Advanced Components & Signal Integrity
11. Cable: What is Passive Intermodulation (PIM) and why is it critical for LTE/5G? A: PIM is interference generated when two or more carrier frequencies mix at a non-linear junction (e.g., a rusty bolt, loose connector, or poor solder joint). Unlike thermal noise, PIM rises with Transmit power. In LTE/5G, PIM often lands directly in the Uplink (Rx) band, raising the noise floor and effectively shrinking the cell’s coverage area and reducing upload throughput.
12. Antenna: Why do we use Cross-Polarization (+45°/-45°) instead of Vertical/Horizontal polarization? A: V/H polarization suffers from high signal correlation in cluttered environments (urban areas), meaning if one fades, the other likely fades too. +45°/-45° (Slant) polarization provides better decorrelation and diversity gain. Additionally, Slant pol is more consistent because user devices (phones) are rarely held perfectly vertically or horizontally.
13. Power: What is the difference between RRU Output Power and EIRP? A:
- RRU Output Power: The raw power generated by the radio unit (e.g., 40W or 46dBm).
- EIRP (Effective Isotropic Radiated Power): The actual power leaving the antenna.
- Formula:
EIRP = RRU Power (dBm) - Cable/Connector Losses (dB) + Antenna Gain (dBi). RF Engineers plan coverage based on EIRP, but dimension power supplies based on RRU Output.
14. Mech vs. Electrical Tilt: What is “Pattern Blooming”? A: Pattern Blooming is a negative side-effect of excessive Mechanical Tilt. When you physically tilt an antenna more than 10-15 degrees, the horizontal beam pattern distorts—the main lobe widens, and side lobes can “bloom” (expand) outward and upward. This can inadvertently cause interference to neighboring sectors that were previously isolated, a problem that Electrical Tilt avoids.
15. Splitters: When should I use a Tapper instead of a Standard Splitter? A: Use a Tapper (Unequal Splitter) in DAS (Distributed Antenna Systems) or stadium builds when you need to daisy-chain antennas. A standard splitter divides power 50/50. A Tapper allows you to “tap off” a small amount of power (e.g., -10dB) for a nearby antenna while sending the bulk of the signal down the line to antennas further away, ensuring uniform coverage levels.
16. PCI: How does PCI Modulo 30 planning in 5G NR differ from LTE Modulo 3? A: In LTE, PCI Mod 3 determines the Reference Signal (RS) frequency shift; collisions cause severe interference. In 5G NR, the DMRS (Demodulation Reference Signal) positions are determined by PCI Mod 30. Therefore, in 5G, you must ensure neighbors do not share the same PCI Mod 30 to avoid DMRS collisions, which would prevent the UE from decoding the channel.
17. RSI: How does the “Zero Correlation Zone” (ZCZ) relate to cell size? A: The ZCZ config defines the cyclic shift applied to the Root Sequence for RACH preambles. A larger ZCZ allows the cell to handle RACH attempts from users further away (larger cell radius) without ambiguity. However, a larger ZCZ reduces the number of available preambles per root sequence. If you configure a small ZCZ for a massive rural cell, distant users may fail to access the network (RACH Ghosting).
18. Beamwidth: How does Vertical Beamwidth affect the “Cell Radius”? A: There is a direct trade-off: Narrower Vertical Beamwidth = Higher Antenna Gain.
- High Gain (Narrow V-Beam): Pushes signal further (longer radius) but is harder to contain; difficult to create a sharp drop-off at the cell edge.
- Low Gain (Wide V-Beam): Shorter radius, but easier to tilt down and contain within a dense urban grid.
19. Diplexer: Can a Diplexer be used as a filter to block interference? A: Yes, inherently. Because a Diplexer consists of a High-Pass and a Low-Pass filter (or two Band-Pass filters), it provides isolation. For example, if you have an external interferer at 800MHz and you are operating at 1900MHz, passing your signal through a 1900MHz port on a diplexer will naturally attenuate the 800MHz noise, acting as a “poor man’s filter.”
20. Combiner: What happens if I ignore the “Port-to-Port Isolation” spec? A: Disaster. Isolation ensures that the high-power Tx signal from Radio A doesn’t flow backwards into the Tx port of Radio B. If isolation is poor (<20dB), the energy from Radio A enters Radio B’s output stage, causing Intermodulation Distortion (IMD) and potentially burning out the amplifiers in Radio B.