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Introduction to co-sitting solutions
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Ideally, every technology should have its own antenna.
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But there are locations where space is at a premium or it is impossible to allocate more than a certain number of antennas or coax lines.
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These restrictions drive the RF designer to share antennas and transmission lines between technologies.
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Depending on the constrains of the project, one of three popular co-sitting strategies (or in extreme cases a combination of them) can be used.
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Ideal configuration diagram
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Ideal configuration the PROs
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One antenna per technology frequency allows flexibility for different azimuths, radiation patterns and downtilt angles.
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As only one technology frequency is transmitted over one pair of transmission lines, there is no risk of intermodulation between the different technologies or frequency bands.
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Reduced losses on the transmitted signal since there are no additional elements are required to combine technology frequencies.
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The ability to perform maintenance on one system without disrupting the others.
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Ideal configuration the CONs
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Local zoning ordinances may restrict the quantity, size, location and visibility of the antennas.
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The increased weight may over-stress the structure or building.
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Additional antennas and transmission lines are expensive, difficult to install.
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Scheduling demands/Maintenance windows restrict the amount of time available for construction.
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Benefits of co-sitting
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The co-siting or sharing of infrastructure provides a good solution for carriers, and that solves several of
the mentioned difficulties, while representing a good compromise of cost savings and service performance.
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Better use of resources. Each transmission line is capable of carrying more than one frequency band. A dual-band antenna can take the place of a single band antenna.
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Co-sitting solutions are based on specific equipment and configurations designed to improve performance within a specific set of circumstances and limitations.
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Types of co-sitting
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Multi-Band combining: Multiple frequency bands can be funneled into a single transmission line or antenna. Care must be taken to avoid the different frequency bands from interfering with each other, this is known as intermodulation.
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Antenna combining physically combines two single-band antennas on the same enclosure.
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Diplexers combines and separates two different frequency bands on the same transmission line.
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Triplexers combines and separates three different frequency bands on the same transmission line.
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Same-band combining: Two or more services operating on the same frequency band are funneled on one transmission line. It requires more complex configurations and adds higher losses to the system.
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RxAIT (Rx Antenna Interface Tray) and LLC: Combine two different technologies operating on the same frequency band.
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Antenna Combining
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Is the simplest type of co-sitting. Combines two single-band antennas into the same panel. It is best fitted in situations where it is required to reduce the number of antennas and no need to remove any existing transmission lines.
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Diplexers and Triplexers
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Also known as crossband couplers, combine and separate two or three different frequency bands on the same transmission line.
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Low cost, compact devices.
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Introduce low losses to the system.
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Help reduce the amount of required transmission lines, usually used in conjunction with antenna combining.
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RxAIT and Low Loss Combiners (LLC)
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RxAIT: Rx Antenna Interface Tray
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RxAIT and LLC offers a way to combine two different technologies operating on the same frequency band.
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Employed for combining Tx signals, also offer a way of distributing Rx signals.
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High-cost equipment, introduce higher losses to the system compared to Diplexers and Triplexers.
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Large footprint and increased power requirements.
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Impose restrictions on the amount of carriers that can be combined, may reduce the capacity and scalability of the cell site
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Least desirable solution.
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