Ralf-Peter Braun

Link Budget Analysis for Terahertz Fixed Wireless Links

Due to the broad bandwidths, terahertz (THz)-waves offer the possibility for wireless transmission of high data rates. Especially, broadband wireless access over short ranges and fixed wireless links based on THz-waves are very promising. They can be incorporated as a bridge for optical networks or an alternative for the connection of wireless stations in difficult environments, to transmit next generation HDTV signals or for the broadband connection of servers in a data center, for instance. The frequency range between 300 and 900 GHz is very promising for these applications since the possible bandwidth is very high and first electronic circuits will become available on the market soon. However, contrary to wireless links in the lower GHz-bands, the free-space path-loss is quite high and the attenuation due to molecules in the air or water droplets can significantly decrease the transmittable data rates in this frequency range. Here the basic properties of THz-waves will be investigated and the maximum achievable data rates for fixed wireless THz-links will be derived. In order to keep the considerations as general as possible, the derivations are based on simple assumptions and equations. Additionally, conclusions for the applicability of THz-waves for fixed wireless links with distances up to 1 km will be given and the special requirements for these systems will be discussed. As we will show, high data rates can only be transmitted via these links if transmitter and receiver antennas with very high gains are used. This requires an adaptive control of mechanical fluctuations.

Generation of ultra-narrow, stable and tunable millimeter- and terahertz- waves with very low phase noise.

The interference between two spectral lines of the frequency comb of a fiber femtosecond laser is used to generate millimeter-wave and terahertz tones. The two lines are selected by stimulated Brillouin scattering (SBS) amplification. All other modes are strongly rejected based on polarization discrimination, using the polarization-pulling effect that is associated with SBS. The inherent high spectral quality of a femtosecond fiber laser comb allows generation of millimeter- and terahertz waves with linewidths below 1 Hz, and a phase noise of -105 dBc/Hz at 10 kHz offset. The generation, free-space transmission and detection of continuous waves at 1 THz are demonstrated as well. Lastly, the generated millimeter-wave carriers are modulated by 40 Gbit/s data. The entire system consists of a fiber laser and standard equipment of optical telecommunications. Besides metrology, spectroscopy and astronomy, the method can be utilized for the emergent field of wireless millimeter-wave and THz-communications at ultra-high data rates.

512-Gb/s DP-16-QAM field trial over 734 km installed SSMF with co-propagating 10 Gb/s NRZ neighbors incorporating soft-FEC decoding

We report on a 512-Gb/s dual-carrier DP-16-QAM long haul field trial over 734 km of dispersion compensated SSMF with 10-Gb/s neighbors. Soft FEC decoding was utilized in the receiver DSP confirming successful transmission.

Impact of adaptive-rate transponders and fiber attributes on the achievable capacity

We use the analytical Gaussian noise model to assess the achievable capacity over a fully transparent network with 18 nodes and 26 physical connections. By applying a network-centric approach, we show that the use of adaptive modulation in combination with ultralow loss fiber can provide an increase in the achievable capacity. The use of ultralow loss, large effective area fibers can allow for further capacity increase.

Field Trial of a 1 Tb/s Super-Channel Network Using Probabilistically Shaped Constellations

We successfully tested the suitability of probabilistically shaped constellations in a German nationwide fiber ring of the R&D field test network of Deutsche Telekom. In this ring, eight German cities can be reached by add/drop nodes implemented by passive optical add/drop multiplexers, offering 12xa0bands of 400xa0GHz widths in the extended C-band. Multiple cascaded add/drop multiplexers are reducing the available bandwidth. We transmitted a 1xa0Tb/s four-carrier super-channel with an extended family of probabilistically shaped constellations using 16QAM, 36QAM, and 64QAM and variable bandwidth. We verified in the field environment by two examples that probabilistic shaping is outperforming standard 64QAM and 16QAM formats in reach. We experimentally tested two typical 1xa0Tb/s working paths with 419 and 951xa0km, demonstrating spectral efficiencies of 6.75 and 5xa0bit/s/Hz, respectively. The maximum reach of about 1500xa0km was extrapolated for a 1xa0Tb/s probabilistically shaped super-channel transmitted in a 200xa0GHz bandwidth. For protection paths with longer distances than 1500xa0km, the spectral efficiency has to be reduced with the net-bitrate in steps of 100xa0Gb/s. For the longest bidirectional path of the test network, i.e., a protection path of 2159xa0km, the maximum achieved net-bitrate was 800xa0Gb/s.

Maximum transmittable data rates for Millimeter-wave fixed wireless links

Smartphones, video streaming and social networks lead to an increase of the transmitted data rates in the telecommunications networks by about 40% per year. To keep pace with the increasing data rates, new ideas for spectrum efficient data transport with low energy consumption are required. Electromagnetic waves in the Millimeter-region of the spectrum are very promising for this purpose since they offer large unregulated bandwidths. However, weather conditions like rain or fog can drastically decrease the transmittable data rates at these wavelengths. Here we will show that fixed wireless links with a distance of 1 km and maximum data rates of around 1 Tbps are possible with just one source. This opens the way to solve the so-called last mile problem.