Image Processing Reference
In-Depth Information
110
433 MHz CC1010
868 MHz CC1010, ESB
915 MHz CC1010
2,4 GHz Bluetooth, WLAN
3,5 GHz WiMax
100
90
PL
max
= 80.7
80
70
60
50
40
30
20
1
10
100
1000
Distance (m)
d
max
=297 m
FIGURE .
PL of radio signals over distance between sender and receiver at different frequencies, e.g., used by
WiMax, WLAN, Chipcon CC, and ESB.
the receiver sensitivity of the embedded sensor boards (ESB) is PL
max
=
. dB [Sch,Dei].
Therefore, signals can be detected in a range
<
d
<
d
max
=
m (Figure .).
λ
π
−
PL
max
d
max
=
(.)
The equation of Friis (Equation .) is an approximation of the received power, but it is valid only
outside the Fraunhofer region
d
f
D
λ
of the sender. he Fraunhofer distance
d
f
depends on the
maximum expansion of the antenna
D
(
d
f
=
/
>>
D
)andthewavelengthλ
(
d
f
>>
λ
).
6.2.10 Minimal Transmission Range
In systems without any analog to digital converters (A/D converter) or without circuits to con-
vert signals, the RSS cannot be determined. In those devices, the signal strength can be measured
indirectly.
In ideal environments, the antenna gains (
G
TX
=
,
G
RX
=
) and system losses
L
can be dis-
regarded. hus, Equation . simplifies to
λ
π
d
PL
(
dB
)=−
log
(.)
Usually, a sender transmits with the maximum transmission power
P
TX
,max
.hissignalcanbe
detected within range
d
max
by measuring the received power if it is higher or equal than
P
RX
,min
.
The ESB for example transmits a signal at
P
TX
,max
=
.mW.hus,theminimalreceivedpoweris
P
RX
,min
=
. pW at
d
max
=
m.
P
TX
,max
PL
max
P
RX
,min
=
(.)
RSS-based methods determine the distance by solving Equation .. Figure .a visualizes the
ideal relation between
P
RX
and
d
.Withinrange
<
d
<
d
max
,thesignalcanbedetectedandthe
distancecanbecalculatedalongthegraph.
