EP0603656B1 - Method and apparatus for eliminating the frequency-offset of received signals in a digital transmission system - Google Patents

Abstract

In this digital transmission system, a control signal is transmitted at the transmitter end, by the evaluation of which signal the frequency offset of the received signal with respect to the transmitted signal can be determined. According to the invention, the frequency correction occurs in the frequency domain, that is to say, after the FFT in the receiver, by readdressing the memory contents which contain the carrier information in accordance with absolute value and phase. <IMAGE>

Description

The invention relates to a method and a device according to the preambles of claims 1 and 7.

A high quality radio broadcast that the of digital storage media (compact disc, DAT) offered quality standard is with the today's analog VHF transmission methods, especially with mobile reception in the motor vehicle or with portable Devices, not possible. Fluctuations in field strength and Multipath reception lead to signal distortions, their Impact only partly through tricky Switching strategies to alternative reception frequencies (e.g. in connection with the radio data system, RDS) can be mitigated.

To improve the transmission quality, always more digital processes are used, such as at Digital Satellite Radio, or DSR for short. Here is a Transmission signal from the transmitter via satellite to the receiver transfer. In order not to have every single recipient with one Antenna and a first low-noise mixer for satellite radio, which are relatively large or expensive, can equip a DSR signal from a central terrestrial receivers also over broadband cable networks be forwarded to the recipient.

A digital radio transmission for mobile reception with the help of satellites is currently failing. the need to use receiving antennas with pronounced directivity due to the comparatively low transmission power.
In order to provide a transmission method for mobile reception that corresponds to the quality standard offered by digital storage media (digital compact cassette, compact disc), work has been underway for a number of years on a standard for a new terrestrial, digital transmission method which is known as "DAB "(Digital Audio Broadcasting) is known, as is known, for example, from" Funkschau - Spezial ":" Digital Sound - From Radio to Cell Phone ", 1989, pages 9-18.

The so-called transmission method is COFDM (Coded Orthogonal Frequency Division Multiplex) is provided, such as in a Report by Frank Müller-Römer, Digital Audio Broadcasting (DAB), Infosat, issue 10, 1992, number 55, Pages 60 - 89 is described. With this Transmission method with which within an area, e.g. the broadcasting area of a state broadcaster, at Utilization of a carrier frequency bandwidth of e.g. 1.5 MHz at the same time approx. 5 ... 6 stereo programs (next to additionally program-related and program-independent Data) can be sent. Within the for The available channel bandwidth becomes one Large number of individual carriers (e.g. 1536 carrier frequencies in equidistant distance on the frequency axis) with a 4-DPSK modulation applied (DPSK = differential phase Shift keying). By scrambling the digital Program data in the time sequence and in the assignment to the individual carrier frequencies is achieved that Transmission errors due to fluctuations in field strength not over longer, time-related Signal sections extend and therefore lighter can be corrected.

In a DAB receiver, according to the current state, the input signal, which has a bandwidth of 1536 kHz, is first mixed down to a lower frequency. The signal is limited with a bandpass filter and mixed down in a second mixer to an intermediate frequency range with the center frequency 3072 kHz. When the center frequency of the received signal changes, the center frequencies obtained at the two mixer outputs also change.
Since such changes in the center frequency lead to errors in the evaluation, it is necessary to make corrections. This is achieved with known methods for automatic frequency control (AFC - Automatic Frequency Control). However, these methods have the disadvantage that they are very complex if sufficient accuracy is achieved. A demodulating circuit for an MSK (Minimum Shift Keying) receiver and a QPSK (Quadrature Phase Shift Keying) receiver is from the German patent application DE 42 16 027 A1 known. The demodulating circuit includes a synchronous state determination device and a control voltage sweep device for sweeping the output of a voltage oscillator. In the asynchronous state, a switch is turned off to interrupt a recovery phase error signal. In this way, the output of the voltage oscillator is run through and the synchronous state is reached. Then the sweep is stopped and the switch is turned on to control the voltage of the voltage controlled oscillator so that the low frequency error component is removed from the phase signal of the demodulating circuit.
A driving circuit is included in the modulation circuit for QPSK-modulated signals. This is arranged so that if a synchronous state is not detected in the digital signal processing unit, a low-frequency deflection or pass signal is drawn into the modulating circuit. This pass signal is overlapped with the control voltage of a voltage controlled oscillator included in the QPSK carrier recovery circuit. When the synchronous state is recognized, the supply of the deflection signal is stopped.
Such a control can only be implemented with great effort so that the control works with sufficient accuracy.

