Patent Description:
Analogue ROF (Radio Over Fiber) exhibits great interest in RRU design especially in massive MIMO scenario for its advantage in volume, weight and power consumption as only O-E (optical-electrical) conversion and RF (radio frequency) amplification are required. In the meanwhile, the digital pre-distortion (DPD) is widely used to compensate RF front-end distortion, for example nonlinearity of the power amplifier. In the scope of analogue ROF, the uplink ROF is used as feedback channel for DPD algorithm, however, due to the distortion introduced by ROF feedback channel, DPD algorithm's performance is respectively degraded: especially, the non-linear distortion will greatly increase the ACLR (adjacent channel leakage ratio). In the following, ROF distortion is denoted by ΦROF(·) as shown in <FIG> which illustrates ROF channel's distortion. In <FIG>, signal xRF(t) <NUM> excites a directly modulated laser (DML) <NUM>. Output signal of laser <NUM> passes single mode fiber (SMF) <NUM> after which post-distortion (PD) <NUM> provides signal yRF(t) <NUM>.

The DPD degradation can be explained with help of <FIG>, where s(f), <NUM> is the desired signal, x(t), <NUM> is the pre-distorted signal, y'(t), <NUM> is the feedback signal, y(t), <NUM> is the transmitted signal. Ψ(·) is used to denote the nonlinear system <NUM>, such like power-amplifier, and Φ(·) to denote the feedback channel <NUM>, like ROF channel. As DPD training algorithm <NUM> targets y'(t)=s(t), the truly transmit signal y(t) <NUM> is close to Φ-<NUM>(s), which is distorted version of s(t), <NUM>.

The common solution is to mitigate the feedback channel's distortion. As discussed in "<NPL>", the major distortion for the ROF is caused by the joint-work of fiber's chromatic dispersion and laser chirp effect. To avoid such degradation, a dispersion chromatic module (DCM) can be used and it has been proven effective in test. But this solution increases significantly the hardware cost. At current state-of-the-art, a digital method operating with baseband signal seems to be more attractive, namely the post-distortion as recommended in "<NPL>".

The dispersion chromatic module (DCM) consists of using passive optic device that compensates the chromatic dispersion introduced in fiber transmission. It operates with optic signal and has proven effective for the non-linear behaviour of analogue ROF. However, this device is relatively expensive and in practice it must be adapted to specific fiber in use, for example <NUM> <NUM>. Moreover, the use of DCM will introduce the attenuation of optic signal's strength that is definitely not wished in real network design.

At current state-of-the-art, a digital method operating with baseband signal seems to be more attractive, namely the post-distortion as recommended in "<NPL>", see <FIG> illustrating training sequence based post-distortion at RRU side <NUM>. This method is composed of: (<NUM>) RRU <NUM> sends a training signal that can approximate the statistical behaviour of the ROF input signal <NUM>; (<NUM>) central unit performs nonlinear system identification by using the known training signal; (<NUM>) perform non-linear post-distortion <NUM> on the received signal <NUM> for both UL <NUM> and feedback <NUM> use. The disadvantages of such post-equalization method are, in order to send the training signal from RRU sides <NUM>, it is required first the use of large memory to store the digital signal, then DAC <NUM> and RF signal modulation with respect to the feedback RF signal <NUM> center frequency requires additional hardware. Moreover, the LO fed to up-converter <NUM> needs to be perfectly synchronized with LO of down-converter at central unit side.

In order to simplify the RRU design <NUM>, the input signal power can be reduced by taking into account that the non-linear distortion is proportional to the cubic of input signal's power such that at certain power level, the non-linear effect can be neglectable and the system ΦROF(·) can be approximated by a linear system: <MAT>.

Although the SNR will be reduced accordingly, this setting is welcome when high linearity of system is required. The linearization requires that the feedback ROF channel contains neglectable linear distortion; however, this condition doesn't hold always true due to hardware impairment.

<CIT> relates to electro-absorption duplexer. <CIT> relates to Millimeter wave fiber-optically linked antenna receiver device. <CIT> relates to normalization methods for automatic frequency compensation in Bluetooth applications. <CIT> relates to wideband enhanced digital injection pre-distortion system and method.

It is the object of the disclosure to provide techniques for solving the above described problems, i.e. to reduce the hardware complexity and costs for implementing radio over fiber (ROF) systems, in particular ROF systems using digital predistortion (DPD) and feedback as shown in <FIG>.

