Signal Quality & Precision Planning
31. Antenna: What is “Beam Squint”?
A: Beam Squint is the phenomenon where the antenna’s main beam direction shifts slightly as the frequency changes. For example, an antenna might point at 0° azimuth at 700MHz but shift to 2° or 3° at 800MHz.
This is critical in wideband antennas. If the squint is severe, users at the sector edge might fall out of coverage on specific frequencies (carriers), causing throughput disparities between bands.
32. Power: What is PAPR (Peak-to-Average Power Ratio) and why does it limit my output?
A: Modern modulation schemes (like OFDM in LTE/5G) behave like noise with high amplitude spikes. PAPR is the ratio between the highest instantaneous power spike and the average power. If the PAPR is high (e.g., 8-10dB), the Power Amplifier (PA) must be “backed off” (run at lower average power) to prevent the peaks from clipping (saturating), which would generate distortion and splatter.
33. Tilt: What is “Upper Sidelobe Suppression” (USLS)?
A: USLS measures how much the antenna suppresses the energy radiating above the main beam (towards the sky/horizon).
This is vital for interference management. If USLS is poor (e.g., only 12dB), energy leaks towards distant cell sites (pilot pollution). Good electrical tilt mechanisms maintain high USLS (>18dB) even as you tilt the main beam down, whereas mechanical tilt often degrades USLS.
34. Combiners: Why is the “Third Order Intermodulation” (IM3) the most dangerous?
A: When two frequencies ($f_1$ and $f_2$) mix, they create products at mathematical intervals. The 3rd order products fall at $2f_1 – f_2$ and $2f_2 – f_1$.
These products often land exactly within the receive band of the same system. Since they are close to the fundamental frequencies, they are difficult to filter out and can totally deafen the receiver (raise the noise floor).
35. Cable: What is “Velocity Factor” ($V_f$) and when does it matter?
A: $V_f$ is the speed of the signal inside the cable relative to the speed of light in a vacuum ($c$). A standard foam-dielectric coax might have a $V_f$ of 0.88 (88% of $c$). This matters critically when cutting “phase-matched” cables for antenna arrays or diversity testing. If you cut two cables to the same physical length but they have different $V_f$ specs, the signals will arrive out of phase.
36. PCI: What is the specific definition of “PCI Confusion” (vs Collision)?
A:
- Collision: A cell and its direct neighbor have the same PCI. The UE cannot decode either.
- Confusion: A cell has two different neighbors that share the same PCI. The serving cell doesn’t know which neighbor to hand over to when the UE reports “PCI 55 is strong.” This leads to handover failures.
37. RSI: What is the “High Speed Flag” or “Unrestricted Set” configuration?
A: This configures how the cell handles Doppler shift for RACH preambles.
- Unrestricted Set: Used for normal static/slow-moving users.
- Restricted Set (High Speed): Used for cells covering highways or high-speed rail (>120 km/h). The cell restricts which root sequences are used to ensure that even with significant Doppler shift, the preamble is correctly identified and doesn’t alias into another preamble ID.
38. Splitters/Couplers: What is the difference between “Through Loss” and “Coupling Loss”?
A: In a Directional Coupler (uneven splitter):
- Coupling Loss: The designed loss to the “tap” port (e.g., -10dB). This is where you pull signal for a nearby antenna.
- Through Loss: The minimal loss to the main output port (e.g., -0.5dB).Knowing the difference is essential for calculating link budgets in daisy-chained DAS systems (e.g., tunnels or subways).
39. Diplexer: Why do I need “Guard Bands” between combined frequencies?
A: Filters in diplexers are not perfect brick walls; they have a slope. The Guard Band is the frequency gap required between the two systems (e.g., between Tx of Band A and Rx of Band B) to allow the filter’s attenuation to roll off sufficiently. Without a guard band, the “skirt” of one filter overlaps the passband of the other, causing high insertion loss and signal degradation.
40. Beamwidth: Why do some datasheets show “10dB Beamwidth” instead of 3dB?
A: The standard HPBW is 3dB, which helps plan the core sector coverage. However, the 10dB Beamwidth (the angle where power drops by 10dB) gives a better indication of the total energy spread and potential overlap with neighbors. In soft handover zones (LTE/5G), knowing the 10dB width helps predict where the handover region actually starts and ends.