DE4239509A1 - Method and receiver for terrestrial digital broadcast transmission - Google Patents

Abstract

The invention concerns a terrestrial digital radio transmission method and the associated receiver. The signal transmitted is a modulated digital signal (CPM signal) with a constant envelope. The frequency bandwidth corresponds to that of an FM-VHF channel. Receivers for the constant envelope CPM signals can be designed in accordance with receiver design concepts normal with FM.

Description

The invention relates to a method and a receiver according to the preamble of claims 1, 8 and 9.

The invention finds use in terrestrial di gital broadcasting.

Several digital ver drive proposed, all to save bandwidth source coding with a data reduction rate of approx. 8 work.

Broadcast Engineering, July 1991 provides an overview: "Tue gital radio formats compared ".  

The methods compared herein are:

• a) Broadband procedure with the bundling of several channels
• b) narrow band method with
• - QPSK modulation
• - Frequency diversity or hopping between different channels
• - sub-carrier or multiple carrier.

When introducing a digital terrestrial broadcast funks the problem arises of an appropriate frequency to find the area for the new transmission method. Considerations, the digital broadcasting in the so far for the to accommodate analog FM radio frequency band used by gradually changing the individual stations to the new one Transfer method to be changed was under at which is also rejected because it turned out had significant adjacent channel interference (Via speech disturbances) in FM-FM receivers.

The invention is therefore based on the object, a di gital transmission process and suitable recipients admit to prevent this adjacent channel interference and an FM-FM transmitter as required by transmitters for the digi tal radio can replace.

The task is solved by the in the characterizing part of claims 1, 8 and 9 specified features. Before partial configurations and / or further training are can be found in the subclaims.  

The invention will be based on exemplary embodiments wrote with reference to schematic drawings.

The invention has the advantage that the modulated digital Signal has a constant envelope with a frequency bandwidth corresponding to an FM-FM radio channel. Of the FM-FM transmitter can advantageously by a Sen which can be replaced for digital broadcasting without

• - the channel structure and the channel spacing in the FM Band needs to be changed
• - the existing FM transmitters will be completely replaced have to,
• - new transmitter antennas are needed,
• - Interferences (by a neighboring frequency digitally modulated transmitter) in the still lead FM FM receivers occur,
• - on inexpensive combination receivers for FM one hand and digital modulation on the other hand ver should be ceded.

One program is provided for each transmitter.

The proposed method uses the frequency modules tion (FM) or the Continuons Phase Modulation (CPM) and Frequency Shift Keying (FSK) for the digital signal. A CPM method for narrowband DAB is specified in which one frequency channel is used per radio channel.

Is used for signal processing z. B. the same with COFDM DAB uses standard source coding methods (Musicam), and the digital symbols are rounded appropriately (e.g. Gaussian filtering GMSK (Gaussian Minimum Shift Keying) or  TFM (Tamed Frequency Modulation)), so you get for that proposed method an RF bandwidth that the in FM band per transmitter corresponds to the usual value. That before proposed procedure is therefore to the existing FM-FM Broadcast bandwidth compatible.

Due to a constant envelope, the transmitter can C operation work. Therefore, it is possible to broadcast stations that are up long broadcast FM radio, for sending CPM signals to be used (transmitter compatibility). This has several decisive advantages:

• 1. No big investments on the transmitter side nope tig.
• 2. Comfortable possibility to introduce the procedure.
• 3. Great efficiency of the transmitter due to C operation.
• 4. Tests on the transmitter side are easily possible.
• 5. Existing transmit antennas can continue to be used become.

If you want to tune to individual stations in the existing FM band Convert DAB or DAB in previously unused channels radiate must not disturb the existing ones VHF receivers are created. Such disturbances arise however, whenever an RF signal is an amplitude mod lation (AM). Therefore, only DAB procedures are used for this usable with constant envelope. CPM signals are thus compatible with reception.  

For the receivers of digital signals, i.a. "left Normal "reception concepts are common. These are quite expensive dig. Linear receivers are used for difficult reception tasks Relationships needed, e.g. B. for mobile reception in the car.

However, the constant envelope of the CPM signal allows also quite simple reception concepts corresponding to the with usual FM receiver concepts (limiter amplifier and discriminator). "Simple" receivers need normal ones Reception relationships. Under this condition there is none There is a difference in quality in the received program.

The simple recipient assumes that the entire In formation of an angle-modulated signal from its zero passages can be obtained. This leads to an emp catching concept, which is identical to the following components a conventional (analog) FM receiver is:

• - RF input stage and tuner (frequency converter)
• - IF amplifier: filter and limiter amplifier
• - FM demodulator: common analog discriminator or similar

The effort for this part of the recipient is thus extremely low. In addition, the power consumption is low, which is particularly important for portable devices.

The analog FM demodulator also has the property that it depends on the small frequency from the target frequency is absolutely uncritical.

The actual data receiver connects to the FM demodulator. This consists of a signal-adapted filter and a decision-making stage. A circuit for bit clock synchronization is also required ( FIG. 1).

A digital reality is available for the data receiver on. Depending on the channel coding, the Ent separator z. B. as a Viterbi decoder with soft decision reali be settled. For this are already integrated switching circles offered, so that the effort is still low remains.

