The relation of sub-carrier and FDMA 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?
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?


There's $K$ pairs of nodes, if you read the text closely. That means you can have, for example, one source with four information sinks, or four sources and three sinks, or…