A cell phone system designer models the coverage that N cell towers can provide to N users in a neighborhood by a matrix equation s$=$Mp$.$

Here, s is a column vector containing the amount of signal power $($i$.$e$.$ bars$)$ that is received at each user location, p is a column vector containing the power supplied by each of the N cell tower transmitters, and M is the N by N matrix that relates the two.

The system designer is required to avoid a situation where none of the user locations receives any power $($i$.$e$.$ a dead zone forms.$)$

Question: What does this requirement mean in terms of the null space of matrix M$?$