Apparatuses, Methods and Computer Programs for Determining Transmission Control Information Technical Field Embodiments relate to apparatuses, methods and computer programs for determining transmission control information, more specifically, but not exclusively, based on a loopback version of a reference signal received from one or more radio units. Background This section introduces aspects that may be helpful in facilitating a better understanding of the invcntion(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art. Demands for higher data rates for mobile services are steadily increasing. At the same time modern mobile communication systems as 3rd Generation systems (3d) and 4th Generation systems (4G) provide enhanced technologies, which enable higher spectral efficiencies and allow for higher data rates and cell capacities. The demands are growing for both directions of transmission, in the Down Link (DL) for transmission from the network infrastructure to a mobile transceiver, as well as in the UpLink (UL) for transmission from a mobile transceiver to the network infrastructure. Current mobile communication systems increasingly rely on small cell base stations. Small cell base stations can be used to cover areas where the macro cell base stations do not provide sufficient coverage and capacity and to improve network efficiency. Small cells may appear to be an efficient approach to increase the capacity and improve network efficiency. It is recognized, however, that the potential attractiveness of small cells might only be realized if installation costs can be controlled. Two requirements for a small cell installation might be electrical power and backhaul, which when provided by wired connections may often be costly. A wired connection for electrical power may be eliminated by deriving power from a small wind turbine or solar panel and charging storage batteries, and by decreasing internal powrer consumption. The second small cell wired connection may be eliminated by using wireless backhaul. To decrease internal power consumption a repeater-type small cell might be used, that on downlink receives a complex-valued analog baseband signal on a carrier frequency from a backhaul link and re-transmits the baseband signal at an access carrier frequency that accommodates one or more small cell users, and on the uplink receives from one or more small cell users their combined transmitted signals on an access link and re-transmits their combined baseband signals on the backhaul uplink. The repeater small cell might mainly comprise pre-amplifiers, frequency converters, filters and power amplifiers so it might be designed for low-power consumption. US patent application 2012/238202 Al discloses a method for transmitting data using relay stations. The method is based on a base station sending a reference signal to relay stations of a mobile communication system via a backhaul link. The relay stations calculate channel parameters based on the received reference signal, and transmit said parameters back to the base station. The base station uses the channel parameters to determine scheduling information for the backhaul link, and transmits the scheduling information to the relay stations. US patent application 2O08/227461 Al discloses a mobile communication system comprising a base station and relay stations. Based on (data or pilot) transmissions by relay stations, the base station calculates adjustments of a transmission power to be used by the relay stations for a link to the base station or for links to other relay stations (for multi-hop relaying). Summary of illustrative Embodiments Some simplifications may be made in the following summary, which is intended to highlight and introduce some aspects of the various exemplary embodiments, but such simplifications are not intended to limit the scope of the invention(s). Detailed descriptions of a preferred exemplary embodiment adequate to allow those of ordinary skill in the art to make and use the inventive concepts will follow in later sections. Various embodiments provide apparatuses, methods and computer programs for determining transmission control information for one or more wireless fronthaul links between a base band unit and one or more radio units of a base station transceiver of a mobile communication system. While the quality of the wireless access link, between the radio units and mobile transceivers, might often be continuously measured and optimized, the quality of the wireless fronthaul link might be seen as near-constant and neglected. To setup the connections an evaluation of the transmission characteristics between the base band unit and the one or more radio units may be necessary. To retain a reduced functionality (and thus complexity and power consumption) at the one or more radio units, the evaluation may occur at the base band unit. The base band unit is configured to transmit a reference signal to the one or more radio units, which are in turn configured to loopback a loopback version of the reference signal. Based on the received loopback version, the base band unit may determine a path loss for the transmission, which is then used to determine transmission characteristics (e.g. per-radio unit transmission power) to be used for the wireless fronthaul links at the one or more radio units (and/or the base band unit). Kmbodiments provide an apparatus for a base band unit of a base station transceiver of a mobile communication system. The base station transceiver further comprises one or more radio units configured to wirelessly communicate with the base band unit using one or more wireless fronthaul links. The apparatus comprises at least one output configured to transmit a downlink component of the one or more wireless fronthaul links to the one or more radio units. The apparatus further comprises at least one input configured to receive an uplink component of the one or more wireless fronthaul links from the one or more radio units. The apparatus further comprises a control module configured to control the at least one output and the at least one input. The control module is further configured to transmit a reference signal via the at least one output to the one or more radio units. The control module is further configured to receive a loopback version of the reference signal via the at least one input from the one or more radio units. The control module is further configured to determine information related to a per-radio unit transmission power to be used by the one or more radio units for transmissions on the one or more wireless fronthaul links using an attenuation of the reference signal determined based on the loopback version of the reference signal. The control module is further configured to determine transmission control information comprising the information related to the per-radio unit transmission power. The control module is further configured to provide the transmission control information to the one or more radio units via the at least one output. Determining the transmission control information at the base band unit enables a calibration or adjustment of transmission parameters for the establishment or refinement of the wireless fronthaul links. Using the loopback version of the reference signal received from the one or more radio units may enable a deployment of low-complexity radio units, as the radio units might not have lo actively determine the transmission control information. Further, no uplink control channel dedicated to transmitting measured characteristics of the one or more wireless fronthaul links may be required. In at least some embodiments, the control module may be configured to determine the per-radio unit transmission power based on an estimated interference and an estimated path loss. The control module may be configured to estimate the path loss based on the reference signal and the loopback version of the reference signal. Determining the per-radio unit transmission power based on the estimated path loss and the estimated interference may enable a determination of the per-radio unit transmission power without requiring measurements performed by the one or more radio units. In various embodiments, the control module may be configured to determine the per-radio unit transmission power based on an optimization function. The control module may be configured to determine the per-radio unit transmission power such, that a target Signa!