• The Journal of Korean Institute of Communications and Information Sciences
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• v.25 no.5A
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• pp.618-633
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• 2000

In this paper, a Gaussian interpolation filter and cubic interpolation filter are presented to do more accurate channel estimation compared to the conventional linear interpolation filter for COFDM systems. In addition to an interpolation filter, a low pass filter using FFT and IFFT is also presented to reduce the noisy components of a channel estimate obtained by an interpolation filter. Channel estimates after low-pass filtering combined with interpolation filters can lower the error floor compared to the use of only interpolation filters. Computer simulation demonstrates that the presented channel estimation methods exhibit an improved performance compared to the conventional linear interpolation filter for COFDM systems in time-varying multipath fading channel and0.1 ~ 0.2 dB of Eb/No difference at BER=10-4 when the perfect channel estimation is compared.

• Journal of Communications and Networks
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• v.16 no.1
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• pp.10-17
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• 2014

This work investigates two important performance metrics of underlay cooperative cognitive radio (CR) networks: Interference cumulative distribution function of licensed users and outage probability of unlicensed users. These metrics are thoroughly analyzed in realistic operating conditions such as imperfect fading channel information and strict transmit power constraint, which satisfies interference power constraint and maximum transmit power constraint, over Nakagami-m fading channels. Novel closed-form expressions are derived and subsequently validated extensively through comparisons with respective results from computer simulations. The proposed expressions are rather long but straightforward to handle both analytically and numerically since they are expressed in terms of well known built-in functions. In addition, the offered results provide the following technical insights: i) Channel information imperfection degrades considerably the performance of both unlicensed network in terms of OP and licensed network in terms of interference levels; ii) underlay cooperative CR networks experience the outage saturation phenomenon; iii) the probability that the interference power constraint is satisfied is relatively low and depends significantly on the corresponding fading severity conditions as well as the channel estimation quality; iv) there exists a critical performance trade-off between unlicensed and licensed networks.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.17 no.1
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• pp.261-279
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• 2023

In this article, the uplink achievable rate is investigated for massive multiple-input multiple-output (MIMO) under correlated Ricean fading channel, where each base station (BS) and user are both deployed multiple antennas. Considering the availability of prior knowledge at BS, two different channel estimation approaches are adopted with and without prior knowledge. Based on these channel estimations, a two-layer decoding scheme is adopted with maximum ratio precoding as the first layer decoder and optimal second layer precoding in the second layer. Based on two aforementioned channel estimations and two-layer decoding scheme, the exact closed form expressions for uplink achievable rates are computed with and without prior knowledge, respectively. These derived expressions enable us to analyze the impacts of line-of-sight (LoS) component, two-layer decoding, data transmit power, pilot contamination, and spatially correlated Ricean fading. Then, numerical results illustrate that the system with spatially correlated Ricean fading channel is superior in terms of uplink achievable rate. Besides, it reveals that compared with the single-layer decoding, the two-layer decoding scheme can significantly improve the uplink achievable rate performance.

• Proceedings of the IEEK Conference
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• 2000.11a
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• pp.165-168
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• 2000

The bit error rate(BER)of the data passed through Wideband-Code Division Multiple Access (W-CDMA) system with turbo-codes structure is presented. The performance of turbo-codes under W-CDMA system is estimated for various users and iteration numbers of decoding. The channel model is Additive White Gaussian Noise(AWGN) and Rayleigh fading channel. When iteration number increases, bit error probability of turbo-codes decreases. and when the number of users increase, bit error probability of turbo-codes increases.

