8 research outputs found
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
New Hybrid Schemes for PAPR Reduction in OFDM Systems
الـ 3GPP قدمت مشروع LTE لتلبية الطلبات المتزايدة لخدمات الاتصالات ذات السرعة العالية والجودة العالية. يستخدم نظام الـ LTE تقنية مضاعفة تقسيم التردد المتعامد (OFDM) في شكل (OFDMA) في الوصلة الهابطة (Downlink) وشكل الـ (SCFDMA) في الوصلة الصاعدة (Uplink) مجتمعة مع تقنية الـ MIMO لتقديم معدل بيانات عالي، قدرة عالية، وحصانة ضد القنوات متعددة المسارات. ومع ذلك لا يزال ارتفاع نسبة القدرة العظمة إلى المتوسط (PAPR) لإشارة الـ LTE المرسلة هي المشكلة الرئيسية التي تعمل على تدهور كفاءة النظام بشك عام وإمكانية استهلاك الطاقة. لذلك كرست الكثير من البحوث للحد من تدهور الأداء بسبب مشكلة الـ PAPR في أنظمة LTE-OFDM. تعتبر طرق ضغط الإشارة (Companding Methods) جزءاً من الطرق المعروفة والتي تعتبر سهلة ومنخفضة التعقيد، وبلا قيود على شكل التضمين وحجم عدد الحوامل (Subcarrier Size)، ولها خصائص طيفية جيدة، ومع ذلك فإن هذه الطرق تقلل الـ PAPR بمقدار ضئيل. وقد اقترح هذا البحث سبعة طرق هجينة جديدة على أساس مزيج من Zaddoff Chu Matrix Transform (ZCT) مع ست أساليب مختلفة من طرق ضغط الإشارة وهي Rooting Companding (RCT)، New Error Function Companding (NERF)، Absolute Exponential Companding (AEXP)، Logarithmic Rooting Companding (LogR)، Cosine Companding (COS)، وTangent Rooting Companding (TanhR). بالإضافة إلى ذلك تم تطوير الطريقة الهجينة السابعة وتجمع الـ Zaddoff Chu Matrix Transform (ZCT) مع طريقة جديدة مقترحة تسمى Advanced AEXP (AAEXP). أظهرت النتائج أن هذه الطرق المتطورة تجمع بين خصائص طريقة الـ ZCT مع خصائص طرق ضغط الإشارة، وتحقق أداء أمثل وانخفاضاً أفضل من حيث PAPR وBER. كما حققت طريقة الـ ZCT+AAEXP أفضل النتائج مقارنة بالطرق الأخرى.The 3rd Generation Partnership Project (3GPP) introduced LTE to meet increasingly demands for communication services with high speed and quality. LTE uses OFDM in the form of OFDMA in the downlink and SCFDMA in the uplink combined with MIMO offering high data rate, high capacity and immunity against multipath channels. However, still the high PAPR of the LTE transmitted signal is the major problem affecting overall system performance degradation and power efficiency. A plenty of research has been devoted to reduce the performance degradation due to the PAPR problem inherent to LTE OFDM systems. A portion of the current techniques such companding methods have low-complexity, no constraint on modulation format and subcarrier size, good distortion and spectral properties; however, they have limited PAPR reduction capabilities. This paper proposes seven new hybrid schemes including Zaddoff Chu Matrix Transform (ZCT) precoding and six modern companding methods; Rooting Companding (RCT), New Error Function Companding (NERF), Absolute Exponential Companding (AEXP), Logarithmic Rooting Companding (LogR), Cosine Companding (COS) and Tangent Rooting Companding (TanhR) companding. Furthermore, the seventh proposed hybrid scheme has been added incorporating ZCT precoding with new proposed companding called Advanced AEXP (AAEXP) companding. The developed methods are combining properties of both ZCT & Compandings, and achieving superior PAPR performance and optimal BER. Simulations results illustrate that the new seven proposed hybrid schemes can achieve better PAPR reduction, and BER performance and the best achievement has been achieved by ZCT+AAEXP scheme
New technique combining the Tone Reservation method with Clipping technique to reduce the Peak-to-Average Power Ratio
