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Satellite Communications/Link budget - carrier in carrier


QUESTION: Good morning Thomas E. Burke,

My question is I had already made a Link budget of two carriers with different frequency, but Know I want to make the calculation of these carriers making then in carrier in carrier

the two carriers in the same frequency , I want to know what parameters I have to include in the link budget to make the calculation.

Thank you,
Have I nice date.

ANSWER: Link Budget: Carrier in Carrier

Señor Saravia:

Thank you for asking.

The Cometech EF Data website ( describes one technique for carrier-in-carrier referred to as DoubleTalk®.

As described on page 3 of the article, the signal received at the downlink demodulator comprises the signal transmitted by the spacecraft plus the interference cause by the much stronger uplink signal. Figure 3 shows how the uplink signal is used to cancel the noise that it causes at the downlink demodulator. The article states that commercially available products provide interfering signal suppression of 28 to 35 dB.

The residual interfering signal appears as noise causing a slight degradation of Eb/No. To compenstate for the residual noise, a small amount of additional link margin is required to maintain the desired BER. Margin requirements depend on the product, modulation, and power ratios. A table near the top of page 5 shows the additional margin requirements for a particular modem and a variety of modulation schemes. The additional margin requirements vary from about 0.3 dB for BPSK to 0.6 dB for 16-QAM.

There is a screen shot at top of page 7 that shows the various parameters needed for a link budget calculation. These parameters include: EIRP, information rate, forward error correction code rate, modulation, rain margin, required availability, and the Eb/No threshold.

I hope that you find this useful and that it answers your question. If it meets your needs, I would appreciate your evaluation. If it does not meet your needs, please get back to me.


Thomas E. Burke

---------- FOLLOW-UP ----------

QUESTION: Hello Thomas,

I read the article and want to know how they calculated The PEB in page 6 and 7?

With and Eb/No desired  I need to add margins in CnC ?

What is it for the " leased BW and leased Cnc"?

Thank you
have a nice day

Dear Mr. Saravia:

I have decomposed your question into three specific questions.

1.  How to calculate PEB

The Appendix (page 15 to page 20) describes the process for calculating the Power Equivalent Bandwidth (PEB).

The figure at the top of page 16 (BER versus Eb/No) pertains only to Comtech EF Data’s CDM-625 Advance Satellite Modem. If using a different modem, you will need its equivalent of this chart.

2.  CnC Margins

Yes, you should add CnC margin.

In the screen shot at top of page 8, the red circle tells us that Eb/No Threshold includes IF-RF Margin and CnC Margin.

The text to the right of the graph at top of page 16 also confirms that Eb/No threshold includes CnC margin and other unspecified margins.

    Eb/No (threshold) = Eb/No (from chart) + X dB (CnC margin) + Y dB (any other margin)

3.  CnC Margins for Leased Bandwidth and CnC Bandwidth

I do not understand “What is it for the "leased BW and leased CnC"? Please clarify to what “it” refers.

If “it” refers to Eb/No, I cannot answer because it depends upon the BER, the modulation scheme, and the characteristics of the mod/demod that you are using.

If my answers are satisfactory, I would appreciate it if you would rate me. If my answers are not satisfactory, please get back to me.


Thomas E. Burke, PhD, PMP

Satellite Communications

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Thomas E. Burke


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Forty years of experience with satellite communications. Have held roles as system engineer on JPL Mariner 9 program and a program manager for a number of classified communication satellite programs. Served as head of TRW / Defense Communications Division / Engineering Development Operation, a 1,400-person organization responsible for all aspects of classified communication satellite design and development.

Ph.D. Chemistry, California Institute of Technology (1969) B.A. magna cum laude Chemistry, University of Minnesota (1962)

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