comprise a processor or processing circuitry 604A, interfaces 604B, and a memory or non-transitory machine-readable medium 604C storing a computer program or instructions 604D. The steps of the method of present embodiments, for example the relevant steps as depicted in Figure 3, may be performed in accordance with the computer program 604D stored on the memory 604C, and may be executed by the processor 604A in conjunction with one or more interfaces 604B.

[0037] As detailed above, in present embodiments the RF carrier tone signal and the data signal may be transmitted from the CRU to a remote radio unit using an optical fibre. Figure 7 depicts embodiments of different optical fibre arrangements.

[0038] Figure 7A depicts an embodiment in which the data signal and the RF carrier tone signal are transmitted on different optical fibres. Accordingly, in some embodiments the method may further comprise transmitting the RF carrier tone signal from the CRU to the remote radio unit using a first optical fibre and transmitting the data signal from the CRU to the remote radio unit using a second optical fibre. That is, in systems of present embodiments the CRU may be further configured to transmit the RF carrier tone signal to the remote radio unit using a first optical fibre, and the CRU may be further configured to transmit the data signal to the remote radio unit using a second optical fibre.

[0039] In the specific embodiments of Figure 7A, the CRU 702 comprises a synthesizer 708, Electro-Optical or Electrical-to-Optical (EO) converter 710 and Digital Front End (DFE) acting as a BPU 706. However, it will be appreciated that other suitable arrangements of CRU 702 are envisaged and that the present embodiments are not limited thereto.Further, in the specific embodiments of Figure 7A the RRU 704 comprises a photodiode (PD) 714, an Opto- Electrical or Optical-to-Electrical (OE) converter 712, a mixer 716, an Analog Front End (AFE) 718, and an emitting element 720. However, it will be appreciated that other suitable arrangements of RRU 704 are envisaged and that the present embodiments are not limited thereto.

[0040] As detailed above, the CRU 702 may obtain a RF carrier tone and in specific embodiments may generate a RF carrier tone. In the specific embodiment of Figure 7A, the CRU comprises a synthesizer 708. The synthesizer 708 of the CRU 702 may be an RF synthesizer, which for example may be integrated into the CRU 702. The RF synthesizer 708 may, for example, be a photonic synthesizer. The synthesizer 708 may be configured to generate an RF carrier tone at a RF carrier wavelength ARF. Accordingly, the CRU 702 may be configured to deliver the RF carrier tone to an optical fibre 722A connected to the RRU 704, such that the RF carrier tone is transmitted from the CRU 702 to the RRU 704 via the optical fibre 722A. Optical fibre 722A may be considered a first optical fibre 722k. In specific embodiments, the synthesiser 708 may be configured to deliver the RF carrier tone to the first optical fibre 722k. Alternatively or additionally, the first optical fibre 722A may be configured to be connected to a photodiode (PD) 714 in the RRU 704.

[0041] As further detailed above, in specific embodiments the CRU 702 may obtain a data signal. In the specific embodiment of Figure 7A, the CRU 702 may comprise a DFE 706. The DFE 706 may be configured to obtain the data signal, and may deliver the data signal to an EO converter 710. The EO converter 710 may convert the data signal to an optical data signal; the optical data signal may be at a data wavelength AD. Accordingly, the CRU 702 may be configured to deliver the data signal (and more specifically, the optical data signal) to an optical fibre 722B connected to the RRU 704, such that the data signal is transmitted from the CRU 702 to the RRU 704 via the optical fibre 722B. Optical fibre 722B may be considered a second optical fibre 722B. In specific embodiments, the EO converter 710 may be configured to deliver the optical data signal to the second optical fibre 722B. Alternatively or additionally, the second optical fibre 722B may be configured to be connected to an OE converter 712 in the RRU 704.

[0042] The OE converter may be an optical receiver including a DAC. Alternatively or additionally, the OE converter may be a digital optical receiver. For example, in present embodiments comprising a delta-sigma modulated data signal as described below, the OE converter may be a digital optical receiver. Accordingly, the OE converter may comprise a photodiode as an opto-electrical conversion element.It will be appreciated that suitable alternative technology options for EO converters are envisaged. In specific examples, the EO converter may be an optical modulator. In further specific examples, the EO converter may be one of a Mach Zehnder Modulator or a Micro Ring modulator.

