Energy per Bit

— energy per information bit over noise density — is the quantity the demodulator’s error performance depends on (Bit Error Rate).

Energy per Information Bit

• Energy per bit = received data power ÷ information bit rate
• Dividing by the noise density:

Available Eb/N0

• in dB-Hz (S/N₀, reduced by the data power fraction if not all power carries data)
• in dB(bit/s)

• At fixed data power, twice as many bits each second means half the energy per bit: −3.0 dB

Lunar Link: Available Energy per Bit

Lunar Link Available Eb/N0

• Carry forward from the lunar link; assume all signal power carries data
• At :
• At :
• Whether that is enough: closing the margin

Question: Doubling the Information Rate

• Keep received data power and noise density fixed; double the information rate from 1.0 to 2.0 Mbit/s. What changes?
  1. improves by 3.0 dB
  2. worsens by 3.0 dB
  3. worsens by 3.0 dB
• Answer: 2 — available falls by 3.0 dB
  • ; the rate term rises from 60.0 to 63.0 dB(bit/s)
  • stays 74.5 dB-Hz; available falls from 14.5 to 11.5 dB
  • 1 has the sign reversed; 3 is false because the received power and noise density stay fixed

NOTE

• Changing BPSK → QPSK at the same information rate does not change (BPSK or QPSK); changing the information rate does