DE29710331U1 - Satellite transmission system for digital data transmission over subcarrier frequencies - Google Patents

Description

Satellite transmission system for digital data transmission via subcarrier frequencies

Description

The invention relates to a satellite transmission system for transmitting digital data via subcarrier frequencies.

A data transmission via satellite generally consists of a data source, a channel coding, a modulation stage, a satellite transmission channel, a demodulation stage, a channel decoding and a data sink.

Today, a 2-PSK or 4-PSK modulated carrier is almost exclusively used for digital data transmission via a satellite channel. Both convolutional codes and block codes are used as channel codes for error correction or detection.

Digital satellite broadcasting (DSR) works with 4-PSK modulation and a (63.44)-BCH code (DE OS 31 34 831). At the reception limit of C/N = 7 dB (in the 27 MHz channel), it achieves a bit error rate of BER < l*10~ ³ at the demodulator output and a BER < 3.7*10~ ⁵ after channel decoding, in which 2 errors are corrected and 3 more are detected. By cleverly dividing the correction capacity 0 according to the importance of the bits and concealing errors through linear interpolation, the DSR still achieves good quality of the transmitted audio signal with this relatively high bit error rate. The gross data rate is 20.48 Mbit/s. All programs (and data) to be transmitted must be fed to an uplink point to form the multiplex signal.

Depending on the data rate, VSAT technology (very small aperture terminal) uses 2-PSK or 4-PSK modulation and a convolutional code with a code redundancy of R = % or R = 3/4 (for example, the Fairchild 5 satellite modem 2800 or the COMSTREAM DBR401 digital broadcast receiver). This achieves a BER of l*10~ ⁵ to 10~ ⁷ at E _b /N _o = 5 to 6 dB. The BER is largely determined by the antenna size. Depending on the transponder used, the code redundancy and the desired BER, antenna diameters between 0.9 m and 1.8 m are common. The data rate can be configured to a large extent between a few kbit/s and a few Mbit/s. SCPC (single carrier per channel) operation, i.e. decentralized data feed to the satellite, is possible.

In the case of Digital Broadcasting Systems for television (DVB) for the satellite channel, the ETSI standard Draft prETS 300421 from June 1994 also prescribes a (gray-coded) 4-PSK for the modulation, while a two-stage process is used for channel coding, consisting of a convolutional code with R = 1/2, 2/3, 3/4, 5/6 or 7/8 as the inner code and a Reed Solomon Code RS (204,188,8) as the outer code (EP 05 51 646 A2). Between the coder or decoder for the outer code and the coder or decoder for the inner code there is an interleaver or deinterleaver with a depth = 12 and a width = 17 to scatter the residual errors (bursts), which are then to be corrected by the RS decoder. With this arrangement, at the reception limit of ε/ν _ω =4.5 to 6.4 dB (depending on the code rate R = 1/2 to 7/8), a BER of l^O" ¹ to io~ ² is obtained at the output of the demodulator and a BER of about 2*10~ ⁴ after the decoder for the inner code. Deinterleavers and Reed Solomon decoders then improve this value to BER < 1*1O~ ¹⁰ to l*10~ ^1:L . This achieves a virtually error-free time of one hour at a gross data rate of 55 Mbit/s. A decentralized uplink of different programs to a satellite transponder is not possible.

ASTRA Digital Radio (ADR) uses a transmission method with double modulation and a convolutional code with R = 3/4 for channel coding. The method is described in the ASTRA standard ASTRA-ADR/Rev. 1.3 SYS-078/02-94 dated December 15, 1994. The coded data is first modulated onto a subcarrier using 4-PSK, then several subcarriers modulated in this way with carrier frequencies from 6.12 (0.18) to 8.46 MHz, the video signal in the frequency range 0 to 5 MHz, at least two further frequency-modulated audio subcarriers and the blurring signal are added (baseband signal). The actual carrier is then transmitted frequency-modulated with this baseband signal over the satellite channel. A net data rate of 256 kbit/s is available per digitally modulated subcarrier. At the reception limit of C/N = 9.5 dB (measured in the 26 MHz satellite channel) the BER at the demodulation output is <4*10"^, at the output of the channel decoder the BER is < l*10~ ⁵ . This BER is sufficient for the reception of MPEG Layer II compressed broadcast programs with good audio quality. SCPC operation is not possible.