[0043] An embodiment comprising an alternative optical fibre arrangement is depicted in Figure 7B.

[0044] In such an arrangement, the synthesizer 708 and EO converter 710 may be configured to generate the RF carrier tone signal and optical data signal such that the RF carrier tone signal has a RF wavelength (ARF), the data signal has a data wavelength (AD), and the RF wavelength ARF is a different wavelength value to the data wavelength AD. In specific embodiments, the EO converter 710 may be configured to output a digital and / or an analog data signal by modulating the amplitude and / or phase of the data signal to a specific data wavelength AD.

[0045] In the specific embodiment of Figure 7B, the CRU 702 may comprise a signal coupler 724 configured to couple the RF carrier tone signal and the optical data signal. That is, the signal coupler 724 may be configured to process the data signal and the RF carrier tone signal to generate a coupled signal. Accordingly, the synthesizer 708 may be configured to configured to deliver the RF carrier tone signal to the signal coupler 724. Alternatively or additionally, the EO converter 710 may be configured to deliver the optical data signal to the signal coupler 724.

[0046] The signal coupler 724 may be configured to deliver the coupled signal to an optical fibre 722 connected to the RRU 704, such that the coupled signal is transmitted from the CRU 702 to the RRU 704 via the optical fibre 722. Accordingly, in such an embodiment both the RF carrier tone signal and the data signal are transmitted from the CRU 702 to the RRU 704 over a single fibre. The RRU 704 may comprise a filer such as an optical filter 726, which may be configured to separate or split the coupled signal into the data signal and the RF carrier tone signal components. The optical filter 726 may be further configured to deliver the RF carrier tone signal component to a PD 714 and to deliver the data signal component to the OE converter 712.

[0047] Accordingly, in specific embodiments the RF carrier tone signal may have a RF wavelength A F, and the data signal may have a data wavelength AD. In further specific embodiments, the RF wavelength may be a different wavelength value to the data wavelength, and the method performed by the system may further comprise processing the data signal and the RF carrier tone signal at the CRU using an optical signal coupler to generate a coupled signal, transmitting the coupled signal from the CRU to the remote radio unit using an the optical fibre,and processing the coupled signal at the remote radio unit using a filter to separate the data signal and the RF carrier tone signal. That is, in systems of specific embodiments the CRU may comprise an optical signal coupler and the remote radio unit may comprise a filter. The CRU may be further configured to process the data signal and the RF carrier tone signal using the optical signal coupler to generate a coupled signal and transmit the coupled signal to the remote radio unit using the optical fibre. The RRU may be further configured to process the coupled signal using the filter to separate the data signal and the RF carrier tone signal.

[0048] In the specific embodiments of Figure 7, the OE converter 712 may be configured to convert the optical data signal into an electrical data signal. The OE converter 712 may be further configured to transmit the electrical data signal to the mixer 716. Similarly, the PD 714 may be configured to convert the RF carrier tone into an electrical RF tone. The PD 714 may be further configured to transmit the electrical RF tone to the mixer 716. The mixer may be configured to process the electrical RF tone and electrical data signal to generate the RoF signal. The mixer may be further configured to deliver the RoF signal to the AFE 718.

[0049] The AFE 718 may be configured to filter and amplify the RoF signal, for example in order to modify the RoF signal such that the RoF signal is suitable for emission by the emitting element 720. The AFE 718 may be further configured to deliver the modified signal to the emitting element 720. In specific embodiments, the RRU 704 may comprise one or more emitting elements 720. In further specific embodiments, the RRU 704 may comprise a sub array of emitting elements 720.

[0050] In specific embodiments the method performed by the system or communications network may comprise emitting the RoF signal by the remote radio unit. Alternatively or additionally, the Remote Radio Unit 704 may be an Antenna Unit (AU).

