Volker Jungnickel

Rate-Maximized Switching Between Spatial Transmission Modes

We evaluate the performance of the common idea to switch between spatial multiplexing and diversity transmission based on the actual channel state, aiming at maximizing the achievable data rate. For different spatial transmission schemes the achievable data rate is evaluated based on the signal to interference and noise ratio (SINR) seen after the equalizer in front of the detector. This process is carried out at the receiver; the decision on the favoured scheme is then signalled to the transmitter via a low-rate feedback link. We provide performance results in terms of spectral efficiency for a 2 x 2 and a 4 x 2 single-user MIMO link as well as for a multi-user system where base station and terminals are equipped with two antennas each.

Low-latency synchronization for OFDM-based visible light communication

5G wireless systems are envisioned to provide ultra-high data rates, increased robustness and low latency for new applications such as machine-to-machine (M2M) and car-to-car (C2C) communications. This paper considers high-speed visible light communication (VLC) as a candidate serving technology to fulfill this set of requirements. Synchronization as the first step towards reliable data detection is studied here thoroughly aiming at high robustness and low complexity. Three schemes are reviewed, namely Schmidl-Cox and Park autocorrelation as well as a cross-correlation based approach, all using a predefined preamble. All schemes are examined concerning their robustness against multipath propagation and their precision in time estimation versus the preamble length. It is found that the simple Schmidl-Cox scheme, originally designed for complex-valued radio waveforms, can be applied also for real-valued optical wireless waveforms. Based on these investigations, an optimized preamble structure is finally proposed. Preamble boosting and fine synchronization can also be applied to reach a similar performance like optimal cross-correlation at reduced complexity. We study the fundamental trade-off between the preamble duration and the minimum signal-to-noise ratio (SNR) required for reliable synchronization and observe that ultra-low latency needs sufficient SNR.

Cooperative Cellular Networks: Overcoming the Effects of Real-World Impairments

The fifth generation (5G) of mobile communication systems will need to support new applications with a variety of requirements, including high data rates, low latency, improved reliability, and sup-port for a large number of devices. We believe that these demands can be satisfied by the combined use of additional spectrum at higher frequencies, small cells, a more advanced air interface, and spectrally efficient transmission using multiple antennas. Base station (BS) cooperation is an advanced multiantenna technique that has the potential to eliminate intercell interference and enhance spectral efficiency. But attaining effective BS cooperation is challenged by a number of real-world channel and synchronization impairments.