Virtual Labs

Study of Frequency division multiplexed schemes

1. Why does OFDM perform better than single-carrier systems in frequency-selective channels?

a: Each subcarrier experiences flat fading due to reduced bandwidth per subcarrier Explanation

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b: All subcarriers experience the same channel response Explanation

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c: OFDM avoids multipath fading by using time-domain filtering Explanation

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d: Subcarriers do not interfere even in the absence of cyclic prefix Explanation

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2. How does timing offset in an OFDM receiver impact system performance?

a: It shifts the center frequency of the signal Explanation

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b: It destroys orthogonality and causes inter-carrier interference Explanation

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c: It increases the signal amplitude Explanation

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d: It improves phase synchronization Explanation

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3. Why does OFDM have a high peak-to-average power ratio (PAPR)?

a: All subcarriers are modulated with BPSK only Explanation

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b: The cyclic prefix increases the peak power Explanation

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c: Multiple sinusoids may add constructively, leading to high peaks Explanation

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d: Frequency offset increases the signal's average power Explanation

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4. How does increasing the number of OFDM subcarriers (with fixed bandwidth) impact the system?

a: It decreases the symbol duration and increases ICI Explanation

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b: It reduces subcarrier spacing and increases sensitivity to Doppler shift Explanation

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c: It increases bandwidth and decreases SNR Explanation

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d: It simplifies channel estimation Explanation

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5. Which of the following best describes the function of the cyclic prefix in OFDM under a dispersive channel?

a: It eliminates the need for channel equalization Explanation

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b: It converts linear convolution into circular convolution, preserving subcarrier orthogonality Explanation

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c: It increases modulation order Explanation

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d: It ensures each subcarrier has equal amplitude Explanation

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6. What is a major disadvantage of OFDM in mobile environments?

a: It is completely immune to Doppler spread Explanation

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b: Time variation within one symbol duration causes loss of orthogonality Explanation

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c: It cannot operate at high data rates Explanation

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d: It has extremely low power efficiency Explanation

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7. In an OFDM system using 64-QAM, what does increasing the modulation order do?

a: Improves noise immunity but reduces data rate Explanation

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b: Increases data rate but also increases BER in noisy channels Explanation

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c: Reduces PAPR due to simpler constellations Explanation

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d: Eliminates the need for cyclic prefix Explanation

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8. Why is the IFFT used at the transmitter in OFDM systems?

a: To convert time-domain data to frequency domain Explanation

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b: To spread the data over the channel using coding Explanation

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c: To synthesize orthogonal time-domain waveform from frequency-domain symbols Explanation

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d: To filter out high-frequency noise Explanation

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9. In an OFDM system, what would be the primary effect of a frequency offset between the transmitter and receiver?

a: It reduces ISI by improving synchronization Explanation

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b: It causes inter-carrier interference and spectral leakage Explanation

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c: It improves orthogonality among subcarriers Explanation

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d: It compresses the time-domain signal Explanation

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10. Which of the following best explains spectral efficiency in OFDM with adaptive modulation?

a: Using different modulation orders across subcarriers depending on channel conditions Explanation

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b: Employing cyclic prefixes of variable length across symbols Explanation

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c: Transmitting pilot symbols in every subcarrier Explanation

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d: Reducing IFFT size to fit more data per symbol Explanation

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