Method for transmitting test sequences for radio transmitters

p0001The invention relates to a digital modulation method and a circuit arrangement for performing the method according to the preambles of claims 1 and 8. FIG.

p0002The desire of the radio listeners for better transmission quality has been in the past with the introduction of frequency modulation in the VHF range and the planned introduction of digital broadcasting (DAB) taken into account in the VHF range and L-band. The DAB method is described in "Final Draft prETS300401" as European Telecommunication Standard. This process is characterized as a multi-carrier method. In this method, a plurality of sub-carriers in accordance with the digitized input signal to be modulated in their phases and transmitted together. Particularly resistant to interference and is thus suitable for the transmission of digitized signals, this method, when a relatively large bandwidth is used. In order to simultaneously achieve a good utilization of the frequency ranges, in this method, a plurality of programs are combined and transmitted in the frequency division multiplex method via a common channel.

p0003The broadcast programs in the frequency ranges long, medium and short wave (AM - bands) are previously transmitted with amplitude modulation and the associated poor quality of the communication channel, whereby these frequency ranges have lost acceptance. Digitizing said frequency ranges will result in an increase in the transmission quality and obtain the benefits of valid for this characteristic frequencies propagation characteristics. It is expected that with higher power transmitters (10 to 100 kW) and digitized transmission will be possible to supply large parts of a continent by means of a long, medium or short wave transmitter with a high quality signal. The known DAB method is suitable for relatively small frequency range of the AM bands do not, because the bandwidth that is used in DAB, in these bands is not available, since the AM transmitter with a range of 9 kHz to 20 kHz are operated. Compliance with these bandwidths is also therefore of transmitters with digital programs, required to keep the adjacent channel interference to still existing analog AM transmitters small.

p0004Besides the described large bandwidth that is necessary for the transmission with a multi-carrier method, the particularly high Aussteuerungsspitzen act in a multi-carrier method is negative, since a transmitter is to be interpreted for this modulation such that the peak power of the signal can still be transmitted.

p0005Because of said high output power of the viewed channel (10 to 100 kW), it seems to be absolutely necessary to put on the efficiency of the transmitter value.

p0006From the literature (L.Kahn, Single-Sideband Transmission by Envelope Elimination and Restoration, Proceedings of the IRE, 1952, S.803 ff) a high-frequency amplifier is known, which is operated with high efficiency. In the specified reference a station is described, which is characterized by the separate control of the power amplifier with magnitude and phase signal. Here, from the input single sideband signal by envelope detection by the envelope signal and limiting the amplitude of the input signal a signal is derived which contains the phase information is limited in amplitude. The limited high-frequency vibration and the envelope signal are fed to the working in the C-spanned operation amplifier, whereby a gain of a Einseitenbandsignales is achieved with high efficiency.

p0007In EP-0193655 a radio transmitter is described with high performance and with high efficiency, which is suitable for emitting a single sideband. In this transmitter, the analog low-frequency modulation signal is sampled, digitized, and by means of signal processing technique is converted into a complex signal consisting of real and imaginary parts. This conversion is effected by means of a Hilbert transformer according to the phase method known from the literature. Then, the magnitude and phase of the signal is determined from these components. the grid of the power tube are then driven with the phase-modulated high-frequency carrier and the anode connected to the magnitude signal corresponding to the envelope signal is driven, as in the circuit mentioned above. In these cases, an analog signal is transmitted with high efficiency.

p0008The object of the invention is to provide a transmission method that produces a low-bandwidth and when specifying a maximum Amplitudenhubes minimum disruption transmission.

p0009This object is achieved by the specified in the patent claims 1 and 8 features. Advantageous developments of the invention are specified in the subclaims.

p0010The invention allows advantageously to achieve a low bandwidth in the digital transmission of signals and thus allows digital broadcasting on long, medium and short wave. Claim 2 shows an advantageous embodiment of the invention with respect to the compensation of transmission errors. In claim 6, an embodiment of the invention is demonstrated with the advantageous use is supported by switching amplifiers. Claim 7 shows the use of the invention in conjunction with an amplification method having a very high efficiency.

p0011The invention is explained with reference to the pictures Fig. 1 to Fig.6.<ul><li>1 shows the symbol constellation of 64-QAM modulation,</li><li>2 shows the symbol constellation of a 32-ALMC modulation in non-compact form,</li><li>3 shows the symbol constellation of a 32-ALMC modulation in a compact form,</li><li>4 shows the symbol constellation of a 64-ALMC modulation with a symbol in the zero point,</li><li>5 shows the symbol constellation of a 64-ALMC modulation, which has no icon at the origin,</li><li>6 shows the frame structure of the preferred modulation,</li><li>7 shows the block diagram of a processing circuit for controlling a transmitter which is controlled separately in the envelope and amount,</li><li>8 shows the block diagram of such a transmitter with a tube-equipped stage and </li><li>9 shows the block diagram of such a transmitter with a transistorized amplifier.</li></ul>

p0012The modulation commonly used 64-QAM, with its symbol alphabet consisting of 64 symbols, depicted in FIG. 1. It is known that reduces the necessary bandwidth for transmission of the signal with increasing number of stages of modulation used. At the same time decreases with the number of symbols at a given maximum amplitude the signal distance between the icons, whereby the susceptibility to interference of the transmission increases again. to the short wave band, a 64- or 32-level modulation appears to be a reasonable compromise between the vulnerability and the necessary bandwidth for transmission in the said Long.

p0013To ensure sound possible utilization of the transmitter, it is necessary to keep the amplitude requirement of the modulation minimum.