-to-Interference and Noise Ratio (SINR), for the uplink component is approximated based on S'NRtarget.u ~ ^txCi ~ °^U( — PlF,z,NF- SlNRtargetu may be the target SINR for the uplink component. Pta-c. may be the per-radio unit transmission power of a radio unit t of the one or more radio units. PLUi may he an the estimated path loss of the uplink component of the radio unit i. PIF,Z,NF maY DC based on an interference power, IF, a thermal noise, z, and a receive noise figure, NF. of the uplink component. Determining the per-radio unit transmission power based on an optimization function on the estimated path loss and the estimated interference may enable a determination of the per-radio unit transmission power without requiring measurements performed by the one or more radio units. In at least some embodiments, the loopback version of the reference signal corresponds to an analog conversion of the reference signal received at the one or more radio units. The analog conversion enables a deployment of low-complexity radio units, as the radio units might not have to digitally decode the reference signal and determine the transmission control information. In various embodiments, the control module may be further configured to determine information related to a transmission power lo be used by the at least one output for transmissions on the one or more wireless fronthaul links based on the reference signal and the loopback version of the reference signal. The control module may be further configured to adapt the transmitting of the downlink component of one or more wireless fronthaul signals at the at least one output based on the information related to a transmission power to be used on the at least one output. The adaptation based on the reference signal and the loopback version of the reference signal may enable a reuse of information determined for the transmission control information, and may enable an adjustment of the transmissions of the output module. In various embodiments, the control module may be configured to determine the transmission power to be used by the at least one output for transmissions on the one or more wireless fronthaul links based on an optimization function. The control module may be configured to determine the transmission power to be used by the at least one output such, that a target Signal-to-Interference and Noise Ratio (SINR), for the downlink component is approximated based on SINRtargetd = Ptxfiubi ~ P^di — PlF,z,NF. SINRtargetct is the target SINR for the downlink component. Ptxhui). 1S tne transmission power to be used by the at least one output for transmissions to a radio unit i of the one or more radio units. PLdi is an estimated path loss of the downlink component for transmissions to the radio unit i. PIFIZINF may be based on an interference power, IF, a thermal noise, z, and a receive noise figure, NF, of the downlink component. Determining the transmission power to be used by the at least one output based on an optimization function on the estimated path loss and the estimated interference may enable a determination of the transmission power without requiring measurements performed by the one or more radio units. The transmission power to be used by the at least one output may further be used to estimate an overall power consumption / demand at the base band unit for the wireless fronthaul links to the one or more radio units. In various embodiments, the control module may be further configured to determine information related to a subset of antenna elements of a plurality of antenna elements to be used for transmitting on the one or more wireless fronthaul links. The information related to the transmission power to be used by the at least one output for transmissions on the one or more wireless fronthaul links may comprise the information related to the subset of antenna elements. Changing the subset of antenna elements to be used for transmitting on the one or more wireless fronthaul links may enable further savings in energy consumption or demand. In at least some embodiments, the control module may be further configured to adapt the receiving of the uplink component of one or more wireless fronthaul signals at the at least one input based on the reference signal and the loopback version of the reference signal. The reference signal and the loopback version of the reference signal may be further used to enable an equalization of the received signal at the input. In various embodiments, the control module may be further configured to determine an uplink channel estimation matrix and a downlink channel estimation matrix based on the reference signal and the loopback version of the reference signal to determine the transmission control information. The control module may be configured to determine a joint uplink/downlink channel estimation matrix based on the reference signal and the loopback version of the reference signal to determine the transmission control information. Determining both uplink and downlink channels at the base band unit may decrease an overall computation effort and may enable a deployment of less complex radio units. In at least some embodiments, the control module may be configured to determine the transmission control information based on a path loss of the downlink component and based on a path loss of the uplink component in the loopback version of the reference signal. Determining the transmission control information based on the path loss may support a determination or adjustment of the per-radio unit transmission power to counter the effects of the path loss. In various embodiments, the control module may be configured to determine information related to a quality of the received uplink component. The control module may be configured to adjust the transmission control information based on the information related to the quality of the received uplink component. Adjusting the transmission control information based on the information related to the quality may enable a continuous adjustment of the one or more wireless fronthaul links, e.g. to account for changes in the external conditions. In some embodiments, the transmission control information may comprise information related to a power threshold for a transmission of ihe one or more wireless fronthaul links. The power threshold may e.g. be used to indicate a lower boundary for the transmission power required to statistically achieve a desired received power. In various embodiments, the control module may be configured to provide the transmission control information to the one or more radio units individually, which may decrease an overhead on the individual wireless fronthaul links. In at least some embodiments, the control module may be further configured to determine the transmission control information based on a cell coverage plan of the one or more radio units. The cell coverage plan may e.g. be used to determine approximate initial transmission control information which may be refined during operation. In various embodiments, the control module may be configured to determine the transmission control information without using information related to a channel estimation of the one or more wireless fronthaul links carried out at the one or more radio units, which