# chirp spread spectrum with 2 users

I am a beginner in digatal communications. I have 2 users and I want use a chirp spread spectrum.

Can we take an up-chirp and a down-chirp as spreading signals for the 2 users? and if yes, why? The down-chirp is $$x_{down}(t)=x_{up}(-t)$$ with $$x_{up}(t)$$ the up-chirp.

• x_{down}(t) = x_{up}(-t)$and$x_{up}(t)\$ don’t overlap in time at all, and so you must be meaning something else for the down chirp. Maybe flipped in time and delayed by the bit duration? – Dilip Sarwate Feb 25 '19 at 0:10

## 1 Answer

Yes, you could. When you're using spread spectrum for multiple access, what's important is the cross-correlation between any two users' spreading signals. Any cross-correlation between two users' spreading sequences will cause "leakage" from one user's channel into another's; this causes an artficial noise floor that will limit your system's performance.

You'll find that an up-chirp and down-chirp have very little correlation with one another, so they would be usable for this purpose.