White Paper - Over Horizon Transmission Utilizing ATSC 3.0
Avateq's engineering team has conducted a research and presents a White Paper with the analysis of long distance transmission utilizing ATSC 3.0 standard.
Over Horizon Diffraction Troposphere Transmission Utilizing ATSC 3.0 Physical Layer at Frequencies Below 1GHz
There is a rising demand in mesoscale over-the-air (OTA) data transmission in areas lacking traditional cable infrastructure. With the recent development of the physical layers targeting digital content delivery (for example, 5G NR and ATSC 3.0 in Digital Television Broadcast, etc.), it becomes more promising and more beneficial to employ OTA data transmission and re-translation infrastructure in remote locations with low density population. Only 5 years ago this would sound ambitious and unrealistic. However, many advancements and outstanding improvements in content delivery has been recently made bringing efficiency in bandwidth utilization. These new techniques include Non-Uniform Constellations (NUC), Layer Division Multiplexing (LDM), wide overall use of Low-Density Parity Check (LDPC) techniques and data interleavers, which significantly facilitates the physical layers usage in terrestrial delivery of data to distant, isolated, and widely spread communities with high potential to contribute into growth of their digital economy and service industries.
The commonly used communication range for terrestrial link budget (LB) planning is up to 200-250 km, which is primarily related to physical goods and services delivery infrastructure and its availability near large cities and transport hubs. This range is perfectly suitable for utilization of the advanced features of modern modulation and transmission techniques running on reasonably priced hardware widely available on the market, redefining a routine link budget planning as a state-of-the-art nowadays.
As a result of rising demands of modern suburban and distant communication, the beyond the horizon signal propagation and diffraction becomes a key topic of many propagation losses estimates projects. This topic is well-known and widely developed and covered in both academic and industrial research within troposphere communication modeling studies. The studies traditionally refer to diffraction caused by propagation over irregular terrain or/and over smooth earth. The troposphere communication scenarios usually face first Fresnel zone ellipse mostly or even completely overlapped with the horizon causing non-line-of-sight. Thus, it becomes even more important to precisely control the complexity of the model in order to maintain the accuracy of the path loss results. Depending on signal polarization, the power degradation may see additional variations in loss estimates and requires extra attention on its own. Generally, horizontal polarization losses vary less than vertical. The range of communication limited to 200 km allows the commonly accepted approximation of troposphere propagation path with either over irregular terrain or over smooth earth to be used. The approximation preserves the model complexity comparable to single obstacle diffraction approach detailed in , equation 8.1 and Figure 8.7. A simplified model based on single isolated obstacle diffraction can be presented by the following equation and illustrated by Figure 1.
Figure 1. Irregular terrain diffraction model based on single isolated obstacle.
A = G0 - F1 - F2 - C1
A - attenuation of the signal along the path from transmitter to receiver (dB);
G0 - free space distance and frequency dependent loss (dB);
F1 - terrain reflections along the path from transmitter to horizon and further to virtual single obstacle (dB);
F2 - terrain reflections along the path from receiver to horizon and further to virtual single obstacle (dB);
C1 - attenuation depending on polarization of the signal, frequency, and the earth radius (dB).
This model is implemented in a popular link budget support software package Radio Link by Roger Coude . Further extension of communication path length would require the model redesign and might be considered as an option for further work.
The important component of the path loss in troposphere communication is unavoidable ground reflections along the path. The propagated signal reflected from the ground wave sums up with non-reflected wave in a vector sum, alters direct wave amplitude and phase, may and will cause partial cancellation of the signal wave, which affects the receiver as a suppression of particular frequencies of the signal bandwidth. The reflection component is usually simplified to an empirical function of the type and quality of the terrain as of signal carrier frequency and would require a map of terrain type to be known along the path of the wavelet from transmitter to receiver. There are other minor contributors to the path losses for over-horizon propagation such as weather and climate. However, due to relatively low carrier frequency range in consideration (way below 1GHz) their marginal contribution into total path estimates is ignored.
With the focus on spatial propagation path losses of the LB task, we provide an example of the signal power degradation estimate for the particular plan to over-the-air troposphere communication between North Bay and Barrie in Southern Ontario with Transmit and Receive sites 200 km separated of irregular terrain ground. The following RF signal parameters are used for the purposes of the example: RF signal carrier frequency 500MHz, with standard ATSC 3.0 6MHz signal bandwidth, the height of the antenna for both receiver and transmitter is 100 m (on a tower). Radio Link model is used which takes advantage of terrain heights map as well as of the provided ground reflection map, see Figure 2.
Figure 2. Signal path losses estimate for over-the-air communication model connecting Transmitter at Barrie (Ontario) with Receiver at North Bay (Ontario) 200 km apart.
The path loss for the communication model connecting Barrie and North Bay is estimated to be approximately 200 dB and can be improved by 5 dB increasing the height of antennas (for example from 100 m to 150 m) and by additional 15 dB by decreasing carrier frequency of the transmitter signal from 500MHz to 200MHz. In both cases, the signal degradation remains significant and the next step is to focus on selecting optimal setting of physical layer. Thorough antenna design based on the recent results in the field of antenna gain and directivity will provide additional increase in the LB.
It is not disputable, that ATSC 3.0 Physical Layer is the best candidate for Internet content delivery using frequency bands below 1GHz. Today, ATSC 3.0 is definitely a superior in terms of frequency utilizing efficiency and reliability to any other widely used broadcasting and OTA communication standards in both digital television and cellular communication. Among factors making ATSC 3.0 the primary candidate for long range broadcasting are the following:
- ➤ wide range of physical level coding options including a range of ModCod settings, different lengths of OFDM symbols, Guarding Intervals, Pilot Patterns ensuring reliable channel equalization;
- ➤ wide range of NUC providing additional general reception improvement as well as robustness of decoding algorithms in variable weather conditions;
- ➤ a number of available time and frequency interleaving settings to choose from to compensate for possible interference along the long signal path including multi-path environment;
- ➤ the use of highly efficient LDPC codes as well as Bose-Chaudhuri-Hocquengham (BCH) codes as primary decoders and errors correction routines;
- ➤ incredibly small service data (both Bootstrap, Layer 1 and Pilot Patterns) overhead in certain modes to get most of the bandwidth and to safely provide the best bit rate possible in given signal propagation conditions.
The most advantages of the ATSC 3.0 features mentioned above are designed to perform the best with signal MER reaching 25-30 dB level. With this signal-to-noise level there is an opportunity to use high resolution NUC constellations (256-QAM and 1024-QAM) with reasonable LDPC codes settings which are quite resistant to immediate MER drops within few dB-s upkeeping decoding without losing receiver synchronization.
In conclusion, it is worth summing up that troposphere communication has a significant potential filling gaps in Internet connectivity of remote communities. However, due to significant and in most cases unavoidable losses of signal power along the propagation path, it is important to take all parameters of LB planning into consideration making reception improvements estimates where possible for each project on case-by-case base. As a result, even physical layer choice becomes an important decision in achieving stable connection with high data throughput.
 Transmission Loss Predictions for Troposheric Communication Circuits Volumes I and II, Technical Note 101, P. L. Rice, A. G. Longley, K. A. Norton, A. P. Barsis