This set of Wireless & Mobile Communications Multiple Choice Questions & Answers (MCQs) focuses on “Spread Spectrum Multiple Access”.
1. SSMA uses signals which have a transmission bandwidth that is smaller than the minimum required RF bandwidth.
Explanation: Spread spectrum multiple access (SSMA) uses signals which have a transmission bandwidth that is several orders of magnitude greater than the minimum required RF bandwidth.
2. PN sequence converts _______ signal to ______ signal.
a) Narrowband, wideband
b) Wideband, narrowband
c) Unmodulated, modulated
d) Low frequency, high frequency
Explanation: A pseudo-noise (PN) sequence converts a narrowband signal to a wideband noise like signal before transmission. SSMA provides immunity to multipath interference and robust multiple access capability.
3. SSMA is bandwidth efficient when used with a single user. State whether True or False.
Explanation: SSMA is not very bandwidth efficient when used by a single user. However, since many users can share the same spread spectrum bandwidth without interfering with one another, spread spectrum systems become bandwidth efficient in a multiple user environment.
4. ______ is a digital multiple access system in which carrier frequencies are varied in pseudorandom order.
Explanation: Frequency hopped multiple access (FHMA) is a digital multiple access system in which the carrier frequencies of the individual users are varied in a pseudorandom fashion within a wideband channel.
5. If the rate of change of the carrier frequency is greater than the symbol rate, then the system is referred as ______
a) Fast frequency hopping system
b) Slow frequency hopping system
c) Time division frequency hopping system
d) Code division multiple access system
Explanation: If the rate of change of the carrier frequency is greater than the symbol rate, then the system is referred as a fast frequency hopping system. If the channel changes at a rate less than or equal to the symbol rate, it is called slow frequency hopping.
6. A frequency hopped system does not provide security. State whether True or False.
Explanation: A frequency hopped system provides a level of security, especially when a large number of channels are used. Since, an unintended receiver that does not know the pseudorandom sequence of frequency slots must retune rapidly to search for the signal it wishes to intercept.
7. All users in CDMA system uses _____- carrier frequency.
Explanation: All users in CDMA system use the same carrier frequency and may transmit simultaneously. Each user has its own pseudorandom codeword which is approximately orthogonal to all other codewords.
8. ______ problem occurs when many mobile users share the same channel.
c) Line of sight
Explanation: The near far problem occurs when many mobile users share the same channel. In general, the strongest received mobile signal will capture the demodulator at a base station.
9. In CDMA, symbol duration is ______ than channel delay spread.
c) Slightly greater
d) Much less
Explanation: In CDMA, symbol (chip) duration is very short and usually much less than the channel delay spread. Thus, channel data rates are very high in CDMA systems.
10. _______ is used to improve reception by collecting time delayed versions of the required signal.
a) RAKE receiver
c) Frequency modulator
d) High pass filter
Explanation: A RAKE receiver can be used to improve reception by collecting time delayed versions of the required signals. Since PN sequences have low autocorrelation, multipath which is delayed by more than a chip appear as noise.
11. _______ arises from the fact that the spreading sequences of different users are not orthogonal.
a) Near-far problem
b) Line of sight
d) Self jamming
Explanation: Self jamming arises from the fact that the spreading sequences of different users are not orthogonal. Hence, in the despreading of a particular PN code, non zero contributions to the receiver decision statistic for a desired user arise from the transmissions of other users in the system.
Sanfoundry Global Education & Learning Series – Wireless & Mobile Communications.
To practice all areas of Wireless & Mobile Communications, here is complete set of 1000+ Multiple Choice Questions and Answers.