DWT OFDM SYSTEMS WITH REDUCED PAPR FOR NEXT GENERATION MOBILE...
Transcript of DWT OFDM SYSTEMS WITH REDUCED PAPR FOR NEXT GENERATION MOBILE...
DWT OFDM SYSTEMS WITH REDUCED PAPR FOR NEXT
GENERATION MOBILE NETWORKS
Mr Y. A. LAFTA 1 and Dr. P. JOHNSON
2
School of engineering, Liverpool John Moores University, Byrom Street, Liverpool, L3 3AF,
UK
ABSTRACT
Peak to power average ratio (PAPR) is a
serious issue in OFDM systems. Several
techniques have been proposed by
researchers to reduce PAPR. A technique
proposed, by Tellambura applies real-
valued modulation schemes to reduce
PAPR in systems using a limited number
of subcarriers. However, the technique has
not been evaluated for its performance of
bit error rate using discrete time
approximations of the continuous-time
PAPR. On other hand, Wong proposed a
technique that applies complex modulation
as a practical technique for computing the
continuous-time peak-to-average to power
ratio (PAPR) of OFDM. However, this
technique does not consider coherent
multilevel modulation scheme. Khalid and
Shah have proposed to select the best
wavelet packet basis function to decrease
the PAPR. This technique results in no less
than 1-2 dB reduction in PAPR. Jiang
proposed a new companding transform for
PAPR reduction in OFDM systems, which
was shown to improve PAPR by no less
than 2.2 dB. In this paper, combination of
DWT OFDM with 64DAPSK is proposed
for better BER performance and improved
PAPR and interference values. It is
demonstrated that including companding
with this system results in further
reduction of PAPR.
Correspondence should be addressed to YHYA LAFTA on
KEYWORDS
Orthogonal Frequency-Division
Multiplexing (OFDM), Discrete Wavelet
Transformer (DWT), Inverse Discrete
Wavelet Transformer (IDWT), Differential
Amplitude and Phase Shift Keying
(DAPSK), Additive Wight Gaussian Noise
(AWGN), Rayleigh channel Peak to power
average ratio (PAPR).
1 INTRODUCTION
1.1 Orthogonal Frequency Division
Modulation (OFDM) Systems
Service providers working towards
4G communication systems are
continuously met with the challenge of
accommodating more and more users
within a limited available bandwidth [5].
The main advantage of the OFDM systems
is that they are immune to multi path
fading [7]. However, their major
disadvantage is that the transmitted signal
has a high peak-to-average power ratio
(PAPR). Many research investigations
have lead to the conclusion that the
wavelet based OFDM is more
advantageous than the Fourier based
OFDM [1], [12], [3]. One of the main
reasons being Fourier based OFDM
systems require allocation of guard bands
between subcarrier frequencies to avoid
inter symbol interference, however
wavelet based OFDM systems do not
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require such guard bands and therefore the
overall system bandwidth is increased.
The OFDM scheme provides an
efficient means to handle high speed data
streams. This is because an OFDM is a
multi carrier modulation (MCM) technique
which divides the entire data stream into
several small number of lower data-rate
subcarrier data streams, thus reducing the
effect of frequency selective fading [8].
Using a large number of sub
carriers to narrow down the spectrum for
each sub carrier is the most straightforward
way to spread the data across a wider
spectrum. However, this increases system
complexity and the cost. This also
increases the sensitivity to Doppler Effect
and demands higher accuracy of frequency
synchronization.
A set of cosinusoidal functions
, n=0,1,….., N-1
representing the signal can be used on
orthogonal basis to achieve the multi
carrier modulation (MCM) scheme that
can be synthesized using a discrete cosine
transform (DCT) [14]. The minimum
frequency spacing required to satisfy the
DCT based OFDM is [16]. The
baseband DCT-OFDM signal x (t) is still a
real value when the data symbols are
obtained using real valued modulation
formats, such as pulse amplitude
modulation (PAM) and binary phase shift
keying ( BPSK).
The discrete wavelet transform
(DWT) is represented by a function of a
countable set of wavelet coefficients, i.e.
individual wavelet functions localized in
space [Zhang, H. et al 2007]. The wavelets
are a family of functions constructed from
translation and dilation of signal
function called the mother wavelet. Due
to an extremely high spectral containment
property of the wavelet filters and the fact
that extremely less energy is contained in
the side lobes, the amount of interference
between carriers in wavelet systems is
much lower than that in Fourier systems
[12]. Therefore, even in the absence of
guard bands, DWT-OFDM can combat
narrowband interference better than the
traditional discrete Fourier transform
systems and is inherently more robust in
terms of ICI (DFT-OFDM) [2]. This
research work proposes a combination of
DWT with DAPSK modulation scheme in
anticipation that this system should have
reduced PAPR compared to the existing
systems.
