The Issues in Wireless Communications
In the seventies, the principal design problem encountered in wireless or
mobile radio was how to overcome the distortion of received signals by a
time-varying and
frequency-selective propagation path. Radio waves near the ground
do not travel over a single well-defined radio path as in
free space; they
are scattered against reflecting obstacles in the vicinity of the mobile
antenna. Reflected waves may add destructively, causing the received signal
to disappear or become heavily attenuated at certain locations. A moving user,
as in vehicular telephony, receives a resulting signal that is rapidly varying
in time. This effect is called `
fading'. Moreover, waves excessively delayed
by remote reflections cause
distortion of the shape of transmitted waveforms.
Modern (digital) signal processing (DSP-)techniques can mitigate these
effects to a great extent. As a result, nowadays the most quoted critical
issues of wireless systems are no longer directly related to multipath fading,
but are
- the scarcity of radio spectrum and the resulting mutual
interference among users
- the power consumption of portable terminals and the inadequacy of existing battery and other energy storage technologies, and
- the complexity of the software needed to support user mobility,
e.g. from cell to cell or from operator to operator.
In the past 10 years, the general emphasis of radio communications designs
has shifted away from maximizing the capacity of individual links, limited by
gaussian noise and available bandwidth, onto optimizing the capabilities of
multi-user networks. The decisive interference now seldom comes from outside
(as in military systems), but is produced by authorized users of the very same
wireless network. Users thus share an interest in developing and adhering to
the best possible protocols and standards for allocating the joint network
resources. Accordingly, wireless systems engineering has developed into a
conscious search for the best common
culture for multiple users in a real
environment. The traffic capacity, spectrum efficiency and cost-effectiveness
of modern radio networks is no longer won from
nature (or an adversary) in a
classical pursuit of individual gain.
Start page of video from Short Course
by
BMRC
Video from Wireless Communications Networks Short Course
"First of all, the receive signal is not a strong line-of-sight signal but are a lot of reflections
from obstacles. That's called multipath reception..... But this can be resolved if you design a system in an appropriate way, using digital processing techniques. What has become very important in the last
few years are three issues ..... "