In Mengyu Liu, Yuan Liu, "Charge-then-Forward: Wireless Powered Communication for Multiuser Relay Networks", it is said that
while information signals are conveyed by using FDMA over $N$ sub-carriers (SCs) in the next phase of WIT(wireless information transmission). For information transmission, we define a binary SC allocation variable $x_{k,n}$ with $x_{k,n} = 1 $ representing that SC $n$ is allocated to pair $k$ for WIT and $x_{k,n} = 0 $ otherwise. Each SC is allocated to at most one pair at slot 1 for WIT to avoid interference
Does it mean that if the source transmits information to relay, it needs a carrier to help it send the message, so if I have $K$ source, then I have $K$ subcarrier?
If so, why does it still need two different variables? And I can't understand that if my thinking is right, why does it still have two situations, $x_{k,n}=0$ and $x_{k,n}=1$, for that?