Patent Description:
One of the major transmission solutions for high-bandwidth wireless communication systems is based on optical for the transmission of radio signals between the central unit and the remote radio unit (RRU) of a base transceiver station (or short "base station"). Such a communication system is generally referred to as a radio over fiber (RoF) system. In a RoF system, the radio signal is used for modulating the intensity of an optical carrier. <FIG> shows a simplified diagram of a conventional RoF link or system, i.e. base transceiver station <NUM>, where the radio signal is converted into an optical signal by the RoF transceiver of the central unit <NUM> comprising an electronic-to-optical (EO) converter. The optical signal is transmitted through the fiber <NUM> and detected by another RoF transceiver at the remote radio unit <NUM>, where an optical-to-electronic (OE) converter recovers the original RF signal, which is amplified and transmitted by the antenna of the RRU. This technique of transmitting the RF signals using an optical carrier over the fiber has numerous advantages over the conventional cooper wire solutions, such as low attenuation loss, large bandwidth, and reduced power consumption to name a few.

There are two main types of RoF communications systems, namely analog RoF systems and digital or digitized RoF systems.

<FIG> shows a conventional analog RoF system comprising a RRU <NUM> and a central unit <NUM> coupled by a RoF link <NUM>. In the exemplary analog RoF system shown in <FIG> the RRU <NUM> just comprises a ROF transceiver or communication interface and a power amplifier (PA). The ADC (Analog to Digital Converter) and DAC (Digital to Analog Converter) are provided in the central unit <NUM> (together with a DSP unit and a ROF transceiver).

<FIG> shows a conventional digital ROF system <NUM>, where the ADC and DAC are provided in the RRU <NUM> instead of the central unit <NUM>, which increases the hardware complexity of the RRU <NUM>. In large size deployments, such as massive MIMO millimeter wave applications, many channels are put into one RRU (for example, <NUM> antennas in one RRU for massive MIMO, which means <NUM> RoF, <NUM> PA and other accessories such as filter and the like also inside the RRU). This substantially increases the size of the RRU and, thus, makes it difficult to deploy in practice.

One of the main challenges of analog RoF systems is to increase the system linearity for long distance transmission (e.g. the chromatic dispersion effect in a <NUM> fiber), as illustrated by the following simple application scenario: OFDM (orthogonal frequency division multiplexing) baseband width <NUM>; radio frequency <NUM>; fiber length <NUM>; directly modulated lasers (DML); a single fiber for a single RF channel, where the downlink optical wavelength is <NUM> and the uplink wavelength is <NUM>. For this simple application scenario the single link performance ACPR (adjacent channel power ratio) may be reduced by <NUM>-<NUM> dBc because of the combined effect of laser chirp and optical fiber dispersion.

There have been some attempts to address the problems of analog RoF systems for long transmission distances and to increase the whole system performance with a few additional hardware components and optimized algorithms.

For instance, it has been proposed to use traditional digital pre-distortion for downlink nonlinearity compensation. A post-distorter can improve the uplink performance, but a training signal is needed as well, which, in turn, increases the complexity of the remote ratio unit, for instance, in terms of hardware. The training signal is required for compensating the nonlinearity of the RoF uplink, because otherwise it will result in a collapse of the performance (see <NPL>; and <NPL>).

The training signal is usually in the digital baseband. If the baseband in the RRU is increased, a RF transceiver, a DAC and other components have to be added to the RRU, which will again increase the size of the RRU and, thus, might render such a RRU impractical for an actual deployment.

As shown in <FIG>, in a conventional ROF system the training signal is added by a training signal module <NUM> of the RRU <NUM>, and an algorithm implemented in the central unit 130a is used to identify the uplink channel 120b. <FIG> illustrates the training signal <NUM> module for generating the conventional training signal used in the system shown in <FIG> in more detail. Typically, the training signal generation in the RRU <NUM> is realized by a digital baseband signal generator 111a, a DAC 111c, a RF modulator 111d, whose frequency is synchronized with the demodulation frequency at the central unit <NUM>, a digital upconverter (DUC) 111b, and a synchronizing unit (Syn) for synchronizing the carrier. As will be appreciated, all of these components will add to the cost, size and weight of the conventional RRU <NUM>.

