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BLAST 1
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
1. INTRODUCTION
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 band
width availability at the frequencies of interest stand in the way of high data rates and low
cost associated with wide access..
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BLAST 2
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
FUNDAMENTAL LIMITATIONS IN WIRELESS
DATA ACESS
Ever since the dawn of information age, capacity has been the principal metric
used to asses the value of a communication system. Since the existing cellular system
were devised almost exclusively for telephony, user data rates low .Infact the user data
were reduced to the minimum level and traded for additional users. The value of a system
is no longer defined only by how many users it can support, but also by its ability to
provide high peak rates to individual users. Thus in the age of wireless data, user data
rates surges as an important metric.
Trying to increase the data rates by simply transmitting more; Power is extremely
costly. Furthermore it is futile in the contest of wherein an increase in everybody¶s
transmit power scales up both the desired signals as well as their mutual interference
yielding no net benefit. Increasing signal bandwidth along with the power is a more
effective way of augmenting the data rate. However radio spectrum is a scarce and very
expensive resource. Moreover increasing the signal bandwidth beyond the coherent
bandwidth of the wireless channel results in frequency selectively. Although well-
established technique such as equalization and OFDM can address this issue, their
complexity grows with the signal bandwidth. Spectral efficiency defined as the capacity
per unit bandwidth has become another key metric by which wireless systems are
measured. In the contest of FDMA and TDMA, the evolutionary path has led to
advanced forms of dynamic channel assessment that enable adaptive and more aggressive
frequency reuse.In the context of multi-user detection and interference cancellation
techniques.
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BLAST 3
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
SPACE: THE LAST FRONTIER
As a key ingredient in the design of more spectrally efficient systems. In recent years
space has become the last frontier. The entire concept of frequency reuse on which
cellular systems are based constitutes a simple way to exploit the spatial dimension. Cell
sectorisation, a widespread procedure that reduces interference can also be regarded as a
form of spatial processing. Moreover, even though the system capacity is ultimately
bounded, the area capacity on a per base station basis. Here, base station antenna array
are the enabling tools for wide range of spatial processing techniques devised to enhance
desired to enhance desired signals and mitigate interference. Coverage can be extended
and tighter user packaging becomes possible, enabling in turn larger cell sizes and higher
capacity can be extended even beyond the point at which every unit of bandwidth is
effectively used in every sector through space division multiple access (SDMA), which
enables the reuse of the same bandwidth by multiple users within a given sector as
long as they can be spatially discriminated.
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BLAST 4
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
LIFTING THE LIMITS WITH TRANSMIT AND
RECEIVE ARRAYS
Until recently, the deployment of antenna arrays in mobile systems was
contemplated-because of size and cost considerations-exclusively at base station sites.
The principle role of those arrays, long before interference suppression and other signal
processing advances were conceived, was to provide spatial diversity against fading.
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 actuallyexploited 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
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BLAST 5
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
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 off 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 withthe number of antennas these rates approaching 90% of Shannon capacity. Rayleigh
scattering of light off the molecules of air, and an 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.
The researchers foun d that the original D-BLAST concept was tough to implement, so
they simplified it to its most current iteration vertical BLAST. The BLAST technology
essentially exploits a concept that other researchers believed was impossible. The prevailing view was that each wireless transmission needed to occupy a separate
frequency, similar to the way in which FM radio within a geographical area are allocated
separate frequencies. Otherwise, the interferences are too overwhelming for quality
communications. The BLAST researchers, however, theorized it is possible to have
several transmissions occupying the same frequency band. Each transmission uses its
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BLAST 6
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
own transmitting antenna. Then, on the receiving end, multiple antennas again are used,
along with innovative signal processing, to separate the mutually interfering
transmissions from each other. Thus the capacity of a given frequency band increases
proportionally to the number of antennas.
The BLAST prototype, built to test this theory, uses an array of eight transmit
and 12 receive antennas. During its first weeks of operation, it achieved unprecedented
wireless capacities of at least 10 times the capacity of today¶s fixed wireless loop
systems, which are used to provide phone service in rural and remote areas. ³This new
technology represents an opportunity for future wireless systems of extraordinary
communications efficiency,´ said Bell Labs researcher Reinaldo Valenzuela, who headed
the BLAST research team. ³This experiment, which was designed to illustrate the basic principle, represents only a first step of using the new technology to achieve higher
capacities.´
The advanced signal-processing techniques used in BLAST were first developed
by researcher Gerard Foschini from a novel interpretation of the fundamental capacity
formulas of Claude Shannon¶s Information Theory, first published in 1948. while
Shannon¶s theory dealt with point-to-point communications, the theory used in BLAST
relies on ³volume-to-volume´ communications, which effectively gives Information
Theory a third, or spatial, dimension, besides frequency and time. This added dimension,
said Foshini, is important because ³when and where noise and interference turn out to be
severe, each bit (of data) is well prepared to weather such impairments.´ The technology
is eventually expected to be deployed in base station equipment and mobile devices such
as note book PCs and PDAs so that mobile operators can deliver higher data services too
substantially greater number of subscribers than is possible today using the best 3G
network technology available.
