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Optimizing LoRa radio performance for embedded devices

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December 19, 2017

clamb3211rwilliams,-December 19, 2017


Long Range Communications on the Horizon (Source: Pxhere.com/CC0)

Introduction

Whether developing a wearable device or industrial battery powered equipment, maximizing range and robustness while minimizing power consumption is critical.  Optimizing RF performance increases flexibility and enables more appealing trade-offs regarding size, battery-life and RF performance.  

After optimizing RF performance, the product development team can consider lowering transmit power to increase battery life or decreasing battery capacity to reduce the product size or perhaps operating exclusively on harvested power and eliminate batteries completely.

Link Budget & Path Loss

So, what factors determine RF range and performance?  Let’s start by examining the link budget.  The link budget is the difference between the strength of the transmitted signal and the minimum required signal at the receiver and equals the total loss from all sources at the maximum range. The simplest equation for link budget is (Figure 1):  


Figure 1: LinkBudget Basic Elements (Source: Device Solutions Inc)

For a typical LoRa radio implementation:  

This configuration provides a link budget of 150dB.

Before range can be estimated using path loss calculations, there are other factors to consider:

  • Transmitter antenna gain in dB, if positive, increases the link budget
  • Receiver antenna gain in dB, if positive, increases the link budget
  • Loss between the transmitter output and the antenna decreases the link budget
  • Loss between the receiver input and the antenna decreases the link budget

Including all these factors provides the link budget available for path loss (Figure 2):


Figure 2: LinkBudget Intermediate Elements (Source: Device Solutions Inc)

Antenna gain is typically expressed in dB relative to an isotropic antenna (dBi), an antenna that radiates equally in all directions.  Typically, an antenna data sheet specifies the “peak gain”, indicating how well the antenna radiates in the optimum direction and the “average gain”, representing the antenna’s effective radiation averaged over all directions.  Generally average gain should be utilized unless the orientation of the devices can be controlled to realize the “peak gain”.  Average antenna gain is equivalent to efficiency, thus, an antenna with an average gain of -3dB is 50% efficient, which can be a more intuitive way to visualize the impact of the antenna performance.  Antenna gain (transmitter or receiver) of -4dB is typical for a compact LoRa device.  If implemented carefully and compactly, receiver and transmitter losses should be approximately 1dB each.  However, the loss can be much higher if the antenna is not well matched to the transmitter and receiver circuits.

Power can only be transferred from the transmitter to the antenna efficiently if the transmitter output impedance is closely matched to the input impedance “load” seen by the transmitter.  That load includes the PCB trace, antenna and any components in the RF path connected to the transmitter’s output pin.  Typically, there is a matching circuit used to transform the antenna impedance (at the desired frequency) to the transmission line characteristic impedance on the PCB and another matching circuit to transform the PCB transmission line impedance (typically 50Ω) to an optimum impedance for the transmitter.  If the antenna and amplifier are poorly matched, then the transmit signal will not be efficiently transferred to the antenna, reducing range. When poorly matched, the transmitter will consume more current, decreasing battery life, and may generate increased harmonics.  Additional harmonic radiation exacerbates the challenge of regulatory approval and may require additional filtering to mitigate - which increases PCB area, increases loss and increases costs.

Combining the typical numbers with the LoRa example mentioned above yields (Figure 3):


Figure 3: LinkBudget Detailed Elements (Source: Device Solutions Inc)

At least 6dB should be subtracted from the link budget to provide margin for real world conditions and operational robustness.  Therefore, in this example, propagation loss at maximum range is approximately 134 dB.

The development team’s decisions directly impact many of components of the link budget and the team can make trade-offs to increase range or to reduce power consumption. Options include increasing transmitter output power or antenna gain, improving receiver sensitivity or minimizing loss. These choices may increase the size and cost of the radio implementation, the battery or the antenna, but it’s important to deliberately consider the performance impact of each decision. Optimizing performance could make the difference between achieving the desired range within the regulatory power limits or being forced to compromise on range to stay within allowed limits.

These trade-offs can be especially difficult when developing wearables, which are extremely size and cost constrained, demand maximum battery life, minimum size and are further constrained by regulatory (FCC, RED) requirements to minimize the RF energy absorbed by the user, known as “Specific Absorption Rate” or SAR.  Cellular devices are further complicated by carrier and industry requirements which require highly optimized antenna performance and high transmit power (compared to Bluetooth or WiFi) while still meeting the SAR limits.  Meeting these requirements, within in a commercially viable package is extremely challenging.

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