Electronics & Comm. (ECE) 16 min read
Digital Communications: QPSK, 16-QAM, OFDM & Bit Error Rate (BER) Analysis
Modern modulation techniques, multicarrier communications, and error control coding.
#Digital Comm#OFDM#QPSK#QAM#BER#Wireless
In-Depth Interview Questions & Model Solutions
Q1Why is OFDM (Orthogonal Frequency Division Multiplexing) robust against Frequency Selective Multipath Fading?
OFDM divides a high-speed broadband serial data stream into N parallel low-rate sub-carriers. Because the symbol duration on each subcarrier (Ts) becomes much larger than the channel multipath delay spread (τ), the wideband frequency-selective channel is transformed into N flat-fading narrowband subchannels. Crucially, a Cyclic Prefix (CP) longer than the channel delay spread is prepended to each OFDM symbol, completely eliminating Inter-Symbol Interference (ISI) and converting linear channel convolution into circular convolution solvable via simple 1-tap frequency domain equalization (FFT).
Key Technical Takeaways:
- Converts frequency-selective fading into flat fading subcarriers.
- Cyclic Prefix preserves subcarrier orthogonality and eliminates ISI.
- High Peak-to-Average Power Ratio (PAPR) requires linear RF power amplifiers.
Technical Panel Interview Strategy Tips
- Be prepared to sketch constellation diagrams for BPSK, QPSK, 8-PSK, and 16-QAM.
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