Wonjun Hwang

An enhanced channel estimation method for MU-MIMO based LTE-Advanced system

In this paper, we propose an enhanced channel estimation method for multi-user multiple-input multiple output (MU-MIMO) based long term evolution (LTE)-Advanced system. In MU-MIMO based LTE-Advanced system, the orthogonality between co-scheduled layers is not perfectly guaranteed. In this case, significant performance degradation in channel estimation can occur due to the interference from transmit signals for co-scheduled users. To overcome this problem, the proposed method partially estimates and eliminates the multi-user interference using a simple pilot matrix inversion based scheme. After the interference mitigation, we apply an iterative two-dimensional minimum mean squared error (2-D MMSE) channel estimation scheme with adaptive noise and interference weighting method to increase the estimation accuracy. Various simulation results verify that the proposed method has superior performance compared with the conventional 2-D MMSE channel estimation scheme, with only a few times of iteration.

An Enhanced Channel Estimation with Partial Interference Cancelation for MU-MIMO System

In this letter, we propose an enhanced channel estimation method for multi-user multiple-input multiple output (MU-MIMO) transmission in long term evolution (LTE)-Advanced system. The proposed method partially estimates and eliminates the interference using simple matrix operations. After that, we apply an iterative channel estimation scheme. Due to the improvement of initial channel estimation performance, the proposed channel estimation method has significantly large performance gain compared to conventional method.

A carrier frequency synchronization method for device-to-device communication network

Device-to-device (D2D) communication has received considerable attention in recent years as one of the key technologies for future communication system. Among the typical D2D communication systems, FlashLinQ (FLQ) adopted single-tone orthogonal frequency division multiplexing (OFDM) transmission which enables wide-sense discovery by time domain spreading and very low peak to average power ratio (PAPR). Although synchronization based on a common external source (CES) is basically assumed in FLQ, a means to support devices when they are unable to use a CES is still necessary. In OFDM system, generally, carrier frequency offset (CFO) induces inter-carrier interference (ICI) which degrades overall system performance drastically. Especially in D2D system, ICI can be amplified due to not only different frequency selectivity but also different path losses between links. Accordingly, a precise estimation and compensation of CFO are very important. Therefore, in this paper, we propose a new CFO estimation method for OFDM based D2D system. In the proposed method, the estimation of CFO is obtained by using two correlation results in a symbol. The estimation procedure of the proposed method enables the independent estimation of CFO between links and the estimation range can be adaptively defined depending on the distance between the two windows. By numerical analysis and performance evaluation, we verified that the proposed method achieves precise estimation performance compared to the conventional method.

A selective transmission power boosting method for D2D discovery in 3GPP LTE cellular system

Third Generation Partnership Project (3GPP) has decided to provide device-to-device (D2D) based UE discovery service in next generation of Long Term Evolution (LTE). By the recent agreement in 3GPP specification meetings, uplink control signal and D2D discovery signal can be co-scheduled in a dedicated D2D discovery channel, thus inter-carrier interference (ICI) between the two signals may occur. In this case, the transmit power of D2D discovery signal should be reduced in order to protect legacy system, but it causes D2D discovery range restriction. Therefore, we propose a novel power control strategy for D2D discovery signal which is found to improve D2D discovery range without reception performance degradation of cellular control signal.

A multi-user interference detection method for MU-MIMO transmission in LTE advanced system

In this paper, multi-user multiple-input multiple output (MU-MIMO) transmission in long term evolution (LTE)-Advanced system is presented. To support dynamic scheduling of MIMO transmission, enhanced node B (eNB) does not inform user equipment (UE) of the existence of other co-scheduled UE. In this case, the UE cannot perform any interference mitigation scheme because it does not know whether the interference actually occured or not, so a multi-user interference (MUI)-detection which precedes the interference mitigation is essential. In order to overcome this problem, we propose a novel MUI detection method. The pro-posed method partially estimates interference using simple matrix operations. After that, we decide the existence of MUI by using a threshold-based interference detection method.

