Category: Wireless & Mobile Communications Objective Questions

Wireless & Mobile Communications Interview Questions and Answers on “Second Generation (2G) Cellular Networks”.

1. Which of the following multiple access techniques are used by second generation cellular systems?
a) FDMA/FDD and TDMA/FDD
b) TDMA/FDD and CDMA/FDD
c) FDMA/FDD and CDMA/FDD
d) FDMA/FDD only
Answer: b
Clarification: First generation cellular system used FDMA/FDD techniques. Second generation standards uses TDMA/FDD and CDMA/FDD multiple access techniques. 2G networks are digital.

2. Which one is not a TDMA standard of second generation networks?
a) GSM
b) IS-136
c) AMPS
d) PDC
Answer: c
Clarification: GSM (Global System Mobile), IS-136 (Interim Standard 136) and PDC (Pacific Digital Cellular) are the three most popular TDMA standards of second generation. AMPS is a first generation standard.

3. Which of the following is a CDMA standard of second generation network?
a) IS-95
b) IS-136
c) ETACS
d) EDGE
Answer: a
Clarification: Interim Standard 95 (IS-95) is the most popular CDMA standard of second generation networks. IS-136 is a TDMA standard of 2G. EDGE is a standard of 2.5G and ETACS is a 1G standard.

4. Popular 2G CDMA standard IS-95 is also known as ______________
a) CdmaOne
b) CdmaTwo
c) IS-136
d) IS-95B
Answer: a
Clarification: The popular 2G CDMA standard, Interim Standard (IS-95) is also known as CdmaOne. The 2.5G CDMA standard, IS-95B is called CdmaTwo. And IS-136 is a TDMA standard for 2G.

5. How many users or voice channels are supported for each 200 KHz channel in GSM?
a) Eight
b) Three
c) Sixty four
d) Twelve
Answer: a
Clarification: GSM is a circuit switched system that divides each 200 KHz channel into eight 25 KHz time slots, i.e. each radio channel is divided into eight voice channels.

6. How many voice channels are supported for each 30 KHz radio channel in IS-136?
a) Eight
b) Thirty
c) Three
d) Sixteen
Answer: c
Clarification: Interim Standard 136 (IS-136) was popularly known as North American Digital Cellular (NADC) system. It divides each 30 KHz radio channel into three time slots, each of 10 KHz.

7. How many users are supported in IS-95 for each 1.25 MHz?
a) Eight
b) Sixty four
c) Sixteen
d) Twenty five
Answer: b
Clarification: IS-95 supports upto 64 users which are orthogonally coded and simultaneously transmitted on each 1.25 MHz. The services of IS-95 standard are short messaging service, slotted paging, over-the-air activation, enhanced mobile station identities etc.

8. Which modulation technique is used by GSM?
a) GMSK
b) BPSK
c) QPSK
d) GFSK
Answer: a
Clarification: GSM uses a form of modulation known as GMSK (Gaussian Minimum Shift Keying). It is a form of modulation with no phase discontinuities and provides data transmission with efficient spectrum usage.

9. IS-95 uses which modulation technique?
a) GMSK
b) BPSK
c) QAM
d) AFSK
Answer: b
Clarification: IS- 95 uses BPSK (Binary Phase Shift Keying) with quadrature spreading. It is regarded as one of the most robust digital modulation technique and is used for long distance wireless communication.

10. IS-136 uses which modulation technique?
a) π/4 DQPSK
b) BPSK
c) GMSK
d) AFSK
Answer: a
Clarification: IS-136 uses π/4 DQPSK modulation technique. This technique allows a bit rate of 48.6 Kbit/s with 30 KHz channel spacing which gives a bandwidth efficiency of 1.62 bit/s/Hz.

11. Which is one of the disadvantages of 2G standards?
a) Short Messaging Service (SMS)
b) Digital modulation
c) Limited capacity
d) Limited Internet Browsing
Answer: d
Clarification: 2G technologies use circuit switched data modems that limits data users to a single circuit switched voice channel. The advantages of 2G network are that they are digital in nature and supports SMS service.

