Patent Publication Number: US-2021195545-A1

Title: Apparatus and method for detecting group delay information and apparatus and method for transmitting a measurement signal via a transmission medium

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from German Patent Application No. 102019220091.5, which was filed on Dec. 18, 2019, and is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to measuring and characterizing a transmission medium, like a coaxial cable, for example, above its normal operating bandwidth. 
     The HelEOS system provided by the Giax GmbH Company comprises modem and switch technology which connects point-to-point Ethernet links via existing coaxial cables to form an Ethernet overlay system. This Ethernet overlay system provides for transmitting data on frequencies which nowadays are not made use of, above the currently used frequencies, which are also referred to as “white spaces”. These frequencies can be accessed by means of a diplexer which makes available the existing upstream and downstream frequencies for well-known systems, like DOCSIS and DVB-C, for example, and a new frequency range above 1 GHz, or 1.2 GHz DOCSIS 3.1, for example. The HelEOS system searches for the optimum frequency range for transmission and is able to adapt, during operation, to potential changes in the transmission route by adjusting the modulation used. Thus, a multiplication of the data rates available for fast Internet connections can be achieved without any great changes in the cable infrastructure and without providing fiber optic cables. The HelEOS system is able to convey up to 10 Gbits/s in downstream operation and 10 Gbits/s in upstream operation so that an overall sum rate of 25-30 Gbits/s can be achieved using a single coaxial cable, together with the data rates achievable in DOCSIS (in the lower frequencies). 
     In order to make use of the coaxial infrastructure range above the normal frequencies provided for the coaxial infrastructure, a precise characterization and measurement of the coaxial infrastructure are performed in order to then be able to drive the transmitter (and consequently also the receiver) correspondingly based on the obtained characterization of the coaxial infrastructure or, expressed generally, the transmission medium. 
     Typically, transmission parameters and, in particular, group delay information of a two-port are measured by a network analyzer. A network analyzer used for a two-port measurement entails synchronization between the transmitter and the receiver in order for precise phase information to be obtained. However, in the application described before, the synchronization cannot be complied with easily since the measurements deal with line lengths in the range of up to several hundreds of meters. Thus, synchronization between transmitter and receiver over such a great distance can only be obtained with difficulties or high complexity, if at all, since the transmitter of a signal and the receiver for this very signal transmitted and changed by the cable, will not be located at one and the same position. 
     WO 2010/081725 A2 discloses a method for measuring a group delay caused by a measurement object to be measured, using a network analyzer, comprising the following method steps: generating an excitation signal consisting of two signals spaced apart by a frequency difference in the network analyzer, exciting the measurement object by the excitation signal and measuring a response signal consisting of two signals which are each phase-distorted by the measurement object relative to the signals, by the network analyzer, determining a phase difference between the signals belonging to the excitation signal and a phase difference between the signals belonging to be response signal, and calculating the group delay from the phase difference of the signals belonging to the excitation signal, the difference of the signals belonging to the response signal and the frequency spacing. 
     DE 28 45 166 C3 discloses a method for increasing the characteristic curve of the group delay in a transmission channel and applying the same to the automatic selection of an equalizer. A measurement signal consisting of three oscillations having three frequencies is transmitted via the transmission channel and the instantaneous phase is derived from the components at the frequencies and, from this, the gradient of the characteristic curve of the group delay is calculated. 
     DE 24 36 011 C3 discloses a method and a circuit arrangement for measuring the group delay characteristic of a transmission route in which a test signal is fed to an input of the transmission route, the test signal containing a carrier which is switched periodically between a measuring frequency and a reference frequency and is amplitude-modulated with a frequency, in which the test signal is received at the output of the transmission route, in which a first time interval is measured from a point during a period of the one carrier frequency and a point during a period of the other carrier frequency, in which a second time interval is measured from a point during a period of the other carrier frequency and a point during a period of the one carrier frequency, and in which the difference between the two time intervals is determined and the group delay characteristic is derived from the difference, wherein specified points of the frequency curve are sampled during the reference frequency period and during the measuring frequency period in the signal received and wherein the first and second time intervals are measures digitally by an impulse counter for clock impulses. 
     SUMMARY 
     According to an embodiment, an apparatus for detecting group delay information over frequency for a transmission medium may have: a receiver for receiving a measurement signal to provide a reception signal, the measurement signal having at least a first carrier signal at a first carrier frequency, a second carrier signal at a second carrier frequency and a third carrier signal at a third carrier frequency, wherein transmission phase information on the carrier signals are known or derivable; a frequency analyzer for analyzing the reception signal to obtain reception phase information on the first carrier signal, the second carrier signal and the third carrier signal; and a processor for forming a first combined piece of phase information from the reception phase information from a first pair of carrier signals and for forming a second combined piece of phase information from the reception phase information from a second pair of carrier signals, the second pair of carrier signals differing from the first pair of carrier signals, for forming a first piece of group delay information from the first combined piece of phase information and the transmission phase information relating to the first pair of carrier signals and for forming a second piece of group delay information from the second combined piece of phase information and the transmission phase information relating to the second pair of carrier signals, and for associating the first piece of group delay information to a first frequency and the second piece of group delay information to a second frequency, the first frequency being derived from frequencies of the first pair of carrier signals, and the second frequency being derived from frequencies of the second pair of carrier signals. 
     According to another embodiment, an apparatus for transmitting a measurement signal via a transmission medium may have: a processor for generating a measurement signal, the measurement signal having at least a first carrier signal at a first carrier frequency, a second carrier signal at a second carrier frequency and a third carrier signal at a third carrier frequency, wherein transmission phase information on the carrier signals are defined; and a transmitter for feeding the measurement signal to the transmission medium, wherein the processor is configured to generate the measurement signal as a cyclic signal, to store the cyclic measurement signal in a transmitter memory, and to transfer the cyclic measurement signal to a digital-to-analog converter successively for a defined number of times to obtain a base band signal having a sequence of analog versions of the measurement signal, and wherein the transmitter is configured to convert the base band signal to a transmission band using a local oscillator and feed the converted base band signal to the transmission medium. 
     According to another embodiment, a method for detecting group delay information over frequency for a transmission medium may have the steps of: receiving a measurement signal to provide a reception signal, the measurement signal having at least a first carrier signal at a first carrier frequency, a second carrier signal at a second carrier frequency and a third carrier signal at a third carrier frequency, wherein transmission phase information on the carrier signals are known or derivable; analyzing the reception signal to obtain reception phase information on the first carrier signal, the second carrier signal and the third carrier signal; forming a first combined piece of phase information from the reception phase information from a first pair of carrier signals and forming a second combined piece of phase information from the reception phase information from a second pair of carrier signals, the second pair of carrier signals differing from the first pair of carrier signals, forming a first piece of group delay information from the first combined piece of phase information and the transmission phase information relating to the first pair of carrier signals and forming a second piece of group delay information from the second combined piece of phase information and the transmission phase information relating to the second pair of carrier signals, and associating the first piece of group delay information to a first frequency and the second piece of group delay information to a second frequency, the first frequency being derived from frequencies of the first pair of carrier signals, and the second frequency being derived from frequencies of the second pair of carrier signals. 
