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How to build a lsfr based on sequence $s_i = s_{i-1} + s_{i-4}$?

us flag

How do I know where my XOR gates go? What does the F2 stand for here? Also the next task is to generate the sequence (with initialisation vector $s_0 = 1, s_1 = s_2 = s_3 = 0$) until it becomes periodic which I'm fairly certain I can do however even a few rows would be helpful as I won't have any other answers to these (not official homework assignment).

enter image description here

edit: my guess is now the xor port goes on top $s_3$, because of unknown reason. How will this affect the sequence? When generating it I shift and do I xor the last $s_3$ and first $s_1$ in order to receive the new bit?

edit2: my solution with sequence enter image description here

us flag
I have tried looking at this among others, but I am still unsure. My guess is that si-1 = s0 and si-4 = s3, but I am still unsure how to draw it and why it becomes as such?
us flag
Also checked that before. This is in different form. I believe I can draw from polynomial form but not from this. Also what F2 stands for is still unknown to me and it isn't mentioned anywhere in your replies.
kelalaka avatar
in flag
The sequence definition already gives you the hint. the next feedback is the sum( xor?) of previous and 4th previous?
us flag
So si-4 = s3 and the xor goes on top of s3?
kelalaka avatar
in flag
Run you solution?
us flag
edited solution
ph flag
$F_2$ is the field with 2 elements. The question is just being explicit that each cell is a single bit.
kelalaka avatar
in flag
From your textbook read from page 40 and you will get it. If you had any problem of understanding the topic your questions are welcome or ask questions for getting hints.
kodlu avatar
sa flag
see also https://crypto.stackexchange.com/questions/53137/lfsr-tap-sequence-15-bit-lfsr
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