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Vblast Presentation Transcript

  • 1. V-BLAST R . Rahul Sekhar
  • 2. THE BIG QUESTION With limited power, scarce and highly precious bandwidth, how to increase the data rate?
  • 3. BLAST ARCHITECTURE Rich-scattering wireless channel is capable of enormous theoretical capacities if the multipath is properly exploited. A novel method used for this is using BLAST architecture Three specific implementations of BLAST, depending on the type of coding employed:1. Diagonal-BLAST (D-BLAST)2. Vertical-BLAST (V-BLAST)3. Turbo-BLAST
  • 4. WHY BLAST? Unlike code division or other spread-spectrum multiple access techniques, the total channel bandwidth utilized in a BLAST system is only a small fraction in excess of the symbol rate. Unlike FDMA, each transmitted signal occupies the entire system bandwidth. Finally, unlike TDMA, the entire system bandwidth is used simultaneously by all of the transmitters all of the time. Taken together, these differences together are precisely what give BLAST the potential to realize higher spectral efficiencies than the multiple-access techniques. An essential feature of BLAST is that no explicit orthogonalization of the transmitted signals is imposed by the transmit structure at all. Instead, the propagation environment itself, is exploited to achieve the signal decorrelation necessary to separate the co-channel signals.
  • 5. D-BLAST It utilizes multi-element antenna arrays at both transmitter and receiver Diagonally layered coding structure in which code blocks are dispersed across diagonals in space time In a Rayleigh scattering environment, this structure leads to theoretical rates which grow linearly with the number of antennas(~90% of Shannon capacity)
  • 6. DIAGONAL LAYERING
  • 7. D BLAST
  • 8. V-BLAST Difference from D-Blast? V-BLAST architecture is a simplified version of D-BLAST, that tries to reduce its computational complexity. The layering is horizontal, meaning that all the symbols of a certain stream are transmitted through the same antenna (one stream per antenna). It eliminates the space time wastage, but loses the transmit diversity, since each stream is “tied” to its antenna.
  • 9. APPLICATIONS V-BLAST is an essential part of MIMO technology. As such it is an integral part of modern wireless communication standards such as IEEE 802.11n (Wi-Fi), 4G, 3GPP Long Term Evolution, WiMAX and HSPA+.
  • 10. SYSTEM OVERVIEW A single data stream is demultiplexed into M sub streams. Each sub stream is then encoded into symbols and fed to its respective transmitter. Transmitters 1 − M operate co-channel at symbol rate 1/ T symbols/sec. Each transmitter is itself an ordinary QAM transmitter. The same constellation is used for each substream.
  • 11.  Receivers 1 − N are, individually, conventional QAM receivers. These receivers also operate co-channel, each receiving the signals radiated from all M transmit antennas. Flat fading is assumed. The matrix channel transfer function is HN×M, where hi j is the (complex) transfer function from transmitter j to receiver i, and M ≤ N.
  • 12. V-BLAST DETECTION Let a = (a1 , a2 , . . . ,aM ) T denote the vector of transmit symbols. Then the corresponding received N vector is r1 = Ha + ν where ν is a noise vector. Each substream in turn is considered to be the desired signal, and the remainder are considered as "interferers".(Nulling) Nulling is performed by linearly weighting the received signals so as to satisfy some performance-related criterion, such as minimum mean-squared error (MMSE) or zero-forcing (ZF). Zero-forcing Nulling can be performed by choosing weight vectors wi , i = 1 , 2 , . . . , M, such that wi T(H) j = δi j where (H) j is the jth column of H, and δ is the Kronecker delta. Thus, the decision statistic for the ith sub stream is yi = wi T ri
  • 13. project thereceived signal yonto thesubspaceorthogonal to theone spanned byh1, h2.......hnt
  • 14.  Superior performance is obtained if nonlinear techniques are used. Use symbol cancellation as well as linear nulling to perform detection. Interference from already-detected components of a is subtracted out from the received signal vector, resulting in modified received vector in which, effectively, fewer interferers are present.
  • 15. 1. Order determination, in which the N, received substreams are to be detected, in accordance with the post detection signal-to-noise ratios of the individual sub streams.2. Detection of the sub stream, starting with the largest signal-to- noise ratio.3. Signal cancellation, wherein the effect of the detected sub stream is removed from subsequent sub streams.4. Repetition of steps 1 through 3 until all the N, received sub streams have been individually detected
  • 16. (V-BLAST) DECODING Initialization: Recursion: i 1 wki (Gi ) ki G1 H H 2 y ki wki ri k1 arg min (G1 ) j j ˆ a ki Q ( y ki ) ri 1 ri ˆ a ki ( H ) ki G H (H H H ) 1 H H Gi 1 H ki G (H H H 2 I) 1H H 2 ki 1 arg min (Gi 1 ) j j k1ki i i 1
  • 17. REFERENCES V-BLAST: An Architecture for Realizing Very High Data Rates Over the Rich-Scattering Wireless Channel P. W. Wolniansky, G. J. Foschini, G. D. Golden, R. A. Valenzuela Modern wireless communication Simon Haykin , Michael Moher BLAST Architectures Eduardo Zacar´ıas B. Fundamentals of wireless communication David Tse , Pramod Performance Analysis of V-BLAST Detectors for the MIMO channel Fenghua Li