Wireless Communication Interview Questions Top 50 Q&A

Hindi: Wireless communication ke interviews/viva/exams mein frequently asked questions with detailed answers. Har question ke saath Hindi explanation bhi hai.

📶 Modulation & Multiple Access (Q11-Q25)

Q11: Compare AM, FM, and PM.

A: AM varies amplitude (simple, noisy), FM varies frequency (noise-immune, wide BW), PM varies phase (base for digital PSK). FM is most widely used for analog audio.

Q12: Why is QPSK preferred over BPSK?

A: QPSK transmits 2 bits/symbol (vs 1 for BPSK) with same BER per bit and half the bandwidth. Double data rate at same performance!

Q13: What is QAM? Why is it important?

A: Quadrature Amplitude Modulation varies both amplitude and phase. Enables high spectral efficiency (256-QAM = 8 bits/symbol). Used in WiFi, 4G, 5G for high-speed data.

Q14: Explain OFDM and why it's used in 4G/5G.

A: OFDM divides wideband channel into many narrow orthogonal subcarriers. Each subcarrier experiences flat fading (easy equalization). Cyclic prefix prevents ISI. Enables OFDMA (multi-user scheduling).

Q15: Compare FDMA, TDMA, CDMA, OFDMA.

A:

• FDMA: Divide by frequency (1G)
• TDMA: Divide by time (2G GSM)
• CDMA: Divide by code (3G)
• OFDMA: Divide by freq+time blocks (4G/5G) – most flexible, highest efficiency

Q16: What is spread spectrum? Types?

A: Spreading narrowband signal across wide bandwidth for interference resistance and security.

• DSSS (Direct Sequence): Multiply with high-rate code
• FHSS (Frequency Hopping): Rapidly change frequency

Q17: What is the near-far problem in CDMA?

A: Strong signal from nearby user overwhelms weak signal from far user. Solution: Fast power control (800-1500 times/sec) to equalize received powers.

Q18: Explain adaptive modulation and coding (AMC).

A: System dynamically changes modulation (QPSK→256-QAM) and coding rate based on channel quality. High SNR = high-order QAM (fast). Low SNR = QPSK (robust). Maximizes throughput under varying conditions.

Q19: What is carrier aggregation?

A: Combining multiple carriers (component carriers) to increase bandwidth. LTE-A: up to 5×20 MHz = 100 MHz. 5G: up to 16 carriers. Dramatically increases peak data rate.

Q20: Explain MIMO spatial multiplexing.

A: Multiple antennas at TX and RX create parallel data streams through multipath channel. N×N MIMO can achieve up to N× the SISO capacity. Requires rich scattering environment.

📱 Cellular & 5G (Q21-Q40)

Q21: What is the cellular concept?

A: Dividing coverage area into small cells with low-power base stations. Same frequencies reused in distant cells. Multiplies capacity by reuse factor.

Q22: What is frequency reuse? Formula for reuse distance?

A: Using same frequencies in geographically separated cells. D = R×√(3N), where N = cluster size, R = cell radius. Smaller N = more capacity but more interference.

Q23: What is handoff/handover?

A: Transferring active call from one cell to another as user moves. Hard handoff (break-before-make) in LTE. Soft handoff (make-before-break) in CDMA.

Q24: Explain the difference between FDD and TDD.

A: FDD: Uplink and downlink on different frequencies simultaneously. TDD: Same frequency, alternating in time. TDD better for asymmetric traffic, FDD better for latency.

Q25: What are the key features of 5G NR?

A: mmWave + sub-6 bands, Massive MIMO, flexible numerology, network slicing, MEC, 10 Gbps peak, 1ms latency, 1M devices/km². Three pillars: eMBB, URLLC, mMTC.

Q26: What is Massive MIMO?

A: 64-256 antennas at base station serving multiple users simultaneously via spatial beamforming. Provides 5-10× capacity increase, array gain for coverage, near-zero intra-cell interference.

Q27: Explain network slicing.

A: Creating multiple virtual networks on one physical infrastructure. Each slice customized (eMBB: speed, URLLC: latency, mMTC: devices). Enabled by NFV + SDN + cloud-native core.

Q28: What is beamforming?

A: Using antenna array phase/amplitude control to focus RF energy in specific direction. Analog (one beam), Digital (multi-beam), Hybrid (practical mmWave). Essential for 5G mmWave coverage.

Q29: What is the 5G core (5GC) architecture?

A: Service-Based Architecture with cloud-native network functions: AMF, SMF, UPF, NSSF, UDM, PCF. All interconnected via HTTP/2 APIs. Supports network slicing, edge computing.

Q30: Compare 4G LTE and 5G NR.

A: 5G: 20× peak speed (20 Gbps), 10× lower latency (1ms), 10× connection density, flexible SCS (15-240 kHz), up to 400 MHz BW, Massive MIMO, network slicing. LTE: 1 Gbps, 10ms, OFDMA, max 20 MHz per carrier.

🔒 Security & IoT (Q31-Q40)

Q31: Why is wireless security challenging?

A: Signals broadcast in air – anyone can intercept. No physical boundary. Must rely on encryption, authentication, access control protocols.

Q32: Compare WEP, WPA2, WPA3.

A: WEP: Broken (RC4, 24-bit IV). WPA2: Good (AES-CCMP, 4-way handshake). WPA3: Best (SAE, forward secrecy, offline attack immune). Always use WPA3!

Q33: What is the KRACK attack?

A: Key Reinstallation Attack on WPA2's 4-way handshake. Tricks client into reusing nonce → decrypts traffic. Fixed via patches. WPA3 not vulnerable.

Q34: What are LPWAN technologies? Compare LoRa and NB-IoT.

A: Low Power Wide Area Networks for IoT. LoRa: Unlicensed, private network, 50 kbps, 15 km. NB-IoT: Licensed (telecom), 200 kbps, QoS guaranteed. Choose based on deployment model.

Q35: What is MQTT? Why used in IoT?

A: Lightweight publish/subscribe messaging protocol. 2-byte header (vs HTTP's 100+), persistent connections, QoS levels, works on constrained devices. Standard for IoT messaging.

🧠 Advanced (Q36-Q50)

Q36: What is cognitive radio?

A: Intelligent radio that senses spectrum, identifies unused bands (spectrum holes), and dynamically uses them without harming primary users. Solves spectrum scarcity.

Q37: Explain Rayleigh vs Rician fading.

A: Rayleigh: No LOS, many scattered paths, severe fading (NLoS urban). Rician: Strong LOS + scattering, milder fading. K-factor measures LOS dominance. K=0 → Rayleigh, K→∞ → AWGN.

Q38: What is OFDMA's advantage over CDMA for 4G?

A: Orthogonal subcarriers = zero intra-cell interference (vs CDMA's self-interference). Flexible scheduling in time+frequency. Better MIMO support. Higher spectral efficiency (5-15 bps/Hz vs 1-2).

Q39: What is MEC (Multi-access Edge Computing)?

A: Processing at network edge (near base station) instead of distant cloud. Enables <5ms latency for AR/VR, gaming, autonomous vehicles. Part of 5G architecture.

Q40: What is the future of wireless (6G)?

A: Expected ~2030: THz frequencies, 1 Tbps speed, AI-native, holographic MIMO, RIS (reconfigurable surfaces), integrated sensing+communication, digital twins, space-air-ground integration.

📝 Quick Revision Tips