Here is Batch 3 of FAQs for telecom.grib.xyz. This batch focuses on hardware constraints, interference management, and distinct planning nuances.
Batch 3: Hardware Specs & Interference Management
21. Antenna: What is the Front-to-Back (F/B) Ratio and why is it critical for frequency reuse? A: The F/B Ratio measures the difference in signal strength (dB) between the main beam (front) and the signal radiating directly backward (180°). A high F/B ratio (e.g., >25dB) is essential for tight frequency reuse (like N=1). If the F/B ratio is poor, the “back lobe” of a sector will interfere with the “front lobe” of a sector on the tower directly behind it, raising the noise floor for the opposing cell.
22. Power: What is the difference between dBi and dBd? A: This is a common calculation trap.
- dBi: Gain relative to an Isotropic radiator (theoretical point source). Used for most link budgets.
- dBd: Gain relative to a Dipole antenna.
- Conversion: 0 dBd = 2.15 dBi.Always check the spec sheet; if an antenna claims “18dB gain” without a unit, assume dBi, but verify. If it’s dBd, the actual gain is higher (20.15 dBi).
23. Tilt: What is “Sub-Band RET” (SRET) in modern antennas? A: Older RET (Remote Electrical Tilt) antennas tilted all frequencies together. SRET allows independent electrical tilting for different frequency bands within the same physical antenna radome (e.g., tilting 1900MHz at 4° while keeping 700MHz at 2°). This is critical because higher frequencies propagate differently than lower frequencies and often require steeper downtilts to match the coverage footprint.
24. Cable: Why are “Dynamic” and “Static” Bend Radii different? A:
- Static Bend Radius: The minimum radius you can bend the cable once into a fixed position (installation).
- Dynamic Bend Radius: The minimum radius allowed if the cable will be moved or flexed repeatedly (e.g., jumpers connected to a swaying tower or temporary deployment). Exceeding these limits deforms the outer conductor, altering impedance and creating permanent VSWR spikes.
25. Combiners: Why can’t I just “T-Connect” two cables to combine them? A: “T-Connecting” without a combiner creates an impedance mismatch. Two 50-ohm loads in parallel create a 25-ohm load. This mismatch causes massive reflection (high VSWR) back to the transmitter. Combiners (Hybrid or Cavity) maintain the 50-ohm system impedance while merging signals.
26. PCI: What is the danger of “PCI Modulo 3” collisions in LTE? A: In LTE, the Primary Synchronization Signal (PSS) has 3 distinct identities (0, 1, 2). If two overlapping cells share the same PCI Mod 3 value (e.g., PCI 3 and PCI 6), their Reference Signals occupy the exact same resource elements in the frequency domain. This prevents the UE from estimating the channel correctly, causing significant throughput degradation (low SINR), even if the RSRP (signal strength) is high.
27. Diplexer: What does “DC Pass” mean on a diplexer spec? A: “DC Pass” indicates that the device allows DC current (and AISG control signals) to pass through specific ports to power Tower Mounted Amplifiers (TMAs) or RET motors. If you install a non-DC-pass diplexer on a line that requires power at the top of the tower, your active components will fail to power up.
28. Beamwidth: Does antenna beamwidth change with frequency? A: Yes. For wideband antennas (e.g., 698-960 MHz), the beamwidth typically narrows as the frequency increases. A sector might have a 68° beamwidth at 700MHz but shrink to 60° at 900MHz. Planners must account for this “breathing” effect, as the coverage overlap at cell edges may decrease at higher frequencies.
29. Splitters: What is a “Hybrid Coupler” (3dB, 90°)? A: A Hybrid Coupler is a specific type of splitter often used for combining transmitters. It splits the input signal into two equal amplitude outputs with a 90° phase shift between them. This phase shift is useful for Balanced Amplifiers or circular polarization applications. It also provides high isolation between the two output ports, protecting transmitters from each other.
30. RSI: How does “Root Sequence Reuse Distance” affect planning? A: Similar to PCI, Root Sequences (used for RACH preambles) must be reused across the network. The “Reuse Distance” is the minimum geographical separation required between two cells using the same RSI to ensure a PRACH preamble sent to Cell A isn’t decoded by Cell B. If the distance is too short, “Ghost RACH” attempts occur, overloading the processor of the distant cell.