may enable a deployment of lower-complexity radio units. Embodiments further provide a base band unit comprising the apparatus the base band unit. Embodiments further provide an apparatus for a radio unit of a base station transceiver of a mobile communication system. The base station transceiver further comprises a base band unit wirelessly communicating with the radio unit using a wireless fronthaul link. The apparatus comprises at least one input configured to receive a downlink component of the wireless fronthaul link from the base band unit. The apparatus further comprises at least one output configured to transmit an uplink component of the wireless fronthaul links to the base band unit. The apparatus further comprises a control module configured to control the at least one input and the at least one output. The control module is further configured to receive a reference signal via the at least one input from the base band unit. The apparatus is further configured to loopback a loopback version of the received reference signal via the at least one output to the base band unit. The apparatus is further configured to receive transmission control information from the base band unit. The transmission control information comprises information related to a per-radio unit transmission power to be used by the radio unit for transmissions on the wireless fronthaul link. The control module is further configured to adapt the transmission power of the uplink component, e.g. a transmission amplifier for the uplink band at the radio unit, via the at least one output based on the transmission control information. Determining the transmission control information at the base band unit enables a calibration or adjustment of transmission parameters for the establishment or refinement of the wireless fronthaul links. Using the loopback version of the reference signal received from the one or more radio units may enable a deployment of low-complexity radio units, as the radio units might not have to actively determine the transmission control information. Further, no uplink control channel dedicated to transmitting measured characteristics of the one or more wireless fronthaul links may be required. In at least some embodiments, the control module may be configured to determine the loopback version of the received reference signal by analogously converting the received reference signal. The analog conversion enables a deployment of low-complexity radio units, as the radio units might not have to digitally decode the reference signal and determine the transmission control information. In various embodiments, the downlink component may use a downlink carrier frequency and the uplink component uses an uplink carrier frequency. The control module may be configured to determine the loopback version of the received reference signal by analogously converting the received reference signal from the downlink carrier frequency to the uplink carrier frequency. The conversion between frequencies may enable a provision of transmission control information for frequency-division duplex networks, where uplink and downlink components use different frequency bands. Embodiments further provide a radio unit providing (he apparatus for the radio unit. Embodiments further provide a base station transceiver comprising the apparatus for the base band unit and the apparatus for the radio unit. Hmbodiments further provide a method for a base band unit of a base station transceiver of a mobile communication system. The base station transceiver further comprises one or more radio units configured to wirelessly communicate with the base band unit using one or more wireless fronthaul links. The method comprises transmitting a reference signal to the one or more radio units. The method further comprises receiving a loopback version of the reference signal from the one or more radio units. The method further comprises determining information related to a per-radio unit transmission power to be used by the one or more radio units for transmissions on the one or more wireless fronthaul links using an attenuation of the reference signal determined based on the loopback version of the reference signal. The method further comprises determining transmission control information comprising the information related lo the per-radio unit transmission power. The method further comprises providing the transmission control information to the one or more radio units. Embodiments further provide a method for a radio unit of a base station transceiver of a mobile communication system. The base station transceiver further comprises a base band unit wirelessly communicating with the radio unit using a wireless fronthaul link. The method comprises receiving a reference signal from the base band unit. The method further comprises looping back a loopback version of the received reference signal to the base band unit. The method further comprises receiving transmission control information from the base band unit. The transmission control information comprises information related to a per radio-unit transmission power to be used by the radio unit for transmissions on the wireless fronthaul link. The method further comprises adapting the transmission power for transmissions on the wireless fronthaul link based on the transmission control information. Embodiments further provide computer program product comprising a computer readable medium having computer readable program code embodied therein, the computer readable program code being configured to implement any of the methods, when being loaded on a computer, a processor, or a programmable hardware component. Embodiments further provide a computer program having a program code for performing the above method, when the computer program is executed on a computer, a processor, or a programmable hardware component. A further embodiment is a computer readable storage medium storing instructions which, when executed by a computer, processor, or programmable hardware component, cause the computer to implement one of the methods described herein. Brief description of the figures Some other features or aspects will be described using the following non-limiting embodiments of apparatuses or methods or computer programs or computer program products by way of example only, and with reference to the accompanying figures, in which: Fig, 1 illustrates a block diagram of an embodiment of an apparatus for a base band unit of a base station transceiver of a mobile communication system; Fig. 2 illustrates a control mechanism for power control of a wireless fronthaul link; Fig. 3 illustrates a processing flow of the wireless fronthaul links' power control of various embodiments; Fig. 4 illustrates an adaptation of the transmission power of at least some embodiments; Fig. 5 illustrates a block diagram of an embodiment of an apparatus 20 for a radio unit 120a of a base station transceiver 100 of a mobile communication system; Fig. 6 illustrates a flow chart of an embodiment of method for a base band unit of a base station transceiver of a mobile communication system; and Fig. 7 illustrates a block diagram of an embodiment of method for a radio unit of a base station transceiver of a mobile communication system. Description of Embodiments Various example embodiments will now be described more fully with reference to the accompanying drawings in which some example embodiments are illustrated. In the figures, the thicknesses of lines, layers or regions may be exaggerated for clarity. Optional components arc illustrated using broken, dashed or dotted lines. Accordingly, while example embodiments are capable of various modifications and alternative forms, embodiments thereof are shown by way of example in the figures and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments to the particular forms disclosed, but on the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of the invention. Like numbers refer to like or similar elements throughout the description of the figures. As used herein, the term, "or" refers to a non-exclusive or, unless otherwise indicated (e.g., "or else" or "or in the alternative"). Furthermore, as used herein, words used to describe a relationship between elements should be broadly construed to include a direct relationship or the presence of intervening elements unless otherwise indicated. For example, when an element is referred to as being "connected" or "'coupled" to another element, the element may be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present. Similarly, words such as "between", "adjacent", and the like should be interpreted in a like fashion. