Instructions: Answer all questions. Each question carries equal marks. Try to answer before checking the solution.
Basic Concepts (Questions 1-10)
1. What does QAM stand for?
Quadrature Amplitude Modulation
2. What two modulation techniques are combined in QAM?
Amplitude Shift Keying (ASK) and Phase Shift Keying (PSK)
3. How many carriers are used in QAM and what is their phase relationship?
Two carriers (in-phase and quadrature) with 90° phase difference
4. What is the main advantage of QAM over simpler modulation schemes?
Higher spectral efficiency (more bits per symbol)
5. What does the "16" represent in 16-QAM?
There are 16 possible symbol states (4 bits per symbol)
6. What graphical representation is used to visualize QAM symbols?
Constellation diagram
7. Name two common applications of QAM.
Cable modems, digital TV (DVB), Wi-Fi, LTE/5G (any two)
8. What is the main disadvantage of higher-order QAM compared to lower-order?
Greater susceptibility to noise and interference (requires better SNR)
9. What mathematical concept is fundamental to QAM's operation?
Orthogonality of sine and cosine waves
10. In QAM terminology, what do I and Q stand for?
In-phase and Quadrature components
Constellation Diagrams (Questions 11-18)
11. How many points are in a 64-QAM constellation diagram?
64 points
12. What is the shape of a standard 16-QAM constellation?
4×4 square grid (16 points arranged in a square pattern)
13. What does the distance between points in a constellation diagram represent?
Noise immunity (larger distances mean better error resistance)
14. What is Gray coding in QAM constellations?
An encoding scheme where adjacent symbols differ by only one bit to minimize bit errors
15. Why are some QAM constellations circular rather than square?
To maintain constant envelope (amplitude) which is better for nonlinear amplifiers
16. How many bits per symbol does 256-QAM carry?
8 bits per symbol (log₂256 = 8)
17. What happens to constellation points when noise is introduced?
Points appear as clouds around their ideal locations
18. What is the relationship between the number of constellation points and bandwidth efficiency?
More points = higher bandwidth efficiency (more bits/Hz) but requires better SNR
Performance and Calculations (Questions 19-26)
19. Calculate the bits per symbol for 32-QAM.
5 bits per symbol (log₂32 = 5)
20. If a 64-QAM system transmits at 6 Msymbols/sec, what is its bit rate?
6 Msymbols/sec × 6 bits/symbol = 36 Mbps
21. What is the bandwidth efficiency of 16-QAM in bits/Hz?
4 bits/Hz (log₂16 = 4)
22. Why does higher-order QAM require better SNR?
Because constellation points are closer together, making them more susceptible to noise
23. What is the theoretical maximum bandwidth efficiency of 1024-QAM?
10 bits/Hz (log₂1024 = 10)
24. If a channel has 6 MHz bandwidth, what is the maximum symbol rate for QAM transmission?
6 Mbaud (symbol rate equals bandwidth for QAM)
25. Calculate the bit rate for 256-QAM at 8 Mbaud.
8 Mbaud × 8 bits/symbol = 64 Mbps
26. What is the minimum SNR required for 64-QAM compared to 16-QAM?
Approximately 6 dB higher (each additional bit/symbol requires about 3 dB more SNR)
Advanced Concepts (Questions 27-30)
27. What is adaptive modulation in QAM systems?
Dynamically changing the QAM order based on channel conditions to maximize throughput
28. What is the purpose of trellis coding in QAM systems?
To add error correction without increasing bandwidth by using redundant coding
29. Why is pulse shaping used in QAM systems?
To limit bandwidth while minimizing intersymbol interference (ISI)
30. What is the difference between QAM and OFDM?
QAM is a single-carrier modulation, while OFDM uses multiple subcarriers each modulated with QAM (QAM is the modulation, OFDM is the multiplexing scheme)