12. GSM (Global System for Mobile) was earlier also known as _____________
a) Group System Mobile
b) Global Special Meaning
c) Group Special Mobile
d) Global Special Mobile
Answer: c
Clarification: GSM was earlier known as Group Special Mobile. As it became more global, the meaning of acronym was changed to Global System for Mobile.

13. 2G CDMA standard, IS-95, was proposed by which company?
a) Nippon Telephone and Telegraph (NTT)
b) Qualcomm
c) Bellcore and Motorola
d) AT&T Bell Laboratories
Answer: b
Clarification: IS-95 was proposed by Qualcomm in early 1990s. Later it was adopted as a standard by Telecommunications Industry Association in TIA/EIA/IS-95 release published in 1995.

14. Which one of the following 2G standard is used in Japan?
a) IS-136
b) GSM
c) PDC
d) AMPS
Answer: c
Clarification: PDC (Personal Digital Cellular) was standardized by Japanese Ministry of Posts and Telecommunication in 1991. It is similar to IS- 136, but with 25 KHz voice channels to be compatible with the Japanese analog channels.

15. The 2G GSM technology uses a carrier separation of _____________
a) 1.25 MHz
b) 200 KHz
c) 30 KHz
d) 300 KHz
Answer: b
Clarification: The Global System for Mobile (GSM) uses a carrier separation of 200 KHz, each channel supporting upto eight users.

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Wireless & Mobile Communications Multiple Choice Questions on “Scattering”.

1. Scattering occurs when medium consists of objects with dimensions _______ compared to the wavelength.
a) Same
b) Small
c) Large
d) Very large
Answer: b
Clarification: Scattering occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength. But the number of obstacles per unit volume is large.

2. Scattered waves are produced at ________
a) Rough surface
b) Shadowed region
c) Smooth surface
d) Horizon
Answer: a
Clarification: Scattered waves are produced by rough surfaces, small objects or by other irregularities in the channel. In practice, foliage, street signs, and lamp posts induce scattering in a mobile communication system.

3. The actual received signal is ______ than what is predicted by reflection and diffraction model.
a) Weaker
b) Equal
c) Stronger
d) Very weak
Answer: c
Clarification: The actual received signal in a mobile radio environment is often stronger than what is predicted by reflection and diffraction model alone. This is because when a radio wave incidence upon the rough surface, reflected energy is spread out in all directions.

4. Scattered energy in all directions provides _________ at a receiver.
a) Channels
b) Loss of signal
c) No energy
d) Additional radio energy
Answer: d
Clarification: Objects such as lamp posts and trees tend to scatter energy in all directions. They provide additional radio energy at a receiver. Scattering may also refer to particle-particle collisions between molecules, atoms, electrons, photons and other particles.

5. Surface roughness are often tested using __________
a) Rayleigh criterion
b) Lawson criterion
c) Barkhausen stability criterion
d) Nyquist criterion
Answer: a
Clarification: Rough surface is often tested using a Rayleigh criterion. It defines the critical height of surface protuberances for a given angle of incidence. The Rayleigh criterion is the criterion for the minimum resolvable detail. The imaging process is said to be diffraction-limited when the first diffraction minimum of the image of one source point coincides with the maximum of another.

6. A surface is considered rough if protuberance is ________ than critical height.
a) Equal
b) Less
c) Greater
d) No relation
Answer: c
Clarification: A surface is considered rough if its minimum to maximum protuberance is greater than the critical height calculated using Rayleigh criterion. It is considered smooth if protuberance is less than critical height.

7. RCS of scattering object is defined as the ratio of _______
a) Power density of signal scattered to power density of radio wave incident
b) Power density of radio wave incident to power density of signal scattered
c) Power density of incident waves to power density of reflected wave
d) Power density of reflected wave to power density of incident waves
Answer: a
Clarification: The radar cross section of a scattering object is defined as the ratio of the power density of the signal scattered in the direction of the receiver to the power density of the radio wave incident upon the scattering object. It has unit of square meters.

