Virtual Labs

Study of Frequency division multiplexed schemes

1. What fundamental mathematical condition ensures orthogonality among OFDM subcarriers?

a: Subcarrier spacing equal to the baud rate Explanation

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b: Subcarrier spacing equal to the inverse of the OFDM symbol duration Explanation

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c: All subcarriers have unique amplitudes Explanation

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d: Cyclic prefix duration equals one symbol duration Explanation

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2. In practical OFDM systems, which impairment causes loss of orthogonality among subcarriers?

a: Phase noise Explanation

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b: Frequency offset Explanation

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c: Nonlinear amplification Explanation

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d: All of the above Explanation

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3. What is the main disadvantage of using a long cyclic prefix in OFDM?

a: It increases the peak-to-average power ratio (PAPR) Explanation

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b: It reduces the bit error rate (BER) Explanation

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c: It increases spectral efficiency Explanation

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d: It decreases useful data rate Explanation

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4. What condition must be satisfied to avoid inter-symbol interference (ISI) in an OFDM system?

a: Symbol duration must be longer than delay spread Explanation

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b: Guard interval must be longer than maximum delay spread Explanation

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c: IFFT size must be equal to FFT size Explanation

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d: Carrier spacing must be larger than coherence bandwidth Explanation

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5. Why is OFDM particularly sensitive to frequency synchronization errors?

a: OFDM uses a single carrier for data transmission Explanation

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b: Orthogonality of subcarriers breaks down with frequency shifts Explanation

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c: Guard intervals cause spectral distortion Explanation

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d: FFT is performed before the cyclic prefix is removed Explanation

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6. In an OFDM system with 1024 subcarriers, what does increasing the number of zero-padded subcarriers achieve?

a: Reduces PAPR Explanation

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b: Increases energy per symbol Explanation

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c: Controls bandwidth and reduces spectral leakage Explanation

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d: Improves timing synchronization Explanation

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7. Why does OFDM inherently suffer from a high peak-to-average power ratio (PAPR)?

a: Because it uses constant-envelope modulation Explanation

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b: Because multiple subcarriers can constructively interfere Explanation

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c: Because the cyclic prefix increases signal power Explanation

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d: Because it lacks time-domain filtering Explanation

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8. In OFDM, what would happen if the cyclic prefix is shorter than the maximum delay spread of the channel?

a: It improves frequency diversity Explanation

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b: It causes inter-symbol and inter-carrier interference Explanation

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c: It lowers the system’s latency Explanation

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d: It reduces the modulation order Explanation

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9. In an OFDM system, what impact does increasing FFT size (with fixed bandwidth) have on system performance?

a: Improves time resolution and increases ICI Explanation

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b: Reduces subcarrier spacing and improves frequency resolution Explanation

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c: Decreases the cyclic prefix overhead Explanation

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d: Improves spectral efficiency by reducing inter-symbol spacing Explanation

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10. Why is equalization in OFDM often simpler than in single-carrier systems in frequency-selective channels?

a: Each subcarrier experiences flat fading, enabling single-tap equalization Explanation

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b: OFDM inherently avoids multipath Explanation

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c: The cyclic prefix eliminates the need for equalization Explanation

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d: OFDM uses time-domain diversity Explanation

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