Hi Sil,
In the world of data comms (not radio), the meanings of terms are generally as follows :
Simplex = One direction only ( A to B only, never in the other direction ) eg a broadcast, NOAA WX stations, etc
HDX Half Duplex = Alternating on both directions, e.g A to B, turnaround, B to A, turnaround, A to B, and so on .....
FDX Full Duplex = simultaneously in both directions at same time, A to B AND B to A.
For Simplex and for HDX only 2 wires (baseband) or 2 FSK frequencies are enough. TX and RX dont clash, because they are used Alternately, like a single track railway.
Turnaround Time was always an important factor, because it slowed down the total amount of data transferred per hour, and required careful synchronisation of modem turnarounds at both ends.
For Full Duplex FDX you need a 2 track railway, or 4 wires, or 4 FSK frequencies, which can send and receive at the same time, for trains to go in both directions, for cars to pass on an an ordinary road, for data to pass (ignoring phase modulation and other techniques.)
Simplex, HDX and FDX have nothing to do with Cross-Band working.
Cross-Band is a completely separate and additional requirement.
FM Repeaters often are FDX with frequencies only 600 khz apart, which is Same-Band FDX in the repeater-radio (and used in HDX mode by human users with HDX radios).
Humans using Satellites at VHF and UHF for Simultaneous Up and Down links, require radios with FDX and Cross-band functions.
To summarise, for sat operations, two different functions are required of a radio, FDX and Cross-Band working.
Combining these 2 separate function names into one name seems to confuse the matter.
73
de M3RRX