Everything about wireless networks


Duplexing in wireless communications

Wireless communication systems have evolvedthe same frequency band is a function of the
through several stages of multiple-accesstechnology available. As can be inferred, TDD
control. The foremost controllable resourcemakes more efficient use of the spectrum as
has always been the frequency spectrum. Othercompared with FDD and is more flexible.
resources such as time, code, and space wereUsually, but not necessarily, an equal number
initially manipulated in a very precariousof windows is dedicated to the forward
and, therefore, ineffective manner. The earlychannels and to the reverse channels, with
systems operated in the simplex mode in thepaired windows symmetrically placed in time
forward link. Halfduplex systems soonfor maximum insulation. On the other hand,
appeared, in which forward link and reverseasymmetrical window assignment is also
link shared the same channel. Access controlpossible and necessary in asymmetrical
was performed on a push-to-talk basis withtraffic operation conditions, and in this
the access point still competing with thecase special attention must be paid regarding
terminals for access. Double halfduplexinterference  issues.
systems, in which forward link uses one
channel and reverse link another channel,Code  Division  Duplexing
granted the access point the privilege of not
having to contend for access. TheIn code division duplexing (CDD), forward and
push-to-talk procedure was the access controlreverse channels simultaneously share the
mechanism used on the reverse channel. Thesame frequency band but are discriminated by
full-duplex mode, or simply duplex mode, wasmeans of orthogonal codes. Therefore, a
then the last stage in this evolutionaryduplex channel is in fact a set of two
cycle in which push-to-talk access controlorthogonal codes within a given carrier,
was  no  longer  necessary.which constitutes the physical channels.
Practical implementation of such a scheme may
Duplex communication can be implemented byrender the circuitry very complex. Because
means of frequency division, time division,transmission and reception occur
code  division,  and  space division methods.simultaneously and continuously within the
same band and because the transmitted signal
Frequency  Division  Duplexingis at a much greater power than the received
signal, the level of interference may impair
In frequency division duplexing (FDD),such a communication scheme. Some sort of
forward and reverse channels use separateinterference cancellation mechanism is
frequencies. Therefore, a duplex channel isnecessary  to  realize  this  scheme.
in fact a set of two distinct carriers, which
constitute the physical channels. BecauseSpace  Division  Duplexing
forward and reverse channels are continuously
on and share the same antenna, the use of aIn space division duplexing (SDD), forward
duplexer (a filter between transmitter andand reverse channels share the same frequency
receiver) is necessary so that reverse andband but are discriminated in space.
forward channels do not interfere with eachTherefore, a duplex channel is in fact a set
other. In the same way, sharp filters withof two distinct locations where signals share
strong out-of-band rejection must be used tothe same frequency band. In a line-of-sight
reduce adjacent-channel interference.condition, directional antennas provide for
Adjacent-channel interference is alsothe required insulation of the signals and
minimized by allowing for a guard band withincan be used in SDD communication. In a
each channel. For a given continuousnon-line-of-sight condition, smart antennas
spectrum, insulation between forward andare  necessary.
reverse channels is maximized if paired
channels are separated by half of theBrief  Remarks  on  Duplexing  Techniques
spectrum. FDD is a well-known technology,
widely  used  in  wireless  systems.FDD is certainly the duplexing technique most
commonly used in wireless networks; it has
Time  Division  Duplexingbeen employed in all the first-generation
wireless systems, in most of the
In time division duplexing (TDD), forward andsecond-generation systems, and its deployment
reverse channels share the same frequencyin higher generations is without question.
band but occupy this band in nonoverlappingTDD is used in some second-generation
periods of time (slots), also known assystems, as well as in higher generations.
windows. Therefore, a duplex channel is inCDD alone does not seem to lend itself to
fact a set of two nonoverlapping windowseasy implementation for sophisticated
within a given carrier, which constitutes theinterference cancellation mechanisms may be
physical channels. Because transmission andrequired. SDD alone can be used in diverse
reception alternate in time, this scheme doessystem applications mainly to increase
not require the use of a duplexer. The numbercapacity. Combination of some of these
of possible access points (windows) withintechniques is a common practice.



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