25-09-2014, 11:43 AM
DOCTOR OF PHILOSOPHY
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Introduction
Traditional single carrier modulation techniques can achieve only limited data rates due to the
restrictions imposed by the multipath effect of wireless channel and the receiver complexity.
High data-rate is desirable in many recent wireless multimedia applications [1]. However, as
the data-rate in communication system increases, the symbol duration gets reduced.
Therefore, the communication systems using single carrier modulation suffer from severe
intersymbol interference (ISI) caused by dispersive channel impulse response, thereby
needing a complex equalization mechanism. Orthogonal Frequency Division Multiplexing
(OFDM) is a special form of multicarrier modulation scheme, which divides the entire
frequency selective fading channel into many orthogonal narrow band flat fading sub
channels. In OFDM system high-bit-rate data stream is transmitted in parallel over a number
of lower data rate subcarriers and do not undergo ISI due to the long symbol duration
Literature Review
Many PAPR reduction techniques are proposed in the literature [3]-[18] to reduce the PAPR
of the OFDM signal. The PAPR reduction schemes are majorly divided into two categories
a) Distortion based Techniques [4]-[10]
b) Non-distortion Techniques [11]-[18]
The schemes that introduce spectral re-growth belong to distortion based category. These
techniques are the most straightforward PAPR reduction methods.
The clipping [3] is one of the simplest distortion based technique to reduce the PAPR of
OFDM signal. It reduces the peak of the OFDM signal by clipping the signal to the desired
level but it introduces both in-band distortion and out-of-band radiation. To limit out-of-band
radiation and PAPR, Jean Armstrong proposed iterative clipping and filtering scheme [4].
Companding is another popular distortion based scheme for PAPR reduction in OFDM
system. In [5], Wang et al. proposed a scheme based on μ-law companding to reduce the
PAPR of OFDM signal. In μ-law companding scheme the peak value of the OFDM signal
before and after companding remains same, which keeps peak power of the OFDM signal
unchanged but the average power of the OFDM signal after companding increases and
therefore the PAPR of the OFDM signal gets decreased. But due to increase in the average
power of the OFDM signal the error performance of μ-law companding scheme degrades.
Motivation
The current implementations of OFDM do not fully exploit the capabilities of OFDM. There
are still several avenues which can be explored to reduce the peak-to-power ratio (PAPR) of
OFDM signal. The PAPR performance of existing ICI cancellation schemes is either same or
worse than normal OFDM signal. Therefore, the necessity to reduce the PAPR of normal
OFDM signal and OFDM signal obtained from ICI cancellation schemes has been a prime
motivating factor for this work. The thesis aims at exploring and arriving at the schemes for
PAPR reduction in OFDM based systems (with and without ICI cancellation scheme) of
practical use.
Work Done
As discussed earlier, companding can reduce the dynamic range or PAPR of the OFDM
signal at the cost of BER performance degradation. In order to improve the BER performance
of companding based PAPR reduction scheme a quadrilateral companding transform (QCT)
is proposed. The proposed QCT transforms the original distribution of OFDM signal into
quadrilateral distribution hence the name “Quadrilateral Companding Transform”. To achieve
this transformation, we have derived the mathematical expression of companding function
used to perform the companding operation on OFDM signals. The proposed companding
function is monotonic increasing function and its inverse function exists. At the receiver an
expanding function to retrieve the original OFDM signal is used, for which we have also
derived the mathematical expression. The proposed scheme has five parameters to design the
companding function
Conclusions
PAPR reduction and ICI cancellation are two major challenges in implementing an OFDM
system. This thesis presents the study of existing PAPR reduction and ICI cancellation
schemes and proposes improved schemes for PAPR reduction in OFDM systems. This thesis
also proposes a PAPR reduction scheme for OFDM system utilizing ICI cancellation scheme.
Based on the study of companding based PAPR reduction schemes it has been found that the
error performance of existing companding schemes degrade by increasing the parameter
controlling the nonlinearity. Based on our findings, an improved companding scheme i.e.
quadrilateral companding transform has been proposed, which has better flexibility to design
the companding function in comparison to existing companding schemes. Quadrilateral
companding transform has better BER performance with good PAPR reduction capability in
comparison to existing companding based PAPR reduction schemes.
Non-distortion PAPR reduction techniques like PTS and SLM are the two most promising
techniques to reduce the PAPR of the OFDM signal but both of them suffer from the problem
of SI, the MPSM-PTS and proposed CCM-PTS schemes completely eliminate the
requirement of SI transmission and therefore these are found to be the good alternatives ofexisting SI embedding schemes. Proposed CCM-PTS scheme can effectively reduce the
PAPR of the OFDM signal and its SER performance over AWGN and fading channel, is
superior than existing MPSM-PTS scheme.
