Alexander N. Lozhkin

A new digital predistorter linearizer for wide band signals

This paper presents a new method of digital adaptive predistortion for linearization of high-power amplifiers (HPAs) that mitigates linearization performance deterioration due to bandwidth limitation of feedback path. With increases of signal bandwidth, the effects of limited bandwidth of the predistorter feedback path cannot be ignored. In the proposed algorithm, adaptation is derived from direct learning for the lookup table (LUT) gains. According to the proposed approach, when the LUT contents are adapted according to an LMS algorithm, several other LUT entries adjacent to the currently processed entry are also updated. However, the level of adaption for adjacent entries is less than for the currently processed entry. This level is defined by the preselected window weight function. The proposed algorithm makes it possible to design a low-complexity, memoryless adaptive predistortion system with narrowband feedback path to compensate for the nonlinearity of an HPA.

Adaptive Linearization through Narrowband Feedback

An adaptive predistorter (PD) is an effective technique to compensate for the nonlinear distortion of power amplifiers. To enable broadband communications, the bandwidth of communication systems has had to continuously increase. With these increases in bandwidth, frequency distortions due to limitations in the bandwidth of the PDs building blocks can no longer be ignored. Such distortions are important factors that define a PDs linearization performance. In this study, we investigated the adaptive PD performance degradation caused by bandwidth restrictions in the feedback (FB) path. Employing a newly proposed graphical approach which we confirmed by computer simulations. We show that achieving PD linearization with narrowband FB strongly depends on the PDs architecture and the FB path bandwidth. Finally, we make practical recommendations regarding the implementation of adaptive PDs having memoryless and memory polynomial architectures that depend on the available FB path bandwidth.

Improving mobility in wireless MAN

Influence of Doppler shift and ICI are the focus of the paper. Newly proposed systems must provide a high mobility level compare with 802.11 standard and support low subcarrier frequency spacing. Moreover, for the higher carrier frequencies and mobile speeds, the influence for Doppler shifts will be large and therefore, the influence of ICI becomes severer. BER performances in such multipath and multiple Doppler shift environment obtained with decision feedback equalizer (DFE) and a newly proposed turbo equalizer (TE) estimated with the aid of computer simulation. The proposed TE provides better ICI suppression than does the DFE at the same level of complexity

Joint Linearization for Radio-over-Fiber Links Equipped with High Power Amplifiers

In this study, we experimentally investigated the adjacent channel leakage ratio (ACLR) performance degradation for the output signal of the RF HPA equipped with the adaptive linearization caused by RoF links placed in both direct and feedback paths of the transmitting system. We show that ACLR exceeding -57 dBc @ 5 MHz offset, which completely satisfy the requirement defined in the 3GPP technical specification, can be achieved for a 20 W class Doherty power amplifier linearized through 1 km fiber with commercial RoF links. With the experimental results, we show that the achieved ACLR strongly depends on the RoF link noise figure; meanwhile most of the nonlinear distortions caused by RoF can be successively suppressed with the proposed joint linearization approach.

Adaptive Peak-to-Average Power Reduction Architecture for Multicarrier Signals with Mixed Modulations

One of the major drawbacks of multicarrier transmission is the high peak-to-average power ratio (PAPR) of the transmit signal. In this paper, we propose a PAPR reduction technique for mixed-modulation multicarrier transmission into subcarriers. We show that the knowledge of the allocation of modulation schemes among the subcarriers gives an extra degree of freedom compared with traditional approaches, and results in the simultaneous enhancement of the PAPR reduction level and achievement of the required error vector magnitude (EVM) performances. We demonstrate that PAPR in the transmitting signal is significantly reduced when the EVM distortion for each modulation schema is allowed to increase up to its upper specification limit. We also show that the PAPR regrowth is prevented when the spectrum shaping filters passband is allowed to increase.

