blast technology
TRANSCRIPT
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BLAST TECHNOLOGY 1
A
SEMINAR REPORT
ON
BLAST TECHNOLOGY
Submitted in the partial fulfillment of the requirements
For the award of degree
BACHELOR OF TECHNOLOGYIn
ELECTRONICS AND COMMUNICATION ENGINEERINGOf
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITYHYDERABAD
Presented by
HARINI RAJAN V (06281A0403)
Department of Electronics and Communication Engineering
KAMALA INSTITUTE OF TECHNOLOGY AND SCIENCE
(Approved by AICTE and Affiliated to JNTU, Hyderabad)Singapur, Huzurabad-505468
(2009-2010)
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BLAST TECHNOLOGY 2
KAMALA INSTITUTE OF TECHNOLOGY AND SCIENCE
SINGAPUR, HUZURABAD
DEPARTMENT OF ELECTRONICS AND COMMUNICATIONENGINEERING
CERTIFICATE
This is to certify that the Technical seminar entitled BLAST
TECHNOLOGY is a bonafide work carried out by HARINI RAJAN V
(06281A0403) in partial fulfillment of the requirements for the award of the
degree of BACHELOR OF TECHNOLOGY in ELECTRONICS AND
COMMUNICATION ENGINEERING by the Jawaharlal Nehru Technological
University, Hyderabad during the academic year 2009-2010.
E.PRADEEP B. RAMESH
Assistant Professor
Seminar Coordinator Head of Department
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BLAST TECHNOLOGY 3
ACKNOWLEDGEMENT
I express my deep sense of gratitude and sincere thanks to my seminar
coordinator Mr. E. Pradeep, Assistant Professor of ELECTRONICS AND
COMMUNICATION ENGINEERING department for his valuable guidance,
inspiration and constant encouragement throughout the course of this work.
My special thanks to Mr. B. Ramesh, Head of ECE department and to all the
faculty members for their valuable assistance extended during the entire seminar
period.
Last but not least, I would like to express heartfelt gratitude to all others
and especially my classmates who directly and indirectly helped me in bringing
out this seminar successfully.
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BLAST TECHNOLOGY 4
ABSTRACT
In wireless transmission the radio waves do not simply propagate from transmit to
receive antenna, but bounce and scatter randomly off objects in the environment.
This scattering is known as multipath as it results in multiple copies of transmitted
signals, arriving at the receiver via different scattered paths. In conventional
wireless systems, multipath represents a significant impediment to accurate
transmission, because the images arrive at the receiver at slightly different times
and can thus interfere destructively, canceling each other out. For this reason,
multipath is traditionally viewed as a serious impairment. A layered space
time technology by Bell labs to exploit the concept of multipath known as BLAST
( Bell labs Layered Space Time Technology). Using the BLAST approach
however, it is possible to exploit multipath, that is, to use the scattering
characteristics of the propagation environment to enhance, rather than degrade,
transmission accuracy by treating the multiplicity of scattering paths as separate
parallel sub channels. By this method the spectrum is used more efficiently.
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BLAST TECHNOLOGY 5
CHAPTER: 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
slow 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 simultaneous deployment of
base stations and terminal arrays that can unleash unprecedented levels of
performance by opening up multiple spatial signaling dimensions. Theoretically,
user data rates as high as 2Mb/sec will be supported in certain environments,
although recent studies have shown that approaching those might be feasible
under extremely favorable conditions- in the vicinity of the base station and with
no other user s competing for bandwidth .Some fundamental barriers related to
nature of radio channel as well as to 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 TECHNOLOGY 6
CHAPTER: 2
FUNDAMENTAL LIMITATIONS IN WIRELESS DATA ACCESS
Ever since the dawn of information age, capacity has been the principal metric
used to assess the value of a communication system. Since the existing cellular
systems were devised almost exclusively for telephony, user data rates were low.
In fact the user data were reduced to a 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 context of wherein an increase in
everybodys 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 date rate. However ratio 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. The entire concept of frequency reuse on which cellularsystems are based constitutes a simple way to exploit the spatial dimension. Cell
sectorisation a wide spread procedure that reduces interference can also be
regarded as a form of spatial processing.
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CHAPTER: 3
LIFTING THE LIMITS WITH TRANSMIT AND RECEIVE ARRAYS
In wireless systems, radio waves do not propagate simply from transmit antenna
to receive antenna, but bounce and scatter randomly off objects in the
environment. This scattering is known as multipath, as it results in multiple copies
("images") of the transmitted signal arriving at the receiver via different scattered
paths. In conventional wireless systems, multipath represents a significant
impediment to accurate transmission, because the images arrive at the receiver at
slightly different times and can thus interfere destructively, canceling each other
out. For this reason, multipath is traditionally viewed as a serious impairment.
