Introduction to RF Signal Distribution using Fiber Optics

By Jack Daniel
The Jack Daniel Company
1-800-NON-TOLL
www.RFSolutions.com

Introduction

In recent years, the use of fiber optic (FO) cable in place of more conventional coaxial cables has become viable in many applications due to advances in analog fiber optic technology . This discussion is intended to make the typical wireless system designer familiar with fiber optic terms, specifications and devices as they would apply in a radio frequency (RF) system design.

For the purposes of introduction to FO technology some generalizations are used. When specific specifications are used, they are based on manufacturers published specifications available at the time this text was written, such as the current Foxcom Wireless specification sheets.

Please note that this presentation does not cover digital data or LAN type products or applications which use digital fiber interfaces instead of analog.

Why use Fiber Optic system for RF applications:

RF system designers are familiar with the two major limiting characteristics of coaxial cables: the RF loss increases with frequency and length. Coaxial cables are limited in the length that they may be used without additional amplification. Long lengths of coaxial cables, such as might be required in tunnels or building-to-building, rapidly become cost prohibitive, especially if in-line RF amplifiers are required.

Fiber optic cables have very low losses compared to coaxial cables. With RF over fiber distances of 10 miles or more being practical, the system designer has a new tool to help solve difficult RF distribution challenges that would normally be impractical using coaxial cables.

The fact that FO cables do not 'leak' or couple RF signals makes them ideal when routing through noisy RF environments or running long lengths parallel to other FO or RF cables. This same feature makes FO cables ideal when used in an application that cannot tolerate EME emissions, such as TEMPEST sites, etc.

The lack of electrical conductivity may also be attractive in some applications, such as electric utilities.

Fiber optic cables can be much smaller and lighter weight than corresponding RF cables and are non-metallic, making installation and routing much simpler.

Fiber Optic System Conventions and Components:

The basic components of a RF - FO system consists of (1) fiber cable (including associated connectors), (2) FO Transmitters and (2) FO Receivers. The transmitter and receiver components may be small stand-alone packages or multiple, rack mounted plug-in units. Additional FO devices, such as splitters and duplexers, that expand the system design options will be discussed later.

The Fiber Optic Cable:

Fiber optic cable comes in many different types, configurations and specifications. Rather than go into great detail of the many variations, we will concentrate on the most important basics relative to RF applications.

There are two primary 'modes' of fiber optic cable operation: 'Multimode' and 'Single mode'.

In general, multimode has been in use longer than single mode and meets the needs for shorter data communications applications where the data rate is comparatively low (i.e. <100 Mbps) when compared to RF signals (100 to 2200 MHz).

While multimode cable can be used for very short cable runs (less than 2000 ft, typically), the distance restriction can limit the usefulness and future expansion of a system.

Single mode is the preferred type of cable for RF applications, especially for longer cable runs where higher output laser type transmitters can be used and where very wide bandwidth and high RF frequencies are required. Single mode losses are less than multimode. It is practical to design a single mode fiber system that has very wide bandwidth with zero net loss, end to end, up to about 12 miles without any in-line amplification.

The physical installation considerations for FO cable is usually simpler than installing 1/2" corrugated coaxial cable, with comparable bending radius requirements. However, FO cable works best with minimum bends and turns of the cable, so installation in conduit or raceways is preferred to give the optimum support and maximum practical bending radius. These considerations are determined by the number of fibers in the same cable sheath and the mechanical construction of the cable. Fire resistant and Plenum rated cables are common. The costs of FO cable are dependent upon the number of fibers and the mechanical construction of the cable. It is common practice to include several spare fibers (called "dark fibers") for growth due to the small incremental cost increase.

Connectors for high performance single mode cables are unique and are generally less expensive than coaxial cable connectors. Most fiber connectors require special equipment to install the connectors or splices for minimum optical loss.

APC (Angle Polished Connectors) are used on single mode fiber to reduce the reflections back to the laser transmitter which could reduce efficiency or cause damage in extreme cases. In APC connectors the fiber end is slightly angled (8 degrees) so that any reflected light is highly attenuated instead of propagating freely within the fiber. Some training and special tools are required to properly attach and test APC connectors.

Unlike coax connectors, many popular versions, such as FC/APC types, are all the same gender and use a "sleeve" (the equivalent of a coax 'splice' or dual female connector) to complete each connection. Sleeves are purely mechanical with the optical coupling occurring dierctly between the butted ends of the two connectors.

For loss approximations, use 0.38 dB/Km or 0.515 dB/Mile for single mode fiber cable plus 0.25 dB loss per FC/APC connector. Note: A splice or in-line connection would use two connectors and one sleeve for a total loss of approximately 0.5 dB per splice.

