5G MAC Layer Interview Questions – Expert Scenario Based (20 Q&A)
This page provides expert-level 5G MAC layer interview questions ranging from core concepts to real-field troubleshooting and lab scenarios. These questions are designed for RAN engineers, protocol testers, system integrators, and L2/L3 support roles.
Conceptual & Design-Level Questions
Q1. Why is MAC layer critical in 5G performance?
MAC controls scheduling, resource allocation and HARQ. It relies on buffer information from the
RLC layer, making it central to throughput and latency.
Q2. How does MAC differ in 5G compared to LTE?
5G MAC supports flexible numerology, multiple BWPs, tighter PHY interaction and granular scheduling.
Q3. What inputs does MAC scheduler use?
BSR reports, QoS priority, channel quality, HARQ feedback and available radio resources.
Q4. How does MAC maintain fairness?
Fairness is achieved using scheduling algorithms balancing priority, buffer size and historical resource usage.
Practical Scheduling & Resource Management
Q5. UE has data but no uplink grant – why?
Missing BSR from
RLC, scheduler congestion, low priority logical channel or inactive BWP.
Q6. How does MAC prioritize control vs user traffic?
Control channels are scheduled first; user data uses remaining resources based on priority.
Q7. Impact of delayed BSR?
Scheduler remains unaware of uplink buffer, causing uplink delay and throughput degradation.
Q8. How are multiple logical channels handled?
MAC multiplexes logical channels based on priority and QoS configuration.
HARQ & Reliability Scenarios
Q9. Why do frequent HARQ retransmissions occur?
Poor radio quality, aggressive MCS or PHY decoding failure; unresolved errors propagate to
RLC retransmissions.
Q10. How does MAC choose retransmission vs new data?
Retransmissions are prioritized to ensure reliability while balancing new data scheduling.
Q11. Which logs indicate HARQ issues?
HARQ process IDs, NACK count, retransmission count and MCS downgrade patterns.
Q12. Can HARQ impact latency?
Yes. Multiple retransmissions increase latency and may affect reordering at the
PDCP layer.
Real Field & Log Analysis Scenarios
Q13. Throughput drops after BWP switching – why?
Reduced bandwidth, scheduling restrictions or delayed BWP activation.
Q14. Control signaling overload impact?
Control traffic consumes resources, starving user-plane scheduling.
Q15. How to correlate MAC and PHY logs?
Match HARQ feedback, MCS changes and scheduling grants with PHY error reports.
Q16. MAC scheduling delay seen in logs?
Delayed BSR, scheduler congestion or MAC-PHY processing latency.
LAB & Hands-On Scenarios
Q17. Lab shows low throughput despite good signal?
MAC scheduler limits, QoS misconfiguration or restricted bandwidth allocation.
Q18. How to verify MAC scheduling in lab?
Analyze grants, HARQ behavior, throughput KPIs and scheduler traces.
Q19. Repeated HARQ NACKs in lab – debug steps?
Check MCS selection, signal quality, timing alignment and PHY decoding.
Q20. How to design a MAC stress test?
Generate heavy traffic, vary priorities and monitor fairness, latency and HARQ behavior.
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