Power Control
Power control manages interