It’s an another important problem that how to produce a
proper RF signal (WiMAX signal in this paper). By now, there
are several methods to produce RF signal such as optical
heterodyne method, modulation, up and down conversion, etc.
Among them, optical heterodyne method can only generate RF
signal through frequency mixing at SS, so that the structure of
the SS becomes more complicated and the cost is more
expensive. Taking the flexibility and cost of the SS into
account, we use modulation so that the RF signal can be
generated at BS, as shown in Figure 8.