LPS - Sending radio signals

A lot of things have to be considered if you want to send and receive a signal. I want to list those and my conclusion about them.

1. Radio-traffic

I intend to send a signal to a reference-location and the reference-location should return the signal. Doing so brings up two problems:

1. The frequencies which can be used for general-purpose transmissions are limited. For example within the EU 433Mhz or 868Mhz may be used. So every garage-door-opener or weather station uses one of this frequencies which means you will receive a lot of noise made by other people's gadgets.
   To filter signals sent by the mowing robot to a reference-location I have to encode the data in a certain way which can only be decoded by the reference-location. To do so dedicated ICs can be used. I prefer HT12E as encoder and HT12D as decoder. But there are also other products available like PT2272 (Encoder) and PT2262 (Decoder). Those ICs encode a couple of bits into a stream signed by an "address". The decoder-ICs are listening for a certain "address" and spit out the bits set to the encoder once the address matches. In short: You don't have to be concerned about other people's signal-noise by using those ICs.
    
2. Another issue is about bidirectional communication. Every communicator needs a sending- and a receiving-modul. So if the the mowing robot sends out a the signal its own receiving-modul will pick up the signal, too. In the same way the reference-location's receiving-modul will pick up the response.
   Sending any signals occupies the receiver. This means that the mowing robot cannot receive the response during transmission. At a first look this does not seem to be a problem, but for longer distances the duration of the sending has to be increased to guarantee a successful transmission. In this situation it might be that the success is already there in the very first milliseconds of sending. If the reference-location starts responding immediately the mowing robot's receiver will no see the signal because its own sender is transmitting a signal much stronger.
   This situation can be avoided in two ways: The easier one is to use another frequency for sending the response. Unfortunally I could not find an easy-to-use sending-modul available in both frequencies. So I have to use the second way which is by responding to the robot's signal after a certain amount of time, when the reference-location can be sure that the robot's transmission is complete. The downside of this way is that a lot of time is wasted. At the moment I cannot tell for sure but I guess using two frequencies is about 2-10 times faster. At a distance of 25 meters this means the robot will be able to contact only 3-4 reference-locations instead of 6-8 per second.

2. Sender-/Receiver modules

I was looking for cheap modules and found this:

• Sender = FS1000A
• Receiver = XY-MK-5V

They are pretty cheap and use very little current in standby mode (available at DX or Semaf). The amount of energy needed to run a reference-location is a critical issue. I will write about this later. For the moment: less is better.

Above, I mentioned that I could not find those modules at 868Mhz. There were other modules which do 868Mhz but they has to be connected by I2C or SPI. A pair of the modules introduced here can conceptionally be used in the same way as a cable. This is pretty easy and can be used in any electronic circuit without the need for a microcontroller.

The most websites reporting about the FS1000A-module tell that the range of transmission is from 2 to 200m. The actual value depends on several attributes:

1. The module can be run at different voltages (3V to 12V). A higher voltage means bigger range.
2. For the receiver-module a 5V supply is required so it would be easiest to apply 5V to the sender-module, too. Due to the noise at the frequency of 433Mhz I cannot tell how far the signal reaches, but something above 50m.
3. The modules itself may send only a couple of centimeters. They have to be extended by antennas (both sender and receiver) to gain more than 1m. I soldered a 1/4-antenna (simple cable in a 17.3cm length) at the proper pins.
4. At the outer ranges the signal is weak. So even if the range is 100m I won't be able to read any data or identify those data as mine at 100m.
5. I was able to build a stable communication at a distance of 30m using a 5V-supply. For that I have chosen to stretch the signal as much as possible (lowest oscillation frequency - see datasheet of HT12E and HT12D). Additionally I set the enabled-pin of HT12E to active for 90ms. In this time the stream generated is repeated again and again. Usually one or two of those sequences could be read by the receiver at a distance of 30m. If the noise was low it worked up to 50m.
6. Noise: I've mentioned it already above. The noise influences the quality of transmission and therefore the distance of possible communication. This has to be kept in mind for processing the ranges for position triangulation: The circuit will measure different values for the same distance in dependence to the current environment conditions.

3. Transmission attributes

I already mentioned that it is necessary to increase the time of sending for longer distances. The greater the distance between receiver and sender the more other radio-traffic of the same signal strength (of other people's gadgets) will disturb the transmission.

In addition to this effect the speed of transmitting has to be reduced for longer distances. This does not mean to modify the frequency. The HT12E-IC encodes the information I want to transmit into a stream of 0 and 1 digits. If I have longer distances the whole transmission has to be stretched not repeated more often. So for short distances a 1-digit might be successfully read if it is 0.1ms but for longer distances the noise scrambles the 1-digit so it has to be 1ms to be detected as a 1.

In the first try I will use constant duration and constant speed of transmission. Each value is selected so that it works at the longest distance I want to reach. To optimize the number of reference-stations to track per second I will try to decrease the duration and increase the speed of transmission in dependence of the distance. I intend to do this by transmitting the distance to the reference-station because the strategy I want to use for measuring the distance does not require transmitting a specific value - it only requires transmitting anything. So why not sending a value which can be used for optimization?

What's next? Let's take a breadboard and build a reference-location. Oh, wait. All my breadboards are in use. It seems I have to get them free. Stay tuned to what this means ;-)