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BLAST 7
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
OVERVIEW OF BLAST SYSTEM
V-BLAST takes single data stream and demultiplexes it in to m substreams. Each
substream is encoded into symbols and feed into separate transmitter. Transmitter 1
through M operate co channel at a symbol rate of 1/T symbols per second. Eachtransmitter utilizes QAM. QAM combines phase modulation with AM. Since all the sub
streams are transmitted in the same frequency band, spectrum is used very efficiently
Since the user¶s data is being sent in parallel over multiple antennas used. QAM is an
efficient method for transmitting data over limited bandwidth channel. It is assumed that
the same constellation is used for each sub streams and the transmission is organized in
to burst of L symbols. The power of each transmitter is proportional to 1/M and total
radiated power is constant irrespective of the number of transmitting antennas. BLAST¶s
receivers operate co channel, each receiving signals emanating from all M of the
transmitting antennas. It is assumed that the channel-time variation is negligible over the
symbol periods in a burst.
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BLAST 8
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
BLAST¶S SIGNAL DETECTION
At the receiver, an array of antennas is again used to pick up the multiple
transmitted sub streams and their scattered images. Each receiver antenna sees the entire
transmitted sub streams super imposed, not separately. However, if the multipath
scattering is sufficient is sufficient, then the multiple sub streams are located at different
points in space .Using sophisticated signal processing, these slight difference in
scattering allow the sub streams to be identified and recovered. In effect the unavoidable
multipath is exploited to provide a useful spatial parallelism that is used to greatly
improve data transmission rates. Thus when using the BLAST technique, the more
multipath, the better, just the opposite of the conventional systems. The blast signal
processing algorithms used at the receiver are the heart of the technique. At the bank of
receiving antennas, high speed signal processors look at the signals from all the receiver
antennas simultaneously, first extracting the strongest signal have been removed as a
source of interference. Again the ability to separate the sub streams depends on the slight
differences in the way the different sub streams propagate through the environment. Let
us assume a signal transmitted vector symbol with symbol-synchronous receiver
sampling and ideal timing. If a= (a1, a2, a3,«. am) T is the vector transmitted symbols,
then the receiver N vector is r1=Ha+v, where H is the matrix channel transfer function
and V is a noise vector. Signal detection can be done using adaptive, antenna array
techniques, sometimes called linear combinational nulling. Each sub stream is
sequentially understood as the desired signal. This implies that the other substream will
be understood as interference. One nulls out this interference by weighting the interfering
signals they go to zero (known as zero forcing). While these linear nullings work, on
linear approaches can be used in conjunction with them for overall result. Symbol
cancellation is one such technique. Using interference from already detected componentsof interfering signals are subtracted to form the received signal vector. The end result is a
modified receiver vector with few interferes present in the matrix. Bell labs actually tried
both approaches. The result showed that adding the nonlinear to the linear yielded the
best performance and dealing with the strongest channel, first (thus removing it as and
interference) give the best overall SNR. If all components of µa¶ are assumed to be the
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BLAST 9
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
part of the same constellation, it would be expected that the component with the smallest
SNR would dominate the overall error performance. The strongest channel then becomes
the place to start symbol cancellation. This technique has been called the ³best-first´
approach and has become the de-facto way to do signal detection from an RF stream. But
what the Bell labs guys found is that if you evaluate the SNR function at each stage of
the detection process, rather than just at the beginning, you come up with a different
ordering that is also (minmax) optimal.
As its core V-BLAST is an iterative cancellation method that depends on
computing a matrix inverse to solve the zero forcing function. The algorithm works by
detecting the strongest data stream from the received signal and repeating the process for
the remaining data streams. While the algorithm complexity is linear with the number of transmitting antennas, it suffers performance degradation through the cancellation
process. If cancellation is not perfect, it can inject more noise in to the system and
degrade detection.
The essential difference between D-BLAST and V-BLAST lies in the vector
encoding process. In D-BLAST, redundancy between the sub streams is introduced
through the use of specialized inter-sub stream block coding. In D-BLAST code blocks
are organized along diagonals in space-time. It is this coding that leads to D-BLAST¶s
higher spectral efficiencies for a given number of transmitters and receivers. In V-
BLAST, however, the vector encoding process is simply a demultiplex operation
followed by independent bit-to-symbol mapping of each sub stream. No inter-sub stream
coding, or coding of any kind, is required, though conventional coding of the individual
sub streams may certainly be applied.
