Virtual Labs

Study of Frequency division multiplexed schemes

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

a: Each subcarrier experiences flat fading due to narrow bandwidth b: All subcarriers experience the same channel response c: OFDM avoids multipath fading using time-domain filtering d: Subcarriers remain orthogonal without a cyclic prefix

What is the effect of increasing the number of OFDM subcarriers while keeping bandwidth fixed?

a: Decreases symbol duration and increases ICI b: Reduces subcarrier spacing and increases sensitivity to Doppler c: Increases total bandwidth and reduces SNR d: Simplifies channel estimation

What is the main purpose of the cyclic prefix in OFDM under dispersive channels?

a: Eliminates the need for channel equalization b: Converts linear convolution to circular convolution, preserving orthogonality c: Increases modulation order d: Equalizes subcarrier amplitudes

What is a major limitation of OFDM in mobile environments?

a: Completely immune to Doppler spread b: Time variation within a symbol breaks orthogonality, causing ICI c: Cannot operate at high data rates d: Has extremely low power efficiency

In OFDM, what is the effect of increasing modulation order?

a: Improves noise immunity but reduces data rate b: Increases data rate but also increases BER in low-SNR channels c: Reduces PAPR due to simpler constellations d: Eliminates the need for cyclic prefix

Why is IFFT used at the OFDM transmitter?

a: To convert time-domain data to frequency domain b: To spread the data using coding c: To generate orthogonal time-domain waveform from frequency-domain symbols d: To filter high-frequency noise

In an OFDM system, how does channel estimation error primarily affect system performance?

a: It increases cyclic prefix overhead b: It leads to incorrect equalization, increasing symbol detection errors c: It changes subcarrier spacing dynamically d: It eliminates inter-symbol interference

How does adaptive modulation improve spectral efficiency in OFDM?

a: By assigning modulation order per subcarrier according to channel conditions b: By varying cyclic prefix length across symbols c: By transmitting pilot symbols on all subcarriers d: By reducing IFFT size to fit more data per symbol

How does timing offset at the OFDM receiver affect performance?

a: Shifts the signal’s center frequency b: Breaks subcarrier orthogonality, causing inter-carrier interference c: Increases signal amplitude d: Improves phase synchronization

In an OFDM system, what is the primary consequence of a channel delay spread exceeding the cyclic prefix duration?

a: Improved spectral efficiency b: Loss of orthogonality causing both ISI and inter-carrier interference c: Reduction in peak-to-average power ratio d: Increased subcarrier spacing