     According to still another embodiment, a method for transmitting a measurement signal via a transmission medium may have the steps of: generating a measurement signal, the measurement signal having at least a first carrier signal at a first carrier frequency, a second carrier signal at a second carrier frequency and a third carrier signal at a third carrier frequency, wherein transmission phase information on the carrier signals are defined; and feeding the measurement signal to the transmission medium, wherein generating has generating the measurement signal as a cyclic signal, storing the cyclic measurement signal in a transmitter memory, and transferring the cyclic measurement signal to a digital-to-analog converter successively for a defined number of times to obtain a base band signal having a sequence of analog versions of the measurement signal, and wherein feeding has converting the base band signal to a transmission band using a local oscillator and feeding the converted base band signal to the transmission medium. 
     Another embodiment may have a non-transitory digital storage medium having stored thereon a computer program for performing a method for detecting group delay information over frequency for a transmission medium having the steps of: receiving a measurement signal to provide a reception signal, the measurement signal having at least a first carrier signal at a first carrier frequency, a second carrier signal at a second carrier frequency and a third carrier signal at a third carrier frequency, wherein transmission phase information on the carrier signals are known or derivable; analyzing the reception signal to obtain reception phase information on the first carrier signal, the second carrier signal and the third carrier signal; forming a first combined piece of phase information from the reception phase information from a first pair of carrier signals and forming a second combined piece of phase information from the reception phase information from a second pair of carrier signals, the second pair of carrier signals differing from the first pair of carrier signals, forming a first piece of group delay information from the first combined piece of phase information and the transmission phase information relating to the first pair of carrier signals and forming a second piece of group delay information from the second combined piece of phase information and the transmission phase information relating to the second pair of carrier signals, and associating the first piece of group delay information to a first frequency and the second piece of group delay information to a second frequency, the first frequency being derived from frequencies of the first pair of carrier signals, and the second frequency being derived from frequencies of the second pair of carrier signals, when said computer program is run by a computer. 
     Still another embodiment may have a non-transitory digital storage medium having stored thereon a computer program for performing a method for transmitting a measurement signal via a transmission medium having the steps of: generating a measurement signal, the measurement signal having at least a first carrier signal at a first carrier frequency, a second carrier signal at a second carrier frequency and a third carrier signal at a third carrier frequency, wherein transmission phase information on the carrier signals are defined; and feeding the measurement signal to the transmission medium, wherein generating has generating the measurement signal as a cyclic signal, storing the cyclic measurement signal in a transmitter memory, and transferring the cyclic measurement signal to a digital-to-analog converter successively for a defined number of times to obtain a base band signal having a sequence of analog versions of the measurement signal, and wherein feeding has converting the base band signal to a transmission band using a local oscillator and feeding the converted base band signal to the transmission medium, when said computer program is run by a computer. 
     The present invention is based on the finding that a measurement signal comprising at least three carrier signals at three different carrier frequencies, the measurement signal advantageously being a cyclic measurement signal, allows determining the group delay information by using a frequency analysis of the measurement signal on the receiver side and downstream processing of the phase information, arriving at the receiver, of the carrier signals beyond frequency limits. 
     In particular, a combined piece of phase information is formed by a processor from the reception phase information from at least two carrier signals in order to then determine the group delay information from the combined phase information and the transmission phase information, which are known or can be transferred from the transmitter to the receiver, for example via a second channel. Since, however, the transmission phase information can be selected as desired and, advantageously, are set to +/−90 degrees for a carrier and 0/180 degrees for the carrier adjacent in terms of frequency, the transmission phase information can be estimated by the receiver on the basis of the received carrier signals, in particular when not only three carrier signals are used in one “pass”, but when a larger number of carrier signals, like 20 to 100 carrier signals, for example, is used per pass. 
     An advantageously cyclic measurement signal having a number of carrier signals which at least equals three and the number of which is limited in an upward direction only by the transmitter/receiver elements used and is 100, for example, is generated on the transmitter side. This typically digitally generated signal in which phases of carrier signals adjacent in frequency are to be spaced apart from one another advantageously by 90 degrees, may be generated digitally and stored in a transmitter memory. Subsequently, this signal is reproduced in a “continuous loop” and converted by a local oscillator from the baseband to the band to be characterized and fed to the transmission medium, like a coaxial infrastructure, for example. 
     On the receiver side, the transmission band to be characterized is downmixed to the baseband by a local oscillator and, after an analog-to-digital conversion, stored in a receiver memory. The stored signal is then subjected to frequency analysis to determine the phase values of the carrier signals on the receiver side. Thus, no synchronization between transmitter and receiver is required. Instead, the group delay determination over frequency is performed by forming a combined piece of phase information of the reception phase information from two neighboring carrier signals, then formed from the combined piece of phase information and transmission phase information transmitted via a second channel and derived on the receiver side. The piece of group delay information is then associated to one of the carrier frequencies each. 
     When a piece of group delay information has been determined by evaluating the reception phase information of the first and the second carrier signal, the piece of group delay information can be associated to a frequency equaling the first carrier frequency or the second carrier frequency, or another carrier frequency between the first and second carrier frequencies, for example. Thus, the piece of group delay information is calculated using the reception phase information from two neighboring carrier signals each so that two pieces of group delay information for two different frequency values are obtained for at least three carrier signals to obtain a piece of group delay information over frequency of the transmission medium. 
     Depending on the implementation, advantageously, 20 to 100 carriers are used in a measurement signal so that (n−1) pieces of group delay information can be obtained per frequency analysis and subsequent evaluation of the reception signal, when the measurement signal had n carriers. In order to be able to measure the entire frequency range between 1 GHz and 4 GHz of a coaxial infrastructure, for example, exemplarily five carrier signals of a width of 600 MHz each can be used, which are upmixed from the baseband using a specifically set local oscillator frequency to detect a corresponding frequency section of the transmission medium. 
     In particular, due to the number of carrier signals in a “trial” and due to the determination of the bandwidth of a trial and due to the corresponding LO control, an optimum compromise can be achieved between a parallel/serial measurement of the transmission medium. When a very large number of carriers per measurement signal is used and a corresponding frequency spacing between the carriers is selected, only a single trial may be entailed for characterizing a corresponding transmission region, i.e. only a single LO setting. This achieves a fully parallel characterization of the transmission medium. If, however, the number of carriers or the carrier spacing is selected to be correspondingly smaller, for a complete characterization of the transmission band, several “trials”, like 2 to 10 trials, for example, will be performed, which represents a rather serial measurement of the transmission band, maybe at a very precise frequency resolution, to detect relatively narrow-banded “events” like clear resonance. 