A basic idea of the disclosure is to use a very compact radio remote unit (RRU) design by employing a simple stimulus signal generation with assumption that there is no knowledge of the stimulus sequence at the receiver side. In such scenario, blind equalization is performed and the following system design, namely BPSK-aided post-distortion, is applied with a carrier signal (LO) modulated by a binary sequence randomly generated at RRU side. The power of RF signal as input to uplink ROF (DML) is controlled by CU. Such a design is described below with respect to <FIG>.

The disclosed RRU design can be characterized by the following features:.

In order to describe the disclosure in detail, the following terms, abbreviations and notations will be used:.

According to a first aspect, the disclosure relates to a remote radio unit, RRU, comprising: a binary phase shift keying, BPSK, modulator, configured to modulate a BPSK waveform by a local oscillator, LO, signal to generate a stimulus signal, wherein the LO signal is derived from a downlink optical signal received via downlink radio over fiber, DL-ROF, from a central unit, CU; and an optical signal generator, in particular a laser, configured to generate an uplink optical signal based on the stimulus signal for transmission via uplink radio over fiber, UL-ROF, to the CU.

Such a RRU design can be implemented at reduced hardware complexity and costs. The RRU design can implement radio over fiber (ROF) systems, in particular ROF systems using digital predistortion (DPD) and feedback.

Such a RRU can be implemented by a compact and low-cost RRU design, the same stimulus signal can be reused for multiple/massive MIMO case. The RRU design provides a high quality UL-ROF channel and a high quality feedback ROF channel for DPD application. At central unit side, simple equalization algorithms can be implemented. Besides, there are no-extra ROF link requirement for TDD system and on-line calibration for adaptive ROF deployment.

In an exemplary implementation form of the RRU, a power of the uplink optical signal is controlled by the CU via control channel.

This provides the advantage that non-linear distortions can be effectively controlled by the CU.

In an exemplary implementation form of the RRU, the RRU comprises a band pass filter, BPF, configured to retrieve the LO signal from the downlink optical signal.

This provides the advantage that the band pass filter can filter out harmonics of the received signal due to non-linear distortions of the channel.

In an exemplary implementation form of the RRU, the downlink optical signal carries the LO signal of carrier frequency fc and harmonics of the carrier frequency fc.

This provides the advantage that the downlink optical signal can be used as control signal for the BPSK modulator as it carries information about the carrier frequency fc from the CU. The transmission efficiency is improved, as less resources are used.

In an exemplary implementation form of the RRU, a pass band frequency range of the BPF includes the carrier frequency fc.

This provides the advantage that the band pass filter is adjusted to pass the carrier frequency fc from CU to the BPSK modulator.

In an exemplary implementation form of the RRU, the RRU comprises a pseudo-random binary sequence, PRBS, generator or a white noise generator configured to generate the BPSK waveform.

This provides the advantage that such a PRBS or white noise generator is easy to implement, e.g. by using a shift register design.

In an exemplary implementation form of the RRU, the RRU is operated in time division duplex, TDD, mode, a downlink, DL, phase of the TDD mode is used for reception of the DL optical signal via DL-ROF, an uplink, UL, phase of the TDD mode is used for transmission of the UL optical signal via UL-ROF, and a DL/UL or UL/DL switch interval is used for training and/or calibration.

This provides the advantage that the design is compact and efficient due to the different phases of the TDD mode in which the different tasks of the RRU can be realized.

According to a second aspect, the disclosure relates to a central unit, CU, comprising: an optical signal generator, in particular a laser, configured to generate a downlink optical signal based on a downlink digital signal for transmission via downlink radio over fiber, DL-ROF, to a radio remote unit, RRU; a digital pre-distorter, DPD, configured to digitally pre-distort the downlink digital signal based on a DPD feedback signal; and a blind linear digital channel equalizer, configured to provide the DPD feedback signal based on an uplink optical signal received via uplink radio over fiber, UL-ROF, from the RRU.

Such a CU can be implemented by a compact and low-cost CU design, the same stimulus signal can be reused for multiple/massive MIMO case. The CU design provides a high quality UL-ROF channel and a high quality feedback ROF channel for DPD application. Simple equalization algorithms can be implemented. Besides, there are no-extra ROF link requirement for TDD system and on-line calibration for adaptive ROF deployment.

In an exemplary implementation form of the CU, the CU is configured to apply a decision-directed least-mean-squares, DD-LMS, algorithm on an uplink digital signal derived from the uplink optical signal to determine equalization coefficients of the blind linear digital channel equalizer.

