Bang Chul Jung

Multi-user diversity in a spectrum sharing system

We investigate the effects of multi-user diversity in a spectrum sharing system where secondary users restrictively utilize a spectrum licensed to primary users only if interference perceived at primary users is regulated below a predetermined level. This interference regulation affects the characteristics of multiuser diversity gains previously known in non-spectrum sharing systems. Our numerical and analytical results show that the multiuser diversity gain in a spectrum sharing system increases differently according to conditions given by the transmit power of secondary users, P, and a predetermined interference temperature, Q - if P is sufficiently larger than Q, the multiuser diversity gain in terms of capacity scales like log2 (W (Ns)) similarly to a previously known scaling law in the non-spectrum sharing systems, where W(middot) and Ns denote a Lambert W function and the number of secondary transmitters, respectively. However, the scaling law of multiuser diversity gain becomes log2(Ns) as P becomes sufficiently larger such that P Gt QNs.

Opportunistic Interference Alignment for Interference-Limited Cellular TDD Uplink

We introduce an opportunistic interference alignment (OIA) for cellular networks, where a user scheduling strategy is utilized in time-division duplexing uplink communication environments with time-invariant channel coefficients and multi-antenna base stations (BSs). In the OIA scheme, each BS opportunistically selects users who generate the minimum interference to the other BSs. More specifically, each BS broadcasts its pre-designed interference directions, e.g., orthonormal random vectors, to all the users in other cells. Then, each user computes the amount of its generating interference, affecting the other BSs, and feedbacks it to its home cell BS. Note that the proposed OIA does not require global channel state information, time/frequency expansion, and a number of iterations, thereby resulting in easier implementation. Simulation results show that the proposed scheme provides significant improvement in terms of sum rates.

Opportunistic Interference Mitigation Achieves Optimal Degrees-of-Freedom in Wireless Multi-Cell Uplink Networks

We introduce an opportunistic interference mitigation (OIM) protocol, where a user scheduling strategy is utilized in K-cell uplink networks with time-invariant channel coefficients and base stations (BSs) having M antennas. Each BS opportunistically selects a set of users who generate the minimum interference to the other BSs. Two OIM protocols are shown according to the number of simultaneously transmitting users per cell, S: opportunistic interference nulling (OIN) and opportunistic interference alignment (OIA). Then, their performance is analyzed in terms of degrees-of-freedom (DoFs). As our main result, it is shown that KM DoFs are achievable under the OIN protocol with M selected users per cell, if the total number of users in a cell, N, scales at least as SNR(K-1)M. Similarly, it turns out that the OIA scheme with S(<;M) selected users achieves KS DoFs, if N scales faster than SNR(K-1)S. These results indicate that there exists a trade-off between the achievable DoFs and the minimum required N. By deriving the corresponding upper bound on the DoFs, it is shown that the OIN scheme is DoF-optimal. Finally, numerical evaluation, a two-step scheduling method, and the extension to multi-carrier scenarios are shown.

Optimal modulation and coding scheme selection in cellular networks with hybrid-ARQ error control

We propose an optimal modulation and coding scheme (MCS) selection criterion for maximizing user throughput in cellular networks. The proposed criterion adopts both the Chase combining and incremental redundancy based hybrid automatic repeat request (HARQ) mechanisms and it selects an MCS level that maximizes the expected throughput which is estimated by considering both the number of transmissions and successful decoding probability in HARQ operation. We also prove that the conventional MCS selection rule is not optimized with mathematical analysis. Through link-level and system-level simulations, we show that the proposed MCS selection criterion yields higher average cell throughput than the conventional MCS selection schemes for slowly varying channels.

Opportunistic Interference Alignment for MIMO Interfering Multiple-Access Channels

We consider the K-cell multiple-input multiple-output (MIMO) interfering multiple-access channel (IMAC) with time-invariant channel coefficients, where each cell consists of a base station (BS) with M antennas and N users having L antennas each. In this paper, we propose two opportunistic interference alignment (OIA) techniques utilizing multiple transmit antennas at each user: antenna selection-based OIA and singular value decomposition (SVD)-based OIA. Their performance is analyzed in terms of user scaling law required to achieve KS degrees-of-freedom (DoF), where S(≤ M) denotes the number of simultaneously transmitting users per cell. We assume that each selected user transmits a single data stream at each time-slot. It is shown that the antenna selection-based OIA does not fundamentally change the user scaling condition if L is fixed, compared with the single-input multiple-output (SIMO) IMAC case, which is given by SNR(K-1)S, where SNR denotes the signal-to-noise ratio. In addition, we show that the SVD-based OIA can greatly reduce the user scaling condition to SNR(K-1)S-L+1 through optimizing a weight vector at each user. Simulation results validate the derived scaling laws of the proposed OIA techniques. The sum-rate performance of the proposed OIA techniques is compared with the conventional techniques in MIMO IMAC channels and it is shown that the proposed OIA techniques outperform the conventional techniques.