[0051] Accordingly, in specific embodiments such as that of Figure 7, the data signal may be an electrical data signal and the method performed by the system may further comprise processing the data signal at the CRU using an Electrical-to-Optical (EO) converter to generate an optical data signal, transmitting the optical data signal from the CRU to the remote radio unit using an optical fibre, and processing the optical data signal at the remote radio unit using an Optical-to-Electrical (OE) converter to obtain the electrical data signal. However, it will be understood that present embodiments are not limited thereto; for example, in a case where the data signal is an optical data signal such conversion steps and associated components may not be present.Similarly, in specific embodiments such as that of Figure 7, the mixer may be an electrical mixer 716. Accordingly, the method performed by the system may further comprise processing the RF carrier tone signal at the remote radio unit using a photodetector to generate an electrical RF tone, processing the data signal at the remote radio unit to generate an electrical data signal, and mixing the electrical RF tone and the electrical data signal to generate the RoF signal. That is, in systems of present embodiments the data signal may be an electrical data signal, the CRU may comprise an Electrical-to-Optical (EO) converter and the remote radio unit may comprise an Optical-to-Electrical (OE) converter. The CRU may be further configured to process the data signal using the EO converter to generate an optical data signal and transmit the optical data signal to the remote radio unit using the optical fibre. The RRU may be further configured to process the optical data signal using the OE converter to obtain the electrical data signal. However, it will be understood that present embodiments are not limited thereto; for example, in a case where the mixer is configured to process the data signal and RF carrier tone signal in a format other than electrical (for example, optical format) such conversion steps and associated components may not be present.

[0052] Figure 8 depicts the results of method steps performed in an embodiment in which the RF carrier tone signal or optical carrier is generated in an analog format (for example, by a synthesiser in the CRU) and processed by a PD 814 to generate a RF tone or RF carrier. That is, Figure 8 presents a carrier for an analog RoF system which may comprise the units depicted in Figure 7. As shown in Figure 8, analog RoF systems may generate a high-frequency analog signal at the CRU with two phase-locked wavelengths which are then transmitted to the RRU over an optical fibre. A PD 814 may be present at the RRU, and the RRU may process the RF carrier tone signal using PD 814 to generate an electrical RF tone or RF carrier. An analog RoF system may provide a simpler configuration, as the RRU may not need to be equipped with a DSP or DAC. However, the CRU may require a dedicated converter with high linearity for analog signals for the EO converter. Accordingly, the dynamic range of the system may be limited by the need to ensure linearity. By contrast, digital RoF may not suffer from problems regarding linearity but may require a DSP and / or DAC for processing the signal which in turn may increase cost and power consumption.

[0053] To provide improved cost and power consumption without introducing constraints on linearity, the data signal may be a delta-sigma modulation signal or 1 -bit signal. That is, 1 -bit modulation or delta-sigma modulation may be used, wherein high-frequency analog signals are converted into 1 -bit pulse signals that are transmitted over the optical fibre(s). Accordingly, in present embodiments the data signal may be transmitted from the CRU to the RRU as a 1 -bit signal and the desired analog signal may be reproduced at the RRU using a filter. In specificembodiments, a general purpose EO converter may be used for the filter. Accordingly, in embodiments wherein the data signal is a delta-sigma modulation signal the RRU may not require a DSP or DAC and the overall system architecture may be simplified, providing reduced cost and power consumption. The system architecture of Figure 7 may accordingly be suitable for use with a data signal that is a delta-sigma modulation signal.

[0054] Figure 9 depicts a system architecture demonstrating a separation between analog and digital signal processing or “split approach” in present embodiments. That is, the specific embodiment of Figure 9 comprises a beamforming Application Specific Integrated Circuit (ASIC) which may beamform signals to one or more DFEs. The DFEs and ASIC may form a part of a CRU or otherwise be onboard a CRU (for example, in a BPU). That is, in Figure 9 all elements to the left of the central “split” may form a part of the RRU. Accordingly, the one of more DFEs are responsible for the digital signal processing of the system and are positioned in the CRU. As shown in Figure 9, the system may comprise one or more Digital-to-Analog Converters (DAC) and / or Analog-to-Digital Converters (ADC) which are located directly in the RRU. That is, in Figure 9 all elements to the right of the central “split” may form a part of the CRU. Accordingly, the one of more DAC / ADCs are responsible for the analog signal processing of the system and are positioned in the RRU. As shown in Figure 9, the DAC / ADCs may in turn be connected to one or more AFE units and sub-array (SA) units. In the specific embodiment of Figure 9, the system comprises a single beamforming ASIC connected to four DFEs, which in turn are each connected to four ADC / DACs via optical fibre. The resulting 16 ADC / DACs are each connected to 16 AFE and SA units, resulting in a total of 256 AFE and SA units.

