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Enhancement Orthogonal Frequency Division Multiplexing (OFDM) in Wireless

Communication Network by Using PTS(Partial Transmit Sequences) Technique

Majed M. Albogame and Khaled M. Elleithy

Department of Computer Science and Engineering

University of Bridgeport, Bridgeport, CT 06604, USA

(malbogam@my.bridgeport.edu, elleithy@bridgeport.edu)

Abstract

Wireless telecommunications has been significantly

growing due to the need of high data rate during the last

decade. In this paper, OFDM with different scenarios are

experimentally tested and the results demonstrate the

strengths of OFDM technique as a key technology that

fulfils the need of high data rate and high performance in

wireless communication. Furthermore, we demonstrate that

the complexity of OFDM technology can be reduced and

the capacity is increased by using Partial Transmit

Sequences (PTS) method. This method shows the best

performance of OFDM where increased capacity, coverage,

and reliability are clearly evident from the test results

presented in this paper.

Keywords: Orthogonal Frequency Division Multiplexing

(OFDM), FFT (Fast Fourier Transform), BPSK (Binary

Phase Shift keying) and QAM (Quadrature Amplitude

Modulation), PAPR (Peak to Average Power Ratio), Partial

Transmit Sequences (PTS).

1. Introduction

Orthogonal Frequency Division Multiplexing (OFDM) is

also known as a multicarrier modulation technique, which

has been the key behind the most advances and

achievements associated with high data rate

communications. OFDM nowadays has become so popular

due to its flexible and efficient management of

inter-symbol interference (ISI). Moreover, OFDM grants

high spectral efficiency and mitigates the multipath

environment effects. OFDM divides the data stream into

multiple sub-streams and sends them through multiple

orthogonal sub-channels. Generally, the overall system

performance and communication link quality is improved

by OFDM technology [1-4].

There is a lot work introduced in literature that focusses

on Orthogonal-Frequency-Division-Multiplexing (OFDM)

technology. In 1960, the parallel data transmission was

proposed to increase data processing and transmission

traffic due to the desired demand of high data rate in

communication networks. Furthermore, Frequency

Division Multiplexing (FDM) was introduced as a

technique in parallel data transmission [3].

FDM divides the frequency spectrum into slots that are

called transmission channels. Furthermore, each slot

should be assigned to one transmission channel. These

frequency sub-bands cannot be overlapped neither can be

placed adjacently, which means that multicarrier FDM

communication link designers should not only assign non-

overlapped frequency bands for transmission channels but

also leave frequency guard bands in between those

assigned frequency bands [5]. Consequently, multicarrier

modulation technique has two disadvantages. The first one

is the implementation complexity and the second one is the

spectral inefficiency.

Weinstein and Ebert proposed a multicarrier system

which works with discrete Fourier Transform (DFT) [14,

15]. DFT was implemented as a part of the modulation and

demodulation process in the multicarrier communication

systems to reduce complexity. Inverse Discrete Fourier

Transform (IDFT) and Discrete Fourier Transform (DFT),

according to Weinstein and Ebert proposed approach, are

used to perform the modulation and demodulation

processes, respectively. Then, many research efforts have

been made to improve multicarrier communication system

complexity even more. The most successful improvement

is the use of Fast Fourier Transform (FFT) and Inverse Fast

Fourier Transform (IFFT) instead of DFT and IDFT

respectively. By that time, both system complexity and

bandwidth inefficiency problems were handled through

modifications being added one after another and the last

one was introduced by Weinstein and Ebert as mentioned

before. Furthermore, OFDM divides the high rate input

data stream into lower rate multiple sub-streams. Then,

those sub-streams are modulated on orthogonal subcarriers

offering high spectral efficiency, frequency flat fading

sub-channels, free ISI communication link, and robust

modulation and multiple access schemes [17] and [18].

2. OFDM Concepts

The straightest forward idea of OFDM is to divide the

high rate data stream into lower rate sub-streams and send

them over through multiple parallel sub-channels [13].

Many advantages, therefore, have been accomplished after

applying the idea of OFDM multi carrier communication

system.

3.1 OFDM Orthogonality

One of the aspects that have made OFDM so popular

multicarrier communication technique is that OFDM

system occupies the frequency spectrum efficiently and

wisely. OFDM satisfies orthogonality of communication

system subcarriers which means that a mathematical

relationship between these subcarriers is developed to allow

these subcarriers to overlap without having any adjacent

sub-channels interference [6]. In any OFDM system, if we

have N equally spaced subcarriers, for instance, we would

satisfy the orthogonality property when we place them

within the specified frequency spectrum with frequency

spacing equals to:

Figure 1: Orthogonality property for five sub carriers [5].