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms "a," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises," "comprising," "includes" or "including," when used herein, specify the presence of stated features, integers, steps, operations, elements or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components or groups thereof. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. For easy deployment of a dense small cell network, it may be beneficial to connect the small cells wirelessly to the mobile communication system. A Poinl-lo-MultiPoint (P2MP) enabled Massive Multiple Input Multiple Output (MM I MO) system may fronthaul (via a MMIMO hub base station) easy to deploy low complexity, low functionality, low cost & energy radio units (or small cells, e.g. Small Cell Cubes, SCCs, which may correspond to radio units of one or more radio units 120 as introduced in the description of Fig. 1), which may act as mere RF shifters, receiving a signal via MMIMO multi-user beamforming and forwarding it in a regular small cells operation to the end user in access. For an additional complexity decrease for Frequency Division Duplex (FDD) small cells, the MMIMO operation may be performed in FDD. This may include a joint downlink / uplink channel matrix estimation. A technical problem that may arise in such a system is the near-far problem of the small cells deployment, although the SCCs are stationary and will not be moved around once deployed. For fronthauling application, the fronthaul may have to fulfill a tough S1NR requirement, which may increase the effort in terms of transmit power and thus energy consumption (among others). Depending on the (distance dependent) path loss, the small cell cube (e.g. a radio unit of one or more radio units 120 as introduced subsequently) may fulfill a SINRmin requirement (minimum Signal-to-Interference-and-Noise-Ratio), while, without additional measures, other SCCs may overachieve because of a reduced path loss (positioned closer to the MMIMO array, e.g. of a base band unit 110 as introduced subsequently), overdrive the hubs receiver, or reduce its dynamic range. Also, for dynamic switch-on/off of SCCs e. g. for networking reasons (no users to be served), the power values of the hub may be adapted for energy saving, as the hubs transmit power may be split over all SCCs in operation. A closed loop power control (the receiver signaling the power level and any parameters to be adapted to the transmitter), as used in conventional solutions, may not be applicable for several reasons, e.g. because the SCCs shall be of low complexity and a control channel from SCC (one or more radio units 120) to Hub (e.g. a base band unit 110 as introduced in the description of Fig. 1) may not be feasible, while a (low effort) control channel from the hub to the SCC may be permissible or even necessary. Furthermore, SCCs being stationary positioned may enable omitting closed loop power control. Embodiments may determine how to set the transmit power levels (fronthaul transmission (TX) amplifier adaptation) of the hub overall, the hub per antenna (as usual in MIMO applications), per radio unit (e.g. the infrastructure element SCC), and the transmission power from SCC to hub - which may be one of the main contributors of the power consumption of the SCCs. In an FDD fronthaul MMIMO system, a channel estimation phase may be used. The system may consist of a MMIMO hub (e.g. at the base band unit 110) with M antennas, and K small cells (e.g. the one or more radio units 120), here called small cell cubes SCC, that may be multi-beam fronthauled via the MMIMO hub. The channel knowledge may be acquired in a training phase. The fronthaul link denotes the link between the hub (base band unit) and the SCCs (one or more radio units) of the base station transceiver. First, the K SCCs may be brought into a loop-back (loop-the-loop) mode at one point of lime, thus simply reflecting the signal it receives from the hub. The hub may transmit a training sequence (e.g. a reference signal as described subsequently) which may be reflected by the SCC and received by the hub. From this training sequence (which is affected by the uplink and downlink channel (e.g. on different frequencies of the FDD)), the UpLink (UL) and/or DownLink (DL) channel matrices may be estimated (e.g. by first deriving the M x M matrix from the training sequence correlations, and then estimating the relative uplink matrix, thus defining the downlink matrix up to an overall phase term). Conventional systems may overlay the SCCs in an orthogonal way for UL matrix estimation, which requires additional effort on SCC side (e.g. some sort of orlhogonalization of the signal, e.g. multiplication with a Walsh code in the analog domain). Now the transmit power of the hub (e. g. overall, per radio unit, per antenna) and the SCCs may be adapted according to different conditions, with variations mainly driven by the fronthaul path toss of the particular different SCC locations. The basic idea of embodiments may cope with an acquisition of the data necessary for power control, processing of the data to derive transmission control information, and procedures to apply this transmission control information to the system. At least some embodiments may perform: ■ Loop-the-loop calibration/training measurement between hub and SCCs. e.g. sequentially ■ FDD joint UL / DL channel matrix estimation, as briefly described above ■ Deriving/estimating initial hub transmit power, SCCs transmit powers and the RF hardware parameters (e. g. gains) may act as transmission control information ■ Calculate path loss from the derived UL / DL channel matrices, assuming same path loss in UL and DL: PL = ^= FLu + PL» 1 2 2 ■ Estimate the downlink. SINRs based on the path loss {e.g. using additional system knowledge/estimates of noise and interference) ■ Calculate a required transmit power of the hub for the beam to radio unit i based on the SiNR - thus equalizing the SINRs at the SCCs. As will be described in more detail later, the hub transmit power may be adapted per radio unit in the digital and analog domain. ■ Employ the new hub transmit power setting (adapt digital and analog transmit power values jointly). ■ From the uplink path loss (which may be similar to the downlink path loss) and the known transmit powrer of the SCC, with the known noise and interference of the hub, the required transmit power of the particular SCC to the hub may be calculated. This parameter may be verified by evaluating the actual received signal of the hub in the data but especially in the training phase where there is no beamforming gain. ■ Employ the new SCCs transmit power settings (e.g. by using a low effort i signaling/training frame to the SCCs) to the at least required SCC transmit power necessary to keep the SCC to hub fronthaul connection. In order to save energy and support e.g. energy autarkic operation of the SCC. especially adapting the TX power amplifier transmit power and thus power consumption (e.g. by supply-voltage adaptation) may be