Flemming Bjerge Frederiksen

Spectrum sharing for next generation wireless communication networks

Future wireless services are expected to provide data-rates in the order of 1 Gbps in local area and 100 Mbps in wide area. A very wide bandwidth is required to support such high rates, in the order of 100 MHz: this wideband will not be available for each operator, thus a shared access to the spectrum among operators is needed. An holistic approach to spectrum sharing is adopted in this paper: not a single layer or component is optimized, but the system as a whole. Advanced physical layer (PHY) and radio resource management (RRM) solutions, utilizing some cognitive radio concepts, are discussed.

Improving Spectral Capacity and Wireless Network Coverage by Cognitive Radio Technology and Relay Nodes in Cellular Systems

Methods to enhance the use of the frequency spectrum by automatical spectrum sensing plus spectrum sharing in a cognitive radio technology context have been presented and discussed in this paper. Ideas to improve the wireless transmission by orthogonal OFDM-based communication and to increase the coverage of cellular systems by future wireless networks, relay channels, relay stations and collaborate radio have been presented as well. A revised hierarchical deployment of the future wireless and wired networks are shortly discussed.

Link Adaptation Strategies for Multi-Antenna Assisted WiMAX-like System

In this work, we have studied bit and power allocation strategies for multi-antenna assisted orthogonal frequency division multiplexing (OFDM) systems. Popularly it is believed that simultaneous bit and power allocation at the same rate is not very much useful in terms of system throughput. Contrary to this popular belief, we have found that in some scenarios and in some system conditions, bit and power allocations across OFDM subchannels are required together for efficient system performance. We have studied a novel link adaptation algorithm in different multi-antenna systems. We have found that, if we cannot find the exact signal to noise ratio (SNR) thresholds due to different reasons, such as reduced link adaptation (LA) rate, channel state information (CSI) error, feedback delay etc., it is better to fix the transmit power across all sub-channels to guarantee the target frame error rate (FER). Otherwise, we can use adaptive power distribution to save power, which can be used for other purposes.

Hybrid Strategies for Link Adaptation Exploiting Several Degrees of Freedom in OFDM Based Broadband Wireless Systems

In orthogonal frequency division multiplexing (OFDM) systems, there are several degrees of freedom in time and frequency domain, such as, sub-band size, forward error control coding (FEC) rate, modulation order, power level, modulation adaptation interval, coding rate adaptation interval and power adaptation interval, which can be exploited for link adaptation to achieve high spectral efficiency. The variable channel parameters such as delay spread, Doppler frequency spread, average signal to noise ratio and instantaneous channel gains influence the link adaptation parameters. Optimal adaptation of the link parameters based on the channel conditions would lead to highly complex systems with high overhead. Hybrid strategies to vary the adaptation rates to tradeoff achievable efficiency and complexity are presented in this work.

Influence of Inter Carrier Interference on Link Adaptation Algorithms in OFDM Systems

The performance of link adaptation (LA) under the influence of inter carrier interference (ICI), which is cause by carrier frequency offset (CFO) and Doppler frequency spread due to mobility, in orthogonal frequency division multiplexing (OFDM) based wireless systems is analyzed in this work. LA maximizes throughput while maintaining a target bit error rate (BER)or block error rate (BLER) at the receiver using the signal to noise ratio (SNR) fed back from the receiver to the transmitter. Since ICI power is proportional to the signal strength, the implications of such an impairment on LA OFDM systems is not obvious and has not received much attention till now. It is seen that when ICI is not captured in the SNR measured then the LA system fails to meet the target error rate. However, it is shown that with suitable modification to the SNR switching threshold for changing the adaptive modulation can over the problem. It is also noted that ICI severely reduces the spectral efficiency of OFDM systems even when LA is used.

European Research towards Future Wireless Communications

This paper presents an overview of four on-going European research projects in the field of mobile and wireless communications leading to the next generations of wireless communications. The projects started in 2004. They investigate requirements and definition of access technology, network architecture, antennas and propagation, security, services, applications and socio-economic impact.