Design, Simulation and Hardware Implementation of a Digital Television System: System Overview

Transcription

1 2006 IEEE Ninth International Symposium on Spread Spectrum Techniques and Applications Design, Simulation and Hardware Implementation of a Digital Television System: System Overview (Invited paper) José M. C. Brito, Luciano L. Mendes, Fabbryccio A. Cardoso, Carlos A. F. Rocha and Dalton S. Arantes Abstract In the year 2005 the Brazilian government suppted many research constia in der to develop an advanced Digital Television System employing the most recent technologies f multimedia broadcasting. One of the proposals f the physical layer of this system was entitled Innovative f the Brazilian Digital TV System (D). The D Project includes high perfmance err crecting codes, transmit spatial diversity and multi-carrier modulation. The aim of this paper is to present an overview of this Innovative System. The building blocks of the system, its characteristics and most relevant innovations are presented. The perfmance of the whole system under different channels is compared with the perfmance of the present-day Digital Television standards. Keywds Digital Television, Channel Coding, OFDM, Spatial Diversity, SBTVD. I. INTRODUCTION The currently available Digital Television standards have been conceived with technologies of the 90's. Since then, however, several imptant contributions have been proposed f the new generations of digital wireless systems. When these techniques are applied to the presentday Digital Television standards, a system with significantly higher capacity and robustness can be obtained. Advanced channel coding, modulation systems with spatial diversity and efficient video compression techniques are the key points to achieve higher capacity in a Digital Television system. This paper presents the most relevant technical innovations that were used in the implementation of the proposed wireless communication system f DTV broadcasting. The system is quite flexible and several different configurations allow transmission f indo, outdo and mobile reception. The innovations reside mainly on the Low Density Parity Code () and the Space-Time Code (STC) to allow transmission diversity. José M. C. Brito and Luciano L. Mendes are with the National Institute of Telecommunications INATEL, Santa Rita do Sapucai, MG, Brazil (brito@inatel.br, lucianol@inatel.br). Fabbryccio A. C. M. Cardoso and Dalton S. Arantes are with the State University of Campinas - UNICAMP, Campinas, SP, Brazil (cardoso@decom.fee.unicamp.br, dalton@decom. fee.unicamp.br). Carlos A. F. Rocha is with the Federal Univ. of Santa Catarina, UFSC, Flianópolis, SC, Brazil (aurelio@eel.ufsc.br). This wk was suppted by FINEP Financiada de Estudos e Projetos and by FAPESP Fundação de Amparo à Pesquisa do Estado de São Paulo. The main motivations that guided the development of the present DTV system are: The SBTVD (Sistema Brasileiro de TV Digital Brazilian Digital Television System) should have characteristics that facilitate the integration of services, such as and other multimedia services, in der to mitigate the so-called digital divide. The system should have high digital capacity, allowing the transmission of High Definition TV (HDTV) multiple programs in Standard Definition TV (SDTV). The system should provide mobile phone reception using in-band transmission. Signals f fixed reception and mobile reception should co-exist through the same 6 MHZ channel. This requirement is imptant since it allows the broadcasters to develop new business models f Digital Television. This paper is ganized as follow. Section 2 presents a sht introduction to the current Digital Television standards; Section 3 presents the characteristics of the current system developed f the SBTVD; Section 4 shows the perfmance of the proposed system under different channels and, finally, Section 5 presents our conclusions. II. CHARACTERISTICS OF THE CURRENT DTV STANDARDS In this section we discuss the three Digital Television standards available today, the ATSC (Advanced Television system Committee) developed in USA, the DVB-T (Digital Video Broadcasting - Terrestrial) developed in Europe and the ISDB-T (Integrated Services Digital Broadcasting - Terrestrial) developed in Japan. The main characteristics of these standards are presented in the next subsections. A. ATSC The ATSC standard has been developed by a group of companies called Grand Alliance. This standard was adopted in USA in 1994 and its main characteristics are presented in Table I [1]. ATSC does not suppt hierarchical transmission, a feature suppted by the two other standards. The signal to noise ratio threshold f acceptable quality is around 15 db in AWGN (Additive White Gaussian Noise) channel. No other standard presents better perfmance in AWGN /06/$ IEEE 193

