Image Processing Reference
In-Depth Information
clear channel assessment (CCA) features—designed to enable other WLAN devices to access the
same system—also reacts to narrow band frequency user inside the used band. herefore, the latency
of a WLAN system will have a large jitter in practical applications. Measurements have shown values
to vary from ms to sometimes over ms in a coexistence situation with other low duty cycle users.
•
Bluetooth: Bluetooth is implemented in many mobile phones and notebooks; there-
fore, it is likely to be present. Bluetooth was made for user device interaction and is
a robust protocol. It is suited for HMI purposes in industrial applications. Bluetooth
devicesdonotuseCSMAandmaythushaveanefectonWLANlinks(especiallyin
the search/pairing mode).
•
IEEE ../Zigbee/WirelessHART: Such devices will be used to cover a larger area for
condition monitoring or building automation use cases. They will typically have a low
dutycycleandarenottobeexpectedinlargequantitiesinfactoryautomation(have,
therefore, not been considered as disturber for WISA in more detail). Such devices might
be disturbed by a WLAN with a high duty cycle if in close distance and with not enough
frequency separation (except Wireless Hart with an FH-option).
To estimate distance-dependent interference effects, an empirical model for the path loss (PL) is often
used for Bluetooth-type applications (see, e.g., []). In the near field of the transmitting antenna, this
attenuation is proportional to
d
,where
d
is the distance ( dB additional PL if distance is doubled).
For larger distances the empirical formula stipulates attenuation proportional to
d
.
( dB addi-
tional PL if distance is doubled). This is a coarse model; hence, the distance values derived below
should be considered only as indicative.
28.4.1.1 Interference from IEEE802.11b
Consider factory staff working in the vicinity of a WISA cell using a laptop computer to upload a
large document from an .b base station. he WISA transmission power is dBm. Using the PL
model above, this results in a carrier reception level C, at the WISA base station at a distance of m, of
C
=−
dBm. IEEE .. receivers require a co-channel carrier-to-interference ratio of
C
/
I
>
dB for adequate reception performance. Hence, the interferer level should be
I
<−
dBm
−
dB
=
−
dBm. The .b transmitter power may be up to mW in MHz bandwidth, equivalent
to . dBm per WISA/IEEE.. receiver bandwidth of MHz. The required PL to ensure safe
operation is therefore . dBm
−(−
dBm
)=
. dB. his translates into a minimum safe distance
requirement of m for an .b transmitter.
28.4.1.2 Interference from Bluetooth
Assume a factory worker using a Bluetooth phone headset. he typical Bluetooth transmitter power
is dBm at MHz bandwidth and the required PL to ensure
C
/
I
>
dB is therefore dB, which
corresponds to a minimum distance requirement of m.
28.4.2 Performance Measurements
WISA operation is at relatively short distances, with reception signal levels of about
dBm, well
above the sensitivity level. Hence, performance is in practice not limited by distance or fading effects,
but mainly by robustness to external radio interference.
Performance evaluations were done by measuring the message delays. Any loss of message in the
uplink (sensor status message) or in the downlink (BS acknowledgment) causes a retransmission in
the next frame, resulting in an additional delay of µs. A measurement setup consisted of
WISA nodes set to transmit at a rate of two messages per second over distances of about cm.
−
