Slide 1

25 July 1998WTD 1 DTTB Transmitter Ratings Compiled by Wayne Dickson SMIREE MIEAust. CPEng. Member SMPTE This presentation seeks to explain the relationship to PAL analogue Transmitters and provides the base for the ratings of DTTB Transmitters. Slide 2 25 July 1998WTD 2 DTTB Transmitter Ratings LINEAR CONSIDERATIONS Slide 3 25 July 1998WTD 3 COFDM Amplitude Distribution RMS voltage = 1 (0dB) Peak voltage =2.8 (9 dB) - COFDM ( for CDF = 95% peak voltage = 1.7 (4.7dB) ) ( CDF - Cumultive Distribution Function ) ( CDF = 99.95%) DIGITAL TV 9 dB peak to average Slide 4 25 July 1998WTD 4 DTTB Peak Amplitude Distribution Slide 5 25 July 1998WTD 5 DTTB Peak Amplitude Distribution (Expanded) 99.95 % CDF Slide 6 25 July 1998WTD 6 PAL-DTTB Relative Levels 0dB 3dB 9 dB Analogue PAL COFDM Average Power Peak PAL/DTTB Ratio = - 6 dB (Peak Sync.) Relative Power Levels for 6 dB (if continuous) Average Power Peak DTTB -2.15dB (Black + Sync.) Difference (in dB) Slide 7 25 July 1998WTD 7 PAL-DTTB Relative Levels 10KW 0 dB 20KW 9 dB Analogue PAL COFDM Average Power Peak (Instantaneous) PAL/DTTB Ratio = - 6 dB (Peak Sync.) Relative Power Levels for 6 dB (if continuous) Average Power DTTB 6KW (Black + Sync.) Difference 20KW 2.5KW (in KW) Peak (Instantaneous) Slide 8 25 July 1998WTD 8 DTTB Rating of a PAL Transmitter To handle a COFDM signal with a 9 dB peak to average character, a PAL transmitter needs to be derated by : 6 dB That is, a 10 KW peak sync transmitter is capable of 2.5 KW COFDM power. Provided linearity is adequate. and other parameters are adequate such as : amplitude and group delay response LO phase noise noise level Slide 9 25 July 1998WTD 9 DTTB Transmitter Ratings NON - LINEAR CONSIDERATIONS Slide 10 25 July 1998WTD 10 DTTB Transmitter Non - linear considerations The non - linear performance is shown by the intermodulation character displayed in the transmitted spectrum. The required intermodulation performance is influenced by the multipath performance of the consumers receivers. The performance of a receiver is influenced by the modulation type eg : whether 64QAM, 16QAM or QPSK and whether FEC is 7/8, 5/6, 3/4, 2/3, or 1/2 Slide 11 25 July 1998WTD 11 DTTB Receiver Multipath Performance An example of a receivers performance operating with modulation of 64QAM and a FEC of 2/3 follows. Differences will occur from : other receiver implementations complex static multipath dynamic multipath eg flutter impulse noise Slide 12 25 July 1998WTD 12 DTTB System Multipath Performance 03 15 30 19 35 Multipath Level ( - dB) COFDM (64QAM,2/3,1/8) C/N Threshold (dB) Indoor Antennas Outdoor Antennas W.T.Dickson 16 April 98 (Conditions: Static simple multipath, No Co-channel or impulse interference) COFDM Current implementations (April 1998) (Nov. 1997) Picture No Picture (above curve) (Below curve) 25 Slide 13 25 July 1998WTD 13 DTTB System Multipath Performance under typical reception conditions 03 15 30 19 35 Multipath Level ( - dB) COFDM (64QAM,2/3,1/8) C/N Threshold (dB) Indoor Antennas Outdoor Antennas W.T.Dickson 16 April 98 Picture No Picture (above curve) (Below curve) 25 Simple multipath Complex multipath (approximate median) Slide 14 25 July 1998WTD 14 DTTB COFDM Decoder Threshold C/N W.T.Dickson 16 April 98 Slide 15 25 July 1998WTD 15 DTTB Transmitter Performance requirements By knowing the worse threshold C/N required by the receiver, the required transmission C/N may be derived. The transmission C/N will be determined by the combination of : the noise floor at the transmitted power level the intermodulation