BLAST by kishorkna


IT Project Topics, Engineering Project Topics, IEEE Project Topics, Project Topics, Project

More Info


 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.


 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.


 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.

 LIFTING         THE LIMITS                  WITH    TRANSMIT            AND

 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 distructively, 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 Rauleigh
 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. Raylegh scattering can be considered to be elastic
 scattering because the energies of scattered photons do not change.

 An overview of radiated power

                                                       Scattering at right angles is
                                                       half the forward intensity
                                                       for Rayleigh scattering
       Rayleigh scattering
       from air molecules

                                              N= No. of Scatters
                                              = Polarizability
                                              R=Distance from scatter

                             Observer     The strong wavelength depend upon
                                          Rayleigh scattering enhances the short
   Fig. 1
                                          wave engines giving as the blue sky.

 The researchers found 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

 The BLAST researchers, however, theorized it is possible to have several
 transmissions occupying the same frequency band. Each transmission uses its
 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.

 “This 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. “This
 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 Shanon’s 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

 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



                                                  RX            signal
             VECTOR                                           processing
            ENCODER                               RX


 V-BLAST takes single data stream and demultiplexes it in to msubstreams.
 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. Each transmitter 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 organised 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.


 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 sub
 stream 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 components of
 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 ‘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 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.


 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    26bps/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 4billion. 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 “diversity” 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 1хRTT
 rollout has shown. Asked       about its plans for BLAST, Verizon’s reps
 indicated that the discussion was premature, but that they 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.


 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 speed-
 spectrum 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
 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.


 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.

      Probability (System Capacity C)

                                        0.7   Transmit
                                              diversity using
                                              a single
                                        0.6   antenna
                                        0.5                                               M antennas per
                                        0.4                          4
                                        0.3                              ¼
                                                                10                  100           1000
                                                                M bits /section
                                                                         Fig 4

 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
 a 3G 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 “fixed” wireless
 technology to rival the capabilities of today’s wired networks would connect
 homes and businesses to copper-wired public telephone service providers.


 The BLAST technology is not is not well suited for mobile wireless
 applications, such as hand-held and car-based cellular phones multiple
 antennas—both transmitting and receiving—are needed. In addition, tracking
 signal changes in mobile applications would increase the computational

 It would require manufacture to invest in the development of new multi-
 antenna devices. It would also require new wireless network infrastructure.


 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 attempt was to “tune” the
 system to the environment, or to modify the environment in anyway.

 The antenna arrays consisted of λ/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 λ/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 were each 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 “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, i.e.
 4bits/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 20.7bps/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 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.

 Single-position performance


                                          SNR (dB)

 Multiple-Position Performance

                               BLER and BER in 24 dB SNR vs position
        BLER /BER 2.1 dB SNR

                                                            * BLER

                                          Position Number


 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


 1. IEEE Communication Magazine. September 2001
 3. theory


 BLAST is a wireless communications technique which uses multi-element
 antennas at both transmitter and receiver to permit transmission rates far in
 excess of those possible using conventional approaches.

 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 known as multipath, as it results in multiple
 copies (“images”) of the transmitted sign arriving at the receiver via different
 scattered paths. In conventional wireless system multipath represents a
 significant impediment to accurate transmission, because the image 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
 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.


 I extend my sincere thanks to Prof. P.V.Abdul Hameed, Head of the
 Department for providing me with the guidance and facilities for the

 I express my sincere gratitude to Seminar coordinator Mr. Berly C.J, Staff
 in charge, for their cooperation and guidance for preparing and presenting
 this seminar.

 I also extend my sincere thanks to all other faculty members of Electronics
 and Communication Department and my friends for their support and

                                                              BILAL ABDU


    INTRODUCTION                          01


    SPACE: THE LAST FRONTIER              03


    OVERVIEW OF BLAST SYSTEM              08

    BLAST’S SIGNAL DETECTION              09

    BLAST IN THE REAL WORLD               11

    BLAST vs. EXISTING SYSTEMS            12

    ADVANTAGES                            13

    DRAWBACKS                             15

    LABORATARY RESULS                     15

    CONCLUSION                            19

    REFERENCES                            20

To top