- Philippe Aubé Senior Manager, Transmission Engineering division
Mobile Digital Television (DTV) is a technology that allows small portable devices such as smartphones, tablets, and automobile-based displays to receive digital television signals over-the-air. In North America, the standard adopted for mobile DTV is ATSC M/H (where M/H stands for Mobile/Handheld) and it is compatible with the existing DTV broadcast standards and infrastructure. One or more mobile DTV services can be added to an existing DTV channel by allocating a certain amount of the main programme’s bandwidth to the new mobile services.
Figure 1 – Examples of Mobile DTV Receivers
In other words, this new technology uses radio waves reserved for digital television broadcasters, rather than those that have been allocated to cellular telephones. Unlike typical streaming services, mobile DTV will allow viewers to watch local and national programming live, without using a single bit of your cell phone data plan.
Figure 1 shows an example of an iPad and an iPod touch using a dongle[i] to receive a mobile DTV signal, as well as a Samsung Galaxy S Lightray™ 4G smartphone [ii] with an integrated mobile DTV receiver.
Field-testing & Measurements
Field-testing and measurements were done in order to assess the potential of this technology in terms of coverage and robustness. Mobile DTV transmission equipment was installed on the CBLFT-DT DTV transmitter at the CN Tower in Toronto and on the CBMT-DT DTV transmitter in Montreal for the duration of the trials. Although field-testing was done in both cities, more in-depth measurements and analysis were made in Toronto, hence why it is the focus of this article. The full report can be found here.
Specifically, the objectives of the field-testing and measurements were to:
- Assess the impact of various forward error correction (FEC) configurations on the coverage;
- Identify the best parameters for realistic prediction algorithms used with our software prediction tools for mobile DTV coverage analysis;
- Recommend optimal FEC configurations that meet CBC/Radio-Canada’s requirements for future installations;
- Gain expertise with this new technology.
There are three additional hardware components required to broadcast mobile DTV from an existing DTV installation: an MPEG-4 encoder that encodes and compresses the signal into the correct standard (video H.264 baseline profile, 416x240 resolution + HE AAC v2 audio), an ATSC M/H multiplexer that will incorporate the ATSC M/H signal into the main ATSC stream and, finally, an ATSC M/H-compatible exciter for post-processing and modulation purposes.
Figure 2 shows a basic ATSC M/H block diagram. Yellow blocks represent the existing DTV hardware and green blocks represent the ATSC M/H hardware. The corresponding devices are shown for Toronto’s setup (from manufacturer Rohde and Schwartz), as well as Montreal’s (from manufacturer Harris).
Figure 2 – Mobile DTV Block Diagram: Toronto & Montreal
Other equipment that is not related to the ATSC standard can be used to add features such as weather data, financial data, news feeds, advertising content, traffic information, and various other types of metadata. Figure 3 shows a screenshot of the mobile DTV signal in Montreal with some of these additional metadata features. The screen layout is customisable and can be modified to meet the broadcaster’s requirements.
Figure 3 – Mobile DTV Screenshot Showing Metadata Features (Montreal Mobile DTV Service Shown)
Signal robustness is key for mobile DTV. In order to achieve the required robustness for mobile reception, two forward error correction (FEC) methods have been implemented in the ATSC M/H standard: the Reed-Solomon Cyclic Redundancy Check (RS-CRC) at the packet layer and the Serial Concatenated Convolutional Coder (SCCC) at the physical layer. Various FEC configurations are obtained by changing the parameters of these FEC methods, and they each have an impact on the robustness and bandwidth of the mobile DTV signal. A very robust mobile DTV signal will result in greater coverage, but it will usually require more bandwidth. Since the bandwidth is shared between the mobile DTV and main DTV services, more bandwidth for mobile DTV means less bandwidth for its main legacy DTV counterpart.
In total, four different FEC configurations have been tested during the measurement campaign. Through careful analysis of the different configurations, it became clear that using the RS-CRC encoder would permit an improvement in the reliability of the mobile DTV signals with low bandwidth requirements. Many coverage gaps were eliminated after increasing the RS-CRC from 24 bytes to 48 bytes. The SCCC also had a major effect on improving the coverage, but its bandwidth requirement is higher. The conclusion reached is that the RS-CRC encoder should always be set to 48 bytes as a first step, and then the effect of the SCCC should be increased for further improvement of the coverage. By changing the parameters of the RS-CRC and the Turbo Encoder, it is possible to obtain different configurations with more or less robustness through the use of more or less bandwidth.
Figure 4 shows the bandwidth allocation applicable to each tested configuration. It shows the portion of the channel allocated for the ATSC M/H payload, the ATSC M/H FEC, and the main ATSC. The bitrate for the main ATSC should not be lower than 13.5 Mbps, because degradation can be noticeable when the bitrate is lower.
Vehicle Mobile Evaluation
After analysing the data collected from the trials, two FEC schemes have been chosen to meet requirements for future installations: Configuration 4 (QQQQ RS48) when only one mobile DTV service per digital channel is used, and Configuration 2 (HHHH RS48) when two mobile DTV services are desired on a single digital channel. Configuration 4 was determined to be the most robust and reliable of the tested configurations. As can be seen in figure 5, in Toronto, the signal reception was reliable from Burlington to Oshawa and up to Aurora in the North in a moving vehicle.
