Range Test Comparison between WLR089U module and SAMR34 chip-down XPRO

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 measurements. The first one is based on a conducted test setup using cables and attenuators in between Tx & Rx DUTs placed within GTEM Cells. The communication link between the GTEM cell’s input port and 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

Test Setup

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

GTEMcell.png
ShieldingRoom.png

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:
$P_{R}$ (dB) $= -20 log ({4\pi \over \lambda}) - 10 n log(d) + G_{T} + G_{R} + P_{T}$
Link budget = $20 log ({4\pi \over \lambda}) + 10 n log(d) + F_{\rm m}$

$F_{\rm m} =$ Fading margin, at least 15 dB
$d = distance$ (in m), $n = 2.5$ for open field
For 915 MHz and 868 MHz, $20 log ({4\pi \over \lambda})$ = ~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
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  1. 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.
  2. 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 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

1.5kmTest.png
  • 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 = $10 n log ({d \over 1500})$, where n = 2.8 for suburban environment
  • The range for 915 MHz is ~ 22.7 km and 868MHz is ~ 18.9 km.
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This is an extrapolated range estimation based on the measured RSSI value at 1.5 km, actual full 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.

RecommendedSetup.png

Tested Samples

TestedSamples.jpg

Antenna Used in the Measurement

Used antenna in WLR089U:

Used antenna in SAMR34 XPro:

Reference

Range calculation application note

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