At the optimal sampling instant, which quantity does a matched filter maximize?
A Symbol count
B Carrier frequency
C Output SNR
D Quantizer range
Answer: C. Output SNR. The matched filter maximizes output signal-to-noise ratio at its decision sample.
For real s(t), what is the matched-filter impulse response?
A Constant pulse
B Time-reversed pulse
C Time-advanced pulse
D Differentiated pulse
Answer: B. Time-reversed pulse. For a real signal over duration T, h(t) is proportional to s(T−t).
Nyquist pulse shaping prevents ISI by requiring what?
A Infinite bandwidth
B No carrier
C Equal symbol energies
D Zero at other symbol times
Answer: D. Zero at other symbol times. At every noncentral symbol-spaced sample, the pulse response must be zero.
Increasing raised-cosine roll-off does what?
A Increases excess bandwidth
B Adds parity bits
C Halves symbol energy
D Removes all noise
Answer: A. Increases excess bandwidth. A larger roll-off factor widens the transition band and uses more bandwidth.
Two root-raised-cosine filters cascade to which response?
A Gaussian
B Raised-cosine
C Rectangular
D Triangular
Answer: B. Raised-cosine. Matched root-raised-cosine transmit and receive filters form an overall raised-cosine response.
Equalizers mainly counter which channel effect?
A Quantizer overload
B Channel-induced ISI
C Source entropy
D Carrier generation
Answer: B. Channel-induced ISI. Equalization compensates for channel distortion that spreads symbols in time.
With equal priors in AWGN, which signal decision is optimal?
A Longest codeword
B Highest-energy point
C Nearest Euclidean point
D Lowest-frequency tone
Answer: C. Nearest Euclidean point. For equal-probability signals in AWGN, maximum likelihood selects the nearest signal point.
What extra information does MAP use beyond likelihood?
A Prior probabilities
B Carrier amplitude
C Sampling interval
D Pulse width
Answer: A. Prior probabilities. MAP decisions use prior probabilities as well as the received-signal likelihood.
BER counts errors among which units?
A Code blocks
B Bits
C Carrier cycles
D Symbols only
Answer: B. Bits. Bit-error rate counts incorrect bits relative to transmitted bits.
At a fixed target BER, coding gain usually reduces what requirement?
A Carrier phase
B Code length
C Sampling frequency
D Required Eb/N0
Answer: D. Required Eb/N0. Coding gain achieves the same error performance with lower required energy per bit.
Minimum distance 4 allows a code to detect up to how many errors?
A 3 errors
B 1 error
C 4 errors
D 2 errors
Answer: A. 3 errors. A code of distance d can detect up to d−1 errors.
After PCM decoding, which stage smooths the analog reconstruction?
A Notch filter
B Limiter
C Matched filter
D Low-pass filter
Answer: D. Low-pass filter. A reconstruction low-pass filter smooths the decoded sample waveform.
Equiprobable binary symbols have what entropy?
A 2 bits per symbol
B 4 bits per symbol
C 1 bit per symbol
D 0 bits per symbol
Answer: C. 1 bit per symbol. Two equally likely symbols carry one bit of entropy each.
What term describes received-signal fluctuations caused by multipath?
A Companding
B Aliasing
C Quantization
D Fading
Answer: D. Fading. Multipath propagation causes time-varying signal amplitude and phase called fading.
Differential phase encoding conveys data through what?
A Phase changes
B Pulse width
C Absolute amplitude
D Carrier spacing
Answer: A. Phase changes. Differential schemes encode information in the phase change between successive symbols.
Why use Gray labels for neighboring QAM points?
A Limit bit errors
B Reduce symbol count
C Remove synchronization
D Increase carrier power
Answer: A. Limit bit errors. A nearest-neighbor symbol mistake then tends to alter only one bit.
How does OFDM arrange its subchannels?
A Unequal sample clocks
B Overlapping random tones
C Serial carrier pulses
D Orthogonal subcarriers
Answer: D. Orthogonal subcarriers. OFDM divides transmission among mutually orthogonal subcarriers.
In OFDM, a cyclic prefix should cover what?
A Codeword length
B Quantizer range
C Maximum delay spread
D Carrier phase
Answer: C. Maximum delay spread. A sufficiently long prefix absorbs channel echoes and prevents intersymbol interference.
Small vertical opening in an eye indicates what?
A Wide bandwidth
B Low noise margin
C High code rate
D Strong carrier
Answer: B. Low noise margin. The eye’s vertical opening represents amplitude/noise margin.
Where should a receiver sample within an open eye?
A Transition edge
B Zero crossing
C Eye center
D Pulse tail
Answer: C. Eye center. The eye center offers maximum timing and amplitude margin.
Spectral efficiency equals which ratio?
A Energy per sample
B Bit rate per hertz
C Errors per symbol
D Phase per bit
Answer: B. Bit rate per hertz. Spectral efficiency is data rate divided by occupied bandwidth.
At fixed average power, higher-order QAM has what adjacent-point spacing?
A Wider pulse period
B Equal symbol energy
C Larger guard interval
D Smaller Euclidean distance
Answer: D. Smaller Euclidean distance. More constellation points share the same power region, so adjacent points are closer.
Symbol synchronization aligns what?
A Quantizer levels
B Carrier amplitudes
C Sampling instants
D Codeword lengths
Answer: C. Sampling instants. Timing recovery places receiver samples at the intended symbol decision instants.
Why interleave encoded bits before transmission?
A Spread burst errors
B Remove quantization
C Increase carrier power
D Reduce symbol alphabet
Answer: A. Spread burst errors. Interleaving distributes a burst across codewords so the decoder can handle it.
Which binary scheme can compare tone energies without carrier phase?
A 16-QAM
B Noncoherent FSK
C Coherent BPSK
D Coherent QPSK
Answer: B. Noncoherent FSK. Noncoherent FSK can detect tones by energy without a phase reference.