From: Edward Mitchell <edward.mitchell@...>
Subject: Re: [SeattleRobotics] frequency hopping technology to avoid interference?????
To: SeattleRobotics@yahoogroups.com
Date: Tuesday, 30 June, 2009, 10:08 PM

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@yahoo. com> 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

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