3GPP TR 36.873
Understanding 3GPP TR 36.873
In the early days of 4G LTE, a cell tower antenna was physically 'dumb.' It blasted a signal in a massive 120-degree wedge. While it was highly optimized in the Horizontal (Azimuth) plane, the Vertical (Elevation) plane was static. The engineers would physically tilt the antenna down a few degrees to cover the street, and that was it.
This completely broke down in high-rise cities like New York or Tokyo.
If a tower aimed the beam at the street, people on the 40th floor of the skyscraper had zero signal. To fix this, the 3GPP invented 3D MIMO (also known as Full-Dimension MIMO), allowing the tower to dynamically steer the beam up and down. But to test this new hardware, they needed a new mathematical model. They wrote TR 36.873.
The Addition of the Z-Axis
Older models (like the 2D SCM) only calculated multipath scattering horizontally. If a wave bounced off a building to the left, it calculated the delay.
TR 36.873 completely rewrote the math to include the Elevation Angle (the Z-axis). It forced RF simulation software to calculate how a radio wave scatters when it bounces off the ground and shoots vertically up into the 20th floor of a building.
The High-Rise Scenario
The most famous addition in this document was the 3D-UMa (Urban Macro) environment. It explicitly required engineers to simulate cell towers interacting with high-rise structures.
- The simulation software had to mathematically calculate the path loss of a signal hitting the 1st floor versus the path loss of hitting the 15th floor.
- It proved that an active 3D MIMO antenna could successfully decouple users vertically. The tower could shoot one beam down at a pedestrian on the sidewalk, and simultaneously shoot a completely different beam up to a CEO in a corner office, reusing the exact same frequency without interference.
Key Equations
3GPP TR 36.873 is a highly significant Technical Report that established the first standardized 3D Spatial Channel Model for advanced 4G LTE networks (Release 12)....
Key specifications:
0 dB | 1 mW | 30 dB | 1 W | 110 GHz | 50 dB
Power: P(dBm) = 10log(PmW), 0dBm = 1mW
Comparison
| Aspect | 3GPP TR 36.873 Spec | Typical Range | Impact | Design Note |
|---|---|---|---|---|
| Primary function | 3GPP TR 36.873 is a highly significant T... | Application-dep. | Critical | Verify in sim |
| Operating range | Prior to this document, RF propagation m... | Application-dep. | Critical | Verify in sim |
| Performance | Understanding 3GPP TR 36.873 In the earl... | Application-dep. | Critical | Verify in sim |
| Integration | While it was highly optimized in the Hor... | Application-dep. | Critical | Verify in sim |
| Trade-off | The engineers would physically tilt the... | Application-dep. | Critical | Verify in sim |
Frequently Asked Questions
Is TR 36.873 still used for 5G?
No, it is largely obsolete. TR 36.873 was designed specifically for advanced 4G LTE frequencies (mostly below 6 GHz). When the 3GPP moved to 5G and introduced massive millimeter-wave frequencies (28 GHz), the physics changed so drastically that TR 36.873 was replaced by the much more advanced TR 38.901, which handles both 3D spatial modeling and extreme mmWave atmospheric absorption.
What is Zenith Angle?
In 3D spatial modeling, you need to know exactly where the beam is pointing. The Azimuth angle dictates left and right (like a compass). The Zenith angle (or Elevation angle) dictates up and down. TR 36.873 was the first major model to rigorously define the statistical probability of Zenith Angle fading in an urban canyon.
Why was FD-MIMO such a big deal?
Capacity. Before FD-MIMO, if 100 people were in a skyscraper, they all fought for the exact same radio wave spreading across the building. With FD-MIMO, the tower slices the building into horizontal floors, shooting a dedicated beam to the 10th floor and a dedicated beam to the 20th floor, multiplying the network capacity by utilizing vertical space.