LoRa Spreading Factors & Throughput — What the Numbers Really Mean

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Read the full technical analysis:

https://www.vladavramut.com/articles/lora-spreading-factors-throughput.html

LoRa spreading factors (SFs) are often treated as magical settings that trade range for speed. But the real behavior of SFs in deployed networks is a function of physics, airtime limits, collision domains, and regulatory duty cycles — not just a knob you turn for “better range.”

Understanding SFs means understanding how LoRa modulation actually interacts with shared spectrum.

What Spreading Factors Actually Do

Each LoRa spreading factor increases:

• symbol duration


• processing gain


• airtime per packet

Higher SF values improve sensitivity — which can increase link margins — but they also inflate airtime dramatically.

This has three real consequences:

1) Longer on-air transmission time


2) Higher collision probability


3) Stricter duty-cycle impact

These physical effects dominate real throughput behavior, especially in mesh and multi-hop networks.

Airtime: The Real Cost of Higher SFs

A common misconception is that higher spreading factors boost throughput because they improve reception. In reality, they do:

• increase packet duration


• increase duty cycle occupation


• reduce usable channel capacity


• escalate collision domains

For a simple comparison:

• SF7 might occupy the channel for tens of milliseconds per packet
  


• SF12 can occupy the channel for multiple seconds per packet

This difference isn’t a tweak — it is a scaling effect that collapses usable capacity once traffic increases.

Collision Domains in Shared Spectrum

LoRa modulation uses a shared medium.

When one node transmits:

• everyone else on that channel cannot


• airtime reservation blocks other packets


• collision risk rises with high SF usage

In simple star networks, this is tolerable. In multi-hop meshes, it becomes a design constraint.

High SFs turn a low-traffic link into a throughput choke point, not a performance improvement.

Regulatory & Duty-Cycle Constraints

Across most jurisdictions, LoRa channels are limited by duty-cycle regulations.

In Europe (ETSI):

• duty limits restrict how long a radio can occupy a channel


• higher SF transmissions consume a disproportionate share

In the US (FCC):

• different constraints still bind cumulative airtime


• overlapping channels complicate spectrum occupancy

Designing for real networks means planning SF usage within these limits — not assuming free-range operation.

Continue reading the full technical analysis:

https://www.vladavramut.com/articles/lora-spreading-factors-throughput.html