DVB-H and DVB-SH-A Performance in Mobile and Portable TV

Transcription

1 VOL. 2, NO. 4, DECEMBER 211 DVB-H and DVB-SH-A Performance in Mobile and Portable TV Ladislav Polák, Tomáš Kratochvíl Department of Radio Electronics, Brno University of Technology, Purkyňova 118, 612 Brno, Czech Republic Abstract This paper deals with the examination of the transmission distortions of the latest digital TV standards DVB-H (Digital Video Broadcasting to Handhelds) and DVB- SH (Satellite to Handhelds) in portable and mobile TV over fading channels. These channel model profiles respect Doppler s shift, which is occur thank to the movement of the receiver. For the comparison of the transmission distortions in DVB-H/SH-A in mentioned channel models an appropriate application was developed in MATLAB. Simulation results of the BER (Bit Error Ratio) after Viterbi decoding (DVB-H), after turbo decoding (DVB-SH-A) and MER (Modulation Error Ratio) on C/N (Carrier-to Noise Ratio) for QPSK and 16QAM modulations were obtained, compared and evaluated. Moreover, typical results and the examples or illustration constellation diagrams for the various transmission scenarios and channel types are also presented. Finally, achieved results are evaluated and clearly discussed. 1 Introduction The number of multimedia applications and their possibilities (mainly mobile TV), which are integrated in present mobile phone terminals, has been increased significantly over time. Therefore, demands for mobile reception of high quality video, audio and data broadcasting services are rapidly increasing. Access to TV and multimedia services on mobile terminals is already possible via an UMTS (Universal Mobile Telecommunication Systems) connection. However, after a certain time there was shown that, this solution has many disadvantages (problems with ensuring of good video and image quality, problems with optimal mobile network architecture and problems with sufficient capacity) [1]-[3]. In an era of digital TV broadcasting there exist several types of DVB (Digital Video Broadcasting) standards, which define the methods of framing, coding and modulation of the broadcasted mobile TV [3]. In case of mobile TV, the standards DVB-H and DVB-SH have been developed [4], []. DVB-H (Handheld) [6], [7] is still the leading global technology standard for the transmission of digital TV to handheld receivers such as mobile and mobile phones, terminals and PDAs. DVB-H substantially comprises of a set of extensions to DVB-T which are oriented to handheld mobile (terminal) use. DVB-H has all the benefits of the standard DVB-T and adds new, mobile-oriented features, focusing on IP datacasting and including better mobility support (4k mode), adaptive perservice error protection and power saving capabilities (time slicing) [1]. DVB-SH (Satellite to Handhelds) [] [6] is a relatively new standard provides an efficient and flexible mean of carrying broadcast services over the hybrid satellite and terrestrial infrastructure operating in S-band, ( ) GHz, to a variety of portable, mobile and fixed terminals having compact antennas with very limited or almost no directivity. Target terminals include handheld defined as light-weight and battery-powered apparatus (e.g. PDAs, mobile phones), vehiclemounted, nomadic (e.g. laptops) and stationary terminals (settop-boxes) [1]. Standard DVB-SH complements and improves the existing DVB-H standard. This improvement pushes the limits on the possibilities of DVB broadcasting to handheld terminals. It allows mobile TV transmission in two principle modes: OFDM (Orthogonal Frequency Division Multiplexing) for satellite and terrestrial mode and TDM (Time Division Multiplexing) for satellite mode). More details are available in [4], [8], [9]. With focus on mobile and portable TV implementation aspects it is most important to determine the reception environment. Depending on the typical scenarios (mobile, portable and fixed reception) it is used different fading channel models for the exploring of the signal distortions. In our case, we are focused only on the mobile ad portable scenarios. The mobile means reception while moving at high speeds in cars, trains, etc. The portable means that the device can easily be carried or taken from one point to another. In the context of DVB-H/SH, mobile antenna reception is defined as the reception at medium to high speed (approx. from 3 km/h to 1 km/h, e.g. vehicular traffic) and portable antenna reception is defined as the reception at no speed very low speed (approx. 