∆f=1/(N.Ts) (1)

Where N. Ts = Symbol duration [6].

This mathematical relationship creates a sine frequency

response for all these N subcarriers where each one of them

has maximum amplitude at a point, whereas others have

nulls [6]. Figure1 shows how OFDM works.

The mathematical proof, which shows there is no

interference between signals if spaced as in equation (1), is

provided in [9] as the following:

(2)

Where, C is constant.

Equation (2) shows that out of 5 OFDM subcarriers,

shown in Figure 1, the amplitude of only one subcarrier can

be non-zero value at a frequency of multiples of 1/(N.Ts)

while the remaining ones should have zero amplitudes.

As a result of this orthogonality aspect, the spectral

inefficiency problem has been taken care of as it has been

shown in Figure 2. Considering that the system frequency

bandwidth is the most valuable component in any

communication system, OFDM system has met the aim of

occupying the communication system spectrum efficiently.

According to [8, 11, 12], OFDM multi carrier system

occupies 50% of the bandwidth that regular FDM

multicarrier system does.

Figure 2: Shows what the differences between the. FDM

and OFDM spectral efficiencies are [16].

3.2 ISI, ICI, guard time, CP in OFDM system

Another reason why OFDM is preferred over regular

single carrier modulation is the fact that OFDM mitigates

the effects of multipath delay spread significantly. By

dividing the data stream into N sub-streams, the symbol

duration time of each sub-stream is smaller than the

original one by a factor of (1/Ns) [8]. This shorter symbol

duration helps reducing the effects of multipath delay

spread by a factor of (1/Ns) as well.

ISI is unwanted phenomenon which happens during the

communication process when one symbol interferes with

the adjacent symbols, or i.e. when the energy of one

symbol spills over the adjacent symbols [1]. When the

channel delay spread time (τ) is larger than the symbol

spread time (T), ISI becomes severe. In addition, ISI

becomes ever more serious issue with high data rate

communication systems so this phenomenon needs to be

overcome to maintain communication quality. Therefore,

one of the goals that communication link designers were

trying to achieve is handling ISI and supporting high data

rates at the same time. The idea of OFDM that has handled

ISI problem is dividing high data rate stream wanted to be

transmitted into (L) lower rate sub-streams such that each

one has (Ts/L) >> (τ). This property gives almost free ISI

communication, as a result of ISI criterion mentioned

above [1]. Moreover, OFDM handles ISI problem from

another angle to improve the results and eliminate effects

even more. That angle is to insert a guard time slots in

between OFDM symbols as it is shown in Figure 3. These

guard slots should be chosen to be larger than the expected

symbol delay spread time. Doing so, OFDM ensures that

multiple components received of one symbol due to

multipath environment do not interfere with the adjacent

symbols components [8].

That inserted guard time can be zero content signals.

However, introducing such guard slots raises the effects of

Inter Carrier Modulation (ICI). ICI is the consequences of

losing subcarriers orthogonality in OFDM system and that

happens when the receiver tries to retrieve one of the

subcarriers while a portion from the adjacent symbol is

being added causing ICI [7]. Simply means when OFDM

subcarriers are not properly synchronized, the problem of

ICI would arise, or i.e. when having frequency offset

between OFDM subcarriers as it is illustrated in Figure 4.

That frequency offset causes ICI between adjacent

subcarriers when trying to demodulate one subcarrier

which is not synchronized properly with the adjacent one.

The proposed solution for this problem is adding cyclic

prefix (CP). CP was proposed first in 1980 by Peled and

Ruiz as in [9]. Their idea was to occupy those time guard

slot of any OFDM symbol with the last portion of that

symbol as it is shown in Figure 5, a & b. That portion

duration chosen to have Ng samples. This time duration

should be longer than the longest expected symbol delay

spread. Therefore, the transmitted signal will have (Ng + N)

samples. The advantage of CP is to create an integer

number of cycles of each subcarrier when being passed

through FFT process or simply to maintain subcarriers

orthogonality [8].

As a result of adding CP, energy from one subcarrier will

not interfere with others within FFT process. At the

receiver side, in order to retrieve the original N samples,

the added Ng samples should be excluded out. Doing so

means that the signal to noise ratio (SNR) gets decreased

due to the fact that these samples are being transmitted and

received and they have no useful information. In addition

to the drop in SNR, power consumption is a drawback of

adding CP. That high power consumption takes place

because the transmitter consumes power to transmit CP

samples which contain no useful data.

a)

b)

Figure 3: (a) & (b) show how the guard band is left In

between OFDM symbols [8].

