Wireless signal domains

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