1.2 Organization of The paper
The organization of this paper is as
follows: section 1.3 reviews the QAM
modulation scheme and 1.4 reviews related
work in the area of DCT (DWT) –OFDM
systems with QAM modulation for high
data rate next generation communication
networks. Finally, section 2 presents the
simulation and the results of the proposed
combination of DWT system with DAPSK
modulation scheme and compares to that
with QAM modulation schemes. The
performance from the system is evaluated
against metrics such as Eb/No, ISI and
PAPR and compared with that of DCT
system.
1.3 Modulation Schemes
Quadrature amplitude modulation
(QAM) is more attractive for the highly
mobile radio environment (e.g., in a car or
a train), which is a high-order and non-
coherent modulation scheme, employed as
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a potential means to give further
improvement to the spectral efficiency.
QAM scheme does not require
complicated channel estimation and
equalization to be detected by the receiver.
Use of pilot symbol can be avoided to
more efficiently transmit the information-
bearing data. Moreover, the non-coherent
modulation can preferably withstand the
Doppler Effect caused by the high mobility
of the transmitter and receiver [10]. The
16QAM has a more efficient signal
distribution than the 16DAPSK, in terms
of the relative power saving in an AWGN
channel [13]. Since highly accurate carrier
recovery and phase tracking are required in
the receiver, applying coherent detection in
the mobile radio environment is a very
difficult task. It is also important to
accurately track the fading envelope for
systems such as 16QAM.
The coded bits are mapped to form
symbols which use gray coding in the
constellation map. The coding rate is (1/2)
for the 16QAM modulation scheme. When
the symbol is normalized, the average
power is unity irrespective of the
modulation scheme used. The data are
converted by the process to corresponding
value of M-ary constellation, which is a
complex word, constituting real and
imaginary parts. The bandwidth B =1/ is
divided into N equally spaced subcarriers
at frequencies (kΔf), k=0, 1, 2… N-1 with
Δf=B/N and as the sampling interval.
Grouping and mapping information bits
into complex symbols will be carried out
at the transmitter. Inverse wavelet
Transform IDWT or inverse discrete
cosine IDCT will be used to modulate
spread data symbol on the orthogonal
carriers, as in conventional OFDM.
1.4 Related work
As a candidate for the 3rd generation
mobile telecommunication system the
OFDM systems have attracted
considerable research attention. The
OFDM system has successfully been
implemented [9] in the wireless
broadcasting applications such as Digital
Video Broadcasting (DVB) and Digital
Audio Broadcasting (DAB). The above
mentioned systems use data rates in the
range of 25Mbits/s, the ETSI-BRAN
system employs this transmission
technique for HIPERLAN 2 (high
performance local area networks), and also
for the extension of the IEEE 802.11
standard for the 5-GHz frequency range.
Future mobile communication systems
with data rates far beyond universal mobile
telecommunication system (UMTS) can be
realized using OFDM as a suitable
transmission technique.
2 2 SIMULATION AND
ANALYSIS
The results of the simulation carried out
on DWT and DCT-OFDM systems with
64DAPSK/64QAM modulation schemes
with AWGN and Rayleigh transmission
channels, and random binary stream input
data are presented in this section in figures
1 to 4. The number of samples for the sub
carrier N is 64, and the number of samples
for the symbols is 1000. If the number of
samples for the sub carriers and symbols
were larger, the time for running the
simulations will be longer. Also, it has
been verified that a larger value than the
present value for the symbols does not
make significant difference to the results.
The figures 1a to 1c represent the
power spectral density of the received
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signal for DWT (and DCT)-OFDM
systems with 64QAM
Fig1a Power spectral densities for DWT and DCT
systems with 64QAM modulation schemes via
AWGN channel
Fig1b Power spectral densities for DWT and DCT
systems with 64QAM modulation schemes via
Rayleigh (Gaussian) channel
Fig1c Power spectral densities for DWT and DCT
systems with 64QAM modulation schemes via
Rayleigh (Jakes) channel
modulation schemes using AWGN
channel with Gaussian spectrum and
Rayleigh channel with Jakes spectrum.