Such a CU can implement simple equalization algorithms such as the DD-LMS, reducing hardware (and/or software) complexity.

In an exemplary implementation form of the CU, the CU is configured to vary a gain of the uplink optical signal generated at the RRU via a control channel with the RRU to identify a non-linear distortion introduced by the UL-ROF.

This provides the advantage that non-linear distortion can be controlled and minimized.

In an exemplary implementation form of the CU, the CU is configured to identify an amplitude-to-amplitude modulation, AM-AM, response of the UL-ROF based on the gain variation of the uplink optical signal.

This provides the advantage that the AM-AM response can be controlled and optimized.

In an exemplary implementation form of the CU, the CU is configured to identify the non-linear distortion introduced by the UL-ROF based on an approximation as a memory-less non-linear system, in particular by an N-L or Hammerstein model.

This provides the advantage that an efficient model can be applied for describing the non-linear distortion and by that model optimizing the MIMO system.

In an exemplary implementation form of the CU, the CU is configured to adjust the blind linear digital channel equalizer based on the relation: <MAT> where uBB denotes digital baseband representation of the uplink optical signal, hUL,CU denotes linear distortion introduced at the CU, ΦUL,ROF denotes non-linear distortion introduced by the UL-ROF, g denotes gain of the uplink optical signal generated at the RRU, rBB denotes digital baseband representation of a BPSK waveform at the RRU used to generate the uplink optical signal and nBB denotes a distortion signal.

This provides the advantage that such adjustment of the blind equalization can be easily implemented and provides fast convergence and good tracking performance.

In an exemplary implementation form of the CU, the CU is operated in time division duplex, TDD, mode, a downlink, DL, phase of the TDD mode is used for transmission of the DL optical signal via DL-ROF, an uplink, UL, phase of the TDD mode is used for reception of the UL optical signal via UL-ROF, and a DL/UL or UL/DL switch interval is used for training and/or calibration.

This provides the advantage that the design is compact and efficient due to the different phases of the TDD mode in which the different tasks of the CU can be realized.

In an exemplary implementation form of the CU, the CU is configured to send a local oscillator, LO, signal of carrier frequency fc via the DL-ROF to the RRU.

This provides the advantage that the RRU has information about the carrier frequency at the CU and hence can optimally adjust the BPSK modulator.

According to a third aspect, the disclosure relates to a multiple-input multiple-output, MIMO, system, comprising: a central unit, CU, according to the second aspect described above; and a remote radio unit, RRU, according to the first aspect described above, coupled to the CU by a single mode fiber, SMF.

Such a MIMO system can be efficiently implemented by a compact and low-cost RRU and CU design, the same stimulus signal can be reused for multiple/massive MIMO case. The MIMO system provides a high quality UL-ROF channel and a high quality feedback ROF channel for DPD application. Simple equalization algorithms can be implemented. Besides, there are no-extra ROF link requirement for TDD system and on-line calibration for adaptive ROF deployment.

According to a fourth aspect, the disclosure relates to a method for generating an uplink optical signal by a remote radio unit, RRU, the method comprising: receiving a downlink optical signal received via downlink radio over fiber, DL-ROF, from a central unit, CU; generating a stimulus signal based on a binary phase shift keying, BPSK, modulation of a BPSK waveform by a local oscillator, LO, signal, wherein the LO signal is derived from the downlink optical signal; and generating, by an optical signal generator, in particular a laser, an uplink optical signal based on the stimulus signal for transmission via uplink radio over fiber, UL-ROF, to the CU.

Such a method can be easily implemented by a compact and low-cost RRU and CU design, the same stimulus signal can be reused for multiple/massive MIMO case. The method provides a high quality UL-ROF channel and a high quality feedback ROF channel for DPD application.

According to a fifth aspect, the disclosure relates to a method for generating a downlink optical signal by a central unit, CU, the method comprising: generating, by an optical signal generator, in particular a laser, a downlink optical signal based on a downlink digital signal for transmission via downlink radio over fiber, DL-ROF, to a radio remote unit, RRU; digitally pre-distorting, by a digital pre-distorter, DPD, the downlink digital signal based on a DPD feedback signal; and providing, by a blind linear digital channel equalizer, the DPD feedback signal based on an uplink optical signal received via uplink radio over fiber, UL-ROF, from the RRU.

Such a method can be easily implemented with a compact and low-cost CU and RRU design, the same stimulus signal can be reused for multiple/massive MIMO case. The method provides a high quality UL-ROF channel and a high quality feedback ROF channel for DPD application. Simple equalization algorithms can be implemented.

