Controlling Uplink ALC for a BDA

Often times, if an indoor BDA (Bi Directional Amplifier) system’s uplink is not properly optimized, it can create a lot of problem at the cell site due to uplink noise issues or excess power. BDAs often try to keep the Uplink (UL) and Downlink (DL) balanced, by keeping same gain in both directions or by a few dB offset. In this post, I am proposing to keep the BDA on its ALC (Automatic Level Control) feature and  limit the UL ALC to a level which can be calculated and then adjusted based on real situations. This discussion is for a CDMA system where power management is an important issue.

Assume that you want your uplink signal to hit the donor (i.e., Cell Site) BTS at -85 dB. Then for the above diagram, a basic link budget becomes,

ALCUL – Indoor side cable & other losses + Donor Antenna Gain – Free Space Path Loss – 
Clutter Loss – Fade Margin + Gain in Cell Site Antenna – Cell Site side cable & other losses = -85

Indoor side cable & other losses is the combined loss from all the elements from BDA to the donor antenna, this will include cable, connector, surge protector losses, etc. Same loss applies on the side of the cell site (i.e., Cell Site side cable & other losses)

Lets simplify the equation by assuming,

R = – Indoor side cable & other losses + Donor Antenna Gain – Clutter Loss – Fade Margin + Gain in Cell Site Antenna – Cell Site side cable & other losses

Overall value of R can be positive (when gain is more than loss) or negative when losses are more than gain.

So the equation becomes: ALCUL – Free Space Path Loss + R = -85

Using the Free Space Path Loss equation from the diagram,

ALCUL = 20 * Log10(F) + 20 * Log10(d) - 48.4 - R

For a typical CDMA solution, lets assume the following:

Frequency = 1920 MHz

Indoor side cable & other losses = 3 dB

Donor Antenna Gain for a corner reflector = 12 dB

Clutter loss and fade margin = 6 dB (when the distance between the donor antenna and the cell site is about a mile, lightly obstructed line of sight)

Gain in Cell Site Antenna = 17 dB

Cell Site side cable & other losses = 5 dB

Now we can calculate R.

R for this situation is equal to = 15 dB

Therefore, ALCUL =  2.266 + 20 * Log10(d)

For a distance of 1 mile between the cell site and indoor site’s donor antenna, ALCUL =  2.266. 
If I were in a situation like this I would first set the ALC for uplink at 2 or 3 dB and then start tweaking.


If you know the power output from the BTS (ask local RF) and input into the repeater, you could also figure out an approximate loss from the BTS to all the way to the repeater. So for example, if the BTS output is 43 dB and you are receiving -65 dB into the repeater. That means the total loss in the downlink is 108 dB [43-(-65)]. That means in order to have -85 at BTS, you can set the uplink ALC at 23 dB (-85+108). Anyway, things do not always go as calculated in the field, but these calculations can provide some starting point of optimization.