Thus, there is a need to provide an improved remote radio unit and an improved central unit for a base transceiver station.

It is an object of the disclosure to provide an improved remote radio unit and an improved central unit for a base transceiver station.

Generally, implementations of the present disclosure are based on the idea to use a noise generator for generating a stimulus signal having known statistical properties at the RRU side and to apply blind identification/equalization on the uplink ROF channel. To this end, in implementations of the present disclosure, SOS-based blind equalization is performed. Implementations of the present disclosure provide the necessary architecture for allowing both linear and nonlinear blind equalization. According to further implementations of the present disclosure a power control unit is introduced in the RRU to make the system robust and adaptive in a real environment.

More specifically, according to a first aspect the disclosure relates to a remote radio unit, RRU, for a base transceiver station. The remote radio unit comprises: a noise generator configured to provide a radio-frequency, RF, noise signal having predefined statistical properties, in particular second-order statistics and/or higher-order statistics; a communication interface configured to transmit, to a central unit of the base transceiver station, the RF noise signal as a stimulus (or excitation) signal over a radio-over-fiber, ROF, uplink channel between the remote radio unit and the central unit of the base transceiver station, wherein the communication interface is further configured to receive, from the central unit, a pre-distorted target signal over a ROF downlink channel between the remote radio unit and the central unit of the base transceiver station; and an antenna configured to transmit the pre-distorted target signal received via the communication interface.

Thus, a compact RRU is provided addressing the problems of conventional analog RoF systems for long transmission distances.

The remote radio unit further comprises a power splitter configured to split the stimulus signal into a first stimulus signal and a second stimulus signal, wherein the communication interface is configured to transmit the first stimulus signal over the radio-over-fiber, ROF, uplink channel between the remote radio unit and the central unit of the base transceiver station and to transmit the second stimulus signal over a further radio-over-fiber, ROF, uplink channel between the remote radio unit and the central unit of the base transceiver station.

The remote radio unit further comprises a first local power control unit, in particular automatic gain controller (AGC), configured to control the power of the first stimulus signal to be transmitted over the radio-over-fiber, ROF, uplink channel between the remote radio unit and the central unit of the base transceiver station and a second local power control unit, in particular automatic gain controller (AGC), configured to control the power of the second stimulus signal to be transmitted over the further radio-over-fiber, ROF, uplink channel between the remote radio unit and the central unit of the base transceiver station.

In a further possible implementation form of the first aspect, the noise generator is configured to provide the noise signal as a RF white noise signal having predefined statistical properties.

In a further possible implementation form of the first aspect, the remote radio unit further comprises a power amplifier configured to amplify the pre-distorted target signal received by the communication interface.

In a further possible implementation form of the first aspect, the remote radio unit further comprises a coupler configured to couple a ROF downlink channel processing chain of the remote radio unit to a ROF uplink channel processing chain of the remote radio unit.

In a further possible implementation form of the first aspect, the remote radio unit further comprises a controller configured to provide a control signal for triggering the noise generator to provide the RF noise signal.

In a further possible implementation form of the first aspect, the remote radio unit further comprises a global power controller configured to control the power of the first stimulus signal and the second stimulus signal by implementing an uplink ROF power control loop.

In a further possible implementation form of the first aspect, the communication interface is configured to receive a desired target gain from the central unit and the global power controller is configured to control the power of the first stimulus signal and the second stimulus signal by implementing an uplink ROF power control loop on the basis of the desired target gain. The desired target gain can be determined by the central unit such that the uplink RoF still operates in the linear regime, for instance, on the basis of the CDF (Cumulative Density Function) or CCDF (Complementary Cumulative Density Function) of the received signal.

In a further possible implementation form of the first aspect, the communication interface is further configured to receive a control signal from the central unit of the base transceiver station for triggering the noise generator to provide the RF noise signal.

