Hyoung-Kyu Song

Frequency-offset synchronization and channel estimation for OFDM-based transmission

This letter is concerned with a frequency offset estimation technique for OFDM based transmission systems. The frequency estimation technique utilizes a repetitive signal structure inside of a OFDM symbol which is used to enlarge the range and increase the accuracy of offset estimation. Also, an averaged decision-directed channel estimation (ADDCE) technique suitable for burst data is proposed.

Peak-to-average power ratio in MIMO-OFDM systems using selective mapping

This letter evaluates the peak-to-average power ratio (PAR) performance in a space-time block coded (STBC) multi-input multi-output orthogonal frequency-division multiplexing (MIMO-OFDM) system using the selective mapping (SLM) approach. The investigated SLM-based MIMO-OFDM system selects the transmitted sequence with the lowest average PAR over all transmit antennas and retrieves the side information very accurately at the expense of only 0.5-dB degradation of the PAR, which can improve the overall detection performance of the MIMO-OFDM system with erroneous side information, compared to the individual SLM approach.

Semi-blind channel estimation and PAR reduction for MIMO-OFDM system with multiple antennas

A combining of MIMO signal processing with OFDM is regarded as a promising solution of enhancing the performance of next generation wireless system. Therefore, in this paper, an OFDM-based wireless system employing layered space-time architecture is considered for a high-rate transmission. With an emphasis on a preamble design for multi-channel separation, we address a channel estimation based on the time-domain windowing and its imperfectness in MIMO-OFDM system. By properly designing each preamble for multiple antennas to be orthogonal in the time domain, the channel estimation can be executed based on semi-blind processing in the case of more than two transmitting antennas. And also we evaluate the PAR performance in the MIMO-OFDM system using the SLM and PTS approaches. The investigated SLM and PTS schemes for MIMO-OFDM signals select the transmitted sequence with lowest average PAR over all transmitting antennas and retrieve the side information very accurately at the expense of a slight degradation of the PAR performance. The low probability of false side information can improve the overall detection performance of the MIMO-OFDM system with erroneous side information compared to the ordinary SLM and PTS approaches, respectively. Also, we provide closed form of the average BER performance in MIMO-OFDM system using analytic approach.

A New PAPR Reduction Technique of OFDM System with Nonlinear High Power Amplifier

High peak-to-average power ratio (PAPR) of the transmit signal is a major drawback of multicarrier transmission such as orthogonal frequency division multiplexing (OFDM). Most of recent works have focused on minimizing the PAPR of OFDM signal to enhance performance. However, after relating this issue with the specific characteristics of high power amplifier (HPA), we propose a new scheme based on iterative partial transmit sequence (I-PTS) scheme that uses the amount of OFDM signals nonlinear distortion to be caused by high power amplifier (HPA) as a new measure. The proposed scheme can have simple subblock combining method by applying iterative PTS and then, a new measure which is called adaptive nonlinear estimator (ANE), is achieved by estimating the mean square error (MSE) between the OFDM signals before and after the nonlinear HPA. The one with the minimum MSE among OFDM candidate signals is selected for transmission. From the results, it can be seen that a proposed scheme based-iterative PTS can be easily implemented with low combining complexity, still maintaining a better BER performance, compared to the conventional PTS approach.

Combined QRD-M and DFE detection technique for simple and efficient signal detection in MIMO-OFDM systems

An efficient MIMO signal detection technique with low complexity is presented for MIMO-OFDM systems. The proposed technique has a feature of combined QRD-M and DFE detection. In our detection technique with M transmit antennas, after QRD-M detection is executed for first T detection steps, DFE detection is executed for last M - T detection steps. This approach is simple and has low complexity, because the computations for QRD-M is limited by the newly adopted parameter T. From simulation results, the complexity of the proposed technique with 4 times 4 and T = 2 is reduced by 61% compared with that of the conventional QRD-M at the expense of about 0.7dB degradation of BER performance at BER=10-4. The simulations are executed in Ricean and Rayleigh channel models. And the simulation of proposed technique is executed by using estimated channel. Through the simulations, it is ascertained that the proposed detection can be used for Ricean and Rayleigh channel models as well as estimated channel.

