V-BLAST

R . Rahul Sekhar

THE BIG QUESTION

With limited power, scarce and highly precious bandwidth, how to increase the data rate?

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

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.

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)

DIAGONAL LAYERING

D BLAST

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.

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+.

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.

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.

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

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

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.

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

(V-BLAST) DECODING

Initialization: Recursion:

1iHG1

2

11 )(minarg jj

Gk

ii kik Gw )(

i

H

kk rwyii

)(ˆii kk yQa

ii kkii Harr )(ˆ1

iki HG 1

2

11 )(minarg1

jikkj

i Gki

1 ii

HH HHHHG 1)(

HH HIHHG 12 )(

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 ArchitecturesEduardo Zacar´ıas B.

Fundamentals of wireless communicationDavid Tse , Pramod

Performance Analysis of V-BLAST Detectors for the MIMO channelFenghua Li