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The DVB-T2/LTE-A+ trial

Tower Overlay over LTE-Advanced+

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After the television analogue switch-off, in many countries DVB-T and DVB-T2 terrestrial networks are being deployed or planned for the delivery of digital television services to domestic receivers. Most of the networks are designed to provide services to fixed terminals with rooftop antenna receiving systems, but there is also an increasing demand for TV viewing in mobility on portable devices, such as tablets, smart-phones, and in-car receivers.
Mobile TV services,  such as those based on the DVB-H standard launched some years ago, had limited success, or even failed, for many reasons, such as transmission efficiency and network costs, unclear business models and poor terminals performance.
Nowadays, major changes occurred in the terminal domain: with the success of tablets and smartphones people now have devices that are capable of displaying video quality that is truly compelling while in mobile or nomadic use. In addition, users are becoming accustomed to VoD and live video content consumption on a great range of devices thanks to 3G/4G networks and WiFi connections widely available either at home or around.
However, multiple cell handovers and network overload make live video streaming over a wireless network a poor user experience. Furthermore, big audiences are seriously challenging mobile operators networks which are trying to serve their users with unicast delivery. When 3G/4G network sites have capacity issues and cannot deliver a high quality of service, a possible option to overcome this problems is, apart from using more spectrum or increase the number of sites, to off-load part of the traffic onto other technologies, for example Wi-Fi or Broadcast networks.
From the technical and economical point of view broadcast networks are certainly the best delivery mode to reach big audiences during peak hours or major events. For this reasons DVB introduced in 2011 a new profile of the standard called T2 Lite, aiming to better support portable and mobile reception.
In this context RAI launched in 2013 an experimental trial in Aosta Valley where HDTV services for fixed reception and T2-Lite mobile TV services coexist on the same frequency, to optimize UHF spectrum exploitation while guaranteeing adequate transmission robustness to services with very different requirements.
Another approach foresees the use of an evolution of the LTE (4G) technology called LTE-A+. The project developed by Rai CRIT together with the French network operator company TDF is based on the original “Tower Overlay” idea implemented by Technische Universitat Braunschweig (Germany) in 2013, involves contributions by US company GatesAir and is supported by Germany’s IRT and France’s Expway.
Adapting LTE Broadcast to traditional broadcasting high towers creates the possibility of cooperation between the cellular and broadcasting networks, thus reducing network load, energy consumption and network costs. Using such high towers with such an extentded-LTE technology opens the possibility to reach all mobile devices without the need to add a specific broadcast receiver in the devices, a hurdle that proved very difficult to overcome in the past.
Also for this approach Rai CRIT launched in 2015 an experimental trial in Aosta Valley: two data streams share the same UHF channel in time-division: one stream is conveying conventional digital HDTV programs targeting domestic DVB-T2 TV sets, and a second embedded stream is conveying a specific broadcast flow, intended for 4G LTE-A+ smart-phones, laptops and tablets.

THE TOOL + PROJECT
The Rai Research Center has tested the T2 Lite technology in Aosta Valley in the two year period 2013-2014. Thanks to this experimentation, conducted with the support and collaboration of Rai Way, a working and impressive demonstrator was built for mobile TV. The DVB-T2 Lite demonstrated enormous strength during the tests: the ability to “hold up” in the harshest reception conditions is truly impressive!
The experimentation was a success that also obtained important international awards, such as the award for the best article received during the IBC 2014, one of the most important events in the world in the world of broadcasting and electronics for TV, radio and web.

One of the greatest difficulties we encountered was the difficulty of “entering” the world of portable devices, such as smart-phones and tablets, with extra-telephone-derived electronics. Only recently did we have the chance to test a smart-phone in which a T2-Lite receiver was integrated inside. These difficulties have convinced us to explore other possibilities, which therefore go beyond the strictly radio television world of the DVB consortium, to try to find mobile TV solutions that are more appealing also from the “telephone world”. So let’s see in which area the first steps of a new mobile TV solution have been made and for what reasons it is considered worth developing it for the introduction of mobile TV services.

