Dongsoo Har

Path-loss prediction model for microcells

Empirical path-loss formulas for microcells in low-rise and high-rise environments are established from measurements conducted in the San Francisco Bay area. Using the 1-km intercepts and slope indexes of the least square fit lines to the measurements at cellular and personal communication services (PCS) frequencies for three base station heights, simple analytic expressions are obtained. Separate formulas are presented for environments of low buildings and for the high-rise urban core. Following the formula development processes for individual test routes, in low-building environments, a single nonline-of-sight (non-LOS) formula that is applicable to all non-LOS routes is derived. Due to the anisotropic property of wave propagation, cell shape of microcells is no longer circular. As examples, cell shape is presented when base stations are on the street in the middle of a block and when they are placed in the backyard.

Comment on diffraction loss of rooftop-to-street in COST 231-Walfisch-Ikegami model

As a small-cell path-loss prediction model, the COST 231-Walfisch-Ikegami model has been used extensively. It facilitates radio frequency (RF) path-loss predictions in typical suburban and urban environments where the building heights are quasi-uniform. However, it was found that the expression accounting for the diffraction loss from the last rooftop to the street was erroneously obtained. In this paper, we show why the error is present in the COST 231-Walfisch-Ikegami model and how it impacts on path-loss prediction.

Pilot-Aided Side Information Detection in SLM-Based OFDM Systems

Selected mapping (SLM) based schemes effectively reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) systems. However, they require side information (SI) transmission, which incurs a loss in the data throughput in addition to the increased system complexity. This paper presents a blind SLM scheme based on a decision metric obtained from pilot sub-channel responses. A novel SI detection method enabling low complexity data decoding is proposed. The SI is detected by exploiting the high autocorrelation between adjacent pilot sub-channel responses. The SI detection error rate is analytically derived and compared with that obtained by simulations. Simulation results of the data decoding scheme based on the proposed SI detection method show the bit error rate performance comparable to that of the simplified maximum likelihood (ML) data decoding scheme, while the computational complexity is close to that of the embedded SI based decoding scheme.

MAC Achieving Low Latency and Energy Efficiency in Hierarchical M2M Networks With Clustered Nodes

Machine-to-Machine (M2M) networks enabling networked nodes (sensors or actuators) to exchange information have been intensively studied lately. Key components of M2M networks include sensors that can be deployed with a large quantity for monitoring or surveillance. This paper deals with M2M networks hierarchically structured to accommodate efficient data transmission from terminal nodes to a sink node via cluster heads. A novel medium access control (MAC) protocol, more specifically backoff time decision rule, fit for hierarchical M2M networks is proposed and evaluated by simulations. Analytical success probability in channel access by cluster heads is derived and compared with simulation results. It is shown by simulation results that the proposed backoff time decision rule achieves smaller average latency and reduced per-node energy consumption in comparison with well-known MAC protocols, the DMAC and the SMAC, for sensor networks.

A Pilot Symbol Pattern Enabling Data Recovery Without Side Information in PTS-Based OFDM Systems

Partial transmit sequence (PTS) scheme is one of the most popular peak-to-average power ratio (PAPR) reduction schemes for orthogonal frequency division multiplexing (OFDM) systems. In the PTS scheme, one OFDM symbol is partitioned into disjointed sub-blocks, and each sub-block is multiplied by a phase factor to generate signals with low PAPR. For data recovery, receivers must have side information (SI), e.g., phase factors, from transmitters. In this letter, a novel data recovery scheme in PTS-based OFDM systems without SI is proposed. In the proposed scheme, an additional pilot symbol is intentionally inserted at the end of each sub-block, so that efficient data decoding based on channel estimation can be executed with known pilot symbols. Simulation results show that the BER performance of the proposed scheme without SI is approximately the same as those of the PTS scheme with perfect SI and a maximum likelihood decoding scheme. Considering the cost for SI encoding and decoding, the proposed scheme is practically more advantageous over the PTS scheme.