Pedestrian Outdoor Evaluation
The pedestrian outdoor subjective evaluation was carried out using a 7” RCA mobile DTV receiver. Table 1 shows the measurement results in terms of percentage of locations. This is simply to demonstrate which configurations performed best overall. As expected, Configuration 4 is the most robust one available, with 83% of locations receiving a perfectly clear signal.
|Table 1 – Subjective Quality During Pedestrian Outdoor Evaluations|
|Configuration 1: HHHH RS24||39%||42%||19%|
|Configuration 2: HHHH RS48||50%||28%||22%|
|Configuration 3: QQHH RS48||43%||25%||32%|
|Configuration 4: QQQQ RS48||83%||8%||8%|
Each location was tested with every FEC configuration and the results are shown in figure 6. Results are displayed on a bar graph, ranging from the least robust (top) to the most robust (bottom). The results were largely as expected, Configuration 4 being the best and Configuration 1 the worst in terms of coverage. The results for Configurations 2 and 3 are in between and fairly similar. In the case of a few locations, the results were lower than expected; this is because these locations had no direct line of sight with the CN Tower antenna.
A downtown walk and evaluation was also performed by our engineers using the same 7” RCA mobile DTV receiver. Their trajectory is shown on figure 7. When following the path on foot during this test, it was noted that only Configuration 1 was the subject of some occasional blocking. Given that these glitches were hard to reproduce, the main hypothesis is that they were due to the heavy traffic. Moving vehicles can momentarily have a negative effect on the quality of the signal reaching the receiver. All the other configurations performed well.
Pedestrian Indoor Reception
The 7” RCA mobile DTV receiver was also used for the pedestrian indoor evaluation. It is obvious that being in proximity to the transmitter location greatly increases the probability of picking up the mobile DTV signal indoors. Table 2 shows that Configuration 4 got a perfect score three times out of four, compared with less than one out of four when dealing with the other configurations.
Therefore, Configuration 4 provides the best indoor reception. Whereas Configuration 2’s coverage is good when it comes to moving vehicles and pedestrian outdoor reception, it did not perform as well when dealing with indoor reception in locations far from the CN Tower’s transmitter. However, Configuration 2 requires half the bandwidth of Configuration 4, which makes it ideal for two ATSC M/H services per digital channel. Moreover, with either of these configuration choices, there should not be any noticeable degradation of the main ATSC DTV signal.
|Table 2 – Subjective Quality During Pedestrian Indoor Evaluations|
|Configuration 1: HHHH RS24||7%||46%||46%|
|Configuration 2: HHHH RS48||22%||44%||34%|
|Configuration 3: QQHH RS48||25%||29%||46%|
|Configuration 4: QQQQ RS48||74%||13%||13%|
Field Strength Threshold Analysis
Figure 8 – Coverage Gaps Compared to Predict v3.21 with an FST of 56 dBuV/m & Configuration 4
The field-testing and measurement results also provided an opportunity to identify the best parameters for our propagation model software and its associated field strength thresholds (FST). FST is used to determine the minimum signal strength required to decode a digital signal. This will be useful when planning the coverage of future mobile DTV services. The chosen propagation model is CRC-Predict v3.21 and FST for the vehicle use cases that were validated during the survey. Figure 8 shows that coverage gaps in the simulation software correspond to the measured values. For this specific example, the FST was determined to be 56 dBuV/m using Configuration 4 during the Vehicle Roof Antenna use case. This is comparable to other documentation that we found on thresholds and coverage analysis.
Mobile DTV field-testing and measurements in Toronto were the first coverage tests and analysis performed by CBC/Radio-Canada with this new technology. A number of different forward error correction configurations were tested with the goal of finding one that is best suited for CBC/Radio-Canada’s requirements. Given the state of current receivers, if the desire is to implement a single mobile DTV program on a DTV channel, the recommendation is for Configuration 4 (QQQQ RS48) to be used. It is, by far, the most robust and reliable configuration, but it comes with higher bandwidth usage requirements compared to other less robust configurations.
Mobile DTV technology allows broadcasters to broadcast additional content to large urban populations with the use of a single DTV transmitter, without significantly compromising the quality of the main DTV service. At the time of this report, there were over 130 mobile DTV services offered in the US. In large cities, such as New York, Los Angeles, Dallas, and Atlanta, there are at least five different stations broadcasting mobile DTV content.
For the viewer, mobile DTV may be seen as a suitable and affordable substitute media platform, as more and more people are streaming television content on their cell phones using costly data plans. For the broadcaster, providing mobile DTV content may represent an added value to an existing DTV service. As per the Open Mobile Video Coalition: “mobile DTV represents a significant new revenue stream for the broadcasting industry as well as a new way to reach more customers[iii]”.
Mobile DTV field-testing and measurements at CBC/Radio-Canada were a team effort and many resources from the following groups have contributed to its successful implementation: CBC/Radio-Canada Transmission Operations, New Broadcast Technologies, and Telecommunications. Special thanks must go to Charles Rousseau, Senior Engineer, and Pierre-Alexandre Nolet, Junior Engineer, for their contribution and for doing the actual field-testing, measurements, data analysis, and compiling everything in a very detailed and comprehensive report.
- Engineering Report ER12-041 – Mobile DTV (ATSC M/H) Field Testing and Measurements by Pierre-Alexandre Nolet, Junior Engineer & Charles Rousseau, Senior Engineer, CBC/Radio-Canada