# Satellite Communications/BW vs DATA RATE

Question
QUESTION: Good night,

I have 2 carriers with these parameters

DATA       MODULATION   FEC  COD   BW      Ftx
RATE
2048KBPS     8PSK      3/4   TPC   1.2MHZ  6194.2400
2048KBPS     8PSK      3/4   TPC   1.2MHZ  6195.4400

Im using a roll off 0.25

But when Im trying to transmit with 2048kbps I cannot receive my signal

But when I used a data rate less than 1024 kbps I can receive the two signal

What could be the problem?

Why I cant receive my signal when I transmit with more than 1024 kbps

Have I nice day

You are seeing adjacent carrier interference when operating at 2048 kbps.

Your Tx carrier frequencies are 1200 kHz apart. You need either approximately 1300 kHz separation or a lower information rate.

The following are calculations for 2048 kbps and for 1024 kbps (the 1024 kbps data are in parentheses)
•   Using 8-PSK ¾-rate Turbo (TPC), your symbol rate is ~902 kSym/s (~455 kSym/s)
•   Occupied BW at -10 dB points is ~1082 kHz (~541 kHz)

The allocated bandwidth needs to be 1.35 to 1.4 times the symbol rate. The following are allocated bandwidths for 1.35 and 1.4 for 2048 kbps and 1024 bps.
•   Allocated BW at 1.35 = 1228 kHz (614 kHz)
•   Allocated BW at 1.40 = 1273 kHz (637 kHz)

Please refer to the following website for more details: http://www.satsig.net/symbol01.htm .

If you find this to be useful, I would appreciate it if you would complete the survey. If not, please let me know.

Thomas E. Burke, Ph.D, PMP

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

QUESTION: Ok but Im going to use a roll off = 0.25

¿So the allocated BW 1.25 x symbol rate?

FEC graph

I know that you said roll off = 0.25. But it does not work at 2,048k whereas it does at 1,024k.

It still appears to me that you have an adjacent carrier interference problem.

The article that I cited recommended roll-off greater than 0.35 or 0.40, and it suggested adjacent carrier interference allowance in link budgets is 28 dB on each side.

•   What is the crossover point (in dB) of the upper and lower waveforms at 2,048 kbps?
•   What is the crossover point (in dB) of the upper and lower waveforms at 1,024 kbps?

I suspect that you will find 2,048 kbps crossover point will be less than 28 dB down from peak and that 1,024 kbps crossover point more than 28 dB down from peak,

The attached chart, which shows spectral efficiency (bps/Hz) versus Eb/No for QPSK, 8-PSK, and 8-QAM suggests that you would be better off with a Rate 7/8 TPC or a Rate 0.95 TPC. Source of the graph is http://www.comtechefdata.com/technologies/fec

I hope that this helps

Thomas E. Burke, PhD, PMP

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

two carriers
QUESTION: So I can work with 1024 kbps with roll off  = 0.25%

but for 2048 kbps I need to work with roll off = 0.35% or 0.40%

I have the image of two Carrier with 1.2 MHz of bandwidth and 2048 kbps of data rate

How can I know  the crossover point (in dB)?

Eduardo:

Figure 1 depicts adjacent channel interference between TX and RX signal.

If you were to re-label RX as TX(f1) and TX as TX(f2), this would depict what you are dealing with.

The frequency at which the two waveforms overlap is referred to as the crossover point. You want to measure the amplitude of the crossover point with respect to the peak amplitude.
The amplitude of the crossover point should be a factor of about 1,000 (-30 dB) down from the peak amplitude.

Figure 2 shows a single waveform with envelope (the heavy line) showing -3 dB, - 33 dB, and -53 dB levels.

I hope that this helps
.
Thomas E. Burke, PhD, PMP

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Satellite Communications

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#### Thomas E. Burke

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##### Experience

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.

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