Digital Communications

Question 1

A representation vaiable takes on continuous values in some intervals

Answer 1

Analog data

Question 2

The frequency spectrum of a data pulse periodic signal is

Answer 2

Discrete

Question 3

If all details ofthe signal must be reproduced accurately, then it refers to

Answer 3

Analog transmission

Question 4

An example of conversion from digital to analog is

Answer 4

PSK

Question 5

Unit of signaling speed equal to the number of signal events per second?

Answer 5

Baud rate or symbol rate

Question 6

Speed at which bits are transmitted
Data rate (bps)

Answer 6

Bit rate

Question 7

The maximum rate at which digital data can be transimtted over a channel of a bandwidth Hz refers to
Channel capacity

Answer 7

Nyquist theorem

Question 8

Which ofthe following pulse modulation scheme is analog

PCM
PWM, PAM, PPM
Differential PCM
Delta modulation

Answer 8

PWM

Question 9

Pulse has a direct pattern of analog data

Answer 9

Analog

Question 10

Converting from an analog to a digital signal transmission is achieved by
*PCM

Answer 10

Sampling the analog signal using pulse amplitde moduation, and formatting the samples using pulse code modulation

Question 11

The basic operationd of PCM in the receiver section ate

Answer 11

Regeneration of impaired signals, Decodin and reconstruction

Question 12

Repeater

Answer 12

Analog - amplifier, fidelity
Digital - regenerator, accuracy

Question 13

In the quantization stage, SNR will increase by ____ for every additional bit used?

Answer 13

6dB

Question 14

Companding is used:

Answer 14

To protect small signals in PCM from quantizing distortion

Small signals (voice signal-low amplitude)

Question 15

PCM tradeoffs includes:

Answer 15

Signal bandwidth determines minimum sample rate (NR)

Desired signal fidelity determines precision of reproduced signal

Signals can be quantized using DAC

Question 16

The most popular alt to PCM where analog signal is approximated by staircase function and only a single binary digit is required for each sample

Answer 16

Delta modulation PCM

Question 17

In baseband digital transmission, which line coding format binary 0 represents by NO pulse and binary 1 is represented by a positive pulse?

Answer 17

NRZ unipolar format

Question 18

____ is employed since the medium is bandpass and/or multiple users need to share the medium

Answer 18

Digital to analog conversion

Question 19

A mod technique where the strength(amplitude) of a carrier signal is varied to represent binary 1 or 0

Answer 19

ASK

Question 20

The most commonly used circuit for demodulating binary FSK is

Answer 20

PLL (phase locked loop)

Question 21

Common FSK generator

Answer 21

VCO

Question 22

When input symbol results in phase change, instead of defining absolute phase (such as receiver only needs to detect relative changes in carrier phase, instead of absolute phase reference), it refers to

Answer 22

Differentiak PSK (no need to use carrier recovery ckt)

Question 23

Transmission of digital data over telephone wire can be achieved using

Answer 23

QAM

Question 24

Used to represent possible symbols that may be selected by a given modulation scheme as points in 2D plane

Answer 24

Constellation diagram (or signal state-space diagram)

Question 25

Achieving the desired data rate within the available spectral bandwidth refers to

Answer 25

Spectral efficiency

Question 26

Which of the following digital modulation technique has the same bandwidth

Answer 26

ASK and PSK

Question 27

Why the number of phase shifts used by a QAM system is always greater than the number of amplitude shifts

Answer 27

Amplitude changes are susceptible to noise

Question 28

What does trellis code modulation used in order to reduce the probability of error thus improving the bit error performance?

Answer 28

Encoding and modulation

Question 29

This technique consists in translating each intelligence to a different position in the frequency spectrum

Answer 29

FDM

Question 30

A technology that enables many optical signals to be transmitted simultaneously by a single fiber cable

Answer 30

WDM

Question 31

The technique of combining signals coming from low speed channels to share time on a high speed path

Answer 31

TDM

Question 32

The process of taking a group or bits from each input line for multiplexing is

Answer 32

Interleaving

Question 33

The ___ at the central office translates between analog customer signals and digital internal signal

Answer 33

Codec

Question 34

A hierarchy of digital lines designed to carry speech and other signals in digital forms

Answer 34

T and E lines

Question 35

The mod technique requires a freq band sliced into several hundred of sub bands, each of which carries a modulated signal as a part of the data steam

Answer 35

Orthogonal modulation

Question 36

A type of spread spectrum technique works by pseudorandomly changing the center frequency of the carrier over a set of freq

Answer 36

Frequency hopping (FHSS)

Question 37

To reduce ISI effects caused by multipath fading in OFDM

Answer 37

OFDM inserts a guard interval between symbols