19 May 2010

Practical Applications for Distributed Antenna Systems

Simon Jones of AIB Wireless looks at the way Distributed Antenna Systems, DAS, can be used to provide widespread wireless coverage

With an ever increasing requirement for high bandwidth mobile broadband on cellular networks, a Distributed Antenna System (DAS) can provide an attractive solution.

One such system on Oxford Street is owned by shared infrastructure provider Wireless Infrastructure Group (WIG). This system has the potential to provide wireless coverage for multiple operators and multiple technologies for 2km along this busy urban location.

Radio System Design

Radio waves are all around us and are used to carry prolific amounts of information to all points of the globe. Even the most modest of transmitting equipment can produce a signal that can effectively be received over millions of cubic meters of space. Radio waves travel at the speed of light and can cover huge distances with little loss of intensity. Radio telescopes routinely detect signals that have travelled billions of miles over millions of years. They do have one drawback however. Unlike other forms of radiation they can be stopped dead in there tracks by something as insignificant as a piece of tin foil. Other materials are more or less 'transparent' to radio signals than others. Air for example has very little effect on a radio transmission, stone walls do.

Because of this 'shadowing' effect the design of a radio system covering a building or dense urban environment is very different to a similar system covering a rural or less cluttered outdoor area.

Fading

As well as attenuation of the radio signal as is passes through walls and floors or any other relatively dense structure, another problem occurs that is generally referred to as 'fading'. This effect is seen in any wave like transmission - even light waves. The result of fading is a large variation in signal strength over a short distance. Fading occurs when a signal arrives at a point simultaneously from 2 directions. E.g. 1 signal arrives directly; the other one has bounced off the side of a building and also arrives at the same point. Because the signals have effectively travelled different distances their wave forms may no longer coincide with each other and have a difference in 'phase'.

The effect of this is to cause partial or complete cancellation of the two radio signals creating a large variation in the signal level detected by the receiver

Signal Phasing from Different Antennas

Signal phasing from different antennas

The above illustration shows firstly two signals in phase. The effect of this is a slight increase in the received level. The second illustration shows 2 signals perfectly out of phase. If both signals are of the same amplitude, the signal can theoretically be completely eliminated. Even a significant increase in the power of the transmitter would make no difference.

Coverage Concepts

To overcome coverage problems the simple answer would be to have a higher power transmitter. However within buildings and dense urban areas the opposite is true. Radiating from a single location exacerbates the effects of shadowing and fading described above. The following illustrations show computer generated coverage predictions for a single high power point source system along with a system using several low power point sources.

 Coverage predictions for a single high power point source

Coverage predictions for a single high power point source

Single point source shows good coverage near the antenna but varying coverage around the building with much of the signal being lost outside.

 Coverage predictions using multiple low power sources

Coverage predictions using multiple low power sources

Multiple point low level point sources result in a more even signal distribution with less radiation outside the building.

Distributed Antenna System DAS Basics

For building and dense urban environments therefore, a system is required to distribute the radio signal more evenly from several point sources rather than a single one, hence the concept of a Distributed Antenna System. The concept of a conventional distributed antenna system is shown below:

 Multiple antennas used by a Base-Station

Multiple antennas used by a Base-Station

In simple terms most Distributed Antenna Systems are made up of a central base station connected to several antennas via conventional copper coaxial cable. This system would be described as 'Passive' in that all of the distribution components such as the cable itself along with any splitters, combiners, couplers attenuators or other components do not require additional power to function.

The high power signal from the base station is distributed along the coaxial cable usually to a collection of 'Nodes' where a small amount of the signal is 'tapped off' to feed individual sections of the DAS. The technique for this is generally referred to as 'Trunk and Spur' due to its tree like topology.

The advantages to this approach are its simplicity and high reliability. In most cases nothing short of actual physical damage to a system component will cause a failure.

The system does, however suffer from one major drawback common to any system employing copper based distribution, resistance!

Like any other form of electrical signal, radio transmissions use electrical currents to travel from the base station to the transmitting antenna and, like any form of electrical current they can be affected by the finite resistance of the copper based cables used to distribute them. The effect of this is a gradual loss of the signal level along the cable. This results in a finite distance the radio signal transmission can travel before it is completely dissipated as heat along the cable. This effect is worse at higher frequencies and is also subject to a phenomenon called 'Skin Effect' where the signal does not travel over the whole thickness of the copper conductor. Other factors also cause loss of the signal along the cable such as partial radiation of the valuable RF signal.

The obvious way around this is to make the cable thicker so reducing the electrical resistance. This has practical limitations such as the cost of the cable and the actual process of installing thick cables within building infrastructure or along streets. A copper based DAS is therefore limited in the area it can cover particularly at higher frequencies. A VHF system at 160MHz could potentially cover cable runs of several hundred meters whereas a cellular 3G 2100 MHz system would struggle over more than 200 metres. Higher frequency systems also require more radiating antennas as their coverage properties through the air are reduced due to the increased wavelength and reduced antenna field strength. Where long distances are involved between the base station equipment and radiating antennas, an alternative distribution technology is required.

RF over fibre

The technology deployed by the Oxford Street DAS is Radio Frequency over Fibre (RFoF). Fibre optic systems have existed for many years and are often used to distribute data over long distances. Signals are converted to optical (light) waves generally using a laser diode. These light waves are injected in to a single piece of very thin glass fibre which is used to connect to a similar apparatus at the remote end.

Fibre optic signals not only offer wide band width due to the extremely high frequency of light waves but are also subject to very little loss along the fibre optic cable. A typical optical system can cover more than 20 km while only suffering from a 0.5 dB (10%) loss in signal per km. Compare this to 100 dB (99.99999999%)loss for a high quality coaxial cable at 1000 MHz and you can begin to see the advantages.

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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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