DVB-SH Digital Television Transmission and its Simulation in MATLAB

Ladislav POLÁK, Tomáš KRATOCHVÍL

Dept. of Radio Electronics, Brno University of Technology, Purky ova 118, 612 00 Brno, Czech Republic

xpolak18@stud.feec.vutbr.cz, kratot@feec.vutbr.cz

Abstract. This paper deals with the MATLAB implementation of the DVB-SH (Digital Video Broadcasting - Satellite services to Handhelds) in AWGN (Additive White Gaussian Noise) transmission channel. The paper is especially focused on the model DVB-SH-A, which is using configuration with OFDM transmission mode. For the simulation there was developed application and its functions are described in the paper too. By using this application the BER (Bit Error Ratio) after turbo decoding, with respecting the number of iterations, can be evaluated. Finally, obtained results are discussed.

Keywords Digital Video Broadcasting - Satellite services to Handhelds, DVB-SH-A, turbo encoding, turbo decoding, number of iterations, BER.

1. Introduction Mobile TV is already booming on existing cellular

infrastructures in point-to-point mode. However this mode is not optimized to deliver the same content of broadcasted stream to many users at the same time. More precisely, in areas where the reception of the transmitted information is bad, especially in urban areas; the classical DVB-H (Handheld) does not support the optimal solution. For this situation the data stream, transmitted in DVB-SH system is optimal [1].

The DVB-SH system provides an efficient and flexible mean of carrying broadcast services over a hybrid satellite and terrestrial infrastructure operating at frequencies below 3 GHz to a variety of portable, mobile and fixed terminals having compact antennas with very limited or no directivity [2]. This configuration of system is optimal for the battery-powered devices (PDAs and mobile phones), vehicle-mounted (laptops) and stationary terminals (set-top boxes). The key feature of DVB-SH is the fact that it is a mentioned hybrid satellite/terrestrial system that will allow the use of a satellite to achieve coverage of large regions. Whenever a line of sight between terminal and satellite does not exist, terrestrial gap

fillers are employed to provide the missing coverage. This method is completely illustrated and described in [2] [3] [4].

OFDM (Orthogonal Frequency Division Multiplexing) is the natural choice for the terrestrial modulation as it has been already selected for DVB-T/H systems. For the satellite transmission, two modulations have been selected, which leads to two reference architectures within the variety of possible hybrid satellite/terrestrial system architectures: SH-A (for OFDM terrestrial and OFDM satellite transmission mode) and SH-B (for OFDM terrestrial and TDM satellite transmission mode). And again, more details are available in [2] [3] [4].

This paper contains the results of recent development of MATLAB application which allows simulation of the DVB-SH-A transmission in AWGN channel.

The organization of the paper is as follows. Section 2 introduces the main block diagram of the transmitter of the DVB-SH-A system. The parameters for the simulation are presented in the Section 3. Dependences of the BER after turbo decoding on C/N ratio and on the number of turbo decoding processing are shown in the Section 4. Finally, achieved results are discussed in Section 5.

2. Block diagram of the DVB-SH-A The conception of the transmitter of the DVB-SH-A

system that has been applied in MATLAB is illustrated in Fig. 1. The structure of the transmitter follows common DVB-SH transmitter block diagram, especially the OFDM signal processing. One of the main technological innovations of the DVB-SH is implemented in the part of the FEC (Forward Error Correction). Details of these codes and the following blocks are described below.

2.1 Mode and Stream Adaptation How it can be seen in the Fig. 1, the first functional

block is the Mode and Stream Adaptation. Mode Adaptation consists of CRC (Cyclic Redundancy Check) encoding, to provide detection every MPEG packet. Stream

978-1-61284-324-7/11/$26.00 ©2011 IEEE

Fig. 1. The functional block diagram of the DVB-SH-A transmitter (OFDM transmission mode).

Adaptation provides padding to complete a constant length of one transmitted frame and performs scrambling [4]. The scrambling process is the same that was used in standard DVB-H [6].

For the simulation of the data transmission this block is omitted, because the scrambling of input data stream has not significant effect on the error correction. The application, therefore in the first step is generating the input data sequence. The length of one data frame for the simulation is 2048 bits.

2.2 Turbo Encoder and Puncturing The next step contains the turbo encoder and

puncturing. The turbo code is used for the FEC encoding of the input data stream. Additional code rates, with respect to originally defined 3GPP2 code rates, have been introduced to both DVB-SH modes (OFDM and TDM) allow better granularity in terms of C/N adjustment [4].

The turbo encoder, as it was standardized by the 3GPP2 organization, shall be used for the DVB-SH. However, this type of turbo coder has several disadvantages from the perspective of simulation (complexity, time of encoding, problems with the methods for the turbo decoding). Therefore, for the simulation there was used a modified type of this coder [5]. Hence, the application is using PCCC (Parallel Concatenated Convolutional Code) turbo encoder. This type is preferred for a very low BER [7]. After the turbo encoding the puncturing of the encoded data is followed, as it is defined in [4]. Within a puncturing pattern, a “0” means that that the symbol shall be deleted and a “1” means that a symbol shall be passed.

