n is indicative of time. It shows how the input and noise impact the received symbol with time n.
If x and w are vectors at time n (let's say of length M), which give the corresponding received vector y, since you take h to be a scalar. Such a system would be a strange MIMO system with only links between transmit and receive antennas pairs and each antenna pair seeing the same fading coefficient. For ex: tx antenna 1 would have a fading coefficient of h towards Rx antenna 1 and complete fade (h=0) towards all the other M-1 antennas at the receiver. Similarly for other tx and Rx antenna pairs
So, n here is represting time and x, w should be scalers for this signal model to make sense given h is scalar