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A REPORT OF V-BLAST TECHINIQUE UTILIZATIONFOR DETECTING SIGNAL UNDER RICH SCATTERING

ENVIRONMENT

I. Introduction why using V-BLAST?Multipath is a phenomenon in wireless communication that causesIntersymbol Interference (ISI). A new idea have been investigate fora long time is that we employ it instead of mitigating it. This idea hada base on MIMO wireless channel while there are many paths totransmit data signal between M transmitters and N receivers.In this system, channel is complex and all channels contribute achannel matrix:

[ ] Where is the a complex path between transmitter j to

receiver iNote that, for simplicity, the channel we only consider here is thetime-invariant channel.The problem is how to detect received symbols under ISI in additionwith noise in MIMO channel. One approach is utilizing V-BLAST. Thistechnique brings out spectral efficiency of 20 40 bps/Hz under rich-scattering wireless channel. This result is unattainable usingtraditional techniques.

II. System Overview

VBLAST is a detection technique based on BLAST (Bell LaboratoriesLayered Space-Time) architecture and the older version of V-BLASTis D-BLAST (Diagonally-Layered Space Time Architecture). D-BLASTutilizes multi-element antenna arrays at both transmitter andreceiver and an elegant diagonally-layered coding structure in which

code blocks are dispersed across diagonals in space-time. In anindependent Rayleigh scattering environment, this processing

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structure leads to theoretical rates which grow linearly with thenumber of antennas (assuming equal numbers of transmit andreceive antennas) with these rates approaching 90% of Shannoncapacity.However, the diagonal approach suffers from certain implementationcomplexities which make it inappropriate for initial implementation.Therefore, in this report we describe a simplified version of BLASTknown as vertical BLAST or V-BLAST.

A high-level block diagram of a BLAST system is shown in this figurebelow.

Figure 1: V-BLAST high level system diagram

A single data stream is demultiplexed into M substreams, and eachsubstream is then encoded into symbols and fed to its respective

transmitter. (The encoding process is discussed in more detailbelow). Transmitter 1 M operate co-channel at symbol rate 1/Tsymbols/sec, with synchronized symbol timing. Each transmitter isitself an ordinary QAM transmitter. The collection of transmitterscomprises a vector-valued transmitter, where components of eachtransmitter M-vector are symbols drawn from a QAM constellation.We assume that the same constellation is used for each substreamand that the transmission are organized into burst of L symbols. Thepower launched by each transmitter is proportional to 1/M so thatthe total radiated power is constant and independent of M

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Receivers 1 N are, individually, conventional QAM receivers. Thesereceivers also operate co-channel, each receiving the signals radiatedfrom all M transmit antennas.

For simplicity, flat fading is assume and matrix channel between Mtransmitters and N receivers is defined

III. VBLAST Detection

We take a discrete-time baseband view of detection process for asingle transmitted vector symbol, assuming symbol-synchronousreceiver sampling and ideal timing.

Let transmit symbol vector is:

a =

The received N-vector is:

Where H is channel matrix ; v is n=Gauss noise vector

In general, we can employ some technique to perform detection ofthe receive symbol vector such as Maximum Likelihood decoding (ML),Zero-Forcing algorithm (ZF) and Minimum Mean Square Error algorithm(MMSE)

For considering here, we use ZF-VBLAST algorithm to detect the receivedsignal. ZF-VBLAST based on ZF criterions those are:

The information about channel is exactly know at receiver Linear combinatorial nulling

Conceptually, in linear combination nulling, each substream in turn isconsidered to be the desired signal, and the remainder are considered as

interf erers.Nulling is perform by linearly weighting the received signals so as to satisfyzero-forcing criterion:

if i j other ise Decision statistic of the i-th substream is:

1

2)

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[ ][] [hhh]

[ ] [][] Assumed here is detected accurately.

This me ns th t the c: [ ]

The defl ted versioH is helpful in eliminate the interference effect ofdetected component of a and reduces constrained condition in choosingnulling vector. In other words, minimize the complexity of overall systemperformance.

Step 1 3 are then performs for components by operating inturn on the progession of modified received vectors The specifically of ZF-VBLAST is we have to choose the nulling vector tosatisfy:

The unique nulling vector satisfying (6) is the -th row of , where thenotation denotes the matrix obtained by zeroing column ofH. This zeroing is perform in step3: symbol cancellation. denotes theMoore-Penrose pseudoinverse of H.

6)

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Because [ ] is MIMO channel matrix, has M

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An approach for obtaining the optimum ordering is at each stage, pick thesymbol that provides the maximum SNR. The sequence of locally optimizedsymbols is identical to the globally optimized sequence. Therefore theoptimum ordering set

S is obtain

The full ZF VBLAST detection algorithm can be described as a recursiveprocedure, including determination of the optimal ordering, as follow:

Intialization:

i 1

(8b)

k rgminj

c

Recursion

d y e (y) f

g

(8h)

k rgminj k k i i i j Where: is the j-th row of

(8c),(8i) determine the elements of optimum ordering set

S

(8d-f) compute respectively the ZF-nulling vector, decisionstatistic, and the estimated component of a

(8g) performs cancellation of the detected component from thereceived vector

(8h) computes the new pseudoinverse is based on a deflatedversion of H, in which column

k k k have been zeroed.