How to calculate position update rate using TDMA?

[SlouchyCat]

Dear all,

I was trying to understand the update rate from this comment:

The system will run with 2^TDMA_NSLOTS_BITS slots. So to fly
between 3 and 4 Crazyflie set TDMA_NSLOTS_BITS=2, up to 8 set
TDMA_NSLOTS_BITS=3, etc .....
Each time slot is 4ms long so adding more slot will slow down
the ranging rate.

Does it mean that:
1) if i have 4 tags with TDMA_NSLOTS_BITS=2, the time taken for a tag to get all ranges from 6 anchors is 4ms, and the position update rate is 1/(0.004*4)=62.5Hz?
2) when set TDMA_NSLOTS_BITS=2 and i only use 2 tags, the time taken for 2 tags is the same as 4 tags situation? Since the tag only wait to request when its time slot comes?

Thanks~

[arnaud]

Hi,

1) 4ms is for each slot, and each slot is used by one Crazyflie to communicate with one anchor. So if you have 4 Crazyflie and 6 anchor, each Crazyflie will make a full scann of the 6 anchors about 10 time per second: 1/(0.004*4*6) = 10.42. It is hard to calculate the rate of the position update: each ranging measurement will allows the kalman fileter to refine its position estimate and every 4 measurement it gets enough information for a full position calculation.

2) This is correct: TDMA_NSLOTS_BITS=2 means that there is 4 timeslot, each Crazyflie only talk on one every 4 timeslot.

Note that this implementation is sub-optimal in a lot of ways and could be optimize to allows for shorter timeslots or even different communication scheme, but this is where we are so far.

[SlouchyCat]

Hi arnaud,

Sorry for my late reply.

Could you please elaborate more on the effect of introducing more tags using TDMA? I am planning to test 10 CFs using TDMA because TDOA seems less accurate than TWR.

Is it possible to reduce the time slot duration to 3ms so that it can range faster? or do you have any solutions to increase the rate?

Thanks!

Sam