[attachment=68515]
Introduction
Traditional single carrier modulation techniques can achieve only limited data rates due to the
restrictions imposed by the multipath effect of wireless channel and the receiver complexity.
High data-rate is desirable in many recent wireless multimedia applications [1]. However, as
the data-rate in communication system increases, the symbol duration gets reduced.
Therefore, the communication systems using single carrier modulation suffer from severe
intersymbol interference (ISI) caused by dispersive channel impulse response, thereby
needing a complex equalization mechanism. Orthogonal Frequency Division Multiplexing
(OFDM) is a special form of multicarrier modulation scheme, which divides the entire
frequency selective fading channel into many orthogonal narrow band flat fading sub
channels. In OFDM system high-bit-rate data stream is transmitted in parallel over a number
of lower data rate subcarriers and do not undergo ISI due to the long symbol duration
Literature Review
Many PAPR reduction techniques are proposed in the literature [3]-[18] to reduce the PAPR
of the OFDM signal. The PAPR reduction schemes are majorly divided into two categories
a) Distortion based Techniques [4]-[10]
b) Non-distortion Techniques [11]-[18]
The schemes that introduce spectral re-growth belong to distortion based category. These
techniques are the most straightforward PAPR reduction methods.
The clipping [3] is one of the simplest distortion based technique to reduce the PAPR of
OFDM signal. It reduces the peak of the OFDM signal by clipping the signal to the desired
level but it introduces both in-band distortion and out-of-band radiation. To limit out-of-band
radiation and PAPR, Jean Armstrong proposed iterative clipping and filtering scheme [4].
Companding is another popular distortion based scheme for PAPR reduction in OFDM
system. In [5], Wang et al. proposed a scheme based on μ-law companding to reduce the
PAPR of OFDM signal. In μ-law companding scheme the peak value of the OFDM signal
before and after companding remains same, which keeps peak power of the OFDM signal
unchanged but the average power of the OFDM signal after companding increases and
therefore the PAPR of the OFDM signal gets decreased. But due to increase in the average
power of the OFDM signal the error performance of μ-law companding scheme degrades.
Motivation
The current implementations of OFDM do not fully exploit the capabilities of OFDM. There
are still several avenues which can be explored to reduce the peak-to-power ratio (PAPR) of
OFDM signal. The PAPR performance of existing ICI cancellation schemes is either same or
worse than normal OFDM signal. Therefore, the necessity to reduce the PAPR of normal
OFDM signal and OFDM signal obtained from ICI cancellation schemes has been a prime
motivating factor for this work. The thesis aims at exploring and arriving at the schemes for
PAPR reduction in OFDM based systems (with and without ICI cancellation scheme) of
practical use.
Work Done
As discussed earlier, companding can reduce the dynamic range or PAPR of the OFDM
signal at the cost of BER performance degradation. In order to improve the BER performance
of companding based PAPR reduction scheme a quadrilateral companding transform (QCT)
is proposed. The proposed QCT transforms the original distribution of OFDM signal into
quadrilateral distribution hence the name “Quadrilateral Companding Transform”. To achieve
this transformation, we have derived the mathematical expression of companding function
used to perform the companding operation on OFDM signals. The proposed companding
function is monotonic increasing function and its inverse function exists. At the receiver an
expanding function to retrieve the original OFDM signal is used, for which we have also
derived the mathematical expression. The proposed scheme has five parameters to design the
companding function
Conclusions
PAPR reduction and ICI cancellation are two major challenges in implementing an OFDM
system. This thesis presents the study of existing PAPR reduction and ICI cancellation
schemes and proposes improved schemes for PAPR reduction in OFDM systems. This thesis
also proposes a PAPR reduction scheme for OFDM system utilizing ICI cancellation scheme.
Based on the study of companding based PAPR reduction schemes it has been found that the
error performance of existing companding schemes degrade by increasing the parameter
controlling the nonlinearity. Based on our findings, an improved companding scheme i.e.
quadrilateral companding transform has been proposed, which has better flexibility to design
the companding function in comparison to existing companding schemes. Quadrilateral
companding transform has better BER performance with good PAPR reduction capability in
comparison to existing companding based PAPR reduction schemes.
Non-distortion PAPR reduction techniques like PTS and SLM are the two most promising
techniques to reduce the PAPR of the OFDM signal but both of them suffer from the problem
of SI, the MPSM-PTS and proposed CCM-PTS schemes completely eliminate the
requirement of SI transmission and therefore these are found to be the good alternatives ofexisting SI embedding schemes. Proposed CCM-PTS scheme can effectively reduce the
PAPR of the OFDM signal and its SER performance over AWGN and fading channel, is
superior than existing MPSM-PTS scheme.