Digital Communication — Part 1 MCQs with Explanations

8 MHz is the correct answer to “A 4 GHz carrier is DSB-SC modulated by a low pass message signal with maximum frequency of 2 MHz. The resultant signal is to be ideally sampled. The minimum frequency of the sampling impulse train should be”. This question is standard in competitive examination syllabi for Digital Communication.

6.4 Gbps is the correct answer to “A video transmission system transmits 625 picture frames per second. Each frame consists of a 400 × 400 pixel grid with 64 intensity levels per pixel. The data rate of the system is”. This question is standard in competitive examination syllabi for Digital Communication.

0.5 is the correct answer to “The raised cosine pulse p(t) is used for zero ISI in digital communications. The expression for p(t) with unity roll-off factor is given by $$p\left( t \right) = \frac{{\sin 4\pi \omega t}}{{4\pi \omega t\left( {1 - 16{\omega ^2}{t^2}} \right)}}.$$ The value of p(t) at $$t = \frac{1}{{4\omega }}$$ is”. This question is standard in competitive examination syllabi for Digital Communication.

$$\frac{1}{{2\pi }}$$ is the correct answer to “A sinusoidal signal of 2 kHz frequency is applied to a delta modulator. The sampling rate and step-size Δ of the delta modulator are 20,000 samples per second and 0.1 V, respectively. To prevent slope overload, the maximum amplitude of the sinusoidal signal (in Volts) is”. This question is standard in competitive examination syllabi for Digital Communication.

Q(4) is the correct answer to “Coherent orthogonal binary FSK modulation is used to transmit two equiprobable symbol waveforms s 1 (t) = αcos2πf 1 t & s 2 (t) = αcos2πf 2 t 1 where α = 4 mV. Assume an AWGN channel with two-sided noise power spectral density $$\frac{{{N_0}}}{2} = 0.5 \times {10^{ - 12}}W/Hz.$$ Using an optimal receiver and the relation $$Q\left( v \right) = \frac{1}{{\sqrt {2\pi } }}\int\limits_v^\infty {{\theta ^{\frac{{{u^2}}}{2}}}du,} $$ the bit error probability for a data rate of 500 kbps is”. This question is standard in competitive examination syllabi for Digital Communication.

np (1 - p)n - 1 + (1 - p)n is the correct answer to “During transmission over a certain binary communication channel, bit errors occur independently with probability p. The probability of AT MOST one bit in error in a block of n bits is given by”. This question is standard in competitive examination syllabi for Digital Communication.

0, π, π, π is the correct answer to “The message bit sequence to a DPSK modulator is 1, 1, 0, 0, 1, 1. The carrier phase during the reception of the first two message bits is π, π. The carrier phase for the remaining four message bits is”. This question is standard in competitive examination syllabi for Digital Communication.

3 kHz is the correct answer to “A deterministic signal has the power spectrum given in figure. The minimum sampling rate needed to completely represent signal is”. This question is standard in competitive examination syllabi for Digital Communication.

(1 - p)n + np(1 - p)n - 1 is the correct answer to “During transmission over a communication channel, bit errors occur independently with probability p. If a block of n bits is transmitted, the probability of at most one bit error is equal to”. This question is standard in competitive examination syllabi for Digital Communication.

Number of quantizing levels is the correct answer to “A signal having uniformly distributed amplitude in the interval (-V to + V), is to be encoded using PCM with uniform quantization. The signal to quantizing noise ratio is determined by the”. This question is standard in competitive examination syllabi for Digital Communication.

Lower amplitudes only is the correct answer to “Companding in PCM systems lead to improved signal to quantization noise ratio. This improvement is for”. This question is standard in competitive examination syllabi for Digital Communication.

0.15 is the correct answer to “Consider a binary digital communication system with equally likely 0's and 1's. When binary 0 is transmitted the detector input can lie between the levels -0.25V and +0.25V with equal probability: when binary 1 is transmitted, the voltage at the detector can have any value between 0 and 1 V with equal probability. If the detector has a threshold of 0.2 V (i.e., if the received signal is greater than 0.2 V, the bit is taken as 1), the average bit error probability is”. This question is standard in competitive examination syllabi for Digital Communication.

B1 = 20 kHz, B2 = 10 kHz is the correct answer to “A source produces binary data at the rate of 10 kbps. The binary symbols are represented as shown in the figure. The source output is transmitted using two modulation schemes, namely Binary PSK (BPSK) and Quadrature PSK (QPSK). Let and B2 be the bandwidth requirements of BPSK and QPSK respectively. Assuming that the bandwidth of the above rectangular pulses is 10 kHz, B 1 and B 2 are”. This question is standard in competitive examination syllabi for Digital Communication.