[0055] This physical separation of signal processing, or “split approach”, may allow for the optimization of the implementation of each signal processing step to provide reduced noise and interference. For example, the frequency synthesizer may be located in the DFE of the BPU, or otherwise located in the CRU of the communication network. Accordingly, this frequency synthesizer may be designed to achieve low noise. For example, the frequency synthesizer may be a local oscillator or RF carrier tone generator designed to achieve low noise using optical processing techniques. The RF carrier tone signal generated by the frequency synthesizer may then be transmitted to the RRU using two phase-locked wavelengths as shown in Figure 8, whose distance determines the RF after beating in front of a PD located at the AFE on board the RRU. Furthermore, the interconnection between the CRU (and more specifically the BPU / DFE of the CRU) and the RRU may be achieved through a hybrid link, for example by combining digital and analog signals transmitted over the optical fibre connection. The digital signals are converted from digital to analog and from analog to digital by dedicated DACs and ADCs respectively onboard the RRU as shown in Figure 9. Thissplit architecture approach may offer greater flexibility and modularity in system design, and thus may facilitate greater adaptation to application requirements and / or greater integration capabilities.

[0056] Present embodiments may allow for any required DFE unit of the communication network to integrate a frequency synthesizer operating at RF in the CRU. That is, in present embodiments the DFE unit may include an integrated frequency synthesizer. Accordingly, the frequency synthesizer may be realized with optoelectronics, which may provide reduced phase noise, improved signal integrity, and / or reduced vulnerability to electromagnetic interference. Similarly, present embodiments may allow for a “split” RRU / antenna architecture wherein any required DFE unit is onboard the CRU and the ADC / DAC units are onboard the RRU. The interconnection between the CRU and RRU may be mixed digital and analog connection to realise a hybrid RoF connection.

[0057] In specific embodiments, the RF carrier tone signal may be an analog signal. Alternatively or additionally, the data signal may be one of: an analog signal, a digital signal, or a delta-sigma modulation signal. In specific embodiments, the RF carrier tone signal may be an analog signal and the data signal may be an analog signal wherein the signals are separated and transmitted separately from the CRU to the RRU; such an arrangement may provide improved ease of nonlinearity correction.

[0058] Present embodiments may transmit an RF carrier as an optical signal; the RF carrier may accordingly require little or no linearity constraints and thus the use of optical signals for the RF carrier may provide improved cost and power consumption without introducing linearity constraints. That is, present embodiments may deliver the RF carrier tone signal from the CRU to the RRU as an analog signal, in order to exploit a photodiode that has no constraints on linearity.

[0059] Simultaneously, the data signal may be transmitted from the CRU to the RRU as a digital signal or as a delta-sigma modulation signal in order to avoid constraints imposed on dynamic range to ensure linearity. As detailed in specific embodiments above, the RF carrier tone signal and the data signal may travel over two wavelengths and / or over two separate fibres and be detected separately at the RRU and processed to generate a RoF signal (for example, by being mixed by a mixer unit).

[0060] In particular embodiments, a RF carrier tone signal may be transmitted from the CRU to the RRU over an optical fibre and detected at the RRU as an analog signal without signalprocessing. A data signal may also be transmitted from the CRU to the RRU over an optical fibre and detected at the RRU as a digital signal, which may use delta-sigma modulation to remove the need for DAC and / or DSP processing. The data signal and RF carrier tone signal may both be detected as or converted to electrical signals, and then be mixed in a mixing stage in the RRU and delivered to one or more RRU emitting elements.

[0061] Accordingly, present embodiments may avoid non-linearities in characterising RoF interconnection, for example by using digital equalisation at the BPU / CRU to compensate for nonidealities in opto-electronic interconnection. Alternatively or additionally, present embodiments may maintain a high dynamic range versus pure RoF interconnection and / or may digitally compensate for Optical Signal to Noise Ratio (OSNR) degradation through regeneration techniques applied in DAC / ADC circuitries. Present embodiments may use a photonic generation of the RF tone that is delivered through the fibre for further phase noise reduction.