enabled. Contrary to hub power adaptation, SCC fronthaul transmit power > reduction may affect only the fronthaul link of the particular SCC which is adapted. " Taking cell planning of the deployed SCC into account, also SCC access TX power (SCC cell sizes) and thus power consumption of the access TX power amplifier may be adapted, which may also have effect on SCC overall power consumption. This might either be done predefined manually when deploying the SCCs or also adaptively later on in the ) field by use of adequate signaling during the loop-the-loop calibration phase. TX access power adaptation might e.g. be done more rarely (e.g. at deployment, night - day, etc.) compared to fronthaul power adaptation. Embodiments may employ a control mechanism for power control of the wireless fronthaul link. In Fig. 2, the dotted box 2002 shows the components of the power control that may be added to the systems functionality. A pilot signal (e.g. a reference signal as introduced subsequently) is generated 2004 and transmitted by a MMIMO Transceiver 2006 (TRX, e.g. the base band unit 110) to the SCCs (e.g. the one or more radio units 120), which serve mobile transceivers / terminals 2000. The SCCs (or distributed radio units, or one or more radio units) loopback the pilot signal to the MMIMO transceiver 2006. The uplink / downlink matrix calculation 2010 may use the loopback version of the pilot signal and may be calculated in the base band unit (e.g. in a control module 16 of the base band unit 110), and the power control may rely on these data to calculate the power control parameters (thus, the hubs per radio unit transmit power Ptxhub.> which may correspond to a transmission power to be used by an output 12, and the individual SCCs transmit power Ptx ., which may correspond to a per-radio unit transmission power as introduced ^ J. subsequently). After the calculation 2012 of these values and subsequent method for selecting 2014 the appropriate hub and SCC power levels (which may ensure, that all SCCs arc connected, as described by method above), the base band unit parameters (e.g. analog gain/attenuator value), the determined appropriate pre-coding matrix1 per-radio unit power scaling matrix, may be set in the base band unit, and the power scaling coefficients for each SCCs may be transmitted 2016 to each respective SCC 120, This may be done via explicit signaling of the value in a downlink control channel - e. g. an already existing channel that is used for training phase signaling for the SCCs, e.g. a downlink component of a wireless fronthaul link as introduced subsequently. Fig. 1 illustrates a block diagram of an embodiment of an apparatus 10 for a base band unit 110 of a base station transceiver 100 of a mobile communication system 300. The base station transceiver 100 further comprises one or more radio units 120 configured to wirelessly communicate with the base band unit 110 using one or more wireless fronthaul links. A base station transceiver, e.g. the base station transceiver 100. can be operable to communicate with one or more active mobile transceivers and a base station transceiver can be located in, overlapping to, or adjacent to a coverage area of another base station transceiver, e.g. a macro cell base station transceiver or small cell base station transceiver. Hence, embodiments may provide a mobile communication system comprising one or more mobile transceivers and one or more base station transceivers, wherein the base station transceivers may establish macro cells or small cells, as e.g. pico-, metro-, or femto cells and a kind of frequency shifters or repealers. A mobile transceiver may correspond to a smartphone, a cell phone, user equipment, radio equipment, a mobile, a mobile station, a laptop, a notebook, a personal computer, a Personal Digital Assistant (PDA), a Universal Serial Bus (USB) -stick, a car, a mobile relay transceiver for D2D communication, etc. A mobile transceiver may also be referred to as User Equipment (UK) or mobile in line with the 3GPP terminology. A base station transceiver, e.g. the base station transceiver 100, can be located in the fixed or stationary part of the network or system. A base station transceiver may comprise a base band unit 110, which may correspond to a macro cell, and one or more radio units 120, which may correspond to a remote radio head, a transmission point, an access point, radio equipment, a small cell, a micro cell, a femto cell, a metro cell etc. A base station transceiver may correspond to a base station understood as a logical concept of a node/entity terminating a radio bearer or connectivity over the air interlace between a terminal/mobile transceiver and a radio access network. A base station transceiver can be a wireless interface of a wired network, which enables transmission of radio signals to a UK or mobile transceiver. Such a radio signal may comply with radio signals as, for example, standardized by 3GPP or, generally, in line with one or more of the above listed systems. Thus, a base station transceiver may correspond to a eNodeB, a Base Transceiver Station (BTS) etc., which may be further subdivided in a radio unit and a base band unit. A radio unit may comprise a phase shifter or repeater for the signals provided and/or processed by the base band unit. A mobile transceiver can be associated, camped on, or registered with a base station transceiver or cell. The term cell refers to a coverage area of radio services provided by a base station transceiver, e.g. a NodeB (NB), an eNodeB (eNB), a remote radio head, a transmission point, etc. A base station transceiver may operate one or more cells on one or more frequency layers, in some embodiments a cell may correspond to a sector. For example, sectors can be achieved using sector antennas, which provide a characteristic for covering an angular section around a remote unit or base station transceiver. In some embodiments, a base station transceiver may, for example, operate three or six cells covering sectors of 120° (in case of three cells), 60° (in case of six cells) respectively. A base station transceiver may operate multiple sectorized antennas. In the following a cell may represent an according base station transceiver generating the cell or, likewise, a base station transceiver may represent a cell the base station transceiver generates. In general, the mobile communication system 300 may, for example, correspond to one of the Third Generation Partnership Project (3GPP)-standardized mobile communication networks, where the term mobile communication system is used synonymously to mobile communication network. The mobile or wireless communication system may correspond to, for example, a 5th Generation system (5G), a Long-Term Evolution (LTE), an LTE-Advanccd (LTE-A), High Speed Packet Access (HSPA), a Universal Mobile Telecommunication System (UMTS) or a UMTS Terrestrial Radio Access Network (UTRAN), an evolved-UTRAN (e-UTRAN), a Global System for Mobile communication (GSM) or Enhanced Data rates for GSM Evolution (EDGE) network, a GSM/EDGE Radio Access Network (GERAN), or mobile communication networks with different standards, for example, a Worldwide Inter-operability for Microwave Access (W(MAX) network IEEE 802.16 or Wireless Local Area Network (WLAN) IEEE 802.11, generally an Orthogonal Frequency Division Multiple Access (OFDMA) network, a Time Division Multiple Access (TDMA) network, a Code Division Multiple Access (CDMA) network, a Wideband-CDMA (WCDMA) network, a Frequency Division Multiple Access (FDMA) network, a Spatial Division Multiple Access (SDMA) network, etc. In at least some embodiments, the one or more wireless fronthaul links may correspond to wireless data connections between the base band unit 110 and the one or more radio units 120. The wireless fronthaul links might be signal agnostic or analog, meaning