Using the BLAST approach however, it is possible to exploitmultipath, that is, to
use the scattering characteristics of the propagation environment to enhance,
rather than degrade, transmission accuracy by treating the multiplicity of
scattering paths as separate parallel sub channels.
Fig.3.1 Over view of radiated power showing multipath
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BLAST TECHNOLOGY 8
CHAPTER: 4
OVERVIEW OF BLAST SYSTEM:
The prevailing view was that each wireless transmission needed to occupy
separate frequency, similar to the way in FM radio with in a geographical area
allocated with separate frequencies. BLAST technology essentially exploits a
concept that other researchers believed impossible. The original scheme
developed was D BLAST, which utilizes multi-element antenna arrays at both
transmitter and receiver and an elegant diagonally layered coding structure in
which code blocks are dispersed across diagonals in space-time. In an independent
Rayleigh scattering environment, this processing structure leads to theoretical
rates which grow linearly with the number of antennas (assuming equal numbers
of transmit and receive antennas) with these rates approaching 90% of Shannon
capacity. Scattering of light off the molecules of the air, and can be extended to
scattering from particles up to about a tenth of the wavelength of light. Rayleigh
can be considered to be elastic scattering because the energies of scattered photons
do not change. The coding sequence used in D BLAST is very complex and
costly. So we move to the most current iteration V BLAST.
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BLAST TECHNOLOGY 9
Fig 4.1. Coding sequence used in V BLAST
Fig 4.2 Coding sequence used in D BLAST
LST code encoding process. Here, n= 3. (a) The Incoming information bitsequence is first demultiplexed into n subsequences. Each subsequenceis then encoded using a constituent code. (b) The coded symbols from theCCs aretransmitted by the n transmitting antennas in turn.
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CHAPTER: 5
MOST CURRENT ITERATION-V BLAST
Fig 5.1 Block diagram of V BLAST
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A single data stream is de multiplexed intoMsub streams .Each sub stream is then
encoded into symbols and fed to its respective transmitter. Transmitters 1 through
operate co channel at symbol rate 1/ T symbols/sec, with synchronized symbol
timing. Each transmitter is itself an ordinary QAM transmitter. QAM combines
phase modulation with AM. Since the entire sub streams are transmitted in the
same frequency band, spectrum is used very efficiently. Since the users data is
being sent in parallel multiple antennas are 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 stream, and that transmissions are organized into
bursts ofL symbols. The power launched by each transmitter is proportional to 1/
M so that the total radiated power is constant irrespective of the number of
transmitting antennas. Blasts receivers operate co channel, each receiving signals
emanating from all M of the transmitting antennas .It is assumed that channel-time
variation is negligible over the symbol periods in a burst.
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CHAPTER: 6
BLASTS 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 receiving antenna "sees"
the entire transmitted sub streams superimposed, not separately. However, if the
multipath scattering is sufficient, then the multiple sub streams are all scattered
differently, since they originate from different transmit antennas that are located at
slightly different points in space. Using sophisticated signal processing, these
differences in scattering of the sub streams allow the sub streams to be identified
and recovered. In effect, the unavoidable multipath in wireless communication
offers a very useful spatial parallelism that is used to greatly improve bit-rates.
Thus, when using the BLAST technique, the more multipath, the better, just the
opposite of 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
all the signals from all the receiver antennas simultaneously, first extracting the
strongest sub stream from the morass, then proceeding with the remaining weaker
signals, which are easier to recover once the stronger signals have been removed
as a source of interference. Again, the ability to separate the sub streams depends
on the differences in the way the different sub streams propagate through the
environment.
Let us assume a signal vector symbol with symbol-synchronous receiver sampling
and ideal timing. If a = ( a1,a2,a3,.am )T is the vector transmitted symbols, then
receiver N vector is R1=Ha+V, where H is the matrix channel transfer function
and V is a noise vector.
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Signal detection can be done using adaptive antenna array techniques, some times
called linear combinational nulling. Each sub stream is sequentially understood as
the desired signal. This implies that the other sub stream will be understood as
interference. One nulls this interference by weighting signals they go to zero
(known as zero forcing).
