wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

2-14 Industrial Communication Systems
Fading is also distinguished by the rate at which the channel changes: When moving a radio
receiver, the radio channel changes due to shadowing of large buildings, which results in a higher
channel attenuation for some time (slow fading). Moving a receiver within an industrial environment
with many reflectors, the channel undergoes fast changes due to the high number of
multipaths (fast fading).
• Doppler Spread: If the environment and/or the radio terminals are moving, Doppler frequency
shift occurs. In multipath environments, this will lead simultaneously to several positive and
negative frequency shifts.
2.4.1.7 Link Budget
For planning a radio system, a link budget calculation needs to be done. This can be either used for
determining a wireless link’s range under a given transmit power or, in the other way round, it can be
used for calculating the required transmit power for a certain link range. For a line of sight radio link,
this can be done by (all values in decibels)
Prx = P
tx + G
tx
− L tx
− L0 − Lm + Grx − Lrx
(2.8)
P
EIRP,tx
with
the received and transmitted power P rx , P tx in dBm
the transmit and receive antenna gain G tx , G rx in dBi
the cable/connector losses on transmitter and receiver side L tx , L rx in dB
the free-space and miscellaneous losses L 0 , L m in dB
The effectively radiated power P EIRP,tx denotes the feed power of an isotropic antenna generating the
same maximum field strength as the antenna used for link budget calculation. Therefore, almost all
radio regulations limit P EIRP,tx instead of the transmitter power P tx as it provides a fair comparison of
maximum power densities.
The miscellaneous losses, L m , are very important for the reliability of the link: They include additional
losses due to changing weather conditions, component variations, imperfect alignment of the antennas
to each other, and so on. In case of multipath environments, they also account for losses due to fading.
2.4.2 types and Media Access
Because of the electromagnetic spectrum being a limited resource, access to frequencies is regulated
by national authorities. Deploying a radio system, therefore, requires getting a local license. Typically,
licensing a system with a wide bandwidth and high transmit power is not as easy as for a low-power
narrowband system. The more bandwidth and transmit power is required, the more difficult it is to get
a license and/or the more expensive the license will be.
Only a small fraction of the bandwidth used nowadays is available for unlicensed use.
Furthermore, frequencies are not equivalent: Depending on the application, a higher or lower frequency
is suitable.
2.4.2.1 Frequencies for Wireless Links
Covering all frequency bands in detail would go beyond the scope of this book. Generally, it can be said
that lower frequencies allow longer ranges due to less free-space attenuation show better penetration
into buildings, but need larger antennas, and allow only low data rates. In contrary, higher frequencies
have higher attenuation, show less penetration-performance, but allow high data-rate applications and
small/integrated antennas.
© 2011 by Taylor and Francis Group, LLC