73 research outputs found
Enhanced Receivers for OFDM signals with super-QAM constellations
Nowadays, there is a high demand for wireless communication systems with higher through-
put. One popular technique widely used in current and developing wireless technologies is
Orthogonal Frequency-Division Multiplexing (OFDM) due to its robustness against fre-
quency selective fading and high spectral efficiency. To further extend OFDM capacity
to meet the near future’s expected demanding needs, OFDM systems with very large
Quadrature Amplitude Modulation (QAM) constellations, the so-called super-QAM, are
being proposed. However, OFDM signals are prone to nonlinear distortion effects due to
their high envelope fluctuations which reduces the system’s performance and this issue
is aggravated by the increase in the size of the constellation. For the implementation of
effective super-QAM OFDM systems, it is crucial to develop receivers that expect and
mitigate the nonlinear distortion on the transmitted signal.
In this work, nonlinear distortion on OFDM small QAM and super-QAM constellations
signals is studied, along with distortion models and methods to estimate them solely
from the transmitted signal, and application of Bussgang noise cancellation receivers and
analysis of their performance over a wide range of scenarios.Nos dias de hoje, há uma grande necessidade de criar sistemas de telecomunicação com
maior ritmo de dados. Uma técnica popular em tecnologias de telecomunicação atuais e
em desenvolvimento é Ortogonal Frequency-Devision Multiplexing (OFDM) devido à sua
robustez contra atenuação seletiva na frequência e alta eficiência espectral. Para aumentar
ainda mais a capacidade do OFDM de forma a preparar para ritmos ainda mais altos que
são expectáveis num futuro próximo, estão a ser propostos sistemas OFDM com enormes
constelações de Quadrature Amplitude Modulation (QAM), o chamado super-QAM. O
problema é que sinais OFDM são suscetÃveis a efeitos de distorção não linear devido à s
altas flutuações de envolvente e que traz pior desempenho do sistema, sendo esse problema
agravado pelo aumento do tamanho da constelação. Para a implementação de sistemas
super-QAM OFDM eficazes é crucial desenvolver recetores que mitiguem a distorção não
linear no sinal transmitido.
Neste trabalho, estuda-se a distorção não linear em sinais OFDM de pequenas cons-
telações QAM e super-QAM, modelos de distorção e métodos para estimá-los a partir do
sinal transmitido, aplicação de recetores de cancelamento de ruÃdo Bussgang e análise de
seu desempenho em diversos cenários
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
PAPR and ICI reduction techniques for OFDM based satellite communication systems
Multi-carrier systems such as orthogonal frequency division multiplexing (OFDM) are significantly affected by peak-to-average-power ratio (PAPR). Unfortunately, the high PAPR inherent to OFDM signals envelopes will occasionally drive high power amplifiers (HPAs) to operate in the nonlinear region of their characteristic curve. The nonlinearity of the HPA exhibits amplitude and phase distortions, which cause loss of orthogonality among the subcarriers (SCs), and hence, inter-carrier interference (ICI) is introduced in the transmitted signal. The ICI power is proportional to the amplitude of the signal at the amplifier input and it may cause a considerable bit error rate (BER) degradation. A plethora of research has been devoted to reduce the performance degradation due to the PAPR problem inherent to OFDM systems. Some of the reported techniques such as amplitude clipping have low-complexity; on the other hand, they suffer from various problems such as in-band distortion and out-of-band expansion. Signal companding methods have low-complexity, good distortion and spectral properties; however, they have limited PAPR reduction capabilities. Advanced techniques such as coding, partial transmit sequences (PTS) and selected mapping (SLM) have also been considered for PAPR reduction. Such techniques are efficient and distortionless, nevertheless, their computational complexity is high and requires the transmission of several side information (SI) bits. In this thesis, a new low-complexity scheme is proposed based on the PTS that employs two inverse fast Fourier transforms (IFFTs) and two circulant transform matrices, in order to reduce complexity and improve the system performance. Furthermore, the low-complexity scheme is simplified by omitting one of the circulant transform matrices in order to reduce both the computational complexity and the number of SI bits at the cost of a small reduction in PAPR and BER performance. It is well known that, accurate PAPR estimation requires oversampling of the transmitted signal, which in turn results in increased complexity. More importantly, minimising the PAPR does not necessarily minimise the distortion produced by the nonlinearity of the HPA. Therefore, minimising PAPR does not necessarily imply that the BER will be minimised too. Efficient and less complex schemes for BER reduction of OFDM systems in the presence of nonlinear HPA and/or carrier frequency offset (CFO) are proposed. These proposed techniques are based on predicting the distortion introduced