EP0603656A2 - 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 transmission, which corresponds to the quality standard offered by digital storage media (Compact Disc, DAT), is not possible with today's analogue VHF transmission method, especially with mobile reception in a motor vehicle or with portable devices. Fluctuations in field strength and multi-path reception lead to signal distortions, the effects of which can only be partially mitigated by tricky switching strategies on alternative reception frequencies (e.g. in connection with the radio data system, RDS).

To improve the transmission quality, more and more digital processes are being used, such as digital satellite radio, or DSR for short. A transmission signal is transmitted from the transmitter to the receiver via satellite. Not every single receiver with an antenna and a first low-noise mixer for satellite radio, which are relatively large or expensive, a DSR signal from a central terrestrial receiver can also be fed to the receiver via broadband cable networks.

A digital radio transmission for mobile reception with the help of satellites is currently failing. the need to use receiving antennas with a pronounced directional effect 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 Mobile Phone ", 1989, pages 9-18.

The so-called COFDM method (Coded Orthogonal Frequency Division Multiplex) is provided as the transmission method, as described, for example, 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 the use of a carrier frequency bandwidth of, for example, 1.5 MHz, approx. 5 ... 6 stereo programs (in addition to additional program-related and program-independent data) can be transmitted simultaneously within an area, e.g. the transmission area of a state broadcaster. Within the for A wide range of individual carriers (e.g. 1536 carrier frequencies at equidistant distances on the frequency axis) are exposed to available channel bandwidth with 4-DPSK modulation (DPSK = differential phase shift keying). By scrambling the digital program data in the time sequence and in the assignment to the individual carrier frequencies, it is achieved that transmission errors due to field strength fluctuations do not extend over longer, temporally connected signal sections and can therefore be corrected more easily.

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 demodulation circuit for an MSK (minimum shift keying) receiver and a QPSK (quadrature phase shift keying) receiver is known from German published patent application DE 42 16 027 A1. 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 to provide a method and a device for regulating the frequency, which has a simple structure with high accuracy.

This object is achieved according to the invention in a generic method by the characterizing features of claim 1 and claim 7. Advantageous refinements and developments of the 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 to determine an offset address as a function of the determined frequency offset, which is taken into account when reading the information from the memory.

The offset address, which is taken into account when reading out, provides frequency-corrected carriers which are stored according to amount and phase, for further processing.

With this type of frequency correction, there are few errors since there is no frequency correction for the analog / digital conversion and the FFT. Individual carriers which lie outside the area covered by the analog / digital conversion and the FFT are thus cut off. However, due to the interleaved transmission using the COFDM method, these errors have little effect compared to 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.

A further advantageous embodiment of the invention provides that the stored data are not redirected within the memory or memory area, but rather into another memory frequency corrected addressing. According to this embodiment of the invention, additional memory is required, but the process of readdressing is simplified.

Another advantageous embodiment of the invention provides that the high-frequency received signal is subjected to at least one intermediate frequency conversion before the analog / digital conversion, which converts the received signal to a lower frequency. This embodiment of the invention ensures that the analog / digital conversion can be carried out with a lower sampling frequency.

It has also proven to be advantageous to carry out frequency control using the known analog methods after the intermediate frequency conversion and then to carry out the digital frequency control after the analog / digital conversion and the FFT. In this way, at least a rough frequency control is achieved before the analog signal is converted into a digital signal in the frequency range, as a result of which later errors due to loss of carriers with useful information are avoided.

The invention further relates to a device for performing the method described above.

This device contains an analog / digital converter for converting the received signal into a digital signal. The digital signal is fed to a functional unit for FFT, in which the digital signal from time to time the frequency range is transferred. Furthermore, at least one memory for storing the digital signal transferred into the frequency range and a control unit are contained in the device. The control unit consists of a functional unit for evaluating the control signal transmitted on the transmitter side for determining the frequency offset and a control unit for address control of the memory or memories.

An advantageous embodiment of the device according to the invention provides for a mixer to be arranged in the signal flow upstream of the analog / digital converter, which mixer mixes the high-frequency received signal down to a lower-frequency intermediate frequency. In this way it can be achieved that the analog / digital converter can be made simpler. In particular, the sampling frequency required for analog / digital conversion is lower, so that there are considerable cost advantages.

A further embodiment of the device according to the invention provides that two mixers are provided in front of the analog / digital converter, a bandpass filter which limits the frequency range being provided after the first mixer. The two intermediate frequency stages further reduce the sampling rate of the analog / digital converter.

