Maximum communication distance is a common term, appearing in most discussions about WSN's (Wireless Sensor Networks) and wireless products. If you have ever worked with panStamps or with other wireless hardware then you probably know that distances mainly depend on four factors:
1. Rx sensibility : high sensibility translates into the ability to catch weaker signals by our device
2. Tx power : high-power signals can travel over even longer distances
3. Carrier frequency : lower frequencies achieve longer distances
4. Data rate : signals with lower bitrates are better "understood" by distant receivers so let's say that distances can be longer.
As a reference, we usually say that panStamps can communicate over 200 meters at 38400 bps (bitrate) and 0 dB (tx power), direct line of sight of course. On the other hand, even if we have not done any extensive test with other values, we can say that more than 500 meter could be achieved for 4800 bps and +12 dB.
But are <1Km communication distances enough for WSN's? It depends. Some hundreds of meters may be sufficient for building automation, personal area networks, green houses and maybe small-to-medium fields but Smart Cities seem to be demanding for longer distances. On the other hand, wireless meshes are no longer an option in some situations, due to the additional complexity. What to do then?
During Smart City Expo it was quite clear, at least for us, that long-range RF coverages were preferred over mesh networks. High-power stations with high-gain antennae can be installed at high positions, providing even longer ranges and wider coverages.
At panStamp we are currently working on a long-range design, a carrier board that will turn our panStamps into high-power and high featured nodes. We are still not sure about the name, maybe "ultranode" but we are open to other suggestions. In the meantime, here you have some pictures of our first prototypes, called "ultraShields":
Figure 1 : ultraShield prototype
ultraShield includes a CC1190 front-end and footprints for SRAM and EEPROM memory IC's. On the RF side, the CC1190 adds higher Rx sensibility thanks to its LNA (Low-noise amplifier) and more Tx power. The good thing about the LNA is that devices featuring the CC1190 should be able to receive signals from standard (low-power) panStamps over longer distances. We are currently expecting to get over 2 kilometers between high-power and low-power modules but this has still to be confirmed and we won't do so until we build the new prototype.
Figure 2 : high-power modem
ultraShield was initially designed to act as a memory and range extender for panStamp but we won't be able to get the maximum capabilities of the CC1190 IC with such a small board.
Instead the new ultraNode will be bigger and will host an on-board linear regulator capable to supply at least 500 mA for the Power Amplifier and probably a RS232 connection to be used with external GPRS/3G modems and computers (via a USB/RS232 cable). As you may guess, ultraNode will specially fit the role of a central node with access to a continuous source of power. Antennae are another issue. We still have to search for outdoor antennae with decent gains, something around 9 dBi should be fine, probably a half-wavelength antenna.
Did we say that we are about to start a crowdfunding campaign around the NRG module? We want to start the campaign in 1-2 weeks and offering this long-range board as an optional pledge would be great. Please visit our announcement page in the next days or subscribe to our twitter account for more news.