Method for receiving radio signals via a radio channel

Translation of description of equivalent WO 0122649 A2

p0001A method for receiving radio signals via a radio channel

p0002State of the art

p0003The invention relates to a method for receiving radio signals over a radio channel according to the preamble of the independent claim.

p0004It is already known that a time synchronization in the radio signals which are divided into frames, is performed in a drip receiver. In this case, a voltage controlled oscillator for generating a clock frequency in the receiver is used in particular in the DAB (Digital Audio Broadcasting) in the receiver. If the clock frequency of the voltage-controlled oscillator equal to the clock frequency, which was used in the transmitter, there is an error in the time synchronization by the actual frame start away due to the frequency error ever adopted by the receiver frame start. This is corrected by the frequency of the voltage controlled oscillator is adjusted by changing the voltage of this oscillator. ADVANTAGES OF THE INVENTION

p0005The method with the features of the independent claim has the advantage that is not lost when a receive interrupt, for example, when passing through a tunnel, the time synchronization. Thus, a radio receiver is improved in a simple manner, so that a user of the radio receiver tätseinbußen no quality experienced by reception interrupt.

p0006Advantageously, a measure is calculated from the past error values ​​to determine the correction of the time synchronization during the receive interrupt. Thus, the last variation in the time synchronization be used to predict the variation in reception interrupt.

p0007men through the measures listed in the dependent claims are advantageous further developments and improvements of the independent claim method for receiving radio signals over a radio channel possible.

p0008It is particularly advantageous that the inventive process in the receiver a fixed-frequency oscillator can be used, which is very cheap and the structure of the radio receiver simplistic.

p0009Moreover, it is of advantage that is used in reception of the DAB TFPR (Time Frequency Phase Reference = phase reference symbol) -Symbols to produce the time synchronization in the receiver. For this purpose, advantageously, is calculated by means of the symbol TFPR an impulse response of the radio channel in order to calculate the deviation of the time synchronization, which is caused by the runtime. Moreover, it is of advantage that very large error values, which are calculated by means of the TFPR symbol are discarded. In particular, error values, caused by a short-term change channels at as a single frequency network, as is the case with DAB are thereby eliminated. This widely differing values ​​for the mean value formation are excluded.

p0010In an advantageous development of the invention, the average value of these error values ​​is a measure that is calculated for the last error values ​​calculated. This short-term deviations of the error values ​​will have no appreciable effect on the correction of the time synchronization in reception failure.

p0011Furthermore, it is advantageous that is detected by comparing successive error values ​​from the receiver whether they are running trend in the development of the error values ​​so that a prediction of future deviations due to this trend is possible. This adaptive control of time synchronization is achieved at reception failure.

p0012Moreover, it is of advantage that the occurrence of a transmitter change is detected at a single frequency network. This will advantageously only error values ​​were calculated for a used for receiving stations, used for averaging.

p0013drawing

p0014Embodiments of the invention are shown in the drawing and achieved greater detail in the following description explains. 1 shows a method for time synchro- sation, Figure 2 shows a DAB frame, Figure 3 shows two impulse responses and 4, a method for adaptive timing synchronization when receiving failure.

p0015Description of Embodiments

p0016In the time synchronization of radio signals that are transmitted in frames, as is the case, for example, digital broadcasting transmission method as in DAB, is synchronized to the beginning of the frame in the receiver. This synchronization on the beginning of the frame is necessary so that the information that is transmitted at the beginning of the frame, can be read by the recipient and so that the frame can be decoded at all. Especially with DAB include informa- tion on the frame initial data on the rest of the data, which are transmitted within this framework. Now if the receiver compared to the sender a different frequency, processes the frame with the receiver, then the synchronization from the beginning of the frame moves away and thus the receiver can not process the information in the context correctly. Therefore, a control of the time synchronization is necessary, especially when the clock frequency in the receiver can not be adjusted.

p0017If the frequency in the receiver is not adjusted, a pointer must be set to the data stream in the receiver where the frame begins. The pointer is a date that is set by the software that controls the receiver. The pointer has a memory address, in this case the memory address at which the frame start is filed.

