To reduce laborious tasks of the phase determination, our previous paper has proposed a method to derive phase reference for two-tone intermodulation distortion (IMD) measurement of a power amplifier (PA) by using small-signal S-parameters. Since the method is applicable to low output power level, this paper proposes an iterative process to extend the applicable power level up to 1-dB compression. The iterative process is based on extraction of linear response: the principle of the extraction is described theoretically by using an accurate model of the PA with memory effect. Measurement of two-tone IMD is made for a GaN FET PA. Validity of the iteration is confirmed as convergence of the extracted linear response to that given by the product of S21 and input signal. Measured results also show validity of the physical model of the memory effect provided by Vuolevi et al. because beat frequency dependences of IMD's are accurately fitted by bias impedances at even order harmonics of envelope frequency. The PA is characterized by using measured results and the third and fifth order inverses of the PA are designed. Improvement of IMD is theoretically confirmed by using the inverses as predistorters.

This paper describes the details of the iteration process used to determine the transfer functions of linear time-invariant (LTI) circuits causing the memory effect of power amplifier (PA). An outline of the method is reported in our work presented at ICCS2012. The accuracy of the method is improved by using cross-correlation spectra at three signal levels, and its validity is confirmed by a computer simulation. The method can be applied to online updating of PAs operating in mobile communication systems. The updating is realized separately from the fast varying nonlinear coefficients. The possibility of updating with a short interval is indirectly shown for the nonlinear coefficients using a procedure similar to that of memoryless PAs. For PAs characterized by the method, this paper also describes the inverses that cancel the nonlinear distortion with minimum complexity. The validity of the inverse is confirmed by a computer simulation on the power spectrum of the PA for orthogonal frequency-division multiplexing (OFDM) signals with 500 subcarriers. The simulated spectra show that the fifth order or higher inverses are effective in keeping adjacent channel leakage power ratio (ACLR) lower than -60dB at the practical signal level. Improvements in the error vector magnitude (EVM) due to the inverse were also confirmed by reductions of gain and phase variations under varying envelope conditions.

Adaptive array antennas, which control their own patterns by means of feed-back or feed-forward control, are effective tools for gain enhancement and interference suppression. However, when applying them to mobile terminals, the problems of hardware complexity and power consumption need to be taken into consideration. One solution is the use of analog device-based adaptive array antennas, such as Reactively Steered Adaptive Array (RESAA) antennas and phased array antennas, which have the attractive characteristics of low cost and power consumption. In this paper, we propose an adaptive beamforming method based on a one-dimension search algorithm for phased array antennas with Micro Electro Mechanical Systems (MEMS) phase shifters, taking into consideration their slow operating speed due to mechanical structure of the devices. Furthermore, a smoothing processing is introduced to prevent the effect of noise and a multi-resolution alogrithm is proposed to help the system form beams more quickly and stably. Numerical results based on the IEEE 802.11a Wireless Local Area Network (WLAN) standard show that the proposed method has good interference suppression and gain enhancement capabilities in multipath fading channels.

Phased array antennas are attractive in terms of low cost and power consumption. This paper proposes a controlling scheme based on a bisection method for phased array antennas employing phase shifters with slow switching speed, which is typical for Micro Electro Mechanical Systems (MEMS) switches. Computer simulation results, assuming the IEEE 802.11a Wireless Local Area Network (WLAN) standard, show that the proposed scheme has good gain enhancement capability in multipath fading channels.

The combination of Multiple-Input Multiple-Output (MIMO) and Orthogonal Frequency Division Multiplexing (OFDM) technologies gives wireless communications systems the advantages of lower bit error rate (BER) and higher data rate in frequency-selective fading environments. However, the main drawbacks of MIMO systems are their high complexity and high cost. Therefore, antenna selection in MIMO systems has been shown to be an effective way to overcome the drawbacks. In this paper, we propose two receive antenna selection methods for a MIMO-OFDM system with radio frequency (RF) switches and polarization antenna elements at the receiver side, taking into consideration low computational complexity. The first method selects a set of polarization antenna elements which gives lower correlation between received signals and larger received signal power, thus achieves a lower BER with low computational complexity. The second method first selects a set of polarization antenna elements based on the criterion of the first method and another set of polarization antenna elements based on the criterion of minimizing the correlation between the received signals; it then calculates the signal-to-interference-plus-noise power ratio (SINR) of the two sets and selects a set with larger SINR. As a result, the second method achieves a better BER than the first one but it also requires higher computational complexity than the first one. We use the measured channel data to evaluate the performance of the two methods and show that they work effectively for the realistic channel.

