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- 1. POLITECNICO DI TORINO III ENGINEERING FACULTY BCH-LDPC Concatenated Coding and High Order Modulations for Satellite Transmitters Advisor Co-Advisor Prof. dr. Roberto Garello Eng. dr. Domenico Giancristofaro a.a. 2007-20081Luglio 2008
- 2. Overview Analysis of the state of art of modern satellite communication systems Study of DVB-S2 outer code (BCH) Theoretical, algorithmical, and architectural study of the BCH encoder Definition of a new algorithm on a parallel basis in order to increase data throughput while being compliant with the specific ASIC technology (suitable for space environment, radiation tolerant, etc.) Module integration with LDPC section and Interleaver Design and definition of SW modules (C/C++) to validate the parallel algorithm (test bench for VHDL) Lab tests over the entire TX Section.2Luglio 2008
- 3. Principal Operating Scenario within Satellite Communications Multimedia communications provided to wide geographical areas with low population densisty Maritime communications (e.g., Inmarsat) and Radio Navigation Systems (e.g., GPS, Galileo) Digital TV Broadcasting (e.g, DVB-S, DVB-S2) and broadband internet access (DVB-S2) for flexible ACM services Earth observation and monotoring (e.g., COSMO-SkyMed): high data rate and short revisit time (LEO), very high data rate required (roughly 1Gbps)3Luglio 2008
- 4. DVB-S2 System and ACM within Ka Band Low Protection 32APSK rate 9/10 High Protection QPSK rate 1/4 S/N MOD S/N Bit Mapping • 4 Modulation Format: DEM QPSK, 8PSK, 16APSK e 32APSK Interleaver • 11 Code Rates spanning Deinterleaver ENC-LDPC between 1/4 and 9/10 DEC-LDPC • Spectral Efficiency η ≈ r∙ log2M ENC-BCH between 0,5 and 4,5 bit/s/Hz DEC-BCH4 • S/N ratio from –2dB to +17dB S DLuglio 2008
- 5. Channel Coding for DVB-S2 (BCH-LDPC) Outer BCH Code Primitive shortened Three level of protection (8, 10, 12) Gives an extra protection against the error floor at higher SNR Inner LDPC Code Performance close to Shannon limit Reasonable level of complexity5Luglio 2008
- 6. BCH Encoding Algorithm 1. Multiplication (shift) and zero padding r m( x)⋅x Polynomial 2. Remainder computation Generator r d ( x)=m( x)⋅x mod g ( x) 3. Remainder bits get appended d ( x)=m( x)⋅x r mod g ( x)6Luglio 2008
- 7. A Simple HW Architecture The computation gets interrupted for k clock ticks g1 g2 gn-k-1 …un-1un x0 x1 x2 xn-k-1 This LFSR yields remainder bits in n clock ticks This kind of architecture is not going to work seamlessy : Parity bits have to be extracted not serially Shift register needs to be reset after each computational cycle7Luglio 2008
- 8. A Typical Serial Architecture Injects zeroes Breaks up the feedback loop S1 g1 g2 gn-k-1 x0 x1 x2 xn-k-1 u(i) 1 Saving of k clock Parity bits extraction c(i) S2 ticks 2 It yields remainder bits in k clock ticks After n clock ticks, it is newly ready for the next computation However, it is still not suitable for our requirements8Luglio 2008
- 9. Algorithm at Higher Throughput: Linear System Modeling [ ] [] State equation g0 1 g1 0 x (i )=A x (i +1)+b u (i ) b= ⋮ b= 0 ⋮ ⋮ g n−k −2 ⋮ [ ] g n−k −1 0 0 0 … 0 g0 1 0 … 0 g1 Input-state transition A= ⋮ 1 … 0 g2 vectors 0 ⋮ ⋱ … ⋮ 0 0 … 1 g r−1 The state matrix is relevant to the system itself and so is common to both system State transition matrix: It models its evolution I-S Vectors change as input position changes9Luglio 2008
- 10. System Parallelization By applying the following This matrix shows some sort recursive substitution of regularity x (i )=A x (i −1)+b u (i −1) x (i −1)=A x (i −2)+b u (i together Putting −2) p Ap= ( 0 C1 I C2 ) column vectors p: parallelismo ) (b Ab L A p −1 b It is common to both system p−1 x (i )=A x (i −p )+ ∑ A k b u (i −k −1) p k =0 Bp= I 0 () LFSRx (ip)=A p x [(i −1) p ]+B p u (ip) Bp Serial encoder10Luglio 2008
