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.
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.
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 everybodyâ„¢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.
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.
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 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
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 communications.
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 todayâ„¢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 Shannonâ„¢s Information Theory, first published in 1948. while Shanonâ„¢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 impaiments.
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
OVERVIEW OF BLAST SYSTEM
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 userâ„¢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. BLASTâ„¢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.
BLASTâ„¢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,Â¦. 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 Ëœaâ„¢ 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-BLASTâ„¢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.
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. Letâ„¢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 Verizonâ„¢s recent 2.5G 1?RTT rollout has shown. Asked about its plans for BLAST, Verizonâ„¢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 companyâ„¢s long range planning.
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 userâ„¢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 â€œis only on the order of 500kb/s with no transmit diversity and just over 1Mb/s there-with.
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 todayâ„¢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 antennasâ€both transmitting and receivingâ€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 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 ?/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 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.
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 Raleighâ„¢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.
1. IEEE Communication Magazine. September 2001
3. http://www.lucent.com/information theory
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 Seminar.
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 seminars.
I also extend my sincere thanks to all other faculty members of Electronics and Communication Department and my friends for their support and encouragement.
Â¢ INTRODUCTION 01
Â¢ FUNDAMENTAL LIMITATIONS IN WIRELESS 02
Â¢ SPACE: THE LAST FRONTIER 03
Â¢ LIFTING THE LIMITS WITH TRANMIT AND 03
Â¢ OVERVIEW OF BLAST SYSTEM 08
Â¢ BLASTâ„¢S SIGNAL DETECTION 09
Â¢ BLAST IN THE REAL WORLD 11
Â¢ BLAST vs. EXISTING SYSTEMS 12
Â¢ ADVANTAGES 13
Â¢ DRAWBACKS 15
Â¢ LABORATARY RESULS 15
Â¢ CONCLUSION 19
Â¢ REFERENCES 20