Kyung-Hun Jang

On the Design of Interference Alignment Scheme for Two-Cell MIMO Interfering Broadcast Channels

The interference alignment (IA) is a promising technique to effectively mitigate interferences in wireless communication systems. To show the potential benefits of such an IA scheme, this letter focuses on a two-cell multiple-input multiple-output (MIMO) Gaussian interfering broadcast channels (MIMO-IFBC) with M transmit antennas and N receive antennas. It corresponds to a downlink scenario for cellular networks with two base stations (BSs) with M transmit antennas per BS, and two users with N receive antennas per user, on the cell-boundary of each BS. In this scenario, we propose a novel IA technique jointly designing transmit and receive beamforming vectors in a closed-form expression without iterative computation. It is also analytically shown that the proposed IA algorithm achieves the optimal degrees of freedom (DoF) of 2N in the case of [¾N] ≤ M <; 2N. The simulations demonstrate that not only the analytical results are valid, but the sum-rate of our proposed scheme also outperforms those of conventional techniques, especially in the high signal-to-noise ratio (SNR) regime.

Performance enhancement of multirate IEEE 802.11 WLANs with geographically scattered stations

In todays IEEE 802.11 Wireless LANs (WLANs), e.g., the popular IEEE 802.11 b, stations support multiple transmission rates, and use them adaptively depending on the underlying channel condition via link adaptation. It has been known that when some stations use low transmission rates due to bad channel conditions, the performance of the stations using high rates is heavily degraded, and this phenomenon is often referred to as performance anomaly. In this paper, we model the WLAN incorporating stations with multiple transmission rates in order to demonstrate the performance anomaly analytically. Note that all the previously proposed models of the IEEE 802.11 assume a single transmission rate. We also develop possible remedies to improve the performance. Our solution is basically to control the access parameters such as the initial backoff window, the frame size, and the maximum backoff stage, depending on the employed transmission rate. Throughout simulations, we demonstrate that our analytical model is accurate, and the proposed mechanism can indeed provide the remedies to the performance anomaly by increasing the aggregate throughput up to six times.

Throughput enhancement of IEEE 802.11 WLAN via frame aggregation

The popular IEEE 802.11 WLAN is known to achieve relatively small throughput performance compared to the underlying physical layer (PHY) transmission rate. This is due mainly to the large overheads composed of medium access control (MAC) header, PHY preamble/header, backoff time, acknowledgement (ACK) transmission, and some inter-frame spaces (IFSs). Since these overheads are added to each frame transmission, the throughput degradation is relatively high with small-size frames. In this paper, we present a frame aggregation (FA) scheme, which can improve the throughput performance. By aggregating small-size frames into a large frame, we can reduce these overheads relatively. We propose a simple method to implement the FA into the real testbed using off-the-shelf products via device driver modifications. The performance of the FA is evaluated by both numerical analysis and actual measurements from the real testbed. According to the measurement results, the FA can improve the throughput performance by 2 to 3 Mbps, when multiple frames are aggregated.

Hierarchical Interference Alignment for Downlink Heterogeneous Networks

This paper focuses on interference issues arising in the downlink of a heterogeneous network (HetNet), where small cells are deployed within a macrocell. Interference scenario in a HetNet varies based on the type of small cell access modes, which can be classified as either closed subscriber group (CSG) or open subscriber group (OSG) modes. For these two types of modes, we propose hierarchical interference alignment (HIA) schemes, which successively determine beamforming matrices for small cell and macro base stations (BSs) by considering a HetNet environment in which the macro BS and small cell BSs have different numbers of transmit antennas. Unlike prior work on interference alignment (IA) for homogeneous networks, the proposed HIA schemes compute the beamforming matrices in closed-form and reduce the feedforward overhead through a hierarchical approach. By providing a tight outer bound of the degrees-of-freedom (DoF), we also investigate the optimality of the proposed HIA schemes with respect to the number of antennas without any time expansion. Furthermore, we propose a new optimization process to maximize the sum-rate performance of each cell while satisfying the IA conditions. The simulation results show that the proposed HIA schemes provide an additional DoF compared to the conventional interference coordination schemes using a time domain-based resource partitioning. Under multi-cell interference environments, the proposed schemes offer an approximately 100% improvement in throughput gain compared to the conventional coordinated beamforming schemes when the interference from coordinated BSs is significantly stronger than the remaining interference from uncoordinated BSs.

A high-throughput MAC strategy for next-generation WLANs

Wireless LAN technology has been shown to he a revolutionary development during the last decade. Recently popularized IEEE 802.11a/g-based products can support up to 54 Mb/s physical layer rate and provide wireless access to the Internet. However, in order to deal robustly with the unreliable wireless nature, the 802.11 medium access control protocol has a relatively large overhead and hence, the throughput performance is much worse than the underlying physical layer rate. Moreover, along with many emerging applications and services over WLANs, such as voice over WLAN and audio/video streaming, the demand lor faster and higher- capacity WLANs has been growing recently. In this article, we propose a new medium access control protocol for the next-generation high-speed WLANs. The proposed medium access control, called multi-user polling controlled channel access, is composed of two components: multi-layer frame aggregation, which performs aggregation at both the medium access control and the physical layers; and multi-user polling, used to reduce the contention overhead and in turn, achieve higher network utilization. Multi-user polling controlled channel access is compared with the 802.11e-enhanced distributed channel access medium access control. Highly enhanced medium access control efficiency can be achieved by applying multi-user polling controlled channel access. We show the improved medium access control performance in terms of the aggregate throughput of non-QoS Hows with relevant QoS requirements.

