Near / Far : Designing for Success

The Code

IFC 510 has recently been updated to include verbiage on antenna density, which is especially important when it comes to possible near-far issues. The new verbiage is in IFC 510.4.2.8: Radio Communication Antenna Density. It states systems shall be engineered to minimize the near-far effect. Radio enhancement systems shall include sufficient antenna density to address reduced gain concerns.  There are two exceptions noted: 1. Class A narrow band signal booster devices with independent AGC/ALC circuits per channel. 2. Systems where all portable devices within the same band use active power control features.

The Design

What this really means is simple: don’t under design your ERRC System. More antennas won’t be a bad thing for system performance (only for total cost). The benefit of more antennas is that you will have a more even uplink signal strength into your BDA, meaning you can have a lower gain setting, hence a lower noise floor coming out of the donor antenna on the BDA.

Every system will always have a “minimum” location and a “maximum” location. There are many ways to describe these two locations, but a simple way is stating your strongest downlink signal is the maximum location and your weakest downlink signal is the minimum location. Downlink is how we measure this, because this is a very easy measurement to make. The reverse link will be the same loss, so you can quickly calculate what the uplink signal levels into the BDA will be. ERRCS Design and Testing always factors in Near / Far effect when designing an ERRC system , learn more about our design services by clicking here

The Math

For example, if the downlink output of your BDA is 20dBm per channel, and your “maximum” measurement is -35dBm and your “minimum” measurement is -75dBm, then your total system losses between the BDA and mobile radio fall in the range of 55dB to 95dB. This gives you a 40dB range of uplink signal level inputs. So now we must consider: do we have a channelized BDA or a wideband BDA? If it is channelized, and the dynamic range of the BDA exceeds 40dB, then we will have a system that will function normally. If we have a wideband BDA, we need to look at the worst case.

The expected uplink signal into the BDA (assuming a 3W portable) will be about -20dBm to -60dBm. The first question to ask is: is -20dBm too high of an input signal? BDAs are usually built to amplify low signals, not high signals, so this could be on the higher side of the input range of many BDAs available today. Assuming that -20dBm is an okay input, we need to consider maximum output power. Let’s assume it is 1/2W (a typical output power on the uplink side of a 700/800 MHz BDA). If we transmit from both the “minimum” and “maximum” locations at the same time, the signal will be attenuated by the total input. At -20dBm and -60dBm, the total input power is rounded to -20dBm. If the gain is set to, say, 75dB, then we will be in wideband AGC, so our gain would be reduced to 47dB (the difference between +27dBm and -20dBm). That means our -60dBm signal, instead of being +15dBm as expected by our gain setting, would actually be reduced to -13dBm output from the BDA. 

In Closing

It is my estimation over 50% of all BDAs installed throughout the country would not pass testing if this near-far testing were to take place. This is unacceptable and needs to change. It is recommended to do two things (although it is understood that sometimes you can only do one of the two): increase your antenna density and use a Class A channelized BDA. At ERRCS Design, we will ensure all radio testing requirements will be met with your designs, and be able to provide the link budgets to prove the design will work.