New growth market applications such as carrier grade services, enterprise networks and mobile backhaul can require significantly higher network uptime and availability than traditional “consumer grade” broadband services.
Traditionally, the bulk of fixed satellite communications have used C-band and Ku-band transmission frequencies. Today, C/Ku-band orbital positions and frequencies have become highly congested, creating demand for use of additional frequency bands including Ka-band. Ka-band systems can also offer several advantages. The available frequency range in the Ka-band is about 4 times larger than that of traditionally C- and Ku-bands used for Fixed Satellite Services (FSS). Furthermore, Ka-band transmission is typically employed with the usage of multiple spot beams, so-called ‘frequency reuse’ operation. This allows for the transmission of different signals at the same frequency, simultaneously to several geographic areas. 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) operated in Ka-band can provide data capacities exceeding 100GBit/s per single satellite, many times that of conventional satellite systems.
Despite all these advantages, satellite systems operated in the Ka-band face some challenges. Ka-band transmission is severely degraded by adverse weather conditions. In particular additional atmospheric losses due to rainfall can exceed 50dB for Ka-band satellite transmission systems. With such high atmospheric losses, conventional fade margin approaches as adaptive-waveform and coding/modulation techniques or adaptive-power control techniques do not always compensate sufficiently for the losses, when it comes to meeting the needs of high availability services.
Ka-band Site Diversity
Since transmission quality on Ka-band frequencies is heavily dependent on atmospheric losses dominated by rainfall, suitable system configurations need to be carefully designed. Within the ITU rain zone model the world is mapped into different “rain regions”, in which the rain rates do not exceed a specific value during the year. For instance a typical rain zone for Central Europe would be categorized as rain “zone F”, meaning that the average rain rate in this region is below 25mm/h for 0.01percent of the time of a year, whereas in a tropical rain region, e.g. zone “P”, the average rain rate does not exceed 145mm/h for 0.01 percent of the time. Beside other factors, for example, frequency, earth station location, and elevation angle, link losses scale with these rain rates for Ka-band satellite systems. Attenuation caused by heavy rainfall (>70mm/h) can exceed values of 50dB. Consequently, for 99.99 percent availability, the satellite system needs to be able to provide corresponding fade margins in terms of appropriate system configuration.
To achieve the highest fade margins, and support higher uptime Ka-band services, Site Diversity configurations can be used. Figure 1 shows the schematic view of a Ka-band Site Diversity configuration. The communication system is built with two antenna sites, one Main Site and one Diverse Site. In the event of adverse weather conditions at the Main Antenna Site, the RF signals are switched over to the Diverse Antenna Site.
The optimum distance between Main and Diversity highly depends on the geographical properties of the sites, in addition to other factors. Generally the diversity performance increases with increasing distance between main and diverse antenna site. Although core rain cells in tropical regions can be as small as 2km in diameter, the distance between a Main and Diverse Site for a reliable Site Diversity configuration with sufficient diversity gain should normally exceed 30km. At about 100km distance, rainfall at Main site and Diverse Site are no longer correlated. Consequently a typical distance for separation of Main Site and Diverse Site in the range between 30 to 100km (15 to 60 miles) is recommended.
It is worth mentioning that to meet higher availability requirements, higher fade margins are also necessary. Service availability can be greatly improved due to the statistical independence of weather conditions on the different signal paths and highest level of network uptime and throughput can be achieved.
Figure 2 shows a schematic view of a Ka-band Site Diversity configuration. A bidirectional optical link is used to transfer the signals between the Data Center, 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. By quasi-instantaneously routing to the site without rain, or the lowest amount of rain losses, live user traffic is only negligibly affected and network uptime and throughput is significantly improved.
For several reasons to be discussed, RF L-band signals should be transmitted via optical fiber. Since Ka-band transmission systems are mainly operated with Time Division Multiplexed (TDM) Signals, the time delay between Main and Diverse Site needs to be addressed. To equalize this time delay, an optical Delay Line, with adjustable time delay (e.g. in 10 ns steps) can be used in the optical link to the Main antenna site. To bridge the quite long optical distances of up to 100km between Data Center and the Antenna sites, an RF-over-fiber system has to be employed. Traditional RF-over-fiber transmission systems employ one optical fiber per RF-signal and a unidirectional transmission path. By using optical Dense Wavelength Division Multiplexing (DWDM) technology, a bidirectional transmission of up to 40 RF signals over one optical fiber is enabled. In addition to the benefit of offering the highest data rates over a single fiber, this can provide significant OPEX savings, as costs for renting a dark fiber can be in the range of several hundred dollars per mile per year, taking a typical example in Europe.
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 Center and the Diversity Site in 1+1 redundancy configuration. For the 1+1 redundancy example, the RF signal is transmitted and converted by main and redundant transmitter and receiver modules. 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.
With a Ka-band Site Diversity configuration, the signal transmission is redirected from the Main Site to a Diverse Site in the case of adverse weather conditions. Site Diversity configurations, can employ DWDM RF-over-Fiber transmission systems and redundancy switching units, and provide and excellent way to ensure maximum Ka-band system reliability and availability, despite the high level of rain attenuation at Ka-band.
The new RF-over-fiber technology, combined with wider availability of fiber lines today allows diverse antenna sites to have direct RF connectivity to a single data center, without installation of all the based band data center equipment at each antenna site. This provides cost-savings, and operational ease compared to the installation of complete antenna or baseband traffic site redundancy configurations. Ka-band networks can achieve higher resiliency using RF-over-fiber antenna diversity. If scaled across multiple gateway sites, this kind of solution can increase overall usable satellite network capacity by allowing traffic be routed to less satellite resource-hungry sites during rain. This will help Ka-band systems maximize their competitiveness and ability to address a broader range of customer needs. VS
Michael Waldow is business development manager DAS - Systems Engineering at DEV Systemtechnik GmbH & Co. KG, a Quintech Electronics & Communications Company.