Suk-Bok Lee

Proportional Fair Frequency-Domain Packet Scheduling for 3GPP LTE Uplink

With the power consumption issue of mobile handset taken into account, single-carrier FDMA (SC-FDMA) has been selected for 3GPP long-term evolution (LTE) uplink multiple access scheme. Like in OFDMA downlink, it enables multiple users to be served simultaneously in uplink as well. However, its single carrier property requires that all the subcarriers allocated to a single user must be contiguous in frequency within each time slot. This contiguous allocation constraint limits the scheduling flexibility, and frequency-domain packet scheduling algorithms in such system need to incorporate this constraint while trying to maximize their own scheduling objectives. In this paper we explore this fundamental problem of LTE SC-FDMA uplink scheduling by adopting the conventional time-domain proportional fair algorithm to maximize its objective (i.e. proportional fair criteria) in the frequency-domain setting. We show the NP-hardness of the frequency-domain scheduling problem under this contiguous allocation constraint and present a set of practical algorithms fine tuned to this problem. We demonstrate that competitive performance can be achieved in terms of system throughput as well as fairness perspective, which is evaluated using 3GPP LTE system model simulations.

Traffic-driven power saving in operational 3G cellular networks

Base stations (BSes) in the 3G cellular network are not energy proportional with respect to their carried traffic load. Our mea- surements show that 3G traffic exhibits high fluctuations both in time and over space, thus incurring energy waste. In this paper, we propose a profile-based approach to green cellular infrastruc- ture. We profile BS traffic and approximate network-wide energy proportionality using non-load-adaptive BSes. The instrument is to leverage temporal-spatial traffic diversity and node deployment heterogeneity, and power off under-utilized BSes under light traf- fic. Our evaluation on four regional 3G networks shows that this simple scheme yields up to 53% energy savings in a dense large city and 23% in a sparse, mid-sized city.

A scalable micro wireless interconnect structure for CMPs

This paper describes an unconventional way to apply wireless networking in emerging technologies. It makes the case for using a two-tier hybrid wireless/wired architecture to interconnect hundreds to thousands of cores in chip multiprocessors (CMPs), where current interconnect technologies face severe scaling limitations in excessive latency, long wiring, and complex layout. We propose a recursive wireless interconnect structure called the WCube that features a single transmit antenna and multiple receive antennas at each micro wireless router and offers scalable performance in terms of latency and connectivity. We show the feasibility to build miniature on-chip antennas, and simple transmitters and receivers that operate at 100-500 GHz sub-terahertz frequency bands. We also devise new two-tier wormhole based routing algorithms that are deadlock free and ensure a minimum-latency route on a 1000-core on-chip interconnect network. Our simulations show that our protocol suite can reduce the observed latency by 20% to 45%, and consumes power that is comparable to or less than current 2-D wired mesh designs.

Secure incentives for commercial ad dissemination in vehicular networks

This paper introduces a promising application over vehicular ad hoc networks (VANETs), where advertisers use VANETs to disseminate their commercial ads via car-to-car communication, targeting a large number of potential customers inside cars. However, due to the noncooperative behavior of selfish or even malicious nodes in real-world scenarios, such a vehicular advertisement system cannot be realized unless proper incentives and security mechanisms are in place. This paper presents signature-seeking drive (SSD), which is a secure incentive framework that stimulates cooperative dissemination of advertising messages among vehicular users in a secure way. Unlike existing incentive systems, SSD does not rely on tamper-proof hardware or game-theoretic approaches but leverages a public key infrastructure to provide secure incentives for cooperative nodes. With a set of ad dissemination designs proposed, we demonstrate that our SSD is robust in both incentive and security perspectives.

Downlink MIMO with Frequency-Domain Packet Scheduling for 3GPP LTE

This paper addresses the problem of frequency domain packet scheduling (FDPS) incorporating spatial division multiplexing (SDM) multiple input multiple output (MIMO) techniques on the 3GPP long term evolution (LTE) downlink. We impose the LTE MIMO constraint of selecting only one MIMO mode (spatial multiplexing or transmit diversity) per user per transmission time interval (TTI). First, we address the optimal MIMO mode selection (multiplexing or diversity) per user in each TTI in order to maximize the proportional fair (PF) criterion extended to frequency and spatial domains. We prove that the SU-MIMO (single-user MIMO) FDPS problem under the LTE requirement is NP-hard and therefore, we develop two approximation algorithms (one with full channel feedback and the other with partial channel feedback) with provable performance bounds. Based on 3GPP LTE system model simulations, the approximation algorithm with partial channel feedback is shown to have comparable performance to the one with full channel feedback, while significantly reducing the channel feedback overhead by nearly 50%.