     This means that the present invention operates as a “distributed” network analyzer, without requiring synchronization between transmitter LO and receiver LO or, generally, synchronization between transmitter and receiver. This procedure is supported by the fact that a cyclic measurement signal is used, which is emitted on the transmitter side in a “continuous loop” so that, on the receiver side, a “snapshot” of the signal arriving on the receiver side is taken irrespective of transmitter timing, provided the receiver comprises information on the LO frequency currently used by the transmitter, which, in embodiments, is achieved using an LO plan which is exchanged between the transmitter and the receiver, typically using a side channel, or is predetermined. However, no precise time synchronizations are necessary here, but a simple non-synchronized clock is sufficient, since the temporal succession of the different LO settings does not have an impact on the precision of the measurement. 
     In embodiments of the present invention, each frequency section is recorded several times in several snapshots, to increase precision and to eliminate statistical errors, and the group delay information calculated from each “snapshot” are combined to obtain corresponding mean values which, considered statistically, exhibit higher a precision than when using only a single snapshot. 
     In addition, it is of advantage to form an overlap region between different frequency sections so that, using different LO settings, at least two pieces of group delay information can be obtained for one and the same frequency value. Correction can be performed using these overlap regions to obtain a continuous course without systematically generated level differences for a characterization of the entire transmission band obtained from the different “trials” with different local oscillator settings. 
     The obtained characteristics of the transmission medium which has been captured in a very broad-band region as regards its group delay information and, in embodiments, also with regard to its attenuation, can be used by a transmitter for useful data to set and, if applicable, dynamically change a corresponding transmitter modulation adjusted to the specific transmission band or a corresponding pre-distortion when a measurement pass having one or several trials has been performed. 
     However, the characterization of the transmission medium can also be used to determine whether there are larger defects due to which replacement of the transmission medium could be entailed, or at least closer examination by visual inspection, etc. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will be detailed subsequently referring to the appended drawings, in which: 
         FIG. 1  shows a block circuit diagram of an apparatus for detecting group delay information over frequency; 
         FIG. 2 a    shows an embodiment of the receiver of  FIG. 1 ; 
         FIG. 2 b    shows a schematic illustration of a spectrum of the measurement signal when being fed to the transmission medium; 
         FIG. 2 c    shows a schematic illustration of the spectrum of the reception signal at the output of the frequency analyzer; 
         FIG. 2 d    shows a schematic illustration of allocating the group delay information to corresponding allocation regions; 
         FIG. 3  shows a flow chart of an implementation of the processor of  FIG. 1 ; 
         FIG. 4  shows a flow chart representation of an advantageous procedure when calculating the group delay information using at least three and, advantageously, 20 to 100, for example, carrier signals with phase difference correction and phase hop correction; 
         FIG. 5  shows a schematic illustration of determining group delay information for different carriers below and above a center frequency; 
         FIG. 6  shows different alternatives of setting the transmission and the reception amplitude characteristic; 
         FIG. 7  shows a schematic illustration for illustrating overlap regions between neighboring frequency sections, each frequency section consisting of a plurality of carrier signals; 
         FIG. 8  shows a schematic illustration of two nodes, each node comprising an apparatus for detecting (RX) and an apparatus for transmitting (TX) and a side channel for the LO plan; 
         FIG. 9 a    shows a table of advantageous value regions for individual parameters; 
         FIG. 9 b    shows an exemplary table for LO plan numbers and associated data, like LO frequencies, for example; 
         FIG. 10 a    shows an illustration of a measurement of the transmission band having six LO frequencies relative to the group delay; 
         FIG. 10 b    shows an illustration in analogy to  FIG. 10 a   , but for amplitudes; 
         FIG. 11  shows an illustration of the group delay information over frequency in the entire transmission band before and after correction due to the overlap regions; 
         FIGS. 12 a  and 12 b    show an illustration of the group delay or amplitude over frequency after the performed correction; and 
         FIG. 13  shows a block circuit diagram illustration of an apparatus for transmitting the measurement signal via a transmission medium. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventive concept serves for identifying a channel transfer function with regard to the group delay and, advantageously, also with regard to the attenuation feature to be able to obtain based thereon optimum signal transmission as regards maximum data rates in both transmission directions. Since a precise determination of the amplitude frequency response is desired for optimizing the transmission rates, it is of advantage to determine the amplitude frequency response and, in particular, the group delay information over frequency in a precisely resolved manner. In order to obtain an efficient measurement over a large frequency range, in embodiments, a high number of samples can be determined at the same time, but still at high a frequency resolution in the range of 10 MHz. By using a dynamic allocation of the center frequency, it is even possible to increase the frequency resolution by the factor 100 so that a resolution in a range of 100 kHz can even be generated. This allows achieving a parallel detection of a large number of samples of the group delay information or amplitudes, wherein particularly a further increase in resolution is obtained by the dynamic allocation of the center frequency by performing, in embodiments, a plurality of trials at different LO frequencies. 
     A precisely resolved characterization of the transmission medium allows identifying so-called “notches”, i.e. declines in the transfer function by resonances. In addition, other narrow-band effects can be identified, which counteract an optimum utilization of the transmission range. Advantageously, a configurable number of carriers are employed, wherein particularly at least three carriers are used. These can be below, above or on both sides of a carrier, like the center frequency  500  in  FIG. 5 , for example. 
     In particular,  FIG. 5  shows the situation where a number of four carriers, referred to by  502 , are arranged below the center frequency and a number of four carriers, referred to by  504 , are arranged above the center frequency. For optimizing the signal-to-noise ratio, depending on the implementation, amplitudes can be set differently to compensate an expected channel attenuation, as will be discussed in greater detail referring to the lower representation in  FIG. 6 . Precision of the measurement values can be achieved by a downstream optimization of the phase positon of the individual carriers in that the transmission and the reception signal have a low crest factor, which has an advantageous effect on the resolution. In particular, the transmission signal comprises the configurable number of carriers and additionally a configuration of different amplitudes can be used, wherein optimization of the phases is of advantage for reducing the crest factor. 
     The final resolution achieved in the frequency range can be increased considerably by synchronously changing the center frequencies or LO frequencies at the transmitter and the receiver. 
       FIG. 5  particularly shows an adjustment of the amplitude characteristics to the channel feature in that the amplitudes of the transmission signal, illustrated in  FIG. 5 , increase with an increasing frequency. This means that the carrier at the smallest frequency, i.e. to the left in the carriers  502  below the center frequency, comprises the smallest amplitude and the carrier at the highest frequency, i.e. to the right in the carriers above the center frequency, comprises the highest amplitude. 
     Determining the group delay distortions in the transfer function is used for the intended maximization of the data rate. The main field of application is finding and precisely characterizing resonant frequencies, notches or other narrow-band effects within the transmission region to be used. An essential advantage of the present invention is that phase synchronization between transmitter and receiver is not necessary. In addition, only transferring phase information via the transmission signal from the transmitter to the receiver via a side channel is entailed in special embodiments. However, the transfer may also not be performed. In this case, the receiver is configured to estimate the transmission phases, or these are predetermined fixedly between the transmitter and the receiver per carrier frequency. For estimating, it is sufficient for the transmitter to have only a rough indication of the transmission phases when the transmission phases are selected in a certain phase pattern. Advantageously, the phases are selected such that a phase of a carrier signal at a certain frequency is set to a randomly determined value, which can be +90 degrees or −90 degrees. The phase of the carrier adjacent in frequency is set to the phase of the previous carrier plus again a random phase, which is either 90 degrees or −90 degrees. This means that the phase between successive carriers will differ by a magnitude of 90 degrees. Other phase settings, not operating based on a random number, but 90 degrees, 180 degrees, −90 degrees, 0 degrees, etc., for example, in a order increasing in frequency, are also conceivable. 
