Seong Keun Oh

An Incremental Frequency Reuse Scheme for an OFDMA Cellular System and Its Performance

We propose an incremental frequency reuse (IFR) scheme that reuses effectively the radio spectrum through systematic segment allocation over a cluster of adjoining cells. It divides the entire frequency spectrum into several spectrum segments. Based on the above segmentation, a set of cell- specific segment allocation sequences is designed for universal frequency reuse. Here, each sequence defines its own base segment and allocation order. The designed sequences are then assigned to respective cells over the cell cluster. In this scheme, the base segments over the cell cluster are mutually non-overlapped and collectively exhausted, and the added segments are interfered with from surrounding cells, but only in an incremental and coordinated manner. Hence, the proposed scheme can provide better reuse efficiency over the conventional ones such as the classical universal frequency reuse scheme and the soft frequency reuse (SFR) scheme. In addition, the simple and flexible IFR scheme can be easily configured as most existing reuse schemes only by redefining the set of segment allocation sequences. A system-level simulator for an orthogonal frequency division multiple access (OFDMA) cellular system covering surrounding cells up to 3rd-tier has been implemented. Simulation results show that the IFR scheme provides quite better reuse efficiency as compared to the classical universal reuse scheme and the SFR scheme, especially at the cell-edge region.

A Per-User Successive MMSE Precoding Technique in Multiuser MIMO Systems

We propose a per-user successive minimum mean-squared error (PU-SMMSE) precoding technique suitable for the use in multiuser multiple-input multiple-output (MU-MIMO) downlink systems. Its preceding complexity is much simpler as compared with that of the successive MMSE (SMMSE) technique (V. Stankovic and M. Haardt, 2004), without any performance loss. The new technique also takes both the high capacity benefit achievable with MIMO multiplexing and the benefit of space division multiple access (SDMA), assuming channel state information (CSI) at both the transmitter and receiver sides. In the new technique, the columns of the precoding matrix are obtained by computing successively reduced-rank MMSE precoding solutions on a per-user basis. Then, a per-user virtual channel which consists of the per-user channel through precoding is decomposed into orthogonal transmission modes. Finally, a proper power assignment can be applied to the decomposed modes according to some assignment policy. We compare the proposed technique with the SMMSE technique in terms of the bit error rate (BER) performance. We also analyze the computational complexity of the PU-SMMSE technique in comparison with the SMMSE and BD techniques. From computer simulations and complexity analyses, we see that the PU-SMMSE and SMMSE techniques have the same BER performance in both cases of dominant mode transmission (DMT) and multiple mode transmission (MMT). However, the computational complexity of the proposed technique is much lower than that of the SMMSE technique, and also is lower than that of the simple block diagonalization (BD) technique.

Design of a high speed OFDM modem system for powerline communications

This paper proposes a high speed OFDM modem architecture for powerline communications. The proposed modem using symmetric communication is designed to be compatible with the HomePlug standard. The HomePlug standard adopts from DC to 25 MHz frequency bandwidth, 128 subcarriers for OFDM transmission, and BPSK, DBPSK, and DQPSK modulations for each subcarrier. In particular, this paper proposes algorithms and the associated architectures for the signal detection, AGC and frame synchronization. The AGC and frame synchronization algorithms are based on the symbol power ratio and the sliding cross-correlation of preamble, respectively. The frame is then synchronized with a position of the minimum correlation value. In addition, an area-efficient integrate-and-dump architecture for frame synchronization is proposed. The proposed architectures have been validated using SPW/spl trade/ and implemented with Verilog-XL. We show that the BER performance of the proposed modem under an AWGN channel is similar to the theoretical one.

A Universal Frequency Reuse System in a Mobile Cellular Environment

We propose a universal frequency reuse (UFR) system that can reuse effectively a given frequency resource in a mobile cellular environment. The key concept is to control mutual interferences among neighboring cells in a systematic manner. The system first assigns the whole frequency resource to all cells, so that the whole resource could be reused without any restrictions in all the cells as in the cellular systems with frequency reuse factor of 1. Then, it defines a cell-specific resource allocation rule for each cell, so as to control mutual interferences among neighboring cells in a cooperative way. In our case, the resource allocation rules are designed so that they can avoid maximally inter-cell interference (ICI) when the loading factor is less than a threshold loading factor, while exceeding the threshold, ICI would be averaged over the whole frequency resource. Hence, the proposed system can maintain ICI always at the minimum level, in turn expecting greater spectrum reuse efficiency. Finally, we perform extensive computer simulations in order to verify the effectiveness of the proposed reuse system, assuming an OFDMA-based cellular system. From simulation results, we show that the UFR system provides quite better spectrum reuse efficiency, regardless of omni-cell and 3-sector cell systems.

