performance evaluation of lte ofdm using adaptive modulation€¦ · web viewaim of the project:...

Performance evaluation of LTE OFDM using adaptive modulation

ABSTRACT:-

The growing requirements of mobile broad band services like MIMO, combined with

OFDM involved in 4g wireless networks were met by Long Term Evolution (LTE). OFDM is used

in communication systems to provide high data rates, and is more immune to intercarrier

interference, and intersymbol interference. OFDM has become the core of LTE system.

The performance of the system is evaluated in terms of signal to noise ratio and spectral

efficiency and simulation of the system is carried out in MATLAB environment. The simulation is

carried out for different bandwidths and a comparison plot is shown for SNR (dB) Vs spectral

efficiency(bits/Hz).

Electronics And Communication Engineering


AIM OF THE PROJECT: To evaluate the performance of LTE OFDM system using

adaptive modulation scheme in indoor and outdoor. The performance of the system is

evaluated in terms of signal to noise ratio and spectral efficiency and simulation is carried out in

LTE’s indoor and outdoor in MATLAB environment.



CHAPTER 1

INTRODUCTION

1.1 overview

OFDM is a multicarrier modulation technique compared to FDM which is a single carrier

modulation technique used in communication systems, to cope with severe channel

conditions without complex equalization filters. OFDM uses many slowly- modulated

narrow band signals instead of one wideband modulated signal. OFDM is able to

eliminate ISI and ICI by making use of a guard interval with low symbol rate.

The aim of our project is to provide an OFDM system, its main structure and analysing

the system by simulation results. This OFDM system supports different modulation

schemes and is used to study the effect of the variation of different design parameters

on OFDM systems for different bandwidths. The bandwidth of an LTE system ranges

from 5MHz to 25MHz.

1.1.1 Inter symbol Interference (ISI)

One fundamental problem for communication systems is ISI. It is a fact that every

transmission channel is time-variant. Two adjacent symbols are likely to experience

different channel characteristics including time delays. This is particularly true in

wireless channels and mobile terminals communicating in multipath conditions. For low

bit rates (narrowband signal), the symbol rate is sufficiently long so that delayed

versions of the signal all arrive with the same symbol. They do not spill over to

subsequent symbols and therefore there is no ISI. As data rates go up and/or the

channel delay increases (wideband signal), ISI starts to occur. Traditionally, this has been

overcome by equalization techniques, linear predictive filters and rake receivers. This

involves estimating the channel conditions. This works well if the number of symbols to

be considered is low. Assuming BPSK, a data rate of 10 Mbps on a channel with a

maximum delay of 10 µs would need equalization over 100 symbols. This would be too

complex for any receiver. In HSDPA, data rate is as high as 14.4 Mbps. But this uses



QAM16 and therefore the baud rate is not as high. Using a higher level modulation

requires better channel conditions and a higher transmit power for correct decoding.

HSDPA also uses multi code transmission which means that not all of the data is carried

on a single code. The load is distributed on the physical resources thus reducing ISI

further. Today the need is for even higher bit rates. A higher modulation scheme such as

QAM 64 may be employed. Orthogonal frequency division multiplexing involves multi

carrier transmission which divide bandwidth into smaller bandwidths and these

bandwidths are provided to each carrier. Instead of transmitting a signal with large

bandwidth, the data stream is divided among the carriers with smaller bandwidths.

When the symbol duration increases, then intersymbol interference (ISI) is eliminated

and less equalisation is required.

1.1.2 Proposals of OFDM

Initial proposals for OFDM were made in the 60s and the 70s. It has taken more than a

quarter of a century for this technology to move from the research domain to the

industry. The concept of OFDM is quite simple but the practicality of implementing it

has many complexities. A single stream of data is split into parallel streams each of

which is coded and modulated on to a subcarrier, a term commonly used in OFDM

systems. Thus the high bit rates seen before on a single carrier is reduced to lower bit

rates on the subcarrier. It is easy to see that ISI will therefore be reduced dramatically.

This sounds too simple. When didn’t we think of this much earlier? Actually, FDM

systems have been common for many decades. However, in FDM, the carriers are all

independent of each other. There is a guard period in between them and no overlap

whatsoever. This works well because in FDM system each carrier carries data meant for

a different user or application. FM radio is an FDM system. FDM systems are not ideal

for what we want for wideband systems. Using FDM would waste too much bandwidth.