Per-node throughput enhancement in Wi-Fi densenets

Wi-Fi networks are widely deployed for provision of Internet-centric data services. Since the inception of the Wi-Fi network in 1997 with its technical specification rooted in the IEEE 802.11 standard, much progress for higher data throughput has been made. Currently popular IEEE 802.11n Wi-Fi network in 2.4/5 GHz can deliver 600 Mb/s over a 40 MHz channel, which works well for most types of Internet-centric data services, and a later version of a Wi-Fi network based on IEEE 802.11ac is able to transmit at about 7 Gb/s. A simple configuration of a Wi-Fi network consisting of an AP and multiple stations for bidirectional data transmission enables low-cost implementation. High data rate provided at low cost as well as the abundance of Wi- Fi-capable mobile stations recently led to dense deployment of Wi-Fi networks, particularly in residential areas, business offices, and indoor/ outdoor hotspots. However, dense deployment of Wi-Fi networks (e.g., Wi-Fi DenseNets) causes significantly increased overall interference, and as a result a significantly lowered achievable data rate. Thus, it is sensible to consider technologies that can resolve or mitigate deteriorated throughput of Wi-Fi DenseNets. In this article, technologies to deal with throughput enhancement of Wi-Fi DenseNets are addressed from three different perspectives: exploiting cellular technology for data transmission, elevating spectral efficiency, and controlling overall interference levels. Evaluation of interference control for Wi-Fi DenseNets is carried out in this article, and it is found that significant per-node throughput enhancement can be achieved.

Adaptive phase rotation of OFDM signals for PAPR reduction

High peak-to-average power ratio (PAPR) of the orthogonal frequency division multiplexing (OFDM) signal causes signal distortion, due to nonlinear high power amplifier at the transmitter, and affects received signal quality at the receiver. To reduce the PAPR, various PAPR reduction schemes have been developed. Of these schemes, the tone reservation (TR) scheme and the active constellation extension (ACE) scheme are used for commercial OFDM systems such as the DVB-T2 system. This paper presents an adaptive allpass filter (AAPF)-based PAPR reduction scheme. The AAPF scheme does not incur mean power increment and spectral efficiency decrement unlike the TR scheme. The AAPF scheme can be used with constellation rotation whereas the ACE scheme has a fundamental limitation with it. Comparison of the PAPR performance and the computational complexity with the TR scheme is made and discussed. Bit error rate (BER) performance demonstrates that adoption of the AAPF scheme does not affect signal reception quality at the receiver.1.

Large step-phase measurement by a reduced-phase triple-illumination interferometer.

We present a reduced-phase triple-illumination interferometer (RPTII) as a novel single-shot technique to increase the precision of dual-illumination optical phase unwrapping techniques. The technique employs two measurement ranges to record both low-precision unwrapped and high-precision wrapped phases. To unwrap the high-precision phase, a hierarchical optical phase unwrapping algorithm is used with the low-precision unwrapped phase. The feasibility of this technique is demonstrated by measuring a stepped object with a height 2100 times greater than the wavelength of the source. The phase is reconstructed without applying any numerical unwrapping algorithms, and its noise level is decreased by a factor of ten.

Energy-Efficient Infrastructure Sensor Network for Ad Hoc Cognitive Radio Network

We propose an energy-efficient network architecture that consists of ad hoc (mobile) cognitive radios (CRs) and infrastructure wireless sensor nodes. The sensor nodes within communications range of each CR are grouped into a cluster, and the clusters of CRs are regularly updated according to the random mobility of the CRs. We reduce the energy consumption and the end-to-end delay of the sensor network by dividing each cluster into disjoint subsets with overlapped sensing coverage of primary user (PU) activity. Respective subset of a CR provides target detection and false alarm probabilities. Substantial energy efficiency is achieved by activating only one subset of the cluster, while putting the rest of the subsets in the cluster into sleep mode. Additional gain in energy efficiency is obtained by two promising propositions: selecting nodes from the active subset for actual sensing and switching the active subset to sleep mode by scheduling. The sensor nodes for actual spectrum sensing are chosen considering their respective time durations for sensing. Even the only active subset is switched to sleep mode for a certain number of time slots, utilizing the history of PU activity. We compare the proposed CR network with existing approaches to demonstrate the network performance in terms of the energy consumption and the end-to-end delay.

Effect of anisotropic propagation modeling on microcellular system design

Microcells for wireless communications can be realized with low base station antennas operating at low power. The low base station antennas expected for microcells make the propagation characteristics dependent on the direction relative to the street grid. Due to this anisotropic propagation, the shape of microcells is no longer circular, as is typically assumed so for macrocellular system planning. Therefore the infrastructure of the microcells should be implemented with a different approach from that for macrocells. This paper aims at finding the effect of microcellular wave propagation on the development of cellular design for channelized systems in residential/commercial environments by examining key aspects of cell layout. Using a measurement-based anisotropic propagation model, cell shape and frequency reuse patterns are investigated for the downlink, and a methodology for frequency planning is presented.