At 08:15 AM 10/18/02 -0700, Bill Swingle wrote: >Since were talking of channel conflicts and shootdowns; What's the status of >spread spectrum technology being used for the R/C world? > >Bill Swingle
Hi All, Spread spectrum can provide a technological solution for our needs in R/C and it can be quite affordable. The big problem lies within the FCC. Without a tremendous presence and a sizeable, well funded market demand, the FCC considers our needs to be "background noise". Spread spectrum has two different subsets, DSSS and FHSS. DSSS or Digital Sequenced Spread spectrum modifies the data signal at the transmitter and combines it with a higher data rate bit sequence, or chipping code, that divides the user data according to a spreading ratio. The chipping code is a redundant bit pattern for each bit that is transmitted, which increases the signal's resistance to interference. If one or more bits in the pattern are damaged during transmission, the original data can be recovered due to the redundancy of the transmission. FHSS or Frequency Hopping Spread Spectrum uses a hopset or grouped set of channels in an identical table contained at the transmitter and at the receiver. Various synchronization techniques are used to match up the receiver hop sequence position with that of the transmitter. Error correction schemes vary based on the data payload size and acceptable error levels. FHSS can operate fairly well in an RF rich environment including DSSS equipment in the same band, while DSSS would experience data rate reductions due to data loss from contention by an FHSS system. The FCC dictates the minimum number of hop channels in a hopset as well as the channel dwell time (how long you can stay on channel before having to move) and don't forget the RF output power. Both strategies are actively used, but FHSS would be the most applicable for R/C use due to the rather low data rates that we require. FHSS generally exhibits greater range for the same output power and perhaps most importantly, the computational requirements at the receiver are much lower for FHSS than DSSS. With that said, my company develops wireless technology for commercial and industrial applications, which includes spread spectrum radios. While working on a commercial proof of concept project a while back, I saw an easy way to adapt technology that we had developed to my R/C interest. Using an Airtronics Stylus (what else???), the decision was made to digitized the PPM data stream (easily available when the RF module is removed) since the PCM data format is not available to the public. The digitized data was then rolled into a fairly simple packet and sent to the FHSS radio. The receiver apparatus used another FHSS radio, unpacked the data, translated the digital data back into channelized PPM data and sent the data to the servos. It was a rather simple implementation that could have easily flown in a plane. The FHSS radio in production (>100K pieces) is less than $25 and about the size of a Hitec Super Slim receiver. The advantage of spread spectrum, as some have correctly noted, is the high immunity to noise (i.e. other transmitters on similar or the same frequency). If sufficiently sized hop tables, random addressing assignments, and a few different RF channels were used, large groups of pilots could fly together without perceived conflicts. Now for the down side of spread spectrum. There is the development and tooling costs for the radio manufacturers, re-design of the receiver with spread spectrum features that more closely resembles a PCM unit, re-design of the transmitter RF module to incorporate spread spectrum features, new and greater costs for implementation, but perhaps the biggest obstacle of all would be the FCC. Asking for spread spectrum operation on 72Mhz from the FCC is akin to asking the IRS to revamp its tax laws. So back to what we've got. The Channel Check from Multiplex on 72Mhz was cancelled, as I understand, due to certification problems that would have required additional engineering costs. Apparently, MPX didn't want to spend the money or the time. The Channel Check would prevent "your" transmitter from sending out an RF signal if your channel had RF activity. On the other hand, if some one else turned on their transmitter on your channel and they did NOT have a Channel Check on their radio, then they would "shoot" you down without any problem. Now if there were as many R/C radios as there are cell phones, we would be in business....scale of economies. Tom Hoopes ([EMAIL PROTECTED]) RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off.