According to a second aspect the disclosure relates to a base transceiver station comprising one or more remote radio units according to the first aspect of the disclosure, a central unit and one or more optical fibers connecting the one or more remote radio units with the central unit.

The central unit comprises: a communication interface configured to receive a RF, from a remote radio unit of the base transceiver station, noise signal as a stimulus (or excitation) signal over a radio-over-fiber, ROF, uplink channel between the central unit and the remote radio unit of the base transceiver station; and a pre-distortion unit configured to pre-distort a target signal to be transmitted by the remote radio unit on the basis of the RF noise signal received by the communication interface and information about the predefined statistical properties, in particular second-order statistics and/or higher-order statistics, of the RF noise signal. The communication interface is further configured to transmit the pre-distorted target signal over a ROF downlink channel to the remote radio unit of the base transceiver station.

In a further possible implementation form of the second aspect, the pre-distortion unit is configured to pre-distort the target signal to be transmitted by the remote radio unit on the basis of the RF noise signal received by the communication interface using a blind channel identification algorithm.

In a further possible implementation form of the second aspect, the blind channel identification algorithm is a constant modulus algorithm, CMA.

In a further possible implementation form of the second aspect, the central unit further comprises a control unit configured to provide a control signal, wherein the communication interface is further configured to transmit the control signal to the remote radio unit for triggering a noise generator of the remote radio unit to provide the RF noise signal.

In a further possible implementation form of the second aspect, the central unit further comprises an analog-to-digital converter and/or a digital-to-analog converter, wherein the pre-distortion unit is configured to pre-distort the target signal in the digital domain, i.e. in the base-band.

The disclosure can be implemented in hardware and/or software.

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

In the various figures, identical reference signs will be used for identical or at least functionally equivalent features.

It will be appreciated that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa.

Moreover, in the following detailed description as well as in the claims implementations with different functional blocks or processing units are described, which are connected with each other or exchange signals. It will be appreciated that the present disclosure covers implementations as well, which include additional functional blocks or processing units that are arranged between the functional blocks or processing units of the implementations described below.

Finally, it is understood that the features of the various exemplary aspects described herein may be combined with each other, unless specifically noted otherwise.

As will be described in more detail further below, implementations of the disclosure are based on the idea to replace the training signal generator of a conventional RRU <NUM> by a noise generator for generating a stimulus signal having well-defined statistical properties as the uplink ROF input signal and to perform a blind channel identification algorithm at the central unit. Thus, advantageously, only the noise generator, preferably a diode noise generator, is necessary to generate the stimulus signal in a remote radio unit according to an implementation.

<FIG> shows a schematic diagram illustrating a base transceiver station <NUM> including a remote radio unit (RRU) <NUM> according to an implementation connected via and ROF downlink 520a and a ROF uplink 520b to a central unit <NUM> according to an implementation. For the sake of clarity only one RRU <NUM> is shown in <FIG>. However, as will be appreciated, the base transceiver station <NUM> can comprise a plurality of remote radio units like the RRU <NUM> shown in <FIG>, which are connected (by a respective plurality of ROF links) to one or more central units like the central unit <NUM> shown in <FIG>.

The remote radio unit <NUM> comprises a noise generator <NUM> configured to provide a RF noise signal having predefined statistical properties, in particular second-order statistics and/or higher-order statistics. In an implementation, the noise generator <NUM> is configured to provide a RF white noise signal having predefined statistical properties, in particular second-order statistics and/or higher-order statistics. In an implementation, the noise generator <NUM> is a diode noise generator.

Moreover, the remote radio unit <NUM> comprises a communication interface <NUM> configured to transmit the RF noise signal as a stimulus signal over a radio-over-fiber, ROF, uplink channel 520b between the remote radio unit <NUM> and the central unit <NUM> of the base transceiver station <NUM> for determining on the basis of the RF noise signal a pre-distortion of a target signal to be transmitted by the remote radio unit <NUM>. The communication interface <NUM> is further configured to receive the pre-distorted target signal over a ROF downlink channel 520a between the remote radio unit <NUM> and the central unit (<NUM>) of the base transceiver station <NUM>.

