Transmission Site Planning for ATSC 3.0 Martyn J
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Transmission Site Planning for ATSC 3.0 Martyn J. Horspool GatesAir Mason, Ohio USA [email protected] Abstract - ATSC 3.0 is an advanced, new-generation, over- wireless internet. Essentially, broadband and broadcasting the-air transmission standard. With it comes a wealth of have now been merged. It also allows interactive and hybrid creative technologically advanced capabilities, which have TV using standard internet protocols. It also provides data the potential to enable some significant and exciting new rates ranging from <2Mb/s to >50Mbps over 6MHz business models. Because the physical layer (PHY) of ATSC bandwidth. This provides far more flexibility for the 3.0 is based on OFDM modulation, rather that 8VSB, several broadcaster and certainly opens up the opportunity for 4K items in the current transmission chain may be affected. TV UHD transmission, along with HD and SD and other broadcasters, who may already be contemplating the services. conversion to ATSC 3.0, have by now likely realized that they In summary, here are five key features of ATSC 3.0, all will be forced to make changes to their existing transmitter of which are significant differentiators over ATSC 1.0: plant. Items subject to change may include the transmitter • Robust mobile reception (including exciters and power amplifiers), inside RF plant • Ultra-High Definition (UHD, or 4K) TV (line, mask filter, combiners), and outside transmission line • Immersive high quality audio and antenna. This paper provides insight into the key • IP Transport differences between the current ATSC 8VSB and the new • Advanced Emergency Alerting (EAS) ATSC 3.0 standard, along with an overview of how these differences may impact the transmitter plant. A review of EAK TO VERAGE OWER ATIO Peak-to-Average Power Ratio (PAR) differences between P - -A P R 8VSB and OFDM, along with power amplifier When one compares the ATSC 1.0 and 3.0 physical layer, the characteristics will be reviewed. Lastly, some biggest impact on the RF waveform and by far the biggest recommendations for planning a smooth transition path from factor affecting the transmitter system is the Peak-to-Average ATSC 8VSB transmission over to ATSC 3.0 will be discussed. Power Ratio (PAPR, or PAR). One advantage of the 8-VSB standard used for ATSC 1.0 is that the transmitted PAR was KEY DIFFERENCES: ATSC 1.0 & ATSC 3.0 close to 6dB [1]. In comparison, an OFDM waveform, such as DVB-T or ATSC 3.0, has a transmitted PAR of around ATSC 1.0 (8-VSB) is a fixed digital TV modulation standard, 8dB [1]. This 2dB difference in PAR can affect the with little flexibility or scalability. It uses 8-level Vestigial transmitter average power rating, which in turn can impact Sideband (8-VSB) modulation. The transmitted data rate is cost, physical size and performance. fixed at 19.39Mb/s and the receive C/N threshold is 15dB. The standard is already more than 20 years old and has proven itself to be a very robust digital transmission method for over-the-air delivery. It provides the platform for excellent quality HD and SD multicast transmissions. The benefits over the analog NTSC standard that it replaced in the USA were tremendous and provided the biggest change since color television was first broadcast in 1953. Compared to some other digital TV standards of today, ATSC 1.0 has a few disadvantages, some of which are listed here: • Fixed data rate / payload • Fixed modulation format • Fixed interleaver • Fixed coding / error correction • Difficult reception in high echo areas • Marginal/difficult SFN implementation • Mobile reception difficult ATSC 3.0 is built around OFDM modulation and utilizes FIGURE 1 - PAR DIFFERENCE BETWEEN ATSC 1.0 & 3.0 much more modern techniques for error correction, along with a host of variable parameters and constellations. With Figure 1 shows the difference in average power between introduction of ATSC 3.0, broadcasting becomes part of the ATSC 1.0 and 3.0, for a constant peak RF power level. If a television transmitter was originally designed and 3.0 can accommodate multiple PLP’s, where each PLP can optimized specifically for 8-VSB transmission, it will be individually tailored to best match the type of service that probably have components sized appropriately for a 6dB may be needed. Add LDM and SFN and it becomes even PAR. This includes the power amplifiers, power supplies, more complex. cooling system, RF components (filters, line, test load) and Figure 3 illustrates six operating models that were other