A method and an arrangement for digital frequency control for a multi-channel transmission system is known from German published patent application DE 41 12 860 A1, which is used in particular in digital broadcasting.
In this method, a correction frequency is generated in the receiver of a multi-channel transmission system via a digital control algorithm, with which the frequency offset of a mixer output signal is compensated. The frequency offset, ie the deviation from the nominal frequency, is determined in a measuring device which consists of a hard decision maker, a modulator and an adder. A complex quantity is generated in the measuring device, the phase of which represents a measure of the frequency offset. The measure for the frequency shift is determined periodically in the symbol cycle. The correction frequency formed from this remains constant for the duration of a symbol cycle.
To determine the frequency offset, for example, a time-frequency-phase control symbol, which is transmitted by the transmitter at the beginning of each transmission frame, can be evaluated.
Despite the digital control algorithm of the known method, the control signal acts on an analog actuator with the known disadvantages of the relative inaccuracy and the high implementation effort.

The object of the invention is therefore a method and to specify a device for regulating the frequency has a simple structure with high accuracy.

This object is achieved with one generic method by the characterizing Features of claim 1 and claim 7 achieved. Advantageous refinements and developments of Invention are specified in the subclaims.

According to the invention, the received signal, which was preferably transmitted using the COFDM method, is first subjected to an analog / digital conversion. In this way, the received analog signal that is modulated with digital data is converted into a digital signal that is initially present in the time domain. This signal is subjected to an FFT (Fast Fourier Transformation), so that the digitized received signal is transferred to the frequency domain and stored in a memory or register.
In the case of a signal transmitted using the COFDM method, n carriers are obtained which are stored in a memory according to magnitude and phase, the memory address implicitly indicating the carrier frequency. The frequency offset of the received signal with respect to the transmitted signal is determined with the aid of the transmitted control signal.
In the following step, the memory contents are redirected as a function of the determined frequency offset, so that the frequency-corrected carriers are stored in the memory according to the amount and phase.

An alternative solution is dependent an offset address to the determined frequency offset determine which when reading the information from the Memory is taken into account.

Through the offset address that is taken into account when reading out will be for further processing frequency-corrected carriers by amount and phase are made available.

With this type of frequency correction, there are few Error because of the analog / digital conversion and the FFT there is no frequency correction yet. So be individual carriers outside of the of the Analog / digital conversion and the FFT detected area are cut off. This However, the nested transmission has errors according to the COFDM method little impact in Comparison of the effects of decoding frequency-shifted carrier frequencies.

An improvement in this regard is achieved if the analog / digital conversion is sampled with a higher sampling frequency than the bandwidth of the transmitted useful signal.
In this way, carriers which have no useful information and are in the frequency band to the right and left of the useful information are also subjected to the FFT and are therefore stored in the memory according to the amount and phase. A frequency offset means that only carriers that do not contain any useful information are lost.

Another advantageous embodiment of the invention stipulates that the stored data is not within of the memory or memory area are redirected, but in another memory frequency corrected addressing. According to this embodiment of the invention, a more memory needed, the process of Readdressing is simplified.

Another advantageous embodiment of the invention provides that the high-frequency received signal before Analog / digital conversion at least one Intermediate frequency conversion is subjected to the received signal converts to a lower frequency. This embodiment of the invention ensures that the analog / digital conversion with a lower one Sampling frequency can be executed.

