Analysis of the bandwidth efficiency of DVB-S2 in a typical data distribution network

Transcription

1 Analysis of the bandwidth efficiency of DVB-S2 in a typical data distribution network Dirk Breynaert, Maximilien d Oreye de Lantremange, Newtec (CCBN2005, Beijing, March modified) Abstract: The new DVB-S2 standard represents a quantum leap in term of bandwidth efficiency compared to the former DVB-S and DVB-DSNG standards. This drastic improvement is due not only to a new error correction code called Low Density Parity Check (LDPC), but also to new modulation schemes and new modes of operations called Variable Coding and Modulation (VCM) and Adaptive Coding and Modulation (ACM). In this article we show with a simple example that the use of these technologies can increase the available throughput of a given satellite channel by up to 29% with LDPC, 66% with VCM and 131% with ACM. Introduction As the very first DVB-S2 products are progressively introduced on the market, the new standard has already acquired a widespread reputation as a major breakthrough in bandwidth efficiency. Indeed, the new error correction codes used by DVB-S2 are notorious for representing a significant improvement over the codes used by the former DVB-S standard. Yet, what is less known is that DVB-S2 is much more than improved error correction coding: "DVB-S2 is a powerful cocktail of technologies that can help service operators save even more bandwidth while offering an unprecedented level of flexibility". In this article we take a concrete example of a typical satellite distribution network to analyse how much bandwidth can be saved by the DVB-S2 technologies compared to a classical DVB-S implementation. The network model As example, we consider a data distribution network consisting of one hub and 20 remotes sites, which are geographically distributed over the satellite downlink beam. These sites have typically also a variation of G/T-values in the downlink. The data is distributed to the remote sites on a 30 Mbaud forward link in Ku band. The remote stations are capable of transmitting a return signal back to the hub, but the rate and characteristics of the return channels are not relevant to this analysis. Such an example is illustrated in Table 1, where the sites are ordered in decreasing order of measured (= availability 95%) C/No-values (from 86.3 to 90.1 dbhz). It is also assumed that the probability rain margin is varying from site to site, between 2.5 and 5.5 db (these values don't represent heavy rain area's, where values can go up to for example 15 db). Fixed modulation and coding selected for the worst case Co/No among all receiving sites (Avail ) QPSK 2/3 (DVB-S) QPSK 4/5 (DVB-S2).. HUB Figure 1. Data distribution network model

2 DVB-S1 Rain Margin (db) Implem. Margin(dB) Avail Table 1. C/No value for 20 remote sites DVB-S implementation First we assume a forward link of the network using DVB-S equipment. We assume that an implementation margin of 2 db is necessary. To decide which modulation and error coding to use, we compute the minimum C/No and Co/No at the worst site. For site number 1 we get: C/No = = 80.8 dbhz and Co/No = = 6.0 db. DVB-S uses the Viterbi+RS error correction code. The performance of this code is such that a coding rate of at least 2/3 is required: For the Newtec DVB-S demodulator NTC/2163 the minimum Ebi/No for QPSK 2/3 (to have a BER < 1E- 7) is 4.5 db. This gives: Co/No = Ebi/No db = = 5.4 db (cfr. Application Note NTC/2063/APN01) which is below the available 6.0 db. The usefull bit rate for 30 Mbaud with QPSK 2/3 is x 30 = Mbps. (cfr. Datasheet NTC/2180) Simple DVB-S2 implementation (CCM) CCM is the simplest mode of DVB-S2, which is similar to the DVB-S, in the sense that all data frames are modulated and coded using the same fixed parameters. This mode is called Constant Coding Modulation (CCM). However, in DVB-S2 the inner error correction code is the Low Density Parity Check (LDPC) code invented by Gallager in the sixties. In the previous example, the performance of this code is such that a coding rate of 4/5 is sufficient for the same channel conditions. For Newtec s new DVB-S2 demodulator NTC/2263, a minimum Es/No of 5.4 db is required for QPSK 4/5. The useful bit rate is then given by 30 Mbaud x = Mbps. Compared to DVB-S, this is an efficiency improvement of 29.1% In fact, the performance of LDPC is within 1dB of the theoretical maximum performance known as the Shannon limit. This represents a 2 to 3 db improvement compared to DVB-S.

