Designing DAS in Pilot Pollution Environment

For CDMA network, an indoor area with Pilot Pollution usually means it's an area with high signal strength, but low signal quality. This is a common phenomenon in high rise buildings in Houston (Houston, we have a problem!)

In CDMA network RSSI (Received Signal Strength Indicator) at a point is the total CDMA power received. So if at any point you are getting a RSSI of -65 dBm on a Sprint phone, that RSSI is the sum of RF Power received at that point point from all the Sprint base stations your handset can see or sniff.

Ec/Io on the other hand is an indicator of signal quality. Ec/Io at any point for a particular server (or base station) is the contribution of power from that base station divided by combined received power at that point from all base stations.

So if your phone is getting a Ec/Io of -3 dB from Sector 1 of a base station, then that means 50% of received power at that point is coming from that particular sector. Here, -3 dB translates into 1/2 or 50%.

Now lets say we are indoor and at Point X our current RSSI is -65 dB. So the signal strength is good, but the quality is bad. Lets say that the Ec/Io of the dominant server is -13 dB; this means, at point X, contribution of the signal strength from the most dominant base station is only -13 dB or 1/20th or 5% of the cumulative signal received. This is a typical indoor area with Pilot Pollution - high signal strength, low signal qulaity! You have been asked to design a DAS so that at point X the Ec/Io for the dominant server becomes -3 dB? What RSSI should you design for? And, what will be the final RSSI? Theoretically of course!

Check out the figure below.

So according to the Ec/Io equation, when the power received at point X from Antenna A, RA becomes equal to the current cumulative power we will get an Ec/Io of -3 dB.
In other words, if RA = (R1 + R2 + R3 + R4), then we end up with an Ec/I0 of ½ or 50% or -3 dB.

If we can make sure that point X will receive -65 dBm from Antenna A, we will get an Ec/Io of -3 dB. That means we should design for -65 dBm. Interestingly, however, your final RSSI will become -62 dBm (in dB math, -65 dBm + -65 dBm = -62 dBm) - that is 3 dB higher than the ambient RSSI.

Basically, on a theoretical basis, if I am in a RF environment where we have high ambient RSSI with no clear dominant server (i.e., PN) or bad Ec/Io, and if I am planning a DAS for this indoor area, then we can set a design goal of RSSI to match the ambient RSSI.