Cellular Network Interview Questions Planning Handoff 5G

Essential interview questions and detailed answers covering cellular network fundamentals including frequency reuse, cell planning, handoff mechanisms, capacity optimization, and 5G architecture for engineering job interviews.

Fundamental Concepts

Q1: Why do cellular networks use hexagonal cells rather than circular cells?

Answer: Hexagonal cells are a mathematical idealization. Real cells have irregular shapes determined by terrain and propagation. We use hexagons in analysis because:

• Hexagons tile a plane without gaps or overlap (circles leave gaps)
• Hexagons approximate circles (closest regular polygon that tessellates)
• The hexagonal geometry simplifies frequency reuse analysis
• In flat terrain with identical base stations, cell boundaries approximate hexagons

In practice, cells are irregular blobs determined by building density, terrain elevation, and antenna configuration.

Q2: Explain frequency reuse factor and how to determine it.

Answer: Frequency reuse factor (N) is the number of cells in a cluster before frequencies repeat. For hexagonal geometry: N = i² + ij + j² where i, j are non-negative integers.

Valid values: N = 1, 3, 4, 7, 12, 13, 19...

Smaller N means more capacity (more channels per cell) but higher co-channel interference (same frequencies closer together). The minimum SIR requirement determines the minimum N:

SIR = (D/R)⁴ / 6 = (√(3N))⁴ / 6

For AMPS (requires SIR ≥ 18 dB): N = 7 For GSM (requires SIR ≥ 12 dB): N = 4 For CDMA (uses spreading codes): N = 1

Q3: What is the difference between cell splitting and sectoring?

Answer:

Handoff and Mobility

Q4: Compare hard handoff and soft handoff with advantages and disadvantages.

Answer:

Hard Handoff (Break-Before-Make):

• Connection to old cell broken before new cell connection established
• Brief interruption (~30-200 ms depending on technology)
• Used in: GSM, LTE
• Advantage: Simpler, uses fewer resources (only one channel active)
• Disadvantage: Risk of call drop if new cell unavailable

Soft Handoff (Make-Before-Break):

• Simultaneous connection to 2-3 cells during transition
• No interruption — seamless transition
• Used in: CDMA/WCDMA (possible because all cells use same frequency)
• Advantage: No dropped calls at boundary, 2-3 dB diversity gain
• Disadvantage: Uses multiple channels simultaneously (capacity cost), requires same-frequency cells

Q5: What is the ping-pong effect and how is it prevented?

Answer: Ping-pong occurs when a mobile at the cell boundary rapidly oscillates between two cells because their signal strengths are nearly equal. Each minor fluctuation triggers a handoff, creating signaling overhead and potential call quality issues.

Prevention mechanisms:

• Hysteresis margin (H): Target cell must be stronger by H dB (typically 2-5 dB) before triggering
• Time-to-trigger (TTT): Condition must persist for a duration (40-640 ms) before action
• Frequency offset: Different cells on different frequencies reduce measurement fluctuation
• A3 event with offset: LTE combines offset and TTT in measurement event configuration

Q6: Explain vertical handoff with an example.

Answer: Vertical handoff occurs between different Radio Access Technologies (RATs). Example: A user streaming video on 5G NR enters an elevator where NR coverage is lost but LTE coverage from a different frequency band remains. The device performs:

1. Measurement gap — UE measures LTE frequencies during configured gaps
2. B1 event — LTE cell quality exceeds threshold
3. Inter-RAT handoff preparation — Source gNB communicates with target eNB
4. Handoff execution — UE tunes to LTE, resumes data session

The session continues on LTE until NR coverage returns, triggering a reverse vertical handoff.

Capacity and Planning

Q7: How do you calculate the capacity of a cellular system?

Answer: System capacity = (Total allocated bandwidth / Channel bandwidth) × (1/N) × Number of cells × Sectors per cell × Technology efficiency factor

Example for GSM:

• Bandwidth: 25 MHz
• Channel: 200 kHz (8 timeslots)
• Available channels: 125
• Reuse factor: N = 4
• Channels per cell: 125/4 ≈ 31
• Timeslots: 31 × 8 = 248
• Control overhead: 248 - 3 BCCH = 245 traffic channels
• With 3 sectors: 245 × 3 = 735 per site
• Trunking (Erlang B, 2% blocking): ~700 Erlangs per site

Q8: What is the Erlang B formula and why is it important?

