Su Min Kim

Spatial Group Based Random Access for M2M Communications

We expect that the number of machine-to-machine (M2M) devices will rapidly increase in the near future due to a growing demand for a wide range of M2M applications such as e-health, public safety, surveillance, remote maintenance and control, and smart metering. Therefore, the future cellular networks should accommodate a large number of M2M devices and their random access (RA) requests at a specific time instant. In this letter, we propose a novel RA scheme to effectively increase the number of available preambles for the future M2M communication environment. The proposed scheme provides additional preambles by spatially partitioning a cell coverage into multiple group regions and reducing cyclic shift size in RA preambles. As a result, the proposed RA scheme can accommodate a significantly larger number of machine nodes with much lower collision probability and shorter random access delay, compared to a conventional RA scheme.

On the Entropy Computation of Large Complex Gaussian Mixture Distributions

The entropy computation of Gaussian mixture distributions with a large number of components has a prohibitive computational complexity. In this paper, we propose a novel approach exploiting the sphere decoding concept to bound and approximate such entropy terms with reduced complexity and good accuracy. Moreover, we propose an SNR region-based enhancement of the approximation method to reduce the complexity even further. Using Monte-Carlo simulations, the proposed methods are numerically demonstrated for the computation of the mutual information including the entropy term of various channels with finite constellation modulations such as binary and quadratic amplitude modulation (QAM) inputs for communication applications.

Message-Embedded Random Access for Cellular M2M Communications

In this letter, we propose a message-embedded random access (MERA) scheme to simultaneously transmit both preambles (PAs) and messages (MSGs) as MSG-embedded PAs based on auto- and cross-correlation properties of Zadoff-Chu sequences using distinct root numbers. The proposed MERA scheme enables machine nodes to transmit small-sized data during a PA transmission on control channel (i.e., PRACH) without using any scheduling mechanism and resources on data channel (i.e., PUSCH). The performance of the proposed MERA scheme is mathematically analyzed and evaluated through simulations in terms of PA detection probability, MSG decoding probability, and the throughput of MSG transmissions on PRACH.

Capacity Analysis of Continuous-Time Time-Variant Asynchronous Uplink Wideband CDMA System

This paper considers the capacity limit of an uplink wideband CDMA system. In order to make the studied results useful for the performance assessment of real cellular networks, various realistic assumptions are included in the problem. An equivalent discrete-time channel model is derived based on sufficient statistic for optimal decoding of transmit messages. The capacity regions are then characterized considering finite constellation and Gaussian input assumptions. For further insight, an analysis on the asymptotic capacity is considered, in which the conditions to simultaneously achieve the individual capacities are derived.

An Early Preamble Collision Detection Scheme Based on Tagged Preambles for Cellular M2M Random Access

In this paper, we propose an early preamble collision detection (e-PACD) scheme at the first step of random access (RA) procedure based on tagged preambles (PAs), which consist of both PA and tag Zadoff–Chu sequences using different root numbers, respectively. The proposed e-PACD scheme enables faster PA collision detection and notification, compared with the conventional RA scheme. Accordingly, it can reduce the RA delay and remove resource wastes which occur at the third step of the conventional RA procedure. The reduced RA delay achieved by the proposed e-PACD scheme can contribute to an ultralow latency requirement in fifth-generation (5G) cellular networks. In addition, the proposed e-PACD scheme enables the eNodeB to monitor the number of RA-attempting nodes (RA load) on each physical RA channel slot. The PACD probability and the RA load monitoring accuracy are mathematically analyzed, and the RA performance enhancement of the proposed scheme is evaluated in terms of RA success probability, average RA delay, and RA resource efficiency, compared with the conventional RA scheme.

Enhanced spatial group based random access for cellular M2M communications

We have proposed a spatial group based random access (SGRA) scheme which is a novel RA scheme to effectively increase the number of available preambles for future cellular M2M communications. In the SGRA scheme, a machine node in the cell should determine its own spatial group (SG) by estimating the distance between the eNodeB and itself through any distance estimation methods. At the moment, the distance estimation error may inevitably occur in the distance estimation and may cause a group-mismatch problem, which affects the performance of the SGRA scheme in terms of RA collision probability and delay. Therefore, in this paper, we propose a group-reselection solution and a shared distance solution in order to resolve the group-mismatch problem in the SGRA scheme. Through numerical results, it is shown that the combination of the proposed solutions provides a significant robustness against the distance estimation errors, which leads to accommodating a significantly larger number of machine nodes with very lower collision probabilities and shorter average RA delay in more practical M2M deployment environments.

Effects of Multiple Antennas and Imperfect Channel Knowledge on Secrecy Multiuser Diversity

In this letter, we investigate the effects of multiple antennas at a legitimate receiver and imperfect channel knowledge on secure user-scaling law in uplink multiuser wiretap networks. We generalize the conventional analysis of the secure user-scaling law for a single antenna system with perfect channel knowledge by considering multiple antennas at the receiver and the channel estimation errors.

SPHERE DECODING INSPIRED APPROXIMATION METHOD TO COMPUTE THE ENTROPY OF LARGE GAUSSIAN MIXTURE DISTRIBUTIONS

The computation of mutual informations of large scale systems with finite input alphabet and Gaussian noise has often prohibitive complexities. In this paper, we propose a novel approach exploiting the sphere decoding concept to bound and approximate such mutual information term with reduced complexity and good accuracy. Using Monte-Carlo simulations, the method is numerically demonstrated for the computation of the mutual information of a frequency- and time-selective channel with QAM modulation.

Secrecy multiuser diversity for distributed antenna systems from the perspective of user-scaling law

In this paper, we introduce the notions of a user-scaling function and a user-scaling exponent and multiuser diversity (UEMD) pair. Our notions for the UEMD pair could be utilized as a basic tool for analyzing user-scaling laws. We derive a secure user-scaling law in terms of SNR, the number of eavesdroppers, the number of distributed antennas in distributed antenna systems, and design parameter λ. Through analytical and numerical results, we characterize the effect of the number of distributed antennas on the user-scaling behavior in terms of achievable secrecy rate. Compared with the conventional results based on a single antenna system, our results indicate that installing a small number of distributed antennas could provide a significant performance gain in terms of the achievable secrecy rate.

Capacity Analysis of Uplink WCDMA Systems with Imperfect Channel State Information

This paper considers the capacity limit of an uplink wideband CDMA (WCDMA) system assuming imperfect channel state information at the receiver (CSIR). In order to make the studied results useful for the performance assessment of real cellular networks, various realistic assumptions are included in the problem. A discrete-time channel model is derived based on the mismatched filtering at the receiver. Capacity inner bounds are then characterized based on the discrete-time channel model considering different assumptions on decoding strategy. Numerical results are also provided to show the effect of imperfect CSIR on the capacity.