It is further provided that a bandpass filter and a frequency control circuit is arranged in front of the analog / digital converter in order to Limit the received signal or the signal obtained after the conversion to an intermediate frequency in the bandwidth and carry out a preliminary frequency control.

Figur 1:

Das Blockschaltbild einer erfindungsgemäßen Anordnung

Figur 2:

die Lage der Trägerfrequenzen im Speicher

In Figur 1 ist das Blockschaltbild einer möglichen Ausführung der erfindungsgemäßen Anordnung angegeben. Gemäß dieser Anordnung wird das Empfangssignal S auf einen Mischer 1 gegeben, der das Empfangssignal mit einer hochfrequenten Mittenfrequenz auf eine erste Zwischenfrequenz heruntermischt. Das erhaltene Signal wird mit dem Bandpaß 2 spektral begrenzt und an den zweiten Mischer 3 gegeben, mit dem das bandbegrenzte Signal auf eine weitere Zwischenfrequenz heruntergemischt wird. Diese Zwischenfrequenz liegt vorzugsweise bei 3072 kHz. Auch nach dieser zweiten Frequenzumsetzung kann eine Bandbegrenzung mit einem Bandpaß, der in der Figur nicht dargestellt ist, durchgeführt werden.
In der Zwischenfrequenzlage mit der vorzugsweisen Mittenfrequenz von 3072 kHz wird das Signal im Analog/Digital-Wandler digitalisiert und mittels der Funktionseinheit 5 einer FFT unterworfen. Am Ausgang der Funktionseinheit 5 liegt das Empfangssignal im Frequenzbereich vor.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, as well as the connections to memory 7 are shown in dashed lines, since they can be used optionally.

The memory controller contained in the control unit 8 can also be designed in such a way that it specifies an offset address which is determined as a function of the frequency offset by evaluating the control signal transmitted by the transmitter. The offset address can be used to read from the memory 6 at the correct frequency even without readdressing.

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 in the center of the memory in accordance with FIG. 2b, ie the carriers are stored at the correct address.

Claims (10)

Method for eliminating the frequency offset in the received signal of a digital transmission system, in which a control signal is transmitted on the transmitter side, the evaluation of which enables the frequency offset of the received signal with respect to the transmitted signal to be determined,
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 - Depending on the frequency offset, the memory contents are redirected so that the frequency offset is compensated for. Method for eliminating the frequency offset in the received signal of a digital transmission system, in which a control signal is transmitted on the transmitter side, the evaluation of which enables the frequency offset of the received signal with respect to the transmitted signal to be determined,
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 - Depending on the frequency offset, an offset address is calculated, which is taken into account when reading out the data. A method according to claim 1 or claim 2,
characterized in that
in the analog / digital conversion is sampled with a higher sampling frequency than it corresponds to the bandwidth of the useful signal. Method according to claim 1 or 3,
characterized in that
when the memory contents are readdressed, the data are written into a second memory. Method according to one of claims 1 to 4,
characterized in that
the analog / digital conversion of the received signal only takes place after at least one intermediate frequency conversion of the input signal to a lower frequency. Method according to claim 5,
characterized in that
after the intermediate frequency conversion, frequency control with at least one analog and / or another digital actuator and after FFT, digital frequency control is carried out. Device for carrying out the method according to one of claims 1 to 6,
characterized by - an analog / digital converter (4) for converting the received signal into a digital signal, a functional unit (5) for the FFT, which converts the digital signal from the time domain to the frequency domain, - At least one memory (6,7) for storing the digital signal transferred into the frequency range, and - A control unit (8) consisting of a functional unit for evaluating the control signal transmitted on the transmitter side to determine the frequency offset and a control unit for address control of the memory or memories (6, 7). Device according to claim 7,
characterized in that
a mixer (3) is arranged in the signal flow upstream of the analog / digital converter (4) and mixes the high-frequency received signal down to a lower-frequency intermediate frequency. Device according to claim 7,
characterized in that a mixer (1) is arranged in the signal flow upstream of the analog / digital converter (4) and mixes the high-frequency received signal (S) down to a first, lower-frequency intermediate frequency, - The signal after the first intermediate frequency stage is band-limited in a bandpass filter (2), and - The band-limited signal is mixed down to a second intermediate frequency in a second mixer. Device according to one of claims 7 to 9,
characterized in that
A bandpass filter and a frequency control circuit are arranged in front of the analog / digital converter (4) in order to limit the bandwidth of the received signal or the signal obtained after conversion to an intermediate frequency and to carry out a preliminary frequency control.

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