p0018This pointer is for example set by a processor in a receiver. If now a fault in the synchronization before, then displays these hands are no longer on the start to be processed at the moment frame. Then, this pointer must are shifted in accordance with an error value, so that the pointer points back to the beginning of the frame. Thus, the error value referred to the time error at the time synchronization.

p0019When DAB is a Multifrequenzträgerübertra- supply system with high bandwidth. Within this digital terrestrial broadcasting system, the modulation method of orthogonal frequency division multiplexing (Orthogonal Frequency Divi- sion Multiplex = OFDM) is used. OFDM is a modulation method in which the signal to be transmitted is distributed to a plurality of subcarriers, whereby not disturb the distributed on these sub-carrier signals mutually. This behavior is described with orthogonal.

p0020In transmission mode II for DAB, a frame is available with a time of 24 ms. The sampling frequency is 2.048 MHz, which arise for the frame 49152 samples. If there is a frequency difference at the sampling frequency between transmitter and receiver look of 0.005%, results in a clock drift of about 2.46 per sampling 24 ms frame. The receiver must therefore move to correct time synchronization with each 24 ms frame pointer to the start of frame by 2.46 samples. With a longer interruption of reception, for example because of driving through tunnels, a correct time synchronization when a new receiver is initially no longer possible. The inventive method avoids then required resynchronization.

p0021In addition to DAB have other wireless transmission methods, such as DVB (Digital Video Broadcasting) and DRM (Digital Radio Mondial) to a frame structure, these broadcasting methods comprise the modulation method OFDM. These procedures differ from DAB to one by the construction of the frame structure and on the other by the transmission frequency ranges and transmission rate. In DVB and DRM are test signals for determining the impulse response and thus used for time synchronization.

p0022An oscillator, that a circuit which generates the vibrations, the write only vibrations of a frequency is known as fixed frequency oscillator. The voltage controlled oscillator on the other hand can be forced by changing one other lying voltage to emit a vibration with another frequency. The fixed-frequency oscillator is cheaper and simpler to manufacture due to its simple structure.

p0023In Figure 1 a process is shown for time synchronization. This method runs here in a DAB receiver, said method is controlled by software running on a processor of the DAB receiver.

p0024In step 1, the process is started. In step 2, it is checked whether at the moment there is a reception of DAB broadcast signals. Which is thereby determined that the signal power is observed. Is signal power above a predetermined threshold to recognize, there is a DAB broadcast signal before, otherwise not.

p0025If there is a reception of DAB radio signals, then in step 3, a comparison of the phase reference symbol that is present in the received frame difference symbol with a Phasenrefe-, which is stored in the receiver made.

p0026This comparison is made to determine an impulse response of a radio channel.

p00272 shows a DAB frame for DAB Mode 1 is shown. The DAB frame here has a width of 96 ms. Within the DAB frame synchronization channel 11 is first transmitted by the null symbol 9 and the phase reference symbol are 10th Then in the fast information channel 12 information is transmitted over where the 13 programs and data associated with the data available on the Main Service Channel, and how the respective channel coding looks. The Main Service Channel 13 is transmitted with the remaining 72 OFDM symbols. The Main Service Channel 13 contains the actual user data, such as audio signals or additional data.

p0028By means of the phase reference symbol of the DAB frame is detected, the impulse response of the radio channel. The Phasenrefe- difference symbol is in each frame, ie every 96 ms is transmitted. This clock is hereinafter referred frame clock. The impulse response is calculated such that first the frequency-dependent transfer function is to determine H (f) H (f) is determined by the reception signal is multiplied by the complex conjugate of the transmission signal. As complex conjugate signal transmission, the original is Phasenrefe- ence symbol used and conjugated. By back-transformation into the time domain is obtained from the transfer function H (f) is the impulse response h (t). Before determining the transfer function H (f) the phase reference symbol was transformed rich by transforming the time domain to the frequency domain.

p0029In Figure 3, two impulse responses of the radio channel are shown. The peak value of the calculated at the moment the impulse response of the radio channel 21 is compared with a further impulse response 20 as a reference, the deviation 14 of this reference, yields the error value for time synchronization.

