Marc C. Necker

Totally blind channel estimation for OFDM on fast varying mobile radio channels

A new blind channel estimation scheme for orthogonal frequency division multiplexing systems is proposed based on the maximum likelihood 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-phase shift keying (PSK) and quarternary PSK (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.

TCP-Stream reassembly and state tracking in hardware

In this paper we consider a new approach to network intrusion detection. Conventional network intrusion detection systems (NIDS) are software based. We propose to selectively implement portions of the functionality of a state-of-the-art software NIDS in reconfigurable hardware. This increases performance even under hostile loads and will enable efficient intrusion detection in future multi-gigabit networks. Specifically, we consider the problem of TCP-stream reassembly. We present a high-performance TCP stream reassembly and state tracking module targeted for incorporation into an agile reconfigurable network interface based on Xilinx Virtex technology.

Local Interference Coordination in Cellular OFDMA Networks

Orthogonal Frequency Division Multiple Access (OFDMA) is a promising concept, which is the basis of the currently emerging 802.16e (WiMax) and 3GPP Long Term Evolution (LTE) cellular systems. OFDMA is basically a combination of FDM and TDM, and therefore suffers from heavy inter-cell interference if neighboring basestations use the same frequency range. However, it is desirable to reuse the complete available frequency spectrum in every cell in order to maximize the resource utilization. One approach to solve this conflict is the application of beamforming antennas in combination with interference coordination mechanisms between basestations. Starting from a global interference coordination scheme with full system knowledge, we investigate how spatially limited interference coordination affects the system performance. Subsequently, we study several realizable interference coordination schemes and show that a locally implementable scheme can almost match the performance of the global scheme with respect to the sector throughput.

Interference Coordination in Cellular OFDMA Networks

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.

A Graph-Based Scheme for Distributed Interference Coordination in Cellular OFDMA Networks

Wireless systems based on orthogonal frequency division multiple access (OFDMA) multiplex different users in time and frequency. One of the main problems in OFDMA-systems 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.

Coordinated fractional frequency reuse

In recent years, Orthogonal Frequency Division Multiple Access (OFDMA) has become an attractive transmission technology, which is part of various emerging system standards for broadband cellular communications. Examples include the 3GPP Long Term Evolution (LTE) and 802.16e WiMAX. In OFDMA, mobile terminals are multiplexed in time and frequency. A major problem in these systems is the inter-cell interference, which is caused by neighboring cells when transmitting on the same time and frequency slots. This problem can be solved by using beamforming antennas and coordinating the transmissions among base stations. This is known as interference coordination. In this paper, we present a distributed algorithm for interference coordination, which enhances the cell edge performance with global information provided by a central coordinator. The signaling delay during the communication with the central coordinator can be on the order of seconds, while an additional local interference coordination in each base station ensures a high performance even in dynamic environments. This combination of global and local coordination enhances the overall spectral efficiency by 50% compared to a Reuse 3 system while maintaining the same cell edge performance.

Towards frequency reuse 1 cellular FDM/TDM systems

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.

Impact of Iub flow control on HSDPA system performance

The recently emerging high speed downlink packet access (HSDPA) enhances conventional WCDMA systems according to the UMTS standard with data rates of up to 14 MBit/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 buffer to the Node Bs 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

Parameter Selection for HSDPA Iub Flow Control

The recently emerging high speed downlink packet access (HSDPA) enhances conventional WCDMA systems according to the UMTS standard with data rates of up to 14 MBit/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 RNCs buffer to the node Bs 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. In particular, we consider the parameter choice for a previously presented algorithm and highlight some inherent tradeoffs

Integrated scheduling and interference coordination in cellular OFDMA networks

The currently emerging 802.16e (WiMAX) and 3GPP Long Term Evolution (LTE) cellular systems are based on Orthogonal Frequency Division Multiple Access (OFDMA). OFDMA suffers from heavy inter-cell interference if neighboring base stations use the same frequency range. One possible approach to solve this issue is the application of beamforming antennas in combination with interference coordination (IFCO) mechanisms between base stations. In this paper, we trace the problem of IFCO back to the graph coloring problem and investigate the achievable resource utilization of the interference coordinated system. We develop a heuristic that allows the combination of arbitrary scheduling algorithms with the IFCO mechanism. This allows an efficient utilization of the radio system’s frequency resources while still obeying scheduling constraints, such as Quality of Service requirements. Finally, we study the tradeoff between fairness and the total system throughput.