Hyunkee Min

Capacity Enhancement Using an Interference Limited Area for Device-to-Device Uplink Underlaying Cellular Networks

A new interference management strategy is proposed to enhance the overall capacity of cellular networks (CNs) and device-to-device (D2D) systems. We consider M out of K cellular user equipments (CUEs) and one D2D pair exploiting the same resources in the uplink (UL) period under the assumption of M multiple antennas at the base station (BS). First, we use the conventional mechanism which limits the maximum transmit power of the D2D transmitter so as not to generate harmful interference from D2D systems to CNs. Second, we propose a δD-interference limited area (ILA) control scheme to manage interference from CNs to D2D systems. The method does not allow the coexistence (i.e., use of the same resources) of CUEs and a D2D pair if the CUEs are located in the δD-ILA defined as the area in which the interference to signal ratio (ISR) at the D2D receiver is greater than the predetermined threshold, δD. Next, we analyze the coverage of the δD-ILA and derive the lower bound of the ergodic capacity as a closed form. Numerical results show that the δD-ILA based D2D gain is much greater than the conventional D2D gain, whereas the capacity loss to the CNs caused by using the δD-ILA is negligibly small.

Reliability Improvement Using Receive Mode Selection in the Device-to-Device Uplink Period Underlaying Cellular Networks

A new interference management scheme is proposed to improve the reliability of a device-to-device (D2D) communication in the uplink (UL) period without reducing the power of cellular user equipment (UE). To improve the reliability of the D2D receiver, two conventional receive techniques and one proposed method are introduced. One of the conventional methods is demodulating the desired signal first (MODE1), while the other is demodulating an interference first (MODE2), and the proposed method is exploiting a retransmission of the interference from the base station (BS) (MODE3). We derive their outage probabilities in closed forms and explain the mechanism of receive mode selection which selects the mode guaranteeing the minimum outage probability among three modes. Numerical results show that by applying the receive mode selection, the D2D receiver achieves a remarkable enhancement of outage probability in the middle interference regime from the usage of MODE3 compared to the conventional ways of using only MODE1 or MODE2.

A note on effect of multiple antennas at the source on outage probability for amplify-and-forward relaying systems

The amplify-and-forward (AF) relaying system considered here consists of a source node with M transmit antennas, and relay and destination nodes with a single antenna each. The closed-form expressions for the outage probabilities for maximum-ratio-transmission (MRT) at the source are presented for some specific values of M. Numerical results show that increasing M from 1 to 2 can achieve roughly a 3 dB SNR gain, guaranteeing the same outage probability. However, we discover that for M ges 3, the additional gain compared to M = 2 decreases quickly as the SNR increases. Therefore, increasing M above 3 does not achieve significantly greater gain over M = 2 at high SNR.This paper analyses the estimation of residual frequency offset in MIMO OFDM systems using pilot subcarriers embedded in the data payload. A closed-form expression for the MSE is derived and it shows how the frequency and spatial correlation influence performance. It is also shown that an orthogonal design ensures robust performance, which is independent of the frequency correlation of the channel.

New OFDM Channel Estimation with Dual-ICI Cancellation in Highly Mobile Channel

The number of coefficients for estimating time-varying channel is generally much larger than the maximum number of observable data. Linearly time-varying channel (LTV) models have been widely used as a way of reducing the number of these channel parameters. This approach allows estimation using the channel slope and channel frequency response between symbols. A fatal problem with this approach, however, is that before it is even initiated, the estimated channel in the frequency domain has already been degraded by inter-carrier interference (ICI). We demonstrate this by analyzing the channel estimation mean square error (MSE) and signal to interference and noise ratio (SINR) after ICI cancellation with respect to the LTV model. We then propose a new channel estimation technique using dual-ICI cancellation which resolves this problem by pre-canceling ICI for the channel estimation using the channel of the previous symbol and post-canceling ICI for symbol detection with the more accurate channel information. The process is initialized using the first symbol as a preamble to estimate the channel in the time domain. Performance comparisons of MSE and SINR show that the proposed method is better suited to fast-fading channels than the conventional method, where ICI dominates the interference and noise power.

