Young-Doo Kim

An Overview of Radio Resource Management in Relay-Enhanced OFDMA-Based Networks

Researchers in both academia and industry have accepted OFDMA as the most appropriate air-interface for the emerging broadband wireless access networks and standards. A number of IEEE working groups and various research forums are focusing on developing relay and mesh-enabled networks with cooperative communication features. Among these research efforts are IEEE 802.11s, IEEE 802.16j/m, and 3GPPs advanced long term evolution (LTE-advanced). The combination of OFDMA with relaying techniques provides rich opportunities for cost-effective and high-performance networks. To exploit such opportunities requires intelligent radio resource management (RRM) algorithms. Although a number of publications have highlighted the important and challenging issues involved in designing RRM algorithms for OFDMA networks, only recently a number of papers have investigated relay-enhanced OFDMA-based multicellular networks. By and large, the literature indicates that these issues constitute a hot research topic that will continue to attract interest. This paper provides a survey of the current literature on OFDMA networks enhanced with decode-and-forward relaying and provides their link to earlier literature in non-OFDMA networks. In addition, a rich list of references is provided to direct the readers toward some of the emerging techniques.

Fairness-aware radio resource management in downlink OFDMA cellular relay networks

Relaying and orthogonal frequency division multiple access (OFDMA) are the accepted technologies for emerging wireless communications standards. The activities in many wireless standardization bodies and forums, for example IEEE 802.16 j/m and LTE-Advanced, attest to this fact. The availability or lack thereof of efficient radio resource management (RRM) could make or mar the opportunities in these networks. Although distributed schemes are more attractive, it is essential to seek outstanding performance benchmarks to which various decentralized schemes can be compared. Therefore, this paper provides a comprehensive centralized RRM algorithm for downlink OFDMA cellular fixed relay networks in a way to ensure user fairness with minimal impact on network throughput. In contrast, it has been observed that pure opportunistic schemes and fairness-aware schemes relying solely on achievable and allocated capacities may not attain the desired fairness, e.g., proportional fair scheduling. The proposed scheme is queue-aware and performs three functions jointly; dynamic routing, fair scheduling, and load balancing among cell nodes. We show that the proposed centralized scheme is different from the traditional centralized schemes in terms of the substantial savings in complexity and feedback overhead.

Degrees of Freedom on the K-User MIMO Interference Channel with Constant Channel Coefficients for Downlink Communications

In this paper, we study degrees of freedom for the K-User multiple-input multiple-output (MIMO)-interference channel (IFC) with constant channel coefficients. In this channel, we investigate how many total number of transmit antennas, M1 + M2 + . . . + MK, are required in minimum to achieve di = 1, ∀i degrees of freedom when all receivers have N = 2 antennas, which is a downlink communication scenario. To answer this question, we propose a new interference alignment scheme based on intersection subspace property of the vector space. The proposed interference alignment scheme can be easily generalized regardless of the number of users. In addition, we investigate degrees of freedom for the partially connected MIMOIFC where some arbitrary interfering links are disconnected due to the large path loss or deep fades. In this channel model, we examine how these disconnected links are considered on designing the beamforming vectors for interference alignment.

Adaptive Compensation for Power Amplifier Nonlinearity in the Presence of Quadrature Modulation/Demodulation Errors

This correspondence proposes techniques that jointly compensate for amplifier nonlinearity and quadrature modulation/demodulation (QM/QDM) errors. The proposed methods are derived based on the polynomial predistortion (PD) employing the indirect learning technique and do not require any additional feedback loop for QM/QDM-error compensation. Compared with the existing joint compensation technique, the proposed methods need some additional parameters to be estimated but exhibit faster convergence and better performance. The advantage of the proposed technique is demonstrated through computer simulation.

Non-Data-Aided Approach to I/Q Mismatch Compensation in Low-IF Receivers

A digital signal processing (DSP) technique is presented that can compensate for the in-phase/quadrature-phase (I/Q) mismatch in low-intermediate frequency (IF) receivers. In particular, a non-data-aided (NDA) I/Q mismatch estimator is derived by exploiting the statistical independence between desired and image signals. The proposed technique obtains two baseband signals (uncompensated desired and image signals) from a digital IF signal and processes them to estimate and compensate for the I/Q mismatch. The mean-square error (MSE) of the estimate is analyzed. Computer simulation results indicate that the proposed technique can outperform existing adaptive DSP techniques that are based on the use of blind signal separation algorithms. It is observed that the image rejection ratio (IRR) of the proposed technique decreases monotonically with the number of observed samples for estimation, while that of conventional methods exhibits some floor.