that they may comprise the radio unit access signal shifted into the analog domain, on a different carrier frequency, e.g. by the one or more radio units 120. Some connections may potentially use sharp beam forming or rely on coherent superposition (e.g. massive MIMO) to deliver the respective signal to the respective radio unit. The wireless fronthaul links may, e.g., be implemented using Frequency Division Duplex (FDD) or Time Division Duplex (TDD). They may comprise a downlink component from the base band unit 110 to the one or more radio units 120, and an uplink component from the one or more radio units 120 to the base band unit 110. In an FDD implementation, the uplink component and the downlink component may be based on different carrier frequencies. In a TDD implementation, the uplink component and the downlink component may use different time resources on the same carrier frequency. Due to reciprocity, channel estimation in the TDD mode can be carried out for one direction and used for the other. Reciprocity applies if the switching time of the TDD mode is equal or shorter than the coherence time of the radio channel. The apparatus 10 comprises at least one output 12 configured to transmit a downlink component of the one or more wireless fronthaul links to the one or more radio units 120. An output, e.g. the at least one output 12 or at least one output 24 as introduced subsequently, may correspond to an interface for transmitting information, which may be represented by digital (bit) values according to a specified code or protocol, within a module, between modules, or between modules of different entities. In at least some embodiments, the at least one output 12; 24 may comprise a, correspond to a, or communicate via a Massive MIMO (MM I MO) module, which may comprise an antenna array. The apparatus 10 further comprises at least one input 14 configured to receive an uplink component of the one or more wireless fronthaul links from the one or more radio units 120. An input, e.g. the at least one input 14 or at least one input 22 as will be introduced subsequently, may correspond to an interface for receiving information, which may be in digital (bit) values according to a specified code, within a module, between modules or between modules of different entities. In at least some embodiments, the at least one input 14; 22 may comprise a, correspond to a, or communicate via a Massive MIMO (MMIMO) module, which may comprise an antenna array. The apparatus 10 further comprises a control module 16 configured to control the at least one output 12 and the at least one input 14. The control module 16 is further configured to transmit a reference signal via the at least one output 12 to the one or more radio units 120. In at least some embodiments, the reference signal may comprise a sequence of known, pre-determined or identifiable symbols of known amplitude, which may be used to identify an impulse response of a channel to determine characteristics of a (wireless) channel, e.g. the wireless channels of the one or more wireless fronthaul links. The control module 16 is further configured lo receive a loopback version of the reference signal via the at least one input 14 from the one or more radio units 120. The loopback version of the reference signal may e.g. correspond to an analog conversion of the reference signal received at the one or more radio units. The loopback version of the reference signal may e.g. correspond lo an analogously converted (e.g. frequency shifted) version of the loopback signal. It may further reflect the influence of the downlink component and the uplink component on the reference signal, e.g. the attenuation of the reference signal as seen in the loopback version of the reference signal. The loopback version of the reference signal may e.g. correspond to a reflected or looped back version of the reference signal (e.g. loop-the-loop). The control module 16 is further configured determine information related to a per-radio unit transmission power to be used by the one or more radio units 120 for transmissions on the one or more wireless fronthaul links using an attenuation of the reference signal determined based on the loopback version of the reference signal. The control module 16 is further configured to determine transmission control information comprising the information related to the per-radio unit transmission power. In at least some embodiments, the transmission control information may comprise information related to an adjustment of amplifiers of the one or more radio units to achieve the per-radio unit transmission power, e.g. supply voltages. This may increase power efficiency as amplifiers may operate at increased or peak efficiency. In an exemplary implementation, the control module 16 may be configured to look up information related to the adjustment of the amplifiers based on the determined per-radio unit transmission power. The information related to the adjustment of the amplifiers may further be based on variations of the amplification of the amplifiers caused by variations of the amplifier supply voltage. The control module 16 may e.g. be configured to determine the transmission control information based on a path loss of the downlink component and based on a path loss of the uplink component in the loopback version of the reference signal. The control module 16 may e.g. be configured to determine the per-radio unit transmission power based on an estimated interference and an estimated path loss. The control module 16 may be configured to estimate the path loss based on the reference signal and the loopback version of the reference signal. The control module 16 may e.g. use the amplitude of the received loopback version of the reference signal and compare it with the original amplitude of the reference signal to calculate or deduce an attenuation / the path loss. In an exemplary implementation, the control module 16 may be configured to determine the per-radio unit transmission power based on an optimization function. The control module 16 may e.g. be configured to determine the per-radio unit transmission power such, that a target Signal-to-Interference and Noise, SINR, for the uplink component is approximated based on SINRtargetu denotes the target SINR for the uplink component, PtXc. denotes the per-radio unit transmission power of a radio unit i of the one or more radio units 120, and PLU. is the estimated path loss of the uplink component of the radio unit i. PIFIZ^F may e.g. be based on an interference power (IF), a thermal noise (z), and a receive noise figure (NF), of the uplink component. In various embodiments, the control module 16 may be configured to determine an uplink channel estimation matrix and a downlink channel estimation matrix based on the reference signal and the loopback version of the reference signal to determine the transmission control information. Alternatively or additionally, wherein the control module 16 may be configured to determine a joint uplink/downlink channel estimation matrix based on the reference signal and the loopback version of the reference signal to determine the transmission control information. In various embodiments, the control module 16 may be further configured to determine the transmission control information based on a cell coverage plan of the one or more radio units 120. The control module 16 may e.g. be configured to calculate an estimated per-radio unit transmission power based on e.g. distances of the one or more radio units 120 lo the base band unit 110 based on the cell coverage plan, and may adjust the estimated per-radio unit transmission power based on the reference signal and the loopback version of the reference signal. In a preferred embodiment, the control module 16 may be configured to determine the transmission control information without using