3km/h, e.g. walking speed) [1]-[12]. In this paper the investigation is focused on the comparison of the DVB-H and DVB-SH-A standards (both using classical OFDM) from the perspective of transmission distortions in well known mobile TV fading channels and their models. These channel model profiles, so called (Rural Area), (Typical Urban), (Portable Indoor) and (Portable Outdoor), are originally from the Celtic Wing TV project [1] and defined by COST27 [11]. The contribution contains the results of MATLAB applications which allow DVB-H and DVB-SH-A transmission in and in mobile and portable channels (with Doppler shift) to be analyzed and simulated. The structure of this paper is organized as follows. The fading channel profiles for mobile and portable environment and their main characteristics are introduced in Section 2. The parameters and typical scenarios for the analysis and simulation are presented in Section 3. Section 4 shows the differences between the corrected (after equalization) constellation diagrams in DVB-H/SH-A standards and also includes 44

2 VOL. 2, NO. 4, DECEMBER Fig. 1: Impulse response of the Rural Area channel. Fig. 3: Impulse response of the Pedestrian Indoor channel Fig. 2: Impulse response of the Typical Urban channel. dependences of BER (Bit Error Ratio) after Viterbi decoding (DVB-H), after turbo decoding (DVB-SH) and MER (Modulation Error Ratio) on C/N ratio. Finally, achieved results are compared, evaluated and discussed in Section. 2 Mobile and Portable Channel Profiles Distribution of DVB-H/SH-A programs by way of terrestrial transmitters is the classical technology of broadcasting. Received signal should be interpreted as the overall effect, the sum of various influences created by noise and interference [3]. These scenarios are modeled by fading channel models, which have generally three important parameters: delays, path losses and the type of Doppler spectrum (Gaussian, Ricean or Rayleigh distribution) [1], [12]. The channel profiles and their typical impulse responses, which were used for this simulation, are shown in Fig. 1 to Fig. 4. More details about the implementation (with examples of source codes for the MATLAB simulation) of the fading transmission channel models, which will be briefly described below, can be found in [12]. Fig. 4: Impulse response of the Pedestrian Outdoor channel. 2.1 Gaussian Channel () Model of the Gaussian channel describes a case, which is based on a direct signal path from transmitter to receiver only. In this case the received signal is only attenuated and includes a definite level of noise. This channel is overlaid with (Additive White Gaussian Noise), which is mainly produced in the receiver itself [1], [13]. 2.2 Mobile Fading Channels In the course of research projects [1], [11], two channel profiles were selected to reproduce the DVB-H/SH-A service delivery situation in a mobile environment. channel profile reproduces the terrestrial propagation in a rural area. It has been defined by COST27 [11] as a Typical Rural Area profile and is made of 6 paths having relatively short delay and small power (see Fig. 1). The first part of this channel model has zero delay and attenuation; therefore it is a direct path (red line in the Fig. 1). The speed of the receiver is equal to 1 km/h [1], [7], [12]. mobile channel profile reproduces the terrestrial propagation in an urban area. It was originally defined by COST27 [11] as a Typical Urban profile and is made of 6 paths having wide dispersion in delay and relatively strong 4