- 11. FEC Section 8 bits BCH 8 bits 8 bits encoder BCH to LDPC LDPC input interface memory 8 bits Download parity controller The parallelism of LDPC input memory suggests a p = 8 as degree of parallelism BCH-LDPC interface is devoted to format data in order to have them compliant with DVB-S2 specifications11Luglio 2008
- 12. Parallel BCH Encoder COMB0 X0 COMB An 1st row X1 COMB An 2nd row ( ) x 184 COMB1 1 X2 … C 1⋅ ⋮ … x 191 COMB2 … … … … … X7 COMB An … 8th row p bit … X8 EXOR x9 COMB An … 9th row From COMB8 X9 ( ) COMB7 COMB An x 184 From x175 10th row From COMB 9 … C 2⋅ ⋮ COMB8 To EXOR x9 … x 191 X183 COMB An … 184th row … … From x181 … … COMB An X190 … From x183 191th row From COMB190 X191 COMB An12 COMB191 192th rowLuglio 2008
- 13. Combinatorial Networks Makes the logic ai,183 or ci,1 more adaptable ai,184 or ci,2 They are suitable … for each protection … level ai,191 or ci,8 … Each combinatorial network performs a row by column product The networks (192) ahead of the state registers performB 8⋅u ( )( ) x The ones behind (192) carry out products C 1 ⋅ 184 ⋮ C2 x 19113Luglio 2008
- 14. BCH-LDPC Interface x(184-8*i) x0 LSB i=0 x1 . x2 . x(184-8*i) . … controller x3 i=23 T x(185-8*i) i=0 o . … 8 bits …From BCH encoder . … L . … D … x(185-8*i) i=23 … P C … … … … i x(190-8*i) … i=0 n p u x184 x(190-8*i) t i=23 8 bits x185 m x(191-8*i) MSB e … i=0 m x190 o r x191 x(191-8*i) From k y14 i=23 informative bits 0Luglio 2008
- 15. Algorithm Validation Pseudo random The position of each error is a r.v. Noise source uniformly distributed between 1 and n, the block length Message Generator Error source Comp Decoding section Error Berlekamp Error BCH Encoder Detection Massey Correction Syndrome Computation Decoding blocks exploit useful precomputed Galois Tables Berlekamp-Massey algorithm figures out error polinomial locator coefficients Error correction block finds error positions by Chien search15Luglio 2008
- 16. Integration and Lab Test: TX Section Parallel Modulator architecture NC O Compensate for DAC distortion B 3 Rs D I I Shaping Farrow i T g 7 Bit Filter Interpolator E Filter i N I N M 0 Three t 8-10 B i t a C T A 0 Branches l INPUT O E P DAC DAC INTERFACE R P U P re co m . P/S D L Q Shaping Farrow P E E E 7 Bit Filter Interpolator C Filter R A V R 0 Three S I 0 Branches t N a G g 3 Rs e NC O Blocks before modulator perform BCH-LDPC concatenated encoding, interleaving, and bit mapping SRRC Shaping filter with three roll-off factors The digital up-conversion stage shift the signal spectrum to an intermediate frequency, just before the next shift to the working frequencies16Luglio 2008
- 17. Setup Measures EVM, the Error Vector Module Stratix II DSP Agilent Infinium MSO Agilent 89600 Vector development board 6054A Signal Analyzer L’FPGA (EP2S180) on Oscilloscope Software demodulation board containing the TX wired to the provides the phase and Section synthesis DAC output amplitude error with Stratix II development respect to constellation board has two 14 bit points (165 Msample/s) DACs Generates scatter plots and measures the signal17 spectrumLuglio 2008
- 18. Modulator Performance (2 MBaud – 16-APSK) EVM Amplitude 2% Error: 0,9% Distance from ideal performance very small The DAC distortion, at low transmission rate, is very small18Luglio 2008
- 19. Performance (30 MBaud – 8-PSK) EVM 9% EVM 4% Without precompensation filter With compensation filter Precompensation Filter Il DAC causes flattens the spectrum distortion at high frequencies since it cuts them (low pass)19Luglio 2008

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