Spread Spectrum ?

Discussion in 'General Electronics Chat' started by Mathematics!, Jan 15, 2010.

  1. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    1,022
    4
    I have a few questions on spread spectrum modulation schemes.

    First

    From this I gathered that all the spectral spread algorithms are
    or combo's

    But my main question I still cann't figure out is how these methods keep reciever and transmitter in sync...?

    For instance
    First problem is the control channel this must be a fix channel to initialize synchronization with reciever /transmitter pairs.
    But if multiple user are using this system won't they all be trying to intialize potientially at the same time on the control channel? And also the exchanges and hopping frequencies must not clash with other users of the system how is this maintained?

    Ok assuming the above problem is worked out and they exchanged keys (or hopping frequencies info ) then step 5 when they start communicating what happens if interference screws up a transmission of data.
    Does the reciever/transmitter have to resync or do they just keep changing their frequencies with respects to time and just resend the data on a different frequency....? What I am getting at is if a packet gets corrupt then do they resend the packet on the same frequency or just keep switching frequencies in sync regardless of if the packet made it or not... and if it didn't make it then the reciever sends the transmitter a new packet on the different frequency rather then wait for the packet to be send on the same frequency as it was interfered with in the first place?

    Hopefully you are not confused by how I worded the above
    I mean does the transmitter/reciever pair keep switching frequencies with respects to time regardless of if the data made it their or does the frequency switching with respects to time only hold when the data reaches without any errors because if it is the latter then each time this happens they would have to resync/resend control frequency hopping info to begain again etc....? ( I think resending data would be at different levels in the OSI layers ,...TCP/IP ,...etc and not at this level)
    I would think this frequency hopping is not concerned about weather the data got their or not ,...that would be the concern of high level OSI layers


    Also when they hand off frequency hopping info is their any way that some third party reciever could listen in on this? Assuming the outside reciever never had acess to the control/frequency hopping info....
    Would their be anyway for them to piece the frequency hopping pattern together and beable to listen in or get are data?

    Thanks
     
    Last edited: Jan 15, 2010
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    The way they keep them in sync is the same way you encrypt a signal, the enemy doesn't have the table and keys, but the allies do. Frequency hopping and other schemes basically involve moving the base frequency around. It isn't as much a modulation as a moving target. If the enemy doesn't know where the carrier is at any one time they can't jam or jink it.
     
  3. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    1,022
    4
    Well cann't you jam this transmission by jamming every radio frequency or the whole band with of the spread spectrum transmission.

    I don't know how to make jammers but they all work on the principle of destructive interference to cancel out the wave with 180degrees out of phase...etc Or at least if they provide enough noise so the transmission is not possible something like a super duper spark-gap emitter device or something. I would think jamming is not impossible it would be pretty easy but ofcourse you would kill every transmission in the Spread-spectrum bandwidth or more....

    Also I understand that it is not a modulation schematic but away to make transmission more secure by varying the carrier frequency in sync with trusted receiviers/transmitters but if the enemy ever listened in on the control channel and got the control keys then would it be possible to generate the table for these keys and get in sync with the devices...?
     
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