All known satellite-based data transmission systems have the disadvantage of either being too expensive for widespread use (VSAT, DVB) or the minimum bit error rate of BER < l*10~ ⁶ or less required for the transmission of digital data cannot be achieved (DSR, ADR).

The object of the invention is to achieve bit error rates between 1*10~ ⁶ and 1*10~ ¹² with an inexpensive satellite-based data transmission system in order to enable digital data broadcasting for large user groups. The data rate to be aimed for should be significantly higher than that of a good telephone modem (currently around 28,000 bit/s), but should not exceed a technically economically reasonable upper limit.

To solve the problem, a transmission system is used for the digital data to be transmitted via a satellite channel, the transmitting and receiving side of which consists of the modules connected to one another in the order listed in claim 1.

Starting from a data source, the data is protected against interference that can occur during transmission by adding redundancy in a channel encoder, which consists of an encoder for an external error-detecting or error-correcting code and an encoder for an internal convolutional code. This data stream is modulated onto a subcarrier in a 4 PSK modulator, which contains a (matched) matched filter in the low-pass range before the modulator, or in the band-pass range after the modulator. For transmission via a satellite channel, the 4 PSK modulated subcarrier is then modulated in a frequency modulator to the uplink frequency of the satellite transponder and limited to the necessary transponder band width. The uplink station sends the signal thus processed to the satellite transponder, which converts it in frequency and sends it back to earth. There, the satellite downlink signal is received using a satellite receiving antenna containing an LNB and converted into the 1st IF position. A filter filters out the converted downlink signal from the satellite transponder. This is followed by a frequency demodulator 10, from whose demodulation product the transmitted 4 PSK modulated subcarrier is filtered out using a (matched) matched filter. The following 4 PSK demodulator delivers a data stream at its output which, apart from possible transmission errors, is identical to the data stream that is fed to the 4 PSK modulator on the transmit side. The matched filter can also be arranged as a low-pass filter after the 4 PSK demodulator. The following decoder for the inner convolutional code and the subsequent decoder are used to correct errors that may occur during transmission.

following decoders for the external error-detecting or error-correcting code. By selecting the code parameters appropriately, the remaining error probability of the data received by the data sink can be set within wide limits according to its importance.

Design features are described in subclaims 2 to .
10

The invention and its mode of operation are explained in more detail below using an exemplary embodiment and an illustration.

The data to be transmitted is available on the transmitting side in the form of a binary file, the data source 1 on the server. To protect against transmission errors, a two-stage (chained) channel coding is used with an encoder 2 for a Reed Solomon code, RS (L,K) as the outer code and an encoder 3 for a punctured convolutional code with R = 3/4 as the inner code. Between both encoders and 3 there is also an interleaver 16 with a depth of T and a width of B, which together with the corresponding components on the receiving side ensures scattering and thus easier correction of burst errors. To synchronize the deinterleaver 17 and the Reed Solomon decoder 14, a block-by-block transmission is used, whereby the block length BL is determined by the code word length L of the Reed Solomon code. A 16-bit long Villard code word is used as a synchronization character, which is inserted into the data stream at the output of interleaver 16 every L bytes. The block length BL is therefore L+2 bytes.

Reed Solomon encoder 2, interleaver 16 and synchronous character inserter 5 are implemented by software on the server. In the first step, the source file is divided into parts of length K bytes and each part is processed by the Reed

Solomon encoder 2 encodes the data into a code word that is L bytes long. The totality of all code words forms the Reed Solomon encoded source file, which is rearranged in the second step by the interleaver 16 of depth T and width B, where B*T=L is assumed. The interleaver 16 inserts T*(T-I)*B empty bytes at the beginning and end of the file. In this Reed Solomon encoded and interleaved file, the 2-byte synchronization characters are inserted after every L bytes in the third step. The file prepared in this way is the transmission file. With the parameters of the Reed Solomon code and the interleaver 16, the residual error probability of the data after channel decoding can be determined for a given error probability of the transmission channel. Therefore, these parameters can be selected within certain limits and are adapted to the importance of the data to be transmitted.

The transmission file is now output by the server via an RS 4 22 interface at 192 kbit/s and transmitted via a corresponding data line, e.g. ISDN or similar, to the transmission equipment of an uplink point. There, this bit stream is fed into the ADR coder instead of a MUSCAM Layer II coded audio signal, which carries out further signal processing in accordance with the ASTRA standard ASTRA ADR/Rev. 1.3 SYS-078/02-94, i.e. V.3 5 scrambling, differential coding, forward error correction using a convolutional coder 3, filtering using a matched filter 4 and modulation with 4 PSK modulator 5 on a subcarrier. This subcarrier modulated with the data of the transmission file and possibly further such data of a transmission file 4-PSK-modulated subcarrier, are then added with the video signal 21, the blurring signal 24, frequency-modulated audio subcarriers 22 and ADR audio subcarriers 23 in an addition stage 18.

