Ki-ho Kim

A random beamforming technique in MIMO systems exploiting multiuser diversity

A random beamforming technique for multiple-input multiple-output (MIMO) systems that simultaneously obtains downlink multiuser diversity gain, spatial multiplexing gain and array gain by feeding back only effective signal-to-noise ratios (SNRs) is described. In addition, power control using waterfilling is employed to improve the throughput of our method in correlated channels. In a slow fading channel, we prove that the throughput of the proposed method converges to that of eigen beamforming when many users are in a cell. The number of users required to achieve capacity bound increases with the number of antennas and SNR was determined. However, the capacity bound is achieved even with a small number of users, e.g., 16 users in a cell, when the SNR is low, e.g., 0 dB, and the number of transmit and receive antenna is small, e.g., two. We also find that the effect of waterfilling is more noticeable in correlated channels.

Design and implementation of a simulator based on a cross-layer protocol between MAC and PHY layers in a WiBro Compatible.IEEE 802.16e OFDMA system

In this article we propose cross-layer design frameworks for 802.16e OFDMA systems that are compatible with WiBro based on various kinds of cross-layer protocols for performance improvement: a cross-layer adaptation framework and a design example of primitives for cross-layer operation between its MAC and PHY layers. In addition, we provide a simulation framework for cross-layer analysis between the MAC and PHY layers in 802.16e systems. Through this cross-layer simulator, we show that average cell throughput can be improved by 25-60 percent by applying careful cross-layer adaptation schemes.

Beyond 3G: vision, requirements, and enabling technologies

This article introduces the vision and requirements for future development of mobile communications systems, and discusses several key enabling technologies such as modulation and multiple access schemes, multiple antenna techniques, and an IP-based network, considered important to realize this vision in real-world systems.

SC-FDMA with iterative multiuser detection: improvements on power/spectral efficiency

This article outlines the basic principles of single-carrier frequency division multiple access with iterative multiuser detection, called grouped frequency division multiple access. GFDMA allows multiple users to share a common set of subcarriers and separates the signals of users by employing distinct interleavers and frequency-domain multiuser detection. Some attractive features of GFDMA are explained, including low-cost iterative multiuser detection, multiuser and frequency diversity gains, and flexibility in resource allocation. We also discuss the applications of GFDMA to spatial diversity schemes and to intercell interference mitigation in the cell edge. It is shown that GFDMA offers advantages over SC-FDMA with respect to power/spectral efficiency. Such power/spectral efficiency can provide considerable performance improvements in future wireless communication networks.

Analysis of 60 GHz band indoor wireless channels with channel configurations

In this paper, 60 GHz indoor broadband wireless channels are measured with various configurations in a typical office environment. Measurements are taken at nine positions of the room and the base-station antenna is placed either at the center or at an edge of the measurement room, and the remote-station antenna is of either sharp beam or broad beam type. The RMS delay spread (RDS) and normalized received power (NRP) are estimated from the measurements. Statistical models of the 60 GHz indoor wireless channels are also presented with model parameters from the measured channel data based on the Salehs (1987) indoor channel modeling method. Bit error rate (BER) simulations are performed using measured and simulated channel impulse responses with DQPSK-OFDM modulation, respectively. Using sharp beam antenna results in superior performances to using broad beam antenna in terms of both bit error rates (BER) and NRP penalty. Also, placing the base-station antenna at the center is superior to placing it at an edge in terms of BER and NRP.