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BLAST 10
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
BLAST IN THE REAL WORLD
Two familiar factors are essential to the success of a BLAST: technology and
economics. On the technology side, scalar systems (those currently in use) are far less
spectrally efficient than BLAST ones. They can encode B bits per symbols using a single
constellation of 2B points. Vector systems can realize the same rate using M
constellation of 2B/M points each. Large spectral efficiencies (that is, a large B) are more
practical. Let¶s take an example. If you want 26 bps/Hz with a 23%roll off, you need to
have (26*1.32)=32bits/symbol.a scalar system would require 232 points, which is around
4 billion. No wireless system will put up 4 billon transmitters. Ever. This means the
vector is the approach is the only one that one can ever hope to fulfill such a bit-per-
second rate. On the economic side, BLAST calls for an infrastructure that will take
considerable resource to develop. Cell antennas will have to be redesigned to evolve with
the increase in data rates. The first change will have to occur at the cell towers, and then
at the receiver. The cell tower will have to go from a switched-beam (phase-swept and
the like) to a steered-beam configuration. On the plus side, much of the development can
be gradual. Older ³diversity´ antennas will most likely retained as a fallback for the
worst-case channel environment (which means single path flat-fading at low mobile
speeds), so new antennas can be added gradually .A carrier could go from one to two
four transmit path per sector, upping the cost of service with each incremental
performance gain. Proceeding with a hardware-based migration will yield balanced gains
in the forward and reverse links. Carriers are very sensitive to the costs, however
incremental, of deploying new systems. Since CDMA systems will upgrade faster than
GSM systems. This means that CDMA carriers will be first to market with higher
bandwidth systems, as Verizon¶s recent 2.5G 1RTT rollout has shown. Asked about its
plans for BLAST, Verizon¶s reps indicated that the discussion was premature, but thatthey might have more to say about it in the first quarter of 2003. That seems enough of a
nom-denial to indicate that BLAST is part of the company¶s long range planning.
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BLAST 11
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
BLAST vs. EXISTING SYSTEMS
What makes BLAST different from any other single-user that uses multiple
transmitters? After all, we can always drive all the transmitters using a single user¶s data,
even if it is sub streams. Well, unlike code-division or a speedspectrum approach, the
total bandwidth those QAM systems require. Unlike a Frequency Division Multiple
Access (FDMA) approach, each transmitted signals occupies the entire signal bandwidth.
And finally, unlike Time Division Multiple Access (TDMA), the entire system
bandwidth is used simultaneously by all of the transmitters all of the time .BLAST can be
best used in CDMA such as Verizon or Sprint, rather than a gem system such as AT&T.
The BLAST system does not impose orthonalization ot transmitted signals. The reason
for this is simple, obvious, and rather elegant. The Blast propagation environment of the
real world provides significant multipath latencies one receiver. Rather than fight against
these latencies, BLAST exploits them to provide the signal decor relation necessary to
separate the co-channel signals blast uses the same effect that cause ghosting in TV
pictures as a sort of clock to allow the various signals to be extracted.
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BLAST 12
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
LABORATARY RESULS
A laboratory prototype of a V-BLAST system has-been constructed for the
purpose of demonstrating the feasibility of the BLAST approach. The prototype operates
at a carrier frequency of 1.9 GHz and a symbol/sec, in a bandwidth of 30 KHz. The
system was operated and characterized in the actual laboratory/office environment not a
test range, with transmitter and receiver separations up to about 12 meters. This
environment is relatively benign in that the delay spread is negligible, the fading rates are
low and there is significant near-field scattering from near by equipment and office
furniture. Nevertheless, it is a representative indoor lab/office situation, and no attemptwas to ³tune´ the system to the environment, or to modify the environment in anyway.
The antenna arrays consisted of /2 wire dipoles mounted in various arrangements. For
the results shown below, the receive dipoles were mounted on the surface of a metallic
hemisphere approximately 20cm in diameter, and transmit dipoles were mounted on a
flat sheet, in a roughly rectangular array with about /2 inter-element spacing. In general,
the system performance was found to be nearly independent of small details of the array
geometry. Figure 6 shows the results obtained with the prototype system, using M=8
transmitters and N=12 receivers. In this experiment, the transmit and receive arrays wereeach placed at a single representative position within the environment, and the
performance characterized. The horizontal axis is spatially averaged receiver SNR. The
vertical axis is the block error rate, where a ³block´ is defined as a single transmission
burst. In this case, the burst length L is 100 symbol duration of which is used for training.
In this experiment, each of the eight sub streams utilized uncoded 16-QAM, i.e.
4 bits/symbol/transmitter, so that the payload block size is 8*4*80=2560 bits. The spectral
efficiency of this configuration is 25.9bps/Hz and the payload efficiency is 80% of the
above, or 20.7 bps/Hz, corresponding to a payload data rate of 621 Kbps in 30 KHz
bandwidth.
