• Journal of the Institute of Electronics Engineers of Korea TC
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• v.48 no.6
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• pp.28-33
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• 2011

Recently, the investigation of the cognitive radio (CR) system is actively progressed as one of the methods for using the frequency resources more efficiently. In CR systems, when the frequency band allocated to the incumbent user is not used, the unused frequency band is assigned to the secondary user. Thus, the FFT input signals corresponding to the actually used frequency band by the incumbent user are assigned as '0'. In this paper, based on the fact that there are many '0' input signals in CR systems, a low-power FFT design method for NC-OFDM is proposed. An efficient zero flag generation technique for each stage is first presented. Then, to increase the utility of the zero flag signals, modified architectures for memory and arithmetic circuits are presented. To verify the performance of the proposed algorithm, 2048 point FFT with radix-24SDFstructureisdesignedusingVerilog HDL. The simulation results show that the power consumption of FFT is reduced considerably by the proposed algorithm.

• The Journal of Information Technology
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• v.5 no.4
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• pp.1-8
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• 2002

This thesis is about power line communication(PLC) over the low voltage grid. The main advantage with power line communication is the use of an existing infrastructure. The PLC channel can be modeled as having multi-path propagation with frequency-selective fading, typical power lines exhibit signal attenuation increasing with length and frequency. OFDM(Orthogonal Frequency Division Multiplexing) is a modulation technique where multiple low data rate carriers are combined by a transmitter to form a composite high data rate transmission. To implement the multiple carrier scheme using a bank of parallel modulators would not be very efficient in analog hardware. Each carrier in an OFDM is a sinusoid with a frequency that is an integer multiple of a base or fundamental sinusoid frequency. Therefore, each carrier is a like a Fourier series component of the composite signal. In fact, it will be shown later that an OFDM signal is created in the frequency domain, and then transformed into the time domain via the Discrete Fourier Transform(DFT).

• Journal of Power Electronics
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• v.16 no.3
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• pp.1226-1235
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• 2016

For wearable health monitoring systems, a fundamental problem is the limited space for storing energy, which can be translated into a short operational life. In this paper, a highly efficient active voltage doubling rectifier with a wide input range for micro-piezoelectric energy harvesting systems is proposed. To obtain a higher output voltage, the Dickson charge pump topology is chosen in this design. By replacing the passive diodes with unbalanced-biased comparator-controlled active counterparts, the proposed rectifier minimizes the voltage losses along the conduction path and solves the reverse leakage problem caused by conventional comparator-controlled active diodes. To improve the rectifier input voltage sensitivity and decrease the minimum operational input voltage, two low power common-gate comparators are introduced in the proposed design. To keep the comparator from oscillating, a positive feedback loop formed by the capacitor C is added to it. Based on the SMIC 0.18-μm standard CMOS process, the proposed rectifier is simulated and implemented. The area of the whole chip is 0.91×0.97 mm², while the rectifier core occupies only 13% of this area. The measured results show that the proposed rectifier can operate properly with input amplitudes ranging from 0.2 to 1.0V and with frequencies ranging from 20 to 3000 Hz. The proposed rectifier can achieve a 92.5% power conversion efficiency (PCE) with input amplitudes equal to 0.6 V at 200 Hz. The voltage conversion efficiency (VCE) is around 93% for input amplitudes greater than 0.3 V and load resistances larger than 20kΩ.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.22 no.9
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• pp.1941-1948
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• 1997

In this paper, the power efficient linear upconversion mixer which is a functional circuit in transmit path of intermediate frequency(IF) part of Code Division Multiple (CDMA) cellular phone was explained. In generally, the low CMOS devices limits the implementation of upconversion mixer especially for lower loads. Using replica transconductor, the linear range is extended up to the limit. Thiscircuit was imprlemented using $0.8{\mu}\textrm{m}$ N-well CMOS technology with 2-poly/2-metal. The active area of chip is $0.53mm{\times}0.92mm$. The power consumption is 30mW with 3.3V suply voltage. The 1dB conpression characteristics is -27.3dB with $25{\Omega}$. load and being applied by 2-tone input signal. The mixer operates properly above 200MHz.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.28 no.3C
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• pp.215-223
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• 2003

Turbo codes are selected as FEC(Forward error correction) codes with convolution code in 3GFP(3rd generation partnership project) and 3GPP2 standard of IMT2000. Especially, l/3 turbo code with K=4 is employed for 3GPP standard. In this paper, we proposed a hardware structure of a turbo decoder and denveloped the decoder for 3GPP standard turbo code. For its efficient operation, we design a SOVA decoder by employing a register exchange decoding block and new path metric normalization block as a SISO constituent decoder. In addition, we estimate its performance under MATLAB 6.0 and designed the turbo decoder including control block, input control buffer, SOVA constituent decoder with VHDL. Finally, we synthesized the developed turbo decoder under Synopsys FPGA Express and verified it with ALTERA EPF200SRC240-3 FPGA device.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.7B
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• pp.728-733
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• 2009