[0062] The methods of the present disclosure may be implemented in hardware, or as software modules running on one or more processors. The methods may also be carried out according to the instructions of a computer program, and the present disclosure also provides a computer readable medium having stored thereon a program for carrying out any of the methods described herein. A computer program embodying the disclosure may be stored on a computer readable medium, or it could, for example, be in the form of a signal such as a downloadable data signal provided from an Internet website, or it could be in any other form.

[0063] In general, the various exemplary embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. For example, some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the disclosure is not limited thereto. While various aspects of the exemplary embodiments of this disclosure may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

[0064] As such, it should be appreciated that at least some aspects of the exemplary embodiments of the disclosure may be practiced in various components such as integrated circuit chips and modules. It should thus be appreciated that the exemplary embodiments of this disclosure maybe realized in an apparatus that is embodied as an integrated circuit, where the integrated circuit may comprise circuitry (as well as possibly firmware) for embodying at least one or more of a data processor, a digital signal processor, baseband circuitry and radio frequency circuitry that are configurable so as to operate in accordance with the exemplary embodiments of this disclosure.

[0065] It should be appreciated that at least some aspects of the exemplary embodiments of the disclosure may be embodied in computer-executable instructions, such as in one or more program modules, executed by one or more computers or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types when executed by a processor in a computer or other device. The computer executable instructions may be stored on a computer readable medium such as a hard disk, optical disk, removable storage media, solid state memory, RAM, etc. As will be appreciated by one of skill in the art, the function of the program modules may be combined or distributed as desired in various embodiments. In addition, the function may be embodied in whole or in part in firmware or hardware equivalents such as integrated circuits, field programmable gate arrays (FPGA), and the like.

[0066] References in the present disclosure to “one embodiment”, “an embodiment” and so on, indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to implement such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0067] It should be understood that, although the terms “first”, “second” and so on may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of the disclosure. As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed terms.

[0068] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”, “comprising”, “has”, “having”, “includes” and / or “including”, when used herein, specify the presence of stated features, elements, and / or components, but do not preclude the presence or addition of one or more other features, elements, components and / or combinations thereof. The terms “connect”, “connects”, “connecting” and / or “connected” used herein cover the direct and / or indirect connection between two elements.

[0069] The present disclosure includes any novel feature or combination of features disclosed herein either explicitly or any generalization thereof. Various modifications and adaptations to the foregoing exemplary embodiments of this disclosure may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings. However, any and all modifications will still fall within the scope of the non-limiting and exemplary embodiments of this disclosure. For the avoidance of doubt, the scope of the disclosure is defined by the claims.

Claims

CLAIMS1. A Radio over Fibre (RoF) signal distribution method comprising:obtaining a radio frequency (RF) carrier tone signal at a Central Radio Unit (CRU);obtaining a data signal at the CRU;transmitting the RF carrier tone signal and the data signal from the CRU to a remote radio unit using an optical fibre; andprocessing the RF carrier tone signal and the data signal at the remote radio unit to generate a Radio over Fibre (RoF) signal.

2. A method as claimed in Claim 1 , wherein the RF carrier tone signal is an analog signal and the data signal is one of: an analog signal, a digital signal, or a delta-sigma modulation signal.

3. A method as claimed in any preceding claim, wherein the method further comprises:transmitting the RF carrier tone signal from the CRU to the remote radio unit using a first optical fibre; andtransmitting the data signal from the CRU to the remote radio unit using a second optical fibre.

4. A method as claimed in any of Claims 1 or 2, wherein the RF carrier tone signal has a RF wavelength (ARF), the data signal has a data wavelength (AD), the RF wavelength is a different wavelength value to the data wavelength, and wherein the method further comprises:processing the data signal and the RF carrier tone signal at the CRU using an optical signal coupler to generate a coupled signal;transmitting the coupled signal from the CRU to the remote radio unit using the optical fibre; andprocessing the coupled signal at the remote radio unit using a filter to separate the data signal and the RF carrier tone signal.