A new channel structure and power control strategy for D2D discovery in LTE cellular network

Device-to-device (D2D) discovery in long-term evolution (LTE) cellular network is a major issue in recent 3rd generation partnership project (3GPP) standardization meeting to support commercial/social services and public safety in disaster environment. For the D2D discovery signal transmission, physical uplink shared channel (PUSCH)-like channel structure is being generally considered. In such a channel structure, the discovery signal may be multiplexed with cellular physical uplink control channel (PUCCH) signal in frequency division multiplexing (FDM) manner. In contrast to cellular PUCCH signal, however, typical open-loop power control is still not considered for D2D discovery signal in order to guarantee the sufficient discovery range, thus inter-carrier interference (ICI) from the discovery signal may severely interfere with the reception of cellular PUCCH signal at evolved node B (eNB). Thus in this paper, we analyze the effect of the ICI in D2D discovery scenario, and evaluate two traditional solutions: the use of guard-band and the application of open-loop power control for discovery signal. Considering disadvantages of the two traditional solutions, we propose a new discovery channel structure and power control strategy for discovery signal transmission in which adaptive open-loop power control based on PUCCH scheduling information is applied. By extensive computer simulation, we verify that the proposed method guarantees relatively high signal-to-interference plus noise power ratio (SINR) of both PUCCH signal and discovery signal.

A multi-hop relay based frequency synchronization for D2D communication in 3GPP LTE system

Device-to-device (D2D) communication has been adopted for one of the next generation technologies to support commercial/ social service and public safety application in 3-rd generation partnership project (3GPP). For D2D operation in partial network coverage (PNC) scenario, synchronization among multiple user equipments (UE) is the most important issue due to asynchronous interference which can occur among in-network coverage (INC) UEs and out-of-network coverage (ONC) UEs. Therefore, 3GPP agreed multi-hop relay based synchronization. In this method, UE considers a source type, the number of hops, and received power of D2D synchronization signal (D2DSS) to select one D2DSS among multiple D2DSSs. However, this method cannot consider actual frequency offsets of D2DSS, which are accumulated through the previous multi-hop links. For that reason, it is not suitable for multi-hop relay based frequency synchronization. Therefore, in this paper, we propose a novel frequency synchronization method where UE considers an absolute sum of frequency offsets which actually occurs in the multi-hop links of D2DSS. Also, by using frequency offset based threshold, the proposed method provides a strict condition to perform D2DSS relay. We verify that the proposed method can efficiently reduce frequency offsets among UEs in PNC scenario.

An interference-robust channel estimation method for MU-MIMO transmission in LTE-Advanced system

To support a transparent multi-user multiple-input multiple output (MU-MIMO) transmission, the orthogonality between co-scheduled layers is not perfectly guaranteed in the long term evolution (LTE)-Advanced system. In this case, however, significant performance degradation in channel estimation can occur due to the interference from transmit signals for co-scheduled users. To overcome this problem, the proposed method partially estimates and eliminates the interference using simple matrix operations. After the partial interference cancellation, we apply a two-dimensional minimum mean squared error (2-D MMSE) channel estimation scheme with an adaptive weighting method considering the remaining interference and noise.

An impedance mismatching compensation based CM noise rejection method for capsule endoscope system

In this paper, common-mode noise (CMN) rejection method is presented. As is well known, CMN causes critical performance degradation in the wireless capsule endoscope (CE) system. The differential operation, a typical method for CMN rejection, can effectively remove CMN by subtracting two signals received by patch sensors. However, it cannot perfectly remove CMN when there is impedance mismatching between the two patch sensors. In order to overcome the impedance mismatching problem, therefore, we propose an impedance ratio estimation method and a mismatching compensation method. In the proposed method, the impedance ratio can be estimated by dividing two received signals. Then, we can compensate the impedance mismatching by weighting one of the received signals considering the estimated ratio, using analog amplifier and digital-to-analog converter (DAC). Also, we apply Lloyd-Max quantization algorithm for optimization of DAC output levels. Simulation results verified that the proposed method can effectively remove CMN even when there is impedance mismatching, thus it guarantees the superior quality of received signal.

A new synchronization sequence generation and correlation output calculation method for heterogeneous network

In this paper, we present an interference problem due to the synchronization sequence collision between adjacent nodes in heterogeneous network environment. One of the possible solutions for the synchronization sequence collision is to newly define some kinds of synchronization sequences in system configuration. From this point of view, we propose an effective sequence generation method and suggest simple correlation output calculation method for the newly defined sequence.