Chapter 2

Literature Review

2.1 Fading Channel:

Mehboob Ul Amin et al (2013) worked on Multiple-Input Multiple-Output Orthogonal

Frequency Division Multiplexing (MIMO-OFDM) which is an attractive air-interface solution

for next generation wireless local area networks (WLANs), wireless metropolitan area networks

(WMANs), and fourth generation mobile cellular wireless systems. However one of the main

disadvantage associated with MIMO-OFDM systems is the high peak-to-average power ratio

(PAPR) of the transmitter’s output signal on different antennas. High Peak to Average Power

Ratio (PAPR) for MIMO-OFDM system is still a demanding area and difficult issue. So far

numerous techniques based on PAPR reduction have been proposed. In this work a new

technique based on the combination of Orthogonal Space Time Block Code (OSTBC) Encoder

and Discrete Cosine Transform based Selective Level Mapping as method of PAPR reduction

technique has been proposed and simulated. The results have been verified in terms of various

graphs and plots and are compared with earlier results of embedded transform techniques.

Simulations show that better

results are obtained in the proposed technique [1]

The work investigates one of the bottleneck problem that exist in MIMO-OFDM systems

i.e high peak to average ratio and suggests a new technique to overcome it. The new technique is

based on the combination of OSTBC encoder and DCT matrix. The proposed OSTBC Encoder

uses variable number of transmit antennas that are adaptive and change either manually or

according to an adaptation algorithm. Simulation results show a greater reduction in PAPR for

the proposed scheme as compared to earlier conventional SLM technique. Also the PAPR

decreases significantly for higher values of M as compared to original signal OFDM signal. The

proposed scheme has a lot of scope in next generation network systems. Moreover with this

improvement it can be considered as a potential candidate for high speed data transmission

systems [1].

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Mr. Apoorva Pandey et al (2013) proposed an algorithm for wireless communication fading of

channels is the serious cause of the received degraded signals. The effect of fading can be

minimized by using various time and space domain techniques. However, space domain

techniques are preferred over the others due to its advantages. In this work, comparison of the

wireless MIMO system under Almouti‘s and maximum ratio combining schemes is presented.

Basic idea in these schemes is to transmit and receive more than one copy of the original signals.

Using two transmitter antennas and one receiver antenna, the scheme provides the nearly same

diversity order as the maximal-ratio receiver combining (MRRC) with one transmitter antenna,

and two receiver antennas. Results for one transmitter and four receivers under MRRC is also

presented and compared. Finally, results are presented while varying the average transmitted

power [2].

In this work, a comparison of diversity technique for estimating the channel performance

of mobile communication signals affected by Rayleigh multipath fading phenomena is discussed.

The performance of Alamouti scheme and Maximum ratio combining techniques are evaluated

under the assumption of BPSK signals affected by reflection, diffraction and scattering

environment. It is shown that in wireless MIMO, system based on Alamouti diversity technique

and Maximum ratio combining a technique can help to combat and mitigate against Rayleigh

fading channel and approach AWGN channel performance with constant transmits power. While

the results are equally applicable if the average transmitted power varies [2].

2.2 MIMO System:

H A Mohammed et al (2012) said that the merging of Orthogonal Frequency Division

Multiplexing (OFDM) with Multiple-input multiple-output (MIMO) is a promising mobile air

interface solution for next generation wireless local area networks (WLANs) and 4G mobile

cellular wireless systems. This work details the design of a highly robust and efficient OFDM-

MIMO system to support permanent accessibility and higher data rates to users moving at high

speeds, such as users travelling on trains. It has high relevance for next generation wireless local

area networks (WLANs) and 4G mobile cellular wireless systems. The work begins with a

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comprehensive literature review focused on both technologies. This is followed by the modeling

of the OFDM-MIMO physical layer based on Simulink/Matlab that takes into consideration high

vehicular mobility. Then the entire system is simulated and analyzed under different encoding

and channel estimation algorithms. The use of High Altitude Platform system (HAPs)

technology is considered and analysed [3].

The possibility of using OFDM-MIMO technology as an robustness communication

system with the help of high altitude platform system (HAPs) against the high Doppler effects on

users travelling within high speed vehicles (highly mobile environment), especially when

accessing various IP broadband services. In this work different simulation models have been

introduced to identify a solution for the main drawback. Demonstrating the work that has been

done until now, which involved designing and building the necessary modules for the model

project that led to OFDM-MIMO prototype transceiver. Also, including modifying and

developing the major parts of each side of the UL/DL communication network (Tx/Rx). Finally,

testing the simulation model with different signal parameters, and trying to reduce as much of

Doppler Effect on data during transmission to users within high speed vehicles and trains,

simulation results are presented to indicate the suitable model which will used later for the real

virtual simulation scenarios as HAPs payload supported with satellite systems in hybrid

architectures [3].