It has also proven to be advantageous, according to the Intermediate frequency conversion a frequency control with the perform known analog processes and then, after the analog / digital conversion and the FFT to carry out the digital frequency control. To this Wise is already before the implementation of the analog signal into a digital signal in the frequency domain, achieved at least a rough frequency control, whereby later errors due to loss of carriers Useful information can be avoided.

The invention further relates to a device for Perform the procedure described above.

This device contains an analog / digital converter for converting the received signal into a digital one Signal. The digital signal becomes a functional unit fed to the FFT, in which the digital signal from time-in the frequency range is transferred. In the further is at least one memory for storing the Frequency range transferred digital signals, and a Control unit included in the device. The Control unit consists of a functional unit Evaluation of the control signal transmitted by the transmitter to determine the frequency offset and a Control unit for address control of the Storage.

An advantageous embodiment of the invention Device provides in the signal flow before Analog / digital converter to arrange a mixer that high-frequency received signal to a lower-frequency Intermediate frequency mixed down. That way can be achieved that the analog / digital converter easier can be executed. In particular, the Analog / digital conversion required sampling frequency less, so that there are considerable cost advantages.

Another embodiment of the invention Device provides that before the analog / digital converter two mixers are provided, being after the first A bandpass mixer is provided, which the Frequency range limited. Through the two Intermediate frequency levels will further decrease the Sampling rate of the analog / digital converter reached.

It is further provided that before Analog / digital converter one bandpass and one Frequency control circuit is arranged to Received signal or that after the conversion to a Intermediate frequency received signal in the bandwidth too limit and a preliminary frequency regulation perform.

Figur 1:

Das Blockschaltbild einer erfindungsgemäßen Anordnung

Figur 2:

die Lage der Trägerfrequenzen im Speicher

The invention is described below using an exemplary embodiment according to FIGS. 1 and 2.
Show it:

Figure 1:

The block diagram of an arrangement according to the invention

Figure 2:

the location of the carrier frequencies in the memory

FIG. 1 shows the block diagram of a possible embodiment of the arrangement according to the invention. According to this arrangement, the received signal S is given to a mixer 1 which mixes the received signal down to a first intermediate frequency at a high-frequency center frequency. The signal obtained is spectrally limited with the bandpass filter 2 and passed to the second mixer 3, with which the band-limited signal is mixed down to a further intermediate frequency. This intermediate frequency is preferably 3072 kHz. Even after this second frequency conversion, band limiting can be carried out with a bandpass filter, which is not shown in the figure.
In the intermediate frequency position with the preferred center frequency of 3072 kHz, the signal is digitized in the analog / digital converter and subjected to an FFT by means of the functional unit 5. The received signal is present in the frequency range at the output of the functional unit 5.

According to the current state of the COFDM method, 1536 carrier frequencies are obtained at an equidistant distance on the frequency axis. According to the current specification, the spacing of the carrier frequencies is 1 kHz. By scanning at 2.048 MHz, 2048 carrier frequencies are obtained according to the FFT, with 1536 carrier frequencies containing useful information. The carrier frequencies obtained are stored in the memory 6 according to the amount and phase, the frequency value resulting from the address of the stored carrier (amount and phase).
The frequency shift of the center frequency in the received signal resulting from a real transmission or caused by oscillator inaccuracies in relation to the transmitted signal causes a shift of the carriers containing the useful data in the memory, ie the carriers are stored under the wrong address. Since the carrier frequency results from the address under which the amount and the phase of the carrier are stored during further processing of the useful data, a frequency correction must be carried out. For this purpose, the data stored in the memory are evaluated in the control unit 8. The control unit 8 consists of a unit for detecting the frequency offset from a control signal transmitted on the transmitter side and a memory control unit which controls the readdressing of the memory contents.
The readdressing can take place in the memory 6 itself or by a transfer to the further memory 7. In the first case the control unit 8 is connected to the memory 6, in the second case to the memory 7. The memory 7 and the connections to the memory 7 are shown in dashed lines as they can be used optionally.