This paper describes a method to design a predistorter (PD) for a GaN-FET power amplifier (PA) by using nonlinear parameters extracted from measured IMD which has asymmetrical peaks peculiar to a memory effect with a second-order lag. While, computationally efficient equations have been reported by C. Rey et al. for the memory effect with a first-order lag. Their equations are extended to be applicable to the memory effect with the second-order lag. The extension provides a recursive algorithm for cancellation signals of the PD each of which updating is made by using signals in only two sampling points. The algorithm is equivalent to a memory depth of two in computational efficiency. The required times for multiplications and additions are counted for the updating of all the cancellation signals and it is confirmed that the algorithm reduces computational intensity lower than half of a memory polynomial in recent papers. A computer simulation has clarified that the PD improves the adjacent channel leakage power ratio (ACLR) of OFDM signals with several hundred subcarriers corresponding to 4G mobile radio communications. It has been confirmed that a fifth-order PD is effective up to a higher power level close to 1 dB compression. The improvement of error vector magnitude (EVM) by the PD is also simulated for OFDM signals of which the subcarrier channels are modulated by 16 QAM.

An RF adaptive array antenna (RF-AAA) configured with variable capacitors is proposed. This antenna system can control the power combining ratio and phase value of received signals. In this paper, we focus on the diversity effects of RF-AAA. First, we show the design methodology of the combiner circuit to realize the effective combining. Second, the perturbation method and the steepest gradient method are compared for the optimization algorithms to provide fast convergence and suboptimum solutions among the variable circuit constants. Finally, in simulation, we show the RF-AAA can achieve diversity antenna gains of 7.7 dB, 10.9 dB and 12.6 dB for 2-branch, 3-branch and 4-branch configuration, respectively, which have higher performance than the selection combining.

Radio frequency over fiber (RoF) advanced technology is already integrated into current 3G and 4G radio access networks in which the digital unit and remote radio head equipped with nonlinear high power amplifiers (HPAs) are connected through the RoF-based fronthaul links. In this study, we investigated the degradation in the adjacent channel leakage ratio (ACLR) of equipment with the adaptive linearizer RF HPA when both the direct and feedback paths of the transmitting system include RoF links. We show that an ACLR exceeding -57dBc @ 5-MHz offset, which completely satisfies the requirements of the 3GPP technical specifications, can be achieved for a 20-W-class Doherty power amplifier linearized through commercial RoF links. Experiments showed that the achieved ACLR strongly depends on the RoF-link noise figure and that most of the nonlinear distortions caused by RoF can be completely suppressed with the proposed joint linearization approach for simultaneous linearization of RoF and HPA nonlinearities with a single common “joint” linearizer. Experimental results confirm significant ACLR performance enhancements as a result of RoF noise floor reduction, which is achieved under RoF driving conditions optimized together with joint RoF and HPA linearization. Our joint linearization approach via RoF links is confirmed to be applicable for next-generation mobile fronthaul architectures.

In recent years, digital predistortion linearizers have been used in power amplifiers for mobile communications because they are simpler and provide higher power efficiency than conventional feedforward systems. However, in systems that cover a wider frequency band, it is impossible to disregard the frequency characteristics of their various parameters since the degradation that can result causes a decline in output power efficiency which is the most important property of a power amplifier. To date, no detailed studies have been carried out on predistortion compensation systems. Thus, we focused our research on these systems and in this paper we report the simulation and experimental results we obtained for clarifying these effects. In our experiments, we used a W-CDMA power amplifier to determine how much the distortion compensation effect is degraded by the frequency characteristics of analog RF circuits. The results of experiments to determine the relationship between the ACLR (Adjacent Channel Leakage power Ratio) and power efficiency are also reported.

This paper experimentally studies and models the angular-delay power spectrum density at the mobile station based on the site-specific measurement in a macrocell in urban area of Tokyo. The authors first show the azimuth power spectral density at the mobile station. It is decomposed into the "classes" which represent specific contributions within limited azimuth range, as well as the residual. The site-specific propagation mechanism of the classes are next discussed. Finally, the angular-delay PSD models of both classes and residual are proposed and verified. The analysis and modeling in this paper are antenna independent with the full polarimetric information. Consequently, the results are useful to evaluate the performance of arbitrary array antennas with mixed polarization. Due to the rare number of antenna-independent and full-polarimetric measurements, the significant contribution of the angular-delay PSD channel model can be expected.