Multi-channel digital radio solution - HELP!!!

[JonathanSA]

Hi All

First time posting here as usually can get enough info on any project by just browsing. Not a complete novice when it comes to electronics but would be first to admit when it comes to RF, always just used piggyback solutions (Wifi /nrf24l01) for projects. This all has to change as I need to develop Wireless Headsets for my final design project for UNI.  The actual project brief is listed below.

Design and implement a wireless headset that can be used by rescue
swimmers to communicate with personnel in the vessel or helicopter that
transports them to the rescue location. The device must be shock
resistant, waterproof and have the battery capacity to last a few days.

Before complete panic sets in would really appreciate some guidance on where to even begin.

Thanks
Jonathan

[Nusa]

Communications requirements.
Power.
Antenna.
Human interface that works in the water.
Protection from the environment.

Break it down into pieces, and start thinking about how they fit together. Identify problems and possible solutions. Find what works.

I'd start by surveying out what's on the market already for swimmers comms, and how they solved some of the problems.

[JonathanSA]

The advice is greatly appricited.

The XBEE range looks ideal such as this https://www.sparkfun.com/products/retired/9099. With a 156kbs max throughput and a low bandwidth codec, one hopes this actually might work.

[dmills]

I would actually probably design an AM or FM solution rather then going digital....

The advantage is that it degrades rather more gracefully, in that the noise floor just rises as the signal becomes weaker, without just suddenly stopping.

The elephant in the room here is that sea water is **MASSIVELY** lossy when it comes to RF (And it gets much worse at higher frequencies), so you will need real power and excellent aerials at the other end of the link, not much you can do about the bit on the swimmer (Head mounted aerials?), where you are TX power and size limited, but the end of the link on the bird or surface vessel could reasonably run several watts of power to allow for the massive path losses when a wave is between the swimmer and the ship.

Do watch operating temperature limits as well, particularly with regard to battery technology, some systems behave badly at low temperature (A trick some of the emergency radios use is to mount the doings under the arm so the body heat maintains things at a reasonable temperature). There might be something to be said for using the standard VHF marine band as most ships and rescue aircraft will already have the appropriate receivers and transmitters and the aerials will usually be mounted high up (Plus experiments could be done with off the shelf kit).

It might actually be worth considering acoustic transmission, with a dipping transducer dropped off the heli or hull mounted on the ship, sea noise drops rapidly with frequency, and sound travels well underwater, so a carrier at maybe 100KHz or so should work.

Transducers are going to be interesting, bone conduction/throat mics may be worth a look.

73 Dan.