Laboratory platform DVB-T technology v1

Transcription

1 Laboratory platform DVB-T technology v1 1. Theoretical notions Television can be defined as a set of principles, methods and techniques used for transmitting moving images. The essential steps in television broadcasts are the emission (when the signal corresponding to the image is formed), the transmission (the transmission of the signal obtained in the emission) and reception (the reconstruction of the image based on the received signal). Based on experiments started in the late nineteenth century and continued in the early twentieth century, the first commercial (analogue) television systems appeared in the late 20s of last century. The development of integrated circuits and of the digital signal processing techniques have led to appearance in the 80s of last century of the first intermediate television systems, which contain both analog and digital elements. 1.1 DVB DVB (Digital Video Broadcasting) is a suite of internationally accepted standards for digital TV. The DVB Project was officially inaugurated in September Digital video broadcasting systems use different communication channels: terrestrial (DVB-T), satellite (DVB-S), cable (DVB-C) and handheld portable devices (DVB-H). These standards define the physical and data link layers of the distribution system. Devices interact with the physical substrate through a synchronous parallel interface, a serial synchronous interface or a parallel asynchronous interface. All data is transmitted in MPEG streams, with some additional constraints (DVBMPEG). Transmission systems vary by the type of modulation and error-correcting codes that are used because of the various technical constraints. DVB-S uses QPSK modulation, 8-PSK or 16-QAM. DVB-S2 uses QPSK, 8PSK, 16APSK or 32APSK, at the choice of the distributor. DVB-C (in VHF-UHF bands) uses 16-QAM, 32-QAM, 64-QAM, 128-QAM or 256-QAM. DVB-T (in the VHF / UHF) uses 16-QAM, 64- QAM or QPSK in combination with COFDM. The DVB-T2 standard has been adopted by ETSI (European Telecommunications Standards Institute) on 9 th of September The standard provides greater robustness to the TV reception and increases the possible bit rate by 30%. The DVB standard allows the transmission of data streams at a speed of 38 Mbit / s with a satellite or cable channel, and at the speed of 24 Mbit / s through a terrestrial radio channel. On the other hand, the video signal created in the TV studio has 166 Mbit / s, which cannot be transmitted. Source coding is a must for digital television as it achieves data compression by eliminating redundant information. 1

2 A fundamental decision in the evolution of DVB was marked by selecting standards MPEG-2 and MPEG- 4 for encoding the audio and video source signals. Using MPEG-2 allows the compression of the video signal at a rate of 5 Mbit / s, which can be restored at reception, providing an image quality close to analog television. Using the MPEG-4 standard is recommended for HD broadcasters. Digital television has advantages over the analog one. The transmission of programs shows greater robustness to interference, with practically no "salt and pepper" disturbances on the screen. When the signal is drowned in noise or when the received signal is too weak, the image "freezes". By coding the image with MPEG4 or H.264, channels can be transmitted in full-hd (1080p). DVB provides quality television services, so it is necessary to adopt it in as many countries as possible and to abandon the analog television services. DVB-S and DVB-C were ratified in 1994 and DVB-T in The first country where DVB-T transmissions were conducted was the UK in late The first country that withdrew from analog television broadcasts was Germany, in Currently, most European countries have abandoned the PAL SECAM system and adopted digital television with DVB. European Union members are forced to abandon analogue TV by 2015 and implement DVB services. Countries that are not part of the European Union announced a voluntary transition to DVB. DVB is used in Europe, Africa, Australia, partly in Africa and Asia. Fig. 1.1: Worldwide use of terrestrial digital television broadcasting The map in Fig 1.1 presents the status of worldwide adoption of terrestrial digital television broadcasting standards. European states, Australia, few countries in South America and the vast 2

3 majority of Asian and African states have adopted DVB-T and/or DVB-T2 transmission standards or have announced the transition to such standards in the next years. Situation is similar for the case of cable television standards DVB-C or DVB-C DVB - T By means of this standard, compressed audio/video streams are transmitted using OFDM modulation (Orthogonal frequency-division multiplexing) with channel coding (Coded OFDM). Instead of a single radio frequency carrier in one radio channel, COFDM operates by dividing the data stream in multiple streams with lower data transfer rate, each one digitally modulating a set of adjacent subcarriers. The two main modes of DVB-T OFDM transmissions are 2K and 8K - the names refer to the number of points used to calculate FFT (2048 or 8192). In fact, only 1705 and 6817 subcarriers are used, separated by an interval of 4 KHz or 1 KHz. Optionally, the standard specifies another mode, 4K, designed for specific DVB-H transmissions. The parameters of the three transmission modes are detailed in the table below. Tabel 1.1 : OFDM modes parameters OFDM Parameters 2K 4K 8K Total no. of carriers No. of modulated carries No. of data carriers Duration of an OFDM symbol (µs) Guard interval (µs) 7, 14, 28, 56 14, 28, 56, , 56, 112, 224 Carriers spacing (KHz) Maximum coverage (km) The operations performed by blocks within a DVB-T transmitter are explained in the following. 3

