Overview

A range test in wireless communications generally refers to the distance covered by a wireless link between a transmitter and a receiver, with acceptable packet loss and without breaking the link. The higher the operating frequency the lower the range will be, as high-frequency signals are more attenuated by free space than low-frequency signals are (e.g.: 2.4 GHz signals are attenuated more than Sub-1 GHz signals). So, if two signals (2.4 GHz and Sub-1 GHz) of the same Tx power are transmitted, the Sub-1 GHz signal (915 MHz or 868 MHz) will reach a longer distance than the 2.4 GHz signal. There is another factor that affects the range: antenna size. High-frequency signals (2.4 GHz) need a smaller antenna than lower-frequency signals (915 MHz) as the antenna size (e.g.: λ/4 for monopole antenna) is directly proportional to the wavelength of a signal and the wavelength is inversely proportional to the operating frequency. λ/4 is the required length for a monopole antenna to achieve long range. If a λ/10 size antenna is used for that application, the range will be reduced.

Next, the range of the same Tx & Rx differs based on the test environments (free space, office, industry, hospital, etc.) as there will be different obstacles that affect the signal strength by reflection, diffraction, and scattering; the range will be different for different environments. Here, we focused on two methods of measurement. The first one is based on a conducted test setup using cables and attenuators in between Tx and Rx DUTs placed within Gigahertz Transverse Electro Magnetic (GTEM) Cells. The communication link between the GTEM cell’s input port and Device Under Test (DUT) is radiated. The second one has been measured in an open area in a suburban environment, which is one of the real use cases.

Link budget measurement by GTEM cell
- Calculated range from the link-budget
Better range projection by real range measurement

Link budget measurement by GTEM cell

Test Setup

GTEM cells are used for link budget measurement including Antenna Gains (Tx & Rx)

Antenna Gain Test Setup

Sheilding Room

RF Path Loss Description

TEM Cell Path Loss (TCPL): 868 MHz: ~18 dB, 915 MHz:~19 dB
Insertion loss of the variable attenuator (IL_VA): ~6 dB
Total Coaxial Cable Loss (CL): ~2 dB
Link budget = 2* TCPL + IL_VA+ the attenuation of the variable attenuator + CL + 30 dB
868 MHz Link budget = 74 dB + the attenuation of the variable attenuator
915 MHz Link budget = 76 dB + the attenuation of the variable attenuator

WLR089U and SAMR34 XPRO Settings

Power level: 15 @868 MHz, 20 @915 MHz
Spread factor: SF12
LoRA bandwidth: 125 kHz
No. of packets: 100
1% Packet error rate

	915 MHz	868 MHz
WLR089 XPro	155 dB	152 dB
SAMR34 XPro	156 dB	152 dB

Both WLR089 XPro and SAMR34 Xplained Pro have similar performance in the Link Budget measurement.

Formula

Friis equation in decibel form:
PR (dB) =−20log(4πλ)−10nlog(d)+GT+GR+PT
Link budget = 20log(4πλ)+10nlog(d)+Fm

Fm= Fading margin, at least 15 dB
d=distance (in m), n=2.5 for open field
For 915 MHz and 868 MHz, 20log(4πλ) = ~32 dB

Calculated range from the link-budget

Fm = 15 dB	915 MHz	868 MHz
WLR089 XPro	20.9 km	15.8 km
SAMR34 XPro	22.9 km	15.8 km

Note that the calculated range from the link-budget is ideal, 1 dB difference in the link budget would not have a significant difference in the actual range. Thus, the performance of the WLR089U and SAMR34 XPro is similar.
The UFL antenna used with the WLR089 XPro has a ~15 cm UFL cable, which introduces additional loss when compared to the SAMR34 XPro using an SMA antenna. So, it is expected for the WLR089U to have a slightly lower link budget.

Better Range Projection by Real Range Measurement at 1.5 km

1.5 km RSSI test :

Read the average Received Signal Strength Indicator (RSSI) value in RX at a distance of 1.5 km from TX. Next, calculate the sensitivity margin to estimate the range. DUT is placed at a height of 1.5 m from the ground.

1.5Km Test Photograph

The sensitivity with SF12 for 915 MHz and 868 MHz is -136 dBm.
The average RSSI reading for both WLR089U and SAMR34 XPro is ~ -103 dBm at 915 MHz and ~ -105 dBm at 868 MHz.
The sensitivity margin is 33 dB at 915 MHz and 31 dB at 868 MHz.
sensitivity margin = 10nlog(d1500), where n = 2.8 for suburban environment
The range for 915 MHz is ~ 22.7 km and 868MHz is ~ 18.9 km.

This is an extrapolated range estimation based on the measured RSSI value at 1.5 km. The actual range will vary based on the test environments.

Recommended Range Test Setup

It is suggested to test in an open area with fewer to no obstacles between Tx and Rx.

Recomended Setup

Tested Samples

Samples under Test

Antenna Used in the Measurement

Used antenna in WLR089U:

RFA-S1-C55H1-150D034 from ALEAD technology
Peak antenna gain: 2 dBi

Used antenna in SAMR34 XPro:

RFA-ZW-C55-B-RP-D034 from ALEAD technology
Peak antenna gain: 2 dBi

Learn More

Range calculation application note