at the transmitted power level Slide 16 25 July 1998WTD 16 DTTB Transmission C/N requirements By observing the previous examples, of a receiver operating in a typical to a worse reception condition, the required C/N by the receiver is varies from approximately 20 dB to 35 dB. Hence the transmitted C/N has to be such that the reception conditions determine the decoding performance, not the transmission conditions. The following plot will be used to derive the required transmission C/N requirements. Slide 17 25 July 1998WTD 17 Transmitter C/N interaction with decoder threshold C/N 0.2dB influence results from a 13dB difference. Hence transmission C/N should be more than 6 dB below decoder C/N for less than 1 dB degrading of decoder C/N Slide 18 25 July 1998WTD 18 The required DTTB Transmitter C/N Considering : (A) @ 35dB required decode C/N, an influence of 1dB by the Transmitted C/N is acceptable. (as such high C/N will not be common the high 1dB influence maybe acceptable) (B) @ 25dB required decode C/N, an influence of 0.2dB by the Transmitted C/N is acceptable. (as a 25dB decoder C/N requirement will be potentially common a small influence is demanded) The Transmitted C/N needs to be : for (A) 41dBfor (B) 38dB Hence 41dB Transmitted C/N appears to be indicated. Slide 19 25 July 1998WTD 19 Transmitter C/N versus Spectrum regrowth Although the spectrum regrowth is a reflection of what is occurring within the band of data, there is a 1 to 2dB higher level of N within the data area. Hence the regrowth level should be allowed to be 2dB higher than the required Transmitted C/N. Aim for a Transmitted C/N of 43 dB. A Transmission C/N of 22 dB is likely to ensure failure of all decoders ! These requirements are for 64QAM @ FEC of 2/3. Slide 20 25 July 1998WTD 20 DTTB Transmission Spectrum COFDM Noise Intermod. Transmission C/N Notes :1. Intermod. level increases at double the rate the input level is increasing. 2. Noise level increases at the same rate the input level is increasing. 3. As COFDM is the same as noise, the display of C/N is independent of the resolution B/W. 4. The shape of the sidebands is influenced by the transmitters response. 5. Either noise or intermod. may dominate. Commonly called : spectrum regrowth or spectrum spread. Spectrum regrowth level Slide 21 25 July 1998WTD 21 DTTB Transmission Implementations Spectrum regrowth or spectrum spread is a mirror into what is happening within the modulated spectrum. Although filtering of the spectrum spread is required for the control of adjacent channel interference, such filtering does not change the the level of intermodulation or noise. Implementations must ensure that under conditions of maintenance or partial failure that the maximum allowable Transmission C/N of 43 dB is not exceeded. Slide 22 25 July 1998WTD 22 COFDM - PAL Spectrum Analyser Display Spectrum Analyser display : DTTB (COFDM) Vision Carrier Sound Carriers Chroma PAL Dependent upon Resolution B/W setting. eg. If Res. B/W = 300KHz A= D + 10Log (6.6 / 0.3) = 23.5 dB A D = DTTB to PAL ratio ( eg D = 10 dB ) DTTB power = average heating power PAL power = the equivalent CW power of peak sync Vision Carrier power Note : It is usually less than this value as resolution B/W shape collects more power than the ideal rectangular filter. When Res. B/W approaches or is less than the separation between the carriers of COFDM : A = D + 10Log (No. of Carriers) Slide 23 25 July 1998WTD 23 DTTB Transmission Spectrum Mask Requirements PAL ADJACENT CHANNEL CONSIDERATIONS Slide 24 25 July 1998WTD 