3 DVB-S2 Variable Coding and Modulation (VCM) A unique feature of the DVB-S2 standard is that different services can be transmitted on the same carrier, each using their own modulation scheme and coding rate. This sort of multiplexing at the physical layer is known as Variable Coding and Modulation (VCM). VCM is particularly powerful when different services do not need the same protection level (for example if it is acceptable to loose a secondary channel in case of rain fade), or, like in our example, different services are intended for different stations with different average receiving conditions. When we consider our example with a rain fade probability of, the available Co/No values at the receive sites vary between 6.0 and 10.6 db, as shown in Table 2. Rain Margin (db) Implem. Avail. Co/No(dBHz) Margin(dB) Mod/Cod (Mbps) Avail. baudrate QPSK4/ PSK3/ PSK2/ PSK3/ PSK3/ PSK3/ PSK3/ PSK3/ PSK2/ PSK2/ PSK3/ PSK3/ PSK2/ PSK2/ PSK2/ PSK2/ PSK3/ APSK2/ APSK2/ APSK2/ Total Table 2: C/No values for 20 sites using DVB-S2 (VCM) Using VCM, a different Coding/Modulation (CM) can be selected for each station according the average Co/No (= Es/No) at each site. These CMs are ranging between QPSK 4/5 and 16APSK 2/3 1. If the total baud rate (30 Mbaud) is distributed ly over each station, each station will use a 30/20 = 1.5 Mbaud part of the carrier. The corresponding bit rates are varying between 2.38 and 3.96 Mbps according to the selected CM. The total available rate is then Mbps. This is an improvement of 65.7% compared to DVB-S. DVB-S2 Adaptive Coding and Modulation (VCM) When a return channel is available from each receiving site to the transmit site, DVB-S2 offers an even more powerful feature known as Adaptive Coding and Modulation (ACM). With ACM it is possible to dynamically modify the coding rate and modulation scheme for every single frame, according to the measured channel conditions where the frame must be received. The return channel is used to dynamically report the receiving conditions at each receiving site. 1 Note that the DVB-S2 standard allows up to 5 bits per symbol with a 32APSK constellation. However, 32 APSK receiver chips are not available commercially yet

4 16APSK 5/6 Variable modulation and coding selected for each service according to measured Co/No at the receiving site Co/No measurements.. 16APSK3/4 16APSK2/3 8PSK 3/4 HUB Figure 2: ACM implementation When we consider our example with a 95% probability of rain fade ; the available clear weather C/No values (incl. implementation margin) will vary between 84.3 and 88.1 dbhz as shown in Table 3. DVB-S2 If 1Mbps Co/No(dBHz) Mod/Cod (Mbps) (Mbps) Table 3: C/No values for 20 sites using DVB-S2 (ACM) Baud Rate (Mbaud) Implem. Avail. Avail. Margin(dB) 95% 95% baudrate bitrate PSK3/ PSK3/ PSK2/ PSK2/ PSK2/ PSK2/ PSK2/ PSK2/ PSK3/ PSK3/ PSK3/ PSK3/ PSK3/ PSK3/ PSK3/ PSK5/ PSK5/ PSK5/ PSK5/ PSK5/ Total 234.1% Using ACM, a different Coding/Modulation (CM) will be selected for each station according to the measured Co/No (= Es/No) at each site. The result of each CM-selection is shown in Table 3 for each site. These CMs are ranging between 8PSK 3/4 and 16PSK 5/6. If the total baud rate (30 Mbaud) is distributed ly over each station, each station will use a 30/20 = 1.5 Mbaud part of the carrier. The corresponding bit rates are varying between 3.34 and 4.95 Mbps to each site. The total available rate is then 86.3 Mbps.

5 This calculation is correct in clear weather conditions. However if we assume that for example station 20 has a rain fade of 3 db, it will reduce the Co/No from 13.3 db to = 10.3 db. When we consider at that site an extra safety margin (for operation) of 1.5 db, the net Co/No would still be 8.8 db, allowing use of 8PSK-3/4. The bit rate would therefore only be reduced from 4.95 to 3.34 Mbps and total IP rate from to Mbps. This will still be 130% higher than the conventional DVB-S solution, described earlier. Typical Margin distributions For the above 3 cases, the histogram of the margin has been calculated and shown in the figure hereafter. The typical regions are for CCM : 6 to 9.5 db for VCM : 4.5 to 7 db for ACM : 2 to 3 db. This clearly indicates that additional to the advantage of increased bandwidth for ACM, there is also a good control of the link margins. Histogram Margin 0,5 0,45 0,4 0,35 Probability 0,3 0,25 0,2 CCM VCM ACM 0,15 0,1 0, Margin (db) Conclusions In this paper we have shown with a simple model how powerful the DVB-S2 technologies are in term of bandwidth efficiency. In our typical example of unicast solutions for Ku-band transmissions to multi sites, the use of DVB-S2 allows an increase of satellite transmission capacity of 29% for CCM, 66% for VCM and 130% for ACM. In case of Satellite Ka-band links to multi sites in regions with heavy rainfall, the improvement will even be higher. About Newtec in the authors: Newtec is a Belgian company that has emerged as a leading manufacturer of satellite telecommunication equipment and solution provider worldwide. The expertise of Newtec relies on 20 years of experience in designing, manufacturing and supplying equipment, earth stations, communication networks and related services for satellite applications such as digital television exchange and broadcast, interactive television, Internet broadband access, corporate networks, telecom operator infrastructure and scientific programs. Newtec is an active contributor to standardization bodies such as DVB, ATSC and SabLabs and offers a wide range of products and solutions compatible with the DVB-S, DVB-DSNG,DVB- RCS and DVB-S2 standards. Dirk Breynaert has founded Newtec in 1985 together with Jean-Marie Maes and is the CEO of the company. Maximilien d Oreye de Lantremange has been with Newtec since 1998 and is the Coordinator of R&D projects as well as the Program Manager for the development of DVB- S2 products and systems. Contacts: [email protected] tel: fax: