One of the critical issues of systems utilizing Orthogonal Frequency Division Multiplexing (OFDM) is the high peak to the average power ratio of OFDM signals. We have used Precoding as a way to mitigate the PAPR problem. Furthermore the performance of Precoded OFDM in fading multi-path channels has been studied. This thesis is based on an efficient technique for reducing the PAPR of OFDM signals. The proposed technique is data-independent and thus, does not require new processing and optimization for each transmitted OFDM block. The reduction in PAPR of the OFDM signal is obtained through a proper selection of a Precoding scheme that distributes the power of each modulated symbol over the OFDM block. The obtained results show that this Precoding scheme is an attractive solution to the PAPR problem of OFDM signals.

Orthogonal frequency division multiplexing (OFDM) has become a popular modulation method in high-speed wireless communications. By partitioning a wideband fading channel into flat narrowband channels, OFDM is able to mitigate the detrimental effects of multipath fading using a simple one-tap equalizer. However, in the time domain OFDM signals suffer from large envelope variations, which are often characterized by the peak-to-average ratio (PAR). High PAR signals, like OFDM, require that transmission amplifiers operate at very low power efficiencies to avoid clipping. In this thesis we review the most popular OFDM PAR-reduction techniques and demonstrate that selected mapping (SLM) is a particularly promising reduction technique. In a SLM system, an OFDM symbol is mapped to a set of quasi-independent equivalent symbols and then the lowest-PAR symbol is selected for transmission. The tradeoff for PAR reduction in SLM is computational complexity as each mapping requires an additional inverse fast fourier transform (IFFT) operation in the transmitter. In additional to an overview of current SLM work, we present a thorough analysis of SLM as well as several novel SLM proposals. First, we derive the closed-form expression for the expected PAR in an SLM system. The expected PAR can be thought of as a metric of PAR reduction capability. Second, we provide a power analysis of SLM to determine if the computational power costs outweigh the power saved through PAR reduction. Through this analysis, we show that SLM is capable of several Watts of net power savings when used in a wireless transmission system. Third, we propose that a PAR threshold should be set in SLM. Such thresholding leads to significant complexity decreases. Fourth, we derive the maximum likelihood (ML) and maximum extit{a posteriori} (MAP) detection metrics for blind SLM (BSLM) and threshold BSLM respectively. Lastly, we demonstrate that by using monomial phase sequences in SLM blind phase sequence detection is possible with a single FFT operation in the receiver.

High peak to average power ratio (PAPR) is one of the major drawbacks in orthogonal frequency division multiplexing (OFDM) systems. In recently years, various methods have been proposed to reduce the PAPR performance. The selected mapping (SLM) scheme is perhaps the most popular one because it provides outstanding PAPR reduction performance. In addition, the subcarrier magnitude remains the same in the SLM scheme. However, there are two major shortcomings in the SLM scheme. First of all, it requires a number of inverse fast Fourier transforms (IFFTs) to produce candidate signals, dramatically increasing the computational complexity. In addition, side information has to be transmitted to the receiver to indicate the candidate signal that results in the best PAPR, leading to the decrease in bandwidth utilization. To overcome these two drawbacks, this thesis proposes a novel SLM scheme that does not need side information. The proposed scheme is based on a low complexity SLM scheme [C.-P. Li, S.-H. Wang, and C.-L. Wang, ¡§Novel low-complexity SLM schemes for PAPR reduction in OFDM systems,¡¨ IEEE Trans. Signal Process., vol. 58, no. 5, pp. 2916¡V2921, May 2010] in pilot-aided OFDM system. Simulation experiments are conducted to verify the performance of the proposed scheme. It is shown that the bit error rate (BER) performance of the proposed scheme is very similar to that of the traditional SLM scheme with perfect knowledge of the side information. Therefore, the proposed scheme not only has the advantages of low complexity and high bandwidth utilization, but also has a superior BER performance.

High peak-to-average power ratio (PAPR) is a serious drawback in orthogonal frequency division multiplexing (OFDM) systems. Various methods have been proposed to reduce PAPR, active constellation extension (ACE) scheme has excellent performance. There are two schemes were proposed in traditional ACE, the one of which is ACE-Smart Gradient-Project (SGP) which can significantly reduce PAPR through first iteration. In fact, optimal solution is not obtained in ACE-SGP, we find the scheme can be formulated as convex optimization problem, that is, we can find out optimal solution to minimize PAPR by convex optimization algorithm. Two proposed schemes are based on two low complexity schemes, respectively, and they were proved to satisfy convex optimization problem. Although the power of transmission and complexity of optimization algorithm in the proposed schemes are higher than that of the traditional ACE-SGP scheme, but proposed schemes has proper improvement in PAPR reduction.