Average SNR and Ergodic Capacity of Reactive DF Relaying System with Outdated Channel State Information

In this paper, we investigate the effects of outdated channel state information (CSI) in opportunistic relaying systems on the ergodic capacity. The CSI in the data transmission instant may be different from that used in the relay selection instant due to channel variation. When the CSI in the relay-destination link is outdated, the average signal-to-noise ratio (SNR) and the ergodic capacity of the reactive decode and forward (DF) relaying system are derived. In addition, the effective ergodic capacity which is the spectral efficiency of the successful transmissions excluding the outage events is also derived. The numerical results show that if the CSI in the relay-destination link is outdated, the loss in the ergodic capacity is remarkable compared to the perfect CSI case. However, the level of the inaccuracy of the CSI does not affect the performance of the ergodic capacity in the reactive DF relaying system. In other words, no capacity degradation occurs even if the CSI inaccuracy becomes much larger.

On the design of ZF and MMSE Tomlinson-Harashima precoding in multiuser MIMO amplify-and-forward relay system

In this paper, we design a zero-forcing (ZF) Tomlinson-Harashima precoding (THP) and a minimum mean square error (MMSE) THP for multiuser MIMO relay system with direct link. Based on these precoding structures, we propose the optimal combining weights of the ZF THP and the MMSE THP in half duplex relay system, and the sub-optimal weights of MMSE THP to reduce feedback channel state information (CSI). Through the Monte Carlo simulation, we verify that the optimal weights combining method outperforms the constant weights combining method. In addition, we confirm that the performance of the sub-optimal weights combining method is close to the optimal method, while reducing the feedback CSI simultaneously.

A New Timing Offset Estimation Algorithm Using Phase Difference Between Adjacent Subcarriers in Interleaved OFDMA Uplink Systems

A new timing estimator using phase difference between adjacent subcarriers is proposed for interleaved orthogonal frequency-division multiple access (OFDMA) uplink systems. The phase difference of adjacent subcarriers is represented as the summation of the information of timing offset which we want to estimate and the phase difference of channel frequency responses which can cause performance degradation. Thus, a method to reduce the phase difference of channel frequency responses is applied in time domain, and then timing offset information is estimated from phase difference in frequency domain. Simulation results confirm that the proposed estimator can decrease the mean square error (MSE) compared with channel impulse response (CIR) based method when the information of channel length is unknown.

Ergodic Capacity of M-1-1 AF Relaying Systems over Rayleigh Fading Channels

The amplify-and-forward relaying system considered here consists of a base station (BS) with M transmit antennas, K relay stations (RSs) with a single antenna each, and a mobile station (MS) with a single antenna. Upper bounds of ergodic capacities are derived for some specific values of M and general values of K where the best relay selection (BRS) is used, and the maximum-ratio-transmission (MRT) is applied for the BS-RS transmission. Numerical results show that increasing M enhances the ergodic capacity due to the reduction of noise propagation, and increasing K also enhances the ergodic capacity due to the K-th order diversity gain from the BRS. In addition, the effect of increasing K on capacity enhancement is much significant compared to that of increasing M. On the other hand, the capacity enhancement from increasing M is limited, whereas the capacity enhancement from increasing K is unlimited. In addition, the capacity enhancement from M and that from K are independent.

New Pilot Designs and ICI Mitigation for OFDM Downlink Systems based on IEEE 802.16m Standards over High Speed Vehicular Channels

Two new pilot patterns and channel estimation methods robust to inter-channel-interference (ICI) caused by fast fading channels are proposed which are applicable to 802.16m standards. A number of clusters of pilot subcarriers are used for channel estimation. In addition, the number of needed parameters for channel estimation is reduced by some properties of ICI in the time and frequency domains. Simulation results show that the proposed channel estimation has good bit-error-rate (BER) performance compared to conventional method based on 802.16m standards at the velocity of 350 km/h and has the same performance at the velocity of 3 km/h. Moreover, if the target BER is lower than the supportable BER of conventional method with uncoded 16QAM, then the proposed method can be better option compared to diminish the modulation scheme as 8PSK considering both BER and throughput.

Asymmetric Pilot Subcarrier Allocation for OFDMA-Based Regenerative Dual-Hop Links

An asymmetric pilot subcarrier allocation scheme (APSA) is proposed for orthogonal frequency division multiple access (OFDMA) systems which use regenerative relays. To achieve the best performance of regenerative relay systems, the performance of each hop should be balanced. When there is a direct link between the source and destination, APSA allocates pilot subcarriers asymmetrically to each hop to balance the performance. We show that APSA can improve the performance of the system based on the symbol error rate (SER) analysis and simulation results.