2 channel but, on the other hand, the perfmance of the receiver under dynamic multipath channel is compromised, since the ATSC has not been designed f mobile reception. TABLE I CHARACTERISTICS OF THE ATSC STANDARD. 8-VSB (Vestigial Side Band) Inner Code TCM 2/3 Outer Code Reed Solomon (207,187,10) Total Symbol rate 10,76 Mbauds Data bit rate 19,28 Mbps B. DVB-T DVB-T standard has been developed in Europe to attend the requirements of all the European countries. Thus, the flexibility of the system has been an initial requirement of the project. The main characteristics of the DVB-T are presented in Table II [2]. The main reason f the use of Coded Orthogonal Frequency Division Multiplexing is the robustness of this scheme under frequency selective channels. If the total time delay spread of the channel is shter than the guard time interval, then the received signal will not suffer intersymbol interference (IIS). COFDM is also used in other high data rate digital standards, such as Wi-MAX [3] and Wi-FI [4]. Due to the robustness of the COFDM technique under multipath channels, DVB-T can be used in Single Frequency Netwks (SFN). TABLE II CHARACTERISTICS OF THE DVB-T STANDARD. Multiplex COFDM (2k and 8k), Inner code Convolutional 1/2, 2/3, 5/6, 7/8 Outer code Reed Solomon, 7MHz 8MHz Data bit rate () Min: 3.73Mbps - Max: 23.7Mbps DVB-T allows hierarchical transmission of up to 2 data streams, which can be used f different applications. This flexibility offers new business models f the TV broadcasters. One option is to use one data stream to broadcast video and audio while the other may transmit data associated with the scene. This allows an interactivity of the user with the scene. Another application is to use one data stream to broadcast SDTV while the other is used to transmit the enhanced layer f the HDTV signal. Thus, the users that are capable to receive both streams can watch the program in HDTV, while the users that can receive only the SDTV stream watch the program in standard definition. DVB-T also presents some problems that must be addressed. OFDM technique results in high Peak to Average Power Ratio (PAPR), which means that the power amplifiers must operate with a high back-off in der to avoid signal clipping. This back-off reduces the efficiency of the amplifier and increases the overall cost of the transmitter. One solution to mitigate this problem is to use signal pre-disttion to avoid high power peaks. The perfmance of DVB-T under mobile channels does not seem satisfacty, because apparently there is no configuration that allows reception in presence of large Doppler spread. C. ISDB-T ISDB-T has been developed in Japan and adopted in this country in This standard has been based on DVB-T, but some features have been added in der to improve the perfmance, especially f mobile reception. The flexibility of ISDB-T has also increased, due to a new concept f hierarchical transmission based on frequency segmentation. In this case, the total channel is divided in 13 independent segments that can be dynamically grouped to transmit up to 3 different data streams. Table III presents the main characteristics of ISDB-T standard [5]. TABLE III CHARACTERISTICS OF THE ISDB-T STANDARD. Multiplex COFDM (2k, 4k and 8k) D, Inner code Convolutional 1/2, 2/3, 5/6, 7/8 Outer code Reed Solomon Data bit rate per segment Min: 280.8kbps - Max: 1.79Mbps III. CHANNEL CODING AND MODULATION SYSTEM FOR THE D This section presents the main characteristics of the channel coding and modulation system that have been proposed f the SBTVD, called D. The main innovations are the f inner code and STC (Space Time Coding) f transmission diversity. Since is such a highly efficient err crection code, it is possible to obtain perfmance close to Shannon s theetical limit [6]. STC is a technique introduted by Alamouti in 1998 that uses up to two transmission antennas and multiple receiving antennas to obtain space-time diversity. This scheme is highly robust against Doppler spread, which makes the system very efficient f mobile reception. Figure 1 presents the block diagram of the transmitter while Figure 2 presents the block diagram of the receiver. The outer code is the same Reed Solomon used in DVB-T and ISDB-T. The inner code is a with codewd length equal to 9792 and code rates 1/2, 2/3, 5/6 and 7/8. The