An Extensive Review on STBC for MIMO OFDM System

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459,ISO 9001:2008 Certified Journal, Volume 4, Issue 8, August 2014)

352

An Extensive Review on STBC for MIMO OFDM System

Gaurav Maurya1, Prof. Pramod Patel 2

1

Master of Engg. Research Scholar, 2Research Guide, TIT College Bhopal

Abstract-- Space-Time Block Coding (STBC) with Multiple-Input Multiple-Output (MIMO) set-up proves to be an efficient method for better BER performance and efficient PAPR. In this review paper we have study the different Methodology for achieving the better BER performance. Finally compare the performance of the STBC-OFDM systems for various systems. The system of using multiple antennas at both transmitter (TX) and one receiver (RX) is, referred to as multiple antennas is called Multiple-Input Multiple-Output (MIMO) system. One of the major advantages of these systems is the substantial increase in the channel capacity, which immediately translates to higher data throughput. Another advantage of MIMO systems is the significant improvement in data transmission reliability, i.e., very low bit error rate (BER). These compensation are achievable without any expansion in the required bandwidth or increase in the transmit power.

Index Term- STBC, OFDM, MIMO, 64-QAM, SLM

I. INTRODUCTION

Orthogonal Frequency Division Multiplexing (OFDM) has become the popular modulation technique in high speed wireless communications. It is more advantageous comparatively to the other technologies. In spite of its advantages it has some obstacles also. The high peak-to- average ratio (PAPR) is the main obstacle which causes non-linearity at the receiving end. The PAPR in the OFDM system, its effect and name some techniques which can be used to reduce the PAPR according to the requirement. New techniques for digital transmission have developed to meet the increasing demand for higher data rates in communications which can be used in both wired and wireless environments. To meet out the high spectral efficiency and high data rate, an efficient modulation scheme is to be employed. A promising modulation technique that is increasingly being adopted in the telecommunication field is Orthogonal Frequency Division Multiplexing (OFDM). Orthogonal Frequency Division Multiplexing (OFDM) is a Multi-Carrier Modulation technique in which a single high rate data-stream is divided into multiple low rate data-streams and is modulated using sub- carriers which are orthogonal each other. OFDM is a ―Multi-Carrier Transmission Scheme‖ ODFM is a good solution for high speed digital communications.

In this the data to be transmitted is separated over a large number of orthogonal carriers, each being modulated at a low rate. The carriers can be made orthogonal by appropriately choosing the frequency spacing among them. But with these benefits there are some issues in using OFDM:

(i) OFDM signal has very high Peak to Average Power Ratio (PAPR)

(ii) Inter carrier Interference between the subcarriers can cause a big problem in the system.

The problem of high PAPR associated in OFDM. We will discuss what is PAPR, how it causes problem in existing OFDM & its effect and give a review of several techniques for sorting out this problem.

II. MIMO-OFDMTECHNIQUE OVER FREQUENCY -SELECTIVE CHANNELS

A schematic of a MIMO-OFDM system utilizing STBC with two transmit and two receive antennas is shown in Figure 1. The transmitter of the system is identical to the MISO-OFDM employing STBC. Assuming that the channel characteristics remain constant over the period of two transmission bursts, the received signals are given by in (1), (2), (3) and (4).

{ ( ) ( ) ( ) ( ) ( )} (1)

{ ( ) ( ) ( ) ( ) ( )} (2)

{ ( ) ( ) ( ) ( ) ( )} (3)

{ ( ) ( ) ( ) ( ) ( )} (4)

Where ( ) are the discrete-time filter representations of the channel from the j-th transmit antenna to the i-th the receive antenna. The STBC decoder at the receiver antenna will estimate the transmitted symbols at the k-th subcarrier frequency using the decoding equations given by [5, 6]

̃ ( ) ( ) ̂( ) ( ) ̂ ( ) ( ) ̂( )

( ) ̂ ( ) (5)

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Where ( ) are the frequency response values of the channel from the j-th transmit antenna to the i-th the receive antenna for the k-th

subcarrier, and ̂( ) are the output of FFT blocks at the k-th subcarrier.

