(Hard) Questions about sync index and trigger found from labview inbuilt example.

[sunson29]

Dear friends and application engineer, 

This question could be hard, I hope someone could help me about. This is about the inbuilt example: MT PSK Transceiver (One Shot)  or attached VI (not sure will ask sub vi or not) . If necessary, I can create a service number to discus more. 

I have 2 questions: 

1. In the case below (with those given input parameters。 e.g #guard bits, # sync Bits ....), if the Eb/N0 is high enough, the sync location will be close to 8. Why? In other words, why not other number 2, 6, 22? What's meaning of this 8. This 8 is calculated from where? Please give me details. Btw, I do got 7.9986 means what? I assume the index should be an integer. 

2. The "BER trigger found" could be true only under first condition that "sycn found" is True. Am I right? I know there is no "sycn found" output here. But When I run this vi, I found that, if the sync found index is not close enough to 8 (e.g. found index = 800), the BER trigger found will be always off. 

3. The BER trigger index is always 20. What's meaning of this ? In other words, this 20 is from where to where in this case.

When I ask "to/from where in this case", this means, in this case (16 gurad bits, 2000 message bit, 20 sync bits, PN=9, QPSK ....), the index 8 and 20 is from which part of bit ? Please give me the example.  

I do know this could be a hard question. Thank you so much again for your time and help. Thank you!   

[James_Battleaxe · Accepted solution]

Hi,

I can try to answer some of your questions about the MT PSK Transceiver example. My answers are below.

1. The sync location should correlate to the number of guard bits you have because that where the sync bit sequence starts. For example, if you change your number of guard bits to 10, the "sync found index" should change to 5. I do agree that it should be an integer. I'm not sure why it's showing up as a decimal.

2. If the "BER Trigger Found?" light is not on, it means the receiver never found the sync bits so it didn't know where the message starts. If your receiver never found the the sync bits, it would also not be able to correctly display the sync found index.

3. I've found that the BER Trigger seems to match the # of Sync Bits. For example, if you change the # of Sync Bits to 30 instead of 20, the BER Trigger Index will change to 30 as well. The BER trigger index is showing the first bit where the bit pattern starts to match the PN sequence you sent. That should occur right after the sync bits so it makes sense that the BER Trigger Index seems to match the length of Sync Bits. 

[sunson29]

Dear James, 

Thank you again for your help and answers. Allow me have more follow up questions. 

1. I see that in VI as well. Why sync location is correlated with # of guard bits?  E.g. if the sync index is 5, this 5 is accounted from where,  from guard bit, from sync bit?

2. Yes, this is what I thought as well.

3. This is should be the same as question 1, what's meaning of index trigger index?  If index = 30,   this 30 is calculated from where,  only from beginning of message bit?  

The package is structured as guard bits + sync bits + message bits .

James, If I may,  may I create a service number with you. So I could talk with you more on the phone.  Thank you again.

[James_Battleaxe · Accepted solution]

Hi,

You can definitely open a SR with our support team. You can create SR's through the web or by calling in at 866-275-6964. Your SR will be directed to a team of supporters who have experience with the hardware or software you're using. So the SR may not come to me, but it'll go to someone with experience in the Modulation Toolkit.

As for your follow up questions I think the main area you're still having questions on is how the sync bits and guard bits work. The message you're sending has sync bits and guard bits around it. The entire bit sequence starts with guard bits, then sync bits and then the message itself. If sync index is 5, it means the sync bit sequence was found 5 symbols into your bit sequence. Since you're doing Quadrature PSK, 2 bits equals 1 symbol so 5 symbols into your bit sequence would equal 10 bits into your sequence. The only thing before the sync bits are the guard bits so that's why adjusting the guard bits changes what the sync index returns.

For the trigger index, it looks like the guard bits are ignored and we only count up the index starting with the sync bits. So if you have 30 sync bits, your message should start right after those 30 sync bits are done. 

[sunson29]

Dear james,

When you say"If sync index is 5, it means the sync bit sequence was found 5 symbols into your bit sequence.  ". My understanding is, this index 5 symbol is count from very beginning of the sequence. In other words, this 5 is count from the guard bits. Yes or Not?

In the in built example, the index is started from 0 or 1 ?   Thank you, James.  

[James_Battleaxe]

Hi,

Your understanding is correct. Guard bits are counted for the Sync Found Index. The index starts at 0. 

[sunson29]

Thank you again!

[sunson29]

Hi James, 

Allow me have some follow up questions again here.  Because I found something strange today. 

If you use the setting below, and make the channel kinda bad @ 0.33dB.

In this case, the length of guard bits is 100, sync bits is 80, how comes the sync found index is 5230 ?  My understanding is that the sync found index should be <=100+80, (Bc it count from guard bits).

339 for BER trigger found index is fine, bc the channel is bad, the right location takes "longer" to find, then the index is larger.  

Notes: one packet composed by guard bit+ sync bits + information bits.    The length of information bit = 50000 by using PN=5.  PN=5 only can generates 2^5-1 =31 bits, Hence, there is repeating for those 31 bits until reach 50000.(If will cut off some bits, bc 50000 /31 is not integer. )    

 Thank you again, James.  Actually. I am doing this for USRP simulations, I found BER trigger found index is the same as sync length, which is good! But my sync found index is not the same as length of guard bit length.  Hence, I need better understanding.  Thank you, James.