Gaussian Minimum Shift Keying (GMSK) is the correct answer to “The modulation scheme commonly used for transmission from GSM mobile terminals is”. This question is standard in competitive examination syllabi for Digital Communication.

p3 + 3p2 (1- p) is the correct answer to “Consider a Binary Symmetric Channel (BSC) with probability of error being p. To transmit a bit, say 1, we transmit a sequence of three 1 s. The receiver will interpret the received sequence to represent 1 if at least two bits are 1. The probability that the transmitted bit will be received in error is”. This question is standard in competitive examination syllabi for Digital Communication.

Increasing with increasing value of 'n' is the correct answer to “The signal to quantization noise ratio in an n-bit PCM system”. This question is standard in competitive examination syllabi for Digital Communication.

The difference of message signal sample with its prediction is quantized is the correct answer to “Which one of the following statements about differential pulse code modulation (DPCM) is true”. This question is standard in competitive examination syllabi for Digital Communication.

Increasing the step size is the correct answer to “In delta modulation, the slope overload distortion can be reduced by”. This question is standard in competitive examination syllabi for Digital Communication.

2.7, 3.3 is the correct answer to “Let x(t) = 2cos(800πt) + cos(1400πt).x(t) is sampled with the rectangular pulse train shown in the figure. The only spectral components (in kHz) present in the sampled signal in the frequency range 2.5 kHz to 3.5 kHz are”. This question is standard in competitive examination syllabi for Digital Communication.

10 dB is the correct answer to “If E b , the energy per bit of a binary digital signal, is 10 -5 Watt-sec and the one-sided power spectral density of the white noise, N 0 = 10 -6 W/Hz, then the output SNR of the matched filter is”. This question is standard in competitive examination syllabi for Digital Communication.

200 μsec is the correct answer to “In a digital communication system employing Frequency Shift Keying (FSK), the 0 and 1 bit are represented by sine waves of 10 kHz and 25 kHz respectively. These waveforms will be orthogonal for a bit interval of”. This question is standard in competitive examination syllabi for Digital Communication.

2n dB is the correct answer to “If the number of bits per sample in a PCM system is increased from a n to n + 1, the improvement in signal to quantization nose ratio will be”. This question is standard in competitive examination syllabi for Digital Communication.

Gives rise to aperture effect is the correct answer to “Flat top sampling of low pass signals”. This question is standard in competitive examination syllabi for Digital Communication.

10 Volts is the correct answer to “The input to a matched filter is given by \[s\left( t \right) = \left\{ {\begin{array}{*{20}{c}} {10\sin \left( {2\pi \times {{10}^6}} \right)}&{0 < t < {{10}^{ - 4}}\sec } \\ 0&{{\text{Otherwise}}} \end{array}} \right.\] The peak amplitude of the filter output is”. This question is standard in competitive examination syllabi for Digital Communication.

3 dB is the correct answer to “At a given probability of error, binary coherent FSK is inferior to binary coherent PSK by”. This question is standard in competitive examination syllabi for Digital Communication.

32 kHz and 40 kHz is the correct answer to “For a bit-rate of 8 Kbps, the best possible values of the transmitted frequencies in a coherent binary FSK system are”. This question is standard in competitive examination syllabi for Digital Communication.

$$\frac{\Delta }{2}$$ is the correct answer to “In the following figure the minimum value of the constant "C", which is to be added to y 1 (t) such that y 1 (t) and y 2 (t) are different, is”. This question is standard in competitive examination syllabi for Digital Communication.

12 kHz and 8 kHz is the correct answer to “A signal x(t) = 100cos(24π × 10 3 )t is ideally sampled with a sampling period of 50 psec and then passed through an ideal lowpass filter with cutoff frequency of 15 kHz. Which of the following frequencies is/are present at the filter output”. This question is standard in competitive examination syllabi for Digital Communication.