The upper curve in fig. 6 shows performance obtained when conventional nulling
is used. The lower curve shows performance using nulling and optimally-ordered
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BLAST 13
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
cancellation. The average difference is about 4 db, which corresponds to a raw spectral
efficiency differential (for this configuration) of around 10 bps/Hz. Figure 7 shows
performance results obtained using the same BLAST system configuration ( M=8, N=12,
16-QAM) when the receive array was left fixed and the transmit array was located at
different positions throughout the environment. In each case, the transmit power was
adjusted so that large received SNR was 24+/-0.5db. Nulling with optimized cancellation
was used. It can be seen that operation at this spectra efficiency is reasonably robust with
respect to antenna position. In all positions, the system had at least 2 orders of magnitude
margin relative to 10^-2 BER. For a completely uncoded system, these are entirely
reasonable error rates, and application of ordinary error correcting codes would
significantly reduce this. At 34 db SNR, spectral efficiencies as high as 40bps/hz have
been demonstrated at similar error rates, though with less robust performance.
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BLAST 14
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
ADVANTAGES
Since the entire sub streams are transmitted in the same frequency band, spectrum
is used efficiently. Spectrally efficiency of 30-40 bps/Hz is achieved at SNR of 24 db.
This is possible due to use of multiple antennas at the transmitter and receiver at SNR of
24 db. To achieve 40bps/Hz a conventional single antenna system would require a
constellation with 10^12 points. Furthermore a constellation with such density of points
would require in excess of 100db operating at any reasonable error rate. A critical feature
of BLAST is that the total radiated power is held constant irrespective of the number of
transmitting antennas. Hence there is no increase in the amount interference caused to
users. Figure 5 displays cumulative distributions of system capacity (in megabits per
second per sector) over all locations with transmit arrays only as well as with transmit
and receive arrays. These curves can also be interpreted as user peaks rates, that is user
data rates (in megabits per second) when the entire capacity of every sector is allocated to
an individual user. With transmit arrays only; the benefit appears significant only in the
lower tail of the distribution, corresponding to users in the most detrimental location. The
improvements in average and peak systems capacities are negligible. Moreover, the gains
saturate rapidly as additional transmit antennas are added. With frequency diversity taken
into account, those gains would be reduced even further. The combined use of transmit
and receive arrays, on the other hand , dramatically shifts the curves offering multifold
improvements in data rate at all levels. Notice that, without receive arrays, the peak data
rate that can be supported in 90 per-cent of the systems locations-with a single user per
sector ±is only on the order of 500kb/s with no transmit diversity and just over 1Mb/s
there-with.
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BLAST 15
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
There is an extraordinary growth in attainable data unleashed by the additional
signaling dimensions provided by the combined use of transmit and receive arrays. With
only M=N=8 antennas, the single user data can be increased by an order of magnitude.
Furthermore, the growth does not saturate as long as additional uncorrelated antennas can
be incorporated into the arrays. Figure 5depicts single-user data rate supported in 90%
location Vs range with transmit and receive arrays. M is the terminal; transmit power
PT=10w; bandwidth B=5MHZ. BLAST technology has reportedly delivered a data
reception at 19.2Mbps on a3G network. With BLAST downloading a song would take
3s, not 30 via cable or DSL.20 novels can be downloaded in a second and HDTV can be
watched on a telephone. This innovation, known as BLAST, may allow so-called ³fixed´
wireless technology to rival the capabilities of today¶s wired networks would connect
homes and businesses to copper-wired public telephone service providers
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BLAST 16
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
DISADVANTAGES
The BLAST technology is not is not well suited for mobile wireless applications,
such as hand-held and car-based cellular phones multiple antennas²both transmitting
and receiving²are needed. In addition, tracking signal changes in mobile applicationswould increase the computational complexity.
It would require manufacture to invest in the development of new multiantennadevices. It would also require new wireless network infrastructure.
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BLAST 17
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
CONCLUSION
Under widely used theoretical assumption of independent Rayleigh scattering
theoretical capacity of the BLAST architecture grows roughly, linearly with the number
of antennas even when the total transmitted power is held constant. In the real world
ofcourse scattering will be less favorable than the independent Raleigh¶s assumption ant
it remains to be seen how much capacity is actually available in various propagation
environments. Nevertheless, even in relatively poor scattering environment, BLAST
should be able to provide significantly higher capacities than conventional architectures.
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BLAST 18
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA
REFERENCES
1. IEEE Communication Magazine. September 2001
2. www.bell-labs.com/projects/blast
3. www.lucent.com/information theory
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BLAST 19
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BLAST 20
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BLAST 21
MOUNT ZION COLLEGE OF ENGINEERING, KADAMMANITTA