SCREEN AFRICA EXCLUSIVE: In the past five years, there has been an influx of video transmission technologies that allow transmission direct from camera via Wi-Fi or Bonded Cellular technologies. How do these technologies differ from the traditional DVB-T/COFDM camera-back transmitter and what are their pros and cons?
Wi-Fi Video links
This technology as the name implies uses traditional Wi-Fi 802.11x technology to stream the video from the transmitter to the receiver. These links can utilise both 2.4 GHz and 5 GHz bands to establish a link between Transmit and Receive units and variable video encoding bitrate is dependent on the data bandwidth available to the receiver. The problem sometimes experienced with these links is that their performance is dependent on spectrum availability in the operational area.
For instance, if the units are deployed inside an empty stadium; the RF spectrum is free from other users, so the system is free to utilise the data channels it needs to establish a stable video stream. However, if the stadium was full of other users, the amount of frequency spectrum drops and the system will now battle to find available channels to initiate a video stream. Typically, the units will connect but the quality of the video drops due to the low bitrate it must now use to get a feed through the reduced spectrum space.
- License-free; these links operate using the standard Wi-Fi specifications.
- Easy to operate and setup.
- Advertised performance is usually based on using a relatively open spectrum with minimal RF noise and interference. Difficult to predict performance if the area is to be filled with other users. In these situations, the link is restricted in performance distance.
- Very low power. In order for these links to remain license-free, their output power needs to be very low, resulting in their use being restricted to very short distance links (less than 100m).
Bonded Cellular Video links
These links are based on using Mobile Service Providers (3G/LTE) connectivity to link the transmitter or mobile unit (usually located with the camera) via the public internet to the receiver which is usually located at a broadcaster. These links are also capable of bi-directional voice communications via the link making it ideal for Live ENG operations in the field with minimal setup. The transmitter is designed to be carried by the cameraman or connected onto the camera. The unit then has the facility to have multiple SIM cards inserted into it to give it access to the internet. The amount of SIM cards varies with manufacturer typically, most units can accommodate 4 SIM cards, but some manufacturers can use up to 16 SIM cards.
This link technology is dependent on two factors:
- 3G/LTE mobile connectivity in the transmitter area.
- Public internet connectivity to the receiver.
Both factors have been getting better in South Africa, so the operational areas for this type of system is getting better and more reliable. However, users should be aware that they have no control over mobile connectivity performance or the public, unmanaged internet, so the use of the link must be judged correctly depending on the footage being covered and amount of uptime required. Using the system for ad-hoc news feeds in mobile coverage areas that are not overly congested, or rapid deployment to disaster areas is ideal for this type of technology. Using it at a sports stadium when it is full of fans will be very difficult to get a stable, usable video feed due to the reduced availability of data bandwidth at the mobile sites situated around the stadium. There is also no guaranteed Quality of Service (QOS), and all these links rely on a ‘best effort’ data throughput. Some manufacturers offer solutions to try work around this problem by using repeaters or stronger 3G/LTE modems which allow the system to connect to mobile sites a bit further away from the immediate vicinity of the event site to gain better 3G/LTE connectivity.
- Cheap, compared to SNG or Licensed Microwave Link costs to get the same connectivity (typically 80 per cent cheaper).
- Once set up; one button to initialise link is possible.
- License Free
- Ideal for minimal equipment live camera standup with IFB.
- Both the Transmit and Receive equipment can be fully controlled via a cloud based Web Portal.
- Reliant on mobile operator coverage; the operator needs to have a few SIM cards of different service providers to maximise 3G/LTE connectivity.
- Not ideal for areas where mobile sites are congested due to too many users.
- Cellular/Mobile phone GSM technology is geared for fast download (consumption) speeds from the base station network to mobile phone. Upload speeds are often restricted and easily congested due to the network design. Fewer 3G/LTE frequencies are available for the portable transmitters to reach the base station.