In recent years, the development of OFDM system has received a lot of attention. Some examples of existing systems where OFDM system is used are digital audio broadcasting, high-definition television terrestrial broadcasting, asymmetric digital subcarrier lines and so on. There are several reasons for using OFDM systems. First, OFDM system is an efficient way to deal with multipath effect. Under a fixed amount of delay spread, the implementation complexity of OFDM system is much less than that of single-carrier system. The reason is that OFDM system can simply use guard time to process delay spread without a complex equalizer. Second, OFDM system can achieve high data rate to transmit by using large number of subcarriers. Third, OFDM system can also efficiently combat with narrow band interference. On the other hand, OFDM system also has two main drawbacks. One is more sensitive to frequency offset, the other is higher PAPR. This thesis focuses on the PAPR problem. Pulse shaping method is an effective way to solve this problem. It can be used for any number of subcarriers of OFDM systems, so it is very flexible. It doesn¡¦t have any additional IFFTs in comparison to the selected mapping or partial transmit sequence method. Its implementation is simpler. And because it also doesn¡¦t distort the OFDM symbols, its bit error performance should be better than the clipping method. According to the pulse shaping method, we get a better waveform that can make the PAPR of OFDM symbols do not exceed about 2.

Selected mapping (SLM) schemes are commonly employed to reduce the peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. It has been shown that the computational complexity of the traditional SLM scheme can be substantially reduced by adopting conversion vectors obtained by using the inverse fast Fourier transform (IFFT) of the phase rotation vector in place of the conventional IFFT operations [21]. Unfortunately, however, the elements of these phase rotation vectors of the conversion vectors in [21] do not generally have an equal magnitude, and thus a significant degradation in the bit error rate (BER) performance is incurred. This problem can be remedied by utilizing conversion vectors having the form of a perfect sequence. This paper presents three novel classes of perfect sequence, each of which comprises certain base vectors and their cyclic-shifted versions. Three novel low-complexity SLM schemes are then proposed based upon the unique structures of these perfect sequences. It is shown that while the PAPR performances of the proposed schemes are marginally poorer than that of the traditional SLM scheme, the three schemes achieve an identical BER performance and have a substantially lower computational complexity.

One of the major drawbacks of multi-carrier systems is the high peak-to-average power ratio (PAPR) of the transmitted signals. In this paper, the proposed novel low-complexity selective mapping (SLM) schemes are applicable to interleaved-4 orthogonal frequency division multiple access (OFDMA) uplink systems for PAPR reduction. The novel scheme just needs one inverse fast Fourier transform (IFFT) block because that the phases of the transmitted signals in frequency domain are rotated by circular convolution with conversion vectors in time domain. Moreover, a special set of conversion vectors are proposed in novel scheme, which are not only computed with low complexity but also reduce the PAPR effectively. In proposed scheme, different conversion vectors and appropriate subcarriers mapping are picked up for different users. The scheme supplies a practicable low-complexity method for PAPR reduction in interleaved-4 OFDMA uplink systems. Besides, the bit error rate (BER) performance is as good as the SLM scheme.

An explosive growth in demand for broadband mobile wireless services is currently being fuelled by cellular telephone users who, encouraged by service providers, are no longer content with voice transmission only but are demanding real-time video services, including multi-user, interactive games and 'movie' programmes. As these applications develop, expectations mount in other mobile user markets, especially the public safety arena, for comparable user features but with greater emphasis on reliability and robustness of the equipment and supporting network in adverse propagation conditions, remote locations and emergencies. These applications all have in common the requirements for efficient use of wireless bandwidth and of battery power, as well as seamless operation when moving, sometimes at high vehicle speeds, from one type of environment to another in a multi-user scenario. Orthogonal frequency-division multiplexed (OFDM) signals have been found to compare favourably with other modulation systems in these applications, the multi-carrier format being more tolerant of delay spread. It has been used in both code-division (MC-CDMA) and frequency-division (OFDMA) multi-user schemes, the latter having the advantage of maintaining orthogonality among users in fading-signal environments, with consequent simplification of signal processing. The major drawback of OFDM has been the high peak-to-average power ratio (PAPR) that is characteristic of signals with multiple sub-carriers. A result of this is that the transmitter requires a linear power amplifier (PA) that generally has to be 'backed off' to accommodate the high PAPR. Additional back-off is required to achieve linearity, as well as sometimes-complex linearisation circuitry. The power usage and cost of such a transmitter is more acceptable in a base station, tending to limit the application of OFDM to downlinks. The potential application to hand-portable terminals has severe constraints of size, cost and battery life, exacerbated by the use of video-capable LCD displays, increasing motivation for the use of MIMO (multi-antenna) technology and the development of mobile ad-hoc networks, the latter being particularly applicable in the public safety arena. Previous efforts to ameliorate the PAPR problem have been principally directed at two areas, the reduction of signal PAPR, by block coding, clipping or other techniques, and methods of achieving PA linearisation with improved power efficiency. The first object of the present research was to establish, as far as practicable, the current state of the art in these areas, to set a performance baseline. The next step was to develop an improved transmitter modulation scheme that would not only be able to take advantage of any existing peak reduction methods but would transmit a signal that would be compatible with existing OFDM receivers. A novel modulation technique is now presented, termed Quadrature Phase-Time Modulation (QPTM), that has been found to meet the requirements for linearity, simplicity and low cost, whilst being able to take advantage of constant-envelope PA technology, with its attendant power efficiency. After final amplification, the signal is restored by a passive narrow-band filter to standard OFDM form, having both phase and amplitude modulation. The QPTM system of modulation relies on a dual baseband pulse-width modulation process, performed at a substantially-higher rate than the upper baseband frequency, followed by direct quadrature modulation of a carrier signal. The work undertaken has been in the nature of a feasibility study, commencing with the theoretical basis of the technique, from which a behavioural system model was designed and simulated. After the system was simulated successfully, in several forms, a model was designed for realisation with available high-frequency integrated circuits. From this design, prototypes were constructed and tested. The prototype circuit boards also included an experimental UHF Class-D PA circuit, excluding the output filter, to facilitate ongoing development of the PA and filter subsystem as a separate project. This type of PA was seen as a potential complement to the QPTM modulator, although the technology was at an early stage of development. The prototype PA has a novel push-pull arrangement of GaAs FETs that employs a broadside-coupled tapered-stripline balun instead of the usual transformer. Preliminary measurements were made on the PA using both a spectrum analyser and a newly-available 8GHz-bandwidth digital oscilloscope to confirm basic operating characteristics. The performance of the QPTM technique at frequencies needed for broadband operation is dependent on its practical implementation, which has therefore been a major focus. The inherent difficulties in realising a highly-linear 40MHz triangle-wave reference generator, with a precise ultra-high-speed comparator and modulator system, have been overcome with the chosen design techniques and attention to several critical aspects. The result has been the successful demonstration of QPTM as an efficient PA modulation technique that is equally applicable to either narrow-band, high-capacity UHF or broadband OFDM microwave systems.