Cognitive Ad-hoc Networks under a Cellular Network with an Interference Temperature Limit

We propose an efficient deployment scenario for cognitive ad-hoc networking under a cellular network with a pre-defined interference temperature limit (ITL). Using this scenario, a cellular radio spectrum can be reused efficiently without disturbing the existing cellular network. It uses a relaxed ITL concept in which the pre-defined ITL is satisfied at the target receiver only instead of all around the cellular service coverage. In this scenario, a cognitive radio (CR)-enabled secondary user (also, called as a cognitive ad-hoc user), equipped with the cellular signal strength monitoring device measures the pathloss from the nearest cellular base station (CBS) (also, called as the target CBS) in a cellular downlink channel. Then, using the measured pathloss, each secondary user computes its allowable maximum transmit power level (AMTPL) for a cellular uplink channel. In this scenario, secondary users farther from the target CBS can transmit more power than nearer users can do, thus making the cellular uplink resource reused more efficiently. Therefore, the proposed scenario can reduce the routing overhead and associated delay, and the infrastructure cost because this secondary user power flexibility can decrease the number of required nodes for cognitive ad-hoc networking, or can increase the utilization efficiency of the cellular radio spectrum.

A Multiplierless 2-D Convolver Chip for Real-Time Image Processing

This paper proposes a new real-time 2-D convolver chip with no multiplier. Several commercial 2-D convolver chips have many multipliers and existing multiplierless architectures have many shift-and-accumulators to meet the real-time image processing requirement, i.e., the standard of CCIR601. Even though the proposed architecture uses only one shift-and-accumulator, it can meet the real-time requirement. Furthermore, because it controls the input data sequence, the proposed chip does not require row buffers to store two adjacent rows as do commercial chips, and it can further reduce the gate count. The proposed architecture can reduce the gate count by more than 70 and 90% compared to HSP48901 and HSP48908, respectively, and the gate count of the computation block itself by more than 70% compared to existing multiplierless architectures. We have implemented the chip using the Samsung™ 0.8 μm SOG cell library (KG60K). The implemented filter chip consists of only 3,893 gates, operates at 125 MHz and can meet the real-time image processing requirement. The proposed architecture is especially suitable for larger size convolutions because of its small gate count.

Reliable anchor node based range-free localization algorithm in anisotropic wireless sensor networks

In the wireless sensor networks, location based applications require an accurate localization algorithm. To estimate location, each normal node, which do not know its own location, uses estimated distances to anchor nodes which have their own location. Hence, the estimation of distance is a key issue in the wireless sensor networks localization. Most previous works assume isotropic networks where approximate the shortest path distances to the Euclidian distances. These approaches cannot obtain accurate distance estimations in anisotropic networks where the shortest path distances are not linearly proportion to the Euclidian distances. In the anisotropic networks, the shortest path between two nodes may be detoured and its length may be estimated much larger than the corresponding Euclidean distance. Because of the phenomenon, it is important to select reliable anchor nodes which offer the precisely estimated distance. In this paper, we propose a reliable anchor node selection algorithm in the anisotropic networks. Each normal node selects the three anchor nodes with Triangular rule to enhance localization accuracy in the proposed algorithm. We also show that our scheme is reasonable through mathematical observation.

An Iterative Network Code Optimization for Three-Way Relay Channels

We consider a three-way relay channel using a network coding in this paper. We propose a network coding scheme for the three-way relay channels and present a Latin square-like condition for the three-way network code. We also propose several schemes to design a new three-way network code by using the cell swapping and applied the methods to obtain a better network code to obtain a fair symbol error rates among the users.

Bit manipulation accelerator for communication systems digital signal processor

This paper proposes application-specific instructions and their bit manipulation unit (BMU), which efficiently support scrambling, convolutional encoding, puncturing, interleaving, and bit stream multiplexing. The proposed DSP employs the BMU supporting parallel shift and XOR (exclusive-OR) operations and bit insertion/extraction operations on multiple data. The proposed architecture has been modeled by VHDL and synthesized using the SEC 0.18m standard cell library and the gate count of the BMU is only about 1700 gates. Performance comparisons show that the number of clock cycles can be reduced about for scrambling, convolutional encoding, and interleaving compared with existing DSPs.

On resource block sharing in 3GPP-LTE system

In this paper, we deal with sharing of a resource block (RB) which is the basic resource allocation unit for scheduling in 3rd-generation partnership project long term evolution (3GPP-LTE) system. In 3GPP-LTE system, a scheduler of evolved node-B (eNB) allocates respectively one or more RBs to user-equipments (UEs) considered, considering their requests, their channel qualities, and the resource availability. In the case of constant-rate service (i.e., the real-time service), some of RBs may not be fully utilized, depending on the channel quality. Especially, in the case of multiple-input multiple-output (MIMO) multiplexing, the degree of resource wastage may become much severer since a much less amount of the resource may be required for the same data rate. In this paper, we introduce the concept of RB sharing where one RB can be shared by multiple UEs depending on their respective required rates and the corresponding channel qualities. Through computer simulations, we evaluate the average sum rates for both cases of RB sharing and non-sharing. From simulation results, we see that the RB sharing scheme can achieve much greater sum rate as compared to the non-sharing one.