Hi all,
I am looking for some tribal wisdom/group opinion on the best wireless option to use. I have an application where I need to meet the following requirements:
Range: 100M in a mixed (indoor/outdoor) environment including 1-2 walls
Data rate: 100-250kps should be fine; maybe even less
Topology: Star (one base station communicating to all other nodes); could be mesh, but really doesn’t need to be
Power: nodes need to be battery powered, but battery life doesn’t need to be great (3-4 hours before a charge would be fine)
Software: Ideally there would be a software layer to provide some robustness. At the bare minimum, I need to be able to have the base station address each of the nodes individually. I will have an ARM controller controlling the transceiver and housing any software stack.
Cost: total BOM <= ~$5
The options I came up with were:
The issue with the ZigBee option is range. I bought two Xbee modules with wire antennas just to test out the performance and I was really not impressed.
The issue with the TI 915Mhz is complexity, regulatory hurdles (have to get and pay for rigorous testing), and the PCB antenna would be rather huge.
Did I miss a real 3rd option?
I looked at Nordic, but it looks like the proactively eschew having any software MAC and I don’t really feel like writing a protocol from scratch. Also range seems to be a non-starter.
I could put a RF front end on the 2.4Ghz option, but I don’t think I have enough GPIO pins to make it work (and my BOM cost would jump considerably).
Any thoughts or feedback would be very welcome.
Cheers,
Dave
Xbee’s are pretty awesome. If you were having bad ranges, maybe you were working on a frequency/ channel that was very cluttered up in your area.
I did some good XBee range trials awhile back. I went to a street in a pretty straight line. Lots of overhead wires. Tons of WAPs on the street. Heavy traffic. Most houses had metal gates that ran up to the sidewalk. It was straight…but not line of site. Lots of bushes on the curb…cars pulling out of drive ways and across intermediary streets. We used the X-CTU loop back range test to do this. If I saw many dropped packets I’d come a little closer and then make sure the signal was solid. So these ranges are for a very acceptable packet drop rate.
Anyways…hope this helps you…
These tests were taken on a busy residential street in santa ana with many wireless routers throughout the street, cars passing by, shrubbery overhaning the sidewalks, and chain link or cast iron fences surrounding nearly every house. These are about the worst conditions that we could ask for and should give some good worst case scenarios to test with.
Pos 1 means having the board perpendicular to the ground and the xBee chip facing away from the receiver. In the case of the whip antennas, the antenna are bent so they are pointing upwards. In the case of dipole antennas this means the antenna is held straight up.
Pos 2 means (I forget, fill in the blanks here Dan)
Pos 3 means the board parallel to the ground and the xBee chip facing upwards. In the case of whip antennas the antenna is pointing straight up.
Pos 5 means the xBee is held very tightly to your body and then your back faces the base unit. This will create a maximum amount of signal degradation caused by your body.
http://local.google.com/maps/ms?ie=UTF8 … &z=17&om=1
- xBee - Chip (pos5)/ xBee - Whip (pos5) / xBee - Dipole (pos 5)= 0.04484 km = 147 ft
- xBee - Chip (pos1) = 0.1616 km = 530 ft
- xBee - Whip (Pos 1)/ xBee - Whip (pos 2) = 0.1755 km = 575 ft
- xBee - Whip (Pos 3) = 0.1931 km = 633 ft
- xBee Dipole (pos 1) = 0.2677 km = 878 ft
- xBee Pro - Whip (pos 5) = 0.2782 km = 912 ft
- xBee Pro - Whip (pos 1) = 0.4529 km = 1485 ft
- xBee Pro - Whip (pos 3) = 0.547 km = 1794 ft
- xBee Pro - Dipole (pos 1) = 1.045 km = 3428 ft
These results tell me that the base antenna should be mounted high. Perhaps 6 ft up or so so as to discourage random people from walking in front of it. The xBees are pretty tolerant of the antennas not being in the perfect place, but a person blocking the signal by being extremely close to the units degrades the signal significantly.
Also, the use of the dipole antennas makes an enormous difference in the usable range. Dan and I both agree that we should put a better antenna on the base station so that we can use the whip antennas with max range on the xBees.
The Chip antennas are very picky about orientation and are not good at going through obstructions. Wherever we can, I would recommend not using the chip antennas. The whip antennas are heads and tails better in every respect (except size).
Note…I did not test the xBee Pro - Chip
XBee Pro Series 1 is my advice. Module. Plug and play.