Furthermore, the remote radio unit <NUM> comprises an antenna <NUM> for transmitting the pre-distorted target signal received by the communication interface <NUM>.

The central unit <NUM> of the base transceiver station <NUM> comprises a communication interface <NUM> configured to receive the RF noise signal as a stimulus signal over the radio-over-fiber, ROF, uplink channel 520b between the central unit <NUM> and the remote radio unit <NUM> of the base transceiver station <NUM>.

Moreover, the central unit <NUM> comprises a pre-distortion unit <NUM> implementing an algorithm configured to pre-distort the target signal to be transmitted by the remote radio unit <NUM> on the basis of the RF noise signal received by the communication interface <NUM> as well as information about the predefined statistical properties, in particular second-order statistics and/or higher-order statistics, of the RF noise signal. In an implementation, this information about the predefined statistical properties of the RF noise signal can be retrieved from a memory of the central unit <NUM>.

The communication interface <NUM> of the central unit <NUM> is further configured to transmit the pre-distorted target signal over the ROF downlink channel 520a to the remote radio unit <NUM> of the base transceiver station <NUM>.

In an implementation, the remote radio unit <NUM> further comprises a power amplifier <NUM> configured to amplify the pre-distorted target signal received by the communication interface <NUM> from the central unit <NUM>.

As illustrated in <FIG>, in an implementation the remote radio unit <NUM> further comprises a coupler <NUM> configured to couple a ROF downlink channel processing chain of the remote radio unit <NUM> (which, in the implementation shown in <FIG> includes the downlink portion of the communication interface <NUM>, the power amplifier <NUM> as well as the antenna <NUM>) to a ROF uplink channel processing chain of the remote radio unit <NUM> (which, in the implementation shown in <FIG>, includes the noise generator <NUM> and the uplink portion of the communication interface <NUM>).

In an implementation, the remote radio unit <NUM> further comprises a controller <NUM> configured to provide a control signal for triggering the noise generator <NUM> to provide the RF noise signal (shown in <FIG>). Alternatively or additionally, the control signal for triggering the noise generator <NUM> can be provided by the central unit <NUM>. Thus, in an implementation, the communication interface <NUM> of the remote radio unit <NUM> is further configured to receive a control signal from the central unit <NUM> of the base transceiver station <NUM> for triggering the noise generator <NUM> to provide the RF noise signal. In an implementation, the central unit <NUM> of the base transceiver station <NUM> further comprises a control unit configured to provide a control signal, wherein the communication interface <NUM> of the central unit <NUM> is configured to transmit the control signal to the remote radio unit <NUM> for triggering the noise generator <NUM> of the remote radio unit <NUM> to provide the RF noise signal.

As illustrated in <FIG>, in an implementation the central unit <NUM> of the base transceiver station <NUM> further comprises an analog-to-digital converter <NUM> and/or a digital-to-analog converter <NUM>, wherein the pre-distortion unit <NUM> is configured to pre-distort the target signal in the digital domain, i.e. in the base-band.

As will be appreciated, the noise generator <NUM>, such as diode noise generator, can be provided by a low cost element having a compact size, which is the only element necessary for providing the stimulus signal (contrary to the additional elements required by the conventional training signal module shown in <FIG>). As mentioned, the uplink ROF channel 520b is then identified by the central unit <NUM> by applying the algorithm implemented in unit <NUM>, in particular a blind channel identification algorithm, using the information about the predefined statistical properties, in particular second-order statistics (SOS) and/or higher-order statistics (HOS), of the RF noise signal, i.e. stimulus signal. These statistical properties of the stimulus signal are available to the central unit <NUM>, as they are defined by physical properties of the noise generator <NUM>.