This is where OFDM makes sense. In OFDM, subcarriers overlap. They are orthogonal

because the peak of one subcarrier occurs when other subcarriers are at zero. This is

achieved by realizing all the subcarriers together using Inverse Fast Fourier Transform



(IFFT). The demodulator at the receiver parallel channels from an FFT block. Note that

each subcarrier can still be modulated independently. Ultimately ISI is conquered.

Provided that orthogonality is maintained, OFDM systems perform better than single

carrier systems particularly in frequency selective channels. Each subcarrier is multiplied

by a complex transfer function of the channel and equalising this is quite simple

1.1.3 Basic considerations

An OFDM system can experience fades just as any other system. Thus, coding is required for all

subcarriers. We do get frequency diversity gain because not all subcarriers experience fading at

the same time. Thus, a combination of coding and interleaving gives us better performance in a

fading channel. Higher performance is achieved by adding more subcarriers but this is not

always possible. Adding more subcarriers could lead to random FM noise resulting in a form of

time-selective fading. Practical limitations of transceiver equipment and spectrum availability

mean than alternatives have to be considered. One alternative is to add a guard band in the

time domain to allow for multipath delay spread. Thus, symbols arriving late will not interfere

with the subsequent symbols. This guard time is a pure system overhead. The guard time must

be designed to be larger than the expected delay spread. Reducing ISI from multipath delay

spread thus leads to deciding on the number of subcarriers and the length of the guard period.

Frequency-selective fading of the channel is converted to frequency-flat fading on the

subcarriers.

1.2 Problem statement

The focus of the future generation(5g) mobile system is on supporting higher data rates and

providing seamless services across a multitude of wireless systems and networks.

1.3 Motivation

The ultimate goal of wireless communication technology is to provide universal personal and

multimedia communication irrespective of mobility and location with high data rates. When the



data is transmitted at high data bit rates over mobile radio channels then the channels may

cause: Severe fading of transmitted signals when passed through channel Inter symbol

interference(ISI)

CHAPTER 2

LTE ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING

2.1 Introduction:

In a single carrier communication system, the symbol period must be much greater than the

delay time in order to avoid inter-symbol interference (ISI). Since data rate is inversely



proportional to symbol period, having long symbol periods means low data rate and

communication inefficiency. A multi carrier system, such as FDM (Frequency Division

Multiplexing), divides the total available bandwidth in the spectrum into sub-bands for multiple

carriers to transmit in parallel. An overall high data rate can be achieved by placing carriers

closely in the spectrum. However, inter-carrier interference (ICI) will occur due to lack of

spacing to separate the carriers. To avoid intercarrier interference, guard bands will need to

be placed in between any adjacent carriers, which results in lowered data rate. OFDM

(Orthogonal Frequency Division Multiplexing) is a multicarrier digital communication scheme to

solve both issues. It combines a large number of low data rate carriers to construct a composite

high data rate communication system. Orthogonality gives the carriers a valid reason to be

closely spaced, even overlapped, without inter-carrier interference. Low data rate of each

carrier implies long symbol periods, which greatly diminishes inter-symbol interference.

Although the idea of OFDM started back in 1966, it has never been widely utilized until the last

decade when it “becomes the modem of choice in wireless applications”. It is now interested

enough to experiment some insides of OFDM. This objective is met by developing a MATLAB

program to simulate a basic OFDM system. From the process of this development, the

mechanism of an OFDM system can be studied; and with a completed MATLAB program, the

characteristics of an OFDM system can be explored. Orthogonal frequency division multiplexing

has also been adopted for a number of broadcast standards from DAB Digital Radio to the

Digital Video Broadcast standards, DVB. It has also been adopted for other broadcast systems

as well including Digital Radio Mondiale used for the long medium and short-wave bands.

Although OFDM, orthogonal frequency division multiplexing is more complicated than earlier

forms of signal format, it provides some distinct advantages in terms of data transmission,

especially where high data rates are needed along with relatively wide band widths.

2.2 What is OFDM? - The concept

OFDM is a form of multi carrier modulation. An OFDM signal consists of a number of closely

spaced modulated carriers. When modulation of any form - voice, data, etc. is applied to a



carrier, then sidebands spread out either side. It is necessary for a receiver to be able to receive

the whole signal to be able to successfully demodulate the data. As a result, when signals are

transmitted close to one another they must be spaced so that the receiver can separate them

using a filter and there must be a guard band between them. This is not the case with OFDM.

