Key-Words: - MER, CBER, COFDM modulation, DVB-T, Frequency spectrum

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1 The analysis of OFDM constellations for Digital Terrestrial Television Iulian UDROIU (1, Ioan TACHE (2,Nicoleta ANGELESCU (1, Ion CACIULA (1 1- "VALAHIA" University of Targoviste, Romania 2- POLITECHNICA University of Bucharest, Romania Abstract: - The analysis of the OFDM (Orthogonal Frequency Division Multiplexing) constellation for the DVB-T (Digital Video Broadcasting Terrestrial) signal are studied in this article. The measurements were made on the Romanian territory in two locations with and without obstacles. The measured quality parameters of the DVB-T signal are: CBER (BER before Viterbi), the signal power and MER (the modulation error). The results show that the DVB-T signal had a better quality and the COFMD (Coded Orthogonal Frequency Divisions Multiplex) constellation is less affected by the noise. Key-Words: - MER, CBER, COFDM modulation, DVB-T, Frequency spectrum 1 Introduction Digital Video Broadcasting-Terrestrial (DVB-T) standard began to be adopted in many countries as efficient transmission technology for the broadcasting of the analogical TV channels [1]. The implementation of DVB-T standard in Romania can provide interactive services to consumers like pay-per-view, pay-tv, superteletext etc. In Romania, the switch-off from analog TV to DVB-T standard is planned for In [2], the authors optimize experimentally by means of a cross layer approach [3], the performance of DVB-T systems affected due to the effects of channel interference. In [3], the authors present an experimental analysis of the effects which produce in-channel interference and affect some performance parameters in DVB-T systems. The optimization on some performance parameters in DVB-T system is simultaneously performed on three layers: PHY, Dl, and AP. In this paper, a study of OFDM constellations for DVB- T signals is experimentally investigated using guidelines from [5]. The purpose of this study is the analysis of OFDM constellation and parameters in DVB-T system in the presence of in-channel interference and to offer some solutions for the optimization of DVB-T systems. 2 Details of the experiment The DVB-T standards are based on COFDM (Coded Orthogonal Frequency Division Multiplexing) modulation that has several variants: 16-QAM (Quadrature Amplitude Modulation), 64QAM and QPSK (Quadrature Phase Shift Keying). The COFDM scheme is better than OFDM scheme, because it has error protection included in the first subcarrier [7]. The bandwidth of DVB-T channel can be 6, 7 or 8 MHz. There are two operating modes: the 2k mode and 8k mode. The length of symbols in 2k mode is 250µs, respectively 1ms in the 8k mode. Table 1 summarizes the main characteristics of COFDM scheme. Table 1. The COFDM general characteristics Characteristics DVB-T No. of carriers subcarriers in 2k DFT subcarriers in 8k DFT Modulation QPSK,16QAM or 64-QAM schemes Capacity Mbps Channel spacing B 6,7 or 8 MHz Spectral regions VHF/UHF band Technology Broadcasting technique Our experiment measures the following parameters : Signal/Noise Ratio (C/N), modulation error ratio (MER) and the signal power. C/N ratio is defined by a signal having the same bandwidth between the ratio of the modulated signal power and the equivalent noise power. It is better if the values of the C/N ratio are higher than the quality of the received DVB-T signal. 3 Experimental results The locations for the measurements of the DVB-T signal are in the rural area near Bucharest, on Bucharest- Ploiesti direction (Herastrau and ProTV station) and in the inner part of Bucharest at 1.5 km distance of emitter. The terrain is flat in the rural area, without major difference of levels, the main obstacle being the forest around Bucharest. Within Bucharest, the buildings are the main obstacles. For the experiment it was used a PROMAX TV Explorer II+ device with a DVB-T demodulator. The measurements are taken using a set of antennas of Televes type. The characteristics of ISSN: ISBN:

2 transmitter and reception antenna are described in the Table 2. Table 2. Emitter and Antenna receiver details Emitter details: Herestrau /ProTv Emitter power 1.3 KW/ 1KW DVB standard DVB-T COFDM Geographic Coordinates of emitter 44 28'37"N / 26 3'9"E 44 26'27"N/26 7'46"E Antenna receiver details Frequency bandwidth Antenna gain UIF Internal antenna amplifier gain UIF Noise factor Maximal level UIF ( Mhz ) 4-7 db 12dB <2 db 102 dbuv Table 4. The measurement values for the DVB-T channels Parameters C30 C54 C59 Dist[Km] C/N[dB] >29.5 >20.7 > MER[dB] Power[dBuV] CBER 1.5E-3 1.4E-2 3.3E C/N[dB] >21.3 >18.2 > MER Power[dBuV] CBER 1.9E-2 6.1E-2 6.6E The Fig. 1, Fig. 2 and Fig. 3 contain the OFDM diagram constellation for the channels 30, 54 and 59 at the distance of d=1.5km. The Fig. 4, Fig. 5 and Fig. 6 contain the OFDM diagram constellation for the channels 30, 54 and 59 at the distance of. We measured the quality of the DVB-T signal from three TV channels: the channel 30 with the frequency f=546 MHz, channel C54 with frequency f=768 MHz and channel 59 with the frequency f=778mhz. The broadcasting parameters of the TV channels are given in the Table 3. Table 3.The broadcasting parameters of the TV programs Channel C30 C54 C59 Network ProTV SRTV SRTV Video MPEG- 4 MPEG- 2/4 MPEG-2 Audio MPEG- 2 MPEG- 1/2 MPEG-1 Frame 50Hz 50Hz 50Hz Bandwidth 8Mhz 8Mhz 8Mhz Interval guard 1/16 1/16 1/16 Carriers 8k 8k 8k Rate code 2/3 2/3 2/3 Modulation 16QAM 64QAM 64QAM The measurements tried to establish the conditions of reception in the studied areas, the evaluation being made by analyzing constellation diagrams for the DVB-T transmissions in UHF channels 30, 54 and 59 on the subcarrier 6816 of every channel and the signal power for every channel was analyzed at 1.5 Km distance, respectively 7.5 Km. Table 4 contains the values measured for every DVB-T analyzed channel. Fig 1. OFDM diagram constellation for the channel 30 at the Fig 2. OFDM diagram constellation for the channel 54 at the ISSN: ISBN:

