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The correct option is (b) Varied, varied

Explanation: Quadrature amplitude modulation (QAM) is OBTAINED by allowing the amplitude to ALSO vary with the phase. Thus, the constellation consists of SQUARE lattice of signal POINTS.

The correct answer is (B) False

Easiest explanation: Gaussian PULSE shaping does not satisfy Nyquist criterion for ISI CANCELLATION. Thus, it REDUCES the spectral occupancy but there is degradation in the performance DUE to increased ISI.

Right CHOICE is (a) Increased

Explanation: ISI leads to increased probability of the receiver making an ERROR in detecting a symbol. When rectangular pulses are passed through a bandlimited channel, the pulses will SPREAD in time, and the PULSE for each symbol will SMEAR into the time intervals of succeeding symbols.

The correct choice is (d) Phase DETECTOR

Explanation: A product detector is also called a phase detector. It forms a COHERENT DEMODULATOR for AM signals. It is a down converter CIRCUIT which converts the input bandpass signal to a BASEBAND signal.

Correct choice is (a) True

Easy explanation: PN sequence is usually generated USING SEQUENTIAL logic circuits. When the feedback logic consists of EXCLUSIVE OR gates, the shift REGISTER is called a LINEAR PN sequence generator.

Right option is (c) Same

The EXPLANATION is: The bit ERROR probability of QPSK is identical to BPSK but twice as much data can be SENT in the same BANDWIDTH. Thus, when compared to BPSK, QPSK provides twice the spectral efficiency with exactly the same efficiency.

The CORRECT answer is (d) Inferior

Explanation: The ENERGY efficiency of DPSK is inferior to that of coherent PSK by about 3 dB. But, it has an advantage of REDUCED receiver complexity.

Correct OPTION is (c) 3 DB bandwidth

Explanation: Half POWER bandwidth is ALSO called the 3 dB bandwidth. It is defined as the INTERVAL between frequencies at which the PSD has dropped to half power, or 3 dB below the peak value.

Right option is (a) Signal power

Explanation: In digital communication SYSTEM, in order to increase noise immunity, it is NECESSARY to increase signal power. However, the amount by which the signal power should be increased to OBTAIN a certain level of FIDELITY depends on the particular type of modulation employed.

Right option is (a) Poor

To explain I would say: SSB systems have the advantage of being very bandwidth EFFICIENT. But their performance in fading CHANNELS is very poor. For proper detection, the frequency of the oscillator at the product detector mixer in the receiver MUST be same as that of the incoming carrier frequency.

The CORRECT choice is (b) False

For EXPLANATION I would say: Bit ERROR rate does not provide information about the type of error. However, bit error rate (BER) evaluation GIVES a GOOD indication of the performance of a particular modulation scheme.

Right choice is (a) VARIABLE envelope

For explanation: MSK has a constant envelope. It is a SPECTRALLY efficient scheme. It possesses PROPERTIES such as constant envelope, spectral efficiency, good BER performance and self-synchronizing CAPABILITY.

The correct answer is (c) DERIVATIVE

For explanation: Gaussian minimum phase shift keying is a simple BINARY modulation scheme. It is VIEWED as a derivative of MSK. GMSK considerably REDUCES the sidelobe levels in the transmitted spectrum.

Correct answer is (b) PHASE

The best I can explain: In binary phase shift keying (BPSK), the phase of a constant amplitude CARRIER signal is switched between two possible values m1 and M2. These two values corresponds to binary 1 and 0 RESPECTIVELY.

Correct answer is (b) False

The best I can explain: Gaussian pulse shaping FILTER uses non Nyquist TECHNIQUE. It is effective when used in conjunction with MINIMUM shift keying (MSK) modulation, or other modulation which is well SUITED for power efficient NONLINEAR amplifiers.

The correct option is (a) Phase LOCKED LOOP

To explain I would say: PLL stands for phase locked loop. The PLL is a closed loop control system which can TRACK the variations in the RECEIVED signal phase and frequency.

The correct CHOICE is (a) Carson’s

Explanation: The approximation of bandwidth is done USING Carson’s rule. Carson’s bandwidth rule DEFINES the approximate bandwidth requirements of communications system components for a carrier signal that is FREQUENCY MODULATED by a continuous or broad spectrum of frequencies rather than a single frequency.

The correct option is (b) Amplitude of message signal

To elaborate: The FREQUENCY modulation index defines the RELATIONSHIP between the message amplitude and the bandwidth of the transmitted signal. If the modulating signal is a LOW pass signal, maximum bandwidth of the modulating signal is equal to the highest frequency COMPONENT PRESENT in the modulating signal.

The correct OPTION is (B) Multipath fading

For explanation: RESISTANCE to multipath fading is one of the fundamental reasons for considering spread spectrum systems for wireless communication. SINCE spread spectrum signals have uniform energy over a very large bandwidth, at any given time only a SMALL portion of the spectrum will undergo fading.

Right OPTION is (b) False

The best I can explain: The constant ENVELOPE FAMILY of modulation has an advantage of satisfying various conditions. In this, power efficient Class C AMPLIFIERS can be used without introducing degradation in the spectrum occupancy of the transmitted signal.

Correct choice is (c) TIME varying Doppler spread

Easiest EXPLANATION: Even if a mobile channel is not frequency selective, the TIE varying Doppler spread due to motion creates an irreducible BER floor. It is caused due random SPECTRAL spreading.

