Brazil is planning to reuse its digital dividend 700 MHz frequencies for new 4G mobile services alongside planned digital television services. Overseen by ATDI, the company reports on the mitigation of potential interference issues
It’s the country with more World Cup successes than anywhere else but few spectrum resources.
So when Brazil began the process of making digital dividend frequencies available, mobile phone companies were first in the queue to lay claim to them. But television broadcasters, sensitive to the needs of providing both digital and analogue services across the vast country, resisted giving new allocations to mobile operators.
The phone companies, therefore, needed to demonstrate to spectrum regulator Anatel (Brazil’s national communications agency) that their proposed new services could coexist with existing TV transmissions. To do this, they – as represented by the GSM Association (GSMA) – approached UK-based planning and modelling company ATDI to complete a coexistence study.
ATDI lead engineer Paul Grant oversaw the project. He comments: “It was an interesting challenge because of the size of the country and the importance of terrestrial TV broadcasting. So any mobile broadband services introduced following a digital dividend must be able to coexist with television services.”
The newly liberated frequencies the GSMA members were hoping to use for their new LTE services were between 703 and 803 MHz.
Both analogue and digital television had to be considered, as there was to be a phased switch-off of the analogue signal.
Measurement data exists for ISDB-T (Integrated Services Digital Broadcasting) and IMT (International Mobile Telecommunications) systems and this, according to Grant, was useful for establishing coexistence rules. He adds: “It was also useful to determine the impact of interference, which is what ATDI’s study endeavoured to do. This was achieved by modelling the likely interference situation for sample areas and equating this in terms of a denial of service to a population.”
The modelling used a planning approach with figures based on ITU sources and data from Anatel. ATDI has successfully used this modelling approach in other countries to inform similar debates and the decision-making process.
In Brazil, the modelling used a number of scenarios to explore the potential interference between the broadcast and cellular services. Three study areas were used: São Paulo, Brasilia and Campinas. They were chosen as they are likely to be some of the most constrained areas and the potential problems were deemed to be less acute in other areas and so easier to mitigate.
Although the original dividend date was planned for 2016 there will be a staggered switch off between 2015 and 2018; the analogue transmitters continuing to operate will serve more rural population areas.
The modelling scenarios considered are:
LTE base station (BS) interferes with ISDB-T fixed rooftop receivers
LTE user equipment (UE) interferes with ISDB-T fixed rooftop receivers ISDB-T interferes with LTE UE
ISDB-T interferes with LTE BS
LTE BS interferes with analogue TV
LTE UE interferes with analogue TV
Analogue TV interferes with LTE BS
Analogue TV interferes with LTE UE
For each scenario the analysis considered two interference mechanisms; out of band interference and blocking interference.
The probability of interference has been categorised into either: high, medium, or low.
The number of people who experience interference is relatively low, with typically less than 50,000 for São Paulo and less than 10,000 for Brasilia and Campinas for out of band interference. With mitigation applied those numbers could be virtually eliminated.
For ISDB-T into LTE base station the separation distances required can be reduced from more than 10 km to less than 600 metres following mitigation.
For LTE base station into analogue TV the separation distance required is typically a few kilometres. For the uplink the separation distance required is generally less than 100 metres.
For analogue TV into LTE base station the separation distance required is greater than 10 km but reduces to below 10 km with mitigation. For the LTE mobile the separation distance required is of the order of 10 km.
A number of mitigation techniques were considered by ATDI to alleviate the potential interference issues:
Limit emission power in top TV broadcast channels
Use of orthogonal polarisation
Domestic TV receiver filtering
Improved domestic antenna
Good quality TV receivers
Of these options applying filters to the LTE and ISDB-T transmitters and limiting the emission power in the top TV broadcast channels were deemed to be the most beneficial by ATDI. But Grant notes: “Applying filters is likely to be the most costly option but does reduce the probability of interference.”
The probability of interference from analogue TV into LTE was classed as medium. Filtering on the analogue transmitters, which would reduce the out of band emissions, was not considered as the analogue transmitters are scheduled for switch off within the next few years and so wouldn’t have been a cost effective solution. A combination of frequency and geographic separation would be required to find an acceptable solution during the transition period from analogue to digital.
“The use of orthogonal polarisation was not considered viable due to the complexity and cost involved,” comments Grant. “The options for the domestic TV receiver should be used on a case by case basis where local interference issues are experienced.”
“This was a great project to be involved in as Brazil is one of the world’s fastest-growing economies,” he adds. “This work is helping the country to adopt world-class mobile data and voice communication systems while ensuring that TV services are not adversely affected.”