8. Which equation is used to calculate the received power due to scattering for urban mobile radio system?
a) Laplace equation
b) Bistatic radar equation
c) Poisson’s equation
d) Maxwell equation
Answer: b
Clarification: For urban mobile radio systems, models based on bistatic radar equation is used to compute the received power due to scattering in the far field. This equation describes the propagation of wave in free space which impinges on a scattering object and then reradiated in the direction of receiver.

9. In ionosphere propagation, waves arriving at the receiving antenna using the phenomenon of _______
a) Scattering
b) Refraction
c) Diffraction
d) Radiation
Answer: a
Clarification: Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more paths. It is due to localized non-uniformities in the medium through which they pass.

10. Power density is basically termed as ________ power per unit area.
a) Reflected
b) Refracted
c) Radiated
d) Diffracted
Answer: c
Clarification: Power density is the amount of power (time rate of energy transfer) per unit volume. It is also termed as radiated power per unit area. In energy transformers including batteries, fuel cells, motors, etc., power density refers to a volume. It is then also called volume power density.

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Wireless & Mobile Communications Multiple Choice Questions on “Pulse Shaping Techniques”.

1. Intersymbol interference (ISI) leads to ________ probability of the receiver for making an error in detecting the symbols.
a) Increased
b) Decreased
c) Zero
d) One
Answer: a
Clarification: 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.

2. ISI is ________ by increasing channel bandwidth.
a) Maximized
b) Minimized
c) Zero
d) Infinite
Answer: b
Clarification: Increasing channel bandwidth is one of the method to minimize intersymbol interference. But mobile communication systems use minimal bandwidth, thus other methods to reduce ISI are desirable.

3. Why is pulse shaping technique used?
a) To increase ISI
b) To increase spectral width of modulated signal
c) To reduce ISI
d) To reduce power spectral density
Answer: c
Clarification: Pulse shaping techniques reduces the intersymbol interference. They are also used to reduce the spectral width of the modulated digital signal.

4. Who was the first to solve the problem of ISI?
a) Manchester
b) Faraday
c) Graham Bell
d) Nyquist
Answer: d
Clarification: Nyquist was the first to solve the problem of ISI. He overcome the problem of ISI while keeping the transmission bandwidth low. He observed that ISI can be completely nullified if at every instant, the response due to all symbols except the current symbol is equal to zero.

5. According to Nyquist, the impulse response of the overall communication system should have ______ decay with _______ magnitude for sample values not equal to zero.
a) Fast, small
b) Slow, small
c) Slow, large
d) Fast. Large
Answer: a
Clarification: According to Nyquist, the impulse response of the overall communication system should have fast decay with small magnitude for sample values not equal to zero. If the channel is ideal then it should be possible to realize approximate shaping filters at both transmitter and receiver.

6. Raised cosine filter does not satisfy Nyquist criteria.
a) True
b) False
Answer: b
Clarification: Raised cosine filter is the most popular pulse shaping filter used in mobile communication. It belongs to the class of filters that satisfy Nyquist criterion.

7. As the roll off factor in raised cosine rolloff filter __________ the occupied bandwidth ________
a) Increases, decreases
b) Decreases, constant
c) Increases, increases
d) Decreases, increases
Answer: c
Clarification: As the rolloff factor increases, the bandwidth of the filter also increases and the time sidelobe levels decrease in adjacent symbol slots. Thus, it implies that increasing rolloff factor decreases the sensitivity to timing jitter but increases the occupied bandwidth.

8. Gaussian pulse shaping filter follows Nyquist criterion.
a) True
b) False
Answer: b
Clarification: 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.

9. Gaussian filter has zero crossings at adjacent symbol peaks.
a) True
b) False
Answer: b
Clarification: Nyquist filters have zero crossings at adjacent symbol peaks and a truncated transfer function. Gaussian filter does not follow Nyquist criterion and has a smooth transfer function with no zero crossings.

