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frequency hopping technology to avoid interference?????

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  • sunil karki
    hey there! Can anyone help me out in understanding how a frequency hopping spread spectrum technique works? I mean how do they work it out in a radio system
    Message 1 of 6 , Jun 30, 2009
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      hey there!
      Can anyone help me out in understanding how a frequency hopping spread spectrum technique works? I mean how do they work it out in a radio system without ever jamming even if they are in a same channel? How do they sync with the hopping and get it right ? and can we do it using a microcontroller?
       sunil karki


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    • docel_bangalorerobotics
      Google is the best teacher......I got 2,970,000 pages when I just asked. Cell phones, Bluetooth and modern communication networks work on Spread Spectrum.
      Message 2 of 6 , Jun 30, 2009
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        Google is the best teacher......I got 2,970,000 pages when I just asked.
        Cell phones, Bluetooth and modern communication networks work on Spread Spectrum. DSSS, CSS, FHSS, TDMA, CDMA......

        Just imagine water from a hose pipe: If the hose is stationary , you collect water in a stationary bucket. If you sweep the hosepipe around , to & fro a 20ft. span, how do you collect the water?? The Bucket will now have to sweep along with and in time with the hosepipe (synchronised), to collect all the water. Now you have water "spread" across a 20ft. "Spectrum"!
        Substitute transmitter for the hosepipe, audio for the water and the receiver for the bucket.......you have Spread spectrum.....




        --- In SeattleRobotics@yahoogroups.com, sunil karki <sunil_tule@...> wrote:
        >
        > hey there!
        > Can anyone help me out in understanding how a frequency hopping spread spectrum technique works? I mean how do they work it out in a radio system without ever jamming even if they are in a same channel? How do they sync with the hopping and get it right ? and can we do it using a microcontroller?
        > �sunil karki
        >
        >
        > Get your new Email address!
        > Grab the Email name you've always wanted before someone else does!
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      • docel_bangalorerobotics
        Radio SS is just the same: The transmitter is tuned (hopping) across a known spectrum of frequencies as you talk into the microphone. The receiver is also
        Message 3 of 6 , Jun 30, 2009
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          Radio SS is just the same:
          The transmitter is tuned (hopping) across a known spectrum of frequencies as you talk into the microphone.
          The receiver is also tuned across the same known spectrum. There can be any number of channels and they wont interfere with each other. This is made better with digital techniques , encoding/decoding, encryption etc.,
          Result is that you cant listen to any one frequency and get any intelligene out of it. All you get is a hiss, since the audio bit at that particular frequency/time is a very small bit of the original audio. If you take AM SS as an example, then the amplitude of the modulating audio will be equal to or slightly more than the noise floor.

          Spread Spectrum can be as simple as a microcontroller varying the Carrier frequency of the TX & varying the Local oscillator of the RX VFO, at the same rate as the Transmitter.

          It is not this simple in real life though......
        • sunil karki
          hey docel!                  thank u for sharing , i was trying to figure out how the 2.4ghz tx/rx used in radio control hobby aircrafts are
          Message 4 of 6 , Jun 30, 2009
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            hey docel!
                             thank u for sharing , i was trying to figure out how the 2.4ghz tx/rx used in radio control hobby aircrafts are doing it as in this spectrum there are bluetooth,uwb,vsat and many other equipments. Dont they get any sort of interference and what is the new government policy as i dont think it is a free channel for everyone else.

            --- On Tue, 30/6/09, docel_bangalorerobotics <bangalorerobotics@...> wrote:

            From: docel_bangalorerobotics <bangalorerobotics@...>
            Subject: [SeattleRobotics] Re: frequency hopping technology to avoid interference?????
            To: SeattleRobotics@yahoogroups.com
            Date: Tuesday, 30 June, 2009, 5:59 PM

            Radio SS is just the same:
            The transmitter is tuned (hopping) across a known spectrum of frequencies as you talk into the microphone.
            The receiver is also tuned across the same known spectrum. There can be any number of channels and they wont interfere with each other. This is made better with digital techniques , encoding/decoding, encryption etc.,
            Result is that you cant listen to any one frequency and get any intelligene out of it. All you get is a hiss, since the audio bit at that particular frequency/time is a very small bit of the original audio. If you take AM SS as an example, then the amplitude of the modulating audio will be equal to or slightly more than the noise floor.

