Legacy LongRange Mode information (mode 1 in the my attached link)

Thread Starter

JimmyCho

Joined Aug 1, 2020
8
link for more information about this mode category : https://www.ti.com/lit/an/swra642/swra642.pdf
I want to understand the packet format, what's my syncword Data in HEXA that this legacy mode use? , what's the payload length in this mode? I didn't understand what's N and DSSS symbols that are found in multiplication sum for length of payload: N8DSS symbols...what should its values? could you guys please explain and illustrate all those parameters of mode 1 by an example of transmitted packet with explaining its transmitted data and how it looks once transmitted? thanks alot!
7853.1.jpg
 

Papabravo

Joined Feb 24, 2006
13,902
DSSS: Direct Sequence Spread Spectrum, is a method of encoding data into a signal with a much wider bandwidth than the bandwidth required by the information content alone. See the following articles for a description of the method.

https://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum
https://www.electronics-notes.com/articles/radio/dsss/what-is-direct-sequence-spread-spectrum.php

The word "symbol" is equivalent to "bit" and represents the actual encoding of transitions and values on the signal wire.

The preamble consists of 40 symbols with no "chips" (DSSS symbol additions) of the given pattern
The sync word is given in the document fragment from your post#1 as 0xFE6B 2840 0194 D7BF followed by the note that it appears on the wire LSB first. No problem just write down the bits of the given 64-bit word in reverse order. I can do that for you, but I think you should give it a try first

In the payload I take N to be the number of bytes. The 8 converts the number of bytes into bits. This number of data bits is multiplied by 2 times the number of DSSS symbols which are introduced into the data stream.

The Termination is 6 databits multiplied by 2 times the number of DSSS symbols.

I'm no expert on spread spectrum technology, but those are my best guesses from 50 years in the business of communications.
 
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Thread Starter

JimmyCho

Joined Aug 1, 2020
8
I appreciate your help, thanks alot!
Regarding to syncword it's in hexa FE6B28400194D7BF this means in binary: (first started to decode from F then E then 6 etc .... in other words from left to write my syncword starts in binary): 1111111001101011001010000100000000000001100101001101011110111111
Now after your explanation about syncword, I didn't understand really anything :) sorry, I've read what you explained after your explanation about syncword but didn't understand, could you please to be more clear to take an exclusive example for a packet with arbitrary Data(you could determine the data packet by your example ..doesn't matter) just to explain the length of payload and termination and how do you determine the N value(is N is constant and equal to number of bytes as always?) ? thanks alot for any help
 

Thread Starter

JimmyCho

Joined Aug 1, 2020
8
And this question is more for me, still mode 1 isn't understand able for me, and just curious about mode 2 in the link that I attached in my thread above, it says that in mode 2 it transmit syncword M times and in the M+1 it transmits the inverted syncword, what does this mean the "inverted" syncword? lets assume that syncword is : 0xFE6B 2840 0194 D7BF so what's the inverted syncword for this? thanks alot @Papabravo
 

Papabravo

Joined Feb 24, 2006
13,902
You missed the part about the sync word going on the wire Least Significant Bit (LSB) first. Start with the F on the right hand end of the sync word and write the bits in reverse order. Reversing the bits like this will yield a sync word that doesn't look anything like the one which is written MSB (Most Significant Bit) first. If you're going to do anything in communications you gotta get used to doing things like this by paying close attention to details. Just take the bit string you wrote in post #4 and write it again reading from right to left but writing the bits from left to right. Now what you have written is what the bits will look like "on the wire". Actually this is RF so there is no wire, so maybe I should say on the scope looking at the receiver.

For the Payload and the Termination I'm guessing that the N*8*2*2 is going to be the total number of symbols for the payload, and 6*2*2 is the total number of symbols for the termination. N is the number of bytes and DSSS = 2, so that each bit is replaced by 4 symbols. So the symbol rate is 4 times the bitrate. That is just a guess.
 

Thread Starter

JimmyCho

Joined Aug 1, 2020
8
You missed the part about the sync word going on the wire Least Significant Bit (LSB) first. Start with the F on the right hand end of the sync word and write the bits in reverse order. Reversing the bits like this will yield a sync word that doesn't look anything like the one which is written MSB (Most Significant Bit) first. If you're going to do anything in communications you gotta get used to doing things like this by paying close attention to details. Just take the bit string you wrote in post #4 and write it again reading from right to left but writing the bits from left to right. Now what you have written is what the bits will look like "on the wire". Actually this is RF so there is no wire, so maybe I should say on the scope looking at the receiver.

For the Payload and the Termination I'm guessing that the N*8*2*2 is going to be the total number of symbols for the payload, and 6*2*2 is the total number of symbols for the termination. N is the number of bytes and DSSS = 2, so that each bit is replaced by 4 symbols. So the symbol rate is 4 times the bitrate. That is just a guess.
Appreciated!
the termination is the same as CRC no?
if given a syncword : 0xFE6B 2840 0194 D7BF , so what's the inversion of this syncword? does it mean that inverted syncword is:
0xD7BF 0194 2840 FE6B (reverse words's order )

secondly, you mean by "that each bit is replaced by 4 symbols" you mean the value of the bit repeats 4 times?, lets assume if I sent just one bit equals 1 in binary (its value is 1) so it would appear 4 times on the receiver(once transmitted) ? .. 1---->1111

thanks alot
 
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