Multiband Distributed Antenna System Design Considerations
Posted 15 July 2014 by Tom Kuklo
The increasing demand for in-building communication systems combined with the high cost of installing a distributed antenna system (DAS) has resulted in installations designed to carry multiple bands. Much of this is driven by local ordinances and the need to accommodate various wireless provider services and local agencies on different frequency bands.
Financially this looks great since it eliminates the costly pulling of duplicate lengths of cable through a building and “piggybacks” on an existing DAS antenna. However, if not engineered correctly there is some risk of costing more in the long run because of intermodulation (IM) and RF noise.
Intermodulation distortion can occur due to the undesired combining of several signals in a nonlinear device. The result is a new set of unwanted frequencies that can then cause RF interference to adjacent receivers located at repeater sites.
Let’s focus on multiband systems, but even one-band distributed antenna systems have potential for IM and the classic A+B-C = intermodulation product.
Let’s consider a UHF T Band repeater on 470.7125 MHz transmit and 473.7125 MHz receive and place it near an 800 MHz system operating on 5 channels from 851.0125 to 855.0125 MHz with uniform 1 MHz spacing between channels.
Computing the potential IM products produces the results:
854.0125 MHz + 470.7125 MHz – 851.0125 = 473.7125 MHz or
855.0125 MHz + 470.7125 MHz – 852.0125 = 473.7125 MHz
This mix could result in IM products in the UHF frequencies, creating a feedback loop with the UHF transmitter. If the system was not designed with this IM potential in mind then the UHF system could be degraded to the point of being unusable while the 800 MHz remains RF interference-free.
However, if the DAS is designed correctly, the interference caused by IM products can be dramatically reduced or eliminated. It is key to maintain sufficient isolation between frequency bands at potential mixing points. Since substantial IM products can be generated by high power amplifier stages, it is important for each frequency band to have its own amplification chain. The bands must remain substantially isolated at the outputs of the high power amplifiers as well. It is also key when selecting DAS components (such as couplers, attenuators, cable and antennas) to select components with low PIM ratings (< -150dBc).
Now let’s look at a real-world example from our Site Optimization Services group. In this example, the multiband DAS tunnel system consisted of a 20-channel 800 MHz SMR system and a 15-channel UHF T-band SMR system. The UHF channels were divided between former TV channels 17 and 18, creating two transmit sub-bands and two receive sub-bands. Each system had individual signal boosters but shared a common leaky coax. The signals were divided between the UHF and 800 MHz by cross band couplers at each booster.
The problem was identified as the A+B-C cross band IM combined with “spectral regrowth”. In spectral regrowth, the nonlinear environment is so conducive for producing IM that the IM products remix until the result appears as noise and the entire receive band is degraded.
It was determined that the best course of action would be to install a second leaky coax to prevent the UHF and 800 MHz signals from mixing.
To get the full case study with detailed information of our investigation, results and lessons learned, download our whitepaper “Multiband Distributed Antenna System Design Considerations”.