time synchronization in wireless sensor network

[Munib]

How can i time synchronize my all nodes in a wireless sensor network.By time synchronization i mean that if i want to use TDMA scheme then all nodes must have some timing reference.Also i want to use the sleep mode to save power such that every node will be in sleep mode untill its specified time slot in TDMA scheme arrives then that node will wake up send its data and will again go to sleep mode.
Now to avoid collisions (i.e no two nodes sending data at same time)i must have to time synchronize my all nodes

Any suggestions how can i do this.

NOTE:The hardware i m using is PIC18f452 as processing unit , TXM-418-LR(RF transmitter) , RXM-418-LR(RF receiver).By combining the transmiitter and receiver through a SPDT RF switch i have got a half duplex RF tranceiver.

 

[aNdreiBuuu]

In my opinion, you can solve this problem with no time synchronization... You have some posibilityes:

1. set 1 device as master and then interogate one by one all the slave devices (polling method).
2. if you want that all the devices act as master, use a random number to start the transmitting procedure. A bad crc means data error...

manny of this applications are using multichannel communications in some free bands... you can see some AN on nordic site... www.nvlsi.com

If you want to optimise your bandwidth, you can use some kind of timeslot multiplexing with clock regeneration, but is much difficult...

hope this helps,
//a

 

[Munib]

i cant use master slave configuration without synchronization because when a nodes time slot is over its radio will go to sleep mode then how can master awake it when its next time slot occurs.
actually i want that my nodes themselve should know when their time slot occurs and after that time slot is over node will go to sleep and by itself wake up again when its next time slot occurs.this cant work without synchronization

I also cant use multichannels because my transmitter and receiver works on a single frequency i.e 418MHz

 

[mosh]

you using a transmitter in each sensor [isn't it], so you simply
[not realy simple!] can solve it with IP system [like internet IP but not exactly same], you sould choose a unique number for each of your sensors & send this number befor your data packet on a string[like data packets on internet].
also your MCU's on sensors shoud can wakeup after sleep period by thire own,[don't worry about sleep period ,it's no matter at all,choose what you want!]

and after all these! we wants to communicate!!!!!!!!!
===>> first I want to explane it for just one sensor,so
[your main receiver[I will call it MR,ok] will always be ON & your sensors will wake up by thire timeout period],so go on
imagean:
timeout is finish and your sensor going on and send data packet[and will stay ON up to(stay here and go to MR part)
MR will take the packet and read that unique number and get the data on the packet (PAUSE! there is two posibelites
1.you reseive your data correctly, so your MR will send a packet(with that
unique number and a small command to turn of that sensor) to[actually all of
your sensors], but most of them are off and couldn't get that packet[advantage!]
and mabe some of your sensors are on and recieve that packet BUT the only sensor that going of is the sensor who send the data and have thet unique number.
2.you reseive your data wrong with that unique number you can call your sensor
to send the data again by a command[as I explained it above for turnning off command]. you get your data correctly so do (1.)

It will work impressively [if you can make it right!!!!(don't worry it seems more complicated than it is!)]
I tryed it befor for an one by one IR system [that was more complicated],and that
worked realy impressive.

we done one by one communication so what about one to many?

it will work fine for one to many too [if not how the internet work???!!!!].
you just need to put a repeat loop on your sensor part, for the times your MR
couldn't get the packet from your sensors [for any reasons]
again imagean:
your sensor sent it's data and wait for a while to receive OFF command from MR
but there is no answer [it means your MR couldn't get your sensor data, because
if your MR get the data it should answer] ,so it sould send data again and waite again[up to a successful Communication] also it will happening if your sensor
couldn't get the of command.

getting ABIT!!!!!!! long .... isn't it !!!
I'm realy slow in typing,I could made this circuit on the time I spend for
typing this!!!!!!!!!! (just as fun...., don't mention it ! )

hope helped
mosh