3 VOL. 2, NO. 4, DECEMBER 211 power (see Fig. 2). Values of the parameters fluctuate dynamically following a Rayleigh law. The speed of the receiver is equal to km/h. This channel profile has been proven to present fairly well the general mobile DVB-T reception by several field tests [1], [7], [12]. 2.3 Portable Fading Channels For the portable transmission scenarios, two channel profiles were selected to reproduce the DVB-H/SH-A service delivery situation in a portable environment [1]. The (Pedestrian Indoor) and (Pedestrian Outdoor) channel models have been developed by the Wing-TV [1] project for describing the slowly moving (at speed approx. 3 km/h) handheld indoor and outdoor reception. Both channels consist of 12 independent paths, from which the first path is the direct (see Fig. 3 and Fig. 4) [1], [7], [12]. The main difference between these channel models is in the lengths of the impulse responses and the delays of output paths. In both channels, the influence of attenuation is significant; therefore the has worse results in simulation. 3 Simulation Parameters Brief description of the most used mobile and portable channel profiles for exploring the signal distortion during the transmission was presented in the previous chapter. Standard DVB-SH, in comparison with the standard DVB- H, offers more types of CR (Code Rate), therefore the settings of the transmission modes varies. For both investigated scenario (mobile and portable), in DVB-H has set the CR to 2/3. This CR represents the case of quite robust transmission and it is typically used in real transmission system configurations. In the standard of DVB-SH this CR represents the lowest robustness of the transmission. Therefore, the CR for DVB-SH was set to 2/7. The settings, which were used for the simulation of the transmission of the DVB-H/SH-A standards, are clearly described in Table 1. Tab. 1: Settings Used for the Simulation of the DVB-H/SH-A Mobile and Portable TV Transmission Settings DVB-H DVB-SH-A Code Rate 2/3 2/7 Modulation OFDM Guard Interval Channel Models Method of Decoding QPSK (mobile), 16QAM (portable) 2k (mobile), 4k (portable) 1/8 (large network), 1/16 (mid network) and (mobile), and (portable) Viterbi (hard decision) SISO (8 iterations) For the mobile TV transmission in the mobile scenario, 2k OFDM mode and QPSK modulation were used (for mobile TV reception and minimizing of the frequency shift). For the transmission in the portable scenario, 4k OFDM mode and 16QAM modulation were used. 4 Simulation results and their evaluation Simulation results of the data transmission in the standards DVB-H/SH-A for varying C/N ratio in the Gaussian channel (), in the mobile (, ) and in the portable (, ) fading channels were obtained. As it was mentioned before, standard DVB-H has been partly developed from the standard DVB-T. Therefore for the evaluation of BER it can be used the QEF (Quasi Error-Free) operation. The QEF operation is defined as a bit error rate after Viterbi decoding less or equal to [3], [12]. Then the BER ratio after Reed-Solomon decoding is less or equal to Standard DVB-SH [8], [9] for the FEC uses turbo coding and advanced channel and time interleaving. Therefore for the evaluation of the BER it can not be used the QEF operation. The limit of the error-free reception is considered for C/N, where the BER is equal to after the turbo decoding [9]. The Fig. (Fig. 7) and Fig. 6 (Fig. 8) illustrate BER after Viterbi (turbo) decoding in Gaussian and four fading channel models and for mobile scenario, and for portable scenario. And of course, obtained BER for modulations QPSK and 16QAM in various types of transmission scenarios are different. For the data transmission in the mobile scenario, and fading channel model were used. Both channel profiles, and, are generally different (see Fig. and 7). Channel consists of one direct path and five reflected (echo) paths. Moreover, in the the expected speed of the receiver is equal to 1 km/h, so the Doppler shift is two times higher than in ( km/h) [12]. Of course, in case of QPSK modulation for achieving the minimum BER it is needed small value of C/N. This is because the modulation QPSK is the most robust to noise and fading. On the other hand, the useful data rate is very low, because modulation