According to a sixth aspect, the disclosure relates to a computer program product including computer executable code or computer executable instructions that, when executed, causes at least one computer to execute the method according to the fourth or fifth aspect. Such a computer program product may include a non-transient readable storage medium storing program code thereon for use by a processor, the program code comprising instructions for performing the methods or the computing blocks as described hereinafter.

Further implementations of the disclosure will be described with respect to the following figures, in which:.

It is understood that comments made in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa.

The methods, devices and systems described herein may particularly be implemented in radio over fiber (ROF) communications using remote radio units and central units.

Radio over fiber (RoF) refers to a technology whereby light is modulated by a radio frequency signal and transmitted over an optical fiber link. Main technical advantages of using fiber optical links are lower transmission losses and reduced sensitivity to noise and electromagnetic interference compared to all-electrical signal transmission. Applications range from the transmission of mobile radio signals (e.g. <NUM>, <NUM>, <NUM> and WiFi), the transmission of cable television signal and satellite communications.

In the area of Wireless Communications one main application is to facilitate wireless access, such as <NUM> and WiFi simultaneous from the same antenna. In other words, radio signals are carried over fiber-optic cable. Thus, a single antenna can receive any and all radio signals (<NUM>, Wifi, cell, etc.. ) carried over a single-fiber cable to a central location where equipment then converts the signals.

A remote radio unit (RRU), also called a remote radio head (RRH) in wireless networks, is a remote radio transceiver that connects to an operator radio control panel via electrical or wireless interface.

In wireless system technologies such as GSM, CDMA, UMTS, LTE, <NUM> the radio equipment is remote to the BTS/NodeB/eNodeB/gNodeB (also referred to as the central unit). The equipment is used to extend the coverage of a BTS/NodeB/eNodeB/gNodeB in challenging environments such as rural areas or tunnels. They are generally connected to the BTS/NodeB/eNodeB/gNodeB via a fiber optic cable using Common Public Radio Interface protocols.

RRUs have become one of the most important subsystems of today's new distributed base stations. The RRU contains the base station's RF circuitry plus analog-to-digital/digital-to-analog converters and up/down converters. RRUs also have operation and management processing capabilities and a standardized optical interface to connect to the rest of the base station. Remote radio units make MIMO operation easier; they increase a base station's efficiency and facilitate easier physical location for gap coverage problems.

The methods, devices and systems described herein may particularly utilize PRBS and BPSK generators.

A pseudorandom binary sequence (PRBS) is a binary sequence that, while generated with a deterministic algorithm, is difficult to predict and exhibits statistical behavior similar to a truly random sequence. Pseudorandom binary sequences can be generated using linear feedback shift registers.

BPSK (binary phase shift keying) is the simplest form of phase shift keying (PSK). It uses two phases which are separated by <NUM>° and so can also be termed <NUM>-PSK. It does not particularly matter exactly where the constellation points are positioned. Therefore, it handles the highest noise level or distortion before the demodulator reaches an incorrect decision. That makes it the most robust of all the PSKs.

The described devices may include integrated circuits and/or passives and may be manufactured according to various technologies. For example, the circuits may be designed as logic integrated circuits, analog integrated circuits, mixed signal integrated circuits, optical circuits, memory circuits and/or integrated passives.

The devices and systems described herein may include processors or processing devices, memories and transceivers, i.e. transmitters and/or receivers. In the following description, the term "processor" or "processing device" describes any device that can be utilized for processing specific tasks (or blocks or steps). A processor or processing device can be a single processor or a multi-core processor or can include a set of processors or can include means for processing. A processor or processing device can process software or firmware or applications etc..

<FIG> shows a block diagram of a MIMO system <NUM> with Central Unit (CU) <NUM> and Radio Remote Unit (RRU) <NUM> applying BPSK-aided post-distortion according to the disclosure.

Such a multiple-input multiple-output (MIMO) system <NUM> comprises a central unit <NUM> and a remote radio unit, RRU <NUM> that is coupled to the CU by a single mode fiber, SMF.

The RRU <NUM> comprises a binary phase shift keying, BPSK, modulator <NUM> that is configured to modulate a BPSK waveform <NUM> by a local oscillator, LO, signal <NUM> to generate a stimulus signal <NUM>. The LO signal <NUM> is derived from a downlink optical signal <NUM> (e.g. as shown in <FIG>) received via downlink radio over fiber, DL-ROF <NUM>, from the central unit, CU <NUM>. The RRU <NUM> further comprises an optical signal generator <NUM>, e.g. a laser (or a photo-diode) that is configured to generate an uplink optical signal <NUM> (e.g. as shown in <FIG>) based on the stimulus signal <NUM> for transmission via uplink radio over fiber, UL-ROF <NUM>, to the CU <NUM>.

