687 research outputs found
On the peak-to-average power of OFDM signals based on oversampling
Orthogonal frequency-division multiplexing (OFDM) introduces large amplitude variations in time, which can result in significant signal distortion in the presence of nonlinear amplifiers. We introduce a new bound for the peak of the continuous envelope of an OFDM signal, based on the maximum of its corresponding oversampled sequence; it is shown to be very tight as the oversampling rate increases. The bound is then used to derive a closed-form probability upper bound for the complementary cumulative distribution function of the peak-to-mean envelope power ratio of uncoded OFDM signals for sufficiently large numbers of subcarriers. As another application of the bound for oversampled sequences, we propose tight relative error bounds for computation of the peak power using two main methods: the oversampled inverse fast Fourier transform and the method introduced for coded systems based on minimum distance decoding of the code
PAPR reduction in OFDM system using combined MCS and DHMT precoding
Orthogonal Frequency Division Multiplexing (OFDM) has become a preferable scheme for most high data rate wireless communication standards. However, the non-linear power amplifier effect experienced in the OFDM system has increases the peak-to-average power ratio (PAPR). This paper proposed a Median Codeword Shift (MCS) as a new solution to alleviate the effect of high PAPR. MCS takes advantage of the codeword structure and bit position changes through the manipulation of the codeword structure and permutation process to achieve a low PAPR value. Additionally, the enhanced version of MCS is also being proposed by merging MCS with the Discrete Hartley matrix transform (DHMT) precoding method to boost the PAPR reduction. Simulation results show that MCS is capable of minimizing PAPR of conventional OFDM with 24% improvement and at the same time outperform Selective Codeword Shift (SCS) with a 0.5 dB gap. A remarkable result was also achieved by MCS-DHMT with a 15.1% improvement without facing any bit error rate (BER) degradation
Peak to Average Power Ratio Reduction and Bit Error Rate Improvement in Wireless Orthogonal Frequency Division Multiplexing Communication Systems
Orthogonal frequency division multiplexing (OFDM) offers high data rate transmission
with high spectral efficiency, immunity to multipath fading, and simple implementation
using fast Fourier transform (FFT). OFDM is readily implemented by present day
processors in many high speed networks. However, one of the major drawbacks of
OFDM systems is the high peak-to-average power ratio (PAPR); this can result in poor
power efficiency, degradation in bit-error-rate (BER) performance, and spectral
spreading. The effective PAPR reduction of OFDM signals by simple processing has
been a challenge for the limited power and processing capability of portable OFDM
applications.
This thesis investigates the problem of high PAPR in OFDM systems and presents
many simple implementation PAPR reduction techniques, and one error-resilient
technique. The first part of this thesis presents two time-domain PAPR reduction techniques, viz,
square-rooting the envelope of the OFDM output signals, and the smoothing technique.
The square-rooting process changes the statistical distribution of the OFDM output
signals from Rayleigh to Gaussian-like distribution and reduces the differences between
the values of peak and average power, which consequently reduces the PAPR
significantly. About 6 dB reduction in PAPR is achieved with moderate degradation in
BER performance. For the smoothing process, which is derived from the image
enhancement technique, the smoothing applied on the OFDM signals mitigates the
PAPR due to its averaging effect. Up to 2.5 dB reduction is achieved by smoothing.
Two new probabilistic based non-iterative frequency-domain PAPR reduction
techniques are introduced in the second part of the thesis. These techniques reduce
PAPR by changing the statistical distribution of the OFDM modulated symbols from
uniform distribution to Gaussian-like distribution. This task is performed by two
different methods in two different PAPR techniques. The first method of PAPR
reduction is done by the addition of complex Gaussian random signals, while the second
one is done by insertion of dummy Gaussian subcarriers. The two techniques provide
PAPR reduction in the order of 5 dB for PSK-OFDM systems with no out-of-band
radiation. The adaptive operation of these techniques enhances significantly both the
BER performance and reduce the transmission power.
The last part of this thesis presents a new modulation-based error resilient technique
referred to as multi-dimensional modulation technique (MDM). In this technique
concatenation of digital modulators of decreasing modulation orders are employed. The
MDM technique improves the BER performance linearly with increased size of modulation order; up to 12 dB improvement in Eb/No ratio is achieved relative to the
conventional OFDM systems at high modulation orders, M≥1024. Also, the MDM
technique offers both error resilience and PAPR reduction when it is combined with the
conventional OFDM systems in time domain.
