This paper presents a low-imbalance and inductor-less active balun. The large immittance of the parasitics increases gain and phase errors in single-ended-to-differential conversion at high frequencies. Positive feedback is effective in reducing these errors. However, there is a trade-off between the stability of the feedback and the imbalance correction. This paper analyzes this trade-off, and the common-mode rejection ratio (CMRR) was improved by adding a capacitor. Besides, we established the feedback loops for the imbalance correction are also available for bandwidth extension, that is the additional capacitor improves not only the CMRR but also the high-frequency gain. This peaking technique removes inductors that consume large chip areas. The balun was fabricated in a 0.18-$\mu \text{m}$CMOS process and achieved a small core area of 0.0058 mm2. In addition, a self-bias scheme using a current mirror was devised. It ensures a good bias current balance and reduces errors. The manufacturing variation of the fabricated baluns was statistically evaluated. To obtain the 99.7% limit of the CMRR, we extended the theory of the random CMRR to the complex plane. The measurement results demonstrated small errors within −0.1±0.2 dB and −0.18±1.17° including a variation of$\pm 3\sigma $from DC to 8.0 GHz.

This paper presents an area-efficient differential distributed amplifier using differential coupled inductors. The artificial transmission lines of the proposed distributed amplifier are modeled as differential transmission lines, and thus, it can be easily designed compared with other area reduction techniques. The dimensions of the coupled inductors have been optimized using a gradient descent algorithm with a scalable model of the coupled inductors. The proposed amplifier was designed using 1P5M 180 nm standard CMOS technology. Post-layout simulation results demonstrated a gain of 6.9 dB and a bandwidth of 20.2 GHz with 4.9 ps Nyquist group delay variation. A core area of the proposed amplifier was 0.28 mm2, which was the smallest value compared with previous works using similar processes.

As a method of dimming support for visible light communications using a massive LED array, we consider a hybrid pulse width modulation/digitally controlled pulse amplitude modulation (PWM/DPAM) system. Especially, in this paper, we consider a digital color shift keying (DCSK) using an RGB-LED array as an optical intensity modulation scheme, that transmits data through the ratio of the optical intensities (i.e., color) emitted by red, green, and blue LEDs. In DCSK, since only one color is activated in each RGB-LED at a time, the color can be represented by the combination of the digitally (i.e., linearly) controlled “ON-OFF” LEDs. In general, for the dimming control system of DCSK, two schemes have been considered. One is the dimming control by PWM, which changes the duty cycle of optical transmit signals, and the other is the dimming control by DPAM, which changes the number of active LEDs in the RGB-LED array. In this paper, PWM and DPAM are combined to realize a higher spectral efficiency than PWM and a wider dimming range than DPAM. We evaluate the error performances of the proposed system, DCSK with PWM, and DCSK with DPAM from a simulation analysis under several measured light dimming levels. The results show that DPAM should be used only for low bit rate systems because the effect of inter-symbol-interference (ISI), caused by the LED frequency response, increases at a high bit rate. While, PWM is significantly robust against ISI because the PWM signal duration is limited in a symbol duration at the low dimming levels and the empty duration can mitigate the effect of ISI even if the bit rate is high. When focusing on symbol error rate performances corresponding to the dimming levels, the hybrid dimming control and the PWM dimming control can achieve lower signal energy-to-noise-ratio at the dimming level of low and high, respectively.

We proposed the PAPR reduction technique of resource block assignment. In the proposed technique, the effect of PAPR reduction can be controlled by RB assignment. If the PAPR reduction is more powerful, the output signal power from power amplifier is enlarged. In multi-cell environment, the cell coverage is decided by transmit power. Therefore, the access between mobile station and base station can be controlled by transmit power. When base stations cooperate each other, the base station of the mobile station's access can be switched by channel state information. As a result, the site diversity gain can be obtained. This paper considers two base station cooperation system with the PAPR reduction of RB assignment.

Switching estimated receiver position (SwERP) scheme is proved to be a promising solution for indoor positioning system, owing to its high achievable accuracy and consistency. The high positioning accuracy is achieved from sensitivity (Rx,S) limit, field-of-view (FOV) limit, and assisted azimuth and tilt information from 6-axis sensor. In this paper, we propose a physical simulation model that can predict the latter three factors. The conventional visible light communication (VLC) model uses only geometric optics (GO) to predict light propagation, which is not enough to define the FOV limit. Thus, we propose a novel method using rotation matrix with cone function and support vector machines (SVMs) to classify the boundary of FOV limit. Based on sensitivity and FOV limits, possible azimuth and tilt angulations are mathematically defined. Moreover, by including FOV limit into the simulation, transmitters and their mirrors that are outside FOV limit can be neglected, of which reduce at least 80% of computation during GO calculation.

Nearest transmitter information is a mandatory requirement for utilizing Switching Estimated Receiver Position (SwERP) scheme, of which was proposed to enhance accuracy of Visible Light ID (VLID) positioning system. The conventional approach is achieved by controlling Sensitivity (RxS) limit. Conversely, general distance estimation by optical received power can also be used. Both of the mentioned methods have disadvantage on implementation complexity and inaccuracy due to noise fluctuation, respectively. In this paper, we propose a nearest transmitter classification (NTC) method by utilizing Optical Orthogonal Code (OOC) with On-Off Keying (OOK) modulation at transmitters (Txs), and perform oversampling at a receiver (Rx) to overcome the limitation of bandwidth resolution (B). The results confirm that the proposed method can classify the nearest Txaccurately, with some tradeoff with computation complexity from the increment of oversampling factor (Oc).

TCP service constructs high quality end-to-end communication link. TCP needs two way communication links. First link and second one are necessary for transmitting data packet from source to destination and ACK packet form destination to source, respectively. Therefore, TCP throughput strictly depends on both links. In wireless communication, the link speed is time selective due to shadowing effect. When the link speed is dramatically changed during the event of downloading, the impact of time selective link speed should be significant. In this paper, we clarify the impact of fluctuating link speed to TCP throughput by network simulator. In addition, we propose the adaptive link speed control of ACK link for achieving large ACK throughput. We also confirm the effect of proposed technique by network simulator.

In cognitive radio, the mobile terminal senses the wireless accessing of the other wireless communications, specifies the vacant frequency channel, and then tries to use the specified frequency spectrum. In sensing technique, the detection accuracy depends on the detection time and thus the number of detectable systems also depends on it. When the other system is in the undetectable area for the cognitive radio, cognitive radio must protect the wireless access of it. In this paper, we consider the cognitive radio with using carrier sensing and transmitting power control. In considered system, the accessing protocol based on carrier sensing is established for protecting detectable other system. In addition, the transmitting power for suppressing the interference to the undetectable system is controlled. Since the sensing time depends on the detectable area, the transmitting power of cognitive radio transmitter and the opportunity of finding vacant spectrum are changed. Therefore, we derive the optimal sensing time for achieving the largest channel capacity.

The novel frequency rotation techniques for suppressing the path interference and increasing the frequency diversity gain are proposed in uplink IFDMA system. In IFDMA, the path interference in multipath fading channel and the reduction of frequency diversity gain degrades the error rate performance, especially more significantly as the number of simultaneous access users becomes large. For suppressing the path interference and enlarging the frequency diversity gain, the frequency rotation has been proposed for down link access but it cannot be applied to the uplink due to the distortion of user frequency orthogonality. In the proposed frequency rotation, multiple users do not share the multiple frequency orthogonal channels and thus the orthogonality among the users can be maintained. The effect of the revised technique is shown by computer simulation.