     Advantageously, frequency synchronization of transmitter and receiver can be performed to increase the precision of the measurement in another step. Additionally, depending on the implementation, phase synchronization of transmitter and receiver can be used in a further step to further increase precision. Both frequency synchronization and phase synchronization are optional steps which typically imply relatively high complexity. Even without such a frequency synchronization or phase synchronization, very efficient and very precise group delays and attenuation values over frequency can be obtained. 
       FIG. 1  shows an apparatus for detecting group delay information over frequency for a wire-bound transmission medium  100 , for example. In embodiments, the transmission medium is a coaxial transmission medium and, in particular, an already existing coaxial transmission structure, like a coaxial transmission structure for cable television reception, for example. Alternative transmission media, like two-wire lines, non-coaxial, like copper lines, and also wireless transmission media, can also be measured using the inventive apparatus. 
     The apparatus comprises a receiver  110  for receiving a measurement signal to provide a reception signal, the measurement signal comprising at least a first carrier signal  301  of  FIG. 2 b    at a first carrier frequency f 1 , a second carrier signal  202  at a second carrier frequency f 2  and a third carrier signal  203  at a third carrier frequency f 3 , wherein transmission phase information on the carrier signals are known or derivable. In particular, the transmission phase information when feeding the measurement signal to the transmission medium  100  are advantageously known or derivable. 
       FIG. 2 b    shows a spectrum of the transmission signal having the carrier signals  201 ,  202 ,  203  at the carrier frequencies f 1 , f 2 , f 3 , wherein additionally a fourth carrier signal  204  at a fourth carrier frequency f 4  is shown in  FIG. 2 b   .  FIG. 2 b    shows the transmission signal spectrum or spectrum of the measurement signal which, when arriving at a receiver, i.e. when having become the reception signal, comprises the spectrum as is shown in  FIG. 2 c   . The reception signal spectrum will again comprise a first carrier signal  211 , a second carrier signal  212 , a third carrier signal  213  and a fourth carrier signal  214  which in turn will be at the frequencies f 2 , f 3 , and f 4 . However, the phases of the reception signal spectrum and, in particular, of the carrier signals in the reception signal spectrum will differ from the transmission phases. The same applies to the amplitudes which, due to cable attenuation, in  FIG. 2 c    are indicated to be smaller than in  FIG. 2   b.    
     Additionally, the apparatus for detecting comprises a frequency analyzer  120  for analyzing the reception signal, i.e. the signal the spectrum of which is illustrated in  FIG. 2 c   , to obtain reception phase information on the first carrier signal, the second carrier signal and the third carrier signal. In an embodiment, the frequency analyzer provides the reception phases phaseRX 1 , phaseRX 2 , phaseRX 3  and also phaseRX 4  for the carrier signals  211  to  214  of  FIG. 2 c    by means of Fourier transform and, advantageously, by means of direct Fourier transform (DFT). Additionally, the apparatus comprises a processor for forming a first combined piece of phase information from the reception phase information from a first pair of carrier signals and for forming a second combined piece of phase information from the reception phase information from a second pair of carrier signals which differs from the first pair of carrier signals. 
     The first pair of carrier signals is, for example, carrier signals  211  and  212  and the second pair of carrier signals exemplarily comprises carrier signals  212  and  213 . Alternatively, the second pair of carrier signals may also comprise carrier signal  211  and carrier signal  213 . However, it is of advantage for the pairs from which the combined phase information are calculated to be pairs of carrier signals mutually adjacent as far as frequency is concerned. A first piece of group delay information is formed by the processor  130  from the first combined piece of phase information. Additionally, the processor  130  is configured to form the second piece of group delay information from the second combined piece of phase information and the transmission phase information of the corresponding second pair. This means that, in accordance with the invention, two pieces of group delay information, which are then associated by the processor  130  to two different frequency values, are calculated from at least three carrier signals in the reception signal. 
     The processor is particularly configured to perform associating the first piece of group delay information to a first frequency and associating the second piece of group delay information to a second frequency, the first frequency differing from the second frequency and further being derived from frequencies of the first pair of carrier signals. The same applies to the second frequency which is derived from the frequencies of the second pair of carrier signals. The first frequency which the first piece of group delay information is associated to thus originates from one of the allocation regions as illustrated in  FIG. 2 d   . Thus, the first piece of group delay information can exemplarily be associated to the frequency f 2 , whereas the second piece of group delay information is associated to the frequency f 3 , whereas the group delay information for the frequency f 1  is set to zero, as per definition. Alternative ways of allocation are also possible. The processor can exemplarily be configured to associate the first piece of group delay information to the frequency f 1  and the second piece of group delay information to the frequency f 2  and a third piece of group delay information derived from a third and a fourth carrier signal to the third carrier frequency f 3 . 
     As a further alternative, any other frequency in the allocation region can be used, for example, the center between f 1  and f 2  for the first piece of group delay information and the center between f 2  and f 3  for the second piece of group delay information. Due to the implementation of the present invention with several trials using different local oscillator frequencies, however, it is of advantage to set the piece of group delay information for the lowest frequency of a trial to zero. This means that, in particular due to an overlap region between two frequency sections, it can nevertheless be achieved that at least one piece of group delay information will be calculated for each frequency value when the overlap region comprises only one frequency interface. Advantageously, however, the overlap region is several frequency interfaces, like 10 or more, for example, so that, due to setting the piece of group delay information for the lowest frequency of a frequency section to zero, nevertheless an overlap region of 9 frequency interfaces, for example, will remain, comprising two different pieces of group delay information each, from which correction values can be calculated, as discussed below. 
     This means that the processor is configured, as is illustrated in  FIG. 3 , by obtaining the phase information of the carrier signals in step  300 , as is illustrated by the frequency analyzer  120 . The result of step  300  are phase values phaseRX 1 , phaseRX 2 , phaseRX 3  and phaseRX 4 . In step  310 , a combined piece of phase information each is formed from a pair of carrier signals. In step  320 , the piece of group delay information is formed each from a pair of pieces of phase information and the respective transmission phase information referred to as phase values phaseTX 1 , phaseTX 2 , phaseTX 3  and phaseTX 4  in  FIG. 2 b   . In step  330 , the group delay information of a pair are each associated to a frequency, as has been illustrated in  FIG. 2   d.    
     The frequency analyzer  120  is configured to obtain a reception phase value phaseTX 1  for the first carrier signal and a second reception phase value phaseTX 2  for the second carrier signal. In addition, the frequency analyzer  120  is configured to obtain a third reception phase value for the third carrier signal. Furthermore, the transmission phase information are given either as absolute phase information or already as a first transmission difference between the first carrier signal and the second carrier signal and as a second transmission difference between the second carrier signal and the third carrier signal. 