Correct CHOICE is (c) Instantaneous bandwidth

Best explanation: The bandwidth of a channel used in the hopset is CALLED the instantaneous bandwidth. And the bandwidth of the spectrum over which the HOPPING OCCURS is called total hopping bandwidth.

Right answer is (a) Frequency separation

To EXPLAIN I WOULD SAY: The name minimum phase shift keying implies minimum frequency separation, i.e. the bandwidth that allows orthogonal detection. A modulation index of 0.5 corresponds to the minimum frequency spacing that allows TWO FSK SIGNALS to be coherently orthogonal.

The correct choice is (b) False

Best EXPLANATION: M-ary MODULATION schemes have poor power efficiency, but they have a better bandwidth efficiency. An 8-PSK system requires a bandwidth that is 3 times SMALLER than a BPSK system, whereas its BER PERFORMANCE is very worse since signals are PACKED more closely in the signal constellation.

The correct option is (d) OFFSET Quadrature Phase SHIFT Keying

The explanation is: OQPSK stands for offset quadrature phase shift keying. It is a modified form of QPSK which is LESS susceptible to deleterious EFFECTS and supports more efficient amplification. OQPSK is sometimes also CALLED staggered QPSK.

Right answer is (C) Product detector

The best I can explain: Various TECHNIQUES such as slope detection, ZERO CROSSING detection, phase locked discrimination and quadrature detection are used to demodulate FM. Product detector is used for demodulating AM SIGNALS.

Correct choice is (b) False

To ELABORATE: NON coherent detection does require the knowledge of PHASE information. However, coherent detection requires knowledge of the transmitted carrier frequency and phase at the receiver.

The correct option is (a) Bandwidth efficiency

The explanation is: Bandwidth efficiency REFLECTS how efficiently the allocated bandwidth is utilized. It is DEFINED as the RATIO of throughput data rate PER Hertz in a given bandwidth. It describes the ABILITY of a modulation scheme to accommodate data within a limited bandwidth.

Right answer is (a) True

The explanation: High capacity mobile systems are interference limited, but they are not noise limited. It was clearly seen that when carrier to interference RATIO (C/I) is large, the ERRORS are primarily due to fading, and interference has very little effect. HOWEVER, as C/I drops below a CERTAIN level, interference dominates the link PERFORMANCE.

The CORRECT answer is (a) √Es

The best explanation: The M-ary message points are equally spaced on a circle of radius √Es centred at the origin.Here Es is ENERGY per SYMBOL. Thus, MPSK is a constant envelope signal when no pulse shaping is USED.

The correct answer is (b) Fast FSK

Easiest EXPLANATION: Minimum shift KEYING is sometimes ALSO referred as fast FSK. It is so CALLED because frequency spacing used is only half as much as that used in CONVENTIONAL noncoherent frequency shift keying.

Right OPTION is (d) Sharp cut off

Best explanation: The Gaussian filter has a narrow absolute bandwidth, and has a sharp cut off, LOW overshoot and pulse area preservation properties. This makes it attractive for USE in mobile communication that USES nonlinear RF AMPLIFIERS.

The correct choice is (C) To reduce ISI

Explanation: PULSE shaping techniques reduces the intersymbol interference. They are also used to reduce the spectral WIDTH of the modulated digital SIGNAL.

Correct choice is (c) Power efficiency

The EXPLANATION: Power efficiency is often expressed as the ratio of signal ENERGY per bit to NOISE power spectral density required at the receiver input for a certain PROBABILITY of ERROR. Power efficiency is a measure of how favourably the trade-off between fidelity and signal power is made.

The correct answer is (a) Symbol offset interference does not exist

Explanation: BER performance of BPSK is best among all the modulation schemes compared. This is because symbol offset interference does not exist in BPSK. Symbol offset interference is also CALLED cross RAIL interference due to the fact that the EYE diagram has multiple RAILS.

Right choice is (c) Outage event

Explanation: Outage event is specified by a specific number of BIT errors OCCURRING in a given TRANSMISSION. Evaluating the probability of outage is one of the means to judge the effectiveness of the signalling scheme in a mobile RADIO channel.

Correct choice is (d) M-SIGNALS are orthogonal

Explanation: In M-ary FSK, all the M signals are orthogonal and there is no crowding in the SIGNAL space. HENCE, POWER EFFICIENCY of M-ary FSK increases with M.

The correct choice is (c) 0.5

To explain I would SAY: MINIMUM shift KEYING is a special type of CPFSK. Its peak FREQUENCY deviation is EQUAL to ¼ the bit rate. In other words, MSK is continuous phase FSK with a modulation index of 0.5.

Right option is (b) Minimum shift KEYING

To EXPLAIN I would SAY: MSK stands for minimum shift keying. It is a SPECIAL type of continuous phase shift keying. It is form of digital modulation technique that was developed in 1950s.

Correct choice is (b) False

For explanation: BPSK uses coherent or synchronous demodulation. It REQUIRES the INFORMATION about the phase and frequency of the carrier be AVAILABLE at the RECEIVER.

Right OPTION is (C) FSK

The best I can explain: OQPSK, π/4 QPSK and BPSK are the most popular linear modulation techniques. They have very good SPECTRAL efficiency. However, FSK is an non-linear modulation technique.