Power Amplifiers Linearization Based On Complex Gain Memory Predistortion

Power Amplifiers (PAs) are important components in communication systems and are nonlinear. The nonlinearity creates out of band distortion beyond the signal bandwidth, which interferes with adjacent channels. It also causes distortions within the signal bandwidth, which decreases the bit error rate at the receiver. Digital predistortion is one of the most cost effective ways among all linearization techniques to compensate for these nonlinearities. In this thesis a novel technique for compensating memory effects and out of band distortions is proposed and is called Complex Gain Memory Predistortion (CGMP). The main advantage of the CGMP technique as compared to the memory polynomial technique is the ability of this technique to compensate all the memory effects inside the PA. Two structures of the CGMP technique are proposed. The CGMP technique is examined using two approaches, simulation and experiment. Power amplifiers are modeled with memory polynomial technique to examine the effects of the memory that causes increment in Adjacent Channel Leakage Ratio (ACLR). To implement this method, the complex divider is required. This complex divider is then designed and implemented in Field Programmable Gate Array (FPGA) and combined with other parts to make the predistortion block. The CGMP is implemented in Virtex 5 FPGA and simulated using Xilinx blocks in Matlab. In the experimental approach the CGMP is examined with the actual power amplifier ZVE-8G from Mini Circuit. Finally the CGMP technique is compared with memory polynomial method and validated using a 1.9 GHz 60W LDMOS power amplifier that is designed in simulation and various signals such as 2-carrier WCDMA with 10 MHz carrier spacing and Mobile WiMAX with 10 MHz bandwidth. The simulations results showed between 25 to 30 dB improvement in ACLR and almost 5 dB improvement as compared to the memory polynomial method. The experimental results also show around 10 dB reduction in ACLR with applying QPSK signal with 1 MHz bandwidth. The improvement of 7 percent in Power Added Efficiency (PAE) is also achieved

Partial transmit sequence scheme with new phase sequence for PAPR reduction in OFDM systems

One of the effective methods used for reducing peak to average power ratio (PAPR) in OFDM systems is partial transmit sequence (PTS). In the conventional PTS (CPTS) several inverse fast Fourier transform (IFFT) operations and complicated calculation to obtain optimum phase sequence, increase the computational complexity of C-PTS. In this paper, we propose a technique to reduce the number of IFFT operations to half at the expense of a slight PAPR degradation. Simulations are performed with QPSK modulation with OFDM signal and Saleh model power amplifier. The effects of digital predistortion (DPD) to increase the linearity and efficiency of the Saleh model power amplifier (PA) are also examined

Low Complexity Partial Transmit Sequence with Complex Gain Memory Predistortion in OFDM Systems.

In this paper the peak to average power ratio (PAPR) reduction and digital predistortion effects in orthogonal frequency division multiplexing (OFDM) systems are investigated. By applying a predistortion technique called complex gain memory predistortion (CGMP), power amplifier works at higher power efficiency. The proposed enhanced partial transmit sequence scheme is applied for PAPR reduction and integration with CGMP technique results in increasing in OFDM system efficiency and prolonged battery life. Simulation and results are examined with actual power amplifier and OFDM signal with quadrature phase shift keying (QPSK) modulation

A crest factor reduction scheme based on recursive optimum frequency domain matrix

One of the main imperfections in orthogonal frequency division multiplexing (OFDM) techniques is a peak-to-average power ratio (PAPR) or crest factor (CF). High crest factor signal forces a power amplifier in communication systems to back-off which leads to power efficiency degradation. Several techniques have been proposed in literature to mitigate the high peaks in OFDM signal; however most of them suffer from complexity and PAPR performance. In conventional SLM (C-SLM), increasing the number of inverse fast Fourier transform (IFFT) leads to excessive hardware resources requirement and increases system cost. In this paper a novel crest factor scheme based on recursive optimum frequency domain matrix is proposed which requires only two IFFT which results in significant reduction in the computational complexity. Simulation results show both the complexity and crest factor are reduced significantly compared to the conventional methods

Implementation of a low complexity peak-to-average power ratio reduction scheme on field programmable gate array

Orthogonal frequency division multiplexing systems suffer from one main drawback which is a high peak-to-average power ratio (PAPR) that leads to power efficiency degradation and when feed to a power amplifier operating in the nonlinear region yields adjacent channel interference and inferior bit error rate performance. There are many methods to overcome this drawback, one example of which is partial transmit sequence (PTS). Here implementation of a proposed PTS on field programmable gate array platform to show the feasibility of the PAPR reduction scheme is carried out. It has been shown that the proposed PTS scheme is significantly simpler as compared to the conventional methods and integration of this scheme with digital pre-distortion technique improves the system efficiency, suppressed out-of-band distortion and hence, prolong battery life. The simulations have been carried out with QPSK modulation and inverse fast Fourier transform with 512 subcarriers. The implementation results show in average 21% reduction in hardware resource and almost 18% conservation in total power consumption by applying the new PTS scheme while its PAPR performance remains comparable to the conventional PTS method

A low complexity PAPR reduction scheme based on radix-II IFFT

Due to no feedback process and simplicity in searching algorithm, conventional selected mapping (CSLM) is an efficient crest factor reduction (CFR) technique in orthogonal frequency division multiplexing (OFDM) systems. However high number of inverse fast Fourier transform (IFFT) block is required to achieve the desired PAPR reduction performance. In this paper a PAPR reduction method based on N point radix-2 IFFT is proposed in which the number of IFFTs is reduced to one. The gist of the proposed method is based on storing a part of calculations and using them for the next searching operation results in elimination of the redundant calculations. Simulation results show at least 46.8% complexity reduction compared to CSLM by comparable PAPR performance

A Low Complexity NARX Structure using Indirect Learning Architecture for Digital Pre-Distortion

In this paper, we demonstrate a nonlinear autoregressive with exogenous input (NARX) DPD technique which is more compact, less computationally intensive and less susceptible to errors caused by noise in the PA output compared to an equivalent memory polynomial based DPD. Experimental validation is performed with a 20 MHz LTE signal for a GaN Doherty power amplifier

Low complexity ADRG-PTS scheme for PAPR reduction in OFDM systems

In this paper a novel technique for reducing the peak to average power ratio (PAPR) reduction based on the addition of random signals with a complex Gaussian distribution (ADRG) and combining it with partial transmit sequence (PTS) is proposed. Unlike the conventional PTS (C-PTS) which needs several inverse fast Fourier transform (IFFT) operations, the proposed ADRG-PTS technique requires only half IFFT operations. Simulation and results are examined with 16 QAM OFDM signal

Enhanced handover mechanism in long term evolution (LTE) networks

Over the past decade, there have been great interests in cellular and fixed radio access technologies for providing mobile, nomadic and fixed telecommunication services. The fast pace development of this technology and the challenges it presents due to the increasing number of user equipments and the demand to have the service on-the-go, have presented new challenges on base stations capability and the handover (HO) techniques. To address these challenges intensive researches are being carried out to define algorithms that can handle the HO decisions based on user equipment (UE) requirements and quality of service (QoS) expectations. This paper investigates the improvement steps for HO mechanisms in long term evolution (LTE) system which is being formally submitted as a candidate 4G system. LTE network is expected to support mobility with speeds of up to 500 km/h, when the HO will then become more frequent and fast. The basis of the approach is to reduce the number of unnecessary HOs. The strengths and weaknesses for each algorithm are discussed, and conclusions are subsequently made