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