Link Margin

Margin is how far the available performance exceeds what the service requires. Positive margin means the link closes; how much margin is enough is a design (and risk) decision.

Margin Against a Received-Power Threshold

• Start from the aligned, polarization-matched received power and subtract the pointing and polarization losses:
  • , = transmit and receive pointing losses
• Margin against a required received power:

Pointing and Polarization in the Link Budget

Illustrative link	Value
: aligned, polarization-matched power	−120 dBW
Pointing loss: transmit / receive	1 dB / 2 dB
: polarization mismatch loss	3 dB
Actual received power	−126 dBW

• For a −125 dBW threshold, the margin is −1 dB
• These losses change whether this link supports the selected service

Don't double count

• The aligned power already includes the other link terms
• Use peak gains with pointing losses, or actual-direction gains without subtracting pointing loss again

• More received-power-threshold margins from Homework 3: polarization and margin during a pass

Available and Required Eb/N0

• Received power alone does not say whether the bits are recovered; the real comparison is in energy per bit
• Available comes from received data power, noise density, and information rate
• Required comes from a performance curve or a tested receiver specification
  • Example: ideal coherent uncoded BPSK needs about 9.6 dB for BER in AWGN (BER curve)
• Real receiver losses (implementation loss) and design reserve must also fit in the budget

Eb/N0 Margin

• The margin must then cover any required design reserve

Lunar Link: Closing the Margin

Lunar Link Margin at 1.0 Mbit/s

• Available (from the lunar link budget and energy per bit):
• Assume BER ≤ requires 9.6 dB for ideal uncoded BPSK
• Assume a separate implementation loss of 1.0 dB

Rate and Margin

Information rate	Available	Margin
1.0 Mbit/s	14.5 dB	3.9 dB
2.0 Mbit/s	11.5 dB	0.9 dB
4.0 Mbit/s	8.5 dB	−2.1 dB

• Bandwidth and receiver lock must also be adequate — power margin isn’t the only requirement

Question: The 4 Mbit/s Link

• We need 3.0 dB reserve, but the 4.0 Mbit/s case has −2.1 dB margin. Which change alone is enough?
  1. Switch BPSK to QPSK at the same information rate
  2. Reduce the information rate to 1.0 Mbit/s
  3. Add 20 dB of ideal gain after the receiver front end
• Answer: reduce the information rate (option 2)
  • The 1.0 Mbit/s case has 3.9 dB margin, which exceeds the 3.0 dB reserve
  • QPSK reduces bandwidth but does not change ideal bit-energy performance (BPSK vs. QPSK)
  • Gain after the front end amplifies signal and existing noise together (improving sensitivity)

Maximum Rate With Reserve

Lunar Link: Maximum Rate With 3 dB Reserve (optional slide)

• Require 3.0 dB design reserve beyond the BER requirement and implementation loss
• This is a power-limited result; spectrum, coding, hardware, and acquisition can lower it

• Other ways to buy margin, and what each costs the mission: the remaining trades