Hardware Reference
In-Depth Information
Reading Received Signal Strength
Using Bluetooth Radios
The Bluetooth modules used in Chapters 2 and 6 can also give you an RSSI reading.
To see this, the radio needs to be connected to another Bluetooth radio. The simplest way
is to pair your radio with your computer, as shown in Project #4.
Once you've done this, open a serial connection to the
radio via Bluetooth. Once you're connected, drop out of
data mode into command mode by typing the following:
These are the signal strength values of the link, in hexa-
decimal. FF is the strongest possible value, and 00 is
the weakest. The first of the two values is the current
link quality; the second is the lowest recorded value so
far. As you move the radio closer to or farther from your
computer, the values will change just as it did in the XBee
example in the preceding project. To turn this off and get
back to sending data, type
L
and hit Return again. Then,
type
---
and press Return to leave command mode.
$$$
You'll get a CMD prompt from the radio. Next, type
L
and
hit Return. The radio will respond like so:
RSSI=ff,ff
RSSI=ff,ff
The Multipath Effect
The biggest source of error in distance ranging is what's
called the
multipath
effect (see Figure 8-8). When elec-
tromagnetic waves radiate, they bounce off things. Your
phone may receive multiple signals from a nearby cell
tower if, for example, you're positioned near a large
obstacle, such as a building. The reflected waves off the
building create “phantom” signals that look as real to the
receiver as the original signal. This issue makes it impos-
sible for the receiver to calculate the distance from the
beacon accurately, that causes degradation in the signal
quality of mobile phone reception, as well as errors in
locating the phones. For GPS receivers, multipath results
in a much wider range of possible locations, as the error
means that you can't calculate the position as accurately.
It is possible to filter for the reflected signals, but not all
radios incorporate such filtering.
Actual
beacon
phantom
beacon
phantom
beacon
reflected
signal
reflected
signal
primary
signal
Building
Building
Receiver
Figure 8-8
The multipath effect. Reflected radio waves create phantom
beacons that the receiver can't tell from the real beacon, causing
errors in calculating the distance based on signal strength.