24 DTTB to PAL Adjacent Channel requirements DTTB power and out of band levels will impact upon the interference into the PAL lower and upper adjacent channels. The required Transmission C/N of 43 dB for proper operation of DTTB will provide some inherent protection of the PAL service if it is guaranteed. Deduction from the plot following provides some worse case figures. Slide 25 25 July 1998WTD 25 DTTB to PAL Interference Co-channel (45 dB) Sound (8 dB) Vision (3 dB) SCM40 Avg SCM40 Max LOP-10 dB Protection Ratio D/U (dB) DTTB Frequency Offset (MHz) Slide 26 25 July 1998WTD 26 DTTB Power Levels Without considering spectrum spread from the DTTB, the power which may exist between DTTB and PAL without causing interference is : for adjacent channel operation limited by sound in lower adjacent, DTTB 8 dB below PAL limited by vision in upper adjacent, DTTB 3 dB below PAL for co - channel operation limited by vision, DTTB 45 dB below PAL Slide 27 25 July 1998WTD 27 DTTB Power Levels Allowing for a variation of +/- 2 dB in the DTTB to PAL ratio (>4 Km from the two tower transmissions in Sydney), and combining with the previous -8 dB restriction, a -10 dB DTTB to PAL ratio can be allowed without interference into PAL. This is cautious worse case analysis with the information currently available. Note that the sound of the lower channel is the restricting criteria. If the vision was the limiting factor a -5 dB DTTB to PAL ratio could be allowed. Slide 28 25 July 1998WTD 28 DTTB Side Bands The side bands generated from the spectrum spread or regrowth (from noise or intermod.) may be limited by the Co - channel restrictions imposed upon the DTTB to PAL ratio. As shown the co - channel DTTB to PAL ratio without causing interference is - 45 dB.(flat spectrum across PAL) As the side bands only need to be down this far, the DTTB to PAL ratio can be deducted from this figure to arrive at the Transmission C/N which will produce PAL interference. Hence with a DTTB to PAL ratio of 10 dB, the side band level may be - 35 dB. ( ie 35 dB Transmission C/N) Hence the side band levels are controlled by the DTTB decoding requirements of 43 dB Transmission C/N. Slide 29 25 July 1998WTD 29 Braodcast Spectrum Vision Carrier (peak sync) Sound Carriers Chroma PAL (Upper) PAL (Lower) DTTB COFDM -20 dB -13 dB DTTB to PAL eg -10 dB Carrier only no modulation 0 dB Slide 30 25 July 1998WTD 30 DTTB Spectrum Mask Base Based upon DCA Comms. Laboratory subjective testing using the SCM40 method for vision interference levels and a deduction from the LOP method for sound interference levels. Adjacent channel operation with matching coverage patterns of DTTB and PAL, transmitted from the same area. DTTB to PAL ratio of -10 dB required for adjacent channel protection of PAL from DTTB. The DTTB side bands are required to be 45 dB below PAL vision to protect PAL. The mask is conservative allowing for the introduction of DTTB into a 100 % PAL market. Slide 31 25 July 1998WTD 31 DTTB Spectrum Mask 43 dB * Intermod. D = DTTB to PAL ratio (DTTB average power to equivalent peak sync average power) = - 10 dB B/W = -1dB Bandwidth of COFDM = 6.6 MHz P = 10 KW (70 dBm) P- 42 dBm/4KHz (P-10-10LogB/W) P- 77 dBm/4KHz (P-10LogB/W- 45 dB) @ D = - 10 dB & P = 10 KW 28 dBm/4KHz - 7 dBm/4KHz * dependent upon modulation parameters +3.5 MHz -3.5 MHz -10.5 MHz 10.5 MHz (- 15 dBm/4KHz) Slide 32 25 July 1998WTD 32 DTTB Spectrum Mask Frequency relative to centre of DTTB channel (MHz) Spectral Density (dB/4 kHz) PAL peak sync vision carrier power DTTB single carrier power