|System Trade off studies|
System trade off studies
The basic system figure of merit is the carrier to noise ratio C/N. If we calculate the C/N for each of the individual impairments, then the overall C/N of the system is given by,
Figure 7 shows the results of combining the above equation with individual simulations used to compute the individual terms. The independent parameter in this case is the TWTA BO previously described. Note that these components have competing effects. As the BO decreases there is more output power. Since the thermal noise floor is fixed, the C/N component increases as the BO decreases. On the other hand, at the BO decreases the signal is driven further into the nonlinear region of the TWT curve. This, of course, increases the power of the IM components. The net result is an optimum operating point that is determined via the simulation.
Figure 7. Trade off of the system performance as a function of intermod power and signal power
Another system measure is the bit error rate (BER), or sometimes the message or packet error rate. Some user applications require the BER to be in the 10e-6 to 10e-9 range. To achieve such low rates the information data is usually protected by a variety of forward error correcting codes (FEC). The DVB-S system uses a concatenated code, or code within a code structure. The outer code is a [204,188, 8] shortened Reed Solomon (RS) code. This code is used because it is effective against burst errors. The inner code is a rate ˝ punctured to 2/3, constraint length 7 convolutional code. The Viterbi algorithm is used as the decoder. The nature of the convolutional code and this decoder gives rise to errors occurring in bursts that are then ‘cleaned up’ by the RS code. Simulations employing Monte Carlo techniques can combine all of the impairments described here to determine the BER. Also, trade off studies can determine which of these impairments are the most damaging and which are not. This information leads to tolerance requirements for the various components. Thus component costs are controlled with time, energy, and money being devoted only to the extent that is required for performance.
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