Virtual Labs

Study of Frequency division multiplexed schemes

In a practical OFDM system with N-point FFT and sampling rate f_s, what condition ensures orthogonality among subcarriers?

a: Subcarrier spacing equals the baud rate Explanation

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b: Subcarrier spacing equals f_s / N 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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Which impairments in practical OFDM systems can cause loss of subcarrier orthogonality?

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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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 the effective data rate Explanation

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To avoid inter-symbol interference (ISI) in OFDM, what must be ensured regarding the guard interval?

a: Symbol duration must be longer than the channel delay spread Explanation

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b: Guard interval (cyclic prefix) must be longer than the maximum channel delay spread Explanation

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c: IFFT size must be equal to FFT size Explanation

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d: Subcarrier spacing must exceed coherence bandwidth Explanation

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Why is OFDM 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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In an OFDM system with 1024 subcarriers, what is the purpose of zero-padding unused subcarriers?

a: Reduces PAPR Explanation

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b: Increases energy per symbol Explanation

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c: Reduces spectral leakage and controls bandwidth Explanation

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d: Improves timing synchronization Explanation

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Why does OFDM inherently have 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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What happens if the cyclic prefix is shorter than the channel’s maximum delay spread?

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 latency Explanation

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d: It reduces modulation order Explanation

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In an OFDM system with fixed bandwidth, what is the effect of increasing the FFT size?

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 symbol spacing Explanation

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

a: Each subcarrier experiences flat fading, allowing 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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