DVB-T/COFDM Camera links
These links are best compared to the Wi-Fi links mentioned above. Whereas the Wi-Fi Video links are limited to the performance of typical Wi-Fi connectivity parameters, the DVB-T/COFDM link parameters are fully controlled by the user.
High Power Output transmitters.
- Antenna Gains and propagation can be calculated and reliably used at any time and any place.
- Modulation scheme; Guard Intervals; Error Correction and Bit-Rates, allowing for optimal link performance.
- Receiver configuration: multiple diversity RF inputs via fibre.
- Dedicated Frequency Band.
Being able to change all aspects of the link, the user is capable of setting up a system that is guaranteed of performance. The main technical requirement needed is the frequency spectrum. Typically, these types of links are operated in the license-free 2.4 to 2.5 GHz ISM Band; which should be available worldwide however, this is normally not the case. In certain areas, this spectrum is saturated with other users and creates problems in trying to guarantee a clean link. The operator would need to invest in equipment in a licensed RF band. This adds cost to the link operation, but a clean/stable link can be guaranteed.
An advantage of the DVB-T/COFDM links is that the receive system can be customised depending on the operational environment. Namely:
- The receivers are multiple diversity RF inputs; typically the standard input configuration allows for two RF inputs; additional receivers can be cascaded together to create a multiple RF input system. The multiple inputs help with reception in high multi-path environments or it can be used to locate various antenna receive points in locations where the RF camera would roam, thereby eliminating blank areas.
- RF leads from antennas to receivers can be run over fibre minimising RF loss.
- Various antennas can be used depending on the requirement e.g. stadium or indoor environment: low gain omni antennas should be used. For long range or high RF attenuation environments (jungle/bush) – high gain parabolic or sector antennas should be used to offer increased gain in the direction of the transmitter.
A drawback of the DVB-T/COFDM system is that it doesn’t offer an adaptive modulation scheme i.e. the video bit-rate couldn’t be automatically increased if the link could reduce the Forward Error Correction and increase the modulation QAM scheme if the RF link path became better. To guarantee link performance the user will have to do a test run of the area and monitor the feed to see how well it performed and make changes if needed. The main advantage is that the performance would be guaranteed. If the operator is using a licensed frequency, no other users will interfere with the links performance.
- Guaranteed Performance; provided licensed frequency is used.
- Everything is flexible/customisable.
- Expensive, both equipment and license/s required to operate this equipment
- No adaptive modulation schemes.
- Ideally would need to operate in a dedicated Outside Broadcast frequency band, best suited for mobile links.
Guaranteed RF spectrum availability
The main requirement for all the above-mentioned technologies is guaranteed RF spectrum availability. The link’s performance will be dictated by the following:
- Wi-Fi: dependent on the number of users in operation, utilising similar link technologies. Bonded Cellular: dependent on mobile operator coverage and availability of data bandwidth from local mobile sites and the number of users using the same mobile site.
- DVB-T/COFDM: link performance can be guaranteed if a licensed frequency is used.
In short, if link performance needs to be guaranteed then the operator should invest in using a licensed frequency. Using Wi-Fi or Bonded Cellular links; even though they are cheaper, their performance will be dictated by factors that cannot be controlled by the operator.
The use of these links need to be carefully weighed up against how important the link operation needs to be versus cost. Another concerning factor in the industry regarding the Wi-Fi; Bonded Cellular systems and DVB-T/COFDM in the 1.8GHz, 2.1GHz and 2.4-2.5 GHz band is the use of Screamers/Interference Sources used by Government Security Agencies at events when Heads of State/Ministers attend. These links are the worst affected because they are common off the shelf products. The use of licensed frequencies out of the commonly used bands usually help in avoiding interference from these interference devices. In fact, it is illegal for any Government Agency to interfere with a Licensed frequency user’s transmission.
Written by Quentin Barkhuizen, sales manager, Telemedia