In an implementation, the central unit <NUM> can implement, for instance, the well-known CMA algorithm, which requires HOS information and which minimizes the mean cost function E{Ψ(y(n))}:<MAT> where y(n) denotes the equalized signal and a(n) denotes the input signal. Here E{(|y(n)<NUM>-R|)<NUM>} and E{|a(n)|<NUM>} denote the 4th-order statistic (HOS) information, while E{[a(n)<NUM>} denotes the <NUM>nd-order statistic (SOS) information. However, this algorithm doesn't work with an input signal of Gaussian distribution: that suggests the HOS information cannot be exploited or only SOS information can be exploited. In other words, using the HOS information relies on a minimization of the cost function, which requires the ratio of the input signal's high-order statistic to the second-order statistic. This minimization will optimize the equalized signal's distribution (statistic) to approach the distribution of the input's signal. However, it can be proven that such optimization can be performed on a Gaussian signal. This is because the Gaussian signal after a linear channel will always remain a Gaussian signal, i.e. there is no difference in distribution.

Thus, in an implementation, the pre-distortion unit <NUM> is configured to pre-distort the target signal to be transmitted by the remote radio unit <NUM> on the basis of the RF noise signal received by the communication interface <NUM> using a blind channel identification algorithm. As mentioned, in an implementation, the blind channel identification algorithm is a constant modulus algorithm, CMA.

According to further implementations, the remote radio unit <NUM> and the central unit <NUM> of the base transceiver station <NUM> can implement a SIMO architecture, which will be described in more detail in the following under further reference to <FIG> and which is particular useful in massive MIMO scenarios. As illustrated in <FIG>, the SIMO architecture can be natively supported by the RRU <NUM> according to an implementation further comprising a power splitter <NUM> with a respective power control unit <NUM>, such as an AGC (Automatic Gain Control) unit, for stabilizing the uplink ROF input power.

Thus, in an implementation, the remote radio unit <NUM> further comprises a power splitter <NUM> configured to split the stimulus signal at least into a first stimulus signal and a second stimulus signal, wherein the uplink portion of the communication interface <NUM> is configured to transmit the first stimulus signal over the radio-over-fiber, ROF, uplink channel 520b between the remote radio unit <NUM> and the central unit <NUM> of the base transceiver station <NUM> and to transmit the second stimulus signal over a further radio-over-fiber, ROF, uplink channel 520c between the remote radio unit <NUM> and the central unit <NUM> of the base transceiver station <NUM>.

In an implementation, the remote radio unit <NUM> further comprises a first local power control unit <NUM>, in particular automatic gain controller (AGC), configured to control the power of the first stimulus signal to be transmitted over the radio-over-fiber, ROF, uplink channel 520b between the remote radio unit <NUM> and the central unit <NUM> of the base transceiver station <NUM> and a second local power control unit <NUM>, in particular automatic gain controller (AGC), configured to control the power of the second stimulus signal to be transmitted over the further radio-over-fiber, ROF, uplink channel 520c between the remote radio unit <NUM> and the central unit <NUM> of the base transceiver station <NUM>.

In an implementation, the remote radio unit <NUM> further comprises a global power controller <NUM> (illustrated in <FIG> and described in more detail further below) configured to control the power of the first stimulus signal and the second stimulus signal by implementing an uplink ROF power control loop. To this end, the downlink portion of the communication interface <NUM> of the RRU <NUM> can be configured to receive a desired target gain from the central unit <NUM> and the global power controller <NUM> can be configured to control the power of the first stimulus signal and the second stimulus signal by implementing an uplink ROF power control loop on the basis of the desired target gain. The desired target gain be determined by the central unit <NUM> such that the uplink RoF still operates in the linear regime, for instance, on the basis of a constant CDF (Cumulative Density Function) and/or a constant CCDF (Complementary Cumulative Density Function) of the received signal.

Thus, in an implementation, the multiple ROF channels shown in <FIG> are supposed to be different from each other (diversity condition) and the SOS-based blind equalization algorithm taking advantage of such diversity can be applied by the central unit <NUM> for high accuracy and low complexity in both the linear scenario as well as the and non-linear scenario. According to implementations of the disclosure, these types of algorithms do not require information about the statistical properties of the stimulus signal. Thus, in further implementations of the disclosure, the noise generator <NUM> shown in <FIG> is not necessary and can be replaced by a different stable signal source, such as the downlink signal or the uplink signal.

