Designing a Motorola DAS
Gathering of information and documents
All active equipment and battery backups shall be in a NEMA 4 or 4x enclosure
The Bi-directional Amplifier shall have oscillation control.
All active devices shall have a 12 hour battery backup or 2- Hours Battery w/ Emergency Generator
Co-located, dedicated monitor w/Fire Alarm panel integration capability
FCC Type Accepted Equipment Only
All coax utilized in the system shall be low PIM. (Low PIM coax types are typically hardline, not Superflex or braided)
Exterior coaxial cables subjected to weather shall be rated for this type of use. NEVER use air dielectric or Plenum coax outside.
Internal coaxial cable shall be Plenum-rated, "Hard-line" type. (Superflex style is not PIM-rated, and only allowed when there is only one frequency used. It is not used on BDA/DAS distribution.)
Coaxial connectors shall be “N”, “DIN”, 4.3-10 or equivalent, Low-PIM is required!
Proper Level and Audio Quality
A properly designed DAS will provide adequate RF levels, excellent DAQ, and low noise.
A properly aligned and commissioned BDA/DAS system should be virtually invisible to the Macro system. The Macro system noise floor shall not increase at any time, while providing sufficient signals and data throughput to the subscriber units.
The Near-Far Effect
The near-far effect as it relates to Public Safety DAS systems is when a portable is keyed near an indoor antenna and a second portable is keyed simultaneously at some distance X that represents halfway point between two antennas. When the near radio is keyed, it forces the DAS amplifier to lower its gain to prevent saturation or destructive overload and consequently the radio positioned distance X away may not have sufficient gain available, and go “out of range”. The best way to prevent this effect is to design with more indoor antennas, instead of less.
Prevent System Oscillation
The most frequent problem with an in-building installation is inadequate isolation (path loss) between the roof antenna and those within the building. When insufficient the system ‘oscillates’ and causes interference to yourself and others. It is illegal to operate a signal booster that oscillates.
The industry standard for minimum antenna to antenna isolation uses this formula; BDA gain +15 dB. Example, 80 dB BDA gain + 15 dB = 95 dB minimum ant – ant isolation.
Careful choices of antennas types and mounting can improve the desired channel levels and reduce undesirable channel power levels. Obviously, an antenna with high directivity and high front-to-back rations should always be used. This includes locations where the benefits of the antenna gain is not important because we are looking for the directivity.
High donor antennas also ‘see’ more sites in the distance. The antenna elevation should be as low as practical and still maintain a line of sight path.
Passive Intermodulation (PIM)
Motorola Solutions states that Low-PIM components should always be used.
PIM is a form of intermodulation distortion that occurs in passive components, such as cables, connectors and antennas. PIM signals are generated late in the signal path and cannot be filtered out. PIM shows up as a set of unwanted signals created by the mixing of two or more strong RF signals in a nonlinear device, such as a loose or corroded connector, or nearby rust.
Both “N” and “4.3-10” connectors provide low-PIM when installed properly with enough torque. The difference is in the design. The 4.3-10 uses “radial” contact and the N connector uses “axial” contact. When the N connector gets even a little bit loose, or less than proper torque, it can produce PIM. The contrast is with the 4.3-10 connector, it can be loose and still provide low-PIM.
Some industry analysts have depicted the propagation delay through RF boosters as a universally bad thing. In fact, propagation delay is simply a part of the device physics and must be factored into any properly engineered coverage solution.
Time Delay in RF Boosters
The propagation delay through any filter, whether it is implemented digitally or in the analog domain, is inversely proportional to the bandwidth and (roughly) directly proportional to the order (number of sections) of the filter which determines the filters flatness across the pass band and sharpness of the roll off at the filter’s edges. For filters with bandwidths greater than about 200 kHz, the delay is limited by the electrical delay through the components. This is typically approximately 5 μsec.
TDI (Time Domain Interference)
It is well known from simulcast network design that signal quality degradation can result from the overlap of a signal and a delayed version of the same signal. The worst case occurs in areas where the two signals are the same level. The analysis from the TIA TSB-88B Recommended Methodologies shows that, under these conditions, the maximum relative delay that can be tolerated to maintain a DAQ 3.4 is 33 μsec. For P25 Phase 2 systems, this value is expected to be 15 μsec. When one signal is stronger, higher relative delays can be tolerated.
The properly designed system can overcome propagation delay and TDI by designing each in-building installation so as there are no overlap areas with less than 16 dB (20 dB is the Motorola standard) dominance of a signal from either the direct or delayed signal path.
Power per Channel Considerations
Since the bandwidth of a broadband amplifier allows amplification of more than one communications channel, the total power of all the channels together is called the “composite power”. A power measurement of the total power out of a broadband amplifier is the sum of all the carriers within that passband, not any one single channel.
The more input channels, the less power out per channel. Good engineering practice is to assume the worst case based on the spectrum activity within the signal boosters passband.
DAS Downlink Optimized for Level (dBm)
Remember to calculate for the composite signal level, not just a single channel!
More antennas rather than more power – prevents the “Near/Far” issue
Maximum gain is rarely needed. Only use the amount of gain required to achieve the required coverage.
In an active DAS, set the BDA gain for the proper input level for the fiber headend
DAS Uplink Optimized for Noise
The goal of uplink commissioning is to eliminate the noise rise due to the system, i.e. there should be no change in the UL noise power from Remotes to the BTS in order to maximize the receive sensitivity of the BTS
Proper Commissioning of a DAS requires that system attenuation on the uplink path matches the noise rise through the system path. The attenuation is everything present between the DAS head end and the RF source (DAS tray, jumpers, connectors, cables, splitters, duplexers, etc.) The formula to calculate the total UL attenuation needed is:
10 log (# 𝑜𝑓 𝑅emotes)+𝑆𝑦𝑠𝑡𝑒𝑚 𝑈𝐿 𝑔𝑎𝑖𝑛+𝑁𝐹 𝑜𝑓 𝑏𝑎𝑛𝑑 𝑜𝑓 𝑅emote
Shall not raise system noise floor on the Public Safety donor tower site receiver preamp
Certificates of Occupancy may be denied without adequate performance.