In recent years, the introduction of COFDM digital wireless camera systems has revolutionized the broadcasting industry. The ability to work in built-up areas in non 'line of sight' situations, has enabled production teams to cover live events which were previously impossible, due to the health and safety considerations associated with running camera cables across roads and in busy public places. Adoption has been rapid and it is now practically impossible to watch a news broadcast or sporting event without at least some of the pictures originating from one of these systems.
Until now, most receive systems were either built into OB vehicles or rigged and de-rigged on a job-by-job basis. However, attention is now being turned to the possibilities of fixed receive networks in city centers or major venues enabling camera crews and producers to operate live without the need for an ENG/SNG vehicle and engineering staff. Indeed in some locations, parking restrictions and the proximity of tall buildings have rendered some events impossible to cover with a truck.
COFDM and Diversity
COFDM works by taking a standard compressed digital signal and instead of simply modulating it as one 8MHz-wide carrier, converts it into 2048 separate narrow bandwidth carriers spread across the 8MHz channel. At the receiver, the carriers are used to reconstruct the original signal and powerful error correction is used to compensate for any carriers distorted or missing due to flat-fade cancellations. The narrow bandwidth of each carrier means that its time period is much longer with respect to any reflections than a single 8MHz carrier. This, coupled with a guard interval (a time window during which no data is transmitted, thus giving time for late-arriving reflections to be counted as valid) means that reflected signals actually help build the original signal rather than cause problems, as in conventional analogue systems.
This process produces a robust signal, but there will still be situations where most of the carriers are lost due to a heavy fade. Diversity reception minimizes the effects of heavy flat-fades by spacing two or more receive antennas apart so that if one antenna is experiencing a fade, another isn't and the resultant signal is reconstructed without any picture break-up.
It is possible to perform the diversity function at ASI or packet level. This means that rather than perform diversity on the received RF signal, a 4 input receiver has 4 demodulators each producing an ASI digital stream. These streams are then fed into an ASI diversity switch, which uses available valid data from any of the 4 streams to construct the final error-free signal.
As well as producing a much more robust receive capability, a 4 input diversity system has the additional benefit of allowing higher gain directional antennas to be used to provide 360° coverage without the need for complex (and expensive) mechanical or GPS controlled steerable receive antennas.
Performing diversity at ASI level enables receivers to be cascaded together to form multiple antenna systems--up to 64 antennas can be can be combined in this way. Although in practice it is unlikely that such a high number of antennas would be required in a single system, it opens the door to a couple of extremely useful scenarios. First, a 10 input system could be assembled using 3 receiver units. This would have 4x 90° antennas to provide overall 360° coverage.
However, if there were particular areas of interest, such as court buildings, government buildings, or financial centers, additional narrow beam, ultra high gain directional antennas could be used to boost reception of these areas.
It is also possible to use ASI diversity to link individual receive sites together to form a cellular network. Such a network would allow a camera with a COFDM TX unit to move seamlessly within the combined coverage area of all the receive sites in exactly the same way as a mobile phone. The received signal is automatically tracked and sent back to the station without the need for ENG/SNG trucks or inject points.
Coverage Area
Research into the viability of city center receive sites has been extremely encouraging. The following tests were carried out in Central London using BT Telecom Tower as a receive point. Four 13dBi sector antennas pointing North, South, East, and West were used in conjunction with a standard 4 input diversity receiver to give 360° coverage. Reception was then tested using a standard 100mW camera TX; a 1W booster amplifier (mounted in a backpack); and a 5W booster amplifier mounted in a small flightcase; because of the larger distances covered, this unit was rigged in a car. All tests were conducted with omni TX antennas--no panning in of the signal was required.
Standard 100mW Camera TX: Extensive testing showed that reliable non-line of sight operation could be obtained within a 1km radius around the receive sight. Indeed the more built-up the area, the better the received signal strength appeared to be--probably due to the increased number of reflected paths available. Outside of this area, reception was not guaranteed although reliable operation was possible in certain areas some 2km from the receive site. This was without the benefit of additional ultra high gain directional aerials as discussed previously. A camera operator equipped with a standard, camera-mounted transmitter could cover live stories within this area without any link/SNG vehicle or additional technical support. IFB could be obtained via cell phone.
Camera TX plus 1watt back-pack: Reliable non-line of sight operation is possible in an area approximately 2.5km radius from the receive sight. It was even possible to enter buildings (such as Euston Railway Station) and still operate reliably. A camera operator equipped with a standard, camera-mounted transmitter and lightweight backpack could cover live stories within this area without any ENG/SNG vehicle or additional technical support. IFB could be obtained via cell phone.
Camera TX plus 5 watt booster amplifier: Reliable non-line of sight operation was possible within a 6km radius of the receive point. This included areas in the City of London that are difficult or impossible to cover with conventional satellite trucks due to parking restrictions or the proximity of tall buildings which block the satellite signals. Although a 5 watt system is too bulky to be attached directly to a camera or mounted in a backpack, it can easily be integrated into a motorcycle pannier or small flight case (WxHxD: 300x160x300mm) and then hand carried to the desired location. All that is required is to clip on a battery, plug in the antenna, and switch the unit on. A possible application is a portable inject point enabling a cameraperson and producer to quickly set up a live shot anywhere within the coverage area. Also, live 2-ways are possible from moving vehicles, boats, etc.
Conclusions
While no one is suggesting that city cellular networks will render fleets of ENG/SNG vehicles obsolete, they could reduce producers' dependence on these trucks for simple live 2-ways or tape feeds, freeing them up to cover out-of-town stories or larger events. As the coverage map shows, even in a city as large as London, most of the commonly visited newsgathering locations could be covered by a network of 4 simple 4-antenna receive sites. The areas shown are for standard camera-mounted 100mW transmitters with omni antennas. Coverage could be improved by using additional high gain receive aerials where required. Obviously, utilizing a 1watt backpack would cover the entire city center.