     Additionally, the processor  130  is configured to calculate, as the first combined piece of phase information, a first reception difference from the first reception phase value and the second reception phase value and to calculate, as the second combined piece of phase information, a second reception difference from the second reception phase information and the third reception phase information. Additionally, the processor is configured to calculate the piece of group delay information from the first reception difference and the first transmission difference and associate the same to the first frequency, that is in the allocation region of  FIG. 2 d   . Furthermore, the processor is configured to calculate the second piece of group delay information from the second reception difference and the second transmission difference and allocate the same to the second frequency in the corresponding allocation region between f 2  and f 3 . 
     The step of calculating the transmission difference, which is also referred to by phaseDiff TX , and the step of calculating the reception difference, which is also referred to by phaseDiff RX , are illustrated as an equation at  410  in  FIG. 4 . 
     phaseDiff RX  represents the first combined piece of phase information, wherein ii equals 1 and ii+1 equals 2. The second combined piece of phase information is obtained when ii equals 2 and ii+1 equals 3. The transmission-reception phase difference is calculated in step  420  from the first reception difference and the first transmission difference. These values, which are referred to by p1, p2, are calculated for each pair of transmission-reception phase difference and reception difference, as is illustrated in step  420 . 
     Advantageously, an optional phase correction is performed by correcting the transmission difference or reception difference, in case absolute phase hops occurring are greater than π between successive phase values. In this case, either 2π is added to the corresponding phase, or 2π subtracted, as is shown at  440  in  FIG. 4 . The phase threshold responsible for this is 180 degrees or π, however, other phase thresholds can be used depending on the implementation. Phase unwrap is obtained by the phase correction  440 . 
     Another correction in case absolute hops greater than π occur between successive values, is illustrated in step  450 , which may also be employed optionally. In particular, an uncorrected piece of group delay information is calculated by the processor in step  420  from the first reception difference and the first transmission difference. Additionally, the processor is configured to calculate an uncorrected second piece of group delay information from the second reception difference and the second transmission difference. Additionally, correction is performed in case a difference between the uncorrected first piece of group delay information and the uncorrected second piece of group delay information is greater than a correction threshold. The correction threshold is 180 degrees or π for an absolute phase hop. 
     Then, in step  430 , conversion of the advantageously corrected piece of group delay information to a time value is performed, i.e. to a time value in nanoseconds, as is illustrated in  FIG. 4  by the factor 10 3 , which takes into consideration that the variable freq step  is given in megahertz. The variables len freq  and freq step  will be discussed below. 
     Advantageously, the receiver  110  is configured to determine the transmission phase information, as has been discussed referring to  FIG. 2 b   , by receiving side information on the transmission phase information, as is illustrated in  FIG. 1  by  180 , wherein  180  illustrates a side information channel. Alternatively, the receiver  110  is configured to determine the transmission phase information at the receiver using the knowledge on allowed phase values of the transmission phase information. In particular, the ambiguity eliminated by the corrections in steps  440  and  450  may be used to perform a phase value estimation. 
     Advantageously, the transmission phase information for the carrier of the three carrier frequencies are +90 degrees or −90 degrees and 0 degrees or 180 degrees for another carrier of the three carrier frequencies. This ensures that the a low crest factor of the overall transmission signal can be obtained, which has an increasing importance in particular with an increasing number of carriers in the transmission signal. A reduction in the crest factor is additionally particularly, obtained when a random selection variation is contained either in each phase determination of the transmission signal or in at least every second or every third phase determination of the transmission signal. In particular when setting the phases to +90 degrees or −90 degrees and 0 degrees or 180 degrees for each successive frequency value, the phase threshold in step  440  and the correction threshold in step  450  will each be 180 degrees or π. Advantageously, the number of carrier signals in the measurement signal is at least 30, the 30 carrier signals being associated to 30 different carrier frequencies each which are spaced apart in frequency over a carrier frequency spacing. In particular, the frequency analyzer  120  is configured to calculate a phase value for each carrier signal as reception phase information. Additionally, the processor  130  is configured to determine the group delay information for each carrier frequency except for one carrier frequency so that, for a number of n carrier frequencies received, a number of (n−1) pieces of group delay information are determined which are associated to a number of n−1 frequencies. Additionally, the frequency analyzer  120  is configured to determine one piece of reception amplitude information each for the carrier signals, the processor  130  being additionally configured to determine an attenuation value per carrier frequency using the reception amplitude information. 
       FIG. 2 a    shows an embodiment of the receiver  110  of  FIG. 1 . The receiver comprises a reception front end  112  coupled to a controllable local oscillator  116 , which is controllable by a controller  140 . The output signal of the reception front end  112  is already in the baseband, due to the local oscillator  116  effect, and is converted by an analog-to-digital converter  114 . The output signal of the A/D converter  114  reaches a reception memory  118 . The reception memory  118  is also controlled by the controller  140  to record a defined region of the digital signal at the output of the A/D converter  114 , in temporal order of recording, to drive the frequency analyzer  120 . Depending on the implementation, the A/D converter  114  can be permanently active and the reception memory  118  only records during the time windows indicated by the controller  140  to obtain a “snapshot” each. Alternatively, the A/D converter  114  can be controlled by the controller  140  to only perform an analog-to-digital conversion when recording takes place in the reception memory  118 . In this case, the reception memory  118  does not have to be controlled specifically by the controller, but the A/D converter  114 . Alternatively, both elements can be driven by the controller. 
     In an embodiment of the present invention, the receiver  110  is configured to obtain one or more further measurement signals temporally after the measurement signal. In addition, the frequency analyzer  120  is configured to analyze the one or more further measurement signals. In addition, the processor is configured to determine the group delay information over frequency also for the one or more further measurement signals so that a piece of group delay information is obtained from each individual “snapshot” for each carrier frequency value. Advantageously, the processor  130  is configured to determine, from the group delay information for the measurement signal and the one or more further measurement signals, the group delay information for the individual carrier frequencies by selecting or combining group delay information from different snapshots. 
     Depending on the implementation, the first carrier signal, the second carrier signal and the third carrier signal are sinusoidal. In this case, the frequency analyzer is configured to perform a Fourier analysis of the measurement signal. Alternatively, advantageously periodic carrier signals differing from sinus signals can also be used. In such a case, the frequency analysis is to be adjusted to different “basic functions” than sinus signals from which the measurement signal is set up. 
     In an embodiment of the present invention, the receiver  110  is configured to convert a first frequency section to the baseband using a first local oscillator frequency. In addition, the receiver  110  is configured to convert a second frequency section to the base band using a second local oscillator frequency. In particular, the receiver is configured to set, using a fixed schedule or a schedule received via a side channel, the first local oscillator frequency and the second local oscillator frequency offset in time. In addition, the first local oscillator frequency and the second local oscillator frequency are set such that the first frequency section and the second frequency section overlap in an overlap region so that several pieces of group delay information each are obtained for carrier frequencies in the overlap region from measurements having different local oscillator frequencies. 
     Depending on the implementation, the receiver is configured to perform one-sideband demodulation using the first local oscillator frequency and the second local oscillator frequency.  FIG. 10 a    shows different local oscillator frequencies LO 1 , LO 2 , LO 3 , LO 4 , LO 5  and LO 6 . In addition, the lower sideband is illustrated to be used in the one-sideband demodulation with the local oscillators LO 1 , LO 2 , LO 3 , whereas the upper sideband is used with the local oscillator frequencies LO 4 , LO 5 , LO 6 . Additionally, certain overlap regions  10   a ,  10   b ,  10   c ,  10   d ,  10   e  are illustrated in  FIG. 10 a   . Additionally, the third local oscillator frequency LO 3  is illustrated to be higher in frequency than the fourth local oscillator frequency LO 4  in the overlap region  10   c . The change of the one sideband used for demodulation from the upper to the lower sideband is shown, which is of advantage due to the corresponding frequencies and the local oscillators being easy to handle. 