QPSK allows transmission of two bits in one symbol. For achieving the QEF (after Viterbi decoding) in case of the DVB-H transmission using QPSK modulation in mobile fading channel it is needed C/N = 4.2 db in 2k OFDM mode. In channel it is needed C/N = 8.6 db. DVB-SH standard is used advanced FEC processing (turbo coder/decoder, channel and time interleaver). Thanks for this innovations, the minimal C/N, which is necessary for a good signal quality reception, is smaller. Moreover, thank to the turbo decoding rules and very effective decoding algorithm SISO (Soft Input Soft Output), the decoding processes can be repeated. In this paper, the number of iterations (number of repeated decoding processes) is equal to eight (8) as it is recommended in [9]. For BER = [9] in case of QPSK modulation in mobile fading channel it is needed C/N =.4 db in 2k OFDM mode. In channel it is needed C/N = 2. db. 46

4 VOL. 2, NO. 4, DECEMBER 211 1,E+ 1,E-1 1,E+ 1,E-1 1,E-2 1,E-2 1,E-3 1,E-3 1,E-4 1,E-4 1,E- 1,E- 1,E ,E Fig. : Mobile reception scenario (QPSK, mode 2k, CR 2/3, GI 1/16) and DVB-H performance (BER after Viterbi decoding as a function of C/N ratio) in mobile channel. Fig. 7: Mobile reception scenario (QPSK, mode 2k, CR 2/7, GI 1/16) and DVB-SH-A performance (BER after turbo decoding as a function of C/N ratio) in mobile channel. 1,E+ 1,E-1 1,E+ 1,E-1 1,E-2 1,E-2 1,E-3 1,E-3 1,E-4 1,E-4 1,E- 1,E- 1,E ,E Fig. 6: Portable reception scenario (16QAM, mode 4k, CR 2/3, GI 1/8) and DVB-H performance (BER after Viterbi decoding as a function of C/N ratio) in portable channel. Portable channel profiles and have one direct path, which is shifted in frequency by half of the maximum value of the Doppler shift. The value of the Doppler shift is the same for both channels, because the speed of the receiver, in both channel profiles, equals to 3 km/h. The main difference between these channel models is in the length of the impulse response and the delay of output paths (see Fig. 3 and Fig. 4) [12]. The 16QAM modulation supports a higher bit rate, on the other side the modulation 16QAM with higher number of states is less robust than the modulation with less number of states. However, modulation 16QAM allows transmit two times higher useful data in comparison with QPSK. This advantage is very important in case of portable transmission. For achieving the QEF in case of the DVB-H transmission, using 16QAM modulation in mobile fading channel, it is needed C/N = 12.2 db in 4k OFDM mode. In channel it is needed C/N = 16.2 db (see Fig. 6). For achieving the BER = (after turbo decoding) in case of the DVB-SH-A transmission it is needed less C/N ratio. The reason is again the advanced FEC scheme, which is used in the standard DVB-SH. The minimal C/N ratio, which is needed for achieving of the mentioned BER value, is Fig. 8: Portable reception scenario (16QAM, mode 4k, CR 2/7, GI 1/8) and DVB-SH-A performance (BER after turbo decoding as a function of C/N ratio) in portable channel. C/N = 6.9 db in case of channel model. In the channel this value is a little higher, C/N = 8.4 db (see Fig. 8). The MER (Modulation Error Ratio) is a measure for evaluation used to quantify the performance of a digital transmitter or receiver in a communications system using digital modulation [3]. In area of digital video broadcasting MER is a measure of the sum of all interference effects occurring on the transmission link. Dependences of the MER on C/N in the systems DVB-H (see Fig. 9 and Fig. 1) and DVB-SH (see Fig. 11 and Fig. 12) were obtained. The value of MER is depending on the selected type of modulation and on the CR. The minimal value, which it is necessary to achieve for the QEF, is marked in the graphs by dashed line. How it can be seen in the Fig. 9 and Fig. 11, the values, which are necessary for achieving the min. MER in channel model in standards DVB-H/SH-A, are different. These differences are caused by mainly the different CR (and also system configuration), which was used in the standards DVB-H/SH-A. On the other side, when the transmission channels and were explored, the MER is a little worse in the standard DVB- SH-A. The main reason, except of the mentioned reasons, is that the 16QAM is less resistant to the multipath fadings. 47

