25-04-2014, 02:06 PM
Blast Technology
[attachment=62495]
ABSTRACT
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.
INTRODUCTION
The explosive growth of both the wireless industry and the Internet is creating a huge market opportunity for wireless data access. Limited internet access, at very low speeds, is already available as an enhancement to some existing cellular systems. However those systems were designed with purpose of providing voice services and at most short messaging, but not fast data transfer. Traditional wireless technologies are not very well suited to meet the demanding requirements of providing very high data rates with the ubiquity, mobility and portability characteristics of cellular systems. Increased use of antenna arrays appears to be the only means of enabling the type of data rates and capacities needed for wireless internet and multimedia services. While the deployment of base station arrays is becoming universal it is really the simultaneous deployment of base station and terminal arrays that can unleash unprecedented levels of performance by opening up multiple spatial signaling dimensions .Theoretically, user data rates as high as 2 Mb/sec will be supported in certain environments, although recent studies have shown that approaching those might only be feasible under extremely favorable conditions-in the vicinity of the base station and with no other users competing for band width. Some fundamental barriers related to the nature of radio channel as well as to the limited band width availability at the frequencies of interest stand in the way of high data rates and low cost associated with wide access.
FUNDAMENTAL LIMITATIONS IN WIRELESS DATA ACESS
Ever since the dawn of information age, capacity has been the principal metric used to asses the value of a communication system. Since the existing cellular system were devised almost exclusively for telephony, user data rates low .Infact the user data were reduced to the minimum level and traded for additional users. The value of a system is no longer defined only by how many users it can support, but also by its ability to provide high peak rates to individual users. Thus in the age of wireless data, user data rates surges as an important metric.
Trying to increase the data rates by simply transmitting more; Power is extremely costly. Furthermore it is futile in the contest of wherein an increase in everybody’s transmit power scales up both the desired signals as well as their mutual interference yielding no net benefit.
Increasing signal bandwidth along with the power is a more effective way of augmenting the data rate. However radio spectrum is a scarce and very expensive resource.Moreover increasing the signal bandwidth beyond the coherent bandwidth of the wireless channel results in frequency selectively. Although well-established technique such as equalization and OFDM can address this issue, their complexity grows with the signal bandwidth. Spectral efficiency defined as the capacity per unit bandwidth has become another key metric by which wireless systems are measured. In the contest of FDMA and TDMA, the evolutionary path has led to advanced forms of dynamic channel assessment that enable adaptive and more aggressive frequency reuse.In the context of multi-user detection and interference cancellation techniques.
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.
BLAST’S SIGNAL DETECTION
At the receiver, an array of antennas is again used to pick up the multiple transmitted sub streams and their scattered images. Each receiver antenna sees the entire transmitted sub streams super imposed, not separately. However, if the multipath scattering is sufficient is sufficient, then the multiple sub streams are located at different points in space .Using sophisticated signal processing, these slight difference in scattering allow the sub streams to be identified and recovered. In effect the unavoidable multipath is exploited to provide a useful spatial parallelism that is used to greatly improve data transmission rates. Thus when using the BLAST technique, the more multipath, the better, just the opposite of the conventional systems.
The blast signal processing algorithms used at the receiver are the heart of the technique. At the bank of receiving antennas, high speed signal processors look at the signals from all the receiver antennas simultaneously, first extracting the strongest signal have been removed as a source of interference. Again the ability to separate the sub streams depends on the slight differences in the way the different sub streams propagate through the environment.
BLAST IN THE REAL WORLD
Two familiar factors are essential to the success of a BLAST: technology and economics. On the technology side, scalar systems (those currently in use) are far less spectrally efficient than BLAST ones. They can encode B bits per symbols using a single constellation of 2B points. Vector systems can realize the same rate using M constellation of 2B/M points each. Large spectral efficiencies (that is, a large B) are more practical. Let’s take an example. If you want 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.
[attachment=62495]
ABSTRACT
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.
INTRODUCTION
The explosive growth of both the wireless industry and the Internet is creating a huge market opportunity for wireless data access. Limited internet access, at very low speeds, is already available as an enhancement to some existing cellular systems. However those systems were designed with purpose of providing voice services and at most short messaging, but not fast data transfer. Traditional wireless technologies are not very well suited to meet the demanding requirements of providing very high data rates with the ubiquity, mobility and portability characteristics of cellular systems. Increased use of antenna arrays appears to be the only means of enabling the type of data rates and capacities needed for wireless internet and multimedia services. While the deployment of base station arrays is becoming universal it is really the simultaneous deployment of base station and terminal arrays that can unleash unprecedented levels of performance by opening up multiple spatial signaling dimensions .Theoretically, user data rates as high as 2 Mb/sec will be supported in certain environments, although recent studies have shown that approaching those might only be feasible under extremely favorable conditions-in the vicinity of the base station and with no other users competing for band width. Some fundamental barriers related to the nature of radio channel as well as to the limited band width availability at the frequencies of interest stand in the way of high data rates and low cost associated with wide access.
FUNDAMENTAL LIMITATIONS IN WIRELESS DATA ACESS
Ever since the dawn of information age, capacity has been the principal metric used to asses the value of a communication system. Since the existing cellular system were devised almost exclusively for telephony, user data rates low .Infact the user data were reduced to the minimum level and traded for additional users. The value of a system is no longer defined only by how many users it can support, but also by its ability to provide high peak rates to individual users. Thus in the age of wireless data, user data rates surges as an important metric.
Trying to increase the data rates by simply transmitting more; Power is extremely costly. Furthermore it is futile in the contest of wherein an increase in everybody’s transmit power scales up both the desired signals as well as their mutual interference yielding no net benefit.
Increasing signal bandwidth along with the power is a more effective way of augmenting the data rate. However radio spectrum is a scarce and very expensive resource.Moreover increasing the signal bandwidth beyond the coherent bandwidth of the wireless channel results in frequency selectively. Although well-established technique such as equalization and OFDM can address this issue, their complexity grows with the signal bandwidth. Spectral efficiency defined as the capacity per unit bandwidth has become another key metric by which wireless systems are measured. In the contest of FDMA and TDMA, the evolutionary path has led to advanced forms of dynamic channel assessment that enable adaptive and more aggressive frequency reuse.In the context of multi-user detection and interference cancellation techniques.
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.
BLAST’S SIGNAL DETECTION
At the receiver, an array of antennas is again used to pick up the multiple transmitted sub streams and their scattered images. Each receiver antenna sees the entire transmitted sub streams super imposed, not separately. However, if the multipath scattering is sufficient is sufficient, then the multiple sub streams are located at different points in space .Using sophisticated signal processing, these slight difference in scattering allow the sub streams to be identified and recovered. In effect the unavoidable multipath is exploited to provide a useful spatial parallelism that is used to greatly improve data transmission rates. Thus when using the BLAST technique, the more multipath, the better, just the opposite of the conventional systems.
The blast signal processing algorithms used at the receiver are the heart of the technique. At the bank of receiving antennas, high speed signal processors look at the signals from all the receiver antennas simultaneously, first extracting the strongest signal have been removed as a source of interference. Again the ability to separate the sub streams depends on the slight differences in the way the different sub streams propagate through the environment.
BLAST IN THE REAL WORLD
Two familiar factors are essential to the success of a BLAST: technology and economics. On the technology side, scalar systems (those currently in use) are far less spectrally efficient than BLAST ones. They can encode B bits per symbols using a single constellation of 2B points. Vector systems can realize the same rate using M constellation of 2B/M points each. Large spectral efficiencies (that is, a large B) are more practical. Let’s take an example. If you want 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.