Evaluation of Radio Resource Management Impact on RoF Signal Transmission for Downlink LTE

Analog radio-over-fiber (RoF) is a suitable technology for efficiently developing the cloud-RAN concept in fifth generation deployments based on long-term evolution advanced (LTE-A). Distortion and radiofrequency (RF) power amplifier gain should be taken into account in order to achieve the desired transmission power at the base station. Both mainly depend on several parameters that characterize the optical link, such as the RF input power and bias current. This paper analyzes the link performance due to variations on the traffic load as expected under real operation. In this paper, we show that it is possible in this scenario to adaptively choose the optimum RF input power to minimize the measured error vector magnitude and reduce the RF subsystem gain. Results show that this adaptation allows to relax the RF amplifier gain requirements up to 3-4 dB for medium load conditions

Predistorsión Digital mediante Señales Enventanadas Tipo Chirp para la Linealización de Amplificadores de Potencia

Digital PreDistortion (DPD) is a well-known method to reduce nonlinear distortion in power amplifiers (PA). In a LTE uplink transmission, the modulation schemes are adaptive. Thus, the DPD function will have to be re-calibrated once the modulation changes. This fact increases the DPD computational cost and the required memory. If the DPD is not re-trained its linearity performance will decrease. This effect can be reduced using a suitable training signal. We propose several novel non-stationary calibration signals based on a windowed-chirp waveform. The envelope of these sequences is bounded by different windows used in signal processing such as Bartlett, Blackman, Hamming or Welch. Bartlett and Blackman windows generate calibration sequences with higher peak to average power ratio values than Welch. Welch envelope creates a signal with a more uniform probability density function (pdf) than the Hamming-window, whose pdf is similar to a Rayleigh distribution. The linearization strategy is based on capturing the described sequences at the PA input and output to extract the predistortion parameters. Once the predistorter functions are computed, we apply them on various LTE-transmissions and perform linearity measurements in terms of the adjacent channel leakage ratio to compare with the standard requirements. In all cases the maximum nonlinear distortion reduction is accomplished with the Chirp-Bartlett sequence (up to 37dBc when transmitting a QPSK-LTE-signal). Thus, good DPD performance is achieved when using the proposed Bartlett-Chirp in the DPD-training-stage. This avoids generating specific DPDs for each modulation scheme, saving computational cost, required memory, and increasing the system efficiency

Minimization of Feedback Loop Distortions in Digital Predistortion of a Radio-Over-Fiber System with Optimization Algorithms

This article proposes the linearization of an intensity modulation/direct detection radio-over-fiber (RoF) link with feedback loop. The goal is to carry out the predistortion process in a real scenario, in which the output signals are a few kilometers far from the baseband unit (BBU). First, the feedback loop is considered ideal, so the output signals are captured at the remote radio head side. Then, the feedback loop is taken into account, and both the input and output signals are captured at the BBU side. Applying optimization algorithms, such as Fibonacci, Golden, or Powell, it is possible to seek the optimal attenuation value within only a few iterations, which minimizes the distortion of the feedback loop. Experiments are carried out in a RoF system with 10 and 25 km length fiber within the long-Term evolution (LTE) standard. Measured results show how with a proper choice of the attenuation it is possible to reach analogous results regarding to an ideal feedback loop in terms of adjacent channel power ratio, the output signal power and error vector magnitude

DPD Linearization Complexity Reduction of Remote Radio Heads in C-RAN with Radio over Fiber Fronthaul

Radio-over-Fiber (RoF) is a suitable technology for efficiently developing the cloud-RAN concept in Fifth Generation (5G) deployments based on Long Term Evolution Advanced LTE-A. Distortion coming out from the radiofrequency (RF) power amplifier should be taken into account in order to achieve the desired performance at the base station. In this paper, a complexity reduction technique based on a variable degree polynomial predistorter is proposed. The obtained results show that it is possible to achieve a reduction of operations with a penalty of 1.6 dB in ACLR and negligible impact on the EVM

Radio-over-fiber linearization with optimized genetic algorithm CPWL model

This article proposes an optimized version of a canonical piece-wise-linear (CPWL) digital predistorter in order to enhance the linearity of a radio-over-fiber (RoF) LTE mobile fronthaul. In this work, we propose a threshold allocation optimization process carried out by a genetic algorithm (GA) in order to optimize the CPWL model (GA-CPWL). Firstly, experiments show how the CPWL model outperforms the classical memory polynomial DPD in an intensity modulation/direct detection (IM/DD) RoF link. Then, the GA-CPWL predistorter is compared with the CPWL model in several scenarios, in order to verify that the proposed DPD offers better performance in different optical transmission conditions. Experimental results reveal that with a proper threshold allocation, the GA-CPWL predistorter offers very promising outcomes

Efficiency enhancement by reconfigurable matching networks in LINC transmitters

This paper proposes the use of a transmitter based on a linear amplification with nonlinear components (LINC) architecture, in which the reconfigurable matching networks (RMNs) are included. By varying the RMN active cell number, it is possible to change the load impedance at the power amplifier (PA) output, improving the amplifier drain efficiency and therefore the efficiency of the whole system. A long-term evolution (LTE) downlink signal with a bandwidth of 1.4 MHz and a peak-to-average power ratio (PAPR) of 11.48 dB is applied in order to carry out the experiments. Results show that the use of the RMNs in a LINC architecture improves the efficiency at all tested frequencies, especially at 927 MHz reaching an enhancement of 36.50%. Regarding the distortion, the adjacent channel leakage ratio (ACLR) values increase in all cases, with an improvement of 3.5 dB at 958 MHz. Finally, in terms of error vector magnitude (EVM), the proposed architecture offers a value of 1.96% at 927 MHz

Design and applications of a 300-800 MHz tunable matching network

In this paper, the optimized design, characterization and applications of a broadband 300-800 MHz ($\sim$ 91% fractional bandwidth) digitally-controlled tunable matching network is presented. The design employs PIN diodes as switching components and a repetitive structure of basic cells using lumped reactive elements. After an intensive and complex optimization process, a Smith chart coverage (return losses better than 10 dB and losses lower than 2 dB) above 60% is obtained in all the bandwidth reaching 75% in the middle of the band (400-700 MHz). The potential of the manufactured tunable matching network for antenna mismatch compensation, antenna bandwidth extension and power amplifier efficiency improvement in back-off is showed. \ua9 2011 IEEE

Mesenchymal Differentiation Mediated by NF-kappa B Promotes Radiation Resistance in Glioblastoma

<p>Despite extensive study, few therapeutic targets have been identified for glioblastoma (GBM). Here we show that patient-derived glioma sphere cultures (GSCs) that resemble either the proneural (PN) or nnesenchymal (MES) transcriptomal subtypes differ significantly in their biological characteristics. Moreover, we found that a subset of the PN GSCs undergoes differentiation to a MES state in a TNF-alpha/NF-kappa B-dependent manner with an associated enrichment of CD44 subpopulations and radioresistant phenotypes. We present data to suggest that the tumor microenvironment cell types such as macrophages/microglia may play an integral role in this process. We further show that the MES signature, CD44 expression, and NF-kappa B activation correlate with poor radiation response and shorter survival in patients with GBM.</p>