User's Manual

3CC12426AAAA TQ BJA 01 Issue 01 - April 2001 - Draft 04 59/346
104
2.8.1 – Selecting BS IF cables
When selecting BS IF cables, the main concern is the IF cable signal loss of the indoor and outdoor cable
runs.
The system requires that all the components in the IF signal path between the IBS card and the X-Pol
RBS Tx/X-Pol RBS Rx (cables, connectors and equipment) provide a fixed decibel loss in each of the
downstream and upstream directions. System performance may be affected if the fixed loss is not met.
The downstream losses must total 27 dB ±1 dB at the upper frequency of the IF band used for the
downstream signal. The upstream losses must total 24 dB ±1 dB at the upper frequency of the IF band
used for the upstream signal.
Table 19 can be used to determine how much loss the IF cables must contribute in order to achieve the
fixed loss requirement. The table lists each component and gives its loss value at 2050 MHz downstream
and 900 MHz upstream. It is assumed that the installation uses a 3 m (10 ft) long SMA cable. The last
row in the table shows that the IF cables, including connectors, must contribute the following losses to
the system at the frequencies used in this example:
– 7.37 dB in the downstream path at 2050 MHz,
– 4.37 dB in the upstream path at 900 MHz.
Use the following formulas along with the cable and connector loss values given in Table 20 to determine
the types and lengths of cable that meet the fixed loss requirements and that are suitable for the
installation. Formula 2 accounts for four connectors (two connectors for each of the indoor- and outdoor-
rated cables).
Formula 1: Calculation of dB loss for a cable segment
Loss
segment
= Length
segment
X (dB loss/distance
segment
¸ 100)
where
Loss
segment
is the loss of the indoor or outdoor segment (in dB)
Length
segment
is the length of the segment (in m [ft])
dB loss/distance is the loss characteristic of the segment (dB/100 m [dB/100 ft])
Formula 2: Calculation of total dB loss for the cable run
Loss
total
= Loss
i
+ Loss
o
+ (2 X Loss
i
connector) + (2 x Loss
o
connector)
= 7.37 dB (at 2050 MHz downstream)
= 4.37 dB (at 900 MHz upstream)
Note - The loss characteristics of all components used in the examples in this section are
based on the worst-case frequencies of 2050 MHz downstream and 900 MHz upstream.
Refer to the manufacturers’ specifications when designing systems that use other
frequencies.
Note 1 - Before the cable is installed, ensure that the correct cable types have been chosen
for the required lengths and losses. Otherwise, new cables may need to be installed.
Note 2 - Non-plenum rated cables can be run through plenum space, if a suitable fireproof
plenum conduit is used.