wireless communications

Wireless Communications Interview questions

1. What is the need for cyclic prefix in OFDM systems
2. What is the minimum length of cyclic prefix in OFDM systems
3. What is the maximum length of an OFDM symbol
4. What is the condition for deciding the number of subcarriers in OFDM systems
5. What is delay spread and coherence bandwidth. How are they related
6. What is doppler spread and coherence time. How are they related
7. What is the relationship between BER of BPSK and QPSK. Why?
8. What is a superheterodyne receiver and why is it so special?
9. What is the relation between baseband bandwidth and passband bandwidth
10. What is the relation between subcarrier spacing of an OFDM system and its symbol duration
11. What is doppler spread and how is it related to velocity
12. What is flat fading, frequency selective fading, slow fading and fast fading
13. Is OFDM a modulation
14. What are the advantages and disadvantages of ofdm systems
15. If we have BPSK and BFSK with same average transmit power, which modulation has low BER
16. What is minimum shift keying. why is the termed minimum coined in this
17. Explain baseband sampling theorem. What is nyquist sampling rate
18. Explain the concept of under sampling. Give its advantages and disadvantages
19. What does white gaussian noise mean
20. Describe Shannon's capacity theorem
21. What is a rake receiver and what are the advantages of this receiver
22. Can we interchange IFFT and FFT in OFDM modem and then also demodulate
23. Can we replace the cyclic prefix with zeros and even then equalize the channel with a single tap
24. What is the effect of frequency offset and timing offset on OFDM systems. Also, how does the constellation change
25. Explain the concept of frequency resuse. Give few more ways of improving capacity
26. What is the reason for considering I and Q streams in communication system
27. What is the effect of FFT hardware on noise in an OFDM receiver
28. What happens to transmit power and channel bandwidth of any M- ary modulation scheme as M increases
29. Which modulation scheme is more sensitive to phase offset or timing offset. PSK or FSK. Why?
30. Given a baseband signal with one sided spectrum from 0 to fm, what is the nyquist rate of this signal.
31. When do you realize a wireless channel as rayleigh fading or a ricean fading channel.
32. What is delay spread and coherence bandwidth. How are they related

Answers for above Interview questions on wireless communications

1. What is the need for cyclic prefix in OFDM systems

There should a guard time for every OFDM symbol to remove the inter block interference. However if we fill up this extra guard samples with a cyclic prefix, we can equalize at the receiver with less complexity.

2. What is the minimum length of cyclic prefix in OFDM systems

There should a guard time for every OFDM symbol to remove the inter block interference. However if we fill up this extra guard samples with a cyclic prefix, we can equalize at the receiver with less complexity.

3. What is the maximum length of an OFDM symbol

OFDM symbol depends on the number of subcarriers. In general it is better to increase this value. However, it can be increased to an extent where the OFDM symbol duration is less than coherence time. Else, the channel response will tend to change in the ofdm symbol itself. This contradicts the aim to divide the data symbols to N streams so that each stream experiences flat fading.

4. What is the condition for deciding the number of subcarriers in OFDM systems

OFDM symbol depends on the number of subcarriers. In general it is better to increase this value. However, it can be increased to an extent where the OFDM symbol duration is less than coherence time. Else, the channel response will tend to change in the ofdm symbol itself. This contradicts the aim to divide the data symbols to N streams so that each stream experiences flat fading.

7. What is the relationship between BER of BPSK and QPSK. Why

BPSK and QPSK has the same BER. This comes from the fact that QPSK can be assumed to be a combination of two BPSK constellations each orthogonal to each other. Hence, there is no effect of one BPSK constellation on another.

9. What is the relation between baseband bandwidth and passband bandwidth

passband bandwidth is twice that of baseband bandwidth.

13. Is OFDM a modulation

No. OFDM is a multiplexing scheme. It is not the modulation.

15. If we have BPSK and BFSK with same average transmit power, which modulation has low BER

BPSK. Let us denote that the coordinates of BPSK constellation points be (a,0) and (-a, 0) and BFSK constellation points be (a,0) and (0,a). This shows that the distance between the constellation points of BPSK is more than that of BFSK. Hence, BER is low for BPSK.

25. Explain the concept of frequency resuse. Give few more ways of improving capacity

Instead of using a single high power transmitter, we can use multiple low power

29. What is delay spread and coherence bandwidth. How are they related

If the channel is realized to be a filter of L taps, cyclic prefix should be at least L-1 length.