What is BLAST?
BLAST stands for Bell Laboratories Layered Space-Time
BLAST is an extraordinarily bandwidth-efficient approach to wireless communication which takes advantage of the spatial dimension by transmitting and detecting a number of independent co-channel data streams using multiple, essentially co-located, antennas.
The central paradigm behind BLAST is the exploitation, rather than the mitigation, of multipath effects in order to achieve very high spectral efficiencies (bits/sec/Hz), significantly higher than are possible when multipath is viewed as an adversary rather than an ally.
The BLAST team recently demonstrated an unprecedented wireless spectral efficiencies, ranging from 20 - 40 bps/Hz. By comparison, the efficiencies achieved using traditional wireless modulation techniques range from around 1 - 5 bps/Hz (mobile cellular) to around 10 - 12 bps/Hz (point-to-point fixed microwave systems). In the 30 kHz bandwidth utilized by our research testbed, the raw spectral efficiencies realized thus far in the lab correspond to payload data rates ranging from roughly 0.5 Mb/s to 1 Mb/s. By contrast, the data rate achievable in this bandwidth using typical traditional methods is only about 50 kbps.
BLAST High-Level Overview
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 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, cancelling 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 subchannels.
BLAST accomplishes this by splitting a single user's data stream into multiple substreams and using an array of transmitter antennas to simultaneously launch the parallel substreams (see Figure). All the substreams are transmitted in the same frequency band, so spectrum is used very efficiently. Since the user's data is being sent in parallel over multiple antennas, the effective transmission rate is increased in roughly in proportion to the number of transmitter antennas used.
At the receiver, an array of antennas is again used to pick up the multiple transmitted substreams and their scattered images. Each receive antenna "sees" all of the transmitted substreams superimposed, not separately. However, if the multipath scattering is sufficient, then the multiple substreams are all scattered slightly differently, since they originate from different transmit antennas that are located at different points in space. Using sophisticated signal processing, these slight differences in scattering allow the substreams to be identified and recovered. In effect, the unavoidable multipath is exploited to provide a very 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 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 substream 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 substreams depends on the slight differences in the way the different substreams propagate through the environment.
Under the widely used theoretical assumption of independent Rayleigh scattering, the 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 favorable than the independent Rayleigh assumption, and it remains to be seen how much capacity is actually available in various propagation environments. Nevertheless, even in relatively poor scattering environments, BLAST should be able to provide significantly higher capacities than conventional architectures. Our laboratory prototype has already demonstrated spectral efficiencies of 20 - 40 bits per second per Hertz of bandwidth, numbers which are simply unattainable using standard techniques.
BLAST stands for Bell Laboratories Layered Space-Time
BLAST is an extraordinarily bandwidth-efficient approach to wireless communication which takes advantage of the spatial dimension by transmitting and detecting a number of independent co-channel data streams using multiple, essentially co-located, antennas.
The central paradigm behind BLAST is the exploitation, rather than the mitigation, of multipath effects in order to achieve very high spectral efficiencies (bits/sec/Hz), significantly higher than are possible when multipath is viewed as an adversary rather than an ally.
The BLAST team recently demonstrated an unprecedented wireless spectral efficiencies, ranging from 20 - 40 bps/Hz. By comparison, the efficiencies achieved using traditional wireless modulation techniques range from around 1 - 5 bps/Hz (mobile cellular) to around 10 - 12 bps/Hz (point-to-point fixed microwave systems). In the 30 kHz bandwidth utilized by our research testbed, the raw spectral efficiencies realized thus far in the lab correspond to payload data rates ranging from roughly 0.5 Mb/s to 1 Mb/s. By contrast, the data rate achievable in this bandwidth using typical traditional methods is only about 50 kbps.
BLAST High-Level Overview
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 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, cancelling 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 subchannels.
BLAST accomplishes this by splitting a single user's data stream into multiple substreams and using an array of transmitter antennas to simultaneously launch the parallel substreams (see Figure). All the substreams are transmitted in the same frequency band, so spectrum is used very efficiently. Since the user's data is being sent in parallel over multiple antennas, the effective transmission rate is increased in roughly in proportion to the number of transmitter antennas used.
At the receiver, an array of antennas is again used to pick up the multiple transmitted substreams and their scattered images. Each receive antenna "sees" all of the transmitted substreams superimposed, not separately. However, if the multipath scattering is sufficient, then the multiple substreams are all scattered slightly differently, since they originate from different transmit antennas that are located at different points in space. Using sophisticated signal processing, these slight differences in scattering allow the substreams to be identified and recovered. In effect, the unavoidable multipath is exploited to provide a very 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 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 substream 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 substreams depends on the slight differences in the way the different substreams propagate through the environment.
Under the widely used theoretical assumption of independent Rayleigh scattering, the 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 favorable than the independent Rayleigh assumption, and it remains to be seen how much capacity is actually available in various propagation environments. Nevertheless, even in relatively poor scattering environments, BLAST should be able to provide significantly higher capacities than conventional architectures. Our laboratory prototype has already demonstrated spectral efficiencies of 20 - 40 bits per second per Hertz of bandwidth, numbers which are simply unattainable using standard techniques.
History of MIMO
1975,1976
A.R. Kaye and D.A. George and W. van van Etten
created earliest ideas
1984,1986
Jack Winters and Jack Salz
published several papers on beamforming
1993
Arogyaswami Paulraj and Thomas Kailath
proposed the concept of Spatial Multiplexing using MIMO
1994
Patent No. 5,345,599 issued 1994 on Spatial Multiplexing
1996
Greg Raleigh and Gerard J. Foschini refine new approaches to MIMO
technology
1998
Bell Labs was the first to demonstrate a laboratory prototype of SM
2006
MIMO-OFDMA based solutions for IEEE 802.16e WIMAX broadband mobile
standard.
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