The growth of traffic in the mobile telephony networks and a limited amount of available spectrum resources is leading to the design and implementation of cellular networks with smaller and smaller cell sizes. With this approach, costs increase linearly with the number of users, since a multiplicity of point-to-point transmissions of the same content are generated between the server and each individual user. It follows that the distribution of video services via the cellular data network is very expensive, especially when distributing, for example, live video of particularly popular events. The use of broadcast transmission, which involves a point-to-multi-point spread, ie the content is sent only once to a multiplicity of users, even within the telephone network is able to overcome this drawback. The fourth-generation LTE-Advanced standard (LTE-A) already offers a broadcast service called eMBMS (Evolved Multimedia Broadcast Multicast Service) with some limitations:

  • It works only in mixed mode with unicast services, thus preventing all resources from being allocated to the broadcast.
  • It operates only with small cells: in this case we are talking about Low Power Low Tower services, that is, low power transmissions from very low towers, that is typical of mobile telephone installations. This type of approach therefore requires a large number of cells to ensure good coverage of the territory and the population.

These limitations have led to the development in Germany, at university level, by the Technische Universitaet Braunschweig (TUBS) of the project “Tower Overlay over LTE-A +”, better known with its acronym TOoL +. This solution makes it possible to provide LTE-A services on a High Tower High Power infrastructure, ie from high power transmitters on high-altitude towers that are the typical transmitters used in the television world and generally in broadcast transmissions. This approach reduces energy consumption and overall network costs. Another great advantage of LTE-A + is that it can operate within the framework of a second-generation digital transmission (DVB-T2), thus allowing transmission of the same services to fixed users (high definition and beyond) on the same frequency. and those for mobile users. The current LTE-A standard, however, was born and has been optimized for a cellular environment and therefore in order to overcome the limits that we have illustrated, changes to the standard are necessary. In a first phase TUBS demonstrated in the laboratory the technical feasibility of the TOoL + system. Subsequently, field tests were carried out in Italy and France to evaluate the proposed changes and system performance in a real environment.

TEST IN PARIS
In France, the TDF network operator has turned on a transmitter in Paris, on the Eiffel tower that covers a portion of the city in a densely populated environment. The Paris test allowed the field evaluation of the TOoL + system in a very dense urban environment and in some suburban areas. In the urban area involved in the test, tall buildings with particularly narrow streets alternating with more open areas traveled at higher speeds. The passage from exposed areas with the transmitter to other hidden ones and the echoes generated by the higher buildings make reception particularly complex. The transmitter used for this test is located near the Eiffel Tower and the main lobe of the antenna covers the districts to the south-east of Paris. The receiving antenna is omnidirectional and has been mounted on the roof of the car used during the tests. Due to very heavy traffic the average vehicle speed was around 30km/h with peaks of 70km/h.

TEST IN AOSTA VALLEY
In Italy, the Rai Research and Innovation Center, with the collaboration of Rai Way and the Rai headquarters in Aosta, has activated three transmitters covering a large part of the region. And it is this new experimentation that is taking place in our territory that we wish to speak to you in greater depth.
The Rai Research Center has again chosen Aosta Valley for mobile reception tests. The Aosta Valley region has often been the scene of numerous experiments at the Rai Research Center, especially with the advent of digital technology in the 1990s. For example, in 1995-96, at the dawn of digital radio, an intensive campaign of measures on DAB was conducted; when the digital terrestrial took its first steps, at the end of the 90s, the possibility of transferring the DVB-T signal to analogue radio links was experimented; moreover, in the early 2000s important pre-operative tests of terrestrial digital took place in Aosta Valley.