FO Transmitters:

Most FO transmitters that are designed for data transmission are modulated in a 'digital' (off-on) two state manner. RF FO 'analog' transmitters however are linear in operation, the light source being amplitude modulated at the RF frequency. Analog FO transmitters suitable for wireless RF applications are less common and more expensive than simpler digital FO transmitters, mostly due to the additional circuits needed maintain high linearity and stability over temperature.

Analog FO transmitters may use LED emitters for moderate level output (shorter range/low fiber loss) applications and solid state laser emitters for high power output (longer distance/higher fiber loss) applications.

In older analog designs, the RF input frequency was down converted to a lower "IF" frequency because the linear bandwidth did not extend to the input frequency range. This approach inherently increased the probability of intermodulation, RF distortion and, in some cases, unacceptable RF envelope delays. Fortunately there is no longer a need to convert frequencies below 2200 MHz as improved modulators and emitters have become available.

Modern linear transmitters modulate the optical emitter at the RF frequency directly. RF system frequency bandwidths of 100 to 2200 MHz, or more, using one transmitter and one receiver over one fiber are commonly available at moderate cost.

FO Receivers:

Analog FO receivers are required to convert the FO signals back to RF frequencies. In more advanced designs, the gain of the receiver may be adjustable to prevent overdrive. Overdrive can distort the recovered RF signals. Receivers also include linear compensation circuits and are commonly available.

A basic FO - RF System:

In figure 1, the most basic one way system is shown.

1. The RF source is a decoupled signal from a RF transmitter (RF TX). The full output power of the RF transmitter would damage the FOT. Assuming the RF transmitter is also connected to an outside antenna, the decoupler used is a directional type to further reduce any unwanted signals that may be coupled via the antenna to the FOT. The -50 dB decoupler adds approximately 0.1 dB loss to the power going to the outside antenna. With the RF TX output level of + 52 dBm, we have a +2 dBm level in to the band pass filter, below.

2. A Band Pass Filter (BPF), with 2 dB insertion loss in the example, is used between the RF transmitter and FOT to further reduce any unwanted RF signals such as signals out of the filter passband coming from the antenna, spurious emissions and RF transmitter sideband noise. This approach reduces the possibility of intermodulation in the FO system and conserves optical power to the desired frequency band. Adding the 2 dB loss of the BPF gives a 0 dBm level to the input of the FOT.

This approach is particularly applicable to the combined output of several transmitters (i.e. SMR, AMPS, etc.) when a combline or other wide bandwidth filter is used. The decoupler is placed between the output of the transmitter combiner and the antenna coaxial cable. The amount of decoupling would be set so that the maximum possible output power (i.e. maximum composite power) of all the transmitters does not exceed the maximum allowable input to the FOT.

Most FO transmitters (FOT) are designed for a maximum RF input level of 0 dBm, which is easy to obtain from most base or repeater stations.

Over the air inputs to the FOT usually require additional filters and a low noise amplifier (LNA), such as those in TX RX Systems broadband or channelized signal boosters.

The 9/125 single mode fiber optic cable has an approximate loss of 0.515 dB/mile in the example.

It is important to know the connector losses if the FO cable because connectors and splices can account for the majority of optic loss in 'real' installations. In the example, an estimate of 0.5 dB was used for each paired connection (actually 0.25 dB per connector times two connectors per connection).

The FO receiver (FOR) in the example is designed for a 'zero net loss' system design with up to 9 dB cable loss. This method of stating the system performance is typical of fiber optic system specifications, which use a 'zero loss' basis to determine the optical loss budget in a system. In other words, they design towards a 'zero loss' fiber optic link overall. Naturally, the system may still operate adequately with less or more loss, dependent upon the dynamic range of the system.

The maximum FO receiver optical input level (i.e. that which generates 0 dBm RF output) is usually the highest level where linear reception is assured. If that level is exceeded, RF performance will deteriorate and could even damage the receiver if exceeded greatly. The system designer must avoid overdriving the FO receiver by reducing the gain of the FO receiver (assuming it has an adjustment) or increasing the optical circuit loss. Optical pads are available if needed and are used at the penalty of increasing the system noise floor slightly.

The system designer must also establish the lowest input level of each signal as that will be critical when determining signal to noise performance later. While the signal may remain within the dynamic range of the receiver, it may become too low relative to the noise floor of the system and degrade performance due to too reduced signal-to-noise ration (S/SN).

