Classical FDM/TDM cellular networks, such as GSM, avoid the reuse of the same set of frequencies in close-by cells. This is necessary in order to keep the interference level in the cells below a certain threshold. As a drawback, each cell only uses a fraction of the total frequency resources. Eventually, it would be desirable to fully utilize the complete available frequency spectrum in each cell. In this paper, we demonstrate how beamforming antennas in combination with an intelligent interference coordination in-between cells can be used to achieve this goal. We investigate the tradeoff between the achievable Signal-to-Interference Ratio (SIR) in each cell and the effective utilization of the frequency resources at the example of a state-of-the-art 802.16e system. We conclude that the investigated mechanisms open the way to future wireless access networks with an efficient utilization of the available frequency spectrum.

Abstract-The recently emerging High Speed Downlink Packet Access (HSDPA) enhances conventional WCDMA systems according to the UMTS standard with data rates of up to 14MBit/s in the downlink direction. This is achieved by using adaptive modulation and coding as well as a fast Hybrid Automatic Repeat Request (HARQ) mechanism. This functionality is implemented close to the air interface in the Node B. In addition to the data buffer in the RNC, this requires a second data buffer in the Node B. Consequently, a flow control mechanism is needed which controls the amount of data to be transmitted from the RNC's buffer to the Node B's buffer. The spatial separation of RNC and Node B imposes significant signaling constraints and control dead time limitations to the flow control mechanism. Additionally, due to the time-varying nature of the radio channel, the data rate towards a particular user may be highly variable. In this paper, we study the impact of the flow control on system performance. We will show that it is essential to jointly consider scheduling and flow control in an HSDPA system as the constraints imposed by the flow control may dominate the system performance.

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Orthogonal frequency division multiple access is the basis for several emerging mobile communication systems. Prominent examples are the 3GPP Long Term Evolution as the successor of UMTS high-speed packet access and the IEEE 802.16 system, advanced by the WiMAX forum. On a system level, OFDMA is basically a combination of time and frequency division multiple access. In cellular TDM/FDM systems, inter-cell interference is a major issue that traditionally has been solved by avoiding the use of the same frequency bands in adjacent cells. However, this solution incurs a waste of precious frequency resources. An attractive alternative is the use of beamforming antennas in combination with interference coordination mechanisms, where the transmission of adjacent base stations is coordinated to minimize inter-cell interference. Interference coordination is an important aspect of the system level, which influences many other issues, such as network planning or scheduling mechanisms. In this article, we give an overview of interference coordination as it would apply, for example, to IEEE 802.16e and review the relevant literature. We also discuss and compare interference coordination algorithms, which can be based either on global system knowledge or purely on local system knowledge. Interference Coordination in Cellular OFDMA NetworksO O

in 2002. He joined the Institute of Communication Networks and Computer Engineering at the University of Stuttgart in 2002, where he headed the wireless research group and participated in several privately and publicly funded projects in the area of mobile networks. His research focused on end-toend and system performance of wireless networks, modeling of wireless networks, interference coordination and advanced algorithms, and architectures of future radio access networks. He later joined Daimler Research and Development, where he is currently working on next-generation telematic solutions and architectures for vehicle-centric Internet-services. He received his doctoral degree from the University of Stuttgart in 2009. He is a TPC member of several international workshops and conferences. AbstractWireless systems based on Orthogonal Frequency Division Multiple Access (OFDMA) multiplex different users in time and frequency. One of the main problems in OFDMAsystems is the inter-cell interference. A promising approach to solve this problem is interference coordination (IFCO). In this paper, we present a novel distributed IFCO scheme, where a central coordinator communicates coordination information in regular time intervals. This information is the basis for a local inner optimization in every basestation. The proposed scheme achieves an increase of more than 100% with respect to the cell edge throughput, and a gain of about 30% in the aggregate spectral efficiency compared to a reuse 3 system.

Abstract-Wireless systems based on Orthogonal Frequency Division Multiple Access (OFDMA) multiplex different users in time and frequency. One of the main problems in OFDMAsystems is the inter-cell interference. A promising approach to solve this problem is interference coordination (IFCO). In this paper, we present a novel distributed IFCO scheme, where a central coordinator communicates coordination information in regular time intervals. This information is the basis for a local inner optimization in every basestation. The proposed scheme achieves an increase of more than 100% with respect to the cell edge throughput, and a gain of about 30% in the aggregate spectral efficiency compared to a reuse 3 system.

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Abstract-A new blind channel estimation scheme for OFDMsystems is proposed based on the ML-principle. By avoiding the use of second-and higher-order statistics, a very fast convergence rate is achieved. A novel approach is also proposed for resolving the phase ambiguity of the blind channel estimate without the need for any reference symbols. The approach combines different modulation schemes on adjacent subcarriers, such as 3-PSK and QPSK, to resolve phase ambiguity. Simulations were performed for mobile radio environments with high Doppler frequencies and short-to-medium delay spreads. The achieved performance is comparable to that of pilot-based channel estimation for the case of QPSK-modulation.