• Proceedings of the IEEK Conference
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• 2000.07b
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• pp.765-768
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• 2000

In this paper, we analyze the channel estimation method by using Reference symbols for burst OFDM transmission systems. We set some parameters (the number of Reference symbols, the pilot spacing in Reference symbols, the update constant of equalizer coefficients) to evaluate their performance in the fixed multipath fading channel as like indoor environment.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.3C
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• pp.189-194
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• 2007

This paper derives and computes the detection probability of timing synchronization in an orthogonal frequency division multiplexing (OFDM) system encountered with a multipath Rayleigh fading channel and imperfect noise estimation. The timing synchronization scheme using a simple repeated constant amplitude zero auto-correlation (CAZAC) training symbol and correlation techniques is adopted. With this provision, we focus on the numerical analysis for OFDM timing synchronization scheme employing a preadvancement technique to reduce the inter-symbol interference (ISI). For measuring system performance, the detection performance derived in the considered system is presented in a multipath Rayleigh fading channel.

• Journal of Communications and Networks
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• v.18 no.1
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• pp.8-18
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• 2016

In this paper, we propose a number of blind equalization approaches for time-varying andmulti-path channels. The approaches employ cost reference particle filter (CRPF) as the symbol estimator, and additionally employ either least mean squares algorithm, recursive least squares algorithm, or $H{\infty}$ filter (HF) as a channel estimator such that they are jointly employed for the strategy of "Rao-Blackwellization," or equally called "mixture filtering." The novel feature of the proposed approaches is that the blind equalization is performed based on direct channel estimation with unknown noise statistics of the received signals and channel state system while the channel is not directly estimated in the conventional method, and the noise information if known in similar Kalman mixture filtering approach. Simulation results show that the proposed approaches estimate the transmitted symbols and time-varying channel very effectively, and outperform the previously proposed approach which requires the noise information in its application.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.16 no.12
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• pp.4099-4113
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• 2022

In this paper, we propose improved maximum access delay time, noise variance, and power delay profile (PDP) estimation schemes for orthogonal frequency division multiplexing (OFDM) system in multipath fading channels. To this end, we adopt the approximate maximum likelihood (ML) estimation strategy. For the first step, the log-likelihood function (LLF) of the received OFDM symbols is derived by utilizing only the cyclic redundancy induced by cyclic prefix (CP) without additional information. Then, the set of the initial path powers is sub-optimally obtained to maximize the derived LLF. In the second step, we can select a subset of the initial path power set, i.e. the maximum access delay time, so as to maximize the modified LLF. Through numerical simulations, the benefit of the proposed method is verified by comparison with the existing methods in terms of normalized mean square error, erroneous detection, and good detection probabilities.

• Journal of Electrical Engineering and information Science
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• v.1 no.1
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• pp.23-28
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• 1996

In this series of two papers, adaptive code rate change schemes in DS-SSMA systems are proposed. In the proposed schemes the error correcting code rate is changed according to the channel states. Two channel states having significant effects on the bit error probability are considered: one is the effective number of users considered in Part 1, and the other is the fading environment considered in Part 2. These channel states are estimated based on retransmission requests. The criterion for the change of the code rate is to maximize the throughput under given error bound. Simulation results show that we can transmit maximum amount of information if we change the code rate based on the channel states.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.37 no.9
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• pp.1-10
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• 2000

This paper proposes symbol decoding schemes combined with channel estimation techniques for coded orthogonal frequency division multiplexing (COFDM) systems in fading channels. sThe proposed symbol decoding schemes are consisted of a symbol decoding technique and channel estimation techniques. The symbol decoding based on Viterbi algorithm is achieved by matching the length of branch word from encoder trellis to the codeword length of symbol candidate on decoder trellis. Three combination schemes are described and their error performances are compared. The first scheme is to combine a symbol decoding technique with a training channel estimation technique. The second scheme joins a decision directed channel estimation technique to the first scheme. The time varying channel transfer functions are tracked by the decision directed channel estimation technique and the channel transfer functions used in the symbol decoder are updated every COFDM symbol. Finally, In order to reduce the effect of additive white Gaussian noise (AWGN) between adjacent subchannels, deinterleaved average channel estimation technique is combined. The error performances of the three schemes are significantly improved being compared with that of zero forcing equalizing schemes.