Knowing dBm from dBi

So we understand that outdoor wireless equipment uses electromagnetic waves to transmit data from one point to another, and in doing so, you can add a power ratio to the output power of your radio.  This power ratio is known as dBm.  We use dBm to express the large to very small numbers of power in which outdoor wireless equipment can operate in.

To convert milliwatt (mW) to dBm, we use the rules of 3’s and 10’s, with 0dBM being equal to 1mW.

If you add 3dB, you have to double the mW value and if you add 10dB, you have to multiply your mW value with 10.  For example, going from 0dB to 3dB, you will go from 1mW to 2mW (double).  If you go from 3db to 13db, you will go from 2mW to 20mW (times 10). Likewise if you decrease with 3dB, from 0dB to -3dB, you will go from 1mW to 0.5mW (half) and if you decrease with 10dBm, from -3dBm to -13dBm, you will go from 0.5mW to 0.05mW (divided by 10).

Figure 1. dBm vs mW (www.rfcafe.com)

dBi, on the other hand, is applied to antennas, and this indicates the gain of an antenna.  A low gain antenna, for example, an 8dBi antenna, will have a large beamwidth or radiation pattern.  A high gain antenna will have a narrow or more focused beamwidth.  A high gain antenna is useful in a Point-to-Multipoint (PtMP) scenario where you have one customer that is far away from your access point with low link quality.  By increasing the gain of the antenna at the customer’s premises, you will improve the link quality and the overall performance of your sector network.

So, how does this all fit together?  In our previous blog post, we mentioned that we have to stay within the regulations set out by ICASA and part of this is known as your EIRP or Effective Isotropic Radiated Power.  The total amount of power leaving the RF system is known as the EIRP and is calculated by adding your transmit (TX) Power (dBm from the radio) and the transmit (TX) Gain (dBi from your antenna) and deducting any losses (referring to cable loss and connector loss which is usually measured at about 1dB).

Figure 2: EIRP Formula (www.everythingrf.com)

With that being said, we have to also take FSPL (free space path loss) into consideration.  Free Space Path Loss phenomenon is when the electromagnetic wave propagating from the antenna expands outwards and loses power over distance.  This is an elementary characteristic of waves moving through a dense medium like earth’s atmosphere.

FPSL is taken into consideration when calculating your receive (RX) signal strength, by adding your TX power, TX Gain, minus FSPL (taking distance and frequency into consideration) and adding your RX sensitivity one can calculate the expected link distance.

Figure 3: FSPL Formula (www.ece.nu, 2013)

An important requirement for achieving the best possible performance of your outdoor wireless network is keeping a clear line of sight (LoS).  LoS refers to the elliptically shaped area between the two ends of your link and these areas are divided into progressively larger zones called Fresnel zones.

Within these Zones, obstructions like trees and buildings can have a negative impact on the performance of your network by causing scattered or reflected signals.  A good practice rule is that the first 60% of your Fresnel zones must remain free of obstructions for a successful link.

Figure 4: LoS and Fresnel Zone (www.vias.org).

All of this might sound like a calculus exam, but it needn’t be.  Most outdoor wireless vendors offer some form of link planning tool that takes all these factors into consideration and helps you plan your outdoor wireless network for the best possible performance.

In our next blog post, we will focus on the channel, channel width, and understanding co-location, channel re-use patterns.

References:

http://ece.nu/2014/03/

https://www.everythingrf.com/rf-calculators/eirp-effective-isotropic-radiated-power

http://www.rfcafe.com/references/electrical/decibel-tutorial.htm

http://www.vias.org/wirelessnetw/wndw_04_08b.html