Srikrishna Bardhan et al (2012) worked in wireless communications, spectrum is a scarce

resource and hence imposes a high cost on the high data rate transmission. Fortunately, the

emergence of multiple antenna system has opened another very resourceful dimension i.e. space

for information transmission. Multi-antenna systems are expected to play very important role in

future multimedia wireless communication systems. Such systems are predicted to provide

tremendous improvement in spectrum utilization. Here, the orthogonal space-time block codes

are considered for the capacity and error probability analysis of MIMO systems. The numerical

and simulation results obtained using MATLAB are presented for the multi-antenna system

channel capacity and bit-error rate in Rayleigh fading channels [4].

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In this work, the capacity and BER of MIMO systems in Rayleigh fading channels has

been examined. It has been seen that the use of multiple antennas increases the capacity although

significant improvement can be achieved using equal or higher number of receive antennas

compared to transmit antennas. Similarly, the performance of Alamouti code is worse by about

3dB compared to MRC. This is because in space-diversity-on transmit scheme using Alamouti

code, the transmit power in each of the two antennas is one- half of the transmit power in the

space-diversity-on receive scheme using MRC. But the BER performance of MIMO- STBC [2

by 2] scheme is better than MRC case due to higher diversity order. [4]

Harsh Shah et al (2006) they analyzed the performance of an important class of MIMO

systems, that of orthogonal space time block codes concatenated with channel coding. This

system configuration has an attractive combination of simplicity and performance. We study this

system under spatially independent fading as well as correlated fading that may arise from the

proximity of transmit or receive antennas or unfavorable scattering conditions. We consider the

effects of time correlation and present a general analysis for the case where both spatial and

temporal correlations exist in the system. We present simulation results for a variety of channel

codes, including convolution codes, turbo codes, trellis coded modulation (TCM), and multiple

trellis coded modulation (MTCM), under quasi-static and block-fading Rayleigh as well as

Rician fading. Simulations verify the validity of our analysis [7].

This work presents performance analysis for systems consisting of a concatenation of

channel codes and orthogonal space-time block codes. Such systems are of theoretical and

practical interest. We use the concept of a uniform inter leaver in the context of block fading

channel to calculate bit error probabilities. This analysis is performed both for the case of

spatially uncorrelated fading, as well as spatially correlated fading due to proximity of transmit

or receive antennas. We also consider joint spatio-temporal correlation. We give results for a

wide variety of codes and several types of fading channels. Simulations verify the accuracy of

our analysis. Future work in this area can address bit-interleaved modulation, as well as the case

where only partial channel state information is available at the receiver [7].

2.3 Channel Estimation:

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Soukayna Ghandour-Haidar et al (2011) dealed with the estimation of a flat fading Rayleigh

channel with Jakes’s spectrum. The channel is approximated by a first-order autoregressive

(AR(1)) model and tracked by a Kalman Filter (KF). The common method used in the literature

to estimate the parameter of the AR(1) model is based on a Correlation Matching (CM) criterion.

However, for slow fading variations, another criterion based on the Minimization of the

Asymptotic Variance (MAV) of the KF is more appropriate, as already observed in few works

[5]. This letter gives analytic justification by providing approximated closed-form expressions of

the estimation variance for the CM and MAV criteria, and of the optimal AR(1) parameter [5].

This work addresses the problem of estimating a Rayleigh channel using a first order AR

model. An analytic study clearly shows that the most widely used choice for the AR(1) pole

estimation (the CM criterion) is not accurate for low SNR and low Doppler fdT. Therefore,

switching to an estimation error variance criterion as already proposed in we carry out the

optimization of the AR(1) model and the calculation of its performance. We provide an

approximate expression of the MSE for the CM and MAV criteria first, and of the AR(1) (MAV)

parameter for a given SNR and Doppler scenario. It is demonstrated that the MSE of the AR(1)

KF (MAV) is proportional to the (2=3) power of the product ( fdT ×σ2n ), where σ2n is the

observation noise variance [5].

Angel Lozano et al (20009) worked on a contemporary perspective on the tradeoff between

transmit antenna diversity and spatial multiplexing is provided. It is argued that, in the context of

most modern wireless systems and for the operating points of interest, transmission techniques

that utilize all available spatial degrees of freedom for multiplexing outperform techniques that

explicitly sacrifice spatial multiplexing for diversity. In the context of such systems, therefore,

there essentially is no decision to be made between transmit antenna diversity and spatial

multiplexing in MIMO communication. Reaching this conclusion, however, requires that the

channel and some key system features be adequately modeled and that suitable performance

metrics be adopted; failure to do so may bring about starkly different conclusions. As a specific

example, this contrast is illustrated using the 3GPP Long-Term Evolution system

design [6].

