Atso Hekkala

Adaptive Time Misalignment Compensation in Envelope Tracking Amplifiers

This paper considers the compensation of time misalignment between RF and envelope signals of envelope tracking (ET) amplifier. In particular, we propose methods to compensate the time misalignment and compare them to a previously presented method. Our simulation results indicate that the proposed methods give very accurate timing estimates. In the presence of optimal predistorter, we achieve with 90 percent probability adjacent channel power (ACP) level of -80 dB. With adaptive predistorters, we cannot see any degradation of ACP comparing to case of ideal timing. On the other hand, the proposed methods give roughly 10 dB better ACP than the previously presented method.

Cooperative and noncooperative spectrum sensing techniques using Welch’s periodogram in cognitive radios

Radio spectrum deployment is growing considerably. With this respect finding unutilized frequency channels for new applications has become much more challenging. The problem can be solved by letting unlicensed systems to dynamically use unexploited licensed bands. This kind of flexible spectrum usage requires telecommunication systems to be equipped by an ability to specify unoccupied parts of radio spectrum. One method to identify temporarily unused parts of radio spectrum is spectrum sensing. In this paper, we focus on spectrum sensing using Welchs periodogram. In particular, we generalize and apply the theoretical analysis of the energy detection to the Welchs periodogram. Furthermore, we extend our study to cooperative spectrum sensing. The results indicate that the cooperation between two radios provides the highest cooperation gain.

Predistortion of Radio Over Fiber Links: Algorithms, Implementation, and Measurements

Distributed antenna systems (DASs) have been proposed for use in high-data-rate wireless communications. Connecting the central unit to remote antenna units, radio over fiber (RoF) links become essential parts of the DAS. One of the major problems in the RoF links is the nonlinear distortion experienced in its optoelectronic devices. This paper considers the adaptive compensation of the nonlinear RoF link. In particular, we present an extension to the conventional least-mean-square algorithm for memory-polynomial-based predistortion. In addition, we study different combinations of the algorithms for the predistortion. Moreover, we implement the adaptive predistorter in hardware, build a real RoF link, and verify the performance by measurements. The results demonstrate that distortion of the RoF link can be considerably diminished with a low complexity design.

Performance of adaptive algorithms for compensation of radio over fiber links

This paper considers the adaptive predistortion of the nonlinear distortions in a Radio over Fiber (RoF) link. In particular, we modify and compare two adaptive algorithms developed originally for the compensation of the linear systems, namely LMS and variable step size normalized LMS (VS NLMS). A recursive least squares (RLS) solution is used as a reference. Our simulation results indicate that over 40 dB improvement of adjacent channel power ratios can be achieved via the predistortion. Furthermore, we show that in the compensation of the nonlinear RoF link, the LMS can be used in such a way that its performance is comparable to more complex RLS.

Architectures for Joint Compensation of RoF and PA with Nonideal Feedback

A high capacity wireless communication system requires careful design to minimize interference and distortion effects. This paper considers the adaptive predistortion of the nonlinear distortions induced by a Radio over Fiber (RoF) link and power amplifier (PA) connected in series. In particular, we study the architectures for the joint compensation of the nonlinearities in the presence of nonideal feedback. From the adaptive algorithm point of view, we study different combinations of algorithms for the predistortion. Our simulation results indicate that combined use of least mean squares (LMS) and recursive least squares (RLS) gives the best trade-off between complexity and performance. In addition, we show that the use of nonideal feedback causes a collapse in the performance of the predistortion. However, when using a compensated RoF link for feedback, the degradation of the adjacent channel power is very small compared to the case of the ideal feedback.

