Nak-Myeong Kim

The 3-Bit Gray Counter Based on Magnetic-Tunnel-Junction Elements

A magnetic-tunnel-junction (MTJ) element has been widely studied for data storage applications. An MTJ element can also be used to compute Boolean functions and store the output result. A magnetologic device based on this MTJ element can constitute sequential logic functions as well as combinational logic. Counter is one of the most frequently used sequential logic blocks in digital logic systems. In this paper, a novel architecture of a 3-bit gray counter based on magnetologic elements is presented. It is shown that ten MTJ elements with complementary metal-oxide-semiconductor (CMOS) circuits for sense amplifier and writing-current driver can make a 3-bit gray counter. HSPICE simulation results are presented to verify the functionality of the proposed circuits

Time Slot Allocation Based on Region and Time Partitioning for Dynamic TDD-OFDM Systems

TDD mode is considered as a key solution to cope with the asymmetric traffic services. With the TDD mode, however, the inter-BS/inter-MS interference becomes serious in the cross time slot (CTS) region and this interference seriously degrades the system performance. In this paper, we propose a region and time partitioning dynamic TDD (RTP D-TDD) for OFDM systems. To mitigate the inter-BS/inter-MS interference in the CTS region, a cell is divided into several regions and each time slot in the CTS region is split into minislots by the number of regions in the cell. Each minislot is assigned to the users in its corresponding region. We also propose a time slot allocation based on region and time partitioning (TRTP) under the proposed RTP D-TDD system. In the TRTP, the influence of the CCI in the CTS region is further reduced by assigning the minislots closer to the switching point to the users having higher link gains. By the computer simulation, it has been verified that the proposed TRTP outperforms previous time slot allocation schemes in both the outage probability and the bandwidth efficiency

Pre-RAKE assisted RAKE receiver for TDD WCDMA system

The WCDMA system uses RAKE receiver algorithm or pre-RAKE algorithm to achieve multipath diversity gain. The pre-RAKE algorithm works well in a slow fading environment, but if the channel condition rapidly changes between the uplink and the downlink, the pre-RAKE system performance gets degraded. In this paper, we propose a pre-RAKE assisted RAKE receiver for TDD WCDMA system. The proposed system can simplify the receiver structure, while increasing the total symbol energy of the received signal effectively. By computer simulation, the performance of the proposed system is compared with the conventional RAKE only system and pre-RAKE only system. As a result, the proposed algorithm is proved to achieve better performance than the conventional systems, by more than 3 dB.

An enhanced adaptive time slot assignment using access statistics in TD/CDMA TDD system

The TD/CDMA TDD system with unbalanced timeslot allocation between uplink and downlink is an appropriate method to distribute asymmetric traffic effectively in third generation mobile communication systems. One of the benefits of the TD/CDMA TDD system is an efficient statistical multiplexing on a large common pool of available resources, i.e., slots defined in time and code domain. However, in order to control the time varying asymmetry of traffic between uplink and downlink for the maximum throughput, a dynamic boundary movement is also required to get an optimum timeslot allocation. In this paper, a new optimal boundary movement algorithm for TDD operation is proposed, in which the downlink portion of the slots is maximized according to the intensity of access attempts by mobiles. The proposed algorithm has shown much better performance than the conventional TDD boundary movement algorithms in terms of the mean access delay of data traffic and the blocking probability of voice traffic.

A fast cell search algorithm using code block CPM in asynchronous W-CDMA system

The asynchronous mode W-CDMA system is known to be appropriate for the next generation mobile communication system, especially in a non-homogeneous cellular architecture. In this case, however, each base station needs to use a different spreading code for identification, so it is a demanding task for a mobile terminal to find the best cell site and get an accurate code synchronization at the beginning of a communication. Since slow acquisition of a base station could mean the failure of initiation, a fast algorithm to accelerate the cell search process is essential. In this paper, a new cell search algorithm based on the binary code position modulation (CPM) within a code block is proposed. Different cell sites are identified by different hopping code sequences, and each position modulation is performed by the hopping code. The proposed algorithm is proved to make the cell search time in most places in a cell much shorter than the previous algorithms, and to make the receiver implementation simpler.

