Distributed Antenna System – DAS

What is DAS?

According to the DAS Forum Definition, a Distributed Antenna System, or DAS, is “a network of spatially separated antenna nodes connected to a common source via transport medium that provides wireless service within a geographic area or structure”.

About the DAS concept and its benefits

The basic idea behind the DAS concept is to split the transmitted signal among several antenna sites separated in space to provide the required coverage area. It replaces a single, large high power antenna which otherwise would be required to achieve the same coverage.
As illustrated in the figure, a single antenna radiating at high power (a) is replaced by a group of low-power antennas to cover the same area (b), but with reduced total power and improved reliability and higher data rates.

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What are the advantages of a Distributed Antenna System?

Better coverage using a lower power. Distributed antenna system approach, helps overcome the shadowing and penetration losses because a line of sight link is available more frequently. As a result the levels of absorption are lower and this means:

  • The overall power levels can be reduced and
  • Better defined coverage can be achieved

And Consequently:

  • Higher capacity
  • Higher throughput
  • Higher data rates

In addition the DAS concept offers the following advantages

  • Fewer coverage holes
  • Individual antennas do not need to be as high as a single antenna for the equivalent coverage
  • The ability to place the antennas on existing vertical structures such as light or utility poles
  • Lowers health risk as a result of using lower overall power levels
  • Multiple wireless carriers without separate antenna sites

Today’s smart phones, tablets and other 4G devices demand mobile services with faster data rates. This increased demand impacts network performance and profitability. To retain valuable customers and maximize customer experience, building owners are pressured to expand services from mobile operators and provide better coverage for end users. This caused the idea of distributed antenna systems to become more widespread.

In Building Distributed Antenna System-DAS

A typical in-building DAS system

A passive or active distributed antenna system can be implemented. The size of the desired coverage area is the key factor in deciding whether a passive or active DAS design is needed.
A passive DAS, which is less expensive and easier to install, uses splitters and/or directional couplers and has no amplification.
An active DAS uses power to amplify signals by active repeater amplifiers to overcome the feeder losses and is, of course, more expensive and more complicated to install.
In systems where equalization is applied, it may be desirable to introduce delays between the antenna elements. This artificially increases delay spread in areas of overlapped coverage, permitting quality improvements via time diversity.
DAS are customized for individual installations and designed to suit each facility.

As illustrated in the figure below a typical system consists of bi-directional amplifiers (BDA, or signal booster) relaying and amplifying the RF signal traffic and a network to distribute the signal to every corner of the desired coverage area.
The most common type of the distribution network is a system of fiber/coax cables and indoor antennas called a Distributed Antenna System or DAS.

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What are the in building DAS requirements?

The DAS must provide services for a range of cellular frequencies and technologies including LTE.

Today’s DAS design should meet performance standards demanded by the participating wireless providers and building owner.
Today’s top-tier DAS equipment is built by design to be able to handle a wide range of frequency bands and technologies, allowing for broader use by wireless service providers with modern 3G and 4G/LTE.

DAS resources sharing

The design must consider the economies of scale of sharing DAS resources among carriers. Sharing DAS resources among multiple wireless service providers is an important aspect of designing and operating a neutral-host DAS. This can occur on various levels, from completely independent use of separate DAS equipment for each carrier to fully integrated usage on the same equipment.
For the shared neutral-host architecture, the challenge is how to design it to meet quality of service and growth needs of each participant while sharing resources.

Converged Cellular, PCS, Public Safety and Wi-Fi

An important aspect of ownership is the option to integrate Wi-Fi and cellular services within a building
Flexible DAS Equipment
Over time the DAS equipment manufacturers have designed increased bandwidth into their products. The DAS designer can use these products to provide a flexible design. This can be done either by building in the capability up front with multi-band equipment, or by providing a core architecture that can be easily expanded with modular components.

Build the DAS with the knowledge of the roadmap according to future growth predictions

Project traffic usage and number of channels required to meet subscriber traffic over some foreseeable period of time.
Provide architecture that permits easy addition of more channels and frequencies on existing or expanded DAS equipment as well as migration to LTE which require MIMO implementation.

Provision for a flexible architecture should also include allowing for MIMO transmission.

It may be cheaper to provision up front, such as pulling a higher cable-quantity bundle and installing MIMO in a single enclosure based on dual polarized antennas at each antenna node location.

Passive Intermodulation – PIM elimination

Intermodulation can occur whenever more than one signal is present in an RF system and components display unwanted non-linear frequency responses.
Loss of network capacity caused by PIM is not acceptable for high volume, high speed wireless data networks. The antenna should be PIM optimized and specified with a defined PIM value.

How to choose the right antenna for in building DAS

In addition to the common antenna parameters, there are unique system design aspects that define the indoor antennas requirements. If overlooked or ignored an improperly designed DAS results in, with degraded performance and unnecessary cost increases.
These include:

  • Wide BW to accommodate sharing DAS resources among multiple carriers, multiple cellular frequencies and technologies including LTE, Public Safety and optionally Wi-Fi
  • 3D patterns throughout all supported bands without any nulls/dents
  • Round Omni patterns with the maximum gain focused to the floor
  • No radiation up to the ceiling
  • Support of MIMO in a Single Enclosure based on dual polarized antennas which not only reduce installation time and cost but also achieve significantly higher data-throughput compared to a two spatially separated antennas MIMO
  • PIM (Passive Intermodulation) optimized to minimize performance degradation caused by Intermodulation distortion
  • Divers flexible and convenient mounting

Cross Polarization MIMO Antennas

It is well known that spatially separated antennas can provide uncorrelated signals in a rich scattering environment. Under suitable conditions, orthogonally polarized antennas can provide uncorrelated signals without requiring any or much spatial separation.
Multiple-Input Multiple-Output (MIMO) is a wireless communication technique which uses multiple antennas at both the transmitter and the receiver to provide significant gains in spectral efficiency.
The channel capacity improvement is achieved without any additional power or bandwidth.
Achieving a high spectral efficiency under all channel conditions is required for reliability and robustness.
The use of multiple polarizations of the electromagnetic wave to extract diversity has been well known and understood for a long time.
Polarization diversity has been explored as a means to extract the benefits of space diversity but without the stringent antenna spacing requirements.

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Capacity gains that result from using co-located cross-polarized antennas are the same as would result from using separated antennas.
Cross-polarized antenna configuration provides robust MIMO performance for all the channel scenarios.