User's Manual

5-30 InterReach Unison Accel Installation, Operation, and Reference Manual PN 9000-10
620021-0 Rev. A
5.4.1 Elements of a Link Budget for Narrowband Standards
The link budget represents a typical calculation that might be used to determine how
much path loss can be afforded in a Unison design. This link budget analyzes both the
downlink and uplink paths. For most configurations, the downlink requires lower
path loss and is therefore the limiting factor in the system design. It is for this reason
that a predetermined “design goal” for the downlink is sufficient to predict coverage
distance.
The link budget is organized in a simple manner: the transmitted power is calculated,
the airlink losses due to fading and body loss are summed, and the receiver sensitivity
(minimum level a signal can be received for acceptable call quality) is calculated. The
maximum allowable path loss (in dB) is the difference between the transmitted
power, less the airlink losses, and the receiver sensitivity. From the path loss, the
maximum coverage distance can be estimated using the path loss formula presented
in Section 5.2.1.
Table 5-30 provides link budget considerations for narrowband systems.
Table 5-30 Link Budget Considerations for Narrowband Systems
Consideration Description
BTS Transmit Power The power per carrier transmitted from the base station output
Attenuation between
BTS and Unison
This includes all losses: cable, attenuator, splitter/combiner, and so forth.
On the downlink, attenuation must be chosen so that the maximum power per carrier going into the
Main Hub does not exceed the levels given in Section 5.1.
On the uplink, attenuation is chosen to keep the maximum uplink signal and noise level low enough
to prevent base station alarms but small enough not to cause degradation in the system sensitivity.
If the Unison noise figure minus the attenuation is at least 10 dB higher than the BTS noise figure,
the system noise figure will be approximately that of Unison alone. See Section 5.5 for ways to inde-
pendently set the uplink and downlink attenuations between the base station and Unison.
Antenna Gain The radiated output power includes antenna gain. For example, if you use a 3 dBi antenna at the
RAU that is transmitting 0 dBm per carrier, the effective radiated power (relative to an isotropic
radiator) is 3 dBm per carrier.
BTS Noise Figure This is the effective noise floor of the base station input (usually base station sensitivity is this effec-
tive noise floor plus a certain C/I ratio).
Unison Noise Figure This is Unison’s uplink noise figure, which varies depending on the number of Hubs and RAUs, and
the frequency band. Unison’s uplink noise figure is specified for a 1-1-4 configuration. Thus, the
noise figure for a Unison system (or multiple systems whose uplink ports are power combined) will
be NF(1-1-4) + 10*log(# of Hubs). This represents an upper-bound because the noise figure is lower
if any of the Hub’s RAU ports are not used.