Wireless & Mobile Communications Multiple Choice Questions on “Spread Spectrum Modulation Techniques”.
1. The transmission bandwidth of spread spectrum techniques is equal to the minimum required signal bandwidth.
a) True
b) False
Answer: b
Clarification: Spread spectrum techniques employ a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth. On the other hand, primary objective of all the modulation schemes is to minimize the required transmission bandwidth.
2. Why spread spectrum technique is inefficient for a single user?
a) Large transmission bandwidth
b) Small transmission bandwidth
c) Fixed transmission bandwidth
d) Fixed null bandwidth
Answer: a
Clarification: Spread spectrum systems are bandwidth inefficient for single users. But in spread spectrum systems, many users can simultaneously use the same bandwidth without significantly interfering with one another. It is one of the advantages of spread spectrum.
3. Which of the following is not a property of spread spectrum techniques?
a) Interference rejection capability
b) Multipath fading
c) Frequency planning elimination
d) Multiple user, multiple access interface
Answer: b
Clarification: 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.
4. Which of the following is not a characteristic of PN sequence?
a) Nearly equal number of 0s and1s
b) Low correlation between shifted version of sequence
c) Non deterministic
d) Low cross-correlation between any two sequences
Answer: c
Clarification: Pseudo-Noise sequences are deterministic in nature. Certain characteristics of PN sequence are nearly equal number of 0s and 1s, very low correlation between shifted versions of the sequence, very low cross correlation between any two sequences.
5. PN sequence can be generated using sequential logic circuits.
a) True
b) False
Answer: a
Clarification: 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.
6. The period of a PN sequence produced by a linear m stage shift register cannot exceed _____ symbols.
a) 2m
b) m
c) 2m
d) 2m-1
Answer: d
Clarification: There are exactly 2m-1 non-zero states for an m stage feedback shift register. Thus, the period of a PN sequence produced by a linear m stage shift register cannot exceed 2m-1.
7. DSSS system spreads the baseband signal by ________ the baseband pulses with a pseudo noise sequence.
a) Adding
b) Subtracting
c) Multiplying
d) Dividing
Answer: c
Clarification: A direct sequence spread spectrum (DS-SS) system spreads the baseband data by directly multiplying the baseband data pulses with a pseudo noise sequence. And the pseudo noise sequence is produced by a pseudo noise code generator.
8. Frequency hopping involves a periodic change of transmission _______
a) Signal
b) Frequency
c) Phase
d) Amplitude
Answer: b
Clarification: Frequency hopping involves a periodic change of transmission frequency. A frequency hopping signal is regarded as a sequence of modulated data bursts with time varying, pseudo random carrier frequencies.
9. What is the set of possible carrier frequencies in FH-SS?
a) Hopset
b) Hop
c) Chips
d) Symbols
Answer: a
Clarification: The set of possible carrier frequencies in FH-SS is called hopset. Hopping occurs of a frequency band that includes a number of channels. Each channel is defined as a spectral region with central frequency in the hopset.
10. The bandwidth of the channel used in the hopset is called _________
a) Hopping bandwidth
b) Total hopping bandwidth
c) Instantaneous bandwidth
d) 3 dB bandwidth
Answer: c
Clarification: 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.
11. The processing gain of FH systems is given by ratio of _______
a) Hopping bandwidth and hopping period
b) Instantaneous bandwidth and hopping duration
c) 3 dB bandwidth and bit rate
d) Total hopping bandwidth and instantaneous bandwidth
Answer: d
Clarification: The processing gain of frequency hopping (FH) systems is given by Bss/B. Here, Bss and B denote the total hopping bandwidth and instantaneous bandwidth respectively.
12. FH systems do not have collisions.
a) True
b) False
Answer: b
Clarification: It is possible to have collisions in an FH system where an undesired user transmits in the same channel at the same time as the desired user. Whenever an undesired signal occupies a particular hopping channel in FH, the noise and interference in the channel are translated in frequency.
13. In fast frequency hopping, hopping rate is less than the information symbol rate.
a) True
b) False
Answer: b
Clarification: Fast frequency hopping occurs if there is more than one frequency hop during each transmitted symbol. Thus, in fast frequency hopping the hopping rate equals or exceeds the information symbol rate.
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