V-BLAST is a technique that uses multiple antennas at the transmitter and receiver to increase data rates over wireless channels. It stands for Vertical Bell Laboratories Layered Space Time. V-BLAST is a simplified version of D-BLAST that reduces computational complexity by transmitting each data stream through a single antenna (horizontally layered). At the receiver, signals are detected by first nulling out interference from other streams through linear weighting, then detecting and canceling streams starting from the highest SNR stream. V-BLAST is used in wireless communication standards like 802.11n, 4G, LTE, and WiMAX to implement MIMO technology.

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.

13. 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

14. project the
received signal y
onto the
subspace
orthogonal to the
one spanned by
h1, h2.......hnt

15.  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.

16. 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

18. (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

20. 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