Youngsoo Yuk

Multicarrier technology for 4G WiMax system [WiMAX/LTE Update]

As one of the candidate fourth-generation mobile communication systems, the IEEE 802.16 m-based WiMAX 2.0 system is required to provide up to 1 Gb/s peak transmission rate. The most efficient solution to achieve this challenging objective is to utilize wider channel bandwidth. Multicarrier is the technology to utilize wider bandwidth for parallel data transmission across multiple RF carriers, which is well agreed as one of the key technologies to satisfy ITU-R IMTAdvanced requirements. As the evolution of the IEEE 802.16 e-based WiMAX 1.0 system, IEEE 802.16 m specifies physical and MAC layers to enable multicarrier technology for the WiMAX 2.0 system. This can lead to more than 1 Gb/s peak transmission rate for low-mobility users and 100 Mb/s peak transmission rate for high-mobility users. By having the protocol structure with a common MAC entity to control transmission by multiple physical-layer connections over different RF carriers, the network operator can aggregate either contiguous or non-contiguous spectrum resources with higher deployment flexibility, user throughput, and spectrum efficiency. This article provides an overview of the multicarrier technology supported by the IEEE 802.16 m draft standard1 for WiMAX 2.0 system, including not only the general operation principle but also some details of physical layer and MAC layer support.

Channel prediction and feedback in multiuser broadcast channels

Multiuser linear precoding requires channel state information (CSI) at the transmitter. In the absence of channel reciprocity between the uplink and downlink, a feedback mechanism must be designed to communicate CSI estimates from the mobile receivers to the transmitter. Limiting the total feedback rate is an important design goal for multiuser multiple-input, multiple-output systems, as the feedback overhead can potentially consume a large percentage of system resources, especially when the total number of antennas is large. In this paper, we focus on the challenges of feedback delay and reducing feedback rate; we predict N-frames-ahead, based on the one-step Kalman predictor, and derive a theoretical expression for the prediction mean squared error (MSE). We present simulation results that illustrate a tradeoff between prediction MSE and computational complexity, and also demonstrate situations where adaptive delta modulation (ADM) can be used to exploit temporal redundancy and reduce the required feedback rate.

Network coded information raining over high-speed rail through IEEE 802.16j

In [1] and [2], two-hop network architectures, with wireline and 802.16a backhauls, respectively, and 802.11 repeaters/relays, were proposed for high-speed rail. The resulting infrastructure cost in the former, and complexity of dual mode wireless relays in the latter, urge the need for more technically and economically efficient solutions. In this paper, we first propose a two-hop wireless network architecture for high-speed rail employing 802.16j. Due to its backward compatibility with 802.16e, the use of 802.16j not only mitigates the restrictions of the previous two-hop heterogeneous solutions but also allows a third direct communication link from the base-station to the trains, thus providing opportunities for throughput improvements. We then propose a network coded downlink transmission scheme over the proposed network architecture to both eliminate the undesirable ARQ overhead in high-speed rail communications and better exploit relay diversity. We refer to our proposed scheme as network coded information raining. Simulation results show the merits of our proposed solutions.