Adaptive Sensing Threshold Control Based on Transmission Power in Cognitive Radio Systems

Spectrum sensing is a key enabling technology for cognitive radio. Since there is a tradeoff between the probability of missed detection and the probability of false alarm according to a value of sensing threshold, it is very important to determine the sensing threshold suitable for cognitive radio environments. In this paper, we propose a novel method to determine the sensing threshold in the cognitive radio system, in which the secondary user (SU) first decides its transmission power for the communication and then decides the sensing threshold for the coexistence with the primary user (PU). For the coexistence, the SU controls its sensing threshold adaptively according to its transmission power in order to guarantee the minimum decodable SINR for the primary receiver. The analysis results show that the adaptively controlled sensing threshold decreases both the missed detection and the false alarm simultaneously and so enables both SU and PU to coexist in the same channel without interfering each other.

Interference alignment through user cooperation for two-cell MIMO interfering broadcast channels

This paper focuses on two-cell multiple-input multiple-output (MIMO) Gaussian interfering broadcast channels (MIMO-IFBC) with K cooperating users on the cell-boundary of each BS. It corresponds to a downlink scenario for cellular networks with two base stations (BSs), and K users equipped with Wi-Fi interfaces enabling to cooperate among users on a peer-to-peer basis. In this scenario, we propose a novel interference alignment (IA) technique exploiting user cooperation. Our proposed algorithm obtains the achievable degrees of freedom (DoF) of 2K when each BS and user have M = K + 1 transmit antennas and N = K receive antennas, respectively. Furthermore, the algorithm requires only a small amount of channel feedback information with the aid of the user cooperation channels. The simulations demonstrate that not only are the analytical results valid, but the achievable DoF of our proposed algorithm also outperforms those of conventional techniques.

A novel hidden station detection mechanism in IEEE 802.11 WLAN

The popular IEEE 802.11 wireless local area network (WLAN) is based on a carrier sense multiple access with collision avoidance (CSMA/CA), where a station listens to the medium before transmission in order to avoid collision. If there exist stations which can not hear each other, i.e., hidden stations, the potential collision probability increases, thus dramatically degrading the network throughput. The RTS/CTS (request-to-send/clear-to-send) frame exchange is a solution for the hidden station problem, but the RTS/CTS exchange itself consumes the network resources by transmitting the control frames. In order to maximize the network throughput, we need to use the RTS/CTS exchange adaptively only when hidden stations exist in the network. In this letter, a simple but very effective hidden station detection mechanism is proposed. Once a station detects the hidden stations via the proposed detection mechanism, it can trigger the usage of the RTS/CTS exchange. The simulation results demonstrate that the proposed mechanism can provide the maximum system throughput performance

The analysis of coexistence mechanisms of Bluetooth

Bluetooth, IEEE 802.11b WLAN and microwave ovens are likely to be the dominant interferers to Bluetooth in the coming year. There is both frequency dynamic and frequency static interference. This paper provides a theoretical analysis on the Bluetooth noncollaborative coexistence mechanisms. The analysis results show that a hybrid method of power control, listen-before-talk (LBT) and adaptive frequency hopping (AFH) can give the best packet collision and system aggregate throughput. LBT is the carry sense in Bluetooth. Before any Bluetooth packet transmission, in the turnaround time of the current slot, the packet sender senses the channel, whether there is any transmission going on or not. If the channel is busy, packet transmission is withdrawn until another time. In the hybrid method, power control mitigates the number of potential interferers, LBT combats transient interference, and AFH combats static interference.

Optimal Handover Decision Algorithm for Throughput Enhancement in Cooperative Cellular Networks

In fourth generation wireless communication standards, the throughput enhancement for cell edge users is a main issue so that various inter-cell interference (ICI) coordination techniques have been suggested. Since the coordination between different cells requires the resources of cooperating cell, its use should be determined carefully. In this paper, we consider two basic handover schemes (Fast Cell Selection and Soft Handover) for the ICI coordination and propose a new handover decision algorithm from a viewpoint of the improvement of cell edge throughput. For the proposed algorithm, a new measurement parameter named Interference to other-Interferences-plus-Noise Ratio (IINR) is defined and some practical issues are addressed in the OFDMA-based cellular system. Compared to the legacy SINR-based decision algorithm, the proposed IINR-based decision algorithm improves the cell edge throughput with reduced feedback overhead, because it optimally selects mobile stations that have cooperation gains more than cooperation costs.