Impact of radio over fiber links in feedback on linearization performances of RF power amplifiers

Radio over fiber (RoF) advanced technology is already integrated into existing 3G and 4G radio access networks where the digital unit and remote radio head equipped with the nonlinear high-power amplifiers are connected through RoF-based fronthaul links. In this study, we experimentally investigated the adaptive predistorter adjacent channel leakage (ACLR) ratio performance degradation caused by RoF links placed in both direct and feedback paths of the 3G transmitting system. We focused our investigation on the adaptive predistorter performance degradation caused by an RoF link placed in the feedback path because the distortions that originated in the feedback path define the total system distortion. With the experimental results, we show that the achieved ACLR strongly depends on RoF driving conditions. We find that there is an optimum signal level for feedback RoF that maximizes ACLR in high-power amplifier output.

Linearization through Narrowband Feedback: Problems and Solutions.

An adaptive predistorter is an effective technique to compensate for the nonlinear distortion of power amplifiers. To enable broadband communications, the bandwidth of communication systems has been continuously increased. With these increases in bandwidth, frequency distortions due to limitations in the bandwidth of the predistorters building blocks can no longer be ignored. Such distortions are important factors that deteriorate a predistorters linearization performance. Two main problems directly related with linearization through the narrow feedback path have been the subject of investigation in this study: predistorters performances degradation due to feedback signal bandwidth restrictions and selection of appropriate predistorter architecture suitable for the linearization through narrowband feedback path. Also, in this study we propose a new predistorter architecture based on the inverse modeling for the power amplifier model that is robust against feedback signal bandwidth restrictions. Simulation and experimental results show that with the proposed predistorter architecture, the required feedback signal bandwidth can be reduced approximately to 1.15 times of the transmitting signal bandwidth with minimal degradations in the linearization performances.

New Digital Predistorter Architecture for Signals with High Peak-to-Average Power Ratios

Two closely related non-linear distortions are introduced to the frontend of a transmitting multicarrier signal: a clipping noise due to a high peak-to-average-power-ratio (PAPR) reduction and intermodulation distortions from the nonlinearity of high power amplifiers (HPAs). In recent years, there has been growing interest in various PAPR reduction and linearization techniques using predistortion schemes. Although these two impairments are somewhat interdependent, most designs deal with them separately as it relates to problem formulation, optimization objectives and implementation issues. The overall performance may not be maximized even if each of them is optimized under their own criteria due to possible conflicts of interest. In this paper, we present a new predistorter architecture that jointly optimizes PAPR reduction and digital predistortion, and achieves better transmitting signal quality for desired levels of PAPR. During the optimization, only the clipping noise was employed, so the proposed predistorter became an independent PAPR-specific reduction scheme, i.e., it is robust, and it can therefore operate with various standard and non-standard PAPR reduction solutions.

A Low-Complexity Turbo Equalizer for OFDM Communication Systems

With the growing demand for mobile communications, multicarrier (MC) schemes are receiving an increasing amount of attention, primarily because they handle frequency selective channels better than ordinary single-carrier schemes. However, despite offering several advantages, MC systems have certain weak points. One is their high sensitivity to interchannel interference (ICI). The influence of Doppler shift and ICI are the focus of this paper. Newly proposed B3G/4G systems are developed for data transmission rates higher than those of the IEEE 801.11 It is then necessary that the bandwidth of the subcarrier be small. Moreover, for a higher carrier frequency and mobile speed, the influence of the Doppler shift will be large; therefore, the influence of ICI becomes severer. Using a Markov chain approach, we synthesized a turbo equalizer (TE) that minimizes ICI when interference affects the arbitrary number M of adjacent subchannels. This approach shows the complexity of the proposed algorithm exhibits linear growth with respect to M and independence with respect to the total number of subchannels in the multicarrier system. The proposed ICI cancellation scheme can also be effective in the case of multiple Doppler frequency offsets. This makes the proposed approach attractive for practical implementations.