items. It is known that transmitters in ATSC 1.0 service developed by a group of broadcast engineering experts, using today were optimized for best performance and efficiency use case models that have been selected by broadcasters. This with a 6dB transmitted PAR figure. illustrates the type and number of each service, the target type Clearly, if an ATSC 1.0 transmitter is already operating of reception, the modulation parameters of each PLP and the at, or close to, its maximum peak RF power capability, it must payload capability of each PLP [4]. be operated a similar peak power level with ATSC 3.0 PLP Multiplex Targeted Service Channel Loading (BCH Config. Opportunity Capacity modulation. Any attempt to raise the peak power will result Capacity Receivers Assignments On) (Mb/s) in distortions and clipping, which manifests itself in degraded PLP 1: FFT 32K, GI 148uS, UHD from Lots more Fixed 1 UHD + 1-12 64QAM, LDPC 64800, FEC 20.03 Single Stick RF performance, especially for Third Order Intermodulation HD/SD HD or 11/15, Frame 250mS 1 TDM services space (similar to 1 UHD + 3-6 Distortion (IMD, or “shoulder” level) and Modulation Error Audio PLP 2: FFT 32K, GI 148uS, available today) SD Services QPSK, LDPC 64800, FEC 0.66 Ratio (MER). Figure 2 depicts the effect of increasing peak 5/15, Frame 250mS PLP 1: FFT 32K, GI 148uS, 2-4 HD or 8- power through an amplifier, hard into clipping and beyond its Multicast Fixed 64QAM, LDPC 64800, FEC 17.59 10 SD HD/SD from 11/15, Frame 250mS useful operating range. At this point, even the best pre- 2 5-12 in a single single stick (similar to video stat mux PLP 2: FFT 32K, GI 148uS, TDM today) correction techniques may not be successful in providing a pool QPSK, LDPC 64800, FEC 1.03 5/15, Frame 250mS mask compliant signal. PLP 1: FFT 32K, GI 148uS, 1 UHD Fixed 256QAM, LDPC 64800, 20.8 (+ audio) UHD + Mobile FEC 11/15, Frame 250mS 3 2-5 HD 4 SD or qHD PLP 2: FFT 8K, GI 148uS, Mobile & Mobile + 16QAM, LDPC 64800, FEC 2.23 Indoor audio 5/15, Frame 250mS PLP 1: FFT 16K, GI 148uS, 2 HD in video Fixed 64QAM, LDPC 64800, FEC 8.7 stat mux pool 5/15, Frame 250mS Multicast +3-5 SD in PLP 2: FFT 16K, GI 148mS, HD/SD + Robust 4 5-7 video stat mux 16QAM, LDPC 64800, FEC 2.97 Robust Core Services pool 5/15, Frame 250mS SFN PLP 3: FFT 16K, GI 148uS, + audio QPSK, LDPC 64800, FEC 0.66 5/15, Frame 250mS 2 HD in video PLP 1: FFT 16K, GI 148uS, stat mux pool 64QAM, LDPC 64800, FEC 5.38 (+ audio) 7/15, Frame 250mS Deep Indoor Fixed + 4-6 SD in PLP 2: FFT 16K, GI 148uS, HD Core + portable 5 6-8 video stat mux QPSK, LDPC 64800, FEC 2.13 Mobile Single deep indoor pool (+ audio) 7/15, Frame 250mS Stick receivers PLP 3: FFT 16K, GI 148uS, + audio QPSK, LDPC 64800, FEC 0.;66 5/15, Frame 250mS FIGURE 2 - EFFECT OF AMPLIFIER COMPRESSION ON SHOULDER LEVEL Portable 4-5 SD or Deep Indoor receivers PLP: FFT 8K, GI 222uS, qHD in a 6 & Mobile SFN Roughly 5 indoor and 16QAM, LDPC 64800, FEC 5.74 video stat mux TDM high-speed 5/15, Frame 250mS TRANSMITTER POWER FOR ATSC 3.0 pool (+ audio) Mobile In the broadcast transmitter business, a question that we are FIGURE 3 – SIX USE CASE EXAMPLES FOR ATSC 3.0 asked frequently is: “What transmitter power will I need for my future ATSC 3.0 service?”. A simple question indeed, but one that may not have a simple answer. Due to the very TRANSMITTER TECHNOLOGY AND ATSC 3.0 flexible nature of the ATSC 3.0 physical layer standard, there Newer transmitters for both UHF and VHF transmission have are a lot of variables and many different scenarios. Some emerged in the market over the past 2 or 3 years. These industry leaders have stated that it could be the same average designs bring vastly improved AC to RF efficiency, along power as for the 1.0 transmission. However, a pure side-by- with much better system level redundancy than many earlier side, apples-to-apples, comparison with 15dB C/N rooftop designs. Improved RF devices and Doherty implementations reception and a data payload of 19.39Mb/s will result in a for the final amplifier have played a dramatic role in lower ERP requirement for 3.0 versus 1.0. This is providing large increases in efficiency. attributable to the better coding efficiency and advanced error The newest generation of “Asymmetrical Doherty” 50 correction capabilities built into the new standard. This does Volt LDMOS RF transistors are rated at OFDM power levels not even consider the significant improvements in video of up to 150W across the UHF-TV spectrum and can provide compression that HEVC offers, which will increase the efficiencies approaching 55% at the pallet (board) level.