A Robust Cognitive Radio Based Adaptive Frequency/Time Spreading for OFDM Systems

The cognitive radio (CR) technology is an effective technology for a flexible use of the radio spectrum. However, the interference between the primary and the CR users becomes a critical problem for the CR system. In this paper, we propose a cognitive radio based adaptive frequency/time spreading for the orthogonal frequency division multiplexing (OFDM) systems. This new approach is characterized by the adaptive frequency spreading and time spreading, and the adaptive power control of the subcarriers at the borders of the CR users spectrum. Specifically, we propose a frequency spreading scheme according to the amount of the adjacent channel interference (ACI) coming from nearby primary users, which improves the power gain of data under severe interference. On the other hand, in order not to give an excessive interference the primary system, we also propose to reduce the power of the subcarriers near the edges of the bandwidth by spreading the signal in time. By the computer simulation, it has been proved that the proposed CR based OFDM system is very robust against ACI in comparison with the conventional OFDM system having adaptive guard bands. We also confirmed that the performance of the primary user is maintained with only a nominal degradation in the coexistence environment of the primary and CR system.

A Cooperative ARQ Strategy with Adaptive Back-Off for Mobile Multimedia Communication Using Cognitive Relays

Cooperative ARQ schemes are effective to enhance the quality of communication for the next generation mobile communication systems. In this paper, we propose a cooperative ARQ strategy for supporting instantaneous cooperation in MANET environment using ad hoc cognitive relays. In the proposed strategy, whenever a data frame is transmitted from the source, nearby relays actively sense the SINR of the signal, and proactively propose a cooperative retransmission, if necessary, before the receiver replies with NACK feedback. By computer simulation, it is shown that the proposed cooperative ARQ scheme outperforms the conventional schemes with respect to frame transmission delay and the frame loss probability.

Differentiated resource reservation scheme with handover clearing relay for hybrid duplexing systems

Recently, there have been a lot of researches on hybrid duplexing (HD) to exploit advantages of both time division duplexing (TDD) and frequency division duplexing (FDD). In the next generation communication, heterogeneous cell structure in which each cell would apply proper duplexing mode independently will be raised instead of homogeneous cell structure. However, in heterogeneous cell environment, it would be difficult for handover calls to be guaranteed resource because each duplexing mode has different extent to accommodate asymmetric traffic. In this paper, we propose a differentiated resource reservation scheme to guarantee resource for mobile expected to handover, and also introduce a new handover operation using handover clearing relay to reduce essentially inefficient resource utilization due to asymmetric traffic. The computer simulations show our differentiated resource reservation scheme with handover clearing relay outperforms the conventional schemes with regards to dropping probability of handover call and resource utilization ratio.

An Efficient Heterogeneous Wireless Access Based on Region and Time Partitioning in D-TDD OFDM Systems

Under dynamic time division duplexing (D-TDD) architecture in OFDM cellular systems, the inter-BS and inter-MS interference is inevitable during the cross time slot (CTS) period, and this interference seriously degrades the wireless access system performance. To mitigate such interference, we propose a region and time partitioning D-TDD architecture for OFDM systems. Each time slot in the CTS period is split into several minislots, and then each cell is divided into as many regions as the number of minislots per time slot. We then assign each minislot only to the users in its corresponding region. On top of such architecture, which inherently separates interfering entities farther from each other, we design a robust time slot allocation scheme considering the channel status of each mobile station. By computer simulation, it has been verified that the proposed scheme outperforms conventional time slot allocation methods in both the outage probability and the bandwidth efficiency.

Cognitive Radio Based Bandwidth Sharing Among Heterogeneous OFDM Access Systems

Cognitive radio (CR) has been proposed as an effective technology for flexible use of the radio spectrum. The interference among primary users and CR users, however, becomes a critical problem when they are operating nearby using adjacent frequency channels with different transmission powers. In this paper, a CR-based bandwidth sharing architecture is proposed, which can effectively suppress adjacent channel interference (ACI) between them. This new approach is characterized by the adaptive frequency spreading, and the adaptive time spreading with power control for the subcarriers near the borders of the CR users spectrum. The frequency spreading scheme provides with extra power gain against ACI, whereas the time spreading with power control guarantees the minimal interference to primary users from CR users. By computer simulation, it has been proved that the proposed system outperforms the conventional OFDM systems in both throughput and BER performance.