3 Fig 3. OFDM diagram constellation for the channel 59 at the Fig 6. OFDM diagram constellation for the channel 59 at the distance of d=7.5 km The Fig. 7, Fig. 8 and Fig. 9 contain the frequency spectrum for the channels 30, 54 and 59 at the distance of d=1.5km. The Fig. 4, Fig. 5 and Fig. 6 contain the frequency spectrum for the channels 30, 54 and 59 at the distance of. Fig 4. OFDM diagram constellation for the channel 30 at the distance of d=7.5km Fig 7. Frequency spectrum for the channel 30 at the distance of d=1.5km Fig 5. OFDM diagram constellation for the channel 54 at the distance of d=7.5km Fig 8. Frequency spectrum for the channel 54 at the distance of d=1.5km ISSN: ISBN:

4 Fig 9. Frequency spectrum for the channel 59 at the distance of d=1.5km Fig 10. Frequency spectrum for the channel 30 from distance Fig 11. Frequency spectrum for the channel 30 from distance Fig 12. Frequency spectrum for the channel 30 from distance For the channel 30, the maximum signal power is 54.9 dbµv, for the signal reception from the distance d=1.5 Km and 45.8 dbµv for the distance d=7.5 Km. For the channel 54, the maximum signal power is 46.6 dbµv for the signal reception from the distance d=1.5 Km and 42.7 dbµv for the distance d=7.5 Km. For the channel 59, the maximum signal power is 57.9dBµV for the signal reception from the distance d=1.5 Km and 49 dbµv for the distance d=7.5 Km. In the ideal case, the spectrum is constant in the all frequencies interval. The acquired spectrum has many differences due to the linear distortions in the entire broadcasting channel. The DVB-T signal uses the COFDM modulation scheme which provides a good reception of the TV content with low levels of the noise and distortions. The measured values for the DVB-T channels from the Table 4 confirm the good reception of the DVB-T channels. The error correction systems remove the all false bits from the data stream. In Fig. 1 and Fig. 4 are represented the OFDM diagram constellations for the channel C30 with 16QAM and the corresponding parameters. The DVB-T signal for the channel C30 is little affected by noise. In Fig. 2 and Fig. 5 are shown the OFDM diagram constellations for the channel C54 at the different distances and zones. The constellation doesn't differ very much, because the noise levels are close. In Fig. 3 and Fig. 6 are shown the OFDM diagram constellations for the channel C59 at the different distances and zones. The OFDM diagram constellation from the Fig. 3 highlights that the DVB-T signal has a greater noise level than the DVB-T signal measured from the distance d=7.5 Km due to the obstacles which are represented by the buildings in the center of Bucharest. ISSN: ISBN:

5 4 Conclusion In Romania, the DVB-T standard is implemented according to [1]. The quality of the received DVB-T signal is good under various conditions. The measured values of the DVB-T signal parameters confirm the affirmation above. The using of the COFDM scheme is obviously better than the using of analogical transmission, because due to MPEG-2 or MPEG-4 compression schemes, more TV programs can be broadcasted on one TV channel, so the frequency spectrum is efficiently allocated. References: [1] Digital Video Broadcasting (DVB); Implementation Guidelines for DVB Terrestrial Services; Transmission Aspects, Nov v ETSI TR [2] L. Angrisani, M. Vadursi,Performance Assessment of Wireless Communication Networks through Cross-Layer Measurements, Proc. of IMTC 2006, Sorrento, Italy, April 24-27, 2006,pp [3] L. Angrisani, M. Farias, D. Fortin, A. Sona, Performance assessment and optimization of DVB-T systems affected by in-channel interference, IMTC IEEE Instrumentation and Measurement Technology Conference Warsaw, Poland, May 1-3, 2007 [4] Leopoldo Angrisani, Mario Farias, Daniele Fortin, Alessandro Sona, Experimental Analysis of In- Channel Interference Effects on the Performance of a DVB-T System, Ieee Transactions On Instrumentation And Measurement,Vol. 58, No. 8, August 2009 [5] Digital Video Broadcasting (DVB);Meaurement guidelines for DVB systems, ETSI TR V1.2.1, [6] W.Fisher, Digital Video and Audio Broadcasting Technology:A practical Engineering Guide-Second Edition, Springer-Verlag Berlin Heidelberg,2008 [7] József Biró, Endre Borbély, DVB-T Systems in Hungary,3rd Slovakian-Hungarian Joint Symposium on Applied Machine Intelligence,Herl any, Slovakia, January 21-22, 2005 ISSN: ISBN: