GATE ECE
Communications
Digital Communication Systems
Previous Years Questions

## Marks 1

Let H(X) denote the entropy of a discrete random variable X taking K possible distinct real values. Which of the following statements is/are necessari...
A symbol stream contains alternate QPSK and 16-QAM symbols. If symbols from this stream are transmitted at the rate of 1 mega-symbols per second, the ...
Consider a channel over which either symbol xA or symbol xB is transmitted. Let the output of the channel Y be the input to a maximum likelihood (ML)...
In a digital communication system, the overall pulse shape p(t) at the receiver before the sampler has the Fourier transform P(f). If the symbols are ...
Which one of the following statements about differential pulse code modulation (DPCM) is true?
A speech signal is sampled at 8 kHz and encoded into PCM format using 8 bits/sample. The PCM data is transmitted through a baseband channel via 4-lev...
Consider binary data transmission at a rate of 56 kbps using baseband binary pulse amplitude modulation (PAM) that is designed to have a raised...
The modulation scheme commonly used for transmission from GSM mobile terminals is
In a code - division multiple access (CDMA) system with N = 8 chips, the maximum number of users who can be assigned mutually orthogonal signatu...
The bit rate of a digital communication system is R kbits/s. The modulation used is 32-QAM. The minimum bandwidth required for ISI free transmission i...
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
In the output of a DM speech encoder, the consecutive pulses are of opposite polarity during time interval t1 $$\le$$ t $$\le$$ t2. This indicates...
For a bit-rate of 8 kbps, the best possible values of the transmitted frequencies in a coherent binary FSK system are
A band limited signal is sampled at the Nyquist rate. The signal can be recovered by passing the samples through
Compression in PCM refers to relative compression of
In a PCM system with uniform quantization, increasing the number of bits from 8 to 9 will reduce the quantization noise power by a factor of
For a given data rate, the bandwidth Bp of a BPSK signal and the band width B0 of the OOK signal are related as
The line code that has zero dc component for pulse transmission of random binary data is
A deterministic signal has the power spectrum given in figure. The minimum sampling rate needed to completely represent this signal is ...
If the number of bits per sample in a PCM system is increased from n to n+1, the improvement in signal-to-quantization noise ratio will be
The signal to quantization noise ratio in an n-bit PCM system
The bandwidth required for the transmission of a PCM signal increases by a factor of ________ when the number of quantization levels is increased fro...

## Marks 2

In binary frequency shift keying (FSK), the given signal wave forms are $$\,{u_o}(t) = 5\,\cos \,(20000\,\pi \,t);\,0 \le \,\,t\, \le \,T,$$ and $$... The bit error probability of a memoryless binary symmetric channel is$${10^{ - 5}}$$. If$${10^{ - 5}}$$bits are sent over this channel, then the pr... An ideal band-pass channel 500Hz-2000 Hz is deployed for communication. A modem is designed to transmit bits at the rate of 4800 bits/s using 16-QAM. ... A sinusoidal signal of 2 kHz frequency is applied to a delta modulator. The sampling rate and step-size$$\Delta $$of the delta modulator are 20,000 ... The transmitted signal in a GSM system is of 200 kHz bandwidth and 8 users share a common bandwidth using TDMA. If at a given time 12 users are talkin... An M - level PSK modulation scheme is used to transmit independent binary digits over a band-pass channel with bandwidth 100 kHz. The bit rate is 200 ... In a PCA system, the signal$$m(t) = \{ \sin (100\,\pi \,t)\, + \cos (100\,\pi \,t\} $$V is sampled at the Nyquist rate. The samples are processed by... A binary random variable X takes the value of 1 with probability 1/3. X is input to a casade of 2 independent identical binary symmetric channels (BSC... The input to a 1-bit quantizer is a random variable X with pdf$${f_x}(x) = 2{e^{ - 2x}}\,\,for\,\,x \ge 0$$and$${f_x}(x) = 0\,\,for\,\,x\, < \,0...
Four messages band limited to W, W, 2 W and 3 W respectively are to be multiplexed using Time Division Multiplexing (TDM). The minimum bandwidth requi...
A speech signal, band limited to 4 kHz and peak voltage varying between + 5V and - 5V is sampled at the Nyquist rate. Each sample is quantized and rep...
A speech signal, band limited to 4 kHz and peak voltage varying between + 5V and - 5V is sampled at the Nyquist rate. Each sample is quantized and rep...
A speech signal, band limited to 4kHz and peak voltage varying between +5V and -5V is sampled at the Nyquist rate. Each sample is quantized and repres...
In delta modulation, the slope overload distortion can be reduced by
In a GSM system, 8 channels can co-exist in 200 KHz bandwidth using TDMA. A GSM based cellular operator is allocated 5 MHz bandwidth. Assuming a frequ...
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 $$p(t) = ... In a Direct sequence CDMA system the chip rate is$$1.2288\,\, \times \,\,{10^6}$$chips per second. If the processing gain is desired to be at least ... The minimum step-size required for a Delta-Modulator operating at 32 K samples/sec to track the signal (here u(t) is the unit-step function) x(t) = 1... Three analog signals, having bandwidths 1200 Hz, 600 Hz and 600 Hz are sampled at their respective Nyquist rates, enocodes with 12 bit words, and time... A source produces binary data at the rate of 10 kbps. The binary symbols are represented as shown in fig. The source output is transmitted using t... Choose the correct one from among the alternatives A, B, C, D after matching an item from Group 1 with the most appropriate item in Group 2. Group ... The input to a linear delta modulator having a step size$$\Delta $$= 0.628 is a sine wave with frequency fm and peak amplitude Em. If the sampling f... If S represents the carrier synchronization at the receiver and$$\rho $$represents the bandwidth efficiency, then the correct statement for the cohe... A signal is sampled at 8 KHz and is quantized using 8-bit uniform quantizer. Assuming$$SN{R_q}$$for a sinusoidal signal, the correct statement for P... In a digital communication system employing Frequency Shift Keying (FSK), the 0 and 1 bit are represented by sine waves of 10 KHz and 25 KHz respectiv... Four independent messages have bandwidths of 100Hz, 100 Hz, 200Hz and 400Hz respectively. Each is sampled at the Nyquist rate, and the samples are tim... The number of bits in a binary PCM system is increased from n to n+1. As a result, the signal to quantization noise ratio will improve by a factor The bit stream 01001 is differentially encoded using 'Delay and Ex OR' scheme for DPSK transmission. Assuming the reference bit as a '1' and assigning... Coherent demodulation of FSK signal can be effected using Source encoding in a data communication system is done in order to For the signal constellation shown in the figure, the type of modulation is ________. ... In a BPSK signal detector, the local oscillator has a fixed phase error of 200. This phase error deteriorates the SNR at the output by a factor of ... In binary data transmission DPSK is preferred to PSK because The transfer function of a zero - order hold is The message bit sequence input to a DPSK modulator is 1, 1, 0, 0, 1, 1. The carrier phase during the reception of the first two message bits is$$\pi...

## Marks 8

A rectangular pulse existing between t = 0 and t = T and having an amplitude A is to be received by a matched filter (a) Draw the impulse response o...

## Marks 10

A signal of bandwidth 3.5 kHz has peak-to-peak and rms amplitudes of 4 V and 0.2 V, respectively. This signal is passed through a linear PCM coder and...
For the input signal, x(t), shown in Fig. (a) Sketch the impulse response, h(t), of the matched filter. (b) Sketch the output, y(t), of the matched ...
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