5 VOL. 2, NO. 4, DECEMBER Fig. 9: Modulation error ratio MER in the DVB-H data transmission as a function of C/N ratio in the Gaussian channel and in the fading channels,. Fig. 11: Modulation error ratio MER in the DVB-SH-A data transmission as a function of C/N ratio in the Gaussian channel and in the fading channels, Fig. 1: Modulation error ratio MER in the DVB-H data transmission as a function of C/N ratio in the Gaussian channel and in the fading channels,. Tab. 2: Comparison of the Simulated Results C/N for the BER Equals to1 - in Standards DVB-H/SH-A Scenario Mobile Portable Modulation and type of Channel QPSK 16QAM DVB-H C/N [db] DVB-SH-A C/N [db] The differences in BER results for the various transmission scenarios and channel types for both standards are also easy to compare in Table 2. Typical results and the examples of the constellation diagrams for the various transmission scenarios and channel types are also easy to see in the Fig. 13 a) to f) for DVB-H standard and in the Fig. 13 g) to l) for DVB-SH-A standard. Fig. 12: Modulation error ratio MER in the DVB-SH-A data transmission as a function of C/N ratio in the Gaussian channel and in the fading channels,. Figure 13 a) shows constellation diagrams in channel (as reference) and b) and c) case of mobile scenario within typical DVB-H fading channels. After the receiving of data from the fading channel the IQ diagram is going to high distortions. This is indicated by lower MER. Moreover, thank to the presence of Doppler s shift, rotation between symbols within IQ diagram is also occurred. In channel the speed of receiver is 1km/h. Therefore, the Doppler s shift should cause the higher distortions. On the other hand, the direct path between the transmitter and receiver is also available in this channel model, so the effect of Doppler s shift is minimized (see Fig. 13 b)). In channel the situation is opposite (see Fig. 13 c)). The Doppler s shift is half in comparison of channel, but we have only indirect paths with high attenuations and delays. Figure 13 d) shows constellation diagrams in channel (as reference) and e) and f) in case of portable scenario within typical DVB-H fading channels. How it was described in previous chapter, in and channel, the speed of receiver is very slow (3 km/h). Therefore, the effect of Doppler s shift is low. On the other hand, paths delays and attenuation are higher, which make higher distortions and lower MER in IQ diagrams (see Fig. 13 d) to f)). 48

6 VOL. 2, NO. 4, DECEMBER 211 a) channel, MER = 1.2 db; b) channel, MER = 13. db; c) channel, MER = 7.2 db; d) channel, MER = 1.1 db ; e) channel, MER = 9.4 db; f) channel, MER = 8.3 db; g) channel, MER = 16.4 db; h) channel, MER = 13.8 db; i) channel, MER = 1. db ; j) channel, MER = 1.8 db; k) channel, MER = 9.8 db; l) channel, MER = 8.8dB Fig. 13: I/Q constellation of: a) to c) mobile scenario with QPSK (DVB-H), d) to f) portable scenario with 16QAM (DVB-H), g) to i) mobile scenario with QPSK (DVB-SH-A), j) to l) portable scenario with 16QAM (DVB-SH-A) - all within typical DVB- H/SH-A fading channels and all the constellations incl. channel correction and removed pilots, C/N = 1 db. 49

7 VOL. 2, NO. 4, DECEMBER 211 Conclusion In this paper, the transmission distortions of the standards DVB-H and DVB-SH-A in mobile and portable TV over fading channels were explored. Mobile TV fading channels, which also respect the Doppler shift, were presented graphically and described. Presented results can be used for the transmission distortion revising and evaluation of the DVB- H/SH system parameters influence on transmitted signal of the mobile TV. For the comparison of the signal distortion in different transmission channel models in standards DVB-H/SH-A, two typical scenarios for mobile and portable channel were used. The settings, which were used for the analyzing and simulation in MATLAB, were presented in the Section 3. Dependences of the BER after Viterbi decoding (DVB-H) and after turbo