            Spread Spectrum can be as simple as a microcontroller varying the Carrier frequency of the TX & varying the Local oscillator of the RX VFO, at the same rate as the Transmitter.

            It is not this simple in real life though......



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          • Long, Dan T
            In real life, how does the Rx and Tx synchronize themselves ? And how do they know what frequency to jump to next to continue receiving the right set of
            Message 5 of 6 , Jun 30, 2009
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              In real life,   how does the  Rx and Tx  synchronize themselves ?

               

              And how do they know what frequency to jump to next to continue receiving the right set of data ?

               

               

              --- On Tue, 30/6/09, docel_bangalorerobo tics <bangalorerobotics@ gmail.com> wrote:


              From: docel_bangalorerobo tics <bangalorerobotics@ gmail.com>
              Subject: [SeattleRobotics] Re: frequency hopping technology to avoid interference? ????
              To: SeattleRobotics@ yahoogroups. com
              Date: Tuesday, 30 June, 2009, 5:59 PM

              Radio SS is just the same:
              The transmitter is tuned (hopping) across a known spectrum of frequencies as you talk into the microphone.
              The receiver is also tuned across the same known spectrum. There can be any number of channels and they wont interfere with each other. This is made better with digital techniques , encoding/decoding, encryption etc.,
              Result is that you cant listen to any one frequency and get any intelligene out of it. All you get is a hiss, since the audio bit at that particular frequency/time is a very small bit of the original audio. If you take AM SS as an example, then the amplitude of the modulating audio will be equal to or slightly more than the noise floor.

              Spread Spectrum can be as simple as a microcontroller varying the Carrier frequency of the TX & varying the Local oscillator of the RX VFO, at the same rate as the Transmitter.

              It is not this simple in real life though......

            • Edward Mitchell
              In a frequency hopping system, the transmitter and receiver simultaneously hop rapidly to new radio frequencies, only briefly landing on a frequency for a
              Message 6 of 6 , Jun 30, 2009
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                In a frequency hopping system, the transmitter and receiver simultaneously hop rapidly to new radio frequencies, only briefly landing on a frequency for a fraction of a second before hopping on to another. The sequence of selected radio channels in the hop is coordinated between the transmitter and the receiver, but to the outside world, might appear random. The transmitter and receiver coordinate on a hop sequence and timing.

                Occasionally, a frequency hopper will land on afrequency that is already in use, in which case the transmission of the data packet fails.  So yes, they do sometimes "jam". But the communications protocol will manage that so that the packet is transmitted again on a different hop frequency a moment later. Wired networks, like Ethernet, also have "jams" when more than one network controller attempts to send data at the same time. Again, the protocol is set up to detect and deal with this. Obviously, if you jam a lot, your data thruput goes down due to retransmissions.

                The original, very early 802.11 system used a frequency hopping approach, transmitting data at 1 or 2 mbps on 1 Mhz wide channels between 2400 to 2483.5 Mhz. (I'm writing that from memory so I could be off on some details). Later, 801.11 added direct sequence spread spectrum to spread the signal over a wide frequency range - in away, it accomplishes something similar to a frequency hopper, which spreads the signal over a wide range, in time.

                Could a microcontroller handle this? Certainly a microcontroller can control a radio. However, it depends what type of radio you are thinking of - most radios are not designed and built to rapidly change their frequency. When they hop to a new frequency, the transmitter takes a moment to stabilize on frequency, send its burst, then hop again. A lot of radios would not be able to hop fast enough.

                Ed
                Spokane

                On Tue, Jun 30, 2009 at 4:48 AM, sunil karki <sunil_tule@...> wrote:


                hey there!
                Can anyone help me out in understanding how a frequency hopping spread spectrum technique works? I mean how do they work it out in a radio system without ever jamming even if they are in a same channel? How do they sync with the hopping and get it right ? and can we do it using a microcontroller?
                 sunil karki


                Get your preferred Email name!
                Now you can @ymail.com and @rocketmail.com.



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                Edward Mitchell
                edward.mitchell@...
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