Although the sidebands from each carrier overlap, they can still be received without the

interference that might be expected because they are orthogonal to each another. This is

achieved by having the carrier spacing equal to the reciprocal of the symbol period.

To see how OFDM works, it is necessary to look at the receiver. This acts as a bank of

demodulators, translating each carrier down to DC. The resulting signal is integrated over the

symbol period to regenerate the data from that carrier. The same demodulator also

demodulates the other carriers. As the carrier spacing equal to the reciprocal of the symbol

period means that they will have a whole number of cycles in the symbol period and their

contribution will sum to zero-in other.

One requirement of the OFDM transmitting and receiving systems is that they must be linear.

Any non-linearity will cause interference between the carriers as a result of inter-modulation

distortion. This will introduce unwanted signals that would cause interference and impair the

orthogonality of the transmission.

In terms of the equipment to be used the high peak to average ratio of multicarrier systems

such as OFDM requires the RF final amplifier on the output of the transmitter to be able to

handle the peaks whilst the average power is much lower and this leads to inefficiency. In some



systems the peaks are limited. Although this introduces distortion that results in a higher level

of data errors, the system can rely on the error correction to remove them.

2.3 Data on OFDM

The data to be transmitted on an OFDM signal is spread across the carriers of the signal, each

carrier taking part of the payload. This reduces the data rate taken by each carrier. The lower

data rate has the advantage that interference from reflections is much less critical. This is

achieved by adding a guard band time or guard interval into the system. This ensures that the

data is only sampled when the signal is stable and no new delayed signals arrive that would

alter the timing and phase of the signal.

FIG: OFDM Spectrum

The distribution of the data across a large number of carriers in the OFDM signal has some

further advantages. Nulls caused by multipath effects or interference on a given frequency

only affect a small number of the carriers, the remaining ones being received correctly. By using

error-coding techniques, which does mean adding further data to the transmitted signal, it

enables many or all of the corrupted data to be reconstructed within the receiver. This can be

done because the error correction code is transmitted in a different part of the signal.

2.4 OFDM advantages and disadvantages



2.4.1 Advantages

OFDM has been used in many high data rate wireless systems because of the many advantages

it provides.

Immunity to selective fading: One of the main advantages of OFDM is that is more resistant to

frequency selective fading than single carrier systems because it divides the overall channel into

multiple narrowband signals that are affected individually as flat fading sub-channels.

Resilience to interference: Interference appearing on a channel may be bandwidth limited and

in this way will not affect all the sub-channels. This means that not all the data lost

Spectral efficiency: Using close-spaced overlapping sub-carriers, a significant OFDM

advantage is that it makes efficient use of the available spectrum.

Resilient to ISI: Another advantage of OFDM is that it is very resilient to intersymbol and

inter-frame interference. This results from the low data rate on each of the sub-

channels.

Resilient to narrow-band effects: Using adequate channel coding and interleaving it is

possible to recover symbols lost due to the frequency selectivity of the channel and

narrow band interference. Not all the data is lost.

Simpler channel equalisation: One of the issues with CDMA systems was the

complexity of the channel equalisation which had to be applied across the whole

channel. An advantage of OFDM is that using multiple sub-channels, the channel

equalization becomes much simpler.

2.4.2 Disadvantages

High peak to average power ratio: An OFDM signal has a noise like amplitude variation

and has a relatively high large dynamic range, or peak to average power ratio. This



impacts the RF amplifier efficiency as the amplifiers need to be linear and

accommodate the large amplitude variations and these factors mean the amplifier

cannot operate with a high efficiency level.

Sensitive to carrier offset and drift: Another disadvantage of OFDM is that is sensitive to

carrier frequency offset and drift. Single carrier systems are less sensitive.

2.5 OFDM variants

There are several other variants of OFDM for which the initials are seen in the technical

literature. These follow the basic format for OFDM, but have additional attributes or variations:

2.5.1 COFDM

Coded Orthogonal frequency division multiplexing. A form of OFDM where error correction

coding is incorporated into the signal.

2.5.2 Flash OFDM

This is a variant of OFDM that was developed by Flarion and it is a fast-hopped form of OFDM.

It uses multiple tones and fast hopping to spread signals over a given spectrum band.

2.5.3 OFDMA

Orthogonal frequency division multiple access. A scheme used to provide a multiple access

capability for applications such as cellular telecommunications when using OFDM technologies.