A motivating overview of cognitive radio: Foundations, regulatory issues and key concepts

The radio frequency spectrum is a scarce natural resource and its efficient use is of the utmost importance. A fundamental problem facing the future wireless systems is where to find suitable spectrum bands to meet the demand of future services. A promising technique to improve the spectrum utilization under a lot of investigation is the cognitive radio (CR) concept. This paper provides an overview of a CR study with discussion on enabling techniques for CR, measurements on current spectrum use, forthcoming standardization activities and some findings from our recent research studies on spectrum sensing, frequency management, and power control. The measurements on current spectrum use have identified that in general the overall spectrum occupancy is low. However, the overall spectrum occupancy does not characterize the actual situation adequately and better measures are needed. International Telecommunication Union Radiocommunication sector (ITU-R) has started standards activities on CRs, in particular on the measurement of spectrum occupancy and CR systems in the mobile service, indicating that the inclusion of the CR concept into the future regulatory framework is topical. Reliable spectrum sensing is a crucial task of the CR and proper performance assessment is important. If the future availability times of spectrum holes can be predicted, intelligent channel selection with the predicted channel information can be useful. A power control candidate for the CR system is the truncated inverse power control.

Adaptive Predistortion Architecture for Nonideal Radio Transmitter

A nonideal radio transmitter both distorts the desired signal and generates spectral regrowth causing interference to adjacent channels. Predistortion techniques can be used to minimize these effects. The joint effect of different types of nonidealities is not straightforward to analyze and has not yet been studied in detail in the literature. In this paper, we demonstrate how to adaptively predistort a nonlinear high power amplifier and an I/Q modulator, and to compensate the frequency response of a radio frequency filter. The study considers the spatial order of adaptive predistorters and the temporal sequence of adaptation steps. In this way, both the system capacity and the power efficiency are improved.

Performance Evaluation of Spectrum Sensing Using Welch Periodogram for OFDM Signals

Reliable spectrum sensing is one of the key technologies for enabling spectrum coexistence. In this paper, co-operative spectrum sensing based on the Welch periodogram is studied in frequency selective fading environment for quadrature phase shift keying (QPSK) and orthogonal frequency division multiplexing (OFDM) signals. Parameter selection for sensing OFDM signal is discussed, and near-optimal values for frequency averaging for QPSK and OFDM signals are derived. The results, given as receiver operating characteristic (ROC) curves, indicate that the frequency-selective fading degrades the performance of the sensing. This could be compensated by co-operation of several sensing nodes. Both analytical and simulation results indicate that best performance of spectrum sensing is achieved with different combining rules in the case of signal passing through additive white Gaussian noise (AWGN) or fading channels.

Spectrum sensing in cognitive femto base stations using welch periodogram

Reliable spectrum sensing is one of the key technologies for enabling cognitive communications. In this paper, spectrum sensing based on Welch periodogram is applied to a femtocell scenario where cognitive femto base station (FBS) is located in the same frequency band as the surrounding macrocell and neighboring femtocells. Parameters for the femtocell simulations are selected based on 3G Partnership Project (3GPP) contributions to tie this work to on-going femtocell standardization work. Different interference scenarios are also discussed. Spectrum sensing based on Welch periodogram is presented, and two scenarios including a user equipment (UE) located indoor and outdoor are selected for further study. The results, given as receiver operation characteristic (ROC) curves, clarify the practical regions inside of which spectrum sensing is capable of providing reliable results for sensing the presence of the MUE.

Compensation of linear and nonlinear distortions in envelope tracking amplifier

This paper considers the compensation of linear and nonlinear distortions which are present in RF part of envelope tracking (ET) amplifier. In particular, we consider the compensation of magnitude and phase responses of analog filters, amplifier nonlinearity, and possible time misalignment between signals in RF and envelope branches. We propose a new adaptive predistorter architecture for ET amplifiers. Our analytical results indicate that the proposed architecture is a more general than the state-of-the-art architecture and is guaranteed to work with any ET amplifier. Furthermore, our simulations show that in order to keep adjacent channel power ratio below -60 dB, at least 9th order polynomial predistorter is needed. A comparable performance is obtained with look-up-table based predistorter having only 16 entries.