A power of the uplink optical signal <NUM> may be controlled by the CU <NUM> via a control channel (represented as the dashed line between CU <NUM> and RRU <NUM> in <FIG>).

The RRU <NUM> may comprise a band pass filter, BPF (e.g. a BPF <NUM> as shown in <FIG>) that is configured to retrieve the LO signal <NUM> from the downlink optical signal <NUM>. The downlink optical signal <NUM> may carry the LO signal <NUM> of carrier frequency fc and harmonics of the carrier frequency fc. A pass band frequency range of the BPF <NUM> may include the carrier frequency fc.

The RRU <NUM> may further include a pseudo-random binary sequence, PRBS, generator <NUM> or a white noise generator that is configured to generate the BPSK waveform <NUM>.

The RRU <NUM> may be operated in time division duplex, TDD, mode, e.g. as illustrated in <FIG>. In particular, a downlink, DL, phase of the TDD mode may be used for reception of the DL optical signal <NUM> via DL-ROF <NUM>, an uplink, UL, phase of the TDD mode may be used for transmission of the UL optical signal <NUM> via UL-ROF <NUM>, and a DL/UL or UL/DL switch interval may be used for training and/or calibration, e.g. as described below with respect to <FIG>.

The CU <NUM> comprises an optical signal generator <NUM>, e.g. a laser (or a photo-diode) that is configured to generate a downlink optical signal, e.g. a signal <NUM> as shown in <FIG>, based on a downlink digital signal <NUM> for transmission via downlink radio over fiber, DL-ROF <NUM>, to the RRU <NUM>. The CU <NUM> further includes a digital pre-distorter, DPD <NUM>, that is configured to digitally pre-distort the downlink digital signal <NUM> based on a DPD feedback signal <NUM>. The CU <NUM> further includes a blind linear digital channel equalizer <NUM> that is configured to provide the DPD feedback signal <NUM> based on an uplink optical signal, e.g. signal <NUM> shown in <FIG>, received via uplink radio over fiber, UL-ROF <NUM>, from the RRU <NUM>.

The CU <NUM> may be configured to apply a decision-directed least-mean-squares, DD-LMS, algorithm on an uplink digital signal derived from the uplink optical signal <NUM> to determine equalization coefficients of the blind linear digital channel equalizer <NUM>.

The CU <NUM> may be configured to vary a gain of the uplink optical signal <NUM> generated at the RRU <NUM> via a control channel with the RRU <NUM> to identify a non-linear distortion introduced by the UL-ROF <NUM>. The CU <NUM> may be configured to identify an amplitude-to-amplitude modulation, AM-AM, response (e.g. determined by AM-AM compensator <NUM> shown in <FIG>) of the UL-ROF <NUM> based on the gain variation of the uplink optical signal <NUM>.

The CU <NUM> may be configured to identify the non-linear distortion introduced by the UL-ROF <NUM> based on an approximation as a memory-less non-linear system, in particular by an N-L or Hammerstein model. The Hammerstein model is a special model form for non-linear dynamic systems named after Adolf Hammerstein. Characteristic is the structure consisting of the series connection of a static non-linearity in front of a linear time-invariant dynamic system. The Hammerstein model is defined for both single and multi-size systems.

The CU <NUM> may be configured to adjust the blind linear digital channel equalizer <NUM> based on the relation: <MAT> where uBB denotes digital baseband representation of the uplink optical signal <NUM>, hUL,CU denotes linear distortion introduced at the CU <NUM>, ΦUL,ROF denotes non-linear distortion introduced by the UL-ROF <NUM>, g denotes gain of the uplink optical signal <NUM> generated at the RRU <NUM>, rBB denotes digital baseband representation of a BPSK waveform at the RRU <NUM> used to generate the uplink optical signal <NUM> and nBB denotes a distortion signal.

The CU <NUM> may be operated in time division duplex, TDD, mode, e.g. as illustrated in <FIG>. In particular, a downlink, DL, phase of the TDD mode may be used for transmission of the DL optical signal via DL-ROF, an uplink, UL, phase of the TDD mode may be used for reception of the UL optical signal via UL-ROF, and a DL/UL or UL/DL switch interval may be used for training and/or calibration, e.g. as described below with respect to <FIG>.