2.5.4 VOFDM

Vector OFDM. This form of OFDM uses the concept of MIMO technology. It is being developed

by CISCO Systems. MIMO stands for Multiple Input Multiple output and it uses multiple



antennas to transmit and receive the signals so that multi-path effects can be utilised to

enhance the signal reception and improve the transmission speeds that can be supported.

2.5.4 WOFDM

Wideband OFDM. The concept of this form of OFDM is that it uses a degree of spacing between

the channels that is large enough that any frequency errors between transmitter and receiver

do not affect the performance. It is particularly applicable to Wi-Fi systems.

Each of these forms of OFDM utilise the same basic concept of using close spaced orthogonal

carriers each carrying low data rate signals. During the demodulation phase the data is then

combined to provide the complete signal.

OFDM, orthogonal frequency division multiplexing has gained a significant presence in the

wireless market place. The combination of high data capacity, high spectral efficiency, and its

resilience to interference as a result of multi-path effects means that it is ideal for the high data

applications that have become a major factor in today's communications scene.

ORTHOGONALITY

In a multi carrier transmission system, a very small frequency gap is provided among the

carriers without having intercarrier interference (ICI). When the carriers are orthogonal to each

other, minimum space is reached, and signals overlap with each other without causing

interference. This is called orthogonality in OFDM. IFFT exhibits the orthogonality property.

ADAPTIVE MODULATION

Adaptive Modulation means dynamically varying the modulation in an errorless manner in

order to maximize the throughput under momentary propagation conditions. In other words, a

system can operate at its maximum throughput under clear sky conditions.

The target of adaptive modulation is



1. To increase throughput or data rate

2. To increase resource reliability

3. To increase quality of service (communication quality)

LTE

This LTE (Long Term Evolution) is involved in 4G family, to provide higher data rate from

100Mbps - 200Mbps, to support applications such as Mobile TV, Video conferencing, Tele-

medicine, online gambling, etc. LTE is based on criteria developed by the 3rd Generation

Partnership Project (3GPP). The 3rd Generation Partnership Project (3GPP) is an

organization that defining a mobile system that achieves the IMT-2000 standard. LTE may

also be referred as Evolved UMTS Terrestrial Radio Access (E-UTRA) and Evolved UMTS

Terrestrial Radio Access Network (E-UTRAN). It provides scalable bandwidth stats from

1.25MHz up to 20+ MHz. LTE infrastructures are planned to be as simple through flexible

technology with a broad diversity of frequency bands. The technology used for Downlink is

OFDMA to attain the peak data rate of 100Mbit/s and Uplink is based on Single Carrier

FDMA (SC-FDMA) to attain a peak data rate of 50Mbit/s. LTE provide connected

automobiles, which produce a broad reach of broadband services and also facilitate better

speed on current as well as new mobile applications.

The main methodology in LTE to achieve high data rate, high QOS and bandwidth

efficiency is using MIMO. The OFDM is a multicarrier transmission scheme which provides

several advantages like eliminating the ISI, efficient use of spectrum by overlapping the

subcarriers using the orthogonality principle and provides robustness against Co channel

interference. The existing channel estimation methods, assume an invariant wireless

channel within one OFDM symbol which leads to ICI (Inter Carrier Interference) problem in

high mobility LTE system by losing the orthogonality between the subcarriers reduced by

cyclic prefix.



CHAPTER 3

BLOCK DIAGRAM

3.1 BLOCK DIAGRAM OF LTE OFDM SYSTEM:

Fig 3.1: Block diagram of LTE OFDM system

The transmitter section converts digital data to be transmitted, into a mapping of subcarrier

amplitude and phase. It then transforms this spectral representation of the data into the time

domain using an Inverse Discrete Fourier Transform (IDFT). The Inverse Fast Fourier Transform

(IFFT) performs the 20 same operations as an IDFT, except that it is much more computationally

efficient, and so is used in all practical systems. In order to transmit the OFDM signal the

calculated time domain signal is then mixed up to the required frequency. The receiver

performs the reverse operation of the transmitter, mixing the RF signal to base band for

processing, then using a Fast Fourier Transform (FFT) to analyze the signal in the frequency

domain. The amplitude and phase of the subcarriers is then picked out and converted back to

digital data. The IFFT and the FFT are complementary function and the most appropriate term

depends on whether the signal is being received or generated. In cases where the signal is

independent of this distinction then the term FFT and IFFT is used interchangeably. The high