by the nonlinearity of HPA and/or CFO. Subsequently, techniques such as the PTS and SLM are invoked to minimise the distortion and BER. Three distortion metrics are adopted in this thesis: inter-modulation distortion (IMD), peak interference-to-carrier ratio (PICR) and distortion-to-signal power ratio (DSR). Monte Carlo simulations will confirm that the DSR and PICR are more reliable than the PAPR and IMD for selecting the coefficients of the PTS and SLM to minimise the BER. Furthermore, complexity analyses demonstrate that the proposed schemes offer significant complexity reduction when compared to standard PAPR-based methods. A closed form solution for accurate BER for the OFDM signals perturbed by both the HPA nonlinearity and CFO was derived. Good agreement between the simulation results and the theoretical analysis can be obtained for different HPA parameters and CFOs. Finally, efficient approaches to reduce the impact of nonlinear power amplifiers with respect to the BER of OFDM systems are proposed. These are approaches based on: the well-established PAPR schemes, a power amplifier model and a simple single point cross correlator. The optimum phase sequence within the proposed approaches is selected by maximising the correlation between the input and output of the power amplifier model. Simulation results have confirmed that the BER using the proposed approaches is almost identical to the DSR, while the complexity is reduced significantly for particular system configurations.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Enhanced Multicarrier Techniques for Professional Ad-Hoc and Cell-Based Communications (EMPhAtiC) Document Number D3.3 Reduction of PAPR and non linearities effects
Livrable d'un projet Européen EMPHATICLike other multicarrier modulation techniques, FBMC suffers from high peak-to-average power ratio (PAPR), impacting its performance in the presence of a nonlinear high power amplifier (HPA) in two ways. The first impact is an in-band distortion affecting the error rate performance of the link. The second impact is an out-of-band effect appearing as power spectral density (PSD) regrowth, making the coexistence between FBMC based broad-band Professional Mobile Radio (PMR) systems with existing narrowband systems difficult to achieve. This report addresses first the theoretical analysis of in-band HPA distortions in terms of Bit Error Rate. Also, the out-of band impact of HPA nonlinearities is studied in terms of PSD regrowth prediction. Furthermore, the problem of PAPR reduction is addressed along with some HPA linearization techniques and nonlinearity compensation approaches
New OFDM schemes based on orthogonal transforms for mobile communications systems :
PhD ThesisIn this thesis, two new orthogonal frequency division multiplexing (OFDM)
systems are presented. The first scheme proposes a new OFDM system
transceiver based on the C-transform, which is termed C-OFDM. Over
multipath channels, the C-OFDM achieves 10 dB signal-to-noise ratio
(SNR) gain at 10−4 bit-error-rate (BER), in comparison to the OFDM
that based on the is discrete cosine transform (DCT-OFDM) and the
conventional OFDM schemes. It also reduces the peak-to-average power
ratio (PAPR) of the OFDM signal by about 1 dB and in some cases
up to 3 dB. In the second scheme, a new fast, orthogonal X-transform
is produced. The proposed X-transform is then used in a new OFDM
named X-OFDM to greatly reduce the complexity, the PAPR and the
BER. The proposed scheme achieves around 15 dB SNR gain in comparison
to the conventional OFDM at 10−4 BER and reduces the average
PAPR (over 105 OFDM symbol) by about 6 dB for N =1024 subcarriers.
Furthermore, in this study, the X-transform is utilized to produce a new
Alamouti space-time OFDM (ST-OFDM). The proposed ST-X-OFDM
scheme reduces the transmitter complexity and achieves important SNR
gain over the conventional ST-OFDM systems. The BER performance
of the proposed schemes in the presence of solid-state power amplifiers
(SSPAs) is also investigated analytically and by simulation. It shows that
the X-OFDM is resilient to the SSPAs nonlinear distortion whereas the
C-OFDM may lead to BER impairment in the presence of the SSPA.
Furthermore, a coding technique to mitigate the sensitivity of the COFDM
scheme to the SSPA is also proposed in this study.
In this research, mathematical models for the proposed C-OFDM, XOFDM
and ST-X-OFDM, which tightly match the simulation results
over a diverse range of transmission scenarios and mapping schemes,
are also derived. In addition, the BER performance of the proposed COFDM
and X-OFDM schemes in the presence of the carrier frequency
offset (CFO), with and without frequency synchronization algorithm,
are also investigated. The proposed C-OFDM and X-OFDM schemes
are more sensitive to the CFO than the conventional schemes. However,
when frequency synchronization algorithm is used, both the proposed
schemes retain their significant BER improvement in comparison to the
conventional schemes.Ministry of Higher Education
and Scientific Research (MOHSR), Iraq and to the Iraqi cultural attach-
London for supporting me financially during my study in England
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