Radio Link Calculation for ISM RF Products

Abstract: This article provides a customizable spreadsheet for calculating link performance for Maxim's industrial, scientific, and medical radio band (ISM-RF) products. The spreadsheet takes as input the frequency, transmitter and receiver specifications, and RF channel performance, and calculates performance for free space, outdoor flat ground, and indoor transmission scenarios. The spreadsheet can also be used to estimate link margins for carrier frequencies ranging from 100MHz to 10GHz.

Overview

The Link Budget spreadsheet is used to calculate the link performance of Maxim's industrial, scientific, and medical radio band (ISM-RF) products (Tx, Rx, TRx) and estimate the communication coverage and link margin of a specific RF circuit in several environments. This Excel® spreadsheet can also be used to estimate the link margin of other RF systems in the carrier frequency range of 100MHz to 10GHz. When using this spreadsheet, the user needs to enter the following information:

Carrier frequency

Transmit power

Cable and Connector Losses

Antenna gain and efficiency (receive and transmit)

Transmission delay between free space and flat ground

Receiver and transmitter height

Receiver sensitivity

Obstacle loss

Multipath loss

We will also upgrade the link budget table and add new features to further enrich the existing features, including:

Connector loss information

Calculating multipath loss using scattering models

Transmission medium (humidity, conductivity, dielectric constant, human/animal surface tissue, leaves, etc.) loss

This article briefly introduces some assumptions and mathematical background about the transmission path and provides how to use the link budget table.

Transmission path loss

The spreadsheet includes two basic transmission paths: free space and flat earth. For wireless communications inside buildings and on city streets, multipath, obstacles, and penetration losses must also be considered. Maxim's ISM-RF products are well suited for locations where the wireless unit is above ground, such as parking lots, streets, open fields, and inside buildings. This means that the flat earth model is more suitable for estimating the communication link in such situations. For applications where the transceiver is located on a tower or rooftop, the antenna has a narrow beam, and the free space model is more appropriate for calculations.

If the RF signal is transmitted very close to the ground in the vertical direction relative to the required horizontal transmission distance, its transmission path needs to consider two parts of the signal: the direct (line of sight) transmission signal and the ground reflection signal. The phase of the electromagnetic wave reflected by the ground is opposite to the phase of the electromagnetic wave transmitted directly. The free space transmission model does not take into account the ground reflection.

The transmission path loss formula in free space is:

PR = PTGTGRλ²/(4πR)²(Formula 1)

Where PR is the received power, PT is the transmitted power, GT is the antenna gain of the transmitter, GR is the antenna gain of the receiver, R is the transmission distance, and λ is the wavelength.

The transmission path loss formula for flat ground is:

PR = ½PTGTGRλ²/(4πR)² (1 + a² - 2acos(2πΔR/λ))(Formula 2)

Where ΔR is the distance difference between the direct propagation path and the ground reflection path, and “a” (≤ 1) is the relative strength of the ground reflection path.

ΔR = √(R² + (h2 + h1)²) - √(R² + (h2 - h1)²) (Equation 3)

Note that Equation 2 is derived from Equation 1. Considering the influence of ground reflection loss, the calculation is as follows:

LGB = ½(1 + a² - 2acos(2πΔR/λ))(Equation 4)

In the case of short-distance communication, the path distance difference ΔR is greater than or equal to half the wavelength, LGB changes rapidly with R, and the received power fluctuates significantly. In long-distance communication (usually more than 30 meters), LGB changes with R-2, and the received power is inversely proportional to the fourth power of the transmission distance on flat ground (Equation 2).

The link budget table provides two path loss calculation formulas, which users can choose.

User Form Description

There are five tables in the calculation table that are used for calculations or to provide guidance information to the user:

Link Budget

Link Diagram

Ground multipath

Cable loss

Obstacle loss

Of these tables, only the Link Budget and Ground Multipath tables require user input; the Cable Loss table contains loss indicators for common cables and connectors; and the Obstacle table contains attenuation of walls and glass inside buildings, as well as losses of outdoor buildings, forests, and vegetation. These values ​​can be used to perform calculations and enter the losses of cables and obstacles in the Link Budget table. Connector losses are usually less than 1 dB and can be entered directly without the need for additional tables. The Link Diagram table describes the losses introduced by hardware and on-board transmission.

The ground multipath table requires the user to enter the transmitter and receiver altitudes, the path loss vs. distance, and the losses associated with transmission over the ground.

The input of the calculation table uses different colors to distinguish different parameter sources.

Black: User input data

Dark red: constants, such as the speed of light

Blue: Calculation results

Green: Values ​​obtained from other tables

Use of Calculation Tables

Open the Link Budget table of the calculation spreadsheet, with a screenshot at the end of this article.

Enter the carrier frequency in MHz. The spreadsheet will calculate the wavelength.

Enter the transmit power of the transmitter power amplifier. Measure or estimate the power value and measure as close to the power output pin of the power amplifier as possible.

Enter the Tx matching loss (if any). Most transmitters require several passive components to transform the antenna impedance to optimize the transmitter match.

Enter any cable and connector losses between the transmit circuitry and the antenna. The spreadsheet displays the input power to the transmit antenna.

Enter the gain of the transmitting antenna, including the efficiency of the antenna, additional impedance matching losses and changes in the antenna directional template. Any antenna size less than 0.1 wavelengths will introduce attenuation, not gain.

Enter the required transmission distance in meters.

If necessary, enter the transmission medium loss, for example, when the signal is transmitted through a medium other than air, or when the signal frequency is too high (greater than 2GHz), water vapor and molecules will absorb the signal energy.

Enter the heights of the transmitter antenna and the receiver antenna into the ground multipath table.

Return to the link budget table, which shows the path loss for the free space and flat earth models at the specified distance.

The antenna received power calculated based on free space loss is located in the upper rows of the results calculated based on the flat earth model loss. If the link is a free space link, the free space loss is used for calculation without considering the flat earth model loss.

Enter an estimate of the multipath loss (reflections and scattering along the path). Unless the path is flat and clear (for example, an open area or an empty parking lot), the loss will be higher than 20dB.

Enter an estimate of the obstacle loss.

The antenna received power calculated based on the flat earth model loss is located a few rows below the results calculated based on free space loss.

Enter the receive antenna gain. The same efficiency rules for transmit antenna gain apply here.

Taking into account any connector and cable losses between the antenna and the receive circuitry, the final Rx power at the receiver input is expressed as free space loss and flat-earth model loss.

The input area to the right of Rx Power is the receiver sensitivity, which is the minimum signal level at which the receiver can correctly process link information. When the signal level received by the path (free space or flat earth model) equals this sensitivity value, the entered distance represents the maximum link range. Adjust this range as needed to keep the received signal level consistent with the sensitivity.

To determine the sensitivity value to enter into this region, use the receiver sensitivity calculation function in the link budget table, or select a value from the Maximum RX table in the link diagram. The three input parameters required to calculate receiver sensitivity and SNR are noise figure, receive bandwidth, and operating temperature.

Example 1: Remote Keyless Entry (RKE) control link

Figure 1 shows a link budget table for a 315MHz RKE control link; Figures 2 and 3 show the relationship between link loss and distance for a given Tx and Rx height for ground multipath tables. The meaning of these tables will be discussed later.

Figure 1. Link budget table for 315MHz RKE control link