5. A method as claimed in any preceding claim, wherein the data signal is an electrical data signal and wherein method further comprises:processing the data signal at the CRU using an Electrical-to-Optical (EO) converter to generate an optical data signal;transmitting the optical data signal from the CRU to the remote radio unit using the optical fibre; andprocessing the optical data signal at the remote radio unit using an Optical-to-Electrical (OE) converter to obtain the electrical data signal.

6. A method as claimed in any preceding claim, wherein the method further comprises:generating the RF carrier tone signal at the CRU.

7. A method as claimed in Claim 6, wherein the method further comprises:generating the RF carrier tone signal at the CRU using an optoelectrical Frequency Synthesizer.

8. A method as claimed in any preceding claim, wherein the method further comprises:processing the RF carrier tone signal at the remote radio unit using a photodetector to generate an electrical RF tone;processing the data signal at the remote radio unit to generate an analog electrical data signal; andmixing the electrical RF tone and the analog electrical data signal to generate the RoF signal.

9. A method as claimed in any preceding claim, wherein the method further comprises:emitting the RoF signal by the remote radio unit.

10. A method as claimed in any preceding claim, wherein the remote radio unit is an Antenna Unit (AU).

11. A system for Radio over Fibre (RoF) signal distribution, the system comprising a Central Radio Unit (CRU) and a remote radio unit, wherein:the CRU comprises processing circuitry and a non-transitory machine-readable medium storing instructions, the instructions executable by the processing circuitry, wherein the CRU is configured to obtain a radio frequency (RF) carrier tone signal, obtain a data signal, and transmit the RF carrier tone signal and the data signal to the remote radio unit using an optical fibre; andthe remote radio unit comprises processing circuitry and a non-transitory machine-readable medium storing instructions, the instructions executable by the processing circuitry, wherein the remote radio unit is configured to receive the RF carrier tone signal and the data signal using the optical fibre and process the RF carrier tone signal and the data signal to generate a Radio over Fibre (RoF) signal.

12. A system as claimed in Claim 11 , wherein the RF carrier tone signal is an analog signal and the data signal is one of: an analog signal, a digital signal, or a delta-sigma modulation signal .13 . A system as claimed in any of Claims 11 and 12, whereinthe CRU is further configured to transmit the RF carrier tone signal to the remote radio unit using a first optical fibre; andthe CRU is further configured to transmit the data signal to the remote radio unit using a second optical fibre.

14. A system as claimed in any of Claims 11 or 12, wherein the RF carrier tone signal has a RF wavelength (ARF), the data signal has a data wavelength (AD), the RF wavelength is a different wavelength value to the data wavelength, wherein the CRU comprises an optical signal coupler and the remote radio unit comprises a filter, and:wherein the CRU is further configured to process the data signal and the RF carrier tone signal using the optical signal coupler to generate a coupled signal and transmit the coupled signal to the remote radio unit using the optical fibre; andwherein the remote radio unit is configured to process the coupled signal using the filter to separate the data signal and the RF carrier tone signal.15 . A system as claimed in any of Claims 11 to 14, wherein the data signal is an electrical data signal, wherein the CRU comprises an Electrical-to-Optical (EO) converter and the remote radio unit comprises an Optical-to-Electrical (OE) converter, and wherein:the CRU is further configured to process the data signal using the EO converter to generate an optical data signal and transmit the optical data signal to the remote radio unit using the optical fibre; andthe remote radio unit is further configured to process the optical data signal using the OE converter to obtain the electrical data signal.

16. A system as claimed in any of Claims 11 to 15, wherein the CRU is further configured to generate the RF carrier tone signal.2117. A system as claimed in Claim 16, wherein the CRU further comprises an optoelectrical Frequency Synthesizer and wherein the CRU is further configured to generate the RF carrier tone signal using the optoelectrical Frequency Synthesizer .

18. A system as claimed in any of Claims 11 to 17, wherein the remote radio unit further comprises a photodetector, and wherein the remote radio unit is further configured to:process the RF carrier tone signal using the photodetector to generate an electrical RF tone;process the data signal to generate an analog electrical data signal; andmix the electrical RF tone and the analog electrical data signal to generate the RoF signal.

19. A system as claimed in any of Claims 11 to 18, wherein the remote radio unit is further configured to emit the RoF signal.

20. A system as claimed in any of Claims 11 to 19, wherein the remote radio unit is an Antenna Unit (AU).22

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