Together with Milosch from bitmanufaktur.de, I've spent a couple of days and nights working on building our own 2.4GHz active RFID system. The parameters are: Battery powered, small transponders based on the Nordic Semiconductor nRF24L01 and a ultra-low-power PIC micro-controller. Base-stations have the same nRF chip, plus an Ethernet-enabled micro-controller. Operational range is ~10 meters, we even made it through two walls in our preliminary tests.
So what's the point of all this? The goal is to have that system in operation at the HOPE Nr.6, and to give a practical demonstration on how feasible it is to track people, make protocols on who has spent how much time in which lecture hall, etc.
I'm not involved with HOPE at all, but was only involved in writing firmware and higher-level code for the two embedded systems involved, as well as some testing.
I'm looking forward to learn how the system will perform with several hundreds/thousands of transponders at its first real-world test at HOPE. With some luck, we can have the same system at 23C3 in December this year in Berlin.
Until then I'm certainly going to try to implement my idea for peer-to-peer time slot synchronization of the transponders. I had that idea as a solution to avoid collisions between transponders while working on the project. Unfortunately, the current transponder hardware doesn't have a low-power timing source that is precise enough for such a protocol. Adding a 32.768kHz low-power quartz should do the trick - but will inevitably also make the transponder more costly.
Kudos to bitmanufaktur. I wouldn't even dare to try to design a PCB for 2.4GHz RF...