Using Custom I/Q Sample on "GPS HDL Acquisition and Tracking Using C/A Code" Example

I want to use a custom 8bit I/Q sample data on the example which Mathworks shared:

https://www.mathworks.com/help/satcom/ug/gps-hdl-acquisition-and-tracking-using-cacode.html

What I need to do in order to be able to use my own data? If I feed the I/Q samples directly, I do not receive good results on Acquisition.

Answer 1

[Tom Richter]

Hi Kaan,

it depends on the range of your I/Q samples. In the example the data type is converted from double to fixed-point. Here you must know the range of your data. In the example you can check:

>> max(rxData)

ans =

-5.1297 - 0.0465i

>> min(rxData)

ans =

-1.7579e-04 - 1.3235e-04i

With that you choose the signedness, word length, and fraction length. With fixdt(1,18,14) you will have a range between -8 .. 8-2^-14. When you reduce the word length you also have to reduce the fraction length by the same amount of bits to keep this range. For 8 bit word length: fixdt(1,8,4) which gives a range of -8 .. 8-2^-4. You will have much less precision.

Your data could be complete integter with a range between -128 .. 127. You can interpret the values differently but it would not work using it directly as rxData without changing settings (range would be to high -> overflow). You can do this with the same Data Type Conversion block by changing the "Input and output to have equal:" to stored integer.

It could be necessary to also make fixed-point changes within the "Acquisition and Tracking" subsystem.

Best regards,

Tom