information related to a channel estimation of the one or more wireless fronthaul links carried out at the one or more radio units 120. The control module 16 may e.g. be configured to determine the transmission control information for the one or more radio units 120, while the one or more radio units 120 merely loopback (and e.g. frequency shift) the reference signal. In at least some embodiments, the control module 16 may be configured to determine information related to a quality of the received uplink component. The control module 16 may be configured to adjust the transmission control information based on the information related to the quality of the received uplink component. The control module 16 may e.g. be configured to compare an outcome of the previously determined transmission control information to a desired outcome, e.g. a desired SINR, e.g. by analyzing the received uplink component, and may accordingly adjust the transmission control information, e.g. the per-radio unit transmission power, to achieve the desired outcome. In at least some embodiments, the transmission control information may comprise information related to a power threshold for a transmission of the one or more wireless fronthaul links. The information related to the power threshold may correspond to a projected lower bound for the per-radio unit transmission power based on a desired metric, e.g. a desired SINR or receive power. The control module 16 is further configured to provide the transmission control information to the one or more radio units 120 via the at least one output 12. The control module 16 may e.g. provide the transmission control information to the one or more radio units 120 using a control channel comprised in the one or more wireless fronthaul links, the transmission control information may be comprised in payload data or between payload data, or it may be transported using a further control channel. In embodiments the control module 16, and a control module 26 as introduced subsequently, may be implemented using one or more processing units, one or more processing devices, any means for processing, such as a processor, a computer or a programmable hardware component being operable with accordingly adapted software. In other words, the described function of the control module 16; 26 may as well be implemented in software, which is then executed on one or more programmable hardware components. Such hardware components may comprise a general purpose processor, a Digital Signal Processor (DSP), a micro-controller, etc. In at least some embodiments, the control module 16 may be further configured to determine information related to a transmission power to be used by the at least one output 12 for transmissions on the one or more wireless fronthaul links based on the reference signal and the loopback version of the reference signal. The control module 16 may be further configured to adapt the transmitting of the downlink component of the one or more wireless fronthaul signals at the at least one output 12 based on the information related to a transmission power to be used on the at least one output 12. The control module 16 may e.g. be configured to adjust the transmission power used by the at least one output 12 to achieve a desired SINR or receive power of the one or more wireless fronthaul links at the one or more radio units 120. In various embodiments, the adaptation of the transmitting may comprise an adaptation of a weighting of per-radio unit sub-antenna components or hardware-level adjustments, e.g. an adjustment of attenuators of the transmission component. In at least some embodiments, the supply voltage, e.g. of amplifiers, may be reduced, e.g. in order to operate amplifiers with an increased efficiency, instead of operating with increased backoff (and reduced efficiency). In at least some embodiments, the adaptation of the transmitting may further comprise a deactivation of antenna components or paths (in case radio units are deactivated or an overall required transmission power is reduced). The active antenna components and paths may operate with increased efficiency, as a power consumption of conversion units of deactivated paths may be omitted. As a number of antenna components or sub-antennas is thereby reduced, an impact of the reduction may be further calculated and compared to the requirements. In various embodiments, the control module 16 may be configured to determine the transmission power to be used by the at least one output 12 for transmissions on the one or more wireless fronthaul links based on an optimization function. The control module 16 may e.g. be configured to determine the transmission power to be used by the at least one output 12 such, that a target Signal-to-Interference and Noise, SINR, for the downlink component is approximated based on SlNRtar get.d 's the target SINR for the downlink component, PtxhUh. 's the transmission power to be used by the at least one output 12 for transmissions to a radio unit i of the one or more radio units 120, and PLdi is the estimated path loss of the downlink component for transmissions to the radio unit t. PIF,Z,NF mav be based on an interference power (IP"), a thermal noise (z), and a receive noise figure (NF) of the downlink component. In various embodiments, the control module 16 may be further configured to determine information related to a subset of antenna elements of a plurality of antenna elements to be used for transmitting on the one or more wireless fronthaul links. The information related to the transmission power to be used by the at least one output 12 for transmissions on the one or more wireless fronthaul links comprises the information related to the subset of antenna elements. The plurality of antenna elements may e.g. be comprised in a Multiple Input Multiple Output (MIMO) antenna module, and may e.g. be used for Massive MI MO, e.g. MIMO with a vast plurality of antenna elements. The plurality of antenna elements may e.g. be used for beamforming, and the one or more wireless fronthaul links may be spatially separated by such beamforming. The subset of antenna elements may e.g. correspond to the antenna elements to be used to maintain currently active wireless fronthaul links of the one or more wireless fronthaul links. The control module 16 may be configured to reduce a number of antenna elements in the subset to further decrease an energy consumption or demand of the antenna module, e.g. by deactivating associated conversion units, while maintaining currently active wireless fronthaul links. In at least some embodiments, the control module 16 may be further configured to adapt the receiving of the uplink component of the one or more wireless fronthaul signals at the at least one input 14 based on the reference signal and the loopback version of the reference signal. The control module 16 may e.g. be configured to adjust an equalization of the received uplink component based on the reference signal and the loopback version of the reference signal, e.g. to account for distortion caused by transmission. Fig. 3 illustrates a processing flow of the wireless fronthaul links' power control for various embodiments. In an exemplary embodiment, the calculation of the transmit power of the hub antennas and the transmit power of the SCCs may comprise: Estimating a path loss (PL) 3002 to radio units (e.g. the one or more radio units 120) (approx. equal path loss in uplink (PLU) and downlink (PLd) might be assumed, e.g. PL H PL = —2- = PLU — PLd). The path loss may be estimated from the transmit power Ptxhub at the hubs (e.g. base band unit 110) antennas, and the receive power from radio unit i, Prx., at the Hub. Prx. may be estimated from the receive signal per antenna or from an M x M matrix gained within the training phase per radio unit i or the thereof derived uplink and downlink