SERIAL TO PARALLEL

PARALLEL TO SERIALP

FFTFHSERIAL TO PARALLELS A / DHH

IFFTiiPARALLEL TO SERIAL

D / ADH

CHANNEL


data rate serial input bit stream is fed into serial to parallel converter to get low data rate

output parallel bit stream. Input bit stream is taken as binary data. The low data rate parallel bit

stream is modulated in Signal Mapper. Modulation can be BPSK, QPSK, QAM, etc. The

modulated data are served as input to inverse fast Fourier transform so that each subcarrier is

assigned with a specific frequency. The frequencies selected are orthogonal frequencies. In this

block, orthogonality in subcarriers is introduced. In IFFT, the frequency domain OFDM symbols

are converted into time domain OFDM symbols. Guard interval is introduced in each OFDM

symbol to eliminate inter symbol interference (ISI). All the OFDM symbols are taken as input to

parallel to serial data. These OFDM symbols constitute a frame. A number of frames can be

regarded as one OFDM signal. This OFDM signal is allowed to pass through digital to analog

converter (DAC). In DAC the OFDM signal is fed to RF power amplifier for transmission. Then

the signal is allowed to pass through additive white Gaussian noise channel (AWGN channel). At

the receiver part, the received OFDM signal is fed to analog to digital converter (ADC) and is

taken as input to serial to parallel converter. In these parallel OFDM symbols, Guard interval is

removed and it is allowed to pass through Fast Fourier transform. Here the time domain OFDM

symbols are converted into frequency domain. After this, it is fed into Signal Demapper for

demodulation purpose. And finally the low data rate parallel bit stream is converted into high

data rate serial bit stream.



CHAPTER 4

SIMULATED RESULTS AND DISCUSSIONS

4.1 INTRODUCTION TO MATLAB

MATLAB is widely used in all areas of applied mathematics, in education and research at

universities, and in the industry. MATLAB stands for MATrix LABoratory and the software is

built up around vectors and matrices. This makes the software particularly useful for linear

algebra but MATLAB is also a great tool for solving algebraic and differential equations and for

numerical integration. MATLAB has powerful graphic tools and can produce nice pictures in

both 2D and 3D. It is also a programming language, and is one of the easiest programming

languages for writing mathematical programs. MATLAB also has some tool boxes useful for

signal processing, image processing, optimization, etc.

The name MATLAB stands for MATrix LABoratory. MATLAB was written originally to provide

easy access to matrix software developed by the LINPACK (linear system package) and EISPACK

(Eigen system package) projects.

MATLAB is a high-performance language for technical computing. It integrates computation,

visualization, and programming environment. Furthermore, MATLAB is a modern programming

language environment. it has sophisticated data structures, contains built-in editing and

debugging tools, and supports object-oriented programming. These factors make MATLAB an

excellent tool for teaching and research.

MATLAB has many advantages compared to conventional computer languages (e.g., C,

FORTRAN) for solving technical problems. MATLAB is an interactive system whose basic data

element is an array that does not require dimensioning. The software package has been

commercially available since 1984 and is now considered as a standard tool at most universities

and industries worldwide.



RESULTS:

Figure 4.1: plot for 5 MHz bandwidth.

The above figure shows the plot for both indoor and outdoor for bandwidth of 5MHz. Here the

SNR value is 10-30 dB and the Spectral efficiency is 3.7 bits/Hz for indoor and 2.0 bits/Hz for

outdoor.




The above figure shows the plot for both indoor and outdoor for bandwidth of 10MHz. Here the

SNR value is 10-30 dB and the Spectral efficiency is 4.0 bits/Hz for indoor and 3.9 bits/Hz for

outdoor.




The above figure shows the plot for both indoor and outdoor for bandwidth of 15MHz. Here

the SNR value is 10-30 dB and the Spectral efficiency is 2.7 bits/Hz for indoor and 2.7 bits/Hz for

outdoor.






outdoor.



Figure 5: plot for 25 MHz bandwidth.



outdoor.

CHAPTER 5

CONCLUSION

We conclude that in this project the performance of an OFDM LTE system is evaluated or

measured in terms of SNR and spectral efficiency and a comparison table showing the

differences in indoor and outdoor environment with the help of SNR Vs Spectral efficiency plot

is drawn with different bandwidths 5MHz, 10MHz, 15MHz, 20MHz, 25MHz. From this plots we

are concluding that the strength of the spectral efficiency of the indoor environment is better

than the strength of the spectral efficiency of the outdoor environment. Plots are drawn with

the fixed SNR of 10 to 30dB.



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