How to you the output of gnssBitSynchronize to increase integration time of PLL?

Hello, I have been checking and trying the GPS data decode example found in this link:

https://www.mathworks.com/help/satcom/ug/gps-data-decode.html

At the end of the example, it includes a further exploration which says that:

"Also, this example does not update the bit synchronization status back to the tracking loop. You can also use the bit synchronization value to increase the phase locked loop (PLL) integration time to enhance the PLL performance at low SNR values"

I am interested to do and apply this in the GPS Software receiver I am using. Is the sync index the location of start of navigation data bit? how do I use this to increase the PLL integration time?

Thank you for your help.

Answer 1

[Shubham]

Hi Ellarizza,

In the context of GPS signal processing, bit synchronization is crucial for correctly aligning the incoming data bits with the receiver's processing algorithms. Here's a general idea of how you might use the bit synchronization information to improve the performance of your Phase-Locked Loop (PLL) in a GPS software receiver:

Bit Synchronization

1. Yes, the sync index typically refers to the location of the start of a navigation data bit within the received signal. This index helps in accurately aligning the incoming data bits with the receiver's processing windows.
2. Correct bit synchronization ensures that the receiver accurately decodes the navigation message. Any misalignment can lead to errors in data interpretation.

Using Bit Synchronization to Enhance PLL Performance

1. The integration time in a PLL determines how long the loop integrates the phase error signal before making adjustments. Longer integration times can help smooth out noise, which is beneficial in low Signal-to-Noise Ratio (SNR) environments.
2. Adjusting Integration Time:

• During initial acquisition, use a shorter integration time to quickly lock onto the signal.
• Once bit synchronization is achieved and the sync index is identified, consider increasing the integration time. This helps the PLL maintain lock by averaging out noise over a longer period.
• Implement a feedback mechanism where the PLL integration time is dynamically adjusted based on the bit synchronization status and estimated SNR. For instance, if the bit sync is stable and SNR is low, increase the integration time.

3. Implementation Steps:

• Continuously monitor the bit synchronization status to ensure the receiver remains locked to the correct bit boundaries.
• Use the sync status as feedback to adjust the PLL settings. This can be done by tweaking the loop filter parameters or directly modifying the integration time based on the reliability of the sync index.

4. Algorithm Considerations:

• Ensure that the algorithm for adjusting integration time is robust to sudden changes in noise levels or signal conditions.
• Thoroughly test the modified PLL under various conditions to ensure it maintains lock and performs well across different SNR levels.

[Ellarizza]

Hi Shubham,

Thank you for the informative answer. I think I quite understand it more how to use the sync index to increase the integration time.

Just some clarification:

• Is it correct to check the bit synchronization status every bit period and starting at the sync index since we already know it after the first bit synchronization?
• You mentioned about dynamic integration time. After knowing the sync index, could I extend the integration time beyond the number of samples of data bit (In the case of GPS L1CA, 20 samples every 1 data bit)? My question is that for example I am integrating over 100 samples, but I may have multiple bit transitions during this period. As a result, the integrated values will decrease. When you say dynamic integration time, is it integrating the values before the bit transition occurs?

Thank you so much. Your answer really helped me understand the improvement of PLL.