5 The further processing of this sum or baseband signal in the frequency modulator 6, filtering, conversion to uplink frequency by means of frequency converter 19, amplification and

Uplink 7 to the satellite transponder is specified by the data of the ASTRA satellite system for the transmission of television programs and is independent of whether one or more subcarriers are modulated with the data of a broadcast file or with a MUSICAM Layer II encoded audio signal.

With a satellite antenna 8 with a diameter of 60 to 80 cm and a standard LNB, the transponders of the satellites ASTRA IA, IB, IC and ID are received at the position 19.2° East and converted into the 1st IF, which is typically in the frequency range of 950 - 2150 MHz. The desired transponder is selected in the tuner of the data receiver. For this, the 1st IF coming from the LNB is used.

IF is converted using a frequency converter 2 0 so that the frequency range corresponding to the desired transponder can be filtered out by a 2nd IF filter 9, whose center frequency is 479.5 MHz and whose bandwidth of 27 MHz is adapted to the transponder bandwidth. This IF is amplified and demodulated in an FM demodulator 10. The demodulated signal is the baseband signal, which is composed of the following components depending on the transponder received:

- one or more 4-PSK modulated subcarriers used for data transmission;

- one or more 4-PSK modulated ADR audio subcarriers 23;

In addition, the following signals can be present in any combination:

- 2, 4, 6 or 8 frequency-modulated audio subcarriers 22 (the frequencies of the subcarriers are in the range of 6.12 to 8.46 MHz with a spacing of 180 kHz), - possibly a further frequency-modulated audio subcarrier 2 2 at 6.5 MHz and

- a video signal 21 in the frequency range from 0 to 5 MHz.

The FM demodulator 10 also supplies an AFC voltage, which is used to control the second converter in order to compensate for frequency drifts of the LNB.

For the data receiver, only the 4-PSK modulated subcarriers are of interest for data transmission in the frequency range from 0.36 to 8.82 MHz. This frequency range is filtered out, amplified in a control amplifier and fed to a fast 8-bit A-D converter. Further signal processing is now carried out digitally. The 4-PSK demodulator 12 uses a frequency synthesis block tuned to the frequency of the desired subcarrier to generate two output signals offset by exactly 90°, which are then multiplied with the input signal in a mixer stage.

To filter out the desired subcarrier, the output signals of the two mixers (in-phase and quadrature phase signals) are each filtered in a matched filter 11 with root-raised cosine characteristics, cut-off frequency = 65 kHz, roll off factor = 0.8 in the low-pass range. This is followed by a decision circuit which provides a 3-bit soft decision on the 4-PSK demodulated signal for further processing. In addition, a comparison of the input and output signals of the decision circuit provides the necessary criteria for phase-accurate control of the frequency synthesis block (carrier control) and for controlling a clock VCO to a multiple of the bit clock frequency of 256 kbit/s of the data signal. The two (soft decision) output signals of the 4-PSK demodulator are fed to a Viterbi decoder 13 for decoding the (inner) convolutional code. The parameters of the code used are code rate R= 3/4 (punctuated), polynomials Gl = 17I ₈ , G2 = 133 ₈ , constraint length = 7. The Viterbi decoder 13 does not need a special synchronization word for its synchronization (correct grouping of the input signals and control of the depunctuation logic). If more errors have to be corrected than a set threshold value specifies, the possible combinations are changed cyclically until a state is found,

where the number of errors detected per unit of time is below the set threshold. This theoretically achieves a BER = 4*10~ ³ at the demodulator output and a BER = l*10~ ⁵ at the Viterbi decoder output with an E _b /N ₀ of 5.5 dB.

The data receiver components listed so far are also used in the same form for ADR audio transmission, so that the part of the data receiver described so far can be constructed from circuit parts of a conventional ADR receiver. A complete ADR receiver also contains an MPEG Layer II decoder and a D-A converter in the signal processing branch.

The further processing of this data stream is expediently carried out using software in a PC, which can simultaneously form the data sink 15. For this purpose, the data supplied by the Viterbi decoder are first saved in a file, e.g. on the hard disk.