The input to the modulator is essentially constant is the correct answer to “In the output of a DM speech encoder, the consecutive pulses are of opposite polarity during time interval t 1 ≤ t ≤ t 2 . This indicates that during this interval”. This question is standard in competitive examination syllabi for Digital Communication.

$$Q\left( {\sqrt {\frac{E}{{{N_0}}}} } \right)$$ is the correct answer to “A BPSK scheme operating over an AWGN channel with noise power spectral density of $$\frac{{{N_0}}}{2},$$ uses equiprobable signals $${s_1}\left( t \right) = \sqrt {\frac{{2E}}{T}} \sin \left( {{\omega _c}t} \right)$$ and $${s_2}\left( t \right) = \sqrt {\frac{{2E}}{T}} \sin \left( {{\omega _c}t} \right)$$ over the symbol internal (0, T). If the local oscillator in a coherent receiver is ahead in phase by 45° with respect to the received signal, the probability of error in the resulting system is”. This question is standard in competitive examination syllabi for Digital Communication.

5 × 10-1 cos(8π × 103)t is the correct answer to “Consider a sample signal \[y\left( t \right) = 5 \times {10^{ - 6}}x\left( t \right)\sum\nolimits_{ - \infty }^{ + \infty } {\delta \left( {t - n{T_s}} \right)} \] where x(t) = 10cos(8π × 10 3 )t and T s =100 μsec. When y(t) is passed through an ideal lowpass filter with a cutoff frequency of 5 kHz, the output of the filter is”. This question is standard in competitive examination syllabi for Digital Communication.

1000 is the correct answer to “The Nyquist sampling frequency (in Hz) of a signal given by 6 × 10 4 sinC 3 (400t) ∗ 10 6 sinC 3 (100t) is”. This question is standard in competitive examination syllabi for Digital Communication.

12 is the correct answer to “In a PCM system, if the code word length is increased from 6 to 8 bits, the signal to quantization noise ratio improves by the factor”. This question is standard in competitive examination syllabi for Digital Communication.

57.6 kpbs is the correct answer to “Three analog signals, having bandwidths 1200 Hz, 600 Hz and 600 Hz, are sampled at their respective Nyquist rates, encoded with 12-bit words, and time division multiplexed. The bit rate for the multiplexed signal is”. This question is standard in competitive examination syllabi for Digital Communication.

95.6 is the correct answer to “The peak-to-peak input to an 8-bit PCM coder is 2 Volts. The signal power-to-quantization noise power ratio (in dB) for an input of 0.5cos(ω m t) is”. This question is standard in competitive examination syllabi for Digital Communication.

1200 Hz is the correct answer to “The Nyquist sampling rate for the signal $$s\left( t \right) = \frac{{\sin \left( {500\pi t} \right)}}{{\pi t}} \times \frac{{\sin \left( {700\pi t} \right)}}{{\pi t}}$$ is given by”. This question is standard in competitive examination syllabi for Digital Communication.

$$\frac{R}{{10}}kHz$$ is the correct answer to “The bit rate of digital communication system is R kbits/s. The modulation used is 32-QAM. The minimum bandwidth required for |S| free transmission is”. This question is standard in competitive examination syllabi for Digital Communication.

$$\frac{7}{{80}}$$ is the correct answer to “A binary symmetric channel (BSC) has a transition probability of $$\frac{1}{8}.$$ If the binary transmit symbol X is such that $$P\left( {X = 0} \right) = \frac{9}{{10}},$$ then the probability of error for an optimum receiver will be”. This question is standard in competitive examination syllabi for Digital Communication.

1-S, 2-P, 3-U, 4-Q is the correct answer to “Choose the correct one from among the alternatives a, b, c, dafter matching an item from Group 1 with the most appropriate item in Group 2 . Group 1 Group 2 1. FM P. Slope overload 2. DM Q. H-law 3. PSK R. Envelope detector 4. PCM S. Capture effect T. Hilbert transform U. Matched filter”. This question is standard in competitive examination syllabi for Digital Communication.

$$\frac{1}{6}$$ is the correct answer to “A source emits bit 0 with probability $$\frac{1}{3}$$ and bit 1 with probability $$\frac{2}{3}.$$ The emitted bits are communicated to the receiver. The receiver decides for either 0 or 1 based on the received value R. It is given that the conditional density functions of R as \[\begin{gathered} {f_{R|0}}\left( r \right) = \left\{ {\begin{array}{*{20}{c}} {\frac{1}{4},}&{ - 3 \leqslant r \leqslant 1} \\ {0,}&{{\text{otherwise;}}} \end{array}} \right. \hfill \\ {f_{R|1}}\left( r \right) = \left\{ {\begin{array}{*{20}{c}} {\frac{1}{6},}&{ - 1 \leqslant r \leqslant 5} \\ {0,}&{{\text{otherwise;}}} \end{array}} \right. \hfill \\ \end{gathered} \] The minimum decision error probability is”. This question is standard in competitive examination syllabi for Digital Communication.