Opportunistic Underlay Transmission in Multi-Carrier Cognitive Radio Systems

Underlay transmission in cognitive radio enables a secondary (unlicensed) system to utilize a frequency band of primary (licensed) system as long as the unlicensee interferes less than a certain threshold with the licensee. The secondary system needs to carefully consider not only its own channel to achieve a capacity gain by this sharing spectrum in multi-carrier systems, but also the interference channel to reduce interference at the primary receiver. In this paper, we formulate a capacity maximization problem of the secondary system under an interference-power constraint as well as a conventional transmit-power constraint, and propose an optimal power allocation policy in which we exploit a two-dimensional frequency-selectivity on both channels. Through extensive simulations, we compare the performance of optimal power allocation policy with that of equal power allocation policy and further investigate the effect of the primarys power allocation policy on the performance of the secondary system. Numerical results show that the optimal power allocation policy can achieve a higher capacity in more frequency-selective channels, compared to an equal power allocation policy. Interestingly, a water-filling policy for the primary system also gives additional opportunities to the secondary system than the equal power allocation policy.

Performance Analysis of Two Relay Selection Schemes for Cooperative Diversity

We propose two relay selection in cooperative relay communications. In a fixed scheme, M multiple relays that have strong signal strength are selected out of K relays and forward their received data from a source node to a destination node. As an alternative approach, a threshold-based adaptive relay selection scheme is also proposed to minimize the number of forwarding relays while satisfying a given outage requirement because if the number of forwarding relays increases, then the number of interfering sources also increases. The minimum number of relays that can prevent an outage event are selected to forward data to a destination. The performance of both schemes are evaluated through numerical analysis and Monte-Carlo simulations in terms of end-to-end outage probability and the number of forwarding relays. The result presents a bound that the fixed and adaptive relay selection schemes can achieve information-theoretically. Furthermore, the outage performance of the adaptive relay selection scheme is identical to that of the fixed relay selection scheme with M = K, while the number of forwarding relays is much less than that of the fixed relay selection scheme with M = K.

A Tradeoff Between Single-User and Multi-User MIMO Schemes in Multi-Rate Uplink WLANs

Due to high spectral-efficiency of multiple-input multiple-output (MIMO) transmission techniques, IEEE 802.11n WLAN system adopted a single-user MIMO (SU-MIMO) scheme in which multiple symbol streams are transmitted from a single station (STA) to enhance the system performance. On the other hand, recently, adoption of a multi-user MIMO (MU-MIMO) scheme for multi-packet reception (MPR) in uplink WLAN has also attracted attention. The SU-MIMO scheme achieves a MIMO multiplexing gain at physical (PHY) layer while the MU-MIMO scheme achieves a MIMO multiplexing gain at medium access control (MAC) layer. Thus, there is a fundamental question which scheme is a better solution for uplink WLANs and, in this paper, we analyze and compare these two schemes with random STA distribution scenarios. Moreover, with the adaptation of MAC layer parameters, we also analyze and compare the maximum throughput performance of both the SU- and MU-MIMO schemes in uplink WLANs and we find a proper decision criterion to select the MIMO mode in uplink WLANs.

A MIMO-Based Collision Mitigation Scheme in Uplink WLANs

Although there exist random backoff schemes in WLANs, a collision problem among stations (STAs) is one of the critical factors that degrade the performance of WLAN systems. To mitigate this collision problem, a MIMO-based uplink collision mitigation scheme is proposed by utilizing additional degrees of freedom through multiple antennas, and, consequently, enhances the system performance. Analysis and simulation results show that there is at least 30% throughput enhancement in a basic service set (BSS) with more than 10 STAs, compared to the performance of conventional WLANs in case of saturation traffic.

Power Allocation for OFDM-Based Cognitive Radio Systems under Outage Constraints

This paper investigates power allocation algorithms for OFDM-based cognitive radio systems, where the intra-system channel state information (CSI) of the secondary user (SU) is perfectly known. However, due to loose cooperation between the SU and the primary user (PU), the inter-system CSI is only partially available to the SU transmitter. Two types of PUs are considered to have different capabilities. One is a dumb (Peak Interference-Power tolerable) system that can tolerate a certain amount of peak interference at each subchannel. The other is a more sophisticated (Average Interference-Power tolerable) system that can tolerate the interference from the SU as long as the average interference over all subchannels is within a certain threshold. Accordingly, we introduce an interference power outage constraint, with which the outage is maintained within a target level. The outage is here defined as the probability that peak or average interference power to the PU is greater than a given threshold. With both this interference-power outage constraint along with a transmit-power constraint, we propose optimal and suboptimal algorithms to maximize the capacity of the SU. We evaluate the spectral efficiency through extensive simulations and show that the SU can achieve higher performance (up to two times) with the more sophisticated PU than with the dumb PU.