It's not something that is being used currently, however I can give you as simple an explanation as I can into how it'll work.

At the moment on the UMTS networks every cell site is on the same frequency, around the 2100MHz mark.(Each operator has a different frequency but they're all around this point) Previously on GSM each operator had their own frequency, Voda and O2 were on 900MHz, Orange and T-Mob on 1800. Within these bands they were split into smaller frequency channels, these were reused over the networks footprint, each cell or sector having a specific BCCH channel for your handset to synch to. Every time you made a call you're allocated a specific block of time on one of the operators transmitters. What's important to remember is the fact while you're in a call that little slot of time on that particular transmitters frequency (each transmitter being different) belongs to you, no-one else uses it. (not strictly true if you know about baseband hopping but for the sake of this explanation!)

Within the UMTS network it's slightly different. Say Orange for example, but it's the same for all of the networks, each WBTS site is at the same frequency. When you make a call, pass data etc the network allocates you a dedicated channel which is code multiplexed, this uniquely identifies your data to the network, it then allows it to route the data accordingly. Using this method, with a code for each user thats transmitting or receiving each cell site can be on the same frequency.

Now with HSDPA is based along the same lines as before except instead of a dedicated channel each user shares resource. As most data is bursty it allows data to be sent by one user, once the data is sent another user gets access to the resource and so on. All it's doing is, when some users are silent, not sending any data, more resource is spent on those who want it. Thus resulting in "perceived" high speeds, of up to 10Mbps!! (Again this is also dependant on the link that supplies the WBTS) With HSDPA packet throughput is better which results in more users and quicker response times, latency is another term for it I think.

Hope that makes things a little clearer.

G.