Nonlinear distortions and impairments appear in multicarrier signal with high fluctuations when amplified by a Radio Frequency Power Amplifier (RF PA). Clipping (CL) technique offers a simple way to reduce these fluctuations in Orthogonal Frequency Division Multiplexing (OFDM) Technique, but may degrade seriously the transmission quality. This is why the new mobile standards propose other methods, like the Tone Reservation (TR) technique in the Digital Video Broadcasting-Terrestrial (DVB-T), that reduce the Peak-to-Average Power Ratio (PAPR) without reaching optimal performances. This paper deals with how we can use the TR technique, which exploits null sub-carriers for generating corrective signal, in combining to CL technique in order to improve PAPR reduction without data loss. Also, we show some comparison results on the PAPR reduction obtained with proposed scheme and other techniques. Experiments using a simulated example on a complete WiMax 802.16e transmitter have been made in order to investigate the PAPR reduction performances on presence of the non-linear Power Amplifier model based on gain compression response and phase distortion
Low-complexity frequency synchronization for wireless OFDM systems
Ph.DNUS-TU/E JOINT PH.D. PROGRAMM
Advanced receiver structures for mobile MIMO multicarrier communication systems
Beyond third generation (3G) and fourth generation (4G) wireless communication systems are targeting far higher data rates, spectral efficiency and mobility requirements than existing 3G networks. By using multiple antennas at the transmitter and the receiver, multiple-input multiple-output (MIMO) technology allows improving both the spectral efficiency (bits/s/Hz), the coverage, and link reliability of the system. Multicarrier modulation such as orthogonal frequency division multiplexing (OFDM) is a powerful technique to handle impairments specific to the wireless radio channel. The combination of multicarrier modulation together with MIMO signaling provides a feasible physical layer technology for future beyond 3G and fourth generation communication systems.
The theoretical benefits of MIMO and multicarrier modulation may not be fully achieved because the wireless transmission channels are time and frequency selective. Also, high data rates call for a large bandwidth and high carrier frequencies. As a result, an important Doppler spread is likely to be experienced, leading to variations of the channel over very short period of time. At the same time, transceiver front-end imperfections, mobility and rich scattering environments cause frequency synchronization errors. Unlike their single-carrier counterparts, multi-carrier transmissions are extremely sensitive to carrier frequency offsets (CFO). Therefore, reliable channel estimation and frequency synchronization are necessary to obtain the benefits of MIMO OFDM in mobile systems. These two topics are the main research problems in this thesis.
An algorithm for the joint estimation and tracking of channel and CFO parameters in MIMO OFDM is developed in this thesis. A specific state-space model is introduced for MIMO OFDM systems impaired by multiple carrier frequency offsets under time-frequency selective fading. In MIMO systems, multiple frequency offsets are justified by mobility, rich scattering environment and large angle spread, as well as potentially separate radio frequency - intermediate frequency chains. An extended Kalman filter stage tracks channel and CFO parameters. Tracking takes place in time domain, which ensures reduced computational complexity, robustness to estimation errors as well as low estimation variance in comparison to frequency domain processing.
The thesis also addresses the problem of blind carrier frequency synchronization in OFDM. Blind techniques exploit statistical or structural properties of the OFDM modulation. Two novel approaches are proposed for blind fine CFO estimation. The first one aims at restoring the orthogonality of the OFDM transmission by exploiting the properties of the received signal covariance matrix. The second approach is a subspace algorithm exploiting the correlation of the channel frequency response among the subcarriers. Both methods achieve reliable estimation of the CFO regardless of multipath fading. The subspace algorithm needs extremely small sample support, which is a key feature in the face of time-selective channels. Finally, the Cramér-Rao (CRB) bound is established for the problem in order to assess the large sample performance of the proposed algorithms.reviewe