The simulation results indicate that the
DWT-OFDM with 64QAM scheme has
higher signal power than that for the DCT
based OFDM system using the same
modulation scheme. It is also clearly seen
that the power spectral density (PSD) for
DWT-OFDM is approximately (20-
40dB/rad/sample) better than the DCT-
OFDM system when using the 64QAM
scheme modulation via AWGN
transmission channel type. This huge
difference in PSD between the received
signals which were transmitted with the
same input power leads to the observation
that DWT system has much better signal to
noise ratio. Whereas, the DWT-OFDM
system with 64QAM using Rayleigh with
Jakes spectrum demonstrates to have
higher bandwidth efficiency when
compared to the DCT OFDM system
under exactly same conditions. The DCT-
OFDM seems to have better than PSD
signal using the AWGN and Rayleigh with
Gaussian spectrum. This means that the
spectrum for this system will have smaller
side-lobes and so will be immune to inter-
symbol interference (ISI).
Fig1d Power spectral densities for DWT system
with 64DAPSK modulation schemes via AWGN
channel
Fig1e Power spectral densities for DWT system
with 64DAPSK modulation schemes via Rayleigh
(Jakes) channel
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Fig1f Power spectral densities for DWT system
with 64DAPSK modulation schemes via Rayleigh
(Gaussian) channel
The figures 1d to 1f represent the
power spectral density of the received
signal for DWT -OFDM systems with
64DAPSK modulation scheme using
AWGN or Rayleigh channels with
Gaussian and Jakes spectrum respectively.
The DWT-OFDM system with 64DAPSK
has 75dB higher PSD than that of the
DWT-OFDM system with 64QAM
scheme, though with less bandwidth
efficiency .
The PSD of the DWT and DCT
OFDM systems were also simulated with
and without -law companding, and
exponential transforms. The results are
shown in figures 2a and 2b. Figure 2a
representing the DWT-OFDM system with
64DAPSK scheme using Rayleigh
Channel shows that the signal with no-
companding suffers from out-of-band
interference (OBI) compared to the signal
that has gone through companding or
exponential transforms.
Fig.2a PSD for DWT with 64DAPSK channel
Raleigh ( Jakes and Gaussian)
Fig.2b PSD for DWT with 64DAPSK channel
AWGN
The DWT-64DAPSK system in Fig.2a
using Rayleigh channel with Jakes or
Gaussian spectrum is shown to have at
least 32dB lower PSD with or without
companding when compared to that with
exponential transform. Fig.3 represents
DCT-OFDM system with 64QAM scheme
using Rayleigh Channel and AWGN
channels, the DCT-64QAM is shown to
have at least 15dB lower PSD with
exponential transform when compared to
that with or without companding.
Fig.3 PSD for DCT with 64QAM Rayleigh’s
channel (Jakes and Gaussian) and AWGN
Fig.4 CCDF for DWT/DCT with
64DAPSK/64QAM for no companding,
companding and exponential signals
In figure 4, the continuous line
graphs represent the complementary
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cumulative distribution (CCDF) vs. PAPR
performance for DWT systems, whereas
the dashed lines represent the same for
DCT systems. It can be observed from the
graph that both DCT and DWT systems
with companding have at least 1.9 dB less
PAPR than the systems without
companding and the systems with
exponential transform have 1.3 dB less
PAPR than that without any transform.
3 CONCLUSIONS
In conclusion, the huge difference (20-
40dB/rad/sample) in PSD between the
received signals which were transmitted
with the same input power leads to the
observation that DWT system has much
better signal to noise ratio. Moreover, the
DWT-OFDM system with 64QAM using
Rayleigh with Jakes spectrum
demonstrates to have higher bandwidth
efficiency when compared to the DCT
OFDM system under exactly same
conditions. However, the DCT-OFDM
seems to have better than PSD signal
using the AWGN and Rayleigh with
Gaussian spectrum. This means the
spectrum for this system will have smaller
side-lobes and so will be immune to inter-
symbol interference (ISI). Significant
reduction of PAPR was obtained in
systems using companding signal, i.e. 1.9
dB less PAPR than the systems without
companding. Also, systems using
exponential transform achieved1.3 dB less
PAPR than without any transform. These
performance figures lead us to believe that
the proposed combination of DWT-
DAPSK system with companding or
exponential transforms should be best
suited for the next generation mobile
communication networks.
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