The memory controller contained in the control unit 8 can also be designed to have an offset address, those transmitted by the evaluation of the transmitter Control signals depending on the frequency offset is determined, pretends. Through the offset address a correct reading from the memory 6 even without Readdressing can be achieved.

FIG. 2a shows the position of the carriers in the memory before the frequency correction. FIG. 2b shows the position of the carriers in the memory after the readdressing, ie after the frequency correction.
In the example, the memory contains 2048 memory locations in which a carrier according to amount and phase is stored under each address. In the figure, only the carriers with useful information are shown as vertical lines. According to the current status of the COFDM procedure, there are 1536 carriers. In the event of a frequency offset, the carriers are stored with a shift in frequency. In the pictorial representation according to FIG. 2, the carriers are therefore not in the center of the memory.
After the frequency correction, the carriers come to be centrally located in the memory according to FIG. 2b, ie the carriers are stored at the correct address.

Claims (10)

1. Method of eliminating the frequency offset in the received signal of a digital transmission system, in which a control signal is transmitted at the transmission end whose evaluation can determine the frequency offset of the received signal with respect to the transmitted signal, characterized in that

   the received signal is subjected to an analog/digital conversion in the receiver,

   the digital signal obtained is subjected to an FFT and written into a memory,

   the control signal is evaluated and the frequency offset is determined, and

   as a function of the frequency offset, the memory contents are readdressed so that the frequency offset is compensated for.
2. Method of eliminating the frequency offset in the received signal of a digital transmission system in which a control signal is transmitted at the transmitter end whose evaluation can determine the frequency offset of the received signal with respect to the transmitted signal, characterized in that

   the received signal is subjected to an analog/digital conversion in the receiver,

   the digital signal obtained is subjected to an FFT and is written into a memory,

   the control signal is evaluated and the frequency offset is determined, and

   as a function of the frequency offset, an offset address is calculated which is taken into account when the data are read out.
3. Method according to Claim 1 or 2, characterized in that, during the analog-digital conversion, sampling is performed at a higher sampling frequency than corresponds to the bandwidth of the useful signal.
4. Method according to Claim 1 or 3, characterized in that, during the readdressing of the memory contents, the data are written into a second memory.
5. Method according to one of Claims 1 to 4,
   characterized in that the analog/digital conversion of the received signal is carried out only after at least one intermediate-frequency conversion of the input signal to a lower frequency.
6. Method according to Claim 5, characterized in that, after the intermediate frequency conversion, a frequency control is first carried out using at least one analog and/or a further digital final control element and a digital frequency control is carried out after the FFT.
7. Device for performing the method according to one of Claims 1 to 6 for eliminating the frequency offset in the received signal of a digital transmission system in which a control signal is transmitted at the transmitter end whose evaluation can determine the frequency offset of the received signal with respect to the transmitted signal, characterized by

   an analog/digital converter (4) for converting the received signal into a digital signal,

   an FFT functional unit (5) which transfers the digital signal from the time domain to the frequency domain,

   at least one memory (6, 7) for storing the digital signal transferred to the frequency domain, and

   a control unit (8) which comprises a functional unit for evaluating the control signal transmitted at the transmitter end for determining the frequency offset and a control unit for addressing the memory or memories (6, 7).
8. Device according to Claim 7, characterized in that there is arranged in the signal flow upstream of the analog/digital converter (4) a mixer (3) which down-converts the high-frequency received signal to a lower-frequency intermediate frequency.
9. Device according to Claim 7, characterized in that

   there is arranged in the signal flow upstream of the analog/digital converter (4) a mixer (1) which down-converts the high-frequency received signal (S) to a first, lower-frequency intermediate frequency,

   the signal is band-limited after the first intermediate-frequency stage in a bandpass filter (2), and

   the band-limited signal is down-converted in a second mixer to a second intermediate frequency.
10. Device according to one of Claims 7 to 9,
    characterized in that there is arranged upstream of the analog/digital converter (4) a bandpass filter and a frequency control circuit in order to limit the received signal or the signal obtained after conversion to an intermediate frequency in bandwidth and to perform a preliminary frequency control.

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