As already described above, implementations of the disclosure, one of which is illustrated in <FIG>, provide an uplink ROF power control loop, which is based on the finding that the uplink ROF input signal power is a key factor for the quality of the channel identification for both the uplink 520b and the downlink 520a. Thus, implementations of the disclosure provide a closed-loop power control architecture. This power control is executed by the RRU <NUM>, while the power level, i.e. the target gain, is determined by the central unit <NUM>. To this end, in an implementation, the uplink signal quality and/or the downlink signal quality can be fed to the uplink ROF power control unit <NUM> as information, ex. MSE, EVM, SER or BER. In an implementation, the RRU <NUM> and the central unit <NUM> are configured to implement the following scenario: for a given range of uplink ROF input signal power levels, execute at each power level the loop calibration, including uplink compensation (post-distortion) and downlink compensation (pre-distortion) then find the power level that yields the best downlink signal. More specifically, the respective power control unit <NUM> of the RRU <NUM> will follow the instructions on uplink ROF power level. The blind channel identification/equalization module of the central unit <NUM> can evaluate the equalized uplink signal's quality. The pre-distorter module of the central unit <NUM> can evaluate the equalized looped downlink signal's channel's quality. The uplink ROF power controller optimizes the uplink ROF power level based on the assessment of the UL/DL signal's quality.

In the following different aspects, further background and/or modifications of the implementations shown in <FIG>, <FIG> and <FIG> will be described in more detail.

<FIG> illustrates the signal flow for a downlink transmission, where the signal flows from A to B, C, D, E, F, G, and then is emitted by the antenna <NUM>. In an exemplary implementation, the signal generator <NUM> of the central unit <NUM> generates a <NUM> bandwidth OFDM baseband signal. In this case, the signal at A and B is the same for a <NUM> bandwidth OFDM baseband digital signal. In the DAC <NUM>, the signal is modulated to a <NUM> frequency carrier, and then converted to an analog signal so that the output at C is an analog electronic signal with <NUM>. In the downlink portion of the communication interface <NUM> (i.e. ROF unit), the electronic signal is modulated onto an optical carrier, whose wavelength is <NUM> so that D and E are optical signals.

At the RRU <NUM> the downlink portion of the communication interface <NUM> converts the optical signal to an electronic signal, which is the inverse process of the process performed by the downlink portion of the communication interface <NUM> of the central unit <NUM>. Thus, at F the signal is again an analog electronic signal with <NUM>. As will be appreciated, under ideal channel conditions the signal at F would have the same shape as the signal at C. The power amplifier <NUM> of the RRU <NUM> is provided for signal energy amplification from F to G.

As already described above in the context of the technical background of the present disclosure, for two long distance ROF links (e.g. fibers of lengths <NUM> to <NUM>), the power amplifier <NUM> can have nonlinearity features, which distort the signal and increase out-of-band energy. In the prior art, this distorted signal is very difficult to recover by the receiver. To overcome this issue, it is known to implement a digital pre-distortion algorithm, which is referenced in <FIG> as "Alg1". Thus, in the implementation shown in <FIG>, the signal flowing from A to B is distorted by the digital pre-distortion algorithm "Alg1", which counteracts the nonlinearity features(s) of the power amplifier <NUM> mentioned above. This is illustrated in <FIG>, where the original (Org) signal has the non-linearity feature, the pre-distortion (Pre) algorithm counteracts the non-linearity feature, and the final combined signal (Comb) will have the ideal linearity.

