The 1090 ES is the transponder system used in ADS-B. Why does the 1090 ES use the same frequency (1090MHz) for uplink as well as downlink? 1090 ES is an extension of Mode S. The Mode S used 1030MHz for uplink and 1090MHz for downlink. Why did the 1090 ES change that? Having two frequency channels would be better (More space less noise due to message collisions) for communication right?
There is no uplink in ADS-B. The aircraft is broadcasting information which is received by parties unknown to the ADS-B transmitter. There is no reply and there is no uplink like there is in Mode-S.
The receiving parties listen on 1090MHz for ADS-B broadcast.
A technology related to ADS-B is TIS-B (Traffic Information Service - Broadcast). This is used in the USA to transmit positions of aircraft that are not equipped with 1090ES ADS-B but are detected by FAA radar or UAT ADS-B in an ADS-B like format on 1090MHz. This allows parties listening on 1090MHz for ADS-B traffic to identify non 1090ES equipped aircraft with the same receiver. TIS-B could be seen as form of uplink, but uses the 1090MHz frequency nevertheless as it allows the same ADS-B receiver to be used.