Deinterleaving and Reed Solomon decoding for the received and saved file can now be carried out by the PC with the appropriate software, thus significantly reducing the bit error rate. The parameters of the deinterleaver 17 and the RS decoder 14 (and the corresponding modules on the transmitter side) can be largely adapted to the importance of the data and thus to the necessary BER. For synchronization of the deinterleaver 17 and the RS decoder 14, a block-by-block data transmission is required, in which each block begins with a synchronization character. A 16-bit Villard code word is used, which can still be reliably detected by correlation analysis even if there are one or two bit errors. Once synchronization has been found, this state is maintained even if two consecutive synchronization words are completely destroyed. Only when the third undetectable synchronization word is found does the synchronization break off and begin a new search for a

Iu

Synchronous word. With this strategy, a deinterleaver with a depth T = 12 and a width B = 17 and a Reed Solomon code (204,188,8) of length L = 204 with K = information bytes, the block length BL = 206 bytes. The additional redundancy by which the useful data rate is reduced is (16+2)/188 = 9.5%, so that the useful data rate is 173.6 kbit/s. For very short files, it must also be taken into account that 1224 empty bytes must be inserted at the beginning and end of each file for interleaving.

With this additional redundancy provided by the (outer) RS code, the bit error rate is reduced to BER < l*10~ ¹⁴ if the residual error statistics at the output of the Viterbi decoder are based on burst errors of length 37. The burst error lengths to be expected due to the design of the Viterbi decoder are between 12 and 36.

This residual error rate value is completely sufficient for secure data transmission, the user data rate of approx. 173 kbit/s is 6 times higher than that of a fast telephone modem and the device costs for the data receiver are comparable to the costs for an ADR receiver, assuming that a PC is owned.

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Reference symbol list

1 Data source

2 encoder for outer code 3 encoder for convolutional code

4 matched filters

5 4-PSK modulator

6 Frequency modulator

7 Uplink

8 Satellite reception antenna with LNB

9 Filter for transponder frequency range

10 FM demodulator

11 matched filters

12 4-PSK subcarrier demodulator 13 Convolutional code decoder

14 decoders for outer code

15 Data sink

16 Interleavers

17 Deinterleaver 18 Addition stage

19 Frequency converter

2 0 Frequency converter

21 Video signal

22 frequency-modulated audio subcarriers 23 ADR audio subcarriers

24 Blur signal

Claims (7)

Protection claims 1. Satellite transmission system for digital data transmission via subcarrier frequencies, with the following sequentially arranged modules: - Data source (1), - encoder (2) for an external, error-detecting or error-correcting code, - encoder (3) for a convolutional code, - matched filter (4) and 4-PSK modulator (5) for one subcarrier, whereby the order of both can be swapped, - Frequency modulator (6) - Uplink station (7) to the satellite transponder, - Satellite reception antenna (8) with LNB - Filter (9) for transponder frequency range, - FM demodulator (10), - filter (11) and 4-PSK demodulator (12) for subcarriers, where the order of both can be swapped, - decoder (13) for the convolutional code, - decoder (14) for the outer, error-detecting or error-correcting code, - Data sink (15).
25 2. Satellite transmission system according to claim 1 with several subcarrier frequencies, characterized in that the modules (1) to (5) and (11) to (15) are present for each subcarrier frequency and an addition stage (18) is located in front of the frequency modulator (6). 3. Satellite transmission system according to claims 1 and 2, characterized in that an interleaver (16) is arranged after the encoder (2) and a deinterleaver (17) is arranged before the decoder (14) for beam error scattering. 4. Satellite transmission system according to claims 1 to 3, characterized in that modules for additional signal transmissions, for example video signals (21), blurring signals (24), digitally modulated audio subcarriers (23) or analogue modulated audio subcarriers (22) are present and the transponder is also used for the transmission of a television program. 5. Satellite transmission system according to claims 1 to 4, characterized in that an encoder for Reed Solomon codes is used as the encoder (2) for the error-detecting or error-correcting code and a decoder for Reed Solomon codes is used as the decoder (14). 6. Satellite transmission system according to claims 1 to 5, characterized in that a frequency converter (19, 20) is arranged after the frequency modulator (6) and/or before the FM demodulator (10). 7. Satellite transmission system according to claims 1 to 6, characterized in that modules of commercially available ADR receivers are used to implement the modules (9) to (13).