# Wireless & Mobile Communications Questions & Answers – Orthogonal Frequency Division Multiple Access (OFDMA)

This set of Wireless & Mobile Communications online quiz focuses 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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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.

Sanfoundry Global Education & Learning Series – Wireless & Mobile Communications.

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