DEV Solutions for Ka-Band Diversity
State-of-the art satellite communication systems at the highest data rates are operated on the Ka-Band. As transmission quality on Ka-Band frequencies is heavily dependent on weather conditions, suitable system configurations need to be carefully planned and chosen. In Ka-Band Site Diversity configurations signal transmission is redirected from the Main Site to a Diverse Site in case of adverse weather conditions. These Site Diversity configurations, which rely on DWDM RF-over-Fibre transmission systems and redundancy switching units, are the best-suited solutions and ensure maximum system reliability and availability.
Satellite communications represent a flexible and fast implementation platform for delivering versatile services including voice-, data or video signals. As satellite communication systems do not rely on existing communication infrastructures, like electrical or optical cable networks, all regions of the world can be reached independent of their remoteness or their specific geographical location. Today satellite communication systems with point-to-point and multi-point topologies around the globe for Broadcasting, Data- and Multimedia- Services and Cellular Backhaul are deployed.
Traditionally satellite communication has been based on C-Band and Ku-Band transmission frequencies. Today, orbital positions are highly congested additional bandwidth can essentially only be provided by using additional frequency bands.
For this reason, Ka-Band satellites are gaining a high level of interest to face the increasing demand for higher data transmission rates. Beside the provision of additional frequencies satellite communication systems operating in the Ka-Band offer several other benefits. Firstly, available frequency range in the Ka-Band is about 4 times larger than for satellite communication in conventional C- and Ku-Bands. Furthermore, Ka-Band transmission is typically employed with the usage of multiple spot beams, so-called ‘frequency reuse’ operation, which allow for the transmission of different signals at the same frequency, simultaneously to several geographic areas. Finally, the high transmission frequencies of Ka-Band allow for highly focused spot beams and smaller antennas leading to economically efficient solutions at high data rates. Consequently, state-of-the-art ‘High Throughput Satellites’ (HTS) are operated in the Ka-Band and provide data capacities exceeding 100GBit/s per single satellite.
However, beside their undisputed advantages, satellite communication systems operated in the Ka-Band face some challenges, which need to be addressed appropriately by professional equipment and well thought out system configurations. Ka-Band transmission is severely degraded by adverse weather conditions. In particular additional atmospheric losses due to rainfall can exceed 30dB for Ka-band satellite transmission systems. With such high atmospheric loss, conventional fade margin approaches as adaptive-waveform techniques or adaptive-power control techniques are not sufficient for compensation. For reliable and highly available systems only Site Diversity configurations provide an adequate solution.
Ka-Band Site Diversity
Figure 1 shows the schematic view of a Ka-Band Site Diversity configuration. The communication system is built from two antenna sites, one Main Site and one Diverse Site. In the case of adverse weather conditions the RF signals are switched over to the Diverse Antenna Site. Typically distance for separation of Main Site and Diverse Site is in the range between 30…100km (15…60 miles) and the RF L-Band signals are transmitted via optical fibres.
Figure 2 shows a schematic view of a Ka-Band Diversity site configuration. A bidirectional optical link is used to transfer the signals between the Data Centre, and the Main or Backup Antenna Site respectively. To relay the L-Band signals between the Main and Diverse Site, a fast RF switching unit is used to carry out switchover operations. As Ka-Band transmission systems are mainly operated with Time Division Multiplexed (TDM) Signals, the arising time delay between Main and Diverse Site needs to be compensated.
To equalize this time delay, an optical Delay Line with in steps of 10ns adjustable time delay is used in the optical link to the Main antenna site. To bridge the long optical distances between Data Centre and the Antenna sites, an optical Dense Wavelength Division Multiplexing (DWDM) transmission system is employed. This DWDM system enables the transmission of up to 40 RF signals over one optical fibre, thereby facilitating the transfer of highest data rates.
To further increase the reliability and availability of the system, equipment redundancy can additionally be applied. Figure 3 shows the schematic view of a bidirectional optical link between the Data Centre and the Diversity Site in 1+1 redundancy configuration. For the 1+1 redundancy example, the RF signal is transmitted and converted by one main, one redundant transmitter and a receiver module. In the case of a malfunction, or loss of a main transmitter or main receiver module, the redundancy switch at the receiver side switches over to the backup equipment ensuring a signal transmission with highest possible quality and uptime.
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