Exact BER analysis of distributed alamouti's code for cooperative diversity networks

We analyze the bit-error rate (BER) performance of the distributed Alamoutis code for cooperative diversity networks consisting of a source, two relays and a destination node over Rayleigh fading channels. It is assumed that the relays adopt the amplify-and-forward protocol. Firstly, assuming the existence of the direct path component from the source to the destination, we derive the exact BER expression in a one-integral form for M-pulse amplitude modulation (PAM) and M-quadrature amplitude modulation (QAM) constellations. We also present a series expansion of a very accurate BER approximation, which does not require any numerical calculation, and we prove this series is convergent. Secondly, considering the system where the direct path component does not exist, we obtain the exact BER expression in a one-integral form and a series expansion of the exact BER expression. Numerical results confirm that the two exact BER expressions in a one-integral form perfectly match the simulation results and the two series expressions of BER are very accurate, even with a small truncation window.

Joint Relay-and-Antenna Selection in Multi-Antenna Relay Networks

For the decode-and-forward protocol in relay networks, opportunistic relaying (OR) and selection cooperation (SC) are two major relay selection schemes, which have been studied only for single antenna terminals. We study OR and SC in a multi-antenna relay network where each terminal has multiple antennas. To fully exploit multiple antennas without incurring high feedback overhead, we adopt transmit antenna selection (TAS). Specifically, we first propose two joint relay-and-antenna selection schemes which combine OR and SC, respectively, with TAS: joint OR-TAS and joint SC-TAS. For each joint selection scheme, a single best transmit antenna at the source, a single best relay, and a single best transmit antenna at this selected relay are jointly determined in an optimum sense. In this network, at the first time slot, the selected antenna at the source transmits a symbol to the selected relay; at the second time slot, the selected antenna at the selected relay retransmits the detected symbol to the destination. We derive the outage probability of joint OR-TAS. Also, we obtain the outage probability of joint SC-TAS by proving that the outage probability of joint SC-TAS is identical to that of joint OR-TAS.

Performance Improvement of Cooperative MB-OFDM System Based Coming Home Network

Since ultra wideband (UWB) system aims at low complexity, low cost, and low power consumption, employing multiple antennas seems not to be a good approach to improving error performance for UWB devices. On the contrary, recently developed cooperative diversity technique, which profits the transmit diversity only with the existing hardware by constituting a virtual antenna array, can be a solution. However, most of the introduced cooperative techniques have a common fault of decreased transmission rate because the destination should receive the decodable compositions of symbols from the source and the relay. In this paper, we propose an alternative use of hierarchical modulation and a suitable cooperative space-time block code (C-STBC). This technique is free from the rate-loss and allows seamless cooperative communication while similar diversity gain is benefited. The proposed technique is evaluated in terms of bit error rate (BER) for multi-band orthogonal frequency division multiplexing (MB-OFDM) system. Simulation results show that cooperative system based on our technique outperforms noncooperative one when relatively reliable source-to-relay link (S-R link) is guaranteed.

MULTIPLE ANTENNA TRANSMISSION TECHNIQUE FOR UWB SYSTEM

In this paper, the channel estimation in multi-band OFDM (MB-OFDM) ultra-wideband (UWB) based multipleantenna transmission systems is studied. For multiple antennas, each preamble designed by the Zadoff-Chu sequence satisfies the orthogonal property in the time domain. Therefore, the proposed preambles can improve the system performance and be applied to MB-OFDM specification in the case of more than two transmit antennas.

A simple construction of OFDM-CDMA signals with low peak-to-average power ratio

This paper investigates the peak-to-average power ratio (PAR) reduction scheme employing a simple symbol transform in OFDM-CDMA systems. This approach is very simple because of no additional complexity and works without restriction on the allocation of spreading code, maintaining the original transmission efficiency. Simulation results show that the investigated scheme gives the PAR reduction gain of 2 /spl sim/ 3 dB compared to the original OFDM and OFDM-CDMA signals, and can provide the further PAR reduction by combining the selective mapping (SLM) and partial transmit sequence (PTS) schemes, which are less complex compared to the ordinary SLM and PTS approaches, respectively.