• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2015.05a
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• pp.681-684
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• 2015

Power amplifier is an essential component for transmitting signals to a remote receiver in wireless communication systems. Power amplifier is a non-linear device in general, and the nonlinear distortion becomes severer as the output power increases. The nonlinearity results in spectral regrowth, which leads to adjacent channel interference, and decreases the transmit signal quality. To linearize power amplifiers, many techniques have been developed so far. Among the techniques, digital pre-distortion is known as the most cost and performance effective technique. However, the linearization performance falls down abruptly when the power amplifier operates in its saturation region. This is because of the severe nonlinearity. To relieve this problem, this paper proposes a new adaptive predistortion technique. The proposed technique controls the adaptive algorithm based on the power amplifier input level. Specifically, for small signals, the adaptive predistortion algorithm works normally. On the contrary, for large signals, the adaptive algorithm stops until small signals occur again. By doing this, wrong coefficient update by severe nonlinearity can be avoided. Computer simulation results show that the proposed method can improve the linearization performance compared with the conventional digital predistortion algorithms.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.7A
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• pp.578-586
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• 2005

We propose a random frequency hopping orthogonal frequency division multiple access (RFH-OFDMA) system based on statistical multiplexing for improving downlink user capacity. User capacity is defined as the maximum number of users served with a given basic data-rate in a cell. We compare the downlink user capacity of the proposed RFH-OFDMA system with that of the conventional frequency hopping OFDMA (FH-OFDMA) systems in the worst case where all users are located at the cell boundary. User capacity is limited by either the number of subcarriers or other-cell interference (OCI). Simulation results show that the proposed RFH-OFDMA system can accommodate 262 users in a 3-sectored cell, while the conventional FH-OFDMA systems can accommodate 51 users, when the user channel activity and the required Eb/I0 are 0.1 and 6 dB, respectively, and all users are assumed to be located at the cell boundary.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.24 no.8A
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• pp.1107-1114
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• 1999

In this paper, we analyze the effect of co-channel interferers coming from other systems with composite log-normal shadowing and Nakagami fading on the capacity of the imperfect power-controlled DS/CDMA cellular system. The analytical results are compared with the results of Prasad and Jansen in which they derive the result under the condition of imperfect power-control of DS/CDMA system and without considering interference. And the outage probability is calculated fro the effect of the log-normal/Nakagami faded inferers on DS/CDMA system using Schwartz and Yeh's method. With the deviation of outage probability, we analyzed the decreases in the capacity of DS/CDMA system with the variation of the paramenters of the imperfect power-control, the fadig index, the processing gain, the power ratio, the voice activity factor, and the number of interferers. Analysis reuslts have shown that the capacity of DS/CDMA system is very sensitive to the imperfect power-control and the number of interferers. It is shown that the DS/CDMA system capacity decreased according to increase of the deviation of the impertect power-control, the increase of the fading, and the increase of interferer number.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.29 no.1A
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• pp.9-17
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• 2004

Adapt ive and fixed step size PC (power control) schemes for accommodating voice, video, and data are evaluated according to the different PC command rates and their effects on integrated Voice/Video/Data are investigated. The required minimum power levels are derived as PC thresholds and the effects of PC errors on channel quality and radio 1 ink capacity are investigated. The services with high bit rates and low bit error rates can cause a significant effect on the radio link qualifies of the other types of traffic. The results show that the adapt ive step size PC scheme for voice/video/data services can achieve more capacity and cause less interference to the radio channels because less minimum PIL(Power Increment Level) is required for the specified radio link outage probability.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.39A no.2
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• pp.69-76
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• 2014

Multiple-input and multiple-output (MIMO) technique is used in many communication fields in order to increase the channel capacity. However, this MIMO system has difficulty of miniaturization of antenna size due to the multiple RF chains Also, multiple RF chain raises some problems which increase power consumption at RF circuit and degrade the system performance due to the interference between RF chains. Because of these reasons, beamspace MIMO (BS-MIMO) technique with only single RF chain was proposed for MIMO transmission. This BS-MIMO system basically uses electronically steerable parasitic array radiator (ESPAR) antenna. Existing ESPAR antenna has a 5-element structure. So, it is possible to do only $3{\times}3$ MIMO transmission. Therefore, in order to extend BS-MIMO dimension, extension of ESPAR antenna structure is essential. In this paper, we show that BS-MIMO dimension can be increased according to the extension of structure of the ESPAR antenna, as in the conventional MIMO techniques. For example, we show that it is possible to design the $7{\times}7$ BS-MIMO transmissions with the 13-element ESPAR antenna. Also, when the number of parasitic elements of ESPAR antenna increases by two elements, MIMO dimension is expanded by 1.

