Wireless Technologies in Process Automation - Herkules.oulu.fi - Oulu

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A hybrid topology can combine the benefits of the both aforementioned networks by
combining efficiency and flexibility of star and mesh topologies (Shen et al., 2004). In
hybrid networks, star nodes are controlled by mesh nodes, therefore providing efficiency
and flexibility (Shen et al., 2004).
3.4.6 Radio
As mentioned in Section 3.2, radio technologies can be used to reduce interferences. For
this, spread spectrum radio modulation techniques are applicable because of their
multiple access, anti-multipath fading and anti-jamming capabilities (Low et al., 2005).
The two main spread spectrum techniques employed are direct sequence spread spectrum
(DSSS) and frequency hopping spread spectrum (FHSS) (Low et al., 2005). They have
different physical mechanisms and therefore, react differently in industrial settings (Low
et al., 2005). The choice between radio techniques is dependent on application and
environment (Low et al., 2005).
Due to the nature of frequency hopping in FHSS, other data packets still may reach the
receiver even if one packet fails because of strong narrowband interference. Additionally,
users may choose not to use certain frequencies, if there are known sources of
interference. Therefore, FHSS is suitable for harsh environments, because only part of the
spectrum is affected if packets jam at some hops. In general, FHSS scheme is less likely
to collide with other transmissions since changing of transmitting frequency makes
colliding in certain time frame more difficult than in DSSS scheme. (Low et al., 2005)
DSSS, for its part, can remove the interference completely, if the interfering signal power
is within the jamming margin, especially in case of low two medium narrowband
interference (Low et al., 2005). In case of heavy narrowband interference, though, FHSS
can still perform, but DSSS fails (Low et al., 2005). To summarize, DSSS is ideal for
systems that require high speed and are applied to environments with low or medium
narrowband interference, whereas FHSS is suitable for applications that require low cost,
lower data rate and applicability to high narrowband interference environment (Low et
al., 2005).
Radio modulation can be incorporated with other strategies to improve link quality in
industrial environments. These strategies include channel diversity, path diversity,
temporal diversity an increased transmit power. (Low et al., 2005) Discussion about
reduced radio link quality and countermeasures can be found in (Werb and Sexton, 2005)
The frequencies used for wireless communication in industrial automation are presented
in Table 4. The frequencies can be used also by other wireless applications, e.g. RFID
applications, WLAN applications or ISM applications. There is no guaranty for minimum
radio transmission quality. (VAN, 2005)