An ubiquitous environment has features like peer-to-peer and nomadic environments. Such features can be represented by peer-to-peer systems and mobile ad-hoc networks. P2P systems and MANETs share similar features, appealing for implementing P2P systems in MANET environment. However, if a P2P system designed for wired networks was applied to mobile ad-hoc environment, its performance was not good enough. Subsequently, this paper proposes a P2P system to improve performance using cross-layer design and the goodness of a node as a peer by using routing metric and P2P metric to choose favorable peers to connect. It also utilizes proactive approach for distributing peer information. The simulation results showed that the proposed system produced better performance in query success rate, query response time and energy consumption by reducing the routing path length.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.1A
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• pp.62-70
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• 2010

In this paper, we evaluate the performance of various diversity techniques which can contribute to provide efficient multimedia broadcasting services via hybrid/integrated satellite and terrestrial network. Space-time coding (STC) can achieve the diversity gain in a multi-path environment without additional bandwidth requirement. Recent study results reported that satellite systems can achieve high diversity gains by appropriate utilization of STC and/or forward error correction schemes. Based on these previous study results, we present various cooperative diversity techniques by combining STC and rate compatible turbo codes in order to realize the transmit diversity for the mobile satellite system. The satellite and several terrestrial repeaters operate in unison to send the encoded signals, so that receiver may realize diversity gain. The results demonstrated in this paper can be utilized in future system implementation.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.9B
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• pp.1295-1304
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• 2010

This paper proposes a fault-tolerant data aggregation which provides energy efficient and reliable data collection in wireless sensor networks. The traditional aggregation scheme does not provide the countermeasure to packet loss or the countermeasure scheme requires a large amount of energy. The proposed scheme applies caching and re-transmission based on the track topology to the adaptive timeout scheduling. The proposed scheme uses a single-path routing based on the traditional tree topology at normal, which reduces the dissipated energy in sensor nodes without any countermeasure against packet loss. The proposed scheme, however, retransmits the lost packet using track topology under event occurrences in order to fulfill more accurate data aggregation. Extensive simulation work under various workloads has revealed that the proposed scheme decrease by 8% in terms of the dissipated energy and enhances data accuracy 41% when the potential of event occurrence exists as compared with TAG data aggregation. And the proposed scheme decrease by 53% in terms of the dissipated energy and shows a similar performance in data accuracy when the potential of event occurrence exists as compared with PERLA data aggregation.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.5C
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• pp.556-564
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• 2007

This paper proposes an efficient scheme to track a time variant channel induced by multi-path Rayleigh fading in mobile MIMO-OFDM systems with null sub-carriers. The proposed adaptive channel tracking scheme removes in the frequency domain the interfering signals of the other transmit (Tx) antennas by using a predicted channel frequency response before starting the channel estimation. Time domain channel estimation is then performed to reduce the additive white Gaussian noise (AWGN). The simulation results show that the proposed method is better than the conventional channel tracking method [3] in time varying channel environments. At a Doppler frequency of 300 Hz and bit error rates (BER) of 10-3, signal-to-noise power ratio (Eb/N0) gains of about 2.5 dB are achieved relative to the conventional channel tracking method [3]. At a Doppler frequency of 600 Hz, the performance difference between the proposed method and conventional one becomes much larger.

• ETRI Journal
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• v.37 no.3
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• pp.450-459
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• 2015

Small cell networks, as an important evolution path for next-generation cellular networks, have drawn much attention. Different from the traditional base stations (BSs) always-on model, we proposed a BSs on-off model, where a new, simple expression for the probabilities of active BSs in a heterogeneous network is derived. This model is more suitable for application in practical networks. Based on this, we develop an analytical framework for the performance evaluation of small cell networks, adopting stochastic geometry theory. We derive the system coverage probability; average energy efficiency (AEE) and average uplink power consumption (AUPC) for different association strategies; maximum biased received power (MaBRP); and minimum association distance (MiAD). It is analytically shown that MaBRP is beneficial for coverage but will have some loss in energy saving. On the contrary, MiAD is not advocated from the point of coverage but is more energy efficient. The simulation results show that the use of range expansion in MaBRP helps to save energy but that this is not so in MiAD. Furthermore, we can achieve an optimal AEE by establishing an appropriate density of small cells.