Encapsulation and Framing Efficiency of DVB-S2 Satellite Systems

Transcription

1 Encapsulation and Framing Efficiency of DVB-S2 Satellite Systems M.A.Vázquez Castro Universitat Autónoma de Barcelona Dpt. Telecom. e Ing. de Sistemas Campus Universitario, Bellaterra Spain angeles.vazquez@uab.es Rita Rinaldo European Space Agency Keplerlaan 1, 2200 AG Noordwijk The Netherlands rita.rinaldo@esa.int

2 Contents Introduction Description of the DVB-S2 Adaptive coding and modulation subsystem DVB-S2 layered encapsulation architecture Definition of layered encapsulation efficiency Numerical Results Conclusions

3 Introduction Current DVB-S and DVB-DSNG standards, make use of concatenated Reed-Solomon and Viterbi Forward Error Correction schemes. These techniques do not fully exploit the available bandwidth and are about 4 db away from the theoretical Shannon limit DVB-S2 is a specification for next-generation digital satellite transmission emerging from technical ad-hoc DVB working groups. DVB-S2 standardizes a number of configurations and applications areas. This paper focuses on the DVB-S2 interactive data service in which the use of ACM has been set as normative.

4 coding and modulation subsytem Unlike DVB-S, the second generation of the standard allows for several input stream formats. In addition to MPEG transport stream, generic streams (IP in Fig. 1) are encompassed by the standard Transport Stream rules do not apply and different encapsulation protocols with improved efficiency can be used as an alternative to the Multi Protocol Encapsulation (MPE) [3]. IP datagrams can also be directly mapped on the transmission frame.

5 Functional block diagram of the DVB-S2 encapsulation/framing MPEG p ackets IP Packets IP Packets MPEG ENCAPSUL. n streams MODE ADAPTATION M erging of streams Stream slicing n streams FEC FRAM E or coded bits Header 80bits Data Field (DF) 0<DF <K BCH FRAMING (90 symbols slots) ADAPTIVE MODUL (xpsk) ADAPTIVE CODING (BCH&LDPC) STREAM ADAPTATION (PADD ING)

6 DVB-S2 layered encapsulation architecture IP MPEG/MPE MODE ADAPTATION STREAM ADAPTATION ADAPTIVE CODING ADAPTIVE MODULATION TDM FRAMING 80 bits BB HEADER 0<DFL <K BCH DATA FIELD (DF) BBFRAME FECFRAME (64800 or encoded bits) Header slot PLFRAME Pad ding Pilots (36 symbols every 16 slots) Slot (90 symbols) K BCH -DFL-80 The packet of DVB-S2 is called BBFRAME (in information bits) or FECFRAME (in coded bits) The physical layer packet is called PLFRAME (already symbols)

7 Definition of layered encapsulation efficiency In principle the efficiency can be defined in different ways. We decide on using the following one: ψ ( η, ) i L i = payload bits total transferred ( layer i) bits ( layer i) where the dependency on the spectral efficiency, η, and on the packet length, Li, on a per-layer i-th basis is explicit. Note that the packet to be encapsulated at layer i-th is the packet delivered by the layer (i- 1)-th.

8 Definition of layered encapsulation efficiency Distinction between encapsulation out of the DVB-S2 sub-system and the performed within it ψ tot( ηcod, ηmod, LIP, L) = ψ MPEG/ MPE( LIP ) ψ DVB S 2( ηcod, ηmod, L) The encapsulation performed within the DVB-S2 subsystem is actually performed in two steps and therefore can be expressed as follows ( η, η, L) ψ ( η L) ψ ( η ) ψ, DVB S2 COD MOD = MS COD framing MOD

9 Numerical Results ψ ( η L) MS COD, IP encapsulation efficiency in the Mode and Stream adaptation, when only 1 packet is encapsulated

10 Numerical Results ψ ( η, L) MS COD Efficiency when only packets of 40 or only of 512 bytes (552 after adding the IP header) are encapsulated altogether. It is apparent that short FECFRAME is less efficient for those particular yet most likely packet sizes NORMAL FECFRAME L = 40 bytes L = 552 bytes LDPC coding rate SHORT FECFRAME L = 40 bytes L = 552 bytes LDPC coding rate

11 Numerical Results ( ) ψ framing η MOD Framing is organized in units of 90 symbols (slot) such as after modulation the coded and modulated FECFRAME occupies exactly an integer number of slots. A header is inserted for receiver configuration occupying exactly one slot. Pilots (36 symbols) are inserted every 16 units of 90 symbols (slots). The framing efficiency in therefore given by

12 ψ Numerical Results, framing ( η ) MOD = data symbols ( η ) MOD datasymbol s ( η ) ( S ( η ) + 1) MOD MOD ( ) ψ framing η MOD ( ηmod) ( S ( η ) 1) 90S + Pint MOD 16 (bits/s/ H z ) ( ) ψ framing η MOD NORMAL FECFRAM E SHORT FECFRAME Table Framing efficiency. 97.3

13 Total Efficiency ψ ( η,η, L) tot COD MOD L IP, ψ Recall that tot ( η η, L, L) = ψ ( L ) ψ ( η,, L) COD, MOD IP MPEG / MPE IP DVB S 2 COD η MOD IP over MPEG encapsulation is currently performed via the Multi Protocol Encapsulation MPE, which allows concatenating IP packets. The two following different approaches are possible: MPEG/MPE encapsulation MPEG-TS-based transmission Only the first case is assumed here, IP packets are encapsulated into MPEG packets. However, no MPEG transport stream is assumed for transmission although such a case is also encompassed by the standard

14 Total Efficiency ( η,η, L) tot COD MOD L IP, The effect of the statistical distribution of the IP packet length on the encapsulation efficiency, it is possible to compute analytically for the MPEG/MPE case by L max averaging ψ as follows = ψ L p( L ψ ( ) ) MPEG / MPE MPEG / MPE IP IP L min All packet sizes equally distributed Video streaming packet size distribution PUSI= PUSI= Table 2. MPEG/MPE weighted encapsulation efficiency.

15 Total Efficiency ψ ( η,η, L) tot COD MOD L IP, Total encapsulation efficiency considering MPEG/MPE and PUSI=0 encapsulation on top of DVB-S2 for the packets sizes. Comparison of total and DVB-S2 encapsulation efficiencies when using MPEG/MPE (PUSI=0) and streaming video traffic.

16 Conclusions For streaming video, total efficiency has been found to be above 85% (normal FECFRAME) and above 75% (short FECFRAME) case In case concatenated is allowed, efficiency increases up to 90% and 80 % respectively. IP directly over DVB-S2 shows an average efficiency in the order of the one achieved when using MPEG/MPE but the values show a larger deviation with the coding rate. The deviation is dependent not only on the coding rate but also on the IP packets distribution