While these linear nullings works, on linear approaches can be used in
conjunction with them for overall result. Symbol cancellation is one such
technique. Using interference from already detected components of interfering
signals are subtracted to form the received signal vector. The end result is a
modified receiver vector with little interference present in the matrix. Bell labs
actually tried both approaches. The result showed that adding the non linear to the
linear yielded the best performance and dealing with the strongest channel, first
(thus removing it as interference) give the best overall SNR. If all components of
a are assumed to be the 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 best first approach and 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 (minimax) 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 notperfect, it can inject more noise into 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 this
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BLAST TECHNOLOGY 15
Fig.7.1 Schematic representation of the prototype developed
Fig7.2
Reinaldo Valenzuela (front) and colleagues (l to r) Peter Wolniansky, Glenn
Golden, and Jerry Foschini are shown here with antenna devices they created fortheir experimental BLAST wireless system .They are the persons who headed
BLAST researchers team.
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The advanced signal processing techniques used in BLAST were first developed
by researcher Foschini from a novel interpretation of the fundamental capacity
formulas of Claude Shannons theory dealt with point-to-point communications,the theory used in BLAST relies on volume-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
when and where noise and interference turn out to be severe, each bit of data is
well prepared to weather such impairments.
7.1 LABORATORY RESULTS
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 band width 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 fading 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 attempt was to tune the system to 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 20 cm 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.
Fig. 7.3 shows the results obtained with the prototype system, using M=8
transmitters and N=12 receivers. In this experiment, the transmit and receive
arrays were each placed at a single representative position within the environment,
and the performance characterized. The horizontal axis is spatially averaged
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BLAST TECHNOLOGY 17
receiver SNR. The vertical axis is the block error rate, where a block 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, ie. 4 bits/symbol/transmitter, so that the
payload block size is 8*4*80=2560 bits. The spectral efficiency of this
configuration is 25.9 bps/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 band
width.
The upper curve in fig. shows performance obtained when conventional nulling is
used. The lower curve shows performance using nulling and optimally-ordered
cancellation. The average difference is about 4 db, which corresponds to a raw
spectrally efficiency of around 10 bps/Hz.
Fig. 7.4 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 this case, the transmit power was adjusted so that large received SNR was 24+/-
0.5 db. Nulling with optimized cancellation was used.
It can be seen that at this spectral 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 40
bps/Hz have been demonstrated at similar error rates, though with less robust
performance.
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FIG. 7.3 Single position performance
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Fig. 7.4 Multi position performance
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CHAPTER: 8
BLAST IN THE REAL WORLD
Two familiar factors are there for the success of BLAST: technology and
economics. On 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. That is large spectral efficiencies are more
practical. Lets take an example. If you want 26 bps/Hz with a 23% roll off, you
need to have (26*1.23) = 32 bits/symbol. A scalar system would require 232
points, which is around 4 billion. No wireless system will put up 4 billion
transmitters ever. This means that the vector 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 receivers. The cell
tower will have to go from a switched-beam approach to a steered beam
configuration. On the plus side, much of this development can be gradual. Older
"diversity" antennas will most likely be retained as a fallback for the worst-case
channel environments (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 to
four transmit paths 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 reverselinks. Carriers are very sensitive to the
costs, however incremental, of deploying new systems.
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BLAST TECHNOLOGY 21
CHAPTER: 9
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
users data, even it is sub streams. Well, unlike code-division or a speed spectrum
approach, the total bandwidth those QAM systems require. Unlike a Frequency
Division Multiple Access (FDMA) approach, each transmitted signals occupies
the 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 system does not impose orthogonalisation of
transmitted signals. The reason for this is simple, obvious and rather elegant. The
propagation environment of the real world provides significant latencies. BLAST
exploits them to provide the signal dcor 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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CHAPTER: 10
ADVANTAGES
Since the entire sub streams are transmitted in the same frequency band, the
spectrum is used efficiently. Spectral 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. Further more a
constellation with such density of points would require in excess of 100 db
operating at any reasonable error rate.
A critical feature of BLAST is that the total radiated power remains constant
irrespective of the number of transmitting antennas. Hence there is no increase in
the amount of interference caused ton users.
The BLAST technology has reportedly delivered a data reception at 19.2 Mbps on
a 3G network. With BLAST down loading a song would take 3s, and HDTV canbe watched on a telephone.
This innovation known as BLAST may allow so called fixed wireless
technology to rival the capabilities of todays wired networks would connect
homes and business to copper-wired public telephone service providers.
DRAW BACKS
The BLAST technology is not suited for mobile wireless applications, such as
hand held and car based cellular phones since multiple antennas at both
transmitter and receiver are needed. In addition, tracking signal changes in mobile
applications would increase the computational complexity. It would require
manufacture to invest in the development of new multi antenna devices. It would
require new wireless network infrastructure.
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CHAPTER: 11
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 of course scattering will be less favourable than the independent
Raleighs assumption and 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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