Difference between revisions of "FHSS"

From OpenCircuits
Jump to navigation Jump to search
(quote Part 15)
(fill in some details)
Line 6: Line 6:
  
 
''... mention systems that combine FHSS with mesh networking ...''
 
''... mention systems that combine FHSS with mesh networking ...''
 +
 +
* slow frequency hopping: after hopping to the new frequency, the transmitter sends several symbols before hopping to the next frequency. This gives the fastest data rate when running on hardware that takes a long time to switch to a different frequency.
 +
* fast frequency hopping: The transmitter hops to several different frequencies, sending the same symbol on each one, before transmitting the next data symbol. This is more tolerant of single-frequency interference.
 +
  
  
 
* Other places we [http://opencircuits.com/Special:Whatlinkshere/FHSS talk about FHSS on Open Circuits].
 
* Other places we [http://opencircuits.com/Special:Whatlinkshere/FHSS talk about FHSS on Open Circuits].
 +
 +
== acquisition and tracking ==
 +
 +
When a receiver is turned on, what frequency should it listen to first, and how long should it listen before trying some other channel?
 +
 +
There seem to be 4 possibilities:
 +
* Both systems start at the same "acquisition channel", and stay on that channel until they hear from each other, then start hopping. This gives the quickest time-to-lock, if there is no interference. But if narrowband interference happens to be on that channel, then they never lock.
 +
* The receiver picks a random acquisition channel (a random point on its hop sequence) -- perhaps the quietest channel with the least noise on it -- and stays on that channel until it hears the transmitter. The transmitter always goes through the hop sequence in the standard order, sending a packet on each channel. Eventually the transmitter gets to the channel the receiver is listening on, and the receiver hears that packet, synchronizes its time, and starts hopping.
 +
* The transmitter picks a random acquisition channel (a random point on its hop sequence) -- perhaps the quietest channel with the least noise on it -- and transmits its message over and over on that channel. The receiver goes through the hop sequence in the standard order. Eventually the receiver gets to the channel the transmitter is sending on, and the receiver hears the packet and synchronizes its time. After the transmitter has given the receiver plenty of time to sample every possible channel, both should now be synchronized, and start hopping.
 +
* ...
 +
 +
 +
 +
 +
  
 
== legal considerations ==
 
== legal considerations ==
Line 24: Line 43:
 
permitted.
 
permitted.
 
--[http://edocket.access.gpo.gov/cfr_2005/octqtr/47cfr15.247.htm 47CFR15.247 : FCC Part 15 Section 15.247]
 
--[http://edocket.access.gpo.gov/cfr_2005/octqtr/47cfr15.247.htm 47CFR15.247 : FCC Part 15 Section 15.247]
 +
</blockquote>
 +
 +
Paraphrased in SWRA041 as
 +
<blockquote>
 +
It is not permitted to synchronize transmitters of different frequency hopping systems to each other. This
 +
could lead to a super system that occupies individual frequencies for more than the allowed 0.4 seconds
 +
in the 10s or 20s period. Notwithstanding this, any system may recognize others in the frequency band
 +
and adapt its hop set independently to avoid hopping into an occupied channel.
 
</blockquote>
 
</blockquote>
  
Line 32: Line 59:
 
* [http://interwiki.sourceforge.net/cgi-bin/picawiki.pl/NgARN NgARN : Next Generation Amateur Radio Network]
 
* [http://interwiki.sourceforge.net/cgi-bin/picawiki.pl/NgARN NgARN : Next Generation Amateur Radio Network]
 
* The [http://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrum Wikipedia:FHSS] article
 
* The [http://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrum Wikipedia:FHSS] article
 +
 +
* [http://focus.ti.com/general/docs/techdocsabstract.tsp?abstractName=swra041 TI Application Report SWRA041: "Implementing a Bidirectional Frequency Hopping Application With TRF6903 and MSP430"] by Shreharsha Rao 2004
 +
* [http://focus.ti.com/general/docs/techdocsabstract.tsp?abstractName=swra048 TI Application Report SWRA048: "ISM-Band and Short Range Device Regulatory Compliance Overview"] edited by Matthew Loy, Raju Karingattil, Louis Williams

Revision as of 20:18, 3 August 2008

What should I say here about frequency hopping spread spectrum (FHSS)?

... say a few words about adaptive frequency-hopping spread spectrum (AFH)...

Is it true that adaptive FHSS can give higher bandwidth and use less power (longer battery life) than any fixed-frequency broadcast system?

... mention systems that combine FHSS with mesh networking ...

  • slow frequency hopping: after hopping to the new frequency, the transmitter sends several symbols before hopping to the next frequency. This gives the fastest data rate when running on hardware that takes a long time to switch to a different frequency.
  • fast frequency hopping: The transmitter hops to several different frequencies, sending the same symbol on each one, before transmitting the next data symbol. This is more tolerant of single-frequency interference.


acquisition and tracking

When a receiver is turned on, what frequency should it listen to first, and how long should it listen before trying some other channel?

There seem to be 4 possibilities:

  • Both systems start at the same "acquisition channel", and stay on that channel until they hear from each other, then start hopping. This gives the quickest time-to-lock, if there is no interference. But if narrowband interference happens to be on that channel, then they never lock.
  • The receiver picks a random acquisition channel (a random point on its hop sequence) -- perhaps the quietest channel with the least noise on it -- and stays on that channel until it hears the transmitter. The transmitter always goes through the hop sequence in the standard order, sending a packet on each channel. Eventually the transmitter gets to the channel the receiver is listening on, and the receiver hears that packet, synchronizes its time, and starts hopping.
  • The transmitter picks a random acquisition channel (a random point on its hop sequence) -- perhaps the quietest channel with the least noise on it -- and transmits its message over and over on that channel. The receiver goes through the hop sequence in the standard order. Eventually the receiver gets to the channel the transmitter is sending on, and the receiver hears the packet and synchronizes its time. After the transmitter has given the receiver plenty of time to sample every possible channel, both should now be synchronized, and start hopping.
  • ...




legal considerations

In addition to the physical limitations mentioned above, there are also legal considerations, such as, for example:

(h) The incorporation of intelligence within a frequency hopping spread spectrum system that permits the system to recognize other users within the spectrum band so that it individually and independently chooses and adapts its hopsets to avoid hopping on occupied channels is permitted. The coordination of frequency hopping systems in any other manner for the express purpose of avoiding the simultaneous occupancy of individual hopping frequencies by multiple transmitters is not permitted. --47CFR15.247 : FCC Part 15 Section 15.247

Paraphrased in SWRA041 as

It is not permitted to synchronize transmitters of different frequency hopping systems to each other. This could lead to a super system that occupies individual frequencies for more than the allowed 0.4 seconds in the 10s or 20s period. Notwithstanding this, any system may recognize others in the frequency band and adapt its hop set independently to avoid hopping into an occupied channel.

... What other legal considerations and FCC policies are relevant to designing intelligent radio communication systems? ...

further reading