Secure Incentives for Commercial Ad Dissemination in Vehicular Networks

This paper introduces a promising application over vehicular ad hoc networks (VANETs), where advertisers use VANETs to disseminate their commercial ads via car-to-car communication, targeting a large number of potential customers inside cars. However, due to the noncooperative behavior of selfish or even malicious nodes in real-world scenarios, such a vehicular advertisement system cannot be realized unless proper incentives and security mechanisms are in place. This paper presents signature-seeking drive (SSD), which is a secure incentive framework that stimulates cooperative dissemination of advertising messages among vehicular users in a secure way. Unlike existing incentive systems, SSD does not rely on tamper-proof hardware or game-theoretic approaches but leverages a public key infrastructure to provide secure incentives for cooperative nodes. With a set of ad dissemination designs proposed, we demonstrate that our SSD is robust in both incentive and security perspectives.

Content management in a mobile ad hoc network: Beyond opportunistic strategy

We study the challenging problem of strategic content placement in a dynamic MANET. Existing content placement techniques cannot cope with such network dynamics since they are designed for fixed networks. Opportunistic caching approaches are insufficient as they do not actively manage contents for certain goals. In this paper, we present a novel content management approach called LACMA, which leverages the location information available to mobile devices via GPS. The main idea of LACMA is to bind data to geographic location (as opposed to network nodes). This location-based strategy decouples the content placement problem from the changing network topology, and allows us to design an optimization framework even in a dynamic MANET environment. We present key components of LACMA used for strategic content placement and content-location binding (through proactive content push). We evaluate LACMA and compare its performance with existing caching schemes and show that LACMA considerably outperforms existing schemes over a wide range of scenarios.

Towards MIMO-aware 802.11n rate adaptation

In this paper, we use real experiments to study multiple-input-multiple-output (MIMO) 802.11n rate adaptation (RA) on a programmable access point (AP) platform. Our case study shows that existing RA solutions offer much lower throughput than even a fixed-rate scheme. It is proven that all such algorithms are MIMO-mode oblivious; they do not differentiate spatial diversity and spatial multiplexing modes. We first design MiRA, a novel MIMO RA scheme that zigzags between intra- and inter-MIMO modes to address MIMO 802.11n dynamics. Second, we examine a window-based RA solution, which runs an independent RA in each MIMO mode in parallel and a signal-to-noise ratio (SNR)-based MIMO RA that differentiates modes using SNR measurements. Our experiments show that MIMO-mode aware designs outperform MIMO-mode oblivious RAs in various settings, with goodput gains up to 73.5% in field trials.

GreenBSN: Enabling Energy-Proportional Cellular Base Station Networks

Base station (BS) networks in 3G cellular infrastructure do not consume energy in proportion to their carried traffic load. Our measurements show that the 3G traffic exhibits high fluctuations both in time and over space, thus incurring energy waste. In this paper, we propose Green base station networks (GreenBSN) to approximate network-wide energy proportionality using non-load-adaptive BSes. The instrument is a traffic-driven approach. By leveraging the inherent temporal-spatial traffic dynamics and node deployment heterogeneity, we power off under-utilized BSes under light traffic. Our evaluation on four regional 3G networks shows that GreenBSN yields up to 53 percent energy savings in dense large cities and 23 percent in sparsely deployed regions.

Toward History-Aware Robust 802.11 Rate Adaptation

Rate adaptation is a mechanism unspecified by the IEEE 802.11 standards, yet critical to the system performance by exploiting the multirate capability at the physical layer. In this paper, we conduct a systematic experimental study on rate adaptation over 802.11 wireless networks. Our key contributions are as follows: First, we present a critique on popular design guidelines adopted by many practical algorithms and we uncover their limitations. Our study reveals that these seemingly correct guidelines can be misleading in practice, thus incurring significant performance penalty in certain scenarios. Second, we study the short-term channel dynamics and explore how they guide rate adaptation. To this end, we design and implement a new History-Aware Robust Rate Adaptation Algorithm (HA-RRAA). HA-RRAA uses short-term loss ratio to opportunistically guide its rate change decisions, a cost-effective adaptive RTS filter to prevent collision losses from triggering rate decrease and an adaptive time window to limit transmissions at high loss rates. Our extensive experiments show that HA-RRAA outperforms popular algorithms in all tested scenarios, with goodput gains up to 51.9 percent in field trials.