The CU <NUM> may be configured to send a local oscillator, LO, signal of carrier frequency fc via the DL-ROF <NUM> to the RRU <NUM>.

The MIMO system <NUM> shown in <FIG> can be implemented with a very compact radio remote unit (RRU) design by employing a simple stimulus signal generation with assumption that there is no knowledge of the stimulus sequence at the receiver side. In such scenario, blind equalization is performed and the following system design, namely BPSK-aided post-distortion, is applied with a carrier signal (LO) modulated by a binary sequence randomly generated at RRU side. The power of RF signal as input to uplink ROF (DML) is controlled by CU.

The MIMO system <NUM> design can be characterized by the following features:.

<FIG> shows a block diagram of a remote radio unit (RRU) <NUM> configured to apply BPSK-aided blind equalization according to the disclosure. The RRU <NUM> represents an implementation of the RRU <NUM> described above with respect to <FIG>.

The optic signal <NUM> received from SMF, i.e. via DL-ROF <NUM> as described above with respect to <FIG> passes post-distortion module (PD) <NUM> and power amplifier (PA) <NUM>. After PD <NUM> (and also after PA), the received signal can be represented by a carrier frequency component and a plurality of harmonics. The receive signal after PA <NUM> branches to a first branch with attenuator <NUM> that provides feedback signal x(t) <NUM> that is forwarded via first sub-branch to modulator switch <NUM> and via second sub-branch to bandpass filter (BPF) <NUM> and to BPSK modulator <NUM>. The receive signal after PA <NUM> branches to a second branch with antenna switch <NUM> where it is switched to antenna <NUM> and as uplink signal <NUM> to modulator switch <NUM>. BPSK modulator <NUM> is fed by a modulation waveform <NUM> that is generated by a BPSK generator <NUM>. The BPF <NUM> is configured to filter out the carrier frequency component from the plurality of harmonics of the received optical signal <NUM>. This carrier frequency component controls the BPSK modulator <NUM>. After modulator switch <NUM> the generated output signal is passed to a variable gain amplifier (VGA) <NUM> controlling a gain g of the output signal. Via a control channel (not depicted in <FIG>) the gain can be controlled <NUM> by the CU <NUM>. The amplified output signal is used to excite the optical generator <NUM>, i.e. directly modulated laser <NUM> that generates the optic signal <NUM> fed to SMF <NUM> for transmission via UL/FB-ROF <NUM> as shown in <FIG>.

In the RRU <NUM> depicted in <FIG>, a LO signal is sent by CU <NUM> via DL-ROF <NUM>, denoted by xRF,DL(t) in <FIG> that describes a CU implementation <NUM>.

The received signal yDL,RRU(t) <NUM> (output of DL-ROF <NUM>) or y'DL, RRU(t) (PA output) is actually the LO signal and its harmonics, considering the non-linear effect of DL-ROF <NUM> and PA <NUM>. This RF signal is then fed to BPSK modulator <NUM>, after the filtering the harmonics by using BPF <NUM> for fc (Band Pass Filter), the CU's LO can be perfectly recovered. Since the LO and its harmonics are far spaced in frequency domain, for example <NUM>, the BPF <NUM> is easy to design. The benefit of using CU forwarded LO signal is this can effectively remove the carrier-frequency offset (CFO). The linear or non-linear distortion are all mitigated as well.

At RRU side <NUM>, the BPSK sequence or baseband waveform generator <NUM> is installed and the generated sequence <NUM> is not necessarily known to CU <NUM>. This BPSK signal generator <NUM> can be either a PRBS (pseudo-random binary sequence) generator or filtered white noise generator. The interesting part is the simplified RRU design <NUM> since the random binary sequence generator <NUM>, the BPSK modulator <NUM> and the filter can be implemented as cheap and compact units (hardware circuits), e.g. by using standard integrated circuits.

The modulated RF signal, denoted by rRF(t) is amplified by a gain g, controlled by CU <NUM> via low-rate digital control.

<FIG> shows a block diagram of a central unit (CU) <NUM> configured to apply BPSK-aided blind equalization according to the disclosure. The CU <NUM> represents an implementation of the CU <NUM> described above with respect to <FIG>. It can be combined with the RRU design <NUM> described above with respect to <FIG> to a MIMO system as shown in <FIG>.