p0030This reference is the occurrence of a peak in the impulse response in the middle of the time values ​​for the calculation of the impulse response were used. The reason is that by means of a fast Fourier transformation (eng. Fast Fourier Transformation = FFT) the transfer of a time signal is performed in the frequency domain. A time window is in this case used for the FFT, which is placed over the values ​​from the time domain to the frequency domain tra be nsformiert so are the values ​​for determining the transfer function and therefore of the impulse response set. By means of an inverse fast Fourier transformation is converted into the time domain, the transfer function from the frequency domain.

p0031If the receiver is correctly synchronized, then the time window for the FFT is directly above the Phasenreferenzsym- bol, whereby the peak of the impulse response will occur in the middle of the time slot. If the receiver is not properly synchronized, then the peak value of the impulse response is shifted in accordance with which the error value is calculated for the time synchronization.

p0032Lying in the rest of multiple peaks in the impulse response before, it is important to have all of these peak values ​​within the time window of the FFT and, for example, the time to compute the greatest energy in the impulse response and to determine the deviation from the temporal center of the impulse response to determine the error value at the time of synchronization. One method to determine the position of the FFT window, the strategy is to maximize the distance of the peak values ​​of the edges of the time window.

p0033In step 4, an average of the error values ​​is calculated for 100 frames. Given the failure of the last 100 frames are added and divided by one hundredth Incidentally, a larger or smaller number of frames are used for the averaging, where this is necessary or is made possible due to memory space resources in the receiver.

p0034In step 5, the average value is abgespei- chert. In step 6, the time synchronization in the DAB receiver is corrected by the error value, which was determined for the current frame. For this is the pointer which points to the error value incorrectly accepted frame beginning, now set to the correct frame start. In process step 6 are in the receiver, the data contained in the current frame processed. In DAB ie decoding is performed here.

p0035If there is no DAB broadcast signals, for example when driving through tunnels, then in step 8 of the pointer which points to the frame start and thus the time synchronization to the average value that was last stored, corrected. Here, the average value can be positive or negative, because a faulty synchronization can be in both directions, since the frequency can be by the oscillator in the receiver either too large or too small compared to the frequency in the transmitter.

p0036For DAB, a single-frequency network is present. The stations are thus operated at a certain frequency. Therefore, it can happen that when a station is changed, so if the signal of a transmitter is no longer be received and only the signal of another transmitter is received, a very large error value occurring in the time synchronization. Is this change only temporarily, then discards the recipient this counter, so as not to distort the average of the error values. However, the receiver discards the error value is not equal, because it can be a permanent change of station sign off. Therefore, the receiver determines further Error values, until a predetermined number of error values ​​is achieved. Then, the receiver determines the average value and examines the error values ​​if the values ​​differs strongly from this average. For this, a predetermined threshold in the receiver is stored. The error value is compared with this threshold, the fault value above, the error value is discarded.

p0037If the channel change, however, from time then the receiver the error values, which have arisen for the first station that is no longer used to receive, reject, to form an average of the error values ​​for the sender now used. For the recipient forms two averages, one for an error values ​​that follow after the error value that is far different. For the error values ​​before the error value that is far different from the average, and If the difference between the two averages below a predetermined threshold, then there is no permanent change channels before, but it is the case, then there is a permanent change channels before.

p0038Incidentally, it is conceivable for DAB to operate no single frequency network and provide an area with a transmitter.

p0039In Figure 4, a method is shown that for adaptive

p0040Maintenance of the time synchronization in reception failure is.

p0041In step 15, the error values, described above, is calculated. In step 16, the current error value is compared with at least two previous calculated error values. In step 17, it is checked whether the difference between this error values ​​a monotonically increasing or monotonically decreasing function is zugrunde- so that the statement can be made, whether from the difference between the error values ​​emerges a trend which is based for example on temperature effects. a trend If such detected, then in step 19 by extrapolating the function of future error values ​​estimated off to the time synchronization to correct at a reception failure for these error values.

p0042Extrapolation is here that the future error values ​​are calculated, for example, in such a way that the last calculated error value, as still receiving broadcast signals was present, for the first error value when no transmission of broadcast signals already exists more to the value is changed to to the penultimate error value had changed about the last error value in radio broadcasts. So it is here a simple linear extrapolation.

p0043Is it possible to no trend by comparing the error values ​​read, the time synchronization for receiving broadcast signals is the error value corrected or at a receive interrupt to the average of the last error values ​​in step 18th