[152]

CHAPTER 7 CONCLUSIONS AND FUTURE

SCOPE OF WORK

he conclusions drawn from the analytical and simulation studies made and reported in the

thesis are included in this chapter. This study augments the performance of existing OFDM

system.

7.1 CONCLUSION

The importance of multi-carrier communication system has been established in the present

communication era. The merits and demerits of OFDM system along with its implication are discussed in

first chapter. Multi-carrier communication system especially OFDM has been evolved as one of such

potential candidate which are bandwidth efficient and robust to multipath channel condition (frequency

selective fading). The research activities in OFDM have grown tremendously during last two decades.

Due to its advantageous features like high spectral efficiency, easy equalization and robustness to

frequency selective fading channel, the OFDM has been adopted by many broadband wireless

communication standards like DAB, DVB-T, IEEE 802.11, 802.16 and UWB communication systems.

Besides so many advantageous and favorable features, there exist some major drawbacks of

OFDM which must be resolved for getting all the advantages. Therefore, for overall improvement in the

performance of OFDM system, it is required to handle all these issues separately. This thesis presents a

brief review of major problems of OFDM system with their existing solutions. The main focus of work

was to provide an appropriate solution to each and every major problem like high PAPR, timing

synchronization, frequency synchronization and ICI reduction. After the review of different concerns and

their solutions, following objectives were set for this study:

Development of a suitable timing offset estimator,

Algorithm enumeration for frequency offset estimation,

Establishment of methods for PAPR reduction, and Induction of new pulse shape in OFDM receiver for ICI reduction

T

[153]

A new technique which is based on the use of FRFT and chirp signal is proposed for timing offset

estimation in an OFDM system. The proposed method uses chirp signal as a training sequence and

employed the FRFT as a tool to localize the training sequence (chirp) at the receiver. The comparative

study shows the superiority of the proposed estimator in terms of mean and MSE of timing offset. The

MSE of timing offset with proposed method is found 76% (5 dB SNR) and 63% (8 dB SNR) lower than

Awoseyila et.al method, in HIPERLAN/2 indoor channel-A and in Wi-Max system (strong fading

channel) respectively. However, the betterment in MSE is obtained in the proposed method at the cost of

increased computational complexity, in terms of (푁 4⁄ − 1 + 1.4427 푁 푙표푔 푁) more complex

multiplication than Awoseyila et.al method.

OFDM works on the principle of orthogonality, sub-carriers are orthogonal to each other. This

orthogonality between sub-carriers may be destroyed if carrier frequency offset arises between them due

to Doppler spreading while transmission. The main task of frequency synchronization in OFDM system is

to estimate and correct this carrier frequency offset at the receiver. Lot of frequency offset estimators is

available in the literature. Out of which, chirp signal based estimator (given by Wei et.al) performs better

than others. It is also found that, the FRFT is better tool than FT to determine the frequency offset when

chirp signal is used as a training sequence.

Thus, on the basis of modulation property of FRFT, new frequency offset estimation has been

proposed. With the proposed method, the MSE of frequency offset estimation is found very low in

comparison to other discussed methods, in multipath fading channel. There is an approximately 3 dB

betterment of SNR for obtaining same MSE, as compared with existing methods. In terms of

computational complexity, the proposed algorithm is better than Wei et.al and Schmidl & Cox method.

It is known fact that the high PAPR is one of the major issues associated with the OFDM system.

High PAPR exists due to large dynamic range of input signal. Due to this high PAPR, the efficiency of

high power amplifier at the output of transmitter decreases. Several PAPR reduction methods are available

in the literature. Each technique has its own merit and demerit in terms of PAPR reduction capability,

bandwidth expansion, and implementation complexity. It is found that pre-coding and clipping is the

simplest and efficient method of PAPR reduction.

Therefore, a method is suggested to reduce the PAPR which is the combination of pre-coding and

clipping method. For pre-coding matrix generation, a new window family (MBH window family) is

proposed. Also a new algorithm is proposed for clipping process which is based on iterative clipping. The

[154]

CCDF of PAPR with hybrid method (pre-coding with conventional clipping) at α = 11% is similar to

CCDF of PAPR with only pre-coding method at α = 22%. Therefore, a 50% overhead is reduced.

With normal OFDM system, at 10 CCDF, the value of PAPR is 10.2 dB and with EC method

it is 2.6 dB, whereas with proposed clipping algorithm (푁푂퐼 = 70,푄 = 28), it is obtained 2.1 dB. The

gain of proposed method with respect to Original OFDM signal is 8.1 dB and with respect to EC method

is 0.5 dB.

Literature survey reveals that the results of CFO and STO will appear in the form of inter-carrier

interference. CFO and STO can be estimated and corrected by frequency and timing offset estimators, but

residual errors are inevitable. Therefore, suitable methods are required to reduce ICI at the receiver. It is

also found that receiver windowing is one of the best methods available to reduce ICI. For receiver

windowing, a Nyquist pulse shape is required. Several Nyquist pulse shapes (like RC, BTRC, SOCW,

ISP, PMSP, etc) are available in the literature.

A new window family called as MBH window family is proposed for the improvement in OFDM

system both in terms of ICI power and BER. It is found that many existing pulse shapes are the member

of this window family at different value of pulse shape parameter ′훽′. The MBH window family (with

different β) provides bit error rate better than BTRC, Frank, SOCW and PMSP pulse shapes. With the

help of SIR analysis, the value of 훽 has been determined for given frequency offset and roll-off factor.

In terms of ICI, the performance of MBH pulse shapes is in between BTRC and Frank pulse when the

pulse roll-off factor ′훼′ is less than unity. For 훼 = 1, its performance is equivalent to Frank and SOCW.

The PMSP gives best ICI reduction among all the pulse shapes considered here, but its BER performance

is worst and close to non-windowing option. In this way, the BER performance of MBH window family

is found best amongst all available pulse shapes. This clearly establishes that MBH window family

provides best pulse shapes for improving an OFDM system in terms of both ICI and BER together. With

the proposed window family, the ICI is reduced from -14.28 dB (without windowing) to -36.71 dB at

휀 = 0.1, and BER decreases from 4.559 × 10 to 3. 29 × 10 at 50 dB SNR.

[155]

Figure-7.1: Augmented baseband OFDM system model

After incorporating all the proposed modifications, which are included from Chapter-3 to

Chapter-6, the augmented block diagram of OFDM communication link has been redrawn in Figure 7.1.

The shaded blocks are added to the original OFDM communication link as shown in Figure 2.2.1. This

clearly depicts the contribution made in this study and reported in this thesis.

7.2 FUTURE SCOPE

In this study, different problems of OFDM system have been considered and suitable solutions have

been provided. As it is an established fact, that research is never ending process, a new beginning is

always waiting. Therefore, following are the works that may be considered as a future scope in this

direction:

(a) The channel estimation is an area which required a lot of attention and improper channel

estimation degrades the performance of system. In this work, it is assumed that channel is

estimated perfectly. Hence one can evaluate the performance of proposed work with different

channel estimation method.

[156]

(b) The algorithm of timing offset estimation can be extended for channel estimation in OFDM

system.

(c) The proposed timing offset and frequency offset estimator can be utilized for MIMO OFDM

system.

(d) The proposed PAPR reduction method can be used with MIMO OFDM system.

(e) The closed form expression of BER can be derived for OFDM system with proposed PAPR

reduction method.

(f) The windowing method of ICI reduction can be clubbed with ICI self cancellation scheme.

*******************************************************

[157]

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