4 Fig. 1.2 Block diagram of a DVB-T transmitter Source coding and MPEG-2 multiplexer multiplex the compressed video signal, the compressed audio signal and the data stream. This is the basic digital stream that is sent or received. Allows a data transfer speed that can vary between 5 to 32 Mbps. The splitter allows two MPEG streams to be transmitted simultaneously using a technique called hierarchical transmission: one stream has high priority and the other low priority. It can be used to transmit a SDTV (standard definition) signal and HDTV (high definition) signal on the same carrier. Generally, the SDTV signal is more robust than the HDTV signal. Thus, at reception, should the HDTV signal not be decoded due to weak received signal, SDTV signal can be decoded. The external encoder enables a first error correction of data transmitted using a Reed- Solomon block code that allows correction of up to 8 wrong bytes for each 188 bytes of the packet. External interleaving (of convolutional type ) is used to rearrange the transmitted data sequence in such a way as to make it more resistant to long sequences of errors. The internal encoder is the second level of error correction and is provided by a convolutional code that is called FEC (Forward Error Correction). Five code rates are used: 1/2, 2/3, 3/4, 5/6 and 7/8. During internal interleaving the data sequence is rearranged again in order to reduce the influence of errors. This time a pseudo random scheme is adopted as a technique of interleaving the blocks of data. For coding, a digital bit sequence is mapped into a complex symbol sequence modulating the baseband. Three modulation schemes are used: QPSK, 16-QAM and 64-QAM. Adapting the frame is achieved by grouping symbols into blocks of constant length (1512, 3024 or 6048 symbols per block). A frame has a length of 68 blocks and a superframe consists of 4 frames. To simplify the reception of the radio signal transmitted through the terrestrial channel, additional signals are inserted in each block. The pilot signal is used during synchronization and equalization, while the signal with transmission parameters TPS (Transmission Parameters Signaling) 4

5 sends the parameters of the transmitted signal and identifies the transmission cell. The receiver must be able to achieve synchronization, equalization and signal decoding to obtain access to the information held by TPS signal. Otherwise, the receiver must know this information in advance and then TPS signal is used only in special situations, such as changing parameters and resynchronization. Since the DVB-T transmission system was designed so that digital terrestrial television services can operate within the existing UHF spectrum for analog transmissions, it is necessary for the system to provide sufficient protection against high levels of co-channel interference (CCI) and adjacent channel interference (ACI) produced by the existing PAL / SECAM services. Wireless communication networks must achieve a permanent compromise between providing personalized services a single terminal and offering services to a considerably larger number of terminals. For the distribution of multimedia services to a large number of portable devices, characterized by a limited number of resources, it can become problematic. It is therefore important for network operators and content providers and/or services to provide a mechanism to distribute services in a manner to streamline the allocation of bandwidth and power consumption simultaneously. A Multi-Frequency Network (MFN) is a network in which multiple radio frequencies (or radio channels) are used to transmit content. The main advantage of such a configuration is that it prevents the cochannel interference between transmitters. The frequencies can be reused in accordance with a certain spacing between neighboring stations using the same frequency. Analogue terrestrial broadcasting uses MFN. Relying on COFDM modulation properties, DVB introduces a way of optimizing the use of spectrum for DVB-T and DVB-H, called Single Frequency Network (SFN). The image below illustrates this problem: the MFN used 3 different broadcast frequencies, occupying a band of 24 MHz. In SFN, a single frequency is used, leading to an optimization of bandwidth allocation: only 8 MHz. Fig. 1.3 : MFN and SFN topologies comparison In an SFN network type, two or more transmitters in a particular area can operate using the same frequency. This type of network takes into account the TV coverage characteristics, namely that all 5

