blast technology

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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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BLAST TECHNOLOGY 7

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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BLAST TECHNOLOGY 10

CHAPTER: 5

MOST CURRENT ITERATION-V BLAST

Fig 5.1 Block diagram of V BLAST


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BLAST TECHNOLOGY 11

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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BLAST TECHNOLOGY 12

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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BLAST TECHNOLOGY 13

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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BLAST TECHNOLOGY 16

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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BLAST TECHNOLOGY 19

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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blast technology

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