Why Pulse Position Modulation is used for communication between reader and tag?

Hi Folks,

According to ISO 15693, 1 out of 256 or 1 out of 4 Pulse position coding is used for communication between reader and tag.
Firstly, I didn't understand the concept of 1 out of 256 and 1 out of 4.
Secondly, in the case of PPM, synchronization is required between Tx and Rx though the circuitary is simple.
So, I don't understand the need of PPM in communication from reader to tag.

Hope u folks help me out in understanding this.


-Thanks!

Are you asking why 1-out-of-256/1-out-of-4 PPM has been choosen for ISO/IEC 15693 or how it's working?

In the former case, I wonder if you're involved in a philosphical respectively history of technlogy project? In a technical view, you can analyze, if the protocol has optimal coding properties. Most likely it hasn't. It's rather a kind of legacy, a historical relict. Nevertheless it serves it's purpose and will be probably continued for many years.

Asking for "the need of" is the wrong question. It's just there. You have to manage it.

Hi FvM,

My question is how it works. I have gone through the paper but I didn't understand. I have googled it but I didn't come across any other paper except ISO paper.

Again, my doubt is its difficult to maintain the synchronization between the tag and reader. Since PPM has the big limitation of maintaining synchronization, I got the doubt. May be I am wrong, pls correct me.

Thanks for pointing it as wrong question as I will be little cautious and double-check my words before submitting my post.

But, Please clarify my first question. Is there any document or something which clearly explains 1-out-of-256/1-out-of-4 PPM.

Honestly, I am unable to put the same thing by drawing waveforms using pen and paper. Hope u may help me out.


Thanks[COLOR="Silver"]

Yes, ISO_IEC 15693-2 for the bit level and ISO_IEC 15693-3 for the higher protocol levels.

Basically, there's no synchronization problem for the tag, because everything is synchronized with the 13.56 MHz carrier which is also assumed to be the clock of the tag logic. Pulse positions have to be counted from the first SOF bit of course. In my opinion, possible problems of the protocol are mainly with noise immunity, but the basic algorithm is clear and simple.

Thanks FvM. I have downloaded the protocol documents. Have gone through ISO 15693-2 and got little understanding.

Hi FvM,

Can you please explain why the data rate for 1 out of 256 coding is fc/8192 and 1 out of 4 coding is fc/512? I couldn't realize how 8192 and 512 are linked to data rate.

It's no text book, but an exact and complete specification of the protocol. All your questions are answered within, you may possibly need to read it more than once.
The pulse timebase in both modulation variants is 256/fc or 18.8 us. So an 1-out-of-4 frame has a length 75.5 us and a 1-out-of-256 4.833 ms. If you consider, that the former codes 2 bits and the latter 8 bits, you get the said bit rates.