As a conclusion, the proposed techniques described above offer new solutions to the
problem of high PAPR in OFDM systems, and for one of them offer improvement of
BER performance at the same time. Besides, they can be applied for different systems
parameters and applications requirements. Moreover, the PAPR reduction techniques
proposed in this thesis are data-independent and can be implemented in one-shot; while
the MDM technique uses only digital modulation and dc-offset signal processing, which
can be implemented by simple circuits and/or processors
ON VARIOUS TECHNIQUES IN OFDM AND GFDM: A SURVEY
Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation that divides the available spectrum into a finite number of carriers and applied into a digital transmission system. But it has some drawbacks such as sensitivity in inter-carrier interference, high peak to average power ratio and insufficient cyclic prefix in spectrum. These drawbacks may be reduced by a technique known as Generalized Frequency Division Multiplexing (GFDM). In the present scenario, it is a high speed multi-carrier multiplexing data transfer scheme for the cellular network. This paper deals with a comparison between OFDM and GFDM and focuses on various techniques in OFDM and GFDM
A Physical Layer Security (PLS) approach through Address Fed Mapping Crest Factor Reduction applicable for 5G/6G signals
The privacy and security of 5G/6G infrastructures are receiving great attention together with power consumption and efficiency. Here, Physical Layer Security (PLS) is considered and a technique named Address Fed Mapping (AFM) is proposed which not only enhances the physical layer security, but also reduces the effect of high Peak to Average Power Ratio (PAPR), which results in efficiency improvement in OFDM based signals used in beyond 5G and 6G. The AFM is designed based on the idea of randomly generated signals, modifying the original signal to reduce PAPR. Instead of a typical randomization algorithm, a unique key is generated based on Channel response that is known only transmitter-receiver pairs. This key is used to pick a signal and send it. It is shown that the proposed AFM technique reduces PAPR, which improves the energy efficiency of the system
Performance of OPS-SAP technique for PAPR reduction in IEEE 802.11p scenarios
Vehicular Ad Hoc Networks (VANETs) are wireless networks that emerged thanks to the rapid evolution of wireless technologies and the automotive industry. The IEEE 802.11p standard is part of a group of standards related to all layers of protocols for Wireless Access in Vehicular Environment (WAVE) communications, which defines Medium Access Control (MAC) and Physical (PHY) levels. The PHY layer of IEEE 802.11p is essentially based on Orthogonal Frequency Division Multiplexing (OFDM) due to its advantages. However, OFDM signal suffers from high Peak-to-Average Power Ratio (PAPR) at the transmitter side, which causes a significant power efficiency penalty. An efficient peak power reduction technique is Simple Amplitude Predistortion aided by Orthogonal Pilot Sequences (OPS-SAP), which consists in moving certain outer constellation points of the frequency-domain OFDM symbol. In this paper, we propose the application of this OPS-SAP scheme in the IEEE 802.11p scenario, and, moreover, its evaluation under a complete PHY layer.This work has been supported by the Spanish National Projects GRE3N-SYST (TEC2011-29006-C03-03) and ELISA (TEC2014-59255-C3-3-R) and also by Escuela Politécnica a Nacional (Ecuador) by PII-DETRI-01-2016 Project
NOVEL OFDM SYSTEM BASED ON DUAL-TREE COMPLEX WAVELET TRANSFORM
The demand for higher and higher capacity in wireless networks, such as cellular,
mobile and local area network etc, is driving the development of new signaling
techniques with improved spectral and power efficiencies. At all stages of a
transceiver, from the bandwidth efficiency of the modulation schemes through highly
nonlinear power amplifier of the transmitters to the channel sharing between different
users, the problems relating to power usage and spectrum are aplenty. In the coming
future, orthogonal frequency division multiplexing (OFDM) technology promises to
be a ready solution to achieving the high data capacity and better spectral efficiency in
wireless communication systems by virtue of its well-known and desirable
characteristics.
Towards these ends, this dissertation investigates a novel OFDM system based on
dual-tree complex wavelet transform (D
Spectrally and Energy Efficient Wireless Communications: Signal and System Design, Mathematical Modelling and Optimisation
This thesis explores engineering studies and designs aiming to meeting the requirements of enhancing capacity and energy efficiency for next generation communication networks. Challenges of spectrum scarcity and energy constraints are addressed and new technologies are proposed, analytically investigated and examined.
The thesis commences by reviewing studies on spectrally and energy-efficient techniques, with a special focus on non-orthogonal multicarrier modulation, particularly spectrally efficient frequency division multiplexing (SEFDM). Rigorous theoretical and mathematical modelling studies of SEFDM are presented. Moreover, to address the potential application of SEFDM under the 5th generation new radio (5G NR) heterogeneous numerologies, simulation-based studies of SEFDM coexisting with orthogonal frequency division multiplexing (OFDM) are conducted. New signal formats and corresponding transceiver structure are designed, using a Hilbert transform filter pair for shaping pulses. Detailed modelling and numerical investigations show that the proposed signal doubles spectral efficiency without performance degradation, with studies of two signal formats; uncoded narrow-band internet of things (NB-IoT) signals and unframed turbo coded multi-carrier signals. The thesis also considers using constellation shaping techniques and SEFDM for capacity enhancement in 5G system. Probabilistic shaping for SEFDM is proposed and modelled to show both transmission energy reduction and bandwidth saving with advantageous flexibility for data rate adaptation. Expanding on constellation shaping to improve performance further, a comparative study of multidimensional modulation techniques is carried out. A four-dimensional signal, with better noise immunity is investigated, for which metaheuristic optimisation algorithms are studied, developed, and conducted to optimise bit-to-symbol mapping. Finally, a specially designed machine learning technique for signal and system design in physical layer communications is proposed, utilising the application of autoencoder-based end-to-end learning. Multidimensional signal modulation with multidimensional constellation shaping is proposed and optimised by using machine learning techniques, demonstrating significant improvement in spectral and energy efficiencies
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