Hybrid Division Duplex System for Next-Generation Cellular Services

The paper proposes a new duplexing scheme, which is called the hybrid division duplex (HDD), that is suitable for fourth-generation mobile communication systems. The proposed mobile communication system is much more flexible and efficient in providing asymmetric data service and managing intercell interference by exploiting the advantages of both time division duplex (TDD) and frequency division duplex (FDD) schemes. The HDD scheme has a pair of frequency bands such as the FDD, performing a TDD operation using one of the bands in such a manner that allows for simultaneous FDD and TDD operations. Considering the properties of the HDD system architecture, frequency hopping orthogonal frequency division multiple (OFDM) access is adopted in one band for the TDD operation and code division multiple access (CDMA) in the other band for the FDD uplink operation. The important advantage of the HDD scheme is the robustness against cross time slot interference that is inherent to the TDD system, which is caused by the asynchronous downlink/uplink switching boundaries among all neighbor cells. From the simulation results, the proposed system can achieve approximately 7% and 30% improvement with regard to the downlink and uplink throughput, respectively, as compared to the conventional TDD system under cell-independent downlink/uplink traffic asymmetries. It demonstrates that the HDD scheme is a viable solution for future communication systems that are projected to have a cell-independent asymmetric-traffic-supported hierarchical cell structure.

Content Centric Network-based Virtual Private Community

While the majority of Internet traffic is becoming of high quality multimedia variety, also mostly generated by consumers themselves, efficient content sharing with intuitive useability and security measure in a mobile environment remains an elusive goal. In this paper, we present a Content-Centric Networking (CCN) based Virtual Private Community (VPC) service which aims at supporting intuitive content sharing experience in a secure and distributed manner. VPC is a hierarchical and closed user group which consumers themselves can easily create and manage on their own devices. In VPC architecture, consumer content is shared by VPC-based content forwarding policy which is also based on hierarchical content naming. We prototyped the VPC architecture on the consumer devices to demonstrate the potential of user-oriented and distributed consumer network platform.

Hard Handoff Scheme Exploiting Uplink and Downlink Signals in IEEE 802.16e Systems

We consider hard handoff scheme based on uplink and downlink signals in IEEE 802.16e systems. To exploit the channel reciprocity in TDD systems, the proposed scheme triggers handoff initiation process using uplink signal. A serving base station monitors uplink traffic signal and triggers handoff process when the uplink signal strength or SINR becomes less than the predetermined level. This process can prevent mobile stations from periodically monitoring and reporting downlink signals of neighbor base stations at the non-handoff region. In the handoff decision process, the proposed scheme exploits not only uplink signal but also downlink signal as the handoff criteria to reduce ping-pong and outage probability. To efficiently combine uplink and downlink signal levels, the proposed scheme adopt an uplink and downlink joint hysteresis to determine a handoff direction. Using the joint hysteresis can reduce the outage probability compared with using uplink and downlink hysteresis independently. We evaluate the performance of the proposed scheme using computer simulation. The simulation results show that the proposed algorithm has better performance than the conventional mobile assisted handoff scheme of IEEE 802.16e in terms of the average number of handoff and the outage probability during handoff

Amplitude Clipping and Iterative Reconstruction of STBC/SFBC-OFDM Signals

As with orthogonal frequency-division multiplexing (OFDM), one main disadvantage of multi-input multi-output (MIMO) OFDM is the prohibitively large peak-to-average power ratio (PAPR) of transmitted signals on different antennas, which can be reduced by a deliberate amplitude clipping. However, clipping causes distortion that degrades the system performance. In this letter, we propose clipping noise mitigation techniques for space-time and space-frequency block coded (STBC/SFBC)-OFDMs. A new SFBC transmitter for clipped OFDM is proposed, and the optimum equalizer is derived for reconstruction of the clipped signals. Simulation results show that the proposed receivers effectively recover contaminated STBC/SFBC-OFDM signals with a moderate computational complexity.

Combined effects of RF impairments in the future IEEE 802. 11n WLAN systems

In this paper, the overall performance degradation of an IEEE 802.11n WLAN system due to MIMO RF interference, AM-AM distortion of power amplifier, IQ mismatches, phase noise and their combined effects in transmitter are investigated. To extract more relevant values of RF impairments, we observed constellation diagram and the error vector magnitude (EVM) of the complete RF/modem link as a figure of merit. In addition, we propose the novel and first method for the model of the MIMO RF interference from the neighboring RF chains. We also study the implication of its presence on the performance when coupled with other RF impairments. By analyzing the experimental results, the range of values for MIMO interference, PA backoff, gain/phase error, and phase noise can be determined over which a IEEE 802.11n WLAN transmitter operates while maintaining a tolerable level of performance.