Young Hoon Kwon

Multiuser MIMO in Distributed Antenna Systems With Out-of-Cell Interference

Distributed antenna systems (DAS) augment the base stations transmit capability by adding multiple remote radio units, connected to the base station via a high bandwidth and low latency link. With DAS, the base station operates as if it had multiple antennas, but the antennas happen to be in different geographic locations. DAS have been shown to enhance coverage and capacity in cellular systems, in a variety of different configurations. This paper proposes, analyzes, and compares several downlink multiuser multiple input multiple output (MIMO) DAS strategies in terms of per-user throughput and area spectral efficiency. Zero-forcing transmit beamforming is used for transmission, the remote radio units may have one or more antennas, and the subscriber has a single receive antenna. Techniques considered include beamforming across all remote radio units (full transmission), using the same beamforming vector for each remote radio unit (simplified transmission), and selecting a subset of remote radio units. To facilitate rapid simulation and design space exploration, approximations of the ergodic rate are proposed for each technique assuming path-loss, small-scale Rayleigh fading, and out-of-cell interference. Simulations accounting for multiple interfering cells are used to compare the different transmission techniques. Full transmission is found to have the best performance even accounting for out-of-cell interference, though gains diminish for higher numbers of active users. Simplified transmission improves over no DAS but performance degrades with more active remote radio units.

Detection Timing and Channel Selection for Periodic Spectrum Sensing in Cognitive Radio

In this paper, detection timing and channel selection for periodic spectrum sensing are investigated to improve the performance of cognitive radio (CR) users. Designed to maximize the channel efficiency of CR users under a given level of interference with licensed users, the detection timing scheme utilizes the statistics of the licensed channel occupancy to determine the optimal starting point of each sensing action. A channel selection scheme in the multichannel multiuser environment is also proposed to specify which channel to detect for the upcoming sensing action based on detection timing. Numerical results demonstrate that our schemes considerably improve the overall channel efficiency while protecting the communication among licensed users.

Spatial spectrum holes for cognitive radio with relay-assisted directional transmission

Spectrum hole (SH) is defined as a spectrum band that can be utilized by unlicensed users, which is a basic resource for cognitive radio (CR) systems. Most of existing contributions detect SHs by sensing whether a primary signal is present or absent and then try to access them so that the CR and primary users use the spectrum band either at different time slots or in different geographic regions. In this paper, we propose a novel scheme with relays or directional relays for CR users to exploit new spectrum opportunity, called spatial SH. It can provide higher spectrum efficiency by coexistence of primary and CR users at the same region, time, and spectrum band. In particular, when the spectrum opportunity of a direct link from a CR transmitter to a CR receiver does not appear, our scheme may still establish the communication through indirect links, i.e., other CR users act as relay stations to assist the communication by using other spatial domains. Furthermore, we analyze the successful communication probabilities of CR users and demonstrate that the spectrum efficiency can be considerably improved by our scheme.

Probability-based combination for cooperative spectrum sensing

This letter proposes a probability-based scheme for combination of spectrum sensing information collected from cooperative cognitive radio users. The proposed scheme enables combination of both synchronous and asynchronous sensing information by utilizing the statistics of licensed band occupancy and is superior to conventional schemes in terms of detection performance.

Probability-based optimization of inter-sensing duration and power control in cognitive radio

Probability-based strategies are proposed in this letter to determine the optimal inter-sensing duration and power control for cognitive radio (CR). With utilization of the statistics of licensed band occupancy, appropriate inter-sensing duration is determined to capture the recurrence of spectrum opportunity in time when the licensed signal is detected, or to achieve the maximum spectrum efficiency under a certain level of interference with licensed communication when the licensed signal is declared absent. Transmit power is varied dynamically according to the non-interfering probability at each sample so as to increase the transmission rate and decrease the interference power.

Spatial Spectrum Holes for Cognitive Radio with Directional Transmission

In this paper, we propose a cognitive radio (CR) transmission scheme, which enables secondary users to coexist with primary users by exploiting spatial spectrum holes (SSHs) through directional antennas or antenna arrays with beamforming. To ensure reliable CR links and avoid interference to primary users, some CR users may act as relays . We investigate successful communication probability of CR users when this scheme is applied. We further demonstrate that the spectrum efficiency can be greatly improved by multiplexing CR links with directional transmission.

Multiuser MIMO in distributed antenna systems

Distributed antenna systems (DAS) add remote radio units with one or more antennas to enhance coverage and capacity in cellular systems. DAS works by connecting the remote radio units to the base station via a high bandwidth and low latency link. This paper analyzes multiuser multiple input multiple output (MIMO) DAS and compares it with multiuser MIMO communication without DAS in terms of per-user throughput and area spectral efficiency. Zero-forcing transmit beamforming is used at the transmitter. To facilitate rapid simulation and design space exploration, approximations of the ergodic rate are proposed for each technique assuming path-loss, small-scale Rayleigh fading, and out-of-cell interference. Treating all the remote radio units as a super MIMO transmitter gives the best performance even accounting for out-of-cell interference, though gains diminish for higher numbers of active users.

Probability-Based Transmit Power Control for Dynamic Spectrum Access

To take advantage of time-varying spectrum opportunities, a cognitive radio (CR) user continuously monitors the dynamic usage of the licensed frequency band and is allowed to utilize the spectrum resource when it does not cause unacceptable interference with licensed users. In this paper, the transmit power control scheme for each data transmission between periodic spectrum sensing activities is proposed to improve the performance of the CR user with utilization of the statistics of the licensed band occupancy. While the conventional CR transmission scheme allocates the same transmit power to each data sample, our scheme varies the transmit power dynamically according to the non-interfering probability at each sample so that the effective transmission rate of the CR user increases and the expected interference level with the licensed communication decreases. Detection errors are also incorporated into the analysis. It is demonstrated by numerical results that our scheme considerably improves the overall bandwidth efficiency while ensuring the priority of licensed users.

Probability-Based Combination for Cooperative Spectrum Sensing in Cognitive Radio Networks

To take advantage of time-varying spectrum opportunities, a cognitive radio (CR) network monitors the dynamic usage of the licensed band through cooperative spectrum sensing. In this paper, we propose a probability-based scheme for combination of spectrum sensing information collected from several CR users. Different from conventional cooperative spectrum sensing schemes that assume synchronous local sensing information, our scheme enables combination of both synchronous and asynchronous sensing information by utilizing the statistics of licensed band occupancy. In our scheme, the amount of information from each CR user is flexible and a simplified implementation is also feasible under a symmetrical case. Simulation results demonstrate that our scheme is robust and superior in terms of detection performance.

Distributed Antenna Systems with Power Adjusted Beam Switching

Utilization of distributed antennas provides an additional dimension for performance enhancement in 4G and beyond cellular networks. Intelligent remote antenna allocation and practical transmission scheme are two most important aspects for the design of DAS. In this study, a sectorized distributed antenna structure is introduced and correspondingly, power adjusted beam switching as a practical transmission scheme is proposed with periodic beam switching and dynamic power allocation. Simulation results show that the proposed DAS structure and transmission scheme improve the capacity in all SINR regions especially with optimal power allocation.