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Since the 1970’s, antenna diversity had been a preferred weapon used by mobile wireless

systems against the deleterious effect of fading. While narrowband channelization and non-

adaptive links were the norm, antenna diversity was highly effective. In modern systems,

however, this is no longer the case. Link adaptivity and scheduling have rendered transmit

diversity undesirable for low-velocity users whereas abundant time/frequency selectivity has

rendered transmit diversity superfluous for high-velocity users. Moreover, the prevalence of

MIMO has opened the door for a much more effective use of antennas: spatial multiplexing.

Indeed, the spatial degrees of freedom created by MIMO should be regarded as additional

’bandwidth’ and, for the same reason that schemes based on time/frequency repetition are not

used for they waste bandwidth, transmit diversity techniques waste ’bandwidth’. Of all possible

DMT points, therefore, the zero-diversity one stands out in importance. Techniques, even

suboptimum ones, that can provide full multiplexing are most appealing to modern wireless

systems whereas techniques that achieve full diversity order but fall short on multiplexing gain

are least appealing. Our findings further the conclusion in where a similar point is made solely

on the basis of the multiplexing gain for frequency-flat channels. Although this conclusion has

been reached on the premise that the coded error probabilities of discrete constellations are well

approximated by the mutual information outages of Gaussian codebooks, we expect it to hold in

any situation where the code operates at a (roughly) constant gap to the mutual information.

The trend for the foreseeable future is a sustained increase in system bandwidth, which is bound

to only shore up the above conclusion. LTE, which for our case study was taken to use 10 MHz,

is already moving towards 20 MHz channelization. At the same time, exceptions to the foregoing

conclusion do exist. These include, for example, control channels that convey short messages.

Transmit diversity is fitting for these channels, which do benefit from a lower error probability

but lack significant time/frequency selectivity. Other exceptions may be found in applications

such as sensor networks or others where the medium access control is non-existent or does not

have link adaptation and retransmission mechanisms. Our study has only required evaluating

well-known techniques under realistic models and at the appropriate operating points. Indeed, a

more general conclusion that can be drawn from the discussion in this work is that, over time, the

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evolution of wireless systems has rendered some of the traditional models and wisdoms obsolete.

In particular:

• Frequency and time selectivity should always be properly modeled.

• Performance assessments are to be made at the correct operating point, particularly in

terms of error probability.

• The assumptions regarding transmit CSI must be consistent with the regime being

considered. At low velocities, adaptive rate control based on instantaneous CSI should be

incorporated; at high velocities, only adaptation to average channel conditions should be

allowed.

• Coded block error probabilities or mutual information outages, rather than uncoded

error probabilities, should be used to gauge performance.

Proper modeling is essential in order to evaluate the behavior of transmission and reception

techniques in contemporary and future wireless systems. As our discussion on transmit diversity

and spatial multiplexing demonstrates, improper modeling can lead to misguided perceptions and

fictitious gains [6].

Reference

[1] Mehboob Ul Amin et al “A New Method for PAPR Reduction in MIMOOFDM Using

Combination of OSTBC Encoder and DCT Matrix” International Journal of Recent

Technology and Engineering (IJRTE) ISSN: 2277-3878, Volume-2, Issue-2, May 2013.

[2] Mr. Apoorva Pandey et al “ Comparison of Wireless MIMO System Under Alamouti‘s

Scheme and Maximum Ratio Combining Technique” I.J. Image, Graphics and Signal

Processing, 2013, 2, 31-37 Published Online February 2013 in MECS (http://www.mecs-

press.org/) DOI: 10.5815/ijigsp.2013.02.05.

[3] H A Mohammed et al “Investigation of Doppler Effects on high mobility OFDMMIMO

systems with the support of High Altitude Platforms (HAPs)” Journal of Physics:

Conference Series 364 (2012) 012048 doi:10.1088/1742-6596/364/1/012048.

[4] Srikrishna Bardhan et al “ Capacity and Performance Analysis of MIMO-STBC in

Rayleigh Fading Channels” International Journal of Engineering Research & Technology

(IJERT) Vol. 1 Issue 8, October - 2012 .

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[5] Soukayna Ghandour-Haidar et al’ “On the Use of First-order Autoregressive Modeling

for Rayleigh Flat Fading Channel Estimation with Kalman Filter” Author manuscript,

published in "Signal Processing 92, 2 (2012) 601-606" DOI :

10.1016/j.sigpro.2011.08.014.

[6] Angel Lozano et al, “Transmit Diversity v. Spatial Multiplexing in ModernMIMO

Systems” arXiv:0811.3887v2 [cs.IT] 5 Mar 2009.

[7] Harsh Shah et al, “Performance of Concatenated Channel Codes and Orthogonal Space-

Time Block Codes” IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS,

VOL. 5, NO. 6, JUNE 2006.

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