The design of wireless systems can be a complex task. Environmental variables create difficult to predict losses that can significantly compromise system performance. These effects are compounded in mobile applications where these environmental losses are constantly changing. The factors that affect the performance of an RF path, and thus the effective range of the system are many.
- Antenna polarity
- Cable quality
- Air quality (humidity, temperature, pollution)
- Presence and composition of obstructions (walls, trees, etc.)
- Carrier frequency
- Transmit power
- Receiver Sensitivity
- Proximity of Interferers
Given the variability of application environments, it is impossible to fully characterize the performance of any RF system in all situations. There are some relatively simple calculations that can be performed to predict the approximate range performance by applying a few parameters to a simplified model.
Path Loss Model
The wireless communication path can be generalized by the following model.
This model illustrates the path a wireless signal takes from a single transmitter to a single receiver. While many systems are more complex (fully bi-directional and/or containing many nodes), this model is a useful tool to estimate performance. Typically, the same radios are used on each side of the link, so that the loss in one directional is identical to that in the reverse direction.
Determine the appropriate RF performance parameters for the components in your system
Parameter Description Typical Data Source Transmit Power Conducted RF power from the transmitting radio. This is typically expressed in milliwatts (mW) or milli-decibels (dBm). Configuration tool to determine current setting Receive Sensitivity The minimum signal amplitude that the receiver is sensitive to. This number should be accompanied by the error rate at which it was achieved. Typically this is 10-6 bit error rate (BER). This is typically a negative number expressed in dBm. Device data sheets Fade Margin The amplitude of the signal above the minimum receive sensitivity required to ensure reliable communcations. Minimum of 25 dB. 35dB for conservative estimates. Cable & Connector Losses The losses the signal will experience while traveling through any cabling or connectors added to the system. Component data sheets Antenna Gain
The effective amplification (or attenuation) provided by the attached antenna. These may be different on each side of the link.
Higher Gain Reduces Coverage
It is important to note that higher antenna gain comes at the price of reduced coverage. In applications where the mobility is restricted, this can be very valuable. In applications with highly mobile nodes, high gain antennas may not be feasible.
1.6 dBi for HRI supplied omni-directional (whip) antennas. For other antennas, consult their supplier.
Apply these values to the link model shown earlier. All values need to be converted to decibels (dB).
Effective Transmit Power = (transmit power) + (transmitter antenna gain) + (transmitter cable & connector losses)
Effective Receiver Sensitivity = (receiver antenna gain) + (receiver cable & connector losses) + (receive sensitivity)
Allowable Path Loss = (Effective Transmit Power) + (Effective Receiver Sensitivity) - (fade margin)
The resulting value is the loss that can occur in the distance between the transmitting antenna and receiving antenna.
Watts to Decibel Conversion
A watt to decibel conversion calculator can be found here: http://www.rapidtables.com/electric/dBW.htm
- Convert the path loss to an ideal distance.
This calculator cane used to convert the path loss result from the previous step to a distance in free space.
This is an idealized range in optimal conditions, but can be used as a guideline for system design.