Another band pass filter is added to the receiver output and it may be a minimal filter or even deleted in some cases. The purpose of this filter is to simply reduce unwanted signals and noise that may effect later RF amplifiers or nearby RF receivers.

Since the FOR output should near 0 dBm in the example;

(1) It may be too much signal if it is fed directly to a RF receiver. A high value RF pad (i.e. 90 dB) would reduce the RF signal to -92 dBm (including the 2 dB loss of the second BPF) which is still a very strong RF receiver input level AND reduce the output noise floor at the same time.

(2) If the FO receiver output is to be sent to RF Receivers over the air, a RF amplifier (i.e. 'signal booster') will probably be required to overcome the free space losses and other signal perturbations that will occur over the air. A free space loss of -115 dB and a +25 dB signal booster amplifier is used in the example to deliver a -92 signal level to the RF receiver over the air.

HOW MANY FIBERS TO USE:

The optical signals used in the fiber optic cable have similar interference considerations as RF signals inside a coaxial cable. Since the FO cable is bi-directional two signals on the same optical frequency can't be separated from one another and cause interference to each other.

In wide bandwidth analog FO applications, 1310 nm is the most common and inexpensive optical signal frequency. 1310 nm is near the minimum loss frequency of single mode fibers, making longer distances possible. When one optical frequency is used, it simplifies testing and minimizes spare equipment models. That means two fibers can be used in a two-way system design, one for the 'downlink' (repeater transmit frequency) and one for the 'uplink' (the portable transmit frequency).

Since most fiber optic cable installation contain many 'pairs' (i.e. 6, 24, 50, etc.) in anticipation of future growth, the addition of a few more fibers has minimal effect on overall cable installation costs. Unused single mode fibers may already exist in a previously installed fiber cable.

However, if the optic fiber availability is limited, there is the equivalent of duplexing two optical signals onto one fiber, using a different frequency in each direction. The second frequency is usually 1550 nm. This frequency is also near the lowest loss frequency of single mode fibers, but slightly more expensive to implement in fiber optic transmitters, another reason for favoring 1310 nm if possible.

In fiber optic systems, a 1310/1550 nm duplexer is called a "WDM" which is the abbreviation for "Wavelength-Division Multiplexer". It has three ports: (1) 1310 nm, (2) 1550 nm and (3) duplexed (FO cable) port. It is connected using the standard FO connectors, discussed earlier. These are passive devices not requiring external power.

Unlike RF duplexers, WDM devices can have directional characteristics and come in different configurations. A "unidirectional' type has the two signals going the same direction, which is NOT normally applicable to our requirement. A "Bi-directional" type is most like a RF duplexer as seen below:

In Figure 6 note that each FO receiver output at the repeater site has individual pads to reduce the composite noise floor and provide FOR-to-FOR isolation and minimize receive combiner intermodulation. For example, if 40 dB pads are used, an additional 80 dB of combiner port-to-port isolation occurs.

In real applications, it is also good practice to include decoupled taps as test points to read the RF levels going into the combiner for test and maintenance. Be sure to include the RF combiner losses in signal level calculations and pad estimates.

A similar system design is possible when WDM's are used. If WDMs are used, the number of fibers is reduced by 50% but a WDM must be added at each remote site and another WDM for each fiber added at the repeater site. In the 4 remote site example, it would take 8 WDM's to operate all the fibers full duplex and 4 FO transmitters and 4 FO receivers would have to be 1550 nm models.

MORE COMPLEX SYSTEMS:

With the building blocks described here, much more complex systems can be developed. Many other devices are available to amplify, multiplex and split optical signals, however many are not suitable for linear operation.

System designers using proven products have the choice of;

- Several versions of stand alone, one way 'boxes'.

- Integrated duplex models with integral WDM's in one case.

- Rack mounted systems for more complex applications.

- In-building distribution systems with 'headend' units capable of interfacing with multiple remote fiber - RF 'remote hub units'.

With the many devices discussed above, it is possible to design RF distribution systems that were not possible or practical a few years ago.

One example are new FO devices that are designed to integrate the remote end components into one compact and inexpensive unit that has moderate output RF levels that compare to a distributed antenna system radiator. The big difference is no coaxial cable is required, only the fiber and a power source at the far end. The new Foxcom Litenna* product line is designed to perform these functions.

The devices described in this presentation are available as components or as a system application study from the Jack Daniel Company (wide band analog Fiber Optic Transmitters and receivers) and TX RX Systems (RF splitters. filters, and system design support) and other sources. Common items like fiber optic cables, connectors, WDM's and optic splitters are available from many qualified sources.

For additional information contact:

Jack Daniel
1-800-NON-TOLL
www.RFSolutions.com