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Definition
The explosive growth of both the wireless industry and the Internet is creating
a huge market opportunity for wireless data access. Limited Internet access, at
very low speeds, is already available as an enhancement to some existing cellular
systems. However those systems were designed with purpose of providing voice services
and at most short messaging, but not fast data transfer. Traditional wireless
technologies are not very well suited to meet the demanding requirements of providing
very high data rates with the ubiquity, mobility and portability characteristics
of cellular systems. Increased use of antenna arrays appears to be the only means
of enabling the type of data rates and capacities needed for wireless Internet
and multimedia services. While the deployment of base station arrays is becoming
universal it is really the simultaneous deployment of base station and terminal
arrays that can unleash unprecedented levels of performance by opening up multiple
spatial signaling dimensions. Theoretically, user data rates as high as 2 Mb/sec
will be supported in certain environments, although recent studies have shown
that approaching those might only be feasible under extremely favorable conditions-in
the vicinity of the base station and with no other users competing for band width.
Some fundamental barriers related to the nature of radio channel as well as to
the limited bandwidth availability at the frequencies of interest stand in the
way of high data rates and low cost associated with wide access.
In wireless
systems, radio waves do not propagate simply from transmit antenna to receive
antenna, but bounce and scatter randomly off objects in environment. This scattering
is known as multipath as it result in multiple copies of the transmitted signals
arriving at the receiver via different scattered paths. Multipath has always been
regarded as impairment, because the images arrive at the receiver at slightly
different times and thus can interfere destructively, canceling each other out.
However recent advances in information theory have shown that, with simulations
use of antenna arrays at both base station and terminal, multipath interference
can be not only mitigated, but actually exploited to establish multiple parallel
channels that operate simultaneously and in the same frequency band. Based on
this fundamental idea, a class of layered space-time architecture was proposed
and labeled BLAST. Using BLAST the scattering characteristics of the propagation
environment is used to enhance the transmission accuracy by treating the multiplicity
of the propagation environment is used to enhance the transmission accuracy by
treating the multiplicity of scattering paths as separate parallel sub channels. The
original scheme D-BLAST was a wireless set up that used a multi element antenna
array at both the transmitter and receiver, as well as diagonally layered coding
sequence. The coding sequence was to be dispersed across diagonals in space-tome.
In an independent Rayleigh scattering environment, this processing structure leads
to theoretical rates that grow linearly with the number of antennas with these
rates approaching 90% of Shannon capacity. Rayleigh scattering refers to the scattering
of light of f the molecules of air, and can be extended to. The
original scheme D-BLAST was a wireless set up that used a multi element antenna
array a both the transmitter and receiver, as well as diagonally layered coding
sequence. The coding sequence was to be dispersed across diagonals in space-time.
In an independent Rayleigh scattering environment, this processing structure leads
to theoretical rates that grow linearly with the number of antennas these rates
approaching 90% of Shanon capacity. Rayleigh scattering of light off the molecules
of air, and can be extended to scattering from particles up to about a tenth of
the wavelength of light. Rayleigh scattering can be considered to be elastic scattering
because the energies of scattered photons do not change.
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