Figure1. Schematic of a 2×2MIMO-OFDM System Utilizing STBC

( ) [ , ,

,

-] ( ) ( ) [ ,

,

-]

( ) [

, ,

,

-] ( ) ( ) [

, ,

,

-]

̃( ) [ ̃, ̃,

̃,

-] ̃( ) [ ̃, ̃,

̃, -]

PAPR

The PAPR is the relation between the maximum power of a sample in a given OFDM transmit symbol divided by the average power of that OFDM symbol. PAPR occurs when in a multi- carrier system the different sub-carriers are out of phase with each other. This ratio of the peak to average power value is termed as Peak-to- Average Power Ratio.

OFDM System- Fig. show a block diagram of basic OFDM system model. The input data symbol are first passed through serial to parallel convertor forming a complex vector size N. we call the vector as

, - .After IFFT transform the signal ( ) can be written as

( ) ∑N- _KX K

X

x

I/P

O/P

Figure. 2.1 Block diagram of OFDM System

=1/NT, is the OFDM symbol duration and t is the discrete time. The PAPR of the transmit signal can be expressed mathematically. The root mean square (RMS) magnitude of the OFDM signal is defined as the root of the time average of the power P, where P is defined by.

∫ | ( )| (1)

∑ | ( )|

(2)

The value p in this case corresponds to a single OFDM symbol and depends on the sequence of information carrying coefficients X(k). the average power of OFDM symbols can be written as * +.where E[.] denotes the expected value. Thus, the PAPR of an OFDM signal can be defined as

| ( )|

( )

STBC Encoder

( ) ( )

IFFT Cyclic Prefix

Addition

( ) ( )

IFFT Cyclic Prefix

Addition

FFT Cyclic Prefix

Removal

FFT Cyclic Prefix

Removal

STBC Decoder

Channel Estimation Information 1

2

1

2

( ) ( )

( )

̂( )

̂( ) ̂

̂

̃ ( ) ̃ ( )

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| ( )|

| ( )| ( )

If the input data is normalized, then [| ( )| ]

and we get

| ( )| ( )

It is clearly show that maximum PAPR is equal to the number of subscriber.

Orthogonal frequency division multiplexing (OFDM) is a multi carrier modulation technique where the revolution of 4G wireless communication is focused towards OFDM systems. The main drawback of OFDM system is high Peak to average power ratio.

Selective Mapping Technique (Slm)

[image:3.612.57.283.461.561.2]

Many methods are there to reduce the PAPR, but both complexity and redundancy are high and only small gains in PAPR are achieved. When the phases of different sub-carriers add up in phase the possibility of PAPR being high is for sure. Therefore one method to reduce the in-phase addition is to change the phase before converting the frequency domain signal into time domain. Hence before taking the N point IDFT each block of input is multiplied by an φ vector of length N. Now there is a possibility that the PAPR may turn low.

Fig 2.2 : Scheme of a Modulator with Selective Mapping

The figure 1 shows the scheme of a modulator with selective mapping technique. The algorithm for selective mapping technique is as follows:

Step 1: Got input vector X of length D and let N=integer

Step2: for i=1: N

Step 2. : Generate φ i of length D

Step 2.2: Multiply φ i with the input vector and get Z (Freq domain)

Step 2.3: Compute IDFT and get z (Time domain)

Step 2.4: Determine PAPR using the formula

| ( )|

Step 2.5: Increment the value of i

Step 3: Go-to Step 2

Step 4: PAPR of length N is obtained.

Step 5: Select a threshold Y. One with minimum PAPR is used for transmission

Step 6: If minimum of PAPR>Y then increment a

Counting

Step 7: Perform Steps 1-6 M times Step 8: Obtain final counting

Step 9: Increase the value of N and repeat previous

Steps1-8

Step 10: Plot the Graph for different N values where X axis Line: Threshold values

Y axis Line: [PAPR low>Y]

II. LITERATURE REVIEW

Feng Hu, Libiao Jin, Jianzeng Li, and Jingwen Ji investigated to introduce a novel receive diversity scheme in high-rate, non-orthogonal space-time block coded (STBC) large multiple-input multiple-output (MIMO) systems that achieve high spectral efficiencies and coding gains. While extremely simple scheme (such as those presented in [1] [2]) capable of attaining a significant fraction of the open-loop Shannon capacity of the (4, 1) channel, no such schemes are known for the case of no less than two receive antennas. Following the (4, 1) approach, a new approach in this letter involving linear

receiver processing is designed to receive diversity-multiplexing. The given design procedure can be easily extended to the multi-output system and a code, which outperforms the MISO (4, 1) code. Results show the proficiency of the design procedure, representing that significant improvements in both capacity and BER.

