Hierarchical Modulation & SFN 數位電視系統原理及軟體技術 銘傳大學 : 陳游利 元智大學 : 黃依賢 1 DVB-T Transmitter Diagram MPEG-2 System Channel Coding Modulation 2
Agenda DVB-T Transmission System DVB-T Coded OFDM Tutorial DVB-T Hierarchical Modulation DVB-T SFN Operation 3 High Priority & Low Priority(1/2) If hierarchical modulation is used, the DVB-T modulator has two Transport stream inputs and two FEC blocks. One transport stream with a low data rate is fed into the so-called High priority path (HP) and provided with a large amount of error protection, e.g. by selecting the code rate 1/2. A second transport stream with a higher data rate is supplied In parallel to the low priority (LP) and is provided with less error protection, e.g. with the code rate 3/4. 4
High Priority & Low Priority(2/2) In principle, both HP and LP transport streams can contain the same programs but at different data rates, i.e. with different amounts of compression. On the high priority path, QPSK is used which is a particularly robust type of modulation. On the low priority path, a higher level of modulation is needed due to the higher data rate. 5 DVB-T Hierachical Modulation Hierachical Modulation» two transport streams with high and low priority can be transmitted» the high priority path uses QPSK modulation» the low priority path uses 16 QAM or 64 QAM» hierachical modulation is done by a embedded QPSK inside QAM» there are 2 FEC's COFDM MPEG-2 TS Low Priority Path MPEG-2 TS High Priority Path energy disperal energy disperal outer coder RS outer coder RS outer interl. outer interl. par. par. ser. ser. inner coder convolutional coder with pucturing inner coder demux bit interleaver mapper frequ. interleaver IFFT I Q FIR guard intervall insert. I Q IQ mod. IF RF like DVB-C frequency domain time domain like DVB-S Block Diagram DVB-T Modulator 6
DVB-T Hierachical Modulation Q I α = 2 Hierachical Modulation» portable reception» mobile reception 64 QAM hierarchical continous and scattered pilots TPS Carrier Data Carrier» receiver can select between high and low priority modulation» high priority QPSK modulation is embedded inside low priority modulation (QAM) with = 1, 2, 4» the high priority modulation defines the sector inside constellation diagram 7 DVB-T Hierachical Modulation It can help avoid total signal disruption in areas with low field strength by providing a lower rate, more robust bit stream as part of the transmitted signal. Characteristics of the DVB-T hierarchical modulation to allow broadcasting, on the same radio frequency channel, of two independent MPEG-Transport Streams. to give a dedicated protection, then a dedicated coverage, to each transport stream. LP stream is more sensitive to distortion and requires high C/N. HP stream is more robust and needs lower C/N. Example is HDTV to fixed external antenna receivers and rugged embedded SD service for portables. 8
DVB-T Hierarchical Constellations Another way to use one RF channel A basic 4 QAM named HP (High Priority) is used as a core modulation... 4QAM - HP 4QAM - HP 9 DVB-T Hierarchical Constellations Then, a Low Priority stream (LP) over-modulates the HP one to constitute a... HIERARCHICAL 16 QAM made of 4QAM (LP) over 4QAM (HP) HIERARCHICAL 64 QAM made of 16QAM (LP) over 4QAM (HP) 10
Non-uniform 16 QAM Mapping with α = 2 Hierarchical modulation: High priority (HP) rugged, low-bit rate service to portable or even mobile receivers (SDTV) Low priority (LP) service in the same signal can carry a high bit rate service to roof top antennas (HDTV) 11 Non-uniform 64 QAM Mapping with α = 4 12
Embedded QPSK in 64-QAM A 64-QAM modulation enables 6 bits per symbol to be transmitted. However, since the quadrant information, as QPSK, diverts 2 bits per symbol for the HP stream, only 4 bits per symbol remain for the transmission of the LP stream. The gross data rates for LP and HP thus have a fixed ratio of 4:2 to one another. 13 Hierarchical constellations : Alpha factor to reduce HP-4QAM penalty REG a=1 a=2 a=4 REG a=1 a=2 a=4 REG a=1 a=2 a=4 HIERARCHICAL 16 QAM made of 4QAM (LP) over 4QAM (HP) 0 3 6 9 Alpha = 41 2 12 14
