Considering RF over Fiber links for the Iridium Satellite Network

Originally launched in the 1990s, the Iridium satellite network today provides essential voice and data wireless communications to a range of users including government, personal, maritime and ground transport. The network is made up of a constellation of around 70 satellites operating in Low Earth Orbit (LEO). The network of satellites was recently replaced, extending the service of this popular and profitable LEO satcom service well into the future.

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The air interface operates in the L-band between 1616 MHz and 1626.5 MHz and utilizes both Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). A typical Iridium device is a handset form-factor, similar to that of mobile phones, allowing users to make calls in a similar fashion. As the satellites are moving in relation to the Earth and visible up to around seven minutes per satellite, the call needs to handover between satellites. This is achieved quickly with no noticeable effect to the user. Terminal uplink power averages under 1 watt across a TDMA frame, but can reach 7 watts in a transmit slot. Ground level received signal power level can range between -110 dBm down to -120 dBm.

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Figure 1 Iridium radio signal observed on scanner with spectrum and waterfall plot, showing both FDMA and TDMA signal structure. Scanner was connected to a pre-amplifier and Iridium antenna.

Extending the signal

The vast majority of Iridium users will be in an open location with direct line of sight to the satellite. This is the application the system was primarily designed for. However, a select group of users will be in a location with no line of sight such as an aircraft hangar, secure building or other type of solid roofed structure.

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Figure 2 Iridium handset with direct line of sight to satellites

To solve this issue, in addition to the common application of using an Iridium based handset, there are some other products which take a “feed” from the Iridium satellite network e.g. Precision Network Timers manufactured by companies such as Orolia. Due to the high frequencies used and the link budget available, it may be necessary to have some type of signal repeater from the outside location through to the internal location near the Iridium receiving device.

Off-air radio repeaters can come in various formats; the simplest being two antennas connected by coaxial cable, through to more sophisticated systems with amplification and filtering. Some example Iridium antennas and docking stations can be found on the Iridium website.

Extending the signal using coaxial cable comes with its own challenges. Coaxial cable is good for shorter lengths e.g. less than 100 meters, but as length and frequency increase, so do the losses. A better solution is to use fiber optic cable, which offers a number of distinct advantages.

Some benefits of using RF over fiber:

• With much lower losses, it can be used for far longer distances. This also means much lower losses at higher frequencies and high bandwidths.
• Fiber optic cables are thinner, lighter and more immune to environmental changes. As they do not conduct electrical current, they are immune to electromagnetic interference and can suppress high voltage surges like lightning.
• Fiber based systems offer greater security and are more difficult to intercept. Attempts to “tap-off” light are immediately noticeable and alert the users.

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Figure 3 Fiber optic cable with FC/APC connector

RF over Fiber System Design

RF over Fiber (RFoF) is a technology that has been widely used and accepted in the satellite communications and broadcast industries for at least a couple of decades to move radio signals over fiber optic instead of coaxial cables. The main components are:

1. The transmitter which converts the electrical radio signal into an optical output
2. The receiver which converts the incoming optical signal back into an electrical signal
3. The fiber which links these two conversion modules.

By its conversion process, each link pair is unidirectional. For radio systems which separate the transmit and receive communications by means of frequency separation, the adoption of frequency duplexer filters make it simple to create the unidirectional paths needed by RFoF. An issue that then arises, however, is how to handle signals that are separated by time only.

TDMA-type signals

How can we handle TDMA type signals as used in the Iridium system, where the transmit and receive signals occupy the same frequency channel, but are only separated by time?

The ideal method would be to have a switch positioned just near the antenna which would switch to the uplink and downlink paths in full synchronicity with the TDMA traffic. This would give the best isolation between uplink and downlink paths. Practically though it is difficult to create a synchronizing clock from an off-air signal. Therefore, other methods need to be employed to provide good isolation between uplink transmit and downlink receive paths.

A simple method is to provide separation between the transmit and receive antennas. Free space loss at 1625 MHz is 36 dB for 1 meter and 46 dB for 3 meters. If the antennas also have a beam pattern with a lower gain in the horizontal (x-axis), this would provide more loss between the two antennas – again improving isolation.

An alternative method is to use RF circulators. An RF circulator is a three-port passive device used to control the direction of signal flow in a circuit and is a very effective mechanism to split out a transmit and receive path from a combined radio signal feed. This can be particularly useful if you are wanting to make a “hardwire” connection to the ground based terminal. An example architecture is shown in Figure 3.

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Figure 4 Example Iridium connection architecture utilizing RFoF

There are some other important points to consider when designing a good quality RFoF signal extension system. These are:

• Ensuring the gain in the uplink path is matched to the downlink path. Modern RF communication systems often use the received signal strength indicator (RSSI) to optimize the transmit power level, so it’s important not to imbalance this closed loop mechanism.
• Ensuring the gain in the system does not exceed the isolation in the diverse antennas or circulators. This is to reduce signal feedback ringing or “howl-around”.
• Ensuring signals, which can be presented to the output of amplifiers, are minimized to reduce intermodulation risk.
• Adequate filtering to reduce harmonic output content.

Ideas and techniques for distributed systems

Once the basic RFoF signal extension system solution is proven and performing correctly, one could consider using some other optical components to enable other Iridium RFoF extension based architectures, e.g. distributed systems.

ViaLite has significant experience in creating distributed GPS/GNSS systems, connecting a single rooftop antenna through to many end-points. A common application, in data centers, is connecting a single GPS antenna through to many Precision Network Timers (PNT) for use in financial services. These distribution systems make use of optical splitters. For applications needing many end-points, ViaLite is unique in offering a lossless optical splitter.

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Figure 5 Multizone distribution used in GPS systems

A similar approach used in GPS distribution could be applied in an Iridium type system and makes for a very simple distribution of the downlink to many devices. Combining uplinks from more than one device through to a single antenna requires careful handling (peak power, signal isolation, intermodulation reduction etc.). ViaLite can provide advice and solutions on how best to do this in the optical and radio elements.

Disclaimer: Final words of care needed with Iridium systems

The contents of this paper should be used as a guide and a demonstration of what could be possible using RFoF, but before embarking on creating solutions that repeat radio signals such as Iridium, a few words of care should be heeded:

• Any radio system which is transmitting via an antenna needs to comply with the relevant regulatory radio spectrum licensing and EMC test standards for the region it is operating in.
• Iridium will in addition have their own proprietary technical and commercial licensing requirements in order to receive and transmit on their network.

Written by Richard Jacklin, ViaLite Communications

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