19 May 2010

Practical Applications for Distributed Antenna Systems (Page 2 of 2)

So how are optical system used to distribute radio waves? - Clearly some kind of conversion process needs to take place before a radio system can be connected to a fibre optic cable. This is achieved by connecting the radio base station to an optical 'master' unit. This interfaces to the radio equipment and performs the essential conversion process. Fibre optic cables from the master unit are then used to connect to a series of optical 'remote' units. These powered units convert the optical signal back to radio frequencies which are then distributed to radiating antennas using a conventional copper based network. Although copper is still used at each end, the optical system eliminates the requirement for long runs of copper coaxial cable.

DAS using optical remote stations

DAS using optical remote stations

So, master and remote units carry out the conversion to and from an optical signal in both directions (transmit and receive) with the rest of the DAS consisting of standard passive components. This solves the problem of distance distribution but does have disadvantages of its own.

Additional equipment means additional cost. All of the optical equipment is active and generally complex so is gives rise to reliability and maintenance issues. The remote units also require electrical power at each location which gives rise to the necessity for a separate 240volt power distribution system.

The remote units are also generally limited in the range of frequencies they can accommodate and the amount of power they can generate. Unlike passive systems which for example can generally cover all 3 cellular bands, normally one remote unit per band would be required. Modern cellular systems need to cover all 3 operating bands so this could in effect mean 3 remote units at each location. On complex cellular systems using many individual transmitters (carriers) the coverage from each remote unit can be severely limited due to the power available for each of these carriers.

Hybrid systems

The inclusion of optical components on a typical DAS often gives rise to a Hybrid system. This type of deployment uses antennas connected directly to the base station via copper as well as antennas utilising the optical system.

The reason for this is that most base station equipment is intended for external use and hence has a relatively high output power of 25 to 50 Watts per channel. Optical systems however require very little signal level to drive them, typically 1 mW so there is a significant excess of power to be used conventionally. If all of the antennas are located some distance from the base station then the excess power is often dissipated as heat in a dummy load. This is a waste of energy and in the current energy conservation era in which we live, is a practice becoming ever more frowned upon.

Combining equipment

Another benefit of using optical equipment is the reduced cost of the combining equipment for multiple technologies. With conventional systems where multiple technologies and services are connected, a specialised combiner is required to avoid interaction between systems connected to the DAS. This type of equipment generally takes the form of multiple high quality filtering devices connected to the common DAS output. Isolation requirements for this equipment can be very high particularly for cellular systems which results in an expensive device which can rival the cost of the rest of the DAS. Because the optical system uses very low power levels, the required isolation can be achieved by the simple attenuation devices used to drop the drive power to the required levels. This represents a significant reduction in cost compared to the standard high power combiner.

Unfortunately the Hybrid system loses out as it still requires the high power combiner equipment as well as components to drive the optical system.

Oxford Street DAS

The DAS at Oxford Street in London, UK was originally designed to provide 2G coverage from seven antennas along Oxford Street. Each antenna is connected back to cellular base station equipment located in the basement of the Marble Arch Thistle Hotel.

This particular DAS is not a hybrid design due to the distances between base station and antenna and also because of the installation challenges involved in connecting street level antennas to the base station equipment.

Each remote unit is located within existing CCTV equipment cabinet and connected to a local antenna via conventional copper. The antennas are located on top of street lamp poles at the major road intersections.

The original system design was based on a 2G deployment, following a planned upgrade the system will support 3G and WiFi.

Urban DAS benefits

Being located in a dense urban environment with many local roof top cell sites one could ask the question 'why deploy a DAS in this location'? Surely there is sufficient coverage from these roof tops sites? Adequate cellular coverage would be available without the Oxford Street DAS but the system does have several advantages over a roof top 'macro' based network.

  • Signals from the base station are radiated evenly along the Oxford St DAS resulting in a continuous signal level.

  • Antennas are in close proximity to subscribers resulting in consistent signal quality and higher quality of service.

  • Penetration into nearby shops and cafes is improved compared to the 'coverage from above' of rooftop sites.

  • Traffic capacity in the area is improved and evenly distributed along the DAS In this urban environment DAS has the potential to reach more subscribers than roof top systems.

  • System capacity can be tailored for each radiating antenna to maximize efficiency.

  • The network can be efficiently and cost effectively extended as far as the fibre optic network reaches.

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About the author

AIB Wireless helps organisations gain the maximum benefit - with the minimum level of risk - from wireless technologies. Our portfolio of services includes wireless surveys, audits, wireless network design and management. AIB Wireless assists WIG in the maintenance of the Oxford St DAS.

Wireless Infrastructure Group - WIG is a leading provider of shared communications infrastructure in the UK. The company owns and manages over 1,000 radio sites and is planning to deploy a number of operator neutral DAS solutions over the next five years. WIG works closely with the building, venue or local authority host to agree an acceptable coverage solution before making delivery commitments to the operators.

One of the founders of AIB Wireless, Simon Jones has more than 20 years experience in the design, development and deployment of electronic communication and control systems.

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