Let $s(t)$ an up-chirp with bandwidth $B$ and $y(t)$ an down-chirp with the same bandwidth. For a multi user chirp spread spectrum (2 users), I use the up-chirp and down-chirp as spreading signal. The system bandwidth is $B_c$ with $B_c=B$.
But if I look at the spectrums for the 2 users, there is no superposition. Consequently, can we transmit these signals because we must have a system bandwidth $B_c$?