channel coefficient matrices hu,hd. Fig. 3 further shows calculating the M x M joint channel matrix 3004, calculating the downlink channel coefficients hd 3006 and calculating the uplink channel coefficients hu 3008. For each radio unit, an SINR of at least SINRn,i„ may be achieved. The transmission power may be adapted in a way such that an overachievement of the SINR requirement may be limited. The transmit power necessary per radio unit may vary over the path loss, which may in turn vary mainly over the distance to the hub. The transmit power may also vary over the number of served radio units, so that in case of power down of radio units the hub's transmit power can be reduced. The path loss to radio unit t is denoted PLdp the transmission power portion - including array/transmission scheme gain - transmitted in the signal to radio unit i is Ptxhub. (e-M- at The SINR may be estimated 3010 in several ways, depending on the system. Assuming no interference from other radio units (e. g. perfect zero-forcing), the SINR may be dependent 3012 on thermal noise i ~ PLdi — P\FZ:Nf wherein SlNRtargetd is the target SINR for the downlink component, Ptxhub. is the transmission power to be used by the at least one output (12) for transmissions to a radio unit i of the one or more radio units (120), Pld. is an estimated path loss of the downlink component for transmissions to the radio unit i, and PIF:Z,NF 'S based on an interference power, IF, a thermal noise, z, and a receive noise figure, NF, of the downlink component. and/or wherein the control module (16) is further configured to determine information related to a subset of antenna elements of a plurality of antenna elements to be used for transmitting on the one or more wireless fronthaul links, and wherein the information related to the transmission power to be used by the at least one output (12) for transmissions on the one or more wireless fronthaul links comprises the information related to the subset of antenna elements. 7. The apparatus (10) of claim 1, wherein the control module (16) is further configured to adapt the receiving of the uplink component of one or more wireless fronthaul signals at the at least one input (14) based on the reference signal and the loopback version of the reference signal. 8. The apparatus (10) of claim 1, wherein the control module (16) is configured to determine an uplink channel estimation matrix and a downlink channel estimation matrix based on the reference signal and the loopback version of the reference signal to determine the transmission control information, or wherein the control module (16) is configured to determine a joint uplink/downlink channel estimation matrix based on the reference signal and the loopback version of the reference signal to determine the transmission control information, and/or wherein the control module (16) is configured to determine information related to a quality of the received uplink component and wherein the control module (16) is configured to adjust the transmission control information based on the information related to the quality of the received uplink component. 9. The apparatus (10) of claim 1, wherein the transmission control information comprises information related to a power threshold for a transmission of the one or more wireless fronthaul links. and/or wherein the control module (16) is further configured to determine the transmission control information based on a cell coverage plan of the one or more radio units (120). 10. The apparatus (10) of claim 1, wherein the control module (16) is configured to determine the transmission control information without using information related to a channel estimation of the one or more wireless fronthaul links carried out at the one or more radio units (120). 11. Apparatus (20) for a radio unit (120a) of a base station transceiver (100) of a mobile communication system (300), the base station transceiver (100) further comprising a base band unit (110) wirelessly communicating with the radio unit (120a) using a wireless fronthaul link, the apparatus (20) comprising: - at least one input (22) configured to receive a downlink component of the wireless fronthaul link from the base band unit (110); - at least one output (24) configured to transmit an uplink component of the wireless fronthaul links to the base band unit (110); and - a control module (26) configured to: control the at least one input (22) and the at least one output (24), receive a reference signal via the at least one input (22) from the base band unit (110), loopback a loopback version of the received reference signal via the at least one output (24) to the base band unit (110), receive transmission control information from the base band unit (110), wherein the transmission control information comprises information related to a per-radio unit transmission power to be used by the radio unit (120a) for transmissions on the wireless fronthaul link, and adapt the transmission power of the uplink component via the at least one output (24) based on the transmission control information. 12. The apparatus (20) of claim 11, wherein the control module (26) is configured to determine the loopback version of the received reference signal by analogously converting the received reference signal, and/or wherein the downlink component uses a downlink carrier frequency, wherein the uplink component uses an uplink carrier frequency, and wherein the control module (26) is configured to determine the loopback version of the received reference signal by analogously converting the received reference signal from the downlink carrier frequency to the uplink carrier frequency. 13. Method for a base band unit (110) of a base station transceiver (100) of a mobile communication system (300), the base station transceiver (100) further comprising one or more radio units (120) configured to wirelessly communicate with the base band unit (110) using one or more wireless fronthaul links, the method comprising: transmitting (32) a reference signal to the one or more radio units (120); receiving (34) a loopback version of the reference signal from the one or more radio units (120); determining (36) information related to a per-radio unit transmission power to be used by the one or more radio units (120) for transmissions on the one or more wireless fronthaul links using an attenuation of the reference signal determined based on the loopback version of the reference signal; determining (37) transmission control information comprising the information related to the per-radio unit transmission power; and providing (38) the transmission control information to the one or more radio units (120). 14. Method for a radio unit (120a) of a base station transceiver (100) of a mobile communication system (300), the base station transceiver (100) further comprising a base band unit (110) wirelessly communicating with the radio unit (120a) using a wireless fronthaul link, the method comprising: receiving (42) a reference signal from the base band unit (110); looping back (44) a loopback version of the received reference signal to the base band unit (110); receiving (46) transmission control information from the base band unit (110), wherein the transmission control information comprises information related to a per-radio unit transmission power to be used by the radio unit (120a) for transmissions on the wireless fronthaul link; and adapting (48) the transmission power for transmissions on the wireless fronthaul link based on the transmission control information. 15. A computer program product comprising a computer readable medium having computer readable program code embodied therein, the computer readable program code being configured to implement any of the methods of claim 13 and 14, when being loaded on a computer, a processor, or a programmable hardware component.