Thus, the pre-distortion algorithm can compensate any nonlinearities of the hardware modules. This, however, requires having information about the one or more nonlinearities introduced by a hardware module before designing any pre-distortion algorithm. This can be done using a training signal, as illustrated in <FIG>. The training signal is generated by the signal generation unit <NUM> (or provided thereto) and flows from A, to B, C, D, E, F and G. The goal is to achieve the same signal nonlinearity at G and L. In other words, by means of the algorithm "Alg2" the whole nonlinearity model from A to G has to be represented and then pre-distorted by the algorithm "Alg1". In the case of very long distance fibers and an uplink and a downlink portion of the respective communication interfaces <NUM>, <NUM> from H to K some nonlinearity is involved as well, which is, however, not needed in Alg2.

As already described above in the context of <FIG>, the remote radio unit <NUM> according to an implementation comprises a noise generator <NUM> configured to provide the stimulus signal as a RF noise signal having predefined statistical properties, in particular second-order statistics and/or higher-order statistics, known to the central unit <NUM>. The corresponding signal flow is illustrated in <FIG>. The stimulus signal flows from N, to I, J, K, and L. According to implementations of the disclosure the algorithm "Alg2" uses information about statistical properties of the stimulus signal, such as SOS information, to model uplink channel features. As already mentioned above, in an implementation, the controller <NUM> of the RRU <NUM> can be configured to provide a control signal to the noise generator <NUM> (illustrated at M in <FIG>) for triggering the noise generator <NUM>.

<FIG> illustrates the signal flow in the SIMO implementations shown in <FIG> and <FIG>. As already described above, the SIMO implementations are based on the idea to firstly describe the uplink channel feature and secondly compensate the uplink channel feature (as illustrated by the signal flow from H to L in <FIG>). As used herein, SIMO means one downlink channel and at least two uplink channels. A simple SIMO implementation, similar to the ones shown in <FIG> and <FIG>, is illustrated in <FIG>. As already mentioned above in the context of <FIG> and <FIG>, in an implementation the RRU <NUM> can further comprise a respective power control unit <NUM>, in particular a respective Automatic Gain Controller (AGC) to control different gains for different uplink channels 520b,c. Advantageously, this allows to independently control the different uplink channels 520b,c and to observe the resulting differences by the algorithm "Alg3" implemented in the central unit <NUM>.

A SIMO system can be identified under the following conditions: (i) all channels in the system must be different enough from each other; (ii) the input sequence must be complex enough; and (iii) enough output samples need to be available. As already mentioned above, in implementations of the disclosure respective power control units <NUM> are implemented in the RRU <NUM> for adjusting the input power for every uplink channel, so that they can work differently. The training sequence provided by the central unit <NUM> can be arbitrarily complex and can be kept for sufficient time in order to obtain enough output samples for processing by the algorithm "Alg3".

In an implementation, a two-steps maximum likelihood (TSML) algorithm can be implemented as algorithm "Alg3" in the central unit <NUM>. Thus, if y denotes the received signal (vector) from multiple-channels, according to an implementation the channel can be identified by using the following two-step ML algorithm:.

wherein Y is generated from y, Gc is generated from hc such that: <MAT> equalization:
where H is the Sylvester matrix of h.

The performance of the algorithm "Alg3" as a two-steps maximum likelihood (TSML) algorithm in the central unit <NUM> is illustrated in <FIG>. The simulation result shown in <FIG> is for a linear SIMO system with <NUM> channels, wherein each channel has a length L=<NUM> and the coefficients are randomly generated. NMSE stands for the normalized mean square error between the equalized signal and input signal. It is observed that a low NMSE can be achieved in the high SNR region.

<FIG> shows a schematic diagram illustrating in more detail the global power controller <NUM> of the remote radio unit <NUM> shown in <FIG>. As already mentioned above, in an implementation, the controller <NUM> can have three main functions. The signal identification module 514a of the controller <NUM> is configured to identify a received input signal. In case of a control signal the gain controlling module 514b will generate at a suitable time a gain control signal and send the gain control signal to the power splitter <NUM> for the different uplink channels 520b,c. The power splitter <NUM> of the controller <NUM> is configured to adjust the actual output power for the different uplink channels 520b,c. As will be appreciated, the controller <NUM> is capable of dealing with time-varying channels, for instance, by operating periodically.