Adaptive modulation for MIMO systems with V-BLAST detection

Adaptive modulation for multiple-input multiple-output (MIMO) systems with zero forcing (ZF) V-BLAST is considered. Motivated by the observation that the detection ordering of the original V-BLAST algorithm, called the forward ordering, is not suitable for adaptive modulation, a new detection ordering for V-BLAST-based adaptive modulation is proposed. This ordering, called the reverse ordering, selects the symbol corresponding to the minimum gain at each iteration, in contrast to the forward ordering which always gives priority to the maximum gain. The advantage of the reverse ordering over the forward ordering is demonstrated through analysis and simulation. It is shown that the reverse ordering can provide about 2 dB gain as compared with the forward ordering and almost comparable to the optimal ordering which is found through an exhaustive search. The V-BLAST based adaptive modulation with reverse ordering performs somewhat worse than the singular value decomposition (SVD) based method [G.G Raleigh et al., March 1998], but the former is shown to be considerably simpler to implement than the latter.

Fairness-Aware Joint Routing and Scheduling in OFDMA-Based Cellular Fixed Relay Networks

Relaying and orthogonal frequency division multiple access (OFDMA) are the accepted technologies for emerging wireless communications standards. The activities in many wireless standardization bodies and forums, for example IEEE 802.16 j/m and LTE-Advanced, attest to this fact. The availability or lack thereof of efficient radio resource management (RRM) could make or mar the opportunities in these networks. This paper therefore provides a comprehensive RRM algorithm for OFDMA-based multi-cellular fixed relay networks in a way to ensure fairness among users with minimal impact on the network throughput (in contrast, pure opportunistic RRM techniques always favor users with good channel conditions). Unlike the majority of works in the literature, our proposed scheme is queue-aware and jointly performs routing, fair scheduling, and load balancing among cell nodes. The routing strategy has inherent learning ability and it dynamically converges to better routes.

Radio Resource Management in OFDMA-Based Cellular Networks Enhanced with Fixed and Nomadic Relays

The provision of very high data rates in a ubiquitous manner throughout the service area is a great challenge for 4G and beyond-4G wireless networks. Towards that end, the deployment of fixed relays by the operators has become an accepted radio access network concept in various standardization activities including LTE-A and 802.16j. It is envisaged that next-generation networks will comprise a plethora of wireless relay stations. Worthy of mention is the plug-and-play type of relay known as nomadic relay. We devise novel radio resource management (RRM) schemes to facilitate the operation of fixed relay stations (FRSs) and nomadic relay stations (NRSs) in OFDMA-based multicellular networks. Two schemes of different decentralization levels are devised and classified as distributed and semi-centralized. A novel user-based dynamic routing or link selection that significantly reduces the feedback overhead is employed. We develop methods by which the NRSs act autonomously to acquire radio resources without relying on a central entity. NRS operation is the same in the two schemes and can be extended to any other OFDMA-based RRM scheme. Through the asynchronous opportunistic medium access of the NRSs, smart and opportunistic intra-cell channel reuse is attained. This is different from the static intra-cell reuse patterns often adopted in literature. Furthermore, we introduce a method to enable the cooperation between an NRS and a serving FRS to assist a troubled wireless terminal (WT). We thus establish the concept of nomadic relay-augmented fixed relay networks. To the extent of our knowledge of the literature, no other work has undertaken this task so far.

Empirical analysis of video multicast over WiFi

Video multicast service is becoming one of the most important applications over WiFi, due to the increasing popularity of WiFi for multimedia communication. On the one hand, energy efficient operation is required to WiFi technology due to the limited battery power of most WiFi-equipped devices. In this paper, we empirically study the video multicast operation, especially, along with power management operation in two aspects: (1) whether commercial WiFi devices correctly operate as defined in IEEE 802.11 standard and (2) what problem the standard-compliant operation can induce. From our experimental results, we first figure out that some of commercial WiFi devices do not follow the standard with respect to the power saving operation, and this noncompliance worsens interoperability. We also find that the standard-compliant operation may cause significant delay of voice over IP (VoIP) traffic, when video multicast coexists with VoIP. Through this experimental study, we provide the guidelines for energy-efficient video multicast service.

Designing training sequences for carrier frequency estimation in frequency-selective channels

A procedure for selecting a training sequence (TS) is developed for frequency estimation in frequency-selective channels. An expression for the unconditional Crame/spl acute/r-Rao bound (UCRB) is obtained by averaging the CRB for frequency estimation over the probability density function of Gaussian random channels. In addition, a necessary and sufficient condition for minimizing the UCRB is derived. Based on these results, a procedure for selecting a TS is developed. Through a computer search, binary TSs up to length 24 are found and tabulated. It is observed that periodic TSs tend to be selected when the TS length is twice the channel duration. Simulation results demonstrate that the proposed TSs can enhance the performance of the maximum likelihood (ML) frequency estimate.