Install Manual

BRITECELL System Manual MN010-04 June 2003 Page 73 of 78
The company has a policy of continuous product development and improvement and we therefore
reserve the right to vary any information quoted without prior notice.
65.2. Fibre-optic transmission
Britecell uses an analogue modulation scheme for the transmission of the
composite RF signal through the fibre optic link; the optical carrier is intensity
modulated.
Distortion is critical in cellular systems due to the multiple carriers present and the
regular channel spacing ensures that the products usually occur on the frequency
of another channel.
To minimise these problems Britecell uses high performance single mode lasers as
RF to optical transducers, the characteristics of which are dominant in the link
performance. Single mode optical fibre (SMOF) is employed due to its very low
loss, enabling signals to be carried for long distances when matched with sensitive
optical receivers.
With Britecell optical links in excess of 1.5 km can be achieved without appreciable
degradation of signal characteristics. Care must be taken to keep back reflection of
the optical carrier below -36 dBC to avoid additional link noise and signal
distortion.
We can detect two contributions to back reflection in optical links:
Microscopic variations in the refractive index in the fibre cause scattering of the
light; this depends on fibre length and can be kept within allowable limits by
limiting the length.
Concentrated reflections, due to refractive index discontinuity’s in the fibre are the
dominant source of returned light. These occur mainly at optical connectors so, to
reduce the severity, Britecell uses angled connectors, which direct reflected light
out of the fibre core. Only a negligible portion of light from the interface is
reflected back towards the source; return losses of greater than 60 dB can be
achieved with angled optical connectors.
In case that the system operates on a digital standard, a fundamental parameter
for the transparency is propagation delay through fibre, especially when the indoor
coverage extends an existing outdoor coverage of a BTS because of the need for
an outdoor-indoor overlap area. In this case differential delay between the direct
and repeated paths must be minimised. For example, the upper limit for GSM
systems is 8µs.
The velocity factor of silica glass optical fibre is 0.65, giving a propagation delay in
the fibre of 5ms/km; the Britecell equipment has a negligible contribution.
Optical fibre needs only a little care to be taken in system implementation and
maintenance to avoid possible malfunction. It offers many major advantages in in-
building coverage design, apart from the ability to carry signals long distances.
Significant additional benefits are:
• High bandwidth capacity
• Electromagnetic Compatibility (EMC) of fibre optics is inherent in all
environments; fibre is insensitive to electromagnetic fields, lightning or
radiation and produces no interference to other equipment.
• SMOF cables for indoor applications can be very thin, easy to install and to
hide; thus they permit a low visual impact on the environment and a
minimum effort in their laying.
• Support structures can be minimal and no plenum-rated jacketing is
required for use inside structures (unless copper conductors are
incorporated).
Refer to the following sections for more information on the design and installation
fibre cabling in Britecell systems