Heesoo Lee

An efficient broadcast scheduling algorithm for TDMA ad-hoc networks

Abstract In this paper, we propose an efficient algorithm to find a collision-free time slot schedule in a time division multiple access frame. In order to minimize the system delay, the optimal schedule must be defined as the one that has the minimum frame length and provides the maximum slot utilization. The proposed algorithm is based on the sequential vertex coloring algorithm. Numerical examples and comparisons with the algorithm in previous research have shown that the proposed algorithm can find near-optimal solutions in respect of the system delay. Scope and purpose An ad-hoc network was introduced in order to apply packet switching communication to a shared radio channel. Using a radio channel as the broadcast medium to interconnect users, an ad-hoc network provides flexible data communication services among a large number of geographically distributed, possibly mobile, radio units. In an ad-hoc network, since all users share a single channel by multiple access protocol, unconstrained transmission may lead to the time overlap of two or more packet receptions, called collision, resulting in damaged useless packets at the destination. Collided packets increase the system delay because they must be retransmitted. Therefore, the transmission for each station must be scheduled to avoid any collision, that is, collision-free transmission should be guaranteed. The time division multiple access (TDMA) technology can be used to schedule collision-free transmission. In this paper, we propose an efficient algorithm to find a collision-free time slot schedule in a TDMA ad-hoc network.

An Iterative Decoding Technique for Cooperative STBC-OFDM Systems with Multiple Carrier Frequency Offsets

In this paper, an iterative decoding technique for cooperative space-time block coded orthogonal frequency division multiplexing (STBC-OFDM) systems is proposed to mitigate the intercarrier interference (ICI) caused by multiple carrier frequency offsets (CFOs) in cooperative transmission. The proposed iterative decoding technique is shown to mitigate the noise enhancement effect on the STBC-OFDM signals caused by multiple CFOs, and be effective in reducing ICI especially when the system has a large FFT size and a large multiple CFOs.

Novel Multi-User MIMO Scheme Based on Successive Interference Cancellation

Multiple Input Multiple Output (MIMO) technology using multiple transmit and receive antennas is considered one of essential elements for wireless communication systems in order to achieve the required spectral efficiency and high data rates. In this paper, we propose new multi-user preceding MIMO schemes named successive interference cancellation (SIC) based Per User and Stream Rate Control (S-PUSRC), and modified S-PUSRC, which are characterized by SIC based multiple stream reception and the feedback of decoding order for efficient multi-user scheduling. Additionally, a number of simulation results are presented.

Adaptive code rate for orthogonal code hopping multiplexing (OCHM) in synchronous downlink

We proposed an Orthogonal Code Hopping Multiplexing (OCHM) scheme as a new statistical multiplexing scheme in synchronous downlink. OCHM enables a large number of users to share a limited number of code channels through statistical multiplexing. In this paper, we obtain the optimal code rate in several traffic load environments by simulation and summarize the appropriate traffic load region for each code rate as the optimal code rate. An adaptive code rate control scheme is proposed and the base station adaptively changes the code rate according to traffic environments in order to save power.

Channel Estimation for OFDM SYstems with Transparent Multi-Hop Relays

In this letter, the effect of a propagation delay resulting from the use of an OFDM system with a transparent mobile multi-hop relay (MMR) is initially analyzed. Then, a least square (LS) channel estimation technique for the OFDM system with throughput enhancement (TE) MMR or cooperative MMR is proposed. It is demonstrated by computer simulation that the proposed LS channel estimation technique for OFDM systems with transparent MMR is superior to the conventional technique in terms of mean square error (MSE) and bit error rate (BER).

Data traffic scheduling algorithm for multiuser OFDM system with adaptive modulation considering fairness among users

Orthogonal frequency division multiplexing (OFDM) is regarded as a very promising digital modulation technique for achieving high rate transmission. However, the increasing number of wireless data users and the deployment of broadband wireless networks have brought about issues of fairness among users and system throughput. In this paper, we propose an efficient scheduling algorithm to maximize system throughput while providing a level of fairness among users for non-real-time data traffic in the downlink of a multiuser OFDM system. We establish a practical scheduling procedure to implement our scheme considering fairness among users and also formulate the resource allocation problem for rate, power, and subcarrier allocation as an integer program that maximizes system throughput. Next, we present a computationally efficient heuristic algorithm for a problem based on the Lagrangian relaxation procedure. Through the computing simulation, we show that the proposed scheme performs better than other schemes in terms of both system throughput and fairness among users.

A dynamic channel assignment in low Earth orbit mobile satellite systems

Several low Earth orbit (LEO) satellite constellations have been proposed for mobile satellite systems (MSS). Due to nonstationary coverage regions of individual LEO satellites, the offered traffic load for each cell varies continuously. The variation of the traffic load essentially requires dynamic channel assignment. Furthermore, extremely frequent handover requests require specific techniques that prioritize the service of handover requests. In this paper, we suggest a new dynamic channel assignment (DCA) scheme which eliminates forced call terminations due to handover failure (call dropping). This scheme utilizes frequency spectrum efficiently, and can also provide users a high-quality premium service that guarantees the success of each handover procedure, called guaranteed handover service. This is based on position measurement. Finally, we compare our algorithm with some other DCA algorithms by computational experiments. The results of the experiments show that our algorithm provides systems with higher throughput (higher completed call ratio) as well as guaranteed handovers.

Achieving maximum system throughput with cooperative relaying: A case study of IEEE 802.16j multi-hop relay

Various types of cooperative relaying (CR) schemes exhibit different levels of throughput and outage performance because of their inherent trade-off between diversity gain and opportunity cost; in other words, the overhead that is associated with cooperation. This article attempts to answer whether cooperative communication is beneficial or not from the system-level viewpoint and furthermore, if it is, how its average throughput can be maximized while maintaining the target outage rate. In order to improve throughput at the required outage performance, we propose a unified selection criterion to deal with different levels of combining gain and opportunity cost associated with each scheme, which allows for the employment of different CR schemes for various positions of the mobile station. Our system-level simulation results for an IEEE 802.16j multi-hop relay confirm the varying levels of trade-offs among different CR schemes and furthermore, show that CR will be a useful means of maximizing the average throughput for a multi-hop relay system as long as each type of the cooperating scheme is carefully selected, depending on the position of the mobile stations.

Synchronization and Channel Estimation for OFDM Systems with Transparent Multi-Hop Relays

In this paper, the effect of a propagation delay resulting from the use of an OFDM system with a transparent mobile multi-hop relay (MMR) is initially analyzed. Additionally, a channel estimation technique for the OFDM system with throughput enhancement (TE) MMR or cooperative MMR is proposed. Secondly, the effect of a Doppler shift caused by a mobile MS is analyzed and a compensation technique for the carrier frequency offset (CFO) in the OFDM system with TE MMR is proposed.

MMSE Channel Estimation for OFDM Systems with Transparent Multi-Hop Relays

In this paper, a minimum mean square error (MMSE) channel estimation technique for an OFDM system with a transparent mobile multi-hop relay (MMR) is proposed by taking into account the effect of propagation delays due to relaying. It is demonstrated by computer simulation that the proposed MMSE channel estimation technique for OFDM systems with cooperative MMR is superior to the conventional technique in terms of the mean square error (MSE) and bit error rate (BER).