Difficulty: Medium
For a 2 GHz radio path, transmitter power is 20 dBm, antenna gains are 5 and 2 dBi, path loss is 100 dB, and feed loss is 3 dB. What power arrives?
A -76 dBm
B -66 dBm
C -80 dBm
D -90 dBm
Answer: A. -76 dBm. Add transmitter power and antenna gains, then subtract path and feed losses.
A receiver gets -82 dBm; sensitivity is -90 dBm. What link margin remains?
A -8 dB
B 8 dB
C 82 dB
D 172 dB
Answer: B. 8 dB. Received power is 8 dB above the sensitivity threshold.
In free space, distance doubles while other factors stay fixed. Path loss changes by about what?
A +3 dB
B -6 dB
C +6 dB
D +12 dB
Answer: C. +6 dB. Free-space path loss varies with 20 log10(distance), giving about 6 dB.
At fixed distance, doubling carrier frequency changes free-space loss by about what?
A +3 dB
B -6 dB
C +6 dB
D +12 dB
Answer: C. +6 dB. Free-space loss varies with 20 log10(frequency), when other terms are fixed.
What is the free-space wavelength at 2.4 GHz?
A 12.5 cm
B 1.25 cm
C 25 cm
D 1.25 m
Answer: A. 12.5 cm. Wavelength is c/f, approximately 3 × 10^8 / 2.4 × 10^9 metres.
At 2 GHz, a receiver moves toward the transmitter at 30 m/s. Maximum Doppler shift is about what?
A 200 Hz
B 20 Hz
C 2 kHz
D 20 kHz
Answer: A. 200 Hz. The shift magnitude is approximately v/c times carrier frequency.
For a moving receiver approaching a transmitter, the observed carrier frequency shifts how?
A Lower
B Higher
C Unchanged
D To zero
Answer: B. Higher. Approach reduces wavefront spacing at the receiver, raising observed frequency.
A received signal is 100 times the noise power. What is its SNR?
A 10 dB
B 100 dB
C 20 dB
D 200 dB
Answer: C. 20 dB. 10 log10(100) equals 20 dB.
What absolute power is represented by 0 dBm?
A 0 mW
B 10 mW
C 1 W
D 1 mW
Answer: D. 1 mW. dBm is referenced to one milliwatt.
For 1 MHz bandwidth and 15 dB SNR, Shannon capacity is approximately what?
A 1.00 Mb/s
B 5.03 Mb/s
C 3.00 Mb/s
D 15.0 Mb/s
Answer: B. 5.03 Mb/s. Convert SNR to about 31.6 linear; B log2(1+SNR) is about 5.03 Mb/s.
Wanted power stays fixed while receiver noise doubles. How does SNR change approximately?
A Rises 3 dB
B Drops 10 dB
C Stays constant
D Drops 3 dB
Answer: D. Drops 3 dB. Doubling noise halves the signal-to-noise ratio, a 3 dB decrease.
A link delivers 24 Mb/s over 8 MHz. Its spectral efficiency is what?
A 3 bit/s/Hz
B 1 bit/s/Hz
C 8 bit/s/Hz
D 32 bit/s/Hz
Answer: A. 3 bit/s/Hz. Divide data rate by occupied bandwidth.
How many bits does one uncoded QPSK symbol represent?
A 1 bit
B 4 bits
C 8 bits
D 2 bits
Answer: D. 2 bits. QPSK has four constellation points, representing log2(4) = 2 bits.
How many bits does one uncoded 64-QAM symbol represent?
A 4 bits
B 8 bits
C 64 bits
D 6 bits
Answer: D. 6 bits. A 64-point constellation carries log2(64) = 6 bits per symbol.
An OFDM symbol uses 100 data subcarriers, each carrying four uncoded bits. Bits per symbol?
A 100 bits
B 400 bits
C 200 bits
D 800 bits
Answer: B. 400 bits. The gross count is 100 × 4 bits, before pilots or coding.
An OFDM cyclic prefix is shorter than the channel’s significant delay spread. What can occur?
A Inter-symbol interference
B Higher spectral efficiency
C No fading
D Zero Doppler
Answer: A. Inter-symbol interference. Delayed energy can spill into the next symbol beyond the prefix.
Known pilot symbols in a frame help the receiver estimate what?
A Battery capacity
B Cell identity only
C Channel response
D Antenna height
Answer: C. Channel response. Pilots provide reference values for estimating the radio channel.
Multipath delay spread grows. What usually happens to coherence bandwidth?
A It decreases
B It increases
C It stays fixed
D It becomes zero
Answer: A. It decreases. Larger delay spread generally means the channel is frequency-selective over narrower bands.
A handset moves faster through the same radio environment. Coherence time generally does what?
A Lengthens
B Shortens
C Stays fixed
D Becomes infinite
Answer: B. Shortens. Higher Doppler spread makes the channel change more quickly.
Carrier frequency offset breaks OFDM subcarrier orthogonality. Which impairment rises?
A Thermal runaway
B Frequency reuse
C Shadowing
D Inter-carrier interference
Answer: D. Inter-carrier interference. Misaligned subcarriers leak energy into one another.
An ideal 2 × 2 MIMO channel can carry at most how many independent spatial streams?
A One
B Two
C Three
D Four
Answer: B. Two. The spatial stream count cannot exceed the smaller antenna count.
A diversity receiver uses selection combining. Which branch is chosen?
A Weakest branch
B Oldest branch
C Lowest frequency
D Strongest branch
Answer: D. Strongest branch. Selection combining forwards the branch with the best received quality or power.
A code has rate 1/2. Ignoring overhead, how many coded bits per information bit?
A One-half
B One
C Two
D Four
Answer: C. Two. Rate 1/2 means one information bit per two coded bits.
A data block fails its error check and the receiver sends NACK. What happens in ARQ?
A Retransmission
B Frequency doubling
C Channel release
D Antenna rotation
Answer: A. Retransmission. Automatic repeat request sends the block again after a negative acknowledgement.
In HARQ, how can a receiver improve decoding after a retransmission?
A Discard prior samples
B Change its antenna
C Combine soft information
D Erase the code
Answer: C. Combine soft information. Hybrid ARQ can combine retransmitted information with stored soft decisions.