10. Which of the following is true for a Gaussian filter?
a) Large bandwidth
b) Minimum ISI
c) High overshoot
d) Sharp cut off
Answer: d
Clarification: 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.

11. Gaussian pulse shaping filter reduces the spectral occupancy and ISI.
a) True
b) False
Answer: b
Clarification: 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.

12. Gaussian pulses are used when cost and power efficiency are major factors.
a) True
b) False
Answer: a
Clarification: Gaussian pulses are used when cost and power efficiency are major factors. But the bit error rates due to ISI are deemed to be lower than what is nominally required. Thus, there is a trade-off between desired RF bandwidth and irreducible error due to ISI.

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Wireless & Mobile Communications Multiple Choice Questions on “AMPS and ETACS”.

1. US cellular telephone system, AMPS stands for __________
a) Analog Mobile Phone System
b) Analog Modulation Packet System
c) Analog Machine Precision System
d) Analog Mobile Precision System
Answer: a
Clarification: AMPS stands for Analog Mobile Phone System. It was first deployed in late 1983 in urban and suburban areas of Chicago by Ameritech.

2. Who developed the first US cellular telephone system called AMPS?
a) Motorola
b) Ericsson
c) AT & T Bell Laboratories
d) ETSI
Answer: c
Clarification: In the late 1970s, AT & T Bell Laboratories developed the first US cellular telephone system called the Advanced Mobile Phone System (AMPS). In 1983, a total of 40 MHz of spectrum in the 800 MHz band was located by the Federal Communication Commission for AMPS.

3. The AMPS system uses a ___________ cell reuse pattern.
a) One
b) Five
c) Three
d) Seven
Answer: d
Clarification: The AMPS system uses a seven cell reuse pattern. It also provides sectoring and cell splitting to increase capacity when needed.

4. Which of the following is virtually identical to AMPS?
a) ETACS
b) GSM
c) CDMA
d) IS-54
Answer: a
Clarification: ETACS is virtually identical to AMPS. It was developed in middle 1980s and stands for European Total Access Communication System.

5. AMPS and ETACS use ___________ and _____ for radio transmission.
a) FM, TDD
b) FM, FDD
c) AM, TDD
d) FM, FDD
Answer: b
Clarification: Like all other first generation, analog, cellular systems, AMPS and ETACS use frequency modulation (FM) and frequency division duplex (FDD) for radio transmission.

6. US AMPS system provides 42 control channel for each of the two service providers.
a) True
b) False
Answer: b
Clarification: In the US AMPS system, there are twenty one control channels for each of the two service providers in each market. However, ETACS supports forty two control channels for a single provider.

7. In each cellular market, the non wireline service provider is assigned an odd SID.
a) True
b) False
Answer: a
Clarification: In each cellular market, the non wireline service provider (the ‘A’ provider) is assigned an odd SID (system identification number). The wireline provider (the ‘B’ provider) is assigned an even SID.

8. ETACS use ___________ instead of SID (system identification number).
a) Area identification number
b) Analog identification number
c) Digital identification number
d) Dual identification number
Answer: a
Clarification: For ETACS, area identification numbers (AIDs) are used instead of SIDs. ETACS subscriber units are able to access any control or voice channel in the standard.

9. _________ allows base and mobile to distinguish each other from co-channel users located in different cells.
a) SAT
b) VMAC
c) MAC
d) FVC
Answer: a
Clarification: The SAT (supervisory audio tone) has one of the three different frequencies which allow the base and mobile to distinguish each other from co channel users located in different cells.

10. AMPS and ETACS use same physical channels for transmission of voice and control channels.
a) True
b) False
Answer: b
Clarification: AMPS and ETACS use different physical rate channels for transmission of voice and control channels. A control channel is used by each base station in the system to simultaneously page subscriber units to alert them of incoming calls.

