Orthogonal Frequency Division Multiplexing

A major drawback of OFDM is:

a: High out-of-band radiation Explanation

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b: High Peak-to-Average Power Ratio (PAPR) Explanation

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c: Requires time synchronization only Explanation

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d: Inability to handle multipath fading Explanation

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Which of the following techniques is used to reduce PAPR in OFDM?

a: Zero forcing Explanation

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b: Clipping and filtering Explanation

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c: MMSE equalization Explanation

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d: Adding longer cyclic prefix Explanation

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Why is a cyclic prefix used instead of a simple zero-padding in OFDM?

a: It simplifies time synchronization Explanation

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b: It reduces PAPR Explanation

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c: It preserves circular convolution structure, enabling diagonalization via FFT Explanation

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d: It increases spectral efficiency Explanation

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The number of subcarriers in an OFDM system is typically:

a: Prime number Explanation

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b: Very large to reduce delay Explanation

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c: A power of 2 for efficient FFT implementation Explanation

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d: Equal to the modulation order Explanation

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OFDM achieves robustness to frequency-selective fading primarily because:

a: It transmits in the time domain only Explanation

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b: It averages fading over multiple antennas Explanation

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c: Each subcarrier sees flat fading due to narrowband nature Explanation

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d: It uses non-coherent modulation Explanation

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What does 'orthogonal' refer to in OFDM?

a: The sub-carriers are all orthogonal to each other, which can mitigate inter-carrier interference (ICI) Explanation

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b: The basis signals used to perform QAM encoding are orthogonal Explanation

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c: Both of the above Explanation

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d: None of the above Explanation

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Which one of the following is NOT correct in the context of OFDM?

a: We consider flat fading over the channel corresponding to each sub-carrier Explanation

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b: We consider flat fading over the entire channel Explanation

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c: This scheme provides efficient bandwidth usage Explanation

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d: This scheme can be used in a MIMO system to achieve frequency as well as spatial multiplexing Explanation

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What is the length of the FIR of the channel?

a: N Explanation

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b: N+μ Explanation

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c: N+μ-1 Explanation

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d: μ Explanation

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What is the rate of this scheme (in no. of QAM symbols per sampling time)?

a: N+μ Explanation

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b: μ Explanation

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c: N Explanation

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d: None of the above Explanation

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Which of the following is correct?

a: Channel state information (CSI) at the transmitter is necessary for this scheme to be effective Explanation

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b: Choosing a larger number of sub-carriers will lead to more accurate (less errors) communication Explanation

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c: If too many sub-carriers are used, it might lead to inter-carrier interference (ICI) Explanation

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d: Cyclic prefix is mandatory even if this scheme is implemented in continuous time Explanation

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