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Authors Parismita Gogoi and Kandarpa Kumar Sarma demonstrated in their research paper the Diversity methods provide the receiver with independently faded copies of the transmitted signal with the expectation that at least one of these replicas will be received correctly. This work gives a method based on Space-Time Block Coding (STBC) with Multiple-Input Single-Output (MISO) and Multiple-Input Multiple-Output (MIMO) set-up for use in wireless channels. A special type of STBC called Alamouti code is used for exploiting the performance of MISO and MIMO in frequency selective fading environment. Performance differences are analyzed with and without STBC in frequency selective faded channels. Results demonstrate that spatial diversity along with the power of STBC drastically improves the error performance.

Parismita Gogoi and Kandarpa Kumar Sarma analyzed and estimated in this research work that the combination of coding with spatial diversity opens up new dimensions in wireless communications, and could offer efficient solutions to the challenges faced in realizing reliable high-speed links. Use of Space-Time Block Coding (STBC) with Multiple-Input Multiple-Output (MIMO) set-up proves to be an efficient method for this. This work given an estimation technique based on block type pilot symbols having an orthogonal nature for use in Space-Time Block Coding (STBC) with Multiple Antennas setup. BER performance curves from the estimation technique show a result proving its effectiveness.

Year Author Title Approach Result

2013 K.Sundaravadivu and S.Bharathi.,M.E

STBC Codes for Generalized Spatial

Modulation in MIMO Systems

STBC is used instead of spatial modulator.

BPSK modulation is better for good BER Vs SNR and

FER Vs SNR

2012 Parismita Gogoi & Kandarpa Kumar Sarma

Pilot Assisted Channel Estimation Technique for

Alamouti STBC- MISO System and MIMO Set-up

with

BPSK and QPSK modulation

Use of Space-Time Block Coding

(STBC) with Multiple-Input Multiple-Output

(MIMO)

Highly effective and reliable for high-speed links.

2012

Feng Hu, Libiao Jin, Jianzeng Li, and

Jingwen Ji

Novel Constructions of MIMO STBC Designs Employing Four Transmit

Antennas

MIMO STBC Designs Employing Four Transmit

Antennas

Achieves high spectral efficiencies and coding gains

2012 Minggang Luo, Liping Li

A Rotating Blind Separation Algorithm Suitable for MIMO-STBC Systems

without Precoder

Proposed a rotation transform to maximize the independence between the real and imaginary parts without precoder (rotation)

at the transmitter side.

Separate complex PSK-modulated signals blindly with high Symbol

Error Rate (SER) performance

2012

Parismita Gogoi and Kandarpa Kumar

Sarma

STBC coded MISO and MIMO set-up in frequency selective for wireless fading

channels with BPSK and QPSK modulation schemes

Alamouti (STBC) with Multiple-Input

Single-Output (MISO) and Multiple-Input Multiple-Output (MIMO)

Improves the error performance in frequency

selective wireless fading channels.

K.Sundaravadivu and S.Bharathi., M.E, proposed and analyze a generalized spatial modulation scheme for GSM multiple antenna will be active during the transmission. A spatial modulation (SM) scheme with multiple active transmit antennas, which are called as multiple active spatial modulation (MA-SM) with Compare to SM in STBC the bit error probability is good and signals to noise ratio will be increased.

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III. PROPOSED METHODOLOGY

In our proposed methodology the STBC OFDM system is proposed to reduce PAPR which will enhances the performance with multiple inputs & multiple output (MIMO).Using Efficient 64 QAM modulation technique.

At receiver side filters can be used with PTS or SLM algorithm & these are median/moving average filter is adopted to achieve the higher performance of Bit error rate v/s signal to noise ratio etc. In Figure 4.1 shows the block diagram of proposed methodology.

Fig.4.1 Shows the Block diagram STBC

IV. CONCLUSION

The goal of this review paper is to analyze and study the performance of MIMO systems to the conventional SISO systems. This goal was accomplished by investigating the STBC technique and the corresponding decoding algorithms. The investigated space time coding techniques were combined with OFDM to develop the MIMO-OFDM systems and the BER performance of these systems comprehensive studied. A comprehensive background of the MIMO systems was presented, including the input-output relations, space-coding techniques over flat and frequency selective channels and the decoding algorithms. The different schemes were investigated for MIMO OFDM modulation cases. The various MIMO systems have been studied in this work.

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