11. To increase the capacity of AMPS, Motorola developed ____________
a) P-AMPS
b) N-AMPS
c) H-APMS
d) R-AMPS
Answer: b
Clarification: To increase capacity in large AMPS markets, Motorola developed an AMPS like system called N-AMPS (narrowband AMPS) in 1991. N-AMPS did not become widespread as 2G digital technologies displaced many of the original FM analog systems.

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Wireless & Mobile Communications online quiz on “Orthogonal Frequency Division Multiple Access (OFDMA)”.

1. OFDMA stands for ________
a) omnidirectional frequency division multiple access
b) orthogonal frequency duplex multiple access
c) orthogonal frequency divider multiple access
d) orthogonal frequency division multiple access

Answer: d
Clarification: Orthogonal frequency-division multiple access (OFDMA) is a multi-user version of the popular orthogonal frequency-division multiplexing (OFDM) digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users.

2. Why is a cyclic prefix required in an OFDMA?
a) To ensure symbol time is an integer number
b) To help overcome multipath and ISI
c) To maintain orthogonality
d) To make OFDMA scalable

Answer: b
Clarification: Use of cyclic prefix is a key element of enabling the OFDM signal to operate reliably. The cyclic prefix acts as a buffer region or guard interval to protect the OFDM signals from intersymbol interference.

3. What does the DC subcarrier indicate?
a) Identity of the cell
b) Antenna configuration
c) Center of OFDM channel
d) Format of data channel

Answer: c
Clarification: All the subcarriers of an OFDM symbol do not carry useful data. In OFDM and OFDMA PHY layers, the DC subcarrier is the subcarrier whose frequency is equal to the RF centre frequency of the transmitting station.

4. What processing step combines multiple OFDM subcarriers into a single signal for transmission?
a) FFT
b) IFFT
c) RF combining
d) Channel mapping

Answer: b
Clarification: IFFT combines multiple OFDM subcarriers into a single signal for transmission. These transforms are important from the OFDM perspective because they can be viewed as mapping digitally modulated input data (data symbols) onto orthogonal subcarriers.

5. Which property of OFDMA system allows adjacent subcarriers to be used without interference?
a) Orthogonality
b) Orthodoxy
c) Octagonality
d) Originality

Answer: a
Clarification: Orthogonality of sub-carriers simply means their correlation is zero. Orthogonality in OFDMA system allows adjacent subcarriers to be used without interference.

6. In OFDMA, what is the relationship between the subcarrier spacing f and symbol time t?
a) f=t
b) f=1/2t
c) f=1/t
d) no relation

Answer: c
Clarification: In OFDMA, the relationship between the subcarrier spacing f and symbol time t is f=1/t. They are inversely proportional.

7. OFDM is a technique for 3G mobile communication.
a) True
b) False

Answer: b
Clarification: OFDM has developed into a popular scheme for wideband digital communication, used in applications such as digital television and audio broadcasting, DSL internet access, wireless networks, power line networks, and 4G mobile communications.

8. OFDM uses complex equalizers.
a) True
b) False

Answer: b
Clarification: ODMA does not use complex equalizers. In OFDM, the equalizer only has to multiply each detected sub-carrier (each Fourier coefficient) in each OFDM symbol by a constant complex number, or a rarely changed value.

9. When we divide band of Orthogonal Frequency Division Multiplexing (OFDM) into sub bands, it diminishes effects of __________
a) noise
b) collision
c) interference
d) signals absence

Answer: c
Clarification: OFDM uses the same bandwidth to deliver roughly the same data rate as a single carrier modulation by introducing multiple lower-bandwidth channels. Each of the lower-bandwidth channels has a lower rate, and by combining them together, the original rate is achieved.

10. Common data rates of IEEE 802.11 OFDM are ____________
a) 18 Mbps
b) 200 Mbps
c) 50 Mbps
d) 54 Mbps

Answer: a
Clarification: The IEEE 802.11a standard specifies a modulation that divides a high-speed serial information signal into multiple lower-speed sub signals. Common data rates of IEEE 802.11 OFDM is 18 Mbps.