same band segmentation used in ISDB-T has also been adopted here to allow higher flexibility f the broadcasters, while the modulations are, and. This set of modulations allows the broadcasters to define the trade-off between system throughput and robustness. The matrix interleaver between the inner and outer codes improves the perfmance of the RS decoder. The STC associated with OFDM results in a robust system f mobile and selective channels. The use of two transmission antennas provides diversity gains of der 2 when the receiver has only one antenna, which is an interesting scenario f mobile reception. Table IV summarizes the main characteristics of the proposed system. The overall perfmance of the system will be presented in the next Section and the solution proposed in this paper will be compared with the currently available standards. 194

3 TABLE IV CHARACTERISTICS OF THE D. Multiplex COFDM (2k and 8k), Inner code /2, 2/3, 5/6, 7/8 Outer code Reed Solomon Data bit rate per segment Min: 280.8kbps - Max: 1.79Mbps Diversity scheme STC-OFDM 4, 8 12 wds Number of streams up to 3 IFFT 1/4, 1/8, 1/16, 1/32 Up-Converter Space- Time Encoder IFFT 1/4, 1/8, 1/16, 1/32 Up-Converter Pilot Carriers Figure 1 Block diagram of the transmitter. Signaling Carriers and Sync Info. Detect Detect Detect Multiplexing Channel Estimation Space- Time Decoder FFT Transmission Parameters Recovering Removing Sync Recovering Tunner Figure 2 Block diagram of the receiver. IV. ANALYSIS OF SYSTEM PERFORMANCE The perfmance of the system has been analyzed f different channel models by simulation, including AWGN and Brazil A to E [7]. These channels represent different reception conditions, such as indo and outdo reception. Figure 3 presents the perfmance of the system in AWGN channel f different combinations of modulation and code rate. From this figure it is possible to conclude that the system can operate with a carrier to noise ratio equal to 15.4 db f 64-QAM and code rate 3/4. With this configuration it is possible to achieve 19Mbps of data rate, which is sufficient to suppt HDTV. If SDTV is being transmitted, it is possible to use modulation and code rate 1/2, to achieve a CNR of only 1.3 db. Note that the configuration and code rate 7/8 is not interesting, since it is possible to achieve a better perfmance and a higher bit rate with and code rate 1/2. Figure 4 compares the perfmance of the proposed system with ATSC standard in AWGN channel. Notice that both systems present almost the same perfmance. It is imptant to emphasize, however, that the results obtain f the ATSC standard are measured, while the results f the D are simulated. Canal AWGN após Reed-Solomon - R:1/2 - R:7/8 16-QAM - R:1/2 16-QAM - R:3/4 64-QAM - R:1/2 64-QAM - R:3/ C/N (db) Figure 3 Perfmance under AWGN channel. From Figure 5 it is possible to conclude that, f Brazil-A channel and f a given, the D requires a CNR which is 6 db smaller than that f the ISDB-T. This 195

4 indicates that ISDB-T requires 4 times the power required by D to achieve the same video quality. This large gain in efficiency resulted from the joint action of STC- OFDM and. This gain increases even further when a me adverse channel is used. This fact can be verified in Figure 6 f Brazil-B channel and in Figure 7 f Brazil-E channel. Canal AWGN D - 64-QAM - R:3/4 8-VSB-ATSC Zenith 8-VSB-ATSC Zenith Chip-T C/N (db) Figure 4 Perfmance of ATSC and D in AWGN channel. ISDB-T Figure 5 Perfmance of ISDB-T and D in Brazil-A channel. From Figure 6 one can see that the gain obtained by the D over ISDB-T is approximately of 8dB f Brazil-B channel. Figure 7 shows that the gain of the MI- SBTVD over DVB-T in Brazil-E channel is above 18dB. Figure 5 presents the perfmance of the D and ISDB-T in Brazil-A channel. V. IMPLEMENTATION ASPECTS The D system has been successfully implemented in FPGA transmitter and receiver prototypes as a proof of concept f the technological solution proposed to the Brazilian Digital TV System. The transmitter prototype has been developed using one Altera Stratix II EP2S60 FPGA. This device represents a very small cost of a commercial product. F example, a 5kW commercial transmitter costs around US$ 100,000.00, while such an FPGA chip costs around US$ It is then reasonable to assume that a commercial solution f the transmitter will be implemented using FPGA. On the other hand, the receiver has to be implemented in ASIC to take advantage of a large production scale and to have an industrial price compatible with currently available commercial set-top-boxes. However, the prototype to provide a proof