In the CU design <NUM> a digital input signal <NUM> is passed to digital pre-distortion (DPD) unit <NUM>. The output of DPD <NUM> passes digital-to-analogue converter (DAC) <NUM> and upconverter <NUM> before it excites a directly modulated laser (DML) <NUM> (i.e. optical signal generator) to generate optic signal <NUM> to SMF. In the receive path, optic signal <NUM> is received from SMF <NUM> (see <FIG>) and passes post-distortion (PD) unit <NUM>, down-converter (D/C) <NUM>, analogue-to-digital converter (ADC) <NUM> where it is converted to a digital receive signal uBB(n). This digital receive signal uBB(n) is input to blind channel compensator <NUM> using for example DD-LMS training algorithm. After AM-AM compensation block <NUM> compensated digital signal <NUM> is used to control DPD <NUM>.

At CU side <NUM>, blind channel equalization such like decision-directed least-mean-square (DD-LMS) algorithm is executed to find the linear equalizer's coefficients. Then, the gain applied on rRF(t) <NUM> (see <FIG>) is varied to identify the nonlinear distortion. This processing can be detailed as follows:.

The feedback system can be modelled as: <MAT> where hUL,CU(t) denotes the linear distortion introduced at CU <NUM>, <NUM> (LNA, mixer, LPF, ADC, etc.) and ΦUL,ROF(·) denotes the nonlinear distortion introduced by UL-ROF <NUM>. Note that the linear distortion at RRU side <NUM> can be omitted since typically BPSK modulator <NUM> exhibits less linear distortion (on RF signal) compared with conventional up-conversion approach. Based on the fact that ΦUL,ROF(·) can be approximated as a memory-less non-linear system, see "<NPL>", the ΦUL,ROF(·) can be described by using an AM-AM model <NUM> as shown in <FIG> (equivalent to the absolute value of the complex baseband signal) such that the feedback ROF channel can be approximated by an N-L or Hammerstein model.

<FIG> shows a performance diagram <NUM> illustrating the memoryless non-linear effect for AM-AM. Graph <NUM> represents the non-linear system that can be approximated as linear system (depicted by graph <NUM>). A good approximation can be obtained for input values between <NUM> and <NUM> while a still acceptable approximation can be obtained for input values between <NUM> and <NUM>. Thus the non-linear system can be approximated as memoryless.

<FIG> shows a symbol diagram <NUM> in the complex plane for unequalized ROF transmission and <FIG> shows a symbol diagram <NUM> in the complex plane for equalized ROF transmission using BPSK-aided equalization according to the disclosure.

When using BPSK waveform with constant power (amplitude), the ROF non-linear distortion ΦUL,ROF will not impact the compensation on linear distortion as hUL,CU is compensated: a BPSK waveform after memoryless non-linear distortion is still a BPSK waveform. Thus, the conventional blind equalization technique DD-LMS can be used to compensate hUL,CU and then identify the AM-AM response of ΦUL,ROF by varying the VGA gain g at RRU via control channel. The calibration is the inverse of ΦUL,ROF.

The symbol diagrams <NUM> and <NUM> show that the feedback signal quality can be effectively improved. <FIG> and <FIG> are examples for a <NUM> ROF transmission where the EVM is improved from <NUM>% to <NUM>%.

<FIG> shows a schematic diagram of a TDD system <NUM> illustrating TDD operation and ROF channel calibration according to the disclosure.

The calibration of UL/DL-ROF has been designed for a time division duplex (TDD) system <NUM> as shown in <FIG>. The time intervals <NUM>, <NUM> are used for downlink signal transportation and downlink feedback signal transportation while time intervals <NUM> and <NUM> are used as IDLE state and for uplink signal transportation, i.e. according to a usual TDM system:.

In the new system design according to the disclosure, the DL/UL <NUM>, <NUM> or UL/DL <NUM>, <NUM> switch interval can be used for dedicated calibration:.

Since the calibration procedure can be executed in an adaptive filter manner, the post-distortion can track the quick variation of the environment so compared with conventional off-line training method, this approach is more suitable for on-line calibration.

<FIG> shows a schematic diagram illustrating a method for generating an uplink optical signal by a remote radio unit (RRU) according to the disclosure, for example by a RRU as described above with respect to <FIG> and <FIG>.

The method <NUM> comprises receiving <NUM> a downlink optical signal received via downlink radio over fiber, DL-ROF, from a central unit, CU, e.g. a CU as described above with respect to <FIG> and <FIG>.

