Cellular Diversity - Cellular Diversity Applications
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Introduction
In recent years, the introduction of COFDM Digital Wireless Camera Systems has revolutionised 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. Take up has been rapid and it is now practically impossible to watch a news broadcast or sports event without at least some of the pictures originating from one of these systems. LINK has been the market leader in this area from the outset with the lowest practically usable delay of 40ms (1 frame), extremely compact and lightweight receivers and diversity reception allowing multiple antennas to greatly increase coverage area.
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 centres or major venues enabling camera crews and producers to operate live without the need for a link/SNG vehicle and attendant engineering staff. Indeed in some locations, parking restrictions and the proximity of tall buildings have rendered them impossible to cover even with a satellite truck.
Coverage Area
Research into the viability of City Centre receive sites has been extremely encouraging. After initial trials at Watford, a full test was carried out in Central London using BT Telecom Tower as a receive point. 4 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 back-pack); and a 5W booster amplifier mounted in a small flight-case, 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.
Extensive testing showed that reliable non-‘line of sight' operation could be obtained within the area shown by the circle on the map opposite. The circle represents 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 from Buckingham Palace and St.James Park - some 2Km from the receive site! This was without the benefit of additional ultra high gain directional aerials.
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.
Whilst no one is suggesting that city cellular networks will render fleets of SNG vehicles redundant, they could reduce producer's dependence on these facilities for simple live 2-ways or tape feeds, freeing them up to cover out-of-town stories or larger events.
Such networks could also serve as receive points for dedicated COFDM link vehicles. These vehicles could offer all the facilities of a conventional satellite truck at a fraction of the cost as they require minimal DC power - no expensive and heavy generators - and would not need expensive satellite equipment or a clear view of the sky. Indeed, existing radio cars could be used as a TV feed/live point with very little modification. Powering up is simply a case of selecting a channel and switching on - such systems can be used easily by non-technical staff.
A city receive point can also ‘double-up' as a helicopter receive site. Recent tests undertaken by LINK have achieved distances in excess of 100Km (70 miles) using 5 watt amplifier in the helicopter and a receive site as described above. The diversity automatically tracks the aircraft - no additional aerials or adjustments are required.
Depending on the enviroment and application the specification can vary. Please call to discuss your requirements.