6 transmitters broadcast the same channel. The challenge therefore derives from the desire of ensuring that all transmitters have all the information necessary to broadcast on the same frequency: signals from each transmitter must be correctly aligned in terms of time, by synchronizing information flow, synchronizing each transmitter using GPS reference, guard interval can be chosen as a compromise between transmission rate and SFN capabilities - as the guard interval is greater, the SFN area can be extended. The advantages and disadvantages of the two types of networks are shown compared in the table below: Table 1.2: Comparison of the advantages and disadvantages of SFN and MFN networks SFN MFN Advantages: Excellent spectral efficiency (low bandwidth); Low power consumption; Homogeneous reception for DVB-H; Suitable for indoor receptions. There are no changes in the antenna system; Facilitates coexistence with analogue channels; Can reuse existing infrastructure in analog transmissions; Multiplexing multiple channels. Disadvantages: Requires a high level synchronization (extra-costs); Expensive infrastructure in order to avoid interference. Spectrally inefficient (although much better than analog broadcasts); Not suitable for DVB-H; The DVB-T system allows the use of different types of QAM modulation and different rates of internal coding to achieve an optimum between bit rate and robustness to noise. The system also allows the use of two levels of the hierarchical coding. In this case the functional block diagram of the system will be expanded to include modules represented by a dotted line in Fig. 1.2 The DVB-T receiver adopts techniques dual to the ones used in the broadcast. Below the operation of the blocks of the receiver is briefly explained. 6

7 Fig. 1.3 Block diagram of a DVB-T receiver - The analog RF signal is converted to baseband by the input blocks and then converted to digital signal using an analog-to-digital converter (ADC). -Synchronization in frequency and time is done by identifying frames and blocks from the baseband digital signal. Frequency errors of the components are corrected. The guard interval or cyclic prefix (the copy of the interval from the end of the OFDM symbol which is placed before the symbol to protect it from interference) is used to detect the beginning of the OFDM symbol. In general, the synchronization is done in two stages, before and after the Fast Fourier Transform (FFT), so as to solve both coarse and fine frequency / time errors. The steps before FFT processing involve estimating the signal correlation in time and steps after FFT use correlation between signal frequency and the sequence of pilot carriers. - Removing the guard interval is done by removing the cyclic prefix. - OFDM demodulation is performed using the FFT. - For equalization in frequency the pilot signals are used to estimate the transfer function of the channel (CTF) at each 3 subcarriers. The CTF is determined by interpolation for the rest of subcarriers and then used for smoothing the data received on each subcarrier. CTF is also used to estimate the accuracy of decoded data provided by the Viterbi decoder. - Internal de-interleaving; - Internal decoding by Viterbi algorithm; - External de-interleaving; - External Decoding - Demultiplexing and decoding of MPEG-2 source 7

8 2. Equipment presentation 2.1. DVB-T/T2 Measurement Receiver ReFeree T2 Referee T2 is a DVB-T2/T receiver, achieving real-time capabilities of broadcasting DVB-T2 and DVB-T and the ability of real-time analysis and recording. Fig. 2.1 : Referee Enensys T2 receiver and the front panel ENENSYS Referee T2 allows monitoring DVB-T2 parameters (RF, T2 frame structure, parameters related to physical layer), the DVB-T2 transmission modes (SFN, MISO, etc.) and DVB-T parameters (RF, TPS MIP). As shown in Figure 2.1, is a multiple-input equipment: RF input (for DVB-T and DVB-T2), ASI output, Ethernet connector and two inputs used in the synchronization process SFN specific (see Chapter 1): 1PPS and 10 MHz (for measurements using external references). The device can receive DVB-T2/T signals, with a bandwidth of 7 or 8 MHz, QPSK, 16-QAM or 256- QAM DiviSuite ENENSYS Test Systems include software support for measurements and statistics calculation using the Referee T2 receiver. The application used throughout the paper is DiviSuite - a complete software suite for testing and monitoring digital television networks, both in the RF and baseband. All the aforementioned interfaces can be managed through this application. DiviSuite provides the following features: TS recording - recording analyzed flow in file format (.TS); TS playback - Playing the flow in.ts format on the ASI output; TS forward redirect the analyzed flow onto the Ethernet interface; audio/video output - decoding services, support for H.265 / HEVC, H.264 / MPEG-4 AVC, MPEG-1/2, AAC, MP3; monitoring the bit rate - gross, net, PID values; 8

9 files with records of measurements. Fig. 2.2 : DiviSuite software interface 2.3. ENENSYS LabMod DVB-T2 Transmitter For the DVB-T2 generation it is used the ENENSYS LabMod T2 modulator, which is designed in order to modulate an MPEG2 stream in an IF or RF DVB signal, or a digital I/Q signal. Fig. 2.3 : ENENSYS LabMod DVB-T2 Modulator 9

