User's Manual
044-05156 Rev C 29
4.6.3.1 Standard Gain Overview and Application (Default Configuration)
In the standard gain mode, the gain of the system is dependent on the number of amplifiers operating at a given
time and the power from the amplifiers is used at or near their maximum limits. When the design of the cell site
requires full power from the amplifiers installed in the subrack in order to achieve maximum call capacity, the
standard gain mode should be used. For example, if the sector design calls for 150 watts (measured at the
directional coupler output; assuming 1.5 dB of loss from the subrack to the directional coupler), the sector should
be configured with two 125 watt amplifiers (154 watts at directional coupler; 51.5 dB subrack system gain) to take
full advantage of the available power. The trade-off here, is that if an amplifier fails, the available power drops back
to 77 watts, the overall gain drops by 3.0 dB, and all the transmit channel powers drop by 3.0 dB as well (i.e. 7.5
watts to 3.75 watts per channel).
Gain adjustment via the front panel potentiometer is available.
4.6.3.2 Constant Gain Overview and Application
In constant gain mode, the gain of the subrack remains the same, regardless of the number of amplifiers installed.
Good engineering practice requires careful planning when using constant gain mode. When the design of the cell
site requires much less than maximum power, but must maintain a specific radius of coverage, constant gain mode
is ideal. For example, if the sector design calls for 75 watts output power at the directional coupler, the sector must
be configured with two 125 watt amplifiers (87 watts; 48 dB gain) when constant gain is enabled to allow for
amplifier failure. If an amplifier fails, the available power drops back to 43 watts, the subrack gain adjusts to remain
48 dB (77 watts), and all the transmit channel powers remain at the set power (i.e. 7.5 watts per channel).
However, while in constant gain mode, if the sector power is set to maximum (i.e. 100 watts with two amplifiers
installed), and an amplifier fails, the remaining amplifier will be over-driven and likely to go into an over power
condition. Should this occur, the sector would go into APC control, the input power is attenuated (see paragraph 4-
5), the sector’s footprint shrinks and an alarm is sent to the switch.
Gain adjustment via the front panel potentiometer is available.
4.6.3.3 Gain Mode Control and Theory
The MCR21929-1-2 subrack adds 3 dB of attenuation when constant gain mode is initially activated, reducing the
system gain of the MCR21929-1-2 from a nominal of 51.5 dB to 48.5 dB. Therefore, whenever the gain mode is
changed from standard gain to constant gain or visa-versa, the cell technician must reset the overall system gain or
individual channel power.
In constant gain mode, the gain of each carrier (or the system) must remain constant to avoid reducing the cell
radius. When the subrack detects an amplifier failure, the amplifier subrack reduces input attenuation through the
Voltage Variable Attenuator (VVA) by the amount of gain lost while the amplifier is removed from the circuit. This
allows the system gain to remain constant, and allows the cell site to maintain a consistent footprint. When the
replaced MCPA is enabled, attenuation is again added to the input port of the subrack by the amount of gain
introduced by the replacement MCPA to maintain an overall subrack constant gain.
As MCPAs are disabled and enabled, system gain is recovered within a 1-second time frame. The subrack is
shipped from the factory with the configuration switch set for Standard Gain mode active.
4.6.4 Performance Derating With Lower Supply Voltage
The MCPA system will operate at full power, while meeting all spectrum requirements, over a supply voltage range
of 26 to 28 VDC. The MCPA System will meet derated spectrum requirements at derated output power levels over
a supply voltage range of 21 to 30 VDC. MCPA System power derating levels are outlined in below. The LED
indicator located on the top front right of the subrack blinks when the output power is derated.
Power Derating Versus Voltage Profile
Supply Voltage[VDC] Output Power Derating
28V ≤ V < 30V
0.5dB
26V ≤ V < 28V
0dB
24V ≤ V < 26V
0.5dB
22V ≤ V < 24V
1.0dB
21V ≤ V < 22V
1.5dB