Ingeniero Civil Eléctrico / Los constantes y cada vez más acelerados avances tecnológicos, han generado que los sistemas de comunicación se optimicen considerablemente con el transcurso de los años. Dentro de los cambios más importantes que se han visto en el último tiempo, destaca la disminución del uso de los clásicos sistemas de telecomunicaciones basados en portadoras únicas, dándose paso a sistemas más complejos en donde la información se transmite utilizando múltiples portadoras. Dentro de este último grupo de técnicas, uno de los que más sobresale es orthogonal frequency division multiplexing (OFDM), el cual ha sido y es, ampliamente utilizado en múltiples aplicaciones o estándares de comunicación. El uso extensivo de OFDM se debe a varias ventajas que esta técnica posee, tales como: alcanzar altas tasas de transmisión de datos, generar señales robustas ante canales inalámbricos, tener una alta eficiencia espectral, entre otros. Pero, aun cuando éstos exhiben múltiples ventajas, también presentan ciertos inconvenientes que deben ser tratados como lo son: los altos niveles de peak-to-average power ratio (PAPR) que caracterizan a las señales OFDM y la sensibilidad a errores originados por la desincronización entre el transmisor y receptor. Esto último, facilita la generación de interferencia entre portadoras (inter-carrier interference, ICI) y, por ende, un aumento en la probabilidad de error. En este trabajo se examina el funcionamiento de los sistemas basados en OFDM, partiendo desde la generación de la señal, hasta su comportamiento espectral. Pero por otro lado, también se analiza como la implementación de pulsos que cumplen con el primer criterio de Nyquist (Nyquist-I), favorece al rendimiento de esta clase de sistemas. El uso de pulsos Nyquist-I para combatir los inconvenientes típicos mostrados por los sistemas basados en OFDM ha sido propuesto por múltiples investigadores. En este trabajo se estudia en detalle una nueva familia de pulsos Nyquist-I llamada improved parametric linear combination pulses (IPLCP), la cual se propone para combatir un completo listado de aspectos perjudiciales mostrados por sistemas OFDM reales, a diferencia de otros pulsos que solamente buscan solucionar uno de ellos. Para analizar el rendimiento de la nueva familia de pulsos, se le compara con otras ya conocidas en términos del ICI, la razón señal a interferencia (signal-to-interference ratio, SIR), el PAPR y la tasa de probabilidad de error de bit (bit error rate, BER). Finalmente, el análisis demuestra que la nueva familia es la que mejor se desempeña en promedio en términos de los parámetros de estudio recién mencionados, dejando en claro que la implementación del IPLCP favorece al rendimiento de los sistemas de comunicación basados en la tecnología OFDM.

With the ongoing push for wireless systems to accommodate more users and support higher data rates more efficient modulation schemes have been created that are more advanced than simple FM and AM modulation used for radio broadcasting. These modulation schemes, such as orthogonal frequency division multiplexing (OFDM), suffer from high peak to average power ratios. Standard Class A and Class AB amplifiers cannot simultaneously achieve good linearity and efficiency, and therefore there has been an increase in the development of new topologies to combat this issue. Common features to these circuits is power combining of two or more separate transistors. In this work, we consider various ways of two-source power combining and identify four topologies of interest. We notice that linear power-efficient amplifiers reported to date are based upon two of the identified combining strategies. We believe that no amplifiers have been reported that leverage the other two alternatives. This work produces a fully-functional amplifier based on one of these alternatives. The prototypes are intended to serve as concept verification of the architecture and hence are implemented at lower (1 MHz) frequencies.