ACM (Link)
Understanding the ACM Link (Microwave Backhaul)
When you make a phone call on a cell tower, the tower must send your voice to the main telecom network. If there is no fiber-optic cable in the ground, the tower uses a massive microwave dish to blast your data through the air to the next mountain. That connection is an ACM Link.
The Threat of Weather
Microwave links use extremely high frequencies (e.g., 18 GHz, 80 GHz). These frequencies are incredibly fast, but they are terrified of water. If a heavy rainstorm passes between the two towers, the water droplets violently absorb the radio wave. If the link does not adapt, the connection will break, and thousands of phone calls will drop instantly.
Shifting Gears
An ACM (Adaptive Coding and Modulation) link fixes this by autonomously shifting gears based on the weather.
- High Gear (Sunny Day): The air is perfectly clear. The microwave dish shifts into 4096-QAM. This is highly complex math that crams massive amounts of data into the wave, allowing the tower to blast 10 Gigabits per second (Gbps) of Netflix and YouTube traffic.
- Low Gear (Heavy Rain): The storm hits. The receiving dish sees the signal dying. It instantly commands the transmitting dish to downshift to QPSK (4-QAM). This simple, bulletproof math easily punches through the rain. The total speed of the link drops from 10 Gbps down to 1 Gbps.
The system intentionally drops the Netflix traffic (which causes buffering on the users' phones), but the 1 Gbps connection is more than strong enough to keep the highly critical Voice phone calls and 911 emergency data alive until the storm passes.
Key Equations
ModCod selected by SNRestimated
Throughput with ACM:
T = ΣP(SNRi)×Ri×B
P(SNRi) = probability of state i
Ri = spectral efficiency of ModCod i
Link margin:
Margin = SNRreceived−SNRthreshold dB
Comparison
| ModCod | SNR threshold | SE (bps/Hz) | Application | Notes |
|---|---|---|---|---|
| QPSK 1/4 | −2 dB | 0.49 | Rain fade | Most robust |
| QPSK 1/2 | 1 dB | 0.99 | Normal | Baseline |
| 8PSK 2/3 | 6 dB | 1.98 | Clear sky | Higher SE |
| 16APSK 3/4 | 9 dB | 2.97 | Good conditions | High throughput |
| 32APSK 9/10 | 13 dB | 4.45 | Best conditions | Maximum SE |
Frequently Asked Questions
What is Hitless ACM?
Hitless means "zero dropped packets." When an old microwave link changed gears, it had to reboot the radio for a fraction of a second, causing a "hit" that dropped active phone calls. Modern Hitless ACM is so mathematically advanced that the two radios can downshift from 4096-QAM to QPSK in the middle of a single data frame, without a single byte of data being dropped or delayed.
Does ACM change the transmit power?
Yes, it is almost always paired with ATPC (Automatic Transmit Power Control). When the rainstorm hits, the radio downshifts its modulation (ACM), but it also simultaneously cranks the physical wattage of the amplifier to maximum (ATPC) to try and blast the wave through the water. Once the rain stops, it turns the power back down to save electricity and prevent jamming neighboring towers.
How does the router know which traffic to drop?
Through strict QoS (Quality of Service) VLAN tagging. The cell tower router knows exactly which packets are Netflix (Best Effort) and which packets are 911 calls (Voice). When the ACM link downshifts and the pipe shrinks, the router acts as a ruthless traffic cop, aggressively throwing the Netflix packets into the trash to ensure the 911 calls fit into the smaller pipe.