10 ENENSYS transmitter allows the choice of modulation (BPSK, QPSK, 16-QAM, 64-QAM, 256-QAM) channel bandwidth (6,7,8 MHZ), the guard interval, FFT mode (1k, 2k, 4k, 4k, 8k, 16k, 32k), coding rate (1/2, 3/5, 2/3, 3/4, 4/5, 5/6). The transmission power may be controlled in the range of 2 dbm and - 20dBm. Range of signal frequencies between 174MHz and 858 ranges MHz. The block diagram of equipment is shown in Figure 2.4. Fig. 2.4 : ENENSYS LabMod DVB-T2 Modulator block scheme One can emphasize a series of facilities intermediated by this equipment: bridging interleaving, generating white noise, carrier rejection, output signal in a wide range of levels, reducing PAPR, bit rate adaptation, and so on. It is worth mentioning that can be individually configured up to 6 transmission channels, with different delays. To access and configure the transmitter, it must be connected to a computer via an Ethernet cable (RJ-45). Use a browser window (web-based), in a manner similar to configuring a Wi-Fi router (see Figure 2.5). 10

11 (a) (b) 11

12 (c) Fig. 2.5 : (a) Modulator s config window; (b) Settings Menu; (c) T2 Frame Menu The application s main screen provides information regarding alarms and device status (the user can program certain alarms). In the setup menu ("Settings"), one can configure a number of parameters, such as the identifier of the cell ("Cell ID"), network ("Network ID"), transmitting frequency, type of network (MFN / SFN) flow type. In the "T2 Frame" menu, one can choose parameters such as modulation type, channel bandwidth, guard interval, FFT mode, and so on DiviPitch By appropriate selection of the desired parameter, the MPEG stream (an audio/video file format) is converted to DVB-T2/T stream and further transmitted. This process is achieved using the DiviPitch software. 12

13 Fig. 2.6 : DiviPitch software interface Figures 2.7 (a) and (b) exemplify various properties such as: (a) flow specific, both in terms of audio and video; (b) file specific, such as type, bit rate, size, duration, etc. (a) Fig. 2.7 : DiviPitch Menu (b) 13

14 3. Laboratory work 3.1 Reception of public DVB-T channels Connect the Enensys Referee receiver to PC1 by two USB cables, to the ports on the left side of the laptop. Connect the antenna to the appropriate connector on the receiver. Launch Divi Suite 1.1 application. Start a DVB-T public signal reception by: channel selection, click the Start button, click the Video button, click the Play button in the Video Output Manager window. This will start the VideoLAN program, which will display the current transmission. To measure the bit rate, click the button Monitoring of Divi Suite and then Bitrate tab. This will displays the bit rate chart. The quantitative differences between net and total rates are to be noted. Fill in the following table twice, for public stations stored on 738 Mhz and 778 MHz. To check the reception quality depending on channel propagation, you will move the receiver antenna according to the situation in the table. DVB-T station Perceived quality of video/audio transmission Net bitrate [Mb/s] Total bitrate [Mb/s] Antenna near window Antenna near door Antenna half/fully unscrewed from receiver connector In which situation is the image lost? 3.2 Transmission of DVB-T2 signals in laboratory Connect to 220 V mains the Enensys DVB-T2 modulator and power it on. Connect the modulator to PC2 with a straight LAN cable, connected to the Control 1 connector. Connect the modulator to PC2 also with a USB cable. Open the management window of modulator via a web browser window using the IP address displayed on the modulator screen (such as ). The IP address of PC2 must be in the same family with the address of modulator. In the management window you can view its parameters (Monitoring window) or you can change the settings (Settings window). Transmit power is changed in Settings, Output, Main RF Output Level. To start broadcasting a DVB-T2 choose the transmission frequency (800 MHz) and channel width (8 MHz) in Settings, Output, respectively Settings, T2 Frame. Then start the DiviPitch 3.1 application. From the window ASI / SPI Pitcher choose with Eject / Open button a file type 14

15 transport stream (.ts) or.mpg, and run with the Play button. Check the transmission start in Monitoring window of modulator Management window. What is the total bit rate? The net rate? Using the DiviSuite application on PC1, view the started transmission. Choose the appropriate channel (800 MHz, DVB-T2) and start displaying the video transmission. Fill in the following table for two video files with different bit rates. File name Net bitrate (Mb/s) Transmitted power (dbm) Quality of audiovideo transmission What is the minimum transmitting power for a continuous reception? Compare the quality of the two receptions with the one obtained from public stations. 4. Preparatory questions 1. What is the 2K mode used in DVB T? How many subcarriers does it use? 2. For what are the pilot signals used by the DVB-T receiver? 3. Explain two advantages of using DVB-T in comparison with use of PAL/SECAM standards? 15