<FIG> shows a flow diagram illustrating a method <NUM> of operating the remote radio unit <NUM> of the base transceiver station <NUM>. The method <NUM> comprises the steps of: providing <NUM> a RF noise signal having predefined statistical properties, in particular second-order statistics and/or higher-order statistics; transmitting <NUM> the RF noise signal as a stimulus signal over the radio-over-fiber uplink channel 520b between the remote radio unit <NUM> and the central unit <NUM> of the base transceiver station <NUM> for determining on the basis of the RF noise signal a pre-distortion of the target signal to be transmitted by the remote radio unit <NUM>; receiving <NUM> the pre-distorted target signal over the ROF downlink channel 520a between the remote radio unit <NUM> and the central unit <NUM> of the base transceiver station <NUM>; and transmitting <NUM> the pre-distorted target signal using the antenna <NUM> of the remote radio unit <NUM>.

<FIG> shows a flow diagram illustrating a method <NUM> of operating the central unit <NUM> of the base transceiver station <NUM>. The method <NUM> comprises the steps of: receiving <NUM> a RF noise signal as a stimulus signal over the radio-over-fiber uplink channel 520b between the central unit <NUM> and the remote radio unit <NUM> of the base transceiver station <NUM>; pre-distorting <NUM> a target signal to be transmitted by the remote radio unit <NUM> on the basis of the RF noise signal and information about the predefined statistical properties, in particular second-order statistics and/or higher-order statistics, of the RF noise signal; and transmitting <NUM> the pre-distorted target signal over a ROF downlink channel 520a to the remote radio unit <NUM> of the base transceiver station <NUM>.

While a particular feature or aspect of the disclosure may have been disclosed with respect to only one of several implementations or implementations, such a feature or aspect may be combined with one or more further features or aspects of the other implementations or implementations as may be desired or 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 thereof 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 (<NUM>) for a base transceiver station (<NUM>), wherein the remote radio unit (<NUM>) comprises:
a noise generator (<NUM>) configured to provide a radio-frequency, RF, noise signal having predefined statistical properties;
a communication interface (<NUM>) configured to transmit, to a central unit (<NUM>) of the base transceiver station (<NUM>), the RF noise signal, received from the noise generator (<NUM>), as a stimulus signal over a radio-over-fiber, ROF, uplink channel (520b) between the remote radio unit (<NUM>) and the central unit (<NUM>) of the base transceiver station (<NUM>), wherein the communication interface (<NUM>) is further configured to receive from the central unit (<NUM>) a pre-distorted target signal over a ROF downlink channel (520a) between the remote radio unit (<NUM>) and the central unit (<NUM>) of the base transceiver station (<NUM>); and
an antenna (<NUM>) for transmitting the pre-distorted target signal received by the communication interface (<NUM>);
wherein the remote radio unit (<NUM>) further comprises a power splitter (<NUM>) configured to split the stimulus signal into a first stimulus signal and a second stimulus signal, wherein the communication interface (<NUM>) is configured to transmit the first stimulus signal over the radio-over-fiber, ROF, uplink channel (520b) between the remote radio unit (<NUM>) and the central unit (<NUM>) of the base transceiver station (<NUM>) and to transmit the second stimulus signal over a further radio-over-fiber, ROF, uplink channel (520c) between the remote radio unit (<NUM>) and the central unit (<NUM>) of the base transceiver station (<NUM>);
wherein the remote radio unit (<NUM>) further comprises a first local power control unit (<NUM>) configured to control the power of the first stimulus signal to be transmitted over the radio-over-fiber, ROF, uplink channel (520b) between the remote radio unit (<NUM>) and the central unit (<NUM>) of the base transceiver station (<NUM>) and a second local power control unit (<NUM>) configured to control the power of the second stimulus signal to be transmitted over the further radio-over-fiber, ROF, uplink channel (520c) between the remote radio unit (<NUM>) and the central unit (<NUM>) of the base transceiver station (<NUM>).