• Journal of Communications and Networks
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• v.12 no.2
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• pp.103-113
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• 2010

In this paper, a tradeoff between the total energy consumption-per-bit and the end-to-end rate under spatial reuse in wireless multi-hop network is developed and analyzed. The end-to-end rate of the network is the number of information bits transmitted (end-to-end) per channel use by any node in the network that is forwarding the data. In order to increase the bandwidth efficiency, spatial reuse is considered whereby simultaneous relay transmissions are allowed provided there is a minimum separation between such transmitters. The total energy consumption-per-bit includes the energy transmitted and the energy consumed by the receiver to process (demodulate and decoder) the received signal. The total energy consumption-per-bit is normalized by the distance between a source-destination pair in order to be consistent with a direct (single-hop) communication network. Lower bounds on this energy-bandwidth tradeoff are analyzed using convex optimization methods. For a given location of relays, it is shown that the total energy consumption-per-bit is minimized by optimally selecting the end-to-end rate. It is also demonstrated that spatial reuse can improve the bandwidth efficiency for a given total energy consumption-per-bit. However, at the rate that minimizes the total energy consumption-per-bit, spatial reuse does not provide lower energy consumption-per-bit compared to the case without spatial reuse. This is because spatial reuse requires more receiver energy consumption at a given end-to-end rate. Such degraded energy efficiency can be compensated by varying the minimum separation of hops between simultaneous transmitters. In the case of equi-spaced relays, analytical results for the energy-bandwidth tradeoff are provided and it is shown that the minimum energy consumption-per-bit decreases linearly with the end-to-end distance.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.7A
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• pp.757-767
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• 2008

Start Ongoing next generation networks are expected to be deployed over current existing networks, in the form of overlayed heterogeneous networks, in particular, in hot spot areas. Therefore, it will be necessary to develop an interworking technique such as load balancing, to achieve increased overall resource utilization in the various heterogeneous networks. In this paper, we present a new load balancing mechanism termed 'soft' load balancing where the IP(Internet Protocol) traffic of a user is divided into sub-traffic, each of which flows into a different access network. The terminology of soft load balancing involves the use of both load sharing and handover techniques. Through a numerical analysis, we obtain an optimal LBR (Load Balancing Ratio) for determining the volume of traffic delivered to each network over an overlayed multi-cell environment. Using the optimal LBR, a more reliable channel transmission can be achieved by reducing the outage probability efficiently for a given user traffic.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.45 no.7
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• pp.44-51
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• 2008

A stereo audio digital-to-analog converter (DAC) with a power amplifier using asymmetric pulse-width modulation (PWM) is presented. To adopt class-D amplifier mainly used in high-power audio appliances for head-phones application, this work analyzes the noise caused by the inter-channel interference during the integration and optimizes the design of the sigma-delta modulator to decrease the performance degradation caused by the noise. The asymmetric PWM is implemented to reduce switching noise and power loss generated from the power amplifier. This proposed architecture is fabricated in 0.13-mm CMOS technology. The proposed audio DAC including the power amplifier with single-ended output achieves a dynamic range (DR) of 95-dB dissipating 4.4-mW.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.37 no.1
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• pp.76-87
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• 2000

Multicarrier DS-CDAM is an effective approach to realize wideband CDMA system in a multipath fading channeal. In this paper, we propose a cinvolutionally-coded overlapped multicarrier DS-CDMA system, and show the performance improvement by comparing with conventional multicarrier DS-CDMA system. We consider the case of 50% overlapping with the adjacent subband to utilize the transmission bandwidth more efficiently. In the proposed multicarrier system, each of the rate 1/M convolutionally-encoded symbols is also 1/R repetition coded and transmitted using overlapped multicarriers and we may obtain the coding gain and frequency diversity effect, simultaneously. We also analyze the possibility of reduction in total MUI by considering a raised-cosine wave-shaping filter having a roll-off factor (0< ${\beta}{\le}1$). It will be shown that the proposed system outperforms the multicarrier DS-CDMA system in $^{[3]}$.

• Journal of Broadcast Engineering
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• v.20 no.6
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• pp.837-847
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• 2015

In this paper, the reception performance of spatial modulation multiple-output multiple-input (MIMO) is analyzed for high speed terrestrial broadcasting. The MIMO scheme is required to reduce the inter symbol interference (ISI) and spatial correlation. The spatial modulation scheme solves the problem of ISI, but the spatial correlation degrades the reception performance of SM scheme. The space-time block coded spatial modulation (STBC-SM) is combined the SM system with space-time block code (STBC) for reducing the effects of the spatial correlation. However, the STBC-SM scheme degrades the spectral efficiency by transmitting same data in the two symbol period. The double space-time transmit diversity with spatial modulation (DSTTD-SM) scheme transmits the data with full antenna combination. To adapt these SM MIMO systems into the terrestrial broadcasting system, the reception performance is analyzed using computer simulation in SUI channel environments.