The method <NUM> comprises generating <NUM> a stimulus signal based on a binary phase shift keying, BPSK, modulation of a BPSK waveform by a local oscillator, LO, signal, wherein the LO signal is derived from the downlink optical signal, e.g. as described above with respect to <FIG> and <FIG>.

The method <NUM> comprises generating <NUM>, by an optical signal generator, in particular a laser, an uplink optical signal based on the stimulus signal for transmission via uplink radio over fiber, UL-ROF, to the CU, e.g. as described above with respect to <FIG> and <FIG>.

<FIG> shows a schematic diagram illustrating a method for generating a downlink optical signal by a central unit (CU) according to the disclosure, for example by a CU as described above with respect to <FIG> and <FIG>.

The method <NUM> comprises generating <NUM>, by an optical signal generator, in particular a laser, a downlink optical signal based on a downlink digital signal for transmission via downlink radio over fiber, DL-ROF, to a radio remote unit, RRU, e.g. a RRU as described above with respect to <FIG> and <FIG>.

The method <NUM> comprises digitally pre-distorting <NUM>, by a digital pre-distorter, DPD, the downlink digital signal based on a DPD feedback signal, e.g. as described above with respect to <FIG> and <FIG>.

The method <NUM> comprises providing <NUM>, by a blind linear digital channel equalizer, the DPD feedback signal based on an uplink optical signal received via uplink radio over fiber, UL-ROF, from the RRU, e.g. as described above with respect to <FIG> and <FIG>.

The present disclosure also supports a computer program product including computer executable code or computer executable instructions that, when executed, causes at least one computer to execute the performing and computing steps described herein, in particular the methods and procedures described above. Such a computer program product may include a readable non-transitory storage medium storing program code thereon for use by a computer. The program code may perform the processing and computing steps described herein, in particular the methods and procedures described above.

While a particular feature or aspect of the disclosure may have been disclosed with respect to only one of several implementations, such feature or aspect may be combined with one or more other features or aspects of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms "include", "have", "with", or other variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term "comprise". Also, the terms "exemplary", "for example" and "e.g." are merely meant as an example, rather than the best or optimal. The terms "coupled" and "connected", along with derivatives may have been used. It should be understood that these terms may have been used to indicate that two elements cooperate or interact with each other regardless whether they are in direct physical or electrical contact, or they are not in direct contact with each other.

Claim 1:
A remote radio unit, RRU (<NUM>), comprising, a band-pass filter, BPF (<NUM>), a binary phase shift keying, BPSK, modulator (<NUM>), a variable gain amplifier (<NUM>) and an optical signal generator (<NUM>), wherein:
the BPF (<NUM>) is configured to derive a local oscillator, LO, signal (<NUM>) from a downlink optical signal (<NUM>) received via a downlink radio over fiber, DL-ROF (<NUM>), from a central unit, CU (<NUM>)
the BPSK modulator (<NUM>) is configured to modulate a BPSK waveform (<NUM>) on to the LO signal (<NUM>) to generate a stimulus signal (<NUM>) and provide the stimulus signal to the variable gain amplifier (<NUM>),
the variable gain amplifier (<NUM>) is configured to apply a gain to the stimulus signal and provide the stimulus signal after amplification to the optical signal generator;
the optical signal generator (<NUM>) is configured to generate a to-be-transmitted uplink optical signal (<NUM>) based on the stimulus signal (<NUM>) for transmission via uplink radio over fiber, UL-ROF (<NUM>), to the CU (<NUM>) and transmit the to-be-transmitted uplink optical signal to the CU (<NUM>);
wherein: a received uplink optical signal corresponding to the to-be-transmitted optical signal is defined by the following equation: <MAT>
where uBB denotes a digital baseband representation of the received uplink optical signal (<NUM>), hUL,CU denotes linear distortion introduced at the CU (<NUM>), ΦUL,ROF denotes a non-linear distortion introduced by the UL-ROF (<NUM>), g denotes a gain applied to the stimulus signal by the variable gain amplifier, rBB denotes a digital baseband representation of the BPSK waveform and nBB denotes a distortion signal,
wherein the variable gain amplifier (<NUM>) is further configured to receive via a control channel a varying gain control signal sent by the CU (<NUM>) and vary a gain applied to the stimulus signal in response to a variation in the gain control signal, whereby the CU (<NUM>) is enabled to identify an amplitude-to-amplitude modulation, AM-AM, response (<NUM>) of the UL-ROF (<NUM>) and further to identify a non-linear distortion introduced by the UL-ROF (<NUM>).