Hierarchical constellations : LP-16QAM has same C/N than 64QAM HIERARCHICAL 64 QAM made of 16QAM (LP) over 4QAM (HP) 0 3 R64 a=1 a=2 R16 R64 a=1 a=2 R16 R64 a=1 a=2 R16 6 9 12 15 18 21 24 27 Alpha = 4 21 30 15 DVB-T : Hierarchical modulation for what? To define modulation parameters, the network planning follows usually these rules : to reach a given bitrate, Physical Modulation & Coding rate are determined, 2K / 8K format is selected relative to the transmission cell sizes, Guard Interval value is chosen relative to the nature of the terrain (urban, rural, etc.) The Hierarchical Modulation feature offers a further trade-off : to broadcast TWO services on ONE radio frequency channel 16
Hierarchical Modulation : Stationary vs Portable receivers REG 64QAM 2/3 HP 4QAM 1/2 LP 16QAM 2/3 0 LP REG HP 3 6 9 12 15 18 21 24 17 Hierarchical Modulation : More bitrate globally REG 64QAM 2/3 HP 4QAM 3/4 LP 16QAM 3/4 LP REG HP 0 3 6 9 12 15 18 21 24 27 30 18
Hierarchical Modulation: Mobile vs Static reception Channel RF2 RF1 + RF2 REGULAR LP for Fixed for FIXED Channel RF1 + RF2 RF1 HP REGULAR for Mobile for MOBILE 0 6 12 18 24 30 36 42 48 54 REG RF1 REG RF2 HP RF1.1 RF1.2 LP HP RF2.1 LP RF2.2 16Q 64Q 4Q 16Q 4Q 16Q 1/2 2/3 1/2 3/4 1/2 3/4 19 DVB-T Hierarchical Modulation : Conclusions The flexibility of the DVB-T system allows the broadcaster to trade ruggedness against bit-rate. Hierarchical modulation adds further liberty to adapt RF parameters to Services Regular or Hierarchical modulation? it s your choice! 20
Agenda DVB-T Transmission System DVB-T Coded OFDM Tutorial DVB-T Hierarchical Modulation DVB-T SFN Operation 21 Single-Frequency Networks The modulation scheme offered by the COFDM allows to operate DVB-T as SFN SFNs can offer improved spectrum efficiency» examples for DAB, DVB-T SFN requires an RF channel free over whole service area of network» reason why not chosen for UK DVB-T best results with a dense network of lower-power transmitters 22
Golden Rules of DVB-T SFN Setup an SFN imply to control transmitters in Time & Frequency domains which is to radiate On the SAME FREQUENCY, At the SAME TIME, the SAME DATA BIT(s) To make receivers working with SFN signals, all transmitters involved in the Single Frequency Networks shall respect the above SFN Golden Rules 23 Spectrum & Power Efficiency High Power Transmitters + Less Sites in Network + Lower initial Investment -Lower On-air Redundancy -Higher operational costs due to lower Power Efficiency Low Power Transmitters -More Sites in Network -Higher initial Investment + Higher On-air Redundancy + lower operational costs due to higher Network Efficiency 63 dbµv/m 24
SFN, the receivers point of view... Channel Impulse Response Channel Spectrum To make receivers working with SFN signals, all transmitters involved in the Single Frequency Network shall respect the SFN Golden Rules : To radiate. on thesame FREQUENCY,. at the SAME TIME,. the SAME DATA BIT(s). 25 SFN Golden rules on the same frequency COFDM modulator GPS 10 Mhz Up converter COFDM receiver GPS 10 Mhz Up converter COFDM modulator F0 in few Hertz! frequency frequency 26
SFN : Frequency domain constraints DVB-T (8 MHz) DAB-T (1,5 MHz) Modes 8K 2K I IV II III Subcarriers / symbols 6817 1705 1536 768 384 192. Used for system 769 193 - - - -. Used to carry data 6048 1512 1536 768 384 192 Bits / symbols. D - QPSK - - 3072 1536 768 384. 4 - QAM (QPSK) 12 096 3 024 - - - -. 16 - QAM 24 192 6 048 - - - -. 64 - QAM 36 288 9 072 - - - - Inter-Carrier spacing 1 116 Hz 4 465 Hz 1 khz 2 khz 4 khz 8 khz Absolute precision ~ 1 Hz ~ 4 Hz ~ 10 Hz ~ 20 Hz ~ 40 Hz ~ 80 Hz 27 SFN Golden rules at the same time : T0 in few µs! Primary Distribution Network GPS 1 pps COFDM modulator Up converter COFDM receiver Up converter GPS 1 pps COFDM modulator frequency frequency time time 28
SFN : Time domain constraints DVB-T (8 MHz) DAB-T (1,5 MHz) Modes 8K 2K I IV II III Symbols / frames 68 68 78 78 78 155 Maximum. Used for system - - 2 2 2 2 distance. Used to carry data 68 68 76 76 76 153 between Symbols duration 896 us 224 us 1000 us 500 us 250 us 125 us Tx Guard Interval durations 7 us ~ 3 kms 14 us ~ 6 kms 28 us 28 us 31 us ~12 kms 56 us 56 us 62 us ~24 kms 112 us 123 us ~48 kms 224 us 248 us ~96 kms Delivery Time Accuracy 2..22 us 0..5 us 24 us 12 us 6 us 3 us 29 DVB-T SFN Operation : Conclusions The marvellous modulation offered by the COFDM allows to operate DVB-T as SFN network. Setup an SFN imply to control transmitters in Time & Frequency domains. Numerous DVB-T SFN networks exist today around the word : Sweden, Spain, UK, Taiwan. Improve spectrum & power efficiency with SFNs? it s your choice! 30
SFN Configuration 31 Any Questions? 敬請多 多指教 32