p0014The constellation diagram in Fig. 1 it can be seen that the maximum amount of the symbols only occurs in the four Ecksymbolen the total of 64 symbols. That is, the available amplitude swing of the transmitter is relatively poorly exploited.

p00154 shows the constellation of a 64-valent modulation (64-ALMC) whose symbols lie inside a circle. In comparison with the symbol alphabet shown in Fig. 1 is used in this configuration the maximum amplitude value instead of 4 Ecksymbolen of 24 symbols. This is clearly a relative to the modulation alphabet of 64 QAM shown in Fig.1 improved utilization of Amplitudenhubes.

p0016In Fig.2 a constellation plane of a 32-valent modulation is shown whose icons are located in the interior of a circle. A compact embodiment, exploit the interior of the circular area whose symbols better, is shown in Fig.3. The comparison shows that a further improvement in the utilization of Amplitudenhubes is achieved solely by a more compact arrangement in addition to the circular array. Generally it can be said that the circular boundary of the constellation ensures optimum utilization of the available maximum amplitude over a target icon spacing.

p0017In Figure 5 a slightly different constellation of a 64-valent modulation is shown, which is characterized in that the zero point does not contain a symbol. In particular, for stations that have a distortion at very small amplitudes, this constellation is suitable for transmission, since the distortion-rich region is recessed in the transmission.

p0018This annular in Fig.2 to Fig.5 symbol constellations compared with other constellations improved utilization of the amplitude range is achieved.

p0019As is to be expected in the transmission of signals from transmitter to receiver with interference, the transmission of the data is performed in a frame structure. In Figure 6, an exemplary frame is illustrated. At the beginning of each frame, a preamble and a test sequence is emitted. The preamble contains information independent of the program content (eg station identification, program type, paging, time). The test sequence is formed with predefined symbols, which are used for adaptation of the receiver. If these predefined test symbols received in the receiver distorted, it is possible because the undistorted symbol is assumed to be known to model the transmission channel in the receiver mathematically and calculate back the then incoming similarly distorted symbols with the previously obtained channel model in the unstrained state.

p0020To Errei take a sufficiently accurate modeling of the channel, the test result should be at least twice the channel impulse response length. In the field of medium wave is expected to channel impulse response length of 2ms.

p0021For test suite relatively simple icons are proposed, which are part of a subset of the intended symbol alphabet. In Figure 5, such a subset by crosses in the selected symbol points is shown. As an example, two points which are opposite in the circle is selected. This selection of the points corresponding to the symbol constellation of Zweiphasenumtastung (2-PSK). If elected four offset by 90 degrees circular dots instead of two opposite points on a circle, so this corresponds to a subset of the 4-PSK modulation.

p0022Fig. 7 shows a circuit arrangement for implementing the modulation method of the invention. The digital input signal 2 shown in Fig. 7 the digital processing circuit 1 is in the symbol converter 3, converted into a complex signal which is represented in the form of real and imaginary parts. This selects each data input word from a predefined symbol alphabet of a complex symbol. A band limiting of the real and the imaginary part is effected by the filter 4 and 5. From the thus calculated components of the magnitude of the complex signal is detected in the absolute-value generator 6 which drives on the one hand the envelope amplifier 20 in the figures, Fig. 2 and Fig. 3 and on the other hand the components of the band-limited signals in the dividers 7 and 8 normalized. This thus calculated normalized components are the retarders 10 and 11 supplied to consisting of driver stage 21 to adjust the signal propagation times, one of which is greater in low Hüllkurvenzweig consisting of envelope amplifier 20 with filter 22, generally referred to as the high-frequency phase branch today.

p0023The delayed, normalized signals the carrier or intermediate frequency modulators 12 and 13 supplied to its modulation signal inputs. These modulators are driven at their carrier signal inputs with each other by 90 degrees-shifted components of the carrier or intermediate frequency oscillation and output the modulated output signals to the addition circuit 15. In the case of the use of an intermediate frequency of the adder stage 15 of the frequency converter 16 is connected downstream of the intermediate frequency based on the actual carrier frequency converts.

p0024For the function of the circuit, it is basically the same as if the modulators 12 and 13, the phase division 14 and the addition circuit 15 the necessary operations are carried out in digitized form to work in the form of analog circuits or. When using analog modulators, the retarders 10 and 11 to the respective digital-to-analog converter to complement and the output signal 19 to convert digital to analog in addition, when used in digital form.

p0025Fig.8 and Fig.9 show suitable amplifier arrangements, which are connected together with the processing circuit previously described.

p0026The recovered in the processing circuit phase-modulated and normalized carrier frequency signal is supplied for further amplification of the high frequency driver stage 21, which provides sufficient for the output stage 23 or 30 drive voltage available.

p0027The amount signal 7 is amplified by means of the envelope amplifier 20, which preferably operates as an amplifier with high efficiency. As switching amplifier to amplifier, according to the principle of the step-down converter (pulsdauermodulierender amplifier) ​​or operate on the pulse stage amplifier principle suitable. This amplifier types have in common that they are able to transmit low-frequency signals including the DC components.

p0028As power amplifier is suitable the electron tube shown in Figure 8 23, which draws its anode voltage from the low Hüllkurvenverstärkerzweig, consisting of the amplifier 20 and the downstream filter 22nd The tube is to achieve a high efficiency, spans operated in class C operation (conduction angle less than 90 degrees). Across the anode voltage modulates the tube 23 present at the control grid high-frequency carrier oscillation, which contains the phase modulation in amplitude.

p0029In place of the electron tube 23 in Figure 8 can alternatively be used a transistorized power amplifier 30, as shown in Fig.9. It is advantageous to use the bridge circuit shown in which the transistors are cross-operated in the switching mode. The switching operation used ensures high efficiency.