Channel equalization is for the simple receiver not provided because the Capture property of the FM be known to be echoes, in-channel interferers and adjacent channel interferers largely suppressed. From analog FM receivers announced that capture ratios of <1dB could be achieved sen.

The simple receiver is therefore cheap to manufacture and economical in power consumption. On the one hand, it is suitable for portable devices (handles) and on the other hand for sta tional receivers (stereo systems) with cheap reception conditions.

Since in the case of an analog FM transmission after Fre quenzdemodulator already the multiplex signal available stands with this concept in a simple way Combination receiver for FM-FM on the one hand and DAB on the other on the other hand.  

The possibility of receiving DAB with simple receivers gene ability is a significant advantage of the invention.

The RF input part of the linear receiver differs does not differ in principle from that of the simple recipient. The IF filter can also be of the same type.

The first difference is in the IF amplifier. This must be linear. Associated with this is the requirement that the IF amplifier an automatic gain control be sits so that it has a signal with constant Pe at the output gel supplies.

The IF signal is then multiplied by means of two linear multiplicators, one of which is controlled by means of a cos carrier, the other by means of a sin carrier, in the low-pass position. (Bandpass = <low-pass transformer, Fig. 2).

Other essential distinguishing features of the linear Recipients are:

• a) It has an equalizer to compensate for Multipath propagation (echoes).
• b) The matched filter and equalizer are Channel adaptive.

Various methods are possible for equalization:

• - Decision Feedback Equalizer (DFE) or
• - Viterbi equalizer.

If a DFE is used, the CPM demodulator is largely identical to the PSK demodulator (well known from the literature). As shown in FIG. 3, the DFE additionally contains a CPM modulator in the feedback branch, in contrast to the PSK demodulator. This generates the transmission signal belonging to the symbols already received. The response of the matched filter to this signal is fed to the feedback branch.

The adaptation of matched filter and DFE to the time variant anten channel is done in the same way as with a PSK Demodulator. During a periodically repeated trai ning sequence, the channel is measured. Between two Ka The estimated channel impulse response is nal measurements (Shock response) with an adaptation algorithm that changes changing impulse response of the real channel.

That for the channel measurement (with the help of a correlation measurement) necessary comparison signal sets due to the Non-linearity of the CPM again a model of the transmitter from which during the measurement with the test sequence is fed. But this means no additional effort, since the test sequence is known a priori. It will be heard transmit signal stored in the receiver and it becomes the associated algorithm for the CPM module gate in the receiver is not required.

The transmission signal used for the channel measurement must be spectrally "white". Because of the non-linear properties th of the frequency modulation applies to the test sequence itself not necessarily too.  

Since the linear receiver is critical with respect to a Fre is offset, separate procedures for the divorce Frequency modulation. On the transmitter side, the existing which modulators (control transmitters) are replaced, e.g. B. by an EPROM modulator or an NCO (Numerically Con trolled Oscillator) modulator. With a tax transmitter with an EPROM modulator for equalization of the digi talen frequency modulation (CPM) the temporal courses of the I and Q components for the symbols of the CPM signal elec stored (EPROM) and by means of a control circuit read out.

Claims (11)

1. A method for digital radio transmission, characterized in that a modulation method with constant envelope is used. 2. The method according to claim 1, characterized in that a continuous phase modulation (CPM) is carried out. 3. The method according to any one of the preceding claims, since characterized in that the transmission within the Transmission bandwidth of an FM radio channel leads. 4. The method according to any one of the preceding claims characterized in that the transmitter-side CPM signal with a simple FM receiver is processed.   5. The method according to any one of the preceding claims 1 to 3, characterized in that the transmitter-side DPM Signal is processed with a linear receiver. 6. The method according to claim 5, characterized in that transmitter side for the generation of the digital frequency mod lation (CPM) the temporal profiles of the I and Q compo electronically for the symbols of a CPM signal be saved (e.g. EPROM) and by means of a control circuit can be read out. 7. The method according to claim 5, characterized in that a Numerically Controlled Oscillator (NCO) on the transmitter side used for frequency modulation of a CPM signal becomes. 8. Receiver for a method according to any one of the preceding claims, which contains at least one RF input stage, a frequency converter, an IF amplifier, an FM demodulator and a data recovery device, characterized in that

• - The RF input stage and the frequency converter
• - the IF amplifier
• - The FM demodulator are identical or similar to those of an analog FM receiver, and that the data recovery device of the receiver
• - a matched filter,
• - a decision maker and
• - contains a bit clock synchronization.

9. Recipient for a procedure according to one of the preceding claims, characterized in that the Receiver contains a linearly operable IF amplifier. 10. Receiver according to claim 9, characterized in that at least

• a bandpass-lowpass transformation,
• - a signal-adapted digital filter with adaptation of the filter coefficients,
• - an equalizer with adaptation of the equalizer coefficients,
• - a decision maker,
• - a bit clock synchronization and
• - a channel measurement is included in the receiver.

11. Receiver according to claim 10, characterized in that a defined transmit signal for channel measurement (Test sequence) is stored in the receiver.