2.3 Channel Interleaver and Bit Demultiplexing

The interleavers are introduced to enhance the resistance of the waveform to short-term fading and

medium-term shadowing impairments in satellite and terrestrial channels. The DVB-SH, as a DVB-H, is using bit-wise interleaving. The output of the turbo encoder shall be bit interleaved using a block interleaver. The values for block interleaving are given in [4] and each code rate has own bit-wise interleaver function.

Before the symbol interleaving, the input stream is demultiplexed (and mapped to output modulation symbols) into sub-streams, depending of the modulation used: = 2 for QPSK and = 4 for 16QAM. In contrast with DVB-H the modulation 64QAM is not used in DVB-SH. Each sub-stream from the demultiplexer is then interleaved with own interleaving sequence called permutation function.

2.4 Symbol Interleaver Symbol interleaving is performed as bit-wise

interleaved substream. The purpose of the symbol interleaver is to map bit words onto the 756 (1k mode) 1512 (2k mode), 3024 (4k mode) or 6048 (8k mode) active carriers per one OFDM symbol. As it can be seen, DVB-SH is using more types of the OFDM modes in comparison with DVB-H. This new mode called 1k was added to support higher speeds and/or smaller bandwidths. The symbol interleaver acts on blocks of 756, 1512, 3024 or 6048 data symbols. After previously mentioned FEC blocks and interleavers are ready to modulate symbols into QPSK or 16QAM constellations.

2.5 M-ary QAM Modulation MATLAB functions and tools support some cases for the modulation and demodulation. In the developed application the modem.qammod (M) function was used from Communication Toolbox. This function also enables set up of which symbols will be mapped. However, the symbol order does not match the mapping in the DVB-SH specification [4]. The solution of this problem was described in [8] and it was applied in this application too.

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-1 0 1 2 3 4 5 6C/N [dB]

BER

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QPSK - AWGN; CR=1/5QPSK - AWGN; CR=2/9

Fig. 2. Simulated dependence of BER on C/N ratio after

turbo decoding (It =1, mode: 1k, QPSK).

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0 2 4 6 8 10 12 14C/N [dB]

BER

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16QAM - AWGN; CR=1/516QAM - AWGN; CR=2/9

Fig. 3. Simulated dependence of BER on C/N ratio after turbo decoding (It =1, mode: 1k, 16QAM).

2.6 Frame Adaptation and OFDM Transmission frame adaptation block has to divide

modulated stream, carrying useful data, into OFDM symbols and adds the TPS, scattered, pilot and zero carriers. This block is simple, as the only purpose is to rearrange the data and insert special carriers on their defined positions [4].

2.7 Guard Interval Insertion Once we have OFDM symbols assembled,

guard interval can be inserted. According to the DVB-SH specification the options are 1/4, 1/8, 1/16 and 1/32 of the symbol period. End part of each symbol is copied to the beginning of the previous symbol.

2.8 D/A Conversion and Carrier Modulation Now, complete OFDM signal in the frequency domain is transferred into the time domain by using IFFT (Inverse Fast Fourier Transformation) function. Real Re(t) as well as imaginary Im(t) part of the OFDM signal is upsampled and filtered with Raised Cosine filter with roll-off factor equals to 0.35.

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-1 0 1 2 3 4 5 6C/N [dB]

BER

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QPSK - AWGN; CR=1/5QPSK - AWGN; CR=2/9

Fig. 4. Simulated dependence of BER on C/N ratio after turbo decoding (It =2, mode: 1k, QPSK).

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0 2 4 6 8 10 12 14C/N [dB]

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16QAM - AWGN; CR=1/516QAM - AWGN; CR=2/9

Fig. 5. Simulated dependence of BER on C/N ratio after turbo decoding (It =2, mode: 1k, 16QAM).

2.9 Channel Simulation At this point we have now prepared DVB-SH-A

signal, which can be transmitted by using transmission channel model. There exist several types of channel models (AWGN, Rice, Rayleigh) in MATLAB Communication Toolbox, which can we used for the examination how the signal behaves in the transmission environment. In this paper for the simplified simulation there was chosen AWGN channel, which characteristics are simulated by using the MATLAB function awgn.

3. Simulation The complete functional scheme of the DVB-SH-A

transmitter has been described and it has been implemented in MATLAB. The method of the FEC, used in the DVB-SH was described in [4]. On the other hand, the turbo code decoding process is not described. Decoding systems are based on a SISO (Soft-Input Soft-Output) decoding or LLR (Log Likehood Ratios) and other methods [7]. In this paper for the turbo decoding of received data the SISO method was used. For the simulation of the DVB-SH channel encoder and decoder there was used following settings:

mode: 1k (satellite),

code ratio: 1/5 and 2/9 (robust transmission),

non-hierarchical modulation: QPSK, 16QAM,

transmission channel: Gaussian (AWGN),

turbo decoding method: SISO.