Answer: The Erlang B formula calculates the probability that all channels are busy (Grade of Service) given traffic intensity and number of channels:

B(N, A) = (Aᴺ/N!) / Σₖ₌₀ᴺ (Aᵏ/k!)

Where N = channels, A = offered traffic (Erlangs)

Importance: It determines how many channels are needed for a given traffic load and acceptable blocking rate. With 2% blocking target:

• 10 channels → 5.1 Erlangs capacity (51% efficiency)
• 50 channels → 40.3 Erlangs capacity (80.6% efficiency)
• 100 channels → 87.9 Erlangs capacity (87.9% efficiency)

Key insight: Trunking efficiency improves with more channels — another reason cell splitting (fewer channels per smaller cell) has diminishing returns.

5G Architecture Questions

Q9: What are the key differences between LTE and 5G NR architecture?

Answer:

Q10: Explain network slicing with a practical example.

Answer: Network slicing creates multiple virtual networks on shared physical infrastructure, each optimized for different requirements:

Example — A single 5G network serving three slices:

• eMBB slice (Streaming/gaming): High bandwidth (1 Gbps), moderate latency (10 ms), best-effort reliability
• URLLC slice (Factory robotics): Low bandwidth (1 Mbps), ultra-low latency (1 ms), 99.999% reliability
• mMTC slice (IoT sensors): Very low bandwidth (10 kbps), high latency acceptable (seconds), massive device density

Each slice has dedicated or shared resources at every network layer: radio scheduling priority, dedicated core network functions (SMF, UPF), and specific QoS policies. A factory might purchase all three slices from the same operator for different use cases.

Advanced Questions

Q11: Why does 5G use higher subcarrier spacing at higher frequencies?

Answer: Higher subcarrier spacing (SCS) at higher frequencies addresses two issues:

1. Phase noise — Oscillators at higher frequencies have more phase noise, which causes inter-carrier interference (ICI). Wider SCS increases tolerance to phase noise proportionally.
2. Latency — Higher SCS means shorter slot duration (0.5 ms at 30 kHz vs. 1 ms at 15 kHz), reducing latency.
3. Doppler — Higher frequencies experience larger Doppler shifts. Wider SCS makes the relative Doppler (as fraction of SCS) smaller.

Trade-off: Wider SCS means shorter cyclic prefix, reducing multipath tolerance. This is acceptable at mmWave where cells are small (short delay spread).

Q12: What is the near-far problem and how do CDMA and LTE solve it differently?

Answer: The near-far problem occurs when a strong signal from a nearby user overwhelms a weak signal from a distant user at the base station receiver.

CDMA solution — Power control: All mobiles adjust transmit power so they arrive at the base station with approximately equal power level. Fast closed-loop power control at 1500 Hz compensates for fading.

LTE solution — Orthogonal access: OFDMA assigns different subcarriers to different users — they never overlap in frequency within a cell. Since signals don't interfere, power differences don't cause the near-far problem. Power control in LTE is used for inter-cell interference management, not intra-cell.

Key Takeaways

1. Demonstrate understanding of WHY designs are chosen, not just what they are — interviewers value reasoning over memorization
2. Frequency reuse factor N determines the trade-off between capacity (lower N = more channels/cell) and interference (lower N = worse SIR)
3. Handoff questions test understanding of hard vs soft, ping-pong prevention, and the role of hysteresis and timers
4. Capacity calculations require understanding of Erlang B trunking efficiency — more channels yield disproportionately higher capacity
5. 5G architecture questions focus on network slicing, CU/DU split, beam management, and flexible numerology
6. Always relate theoretical concepts to practical implications (cost, user experience, deployment constraints)
7. Be prepared to compare technologies across generations (why 5G chose X differently from 4G) with clear technical reasoning