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The signal may be described in several domains, and an appropriate manipulation
of the signals within these domains may lead to effective use of the
transmission medium. The signal domains most commonly used for access
purposes are frequency, time, code, and space.
Frequency Domain
The radio spectrum is a precious resource, the quintessence of wireless communication
systems. Because radio propagation does not recognize geopolitical
boundaries and because political, economic, and social aspirations may
vary from country to country, international cooperation leading to an intelligent
and efficient use of the frequency spectrum is mandatory. Because of
the explosion of the demand for and the diversification of wireless services,
special attention have been given to spectrumallocation issues.Wireless communications
systems have been driven to use high frequencies due to the
congestion at the lower portion of the frequency spectrum, where the available
bandwidth is insufficient to satisfy the great demand for mobile services.
However, dealing with high frequencies usually leads to intricate problems
that are severely aggravated by the mobility of the users. Services are assigned
fixed bandwidth, not necessarily in a contiguous fashion. For competition
purposes, for a given service the frequency band is split into subbands, each
of which is allotted to different service operators. Each one of these bands is
then further split into two halves, one for the forward link and the other for
the reverse link. Subsequent divisions are carried out to form the nonoverlapping
frequency slots (channels). The channel bandwidth (channel spacing) is
determined according to criteria such as the services to be provided and the
available technology. Each channel—the physical channel—is identified by a
carrier placed in the middle of the channel band.
Time Domain
Signal insulation in the time domain is accomplished by allowing the information
to use the frequency band during a specific period of time (time
slot)—the physical channel. Nonoverlapping time slots constitute the orthogonal
channels. For any given piece of information, the aim is to transmit
the information in as short a period of time as possible, so that more information
can be conveyed in the same frequency band; this is achieved by
including more time slots per carrier. As before, this certainly depends on
the services to be provided and on the available technology. Access in the
timedomaincharacterizes the transmission occurring in bursts because for the
same source the information will occupy the carrier only in specific periods of
time.
Code Domain
Signal insulation can also be accomplished by assigning each signal a different
code (a key, a password)—the physical channel. A code is built as a
sequence of symbols belonging to an alphabet. In an ideal situation, these
codes must present zero cross-correlation so that they can be univocally discriminated
(e.g., for different sources, different passwords are assigned). For
a finite alphabet, the number of codes is obviously finite. Therefore, the larger
the alphabet, the larger the number of orthogonal codes in the alphabet and
the longer each code. For a given transmission rate, the longer the code, the
longer the time to transmit the code and the longer the time to detect the code.
Should it be transmitted, and detected, in a shorter period of time, the transmission
rate must be increased as well as the required bandwidth. Therefore,
for a limited bandwidth the number of orthogonal codes (code slots) is also
limited.
Space Domain
Signal insulation in the space domain can be achieved in two possible dimensions:
distance and angle—the physical channels. The distance dimension
exploits the fact that the propagation loss increases with the increase of the
distance between transmitter and receiver. Thus, signals using the same frequency
but transmitted by sources sufficiently apart from each other may
not strongly interfere with each other. In the same way, a given signal may
reach the receiver through different paths (due to multiple reflections, for
example). Each multipath signal suffers different attenuations and different
delays, according to the length of the path traveled. Therefore, both attenuation
and delay, jointly or independently, can be used to detect each multipath
signal. The angle dimension exploits the fact that, by illuminating wedges
of a circular area, signals simultaneously using the same frequency may be
discriminated by these very wedges within which they are located. Smart
antennas may be used to keep track of these signals.
Brief Remarks on Signal Domains
The most commonly used and most straightforward way of accomplishing
radio signal insulation is by assigning different frequency carriers to different
signals. This technique is widely employed by both analog and digital
wireless systems. Insulation in the time domain has been boosted by the digital
technology and is widely used in wireless communications. Insulation
in the code domain is a well-known technique that has long been used for
military as well as satellite communications applications. The move toward
high-capacity wireless systems has found great support in this technique.
Insulation in the space domain is widely used in wireless communications.
More specifically, the cellular concept with its frequency reuse philosophy
constitutes an example of such an application. |
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