Cell phone bluetooth range

Question

I've got this phone I'm doing some range testing with...Namely, a Samsung S5 with the latest firmware and I'm testing the WLAN/Bluetooth radio.

This phone has unbelievable range...but, when I replace the phone with a similar homemade class 1 device (20dBm) with this Molex antenna, my range is just about half of the Samsung. The reciever is the same and I think that the power is the same (I can only guess that Samsung will output max power that the class of the device will allow when needed), so the only thing that's different is the antenna and the matching network.

I thought of a LNA, but LNAs that I have seen are unidirectional, and since Bluetooth communications need to be bidirectional (an acknowledgement after each packet), having only one device with a higher transmit power would only solve half the problem.

Am I correct in thinking this? Can I do as well as a cell phones WLAN/Bluetooth RF transmitter circuitry by impedance matching that antenna or is there a common topology with LNA's or something that I'm not aware of?

Answer 1

There are fundamentally two ways to increase the range of a wireless link:

1. increase transmit power
2. improve receiver sensitivity

With a bidirectional protocol such as you have described, then a problem with either the transmitter or receiver would cause the link to fail.

To diagnose a range problem on a link it is common to run some type of bluetooth packet sniffer / protocol analyser on both sides of the link, e.g. wireshark on a PC or android bluetooth snooper. By matching up the logs from both sides it should be possible to see which device is attempting to transmit but not being successfully heard at the other end. The goal is to determine whether it is your transmit power or receive sensitivity which is letting you down.

side note.. An LNA is usually considered a receive device. The amplifier on the transmit side is usually a PA.

and yes, it is possible to get a couple of dBs improvement by matching the antenna, but that depends on how bad the antenna match is to start with.