For any of the transfer functions, to define a new coefficient click . Enter the coefficient value in the new field that appears in the table. Numbers entered are automatically converted to scientific notation. Click to delete an entry from the table. You can make changes to existing values directly in the table. You can add a coefficient in the middle of the table by selecting the table entry that you want to precede the new coefficient and clicking . You can adjust the order of a coefficient by selecting it in the table and clicking the up and down buttons.
The input and output characteristics of a retransmitter are controlled by transfer functions for the transmit frequency, the input/output backoff characteristics, and the carrier to intermod ratio. STK models the transfer functions as Nth-order polynomials by default.
When using polynomial coefficients, the polynomial is defined as:
y = a0x0 + a1x1 + a2x2 + ... + anxn
where x is the input and y is the output. The index column for all the transfer functions strictly refers to either a polynomial coefficient or just the row of a table, if using a table form.
For retransmitters, the Transfer Functions tab has three subtabs described in the sections below. For any of the Transfer Functions subtabs, to define a new coefficient, click . Enter the coefficient value in the new field that appears in the table. STK automatically converts the numbers you enter into scientific notation. Click to delete a selected entry from the table. You can make changes to existing values directly in the table. You can add a coefficient in the middle of the table by selecting the table entry that you want to precede the new coefficient and clicking . You can adjust the order of a coefficient by selecting it in the table and clicking or .
Frequency
Frequency coefficients specify the transmitted frequency as a function of the received frequency. You can only enter them in polynomial form. The coefficient order appears in the left column of the table and updates automatically as you add or remove coefficients. Input and output units are in Hz.
STK uses -7.0e8 and 1.0 as the default coefficients. These model a 700 MHz down conversion. For example, down-converting a 14.5 GHz uplink frequency to a 13.8 GHz downlink frequency entails a -700 MHz shift. The coefficients -7.0e8 and 1.0 reduce the frequency by 700 MHz based on the function:
y = a0 + a1x1 = -7.0e8 Hz + (1.0)14.5 GHz = 13.8 GHz
where x is the input frequency, 14.5 GHz. This setup is just a linear shift of -700 MHz. In most cases, you will keep these default values settings because you won't actually have a polynomial relationship, but you have the option of using a polynomial if you need it.
Power Back Off
You can enter the Power Backoff coefficients in either polynomial form or table data form. Select Table Data as the Input Type to enter the coefficients in table data form. The table will then define coefficients by the Index, Input Backoff, and Output backoff fields. Select the Linear Scale check box if your coefficients are in linear scale rather than dB. In this case, STK will calculate based on the linear scale and then convert the finished values to dB. If you enter coefficients in polynomial form, then the table is the same as for the Frequency coefficients.
STK calculates the Input Back Off (IBO) based on the flux density seen at the receiver, computing the difference between the actual computed flux density in the link information report and the saturation flux density defined in the retransmitter model. If the flux density exceeds the saturation flux density, then IBO is 0. If the flux density is less than the saturation flux density, the IBO is the difference between the values, assuming that you set Operational Mode to Use unadjusted receive flux density in the Model Specs tab. If you set Operational Mode to Use receiver antenna gain delta adjusted flux density, then the IBO is further reduced by the difference between the receiver's max gain and the current gain. From there, STK calculates the Output Back Off (OBO) based on the defined Transfer Function and then subtracts the OBO from the Saturated Power. If the Operational Mode is Use constant output power, the OBO will always be zero, meaning that the retransmitter's transmit power will be exactly the saturation power.
Consider the following example:
- Operational Mode = Use unadjusted receive flux density
- Saturation Flux Density = -90 dBW/m2
- Computed link received Flux Density = -97.5785 dBW/m2
The IBO is then 7.5785 dB. If you change Operational Mode to Use receiver antenna gain delta adjusted flux density, then the IBO will be 7.5785 dB plus the difference in the receiver's max gain (assume 10 dB) and current gain (assume 7 dB). The IBO would then be 10.5785 dB. If the OBO is 5 dB and you set the Saturation Power to 15 dBW, then the retransmitter's actual transmitting power would be 10 dBW.
C/Im
You can enter the Carrier to Intermod ratio (C/Im) coefficients in polynomial or table value form. Select Table Data as the Input Type to enter the coefficients in table value form. The table will then define coefficients by the Index, Input Backoff, and Output backoff fields. Select the Linear Scale check box if your coefficients are in linear scale rather than dB. In this case, STK will calculate based on the linear scale and then convert the finished values to dB. If you enter coefficients in polynomial form, then the table is same as for the Frequency coefficients. When STK computes C/Im, it adds this into the composite C/N for the whole link, thus reducing the composite C/N by the C/Im contribution.