decoding (DVB-SH-A) on C/N ratio were explored for the Gaussian channel, as the reference, and for four main portable and mobile TV fading channels (,, and ). Obtained results were evaluated and discussed in the Section 4. How it can be seen, there are very big differences between the results. In case of the DVB-SH the results are much better than when DVB-H standard was used for the transmission. Thanks for the advanced FEC (turbo coding, channel and time interleaver), which is used in the system DVB-SH-A, for achieving very low BER it is needed lower C/N ratio. The process of turbo decoding was repeated eight (8) times as recommended in [9]. The differences in BER results for the various transmission scenarios and channel types for both standards are also easy to compare in Table 2. According dependences of the MER on C/N were obtained too for both standards. MER is descending by decreasing value of C/N in all cases. Generally, higher MER means less fading in the transmission. How it can be seen from the results presented in Fig. to 8, the DVB-SH offers a very good solution for the mobile TV broadcasting. On the other hand, mobile phones, which are available on the market (for the reception of the signal in DVB-H standard), should be innovated. Therefore, the development of smart and fast hardware devices is unavoidable. Acknowledgments This paper was supported by the Research programme of the Brno University of Technology no. MSM216313, Electronic Communication Systems and New Generation Technology (ELKOM) and project FEKT-S-11-12, Signal processing in mobile and wireless communication systems (MOBYS), grant projects of the Czech Science Foundation no. 12/1/132, Research and modeling of advanced methods of image quality evaluation (DEIMOS), no. 12/8/H27 Advanced Methods, Structures and Components of Electronic Wireless Communication and no. CZ.1.7/2.3./2.7 Wireless Communication Team (WICOMT), financed from the operational program Education for competitiveness, is gratefully acknowledged. References [1] LÁK, L., KRATOCHVÍL, T. DVB-H and DVB-SH- A Performance and Evaluation of Transmission in Fading Channels. In 34 th International Conference on Telecommunications and Signal Processing (TSP 211). Budapest (Hungary), 211, pp [2] KORNFELD, M., DAOUD, K. The DVB-H mobile Broadcast Standard. IEEE Signal Processing Magazine, 28, pp [3] FISHER, W. Digital Video and Audio Broadcasting. A Practical Engineering Guide. 2 nd ed. Springer, pages. ISBN [4] BORKO, F., SYED, A. Handbook of Mobile Broadcasting, DVB-H, DMB, ISDB-T and MEDIAFLO. Taylor & Francis Group, LCC, pages. ISBN [] KUMAR, A. Implementing Mobile TV: ATSC Mobile DTV, MediaFLO, DVB-H/SH, DMB, WiMAX, 3G Systems, and Rich Media Applications (2 nd edition). Oxford: Focal Press, pages. ISBN [6] EN V1.1.1 (24-11). Digital Video Broadcasting (DVB); Transmission System for Handheld Terminals (DVB-H), European Standard ETSI, 24. [7] TR V1.4.1 (29-6). Digital Video Broadcasting (DVB); Implementation guidelines for DVB handheld services (DVB-H), Technical Report ETSI, 29. [8] EN V1.1.2 (21-2). Digital Video Broadcasting (DVB); Framing Structure, channel coding and modulation for Satellite Services to Handheld devices (DVB-SH) below 3GHz, European Standard ETSI, 21. [9] TS V1.2.1 (211-1). Digital Video Broadcasting (DVB); Guidelines for Implementation for Satellite Services to Handheld devices (DVB-SH) below 3GHz, Technical Specification ETSI, 211. [1] SR, D1, CP2-32, Services to Wireless, Integrated, Nomadic, GPRS-UMTS & TV handheld terminals, Wing TV, 26. [11] COST27, Digital land mobile radio communications (final report), Commission of the European Communities, Directorate General Telecommunications, Information Industries and Innovation, [12] LÁK, L., KRATOCHVÍL, T. Simulation and Measurement of the Transmission Distortions of the Digital Television DVB-T/H Part 3: Transmission in Fading Channels. Radioengineering, 21, vol. 19, no. 4, p [13] LÁK, L., KRATOCHVÍL, T. DVB-T/H Digital Television Transmission and its Simulation over Ricean and Rayleigh Fading Channels. In Elektrorevue ( 21, vol. 21, no. 74, pp. 1-7.