     Overlap regions  10   a  to  10   e  are further used to bring the group delay information and, if applicable, the amplitude information, as are illustrated in  FIG. 10 a    and  FIG. 10 b   , to the same level, as is illustrated in  FIG. 11  and will be discussed below. Here, the processor  130  is configured to modify, using the group delay information for the different local oscillator frequencies in an overlap region, group delay information outside the overlap region for the first frequency section and/or the second frequency section to reduce or eliminate discontinuities due to the different local oscillator frequencies. In particular, the processor  130  here is configured to form a first mean value from the group delay information of the first frequency section in the overlap region and to form a second mean value from the group delay information of the second frequency section in the overlap region. Additionally, using the first mean value and the second mean value, a correction value is calculated, which is then used to correct the group delay information of the first frequency section or of the second frequency section so as to obtain corrected group delay information. Depending on the implementation, the correction is a combination of the correction value and the respective group delay information, like an addition or subtraction, for example. 
     In particular, the processor  130  here is configured to eliminate outliers before calculating the first or second mean value, wherein this may be performed using a cumulative distribution function having a maximum and minimum quantile. 
     Depending on the implementation, not only a single overlap region  10   a  is set, but several, like 5 overlap regions, for example, which are illustrated in  FIG. 10 a   . The processor  130  here is configured to calculate the group delay information for a third frequency section, which overlaps with the second frequency section in a second overlap region. Particularly, the processor  130  is configured to use, for calculating a correction value for the third frequency section, a correction value from the overlap region between the first frequency section and the second frequency section, apart from mean values in the third overlap region, so that the correction values “propagate” over the entire frequency range, i.e. the correction value of the highest frequency section, as regards frequency, is calculated from the correction values of the frequency sections of low frequency, in addition to the respective mean values. 
     In embodiments of the present invention, the processor  130  is additionally configured to determine control information for a transmitter for the transmission medium using the group delay information to obtain a desired transfer rate and/or a desired maximum error rate when transmitting via the transmission medium. As has been explained before, the transmission medium advantageously is a coaxial transmission medium and, in particular, an already existing coaxial infrastructure in which a bandwidth of the reception signal is between 50 MHz and 500 MHz and a measurement region of the coaxial infrastructure is between 1000 and 5000 MHz. 
       FIG. 13  shows an apparatus for transmitting a measurement signal via a transmission medium, which is illustrated in  FIG. 13  again as a (coaxial) transmission medium  100 . The apparatus for transmitting comprises a processor  410  for generating the measurement signal, which is illustrated in  FIG. 2 b   , and a first carrier signal  201  at first carrier frequency f 1 , a second carrier signal  202  at a second carrier frequency f 2  and a third carrier signal  203  at a third carrier frequency f 3 . The measurement signal is passed on to a transmitter  420  configured to feed the measurement signal to the transmission medium  100 . 
     Depending on the implementation, the processor  410  comprises a digital adder  412 , a transmitter memory  414  and an digital-to-analog converter  416 . Additionally, the apparatus for transmitting advantageously comprises a controller  430  which is able to transmit transmission phase information via the side channel  180  to the receiver  110  of  FIG. 1 , i.e. from the apparatus for detecting the group delay information. 
     Additionally, in an embodiment of the present invention, the transmitter comprises a local oscillator  424  which can be driven by the controller  430 , and a transmitter front end  422  which particularly comprises an upmixer. The processor  410  is configured to generate the measurement signal in a baseband, and the transmitter  420  is configured to convert, using the local oscillator  424  the measurement signal from the baseband to a transmission band, which is higher in frequency than the baseband. In addition, the processor  410  is configured to generate the measurement signal by digitally summing up, by means of the digital adder  412 , using sinusoidal carrier signals at the individual carrier frequencies, the sinusoidal carrier signals having defined phases to one another, which represent the transmission phase information. The digital signal is then stored in a transmitter memory  414  and converted to an analog form by the digital-to-analog converter  416 . In particular, the processor  410  is additionally configured to cyclically generate the measurement signal using the digital adder  412  controlled by the controller  430 , i.e. generate the same as a cyclic signal comprising a certain length in digital samples, and configured such that there will be no discontinuities when “piecing” the beginning of the cyclic measurement signal again to the end of the cyclic measurement signal. 
     The cyclic measurement signal having been generated by the digital adder  412  is stored in the transmitter memory  414  and subsequently transferred to the digital-to-analog converter  416  for a defined number of times so that a base band signal is obtained at the output of the digital-to-analog converter in the digital domain, comprising a sequence of analog versions of the measurement signals. Additionally, the transmitter is configured to convert the base band signal to a transmission band using the local oscillator  424  and feed it to the transmission medium  100 . For generating the transmission signal in the different frequency portions, the local oscillator  424  may be controlled in the same way as is the local oscillator of the receiver, which performs a corresponding downmixing. Additionally, the transmitter front end  422  is configured to perform a one-sideband modulation to obtain the corresponding transmission signals, which then, at the receiver side, result in the corresponding frequency ranges, as are illustrated in  FIG. 10 a    with the overlap regions  10   a  to  10   b.    
     Subsequently, an implementation of the present invention will be illustrated referring to  FIGS. 8 to 13 . Advantageously, the transmitter illustrated in  FIG. 13  generates a measurement signal, which comprises a special distribution of the amplitude and phase information, and cyclicity. The measurement and evaluation are performed by emitting and receiving the measurement signal, which has a phase distribution, in order for a good measurement signal with regard to the optimized crest factor to be obtained. In particular, the signal may be completely cyclic in the time domain so as to allow any sample time at the receiver. The reference phase is determined from two carriers. Starting from the third carrier, the group delay is calculated while considering the reference phase. The fixed resolution in the frequency range predetermined by the measurement set up can be increased by synchronously changing the center frequency at the transmitter and receiver. It is of advantage that this kind of measurement results can be achieved without a frequency normal on both sides. Thus, the present invention can also be referred to as “distributed network analyzer”. In particular, a measurement signal, i.e. a cyclic measurement signal, is generated, which is suitable for the evaluations as have been described referring to  FIG. 4 , wherein, however, no frequency and phase synchronicity between receiver and transmitter is required, but may optionally be used. 
     Advantageously, the channel is excited by different narrow-band trials, as is illustrated in  FIG. 7 . Here, signal center frequencies alone, signal bandwidths alone or both center frequencies and bandwidths together are adapted, advantageously without interruptions, such that the entire useful spectral range can be “mapped” by means of this measurement, with regard to a signal-to-noise ratio achievable and/or a signal-to-interference ratio achievable. 
     The following may be performed for generating a measurement signal. 
     The measurement signals for the individual trials are generated as follows:
         the sample rate SAMPLE FREQ  is selected in dependence on the ADC and DAC   additionally, the following parameters are given:
           the bandwidth BW use  in MHz   the number of samples used NUM SAMPLES      the spacing, in samples, between the individual frequencies TX SPACING      
           the frequency indices used freq idx  are calculated from these parameters:       