of concept f the proposed system has been ISDB-T Figure 6 Perfmance of ISDB-T and D in Brazil-B channel. DVB-T Figure 7 Perfmance of DVB-T and D in Brazil-E channel. implemented using one Altera Stratix II EP2S60 and two FPGA Xilinx Virtex 4 SX35. In a later development stage the receiver prototype will be implemented over an ASICbased solution. The first proof-of-concept prototype was designed using three FPGA development kits, as shown in Figure 8. The proposed demodulation solution f the D was implemented with a Xilinx Virtex 4 SX35. This solution comprises a digital IF stage, symbol and frequency-offset synchronization, channel estimation, OFDM-based demodulation and Alamouti space-time decoding. The channel decoding was implemented with another Virtex 4 SX35. Interleaving, descrambling, RS decoding and Transpt Stream recovery were implemented with Altera Stratix II. Xilinx s DSP (Digital Signal Process) Design Flow has proven to be very effective f fast prototyping. System Generat has been used to design and to generate VHDL (Very high-speed integrated circuit Hardware Description Language) code f demodulation receiver and f 196

5 decoding. System Generat is a development tool designed f Matlab/Simulink. Complex DSP systems can be designed by building Simulink block diagrams through specific Xilinx blocksets. After translating a Simulink project into a VHDL project, then synthesis, design implementation, simulation and bitstream generation can be accomplished. The device utilization summary is presented in Tables V, VI and VII, f receiver s proof of concept, and Table VIII f the transmitter. TABLE V VIRTEX IV S RESOURCE UTILIZATION SUMMARY FOR DEMODULATION AND SYNCHRONIZATION CIRCUITRY. # Occupied Slices % # BRAM (FIFO16) % # DSP48s % TABLE VI- VIRTEX IV S RESOURCE UTILIZATION SUMMARY FOR DECODER CIRCUITRY. # Occupied Slices % # BRAM (FIFO16) % # DSP48s % Tuner decoder AD Converter Connect J8 Connect J27 Nallatech s Development Kit IV Connect Xilinx Virtex IV SX35 J8 Nallatech s Development Kit IV Connect Xilinx Virtex IV SX35 J10 Altera Stratix II s Development Kit Altera Stratix II EP2S60 Figure 8 Receiver layout. Connect J6 TABLE VII- STRATIX II S RESOURCE UTILIZATION SUMMARY FOR RS, DESCRAMBLER AND TRANSPORT STREAM RECOVERY. # ALUTs ,352 5% # Total RAM bits 1,025,920 2,544,192 40% # DSP block 9 bits % TABLE VIII- STRATIX II S RESOURCE UTILIZATION SUMMARY FOR TRANSMITTER. # ALUTs 42,863 48,352 88% # Total RAM bits 1,579,392 2,544,192 62% # DSP block 9 bits % Figures 9 and 10 show, respectively, the transmitter and receiver assembly used in the first proof of concept f the D project in February IF and baseband stages were implemented using Xilinx FPGAs as mentioned befe. The RF (Radio Frequency) stage was assembled f the transmitter front-end using mixers, power amplifiers and oscillats to transmit a DTV (Digital Television) signal on a MHz carrier. The RF stage f the receiver front-end was assembled using a down-converter test equipment designed by Linear Equipments Ltd. The IF stage has been designed to receive an analog signal centered at MHz and sampled at a rate at least 4 times this frequency. The clock reference f the design was provided by an external source of MHz. VI. CONCLUSIONS The system proposed in this paper, as part of the SBTVD Program, presents a better perfmance when compared to the current Digital Television standards. The gains achieved by the D vary between 4dB and 18dB, depending on the channel profile. The use of and STC-OFDM has proved to be a very efficient combination to achieve high throughput at low carrier-noise ratio, without a significant increase in the total cost of the receiver. Figure 9 - Transmitter assembly with two antennas f the D lab prototype. Figure 10 - Receiver assembly f the D lab prototype. REFERENCES [1] ATSC Document A-54, Guide to the Use of the ATSC Digital Television Standard, [2] European Telecommunication Standard ETS , Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation f digital Terrestrial television (DVB-T), ETSI, [3] H. Córdova, P. Boets, L. Van Biesen, "Insight Analysis into Wi- MAX Standard and its trends", WWAN2005, [4] Amundsen, "IEEE Wireless LAN - Draft Standard", 2 nd IEEE Wkshop on Wireless LANs, Oct [5] ARIB, "Terrestrial Integrated Services Digital Broadcasting (ISDB- T). Specification of Channel Coding, Framing Structure and " September, [6] S. Benedetto and E. Biglieri, Principles of Digital Transmission: With Wireless Applications, Plenum Pub. Cp, pg. 232, [7] AT/SET, Brazilian Tests on Digital Television Systems - Final Rept, May,