number of iterations: 1 and 2

4. Results Dependence of the BER on C/N ratio as a result of the

simulation in the Gaussian channel is shown in Fig. 2 to 5. The Fig. 2 shows the dependence of the BER after turbo decoding on C/N ratio in case of the QPSK modulation. The Fig. 3 shows the same dependence but in case when 16QAM modulation was used. How it can be seen in these figures, there were obtained two BER curves: first one with coding rate 1/5 and second one with 2/9. Both of coding rates (CR) represent higher redundancy of the transmission data. However, the time for the simulating of the transmission is longer. The obtained BER with using these code rates is very low. This is because the modulation QPSK is the most robust to noises and fadings while the modulation 16QAM with higher number of states is less robust than the modulation with less number of states. To achieve a very low BER (approx. 3.10-4) for QPSK modulation it is needed C/N = 1.3 dB in 1k OFDM mode. For 16QAM the ratio is equal to 5.6 dB. In case of the CR = 2/9 it is approx. 2.5 dB for QPSK and 8.1 dB for 16QAM.

One of the main advantages of turbo decoders are that they are enabled to set the number of decoding processes. More precisely, thanks for the drawback between the decoders; the turbo decoder can use the iterative method [7]. The number of repetitions of the decoding processes for one value of C/N is so called number of iteration (“It” – in the Fig. 2 to 5). The iteration of decoding is adopted to improve the performance in differential modulation coded systems. The positive impact of this on the BER can be seen in Fig. 4 to Fig. 5. In all cases the number of turbo decoding iterations is equal to two. To achieve BER = 3.10-4 for QPSK it is needed C/N = 0.35 dB (when the CR = 1/5) and 1.6 dB (when the CR = 2/9). For 16QAM modulation there were obtained better results too. To achieve BER = 3.10-4 it is needed C/N = 4.35 dB (when the CR = 1/5) and 5.5 dB (when the CR = 2/9).

5. Conclusions The application that allows simulation of the DVB-

SH standard (in fact DVB-SH-A) transmission was developed and described in the paper. Innovations of this standard (turbo encoding, OFDM mode 1k, turbo decoding) were slightly modified (for the simulation) and implemented in MATLAB as it is specified in the DVB-SH

documentation [4]. Dependence of the bit error rate after turbo decoding on carrier-to-noise ratio was explored. Differences between the numbers of iterations in turbo decoding processing were also investigated and discussed. Results for all types of DVB-SH modulations were obtained and discussed in the separate paragraph.

This work will continue by finishing the MATLAB application with GUI (Graphical User Interface) and then by improving the turbo encoder/decoder and transmission channel model used in this application. Future work will be also focused on the possibilities to implementation the multipath channel models.

Acknowledgements This paper was supported by the grant project of the

Czech Science Foundation no. 102/08/H027 “Advanced methods, structures and components of the electronic wireless communication”, no. 102/10/1320 “Research and modeling of advanced methods of image quality evaluation” (DEIMOS) and by the Research program of Brno University of Technology no. MSM0021630513, “Electronic Communication Systems and New Generation Technology (ELKOM)” and by the FRVS 1303/2011/G1.

References [1] BORKO, A., SYED, A. Handbook of Mobile Broadcasting, DVB-H,

DMB, ISDB-T and MEDIAFLO. Taylor & Francis Group, LCC, 2008, 726 p., ISBN 978–1-4200–5386-9.

[2] ETSI, Technical Specification 102 584, Digital Video Broadcasting (DVB); Guidelines for Implementation for Satellite Services to Handheld devices (SH) below 3GHz, v.1.1.1, 2008.

[3] ETSI, Technical Specification 102 585, Digital Video Broadcasting (DVB) System Specifications for Satellite services to Handheld devices (SH) below 3 GHz, v.1.1.1, 2008.

[4] ETSI, European Standardization 302 583, Digital Video Broadcasting (DVB); Framing Structure, channel coding and modulation for Satellite Services to Handheld devices (SH) below 3 GHz, v.1.1.2, 2010.

[5] BENMAYOR, D., et al. Rate-Compatible Irregular Repeat-Accumulate Codes for DVB-SH Applications. On International Workshop on Satellite and Space Communications IWSSC 2008. Toulouse (France), 2008, p. 239-243.

[6] POLÁK, L., KRATOCHVÍL, T. Simulation of the DVB-H Channel Coding and Transmission in MATLAB. In Proceedings of the 20th Conference Radioelektronika 2010. Brno (Czech Republic), 2010, p. 57-60.

[7] HANZO, L., LIEW, T.H., YEAP, B.L. Turbo Coding, Turbo Equalisation and Space-Time Coding for Transmission over Fading Channels. John Wiley & Sons, LTD, 2002, 748 p., ISBN 0-470-84726-3.

[8] ŠTUKAVEC, R., KRATOCHVÍL, T. Simulation and Measurement of the Transmission Distortions of the Digital Television DVBT/H Part2: Hierarchical Modulation Performance. Radioengineering, 2010, vol. 19, no. 3, p. 429-436. ISSN: 1210-2512.