     
       
         
           
             
               freq 
               step 
             
             = 
             
               
                 
                   SAMPLE 
                   FREQ 
                 
                 
                   NUM 
                   SAMPLES 
                 
               
               * 
               
                 TX 
                 SPACING 
               
             
           
         
       
       
         
           
             
               
                 freq 
                 idx 
               
               = 
               
                 TX 
                 SPACING 
               
             
             , 
             
               2 
               * 
               
                 TX 
                 SPACING 
               
             
             , 
             … 
              
             
                 
             
             , 
             
               
                 ⌊ 
                 
                   
                      
                     
                       BW 
                       use 
                     
                      
                   
                   
                     freq 
                     step 
                   
                 
                 ⌋ 
               
               * 
               
                 TX 
                 SPACING 
               
             
           
         
       
       
         
           
             
               len 
               freq 
             
             = 
             
               length 
                
               
                 ( 
                 
                   freq 
                   idx 
                 
                 ) 
               
             
           
         
       
         
         
           
             At least three carriers per recording are used to be able to calculate the group delay (see chapter 2.5). 
             The frequencies freq MHz  result from this: 
           
         
       
    
     
       
         
           
             
               freq 
               MHz 
             
             = 
             
               
                 freq 
                 idx 
               
               * 
               
                 
                   SAMPLE 
                   FREQ 
                 
                 
                   NUM 
                   SAMPLES 
                 
               
             
           
         
       
         
         
           
             Calculating the amplitudes amp[idx]: 
           
         
       
    
     
       
         
           
             
               
                 amp 
                  
                 
                   [ 
                   idx 
                   ] 
                 
               
               = 
               
                 
                   
                     ( 
                     
                       
                         
                           freq 
                           idx 
                         
                          
                         
                           [ 
                           idx 
                           ] 
                         
                       
                       - 
                       
                         min 
                          
                         
                           ( 
                           
                             freq 
                             idx 
                           
                           ) 
                         
                       
                     
                     ) 
                   
                   * 
                   
                     
                       
                         amp 
                         factor 
                       
                       - 
                       1 
                     
                     
                       
                         max 
                          
                         
                           ( 
                           
                             freq 
                             idx 
                           
                           ) 
                         
                       
                       - 
                       
                         min 
                          
                         
                           ( 
                           
                             freq 
                             idx 
                           
                           ) 
                         
                       
                     
                   
                 
                 + 
                 1 
               
             
             , 
           
         
       
         
         
           
             wherein idx=1, . . . , len freq  and amp factor  can be selected as desired, so as to obtain the amplitude characteristics of  FIG. 1 . When selecting amp factor =1, the following will result: 
           
         
       
    
       amp[idx]=1.         Calculating the phases phase[idx]:       
       phase[0]=random(90,−90)
 
       phase[idx]=phase[idx]+random(90,−90)
         wherein idx=1, . . . , len freq  and random(90, −90) are selected randomly to be either 90 or −90.   From the amplitude values amp[idx] and phase[idx], dac val [ii] results for the DAC samples:       

     
       
         
           
             
               
                 dac 
                 val 
               
                
               
                 [ 
                 ii 
                 ] 
               
             
             = 
             
               
                 ∑ 
                 
                   idx 
                   = 
                   1 
                 
                 
                   len 
                   freq 
                 
               
                
               
                 ( 
                 
                   
                     amp 
                      
                     
                       [ 
                       idx 
                       ] 
                     
                   
                   * 
                   
                     exp 
                      
                     
                       ( 
                       
                         
                           2 
                            
                           j 
                           * 
                           π 
                           * 
                           
                             ( 
                             
                               ii 
                               - 
                               1 
                             
                             ) 
                           
                           * 
                           
                             
                               
                                 freq 
                                 idx 
                               
                                
                               
                                 [ 
                                 idx 
                                 ] 
                               
                             
                             
                               NUM 
                               SAMPLES 
                             
                           
                         
                         + 
                         
                           ( 
                           
                             
                               phase 
                                
                               
                                 [ 
                                 idx 
                                 ] 
                               
                             
                             * 
                             
                               π 
                               180 
                             
                           
                           ) 
                         
                       
                       ) 
                     
                   
                 
                 ) 
               
             
           
         
       
         
         
           
             wherein ii=1, . . . , NUM SAMPLES  and len freq =(length of freg idx ). 
           
         
       
    
     The factor amp factor &gt;1 is, in accordance with the channel, selected such that the spectrum of the reception signal is no longer decreasing (see  FIG. 6 ). 
     Using these DAC samples, several trials are started pursuant to a predetermined plan lo plan . 
     The plan lo plan  [ii] contains the following parameters:
         lo freq , depending on the parameter iqinvert, indicates the start frequency or the stop frequency.   iqinvert indicates whether lo freq  is the start frequency or the stop frequency.   iqinvert=True means that lo freq  is the start frequency, whereas in the case of iqinvert=False, lo freq  is the stop frequency.   time set  indicates the time indicating after how many seconds after the start of the measurement the plan is to be realized.       

     For making the plan lo plan  [ii], the following steps are used:
         Setting the parameters:
           Number of snapshots NUM SNAPSHOTS  per step of equal center frequency   Number of frequencies OVERLAP MEASURE  which are to overlap at the end of the frequency band of plan [ii] with the beginning of the frequency band of plan [ii+1]   Time time start  in seconds, which the system uses for starting   Time time snapshot  in seconds, which the system uses for a snapshot   
           Used parameters from the calculation of the DAC signal dac val :
           freq step : spacing between two carriers   len freq : number of carriers used   freq max =max(freq MHz )   
           Calculating the center frequency center freq  of the channel to be measured:       

     
       
         
           
             
               
                 center 
                 freq 
               
               = 
               
                 
                   txlowfreq 
                   + 
                   txhighfreq 
                 
                 2 
               
             
             , 
           
         
       
         
         
           
             wherein txlowfreq is the minimum frequency and txhighfreq is the maximum frequency of the channel to be measured. 
             Calculating the start or stop frequency lo freq [ii] and the parameter iqinvert[ii]: 
           
         
       
    
       cur freq [ ii ]=cur freq [ ii− 1]−OVERLAP MEASURE *freq stepping  
         with cur freq  [0]=txlowfreq       

       lo above =cur req [ ii ]freq max    
       lo below =cur freq [ ii ]         In case |lo above −center freq |≤|lo below −center freq  |, the following applies:       
       lo freq [ ii ]=lo above    
       iqinvert[ ii ]=True,         otherwise, the following applies:       
       lo freq [ ii ]=lo below    
       iqinvert[ ii ]=False         This condition ensures that the center frequency is outside the trials (see  FIG. 1  and table 1).   This routine will be performed until the condition cur freq [ii+1]≥(txhighfreq−3/2*freq step ) is fulfilled.   Calculating the time time set [ii]       
       time set [ ii ]=time set [ ii− 1]+time snapshot *NUM SNAPSHOTS    
     For the frequencies of the trials, with the parameters freq MHz , lo freq  and iqinvert=True, the following applies: 
       freq Trial [idx]=lo freq −freq MHz [len freq +1−idx],
 
     with idx=1, . . . , len freq  and lo freq  as the stop frequency. 
     For the frequencies freq Trial [idx] of the trials, with the parameters freq MHz , lo freq , and iqinvert=False, the following applies: 
       freq Trial [idx]=lo freq +freq MHz [idx] 
     with idx=1, . . . , len freq  and lo freq  as the start frequency. 
     Advantageously, the local oscillator control of the receiver-side local oscillator  116  on the one hand and the transmitter-side local oscillator  424  of  FIG. 13  on the other hand is performed pursuant to the LO plan described before. 
     Both directions of the channel are measured at the same time (see  FIG. 3 ). After making the plans on both sides, the plans are performed one after the other on both sides in the TX case using the DAC signal dac val  in a time-controlled manner using the parameter time set  (see  FIG. 8 ). Irrespective of this, in the RX case, the received signals are also stored a time-controlled and temporally cyclic manner on both sides using the corresponding plans, since RX and TX are unsynchronized. 
       FIG. 8  schematically illustrates the simultaneous measurement of the channel in both directions. A first node  1 , referred to by  801 , and a second node  2 , referred to by  802 , cooperate, wherein LO plan  is transferred from node to node  2  via the side channel  180 , and LO plan  is transferred from node  2  to node  1 . The respective transmitter TX corresponds to the apparatus illustrated in  FIG. 13 , whereas the receiver RX each corresponds to the apparatus, illustrated in  FIG. 1 , with the respective described embodiments. 
     After having finished recording and undergone all the plans, the group delay with amplitudes of the individual snapshots is calculated from the received signals. 
       FIG. 9 a    shows advantageous value regions for certain previously mentioned parameters. Correspondingly,  FIG. 9 b    shows a special example of different LO frequencies, which correspond to respective LoPlan numbers, with a respective parameter value iqinvert. These examples roughly correspond to the values as are illustrated, for example, in  FIGS. 10 a , 10 b   ,  11  for the individual frequency portions. 
     Here, the overlap regions of two adjacent frequency bands will be considered. Two mean values are formed from the results within the overlap regions, i.e. for the end of the front frequency band and for the beginning of the successive frequency band. In order not to corrupt the result of the mean value, outliers within the overlap regions are removed for calculating the mean value. 
     The outliers can be found using the following method:
         Calculating the CDFs from the values of the front frequency band for all the overlap regions   Calculating the CDFs from the values of the successive frequency band for all the overlap regions   The quantile values q1 and q3 at 25% and 75% are calculated for all CDFs   All the values for which the following applies, are said to be outliers:       

       val i &lt;( q   1 −( q 3− q 1))*precision
 
       or 
       val i &gt;( q 1+( q 3− q 1))*precision,
         with val i =value within an overlap region, precision=predetermined fixed multiplication value       

     After having removed all the outliers, the mean values are calculated from the remaining values. The correction value correction j+1  for the region (j+1) results as a sum of the correction valued correction of the region j, the mean value firstOverlap j+1  from the end of the front frequency band and the negative mean value secondOverlap j+1  from the beginning of the successive frequency band: 
       correction j+1 =correction j +secondOverlap j+1 −firstOverlap j+1 ,
 
     with correction 1 =0 for the first region and j=1, . . . , (number of frequency bands−1). 
     Depending of the frequency band j, the respective correction values correction j  are added to the group delays or amplitude values. 
     For each frequency bin, the mean value is formed from the shifted values and the result used as a group delay course or amplitude course for further calculation. 
       FIGS. 12 a  and 12 b    show a result, as is obtained by the apparatus of  FIG. 1 , i.e. a continuous course of the group delay information over frequency and the amplitude information over frequency in the entire particularly interesting frequency band from roughly 2500 MHz to 3700 MHz. However, it is to be pointed out that correspondingly a complete continuous high-resolution region recorded efficiently can already be obtained for the transmission medium from the frequency of 1000 MHz to 5000 MHz or above or below. The information from  FIG. 12 a    or  12   b  can then be used to perform repairs in the infrastructure, or to correspondingly drive a transmitter/receiver for useful data so as to obtain good utilization. Alternatively or additionally, the information can also be used for a pre-distortion on the transmitter side. 
     Although some aspects have been described in the context of an apparatus, it is clear that these aspects also represent a description of the corresponding method, such that a block or device of an apparatus also corresponds to a respective method step or feature of a method step. Analogously, aspects described in the context of or as a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus. Some or all of the method steps can be executed by (or using) a hardware apparatus, like, for example, a microprocessor, a programmable computer or an electronic circuit. In some embodiments, some or several of the most important method steps may be executed by such an apparatus. 
     Depending on certain implementation requirements, embodiments of the invention can be implemented in hardware or in software. The implementation can be performed using a digital storage medium, for example a floppy disk, a DVD, a Blu-Ray disc, a CD, an ROM, a PROM, an EPROM, an EEPROM or a FLASH memory, a hard drive or another magnetic or optical memory having electronically readable control signals stored thereon, which cooperate or are capable of cooperating with a programmable computer system such that the respective method is performed. Therefore, the digital storage medium may be computer-readable. 
     Some embodiments according to the invention include a data carrier comprising electronically readable control signals, which are capable of cooperating with a programmable computer system such that one of the methods described herein is performed. 
     Generally, embodiments of the present invention can be implemented as a computer program product with program code, the program code being operative for performing one of the methods when the computer program product runs on a computer. 
     The program code may, for example, be stored on a machine-readable carrier. 
     Other embodiments comprise the computer program for performing one of the methods described herein, wherein the computer program is stored on a machine-readable carrier. 
     In other words, an embodiment of the inventive method is, therefore, a computer program comprising program code for performing one of the methods described herein, when the computer program runs on a computer. 
     A further embodiment of the inventive methods is, therefore, a data carrier (or a digital storage medium or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein. 
     A further embodiment of the inventive method is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein. The data stream or the sequence of signals may, for example, be configured to be transferred via a data communication connection, for example via the Internet. 
     A further embodiment comprises processing means, for example a computer, or a programmable logic device, configured or adapted to perform one of the methods described herein. 
     A further embodiment comprises a computer having installed thereon the computer program for performing one of the methods described herein. 
     A further embodiment according to the invention comprises an apparatus or a system configured to transfer a computer program for performing one of the methods described herein to a receiver. The transmission can be performed electronically or optically. The receiver may, for example, be a computer, a mobile device, a memory device or the like. The apparatus or system may, for example, comprise a file server for transferring the computer program to the receiver. 
     In some embodiments, a programmable logic device (for example a field-programmable gate array, FPGA) may be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field-programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein. Generally, in some embodiments, the methods are performed by any hardware apparatus. This can be a universally applicable hardware, such as a computer processor (CPU) or hardware specific for the method, such as ASIC. 
     While this invention has been described in terms of several embodiments, there are alterations, permutations, and equivalents which will be apparent to others skilled in the art and which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.