Figure 4: Shows frequency offset problem which yields

to ICI.

3.1 OFDM frequency flat fading

OFDM has been the most known modulation scheme

candidate for high data rate wireless communications.

OFDM does group high data rate input stream into smaller

lower rate multiple sub-streams to ensure that each

sub-stream would have a flat fading not a frequency

selective one [10].

Selective frequency fading takes place when having a

wide range of frequencies and some of these frequencies

are more vulnerable to be faded than other frequencies are.

Therefore, when OFDM divides the wideband frequencies

into narrower frequency sub-bands, those narrow bands

experience the same fading effects along that sub-band

which is called flat frequency fading. It is known that flat

frequency fading consequences are lighter and easier to be

taken care of than the selective frequency fading

consequences are. One of the most valued benefits of not

having frequency selective fading is that the receiver no

longer needs complex equalizers and RAKE receivers [10].

Figure 5: a) Shows CP procedure [16].

Figure 5: b) Shows how CP is being added [16]

To be more specific, a communication system with flat

frequency fading needs only simple equalizer at the

receiver side, while the same communication system would

need more than one complex tap equalizers if having

frequency selective fading.

4. PAPR in OFDM

One of the OFDM implementation challenges is peak to

average power ration (PAPR).The major challenge of

implementing OFDM system is the high PAPR. IFFT

procedure in OFDM system involves adding up N

subcarriers which are all sinusoids. This summation of N

subcarriers, at some points gives high peaks values

compared to the average ones and that is well known as

high PAPR. This high PAPR is simply described as an

envelope fluctuation which causes many issues that

degrade the system performance as in [1], [4], and [13].

The most commonly used technique to evaluate the system

PAPR is the cumulative distribution function (CDF). In the

most recent publications, the complementary cumulative

distribution function (CCDF), which can be derived from

the CDF, is also used to evaluate the system PAPR.

5. PTS in OFDM

Partial Transmit Sequences (PTS) is one of the best

techniques that reduce the effect of the peak to average

power ration (PAPR) in OFDM scheme. For this technique,

the input data block is divided into N sub-blocks and each

one of these sub-blocks has multiple subcarriers. For each

sub-block, the subcarriers are weighted by a phase factor.

These phase factors are developed in a way to get the

PAPR of the combined OFDM signal reduced. The block

diagram of PTS is shown in Figure 6.

6. Results

Simulation results are shown in Figure 7 for OFDM

signals by calculating its PAPR (Peak to Average Power

Ratio) based on different sample points Ns, 64,

128,256,512 and 1024. The x-axis represents the diverse

OFDM values, while y-axis represents the CCDF values.

The simulation in Figure 8 shows that OFDM performs

better when the some partial transmit sequences (PTS) are

chosen for different sub blocks points. The x-axis

represents the PAPR (dB) values, while y-axis represents

the probability of (PAPR > PAPR0) values.

The result of OFDM when PAPR is considered shows

that CCDF (Complementary Cumulative Distribution

Function) increases when the number of chosen points of

OFDM increase which make the system fast and more

applicable. The results illustrated the performance of

OFDM signal in response to using FFT, BPSK and QAM.

Moreover, the CCDF of OFDM signal is more accurate

when the number of OFDM points is increased. Using PTS

of PAPR the OFDM performance has clear improvement

and strong ability. The results show that probability of

(PAPR > PAPR0) increased from 10-4 to 10-1.

Figure 6: PTS system block diagram [4]

Figure 7: the relation between OFDM and CCDF for

different points with N= 64, 128, 256, 512, and 1024

7. Conclusion

Orthogonal Frequency Division Multiplexing (OFDM) is

an efficient technique that increases speed, range,

reliability and spectral efficiency for wireless systems. In

this paper we present OFDM using Partial Transmit

Sequences (PTS). The simulation results demonstrate the

improvement in capacity and reliability of OFDM using

PTS. Furthermore, we demonstrate that OFDM signal

becomes more accurate when the number of OFDM points

is increased. Also, using FFT, BPSK and QAM techniques

improve OFDM performance and throughput. Moreover,

OFDM is more efficient and flexible when the PTS PAPR

reduction technique is used.

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Figure 8: the relation of 16 QAM CCDF signal with PTS

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