Wireless Communication Basics
Which is a general property when the modulation is changed from 16QAM to 256QAM at the same symbol rate?
- AThe bit rate doubles, but the signal points are closer together, so a higher S/N (signal-to-noise ratio) is needed to keep the same error rate
- BThe bit rate becomes 16 times higher and the required S/N does not change
- CThe extra signal points make it more robust against noise, so it is easier to use where the signal is weak
Show answer and explanation
Answer: A. The bit rate doubles, but the signal points are closer together, so a higher S/N (signal-to-noise ratio) is needed to keep the same error rate
16QAM sends 4 bits and 256QAM sends 8 bits per symbol, so at the same symbol rate the bit rate doubles. On the other hand, more points are packed into the same average power, so the spacing between points narrows and noise more easily causes a point to be mistaken for a neighbor. For this reason, wireless LANs and mobile phones use adaptive modulation, switching to higher-order modulation when conditions are good and to schemes such as QPSK when they are poor.
- A. Correct. The bits per symbol rise from 4 to 8, but robustness against noise falls.
- B. The number of signal points becomes 16 times larger, but taking log₂ gives a doubling of bits, from 4 to 8. The required S/N also increases.
- C. The narrower spacing between points actually makes it weaker against noise. Where the signal is weak, modulation with fewer levels is used.