The reason for this combination of Rai CRIT Aosta Valley should be sought in the particularity of this region which, due to its orography, offers a particularly complex environment from the point of view of the diffusion and reception of signals. The numerous side valleys and the large number of plants that are necessary to cover the territory make the Aosta Valley perfect for testing in depth the single frequency networks. Moreover, the possibility of traveling along a thick secondary road network, next to the main one of the bottom of the valley, which often climbs up steep mountain slopes frequently hidden by the transmitters, represents an extraordinary testing ground for the reception of moving signals (think of the DAB digital radio or mobile TV as in the case in question). Finally, the Aosta Valley, being so well “shielded” from the rest of the national territory, thanks to its imposing mountain peaks, and having, with respect to the rest of the country, a limited number of occupied channels, has greater flexibility in the allocation of frequencies for experimentation.
For these reasons, in March 2015, after testing the performance of the TOoL + system in the laboratory, testing in the service area began. The aim of the project was the activation of a test in which to associate a DVB-T2 transmission, destined for fixed users and containing high definition programs, a signal using the LTE-A + system developed by TUBS in order to verify the potential and performance under operating conditions. In particular, the possibility of activating several transmitting sites allowed the evaluation of a single-frequency network unlike what was done in Paris in which it was possible to switch on a single transmitter.

In the first phase of the experimentation two transmitters were activated: Aosta-Gerdaz covering the city of Aosta, Aymavilles and the head of Valpelline and Saint Vincent-Salirod covering from Verres to the whole city of Aosta, overlapping the Gerdaz transmitter and guaranteeing the realization of a single frequency network. The only modulator available was installed at the Rai regional headquarters in Saint Christophe and transferred via an analogue radio bridge to Gerdaz where we picked up the signal that was sent to the transmitter. From Gerdaz the modulated signal, coming from the headquarters, was sent to Salirod passing through Saint Nicolas. This double jump is essential since Gerdaz and Salirod are not in sight. The radiant systems and the powers used are the same used for the T2-Lite experimentation: 100 W for Salirod and 50 W for Gerdaz. Reusing the same transmitting antennas and the same power has allowed to be able to activate the experimentation in a simpler way from the administrative point of view and will also allow us to make direct comparisons with the results obtained with the T2-Lite.

Using a single modulator the delay of the single frequency network was calibrated through an analog delay line in order to ensure that the whole area of ??Aosta, including the suburban belt between Nus and Aymavilles, was perfectly synchronized.
The tests in the service area were carried out by means of two specially equipped vehicles: a car for moving measurements and a van equipped with a telescopic pole for measurements at fixed points. The purpose of the mobile measurements was to evaluate the coverage area and to detect the map of the field levels at the height of the car roof, in order to have the data necessary for future planning of the networks. In addition to this, the radio frequency signals received on board the vehicle were also recorded in such a way that they could be reused in the laboratory to verify possible improvements of the receivers.
During the tests we covered more than 200km in urban areas, in Aosta, Saint Vincent, Chatillon etc. at low speed, on the highway at high speeds and on suburban roads often in areas reached only by some reflections of the transmitted signals.

The purpose of the fixed measurements was to detect, in some positions of the service area, the level of the available field, in order to verify the concordance with the predictions obtained by means of computer simulations. This allows to understand if the transmission is in the expected way or if there are some problems on the radiant system or on the devices themselves. These measurements also allowed to evaluate the reception margin on the T2 Base signal. In addition, with the equipment equipped for fixed measurements, we could verify the synchronization of the SFN network, verifying the parameters in a very accurate way.
In the second phase we activated a third transmitter, which was also used during T2-Lite tests: Testa d’Arpy. To make the network more efficient, we decided, after some tests, to change the network configuration, moving the only modulator available from the Rai headquarters to the center of Saint Nicolas. The advantage is that from Saint Nicolas being in sight with Gerdaz, Salirod and Arpy, with a single “jump” you can reach all three transmitters. This significantly improves the quality of the irradiated signal and consequently the signal coverage.
The receiver used in this campaign of measurements is a prototype developed by the University of Braunschweig designed to demonstrate the feasibility of TOoL + and that certainly does not claim to have excellent performance. Despite this, while highlighting the limits that had already been highlighted during laboratory tests and well known to designers, in many areas where there were no particular criticalities such as very strong echoes or particularly weak signals, the receiver worked very well. Having recorded the radiofrequency signal will allow the receiver to be evaluated directly in the laboratory as new versions arrive.

More information

Authors: Andrea Bertella, Vittoria Mignone, Silvio Ripamonti, Bruno Sacco, Mirto Tabone

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