Wireless sensors network, one receiver HELP

I ve been introduced one year ago to a project ( i can't remember the designs that's why i ask here ), where the guy had one receiver hooked up to a mcu ( arduino maybe) and up to 99 !! transmitters serially coded hooked up to a motion sensors.
The designs of the transmitters were very small coin cell batteries, motion sensor and RF ( size of a thumb nail) and cheap...
So basically i want to know what kind of rf were used for that one receiver to 0-99 serially coded transmiters...

oh and the motion sensors weren t supposed to act at the same time so there was no "interferences" problem, just needed for the receiver to know from which sensors it came from.

Any ideas ? (and sorry for the crappy english !).