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

Media 2-15
As an example, the three most popular license-free ISM frequency bands shall be listed:
• 433.05 … 434.79 MHz (ITU-Region 1 only: Europe, Africa, Middle East): Used for speech applications
(baby-phones, two-way radios) and low-speed data communications (remotes); distances of
1.km and more can be achieved; good penetration of walls; high number of users.
• 2.4 … 2.5 GHz (worldwide): Mainly used for wireless LANs (IEEE 802.11b/g/n), Bluetooth radios,
video transmitters, and also by microwave ovens; high channel bandwidths allow for high-speed
data communications; typical ranges are below 200.m; poor penetration of walls; very high number
of users.
• 5.725 … 5.875 GHz (worldwide): Mainly used for wireless LANs (IEEE 802.11a), high transmit
powers allow compensation of higher free-space attenuation; high channel bandwidths allow for
high-speed data communications; very poor wall penetration; still low number of users.
2.4.2.2 Media Access
While cellular systems access the wireless channel coordinated by a master (e.g., a base station), the
opposite is true for ISM bands. As several systems coexist (typically not knowing from each other), a
coordinated access is not possible. Also, for many ISM bands, a system is allowed to transmit a radio
signal, even if the channel is in use by someone else. To prevent ISM bands from becoming useless due
to too much interference, several requirements have to be fulfilled:
• Maximum transmit power: Limiting the transmit power results in a maximum range for the
communication system and, thus, limits the distance where interference can occur.
• Maximum bandwidth: By limiting a system’s radio bandwidth, a single system is not able to block
the entire ISM band and, therefore, collisions are reduced.
• Fixed channel list: Defining a set of center frequencies reduces collisions.
• Maximum duty cycle and maximum transmit time: Both requirements reduce the time a system is
allowed to use the radio channel. As a consequence, the probability for a collision reduces.
• Listen before talk: Allowing a system to use a channel only in case it is not in use gives a high
reduction of collisions but increases access times and latency.
2.4.3 Modulation
Typical modulation formats of present communication systems are as follows (detailed information is
given by [JP95] and [SH88]):
• Gaussian minimum shift keying (GMSK): This nonlinear modulation format has very poor spectral
efficiency (bit rate per bandwidth) and can, therefore, be used for low data rates only. Typical
applications are low power, low data-rate sensor transmitters. GMSK is also used by the cellular
global system for mobile communication (GSM) system.
• Single carrier pulse-amplitude modulation (PAM) with high bit rates: Here, the carrier frequency
is multiplied by a complex value representing the transmission symbol. This factor remains constant
until the next symbol is transmitted. The complex values representing the symbols are
spread across the complex plane but centered around the origin. Therefore, the resulting transmit
signal is altered in magnitude and phase depending around the transmit symbol. Also, several
bits are packed into a single symbol. This can be one bit/symbol for binary phase-shift keying
(BPSK), two bits/symbol for quadrature phase-shift keying (QPSK), or even more bits/symbol for
quadrature amplitude modulation (QAM) formats. A high data rate is achieved by using a large
symbol alphabet (many bits per transmit symbol) and high symbol rates.
• Orthogonal frequency division multiplex (OFDM): Because classic PAM uses a large symbol alphabet
(QAM-16, QAM-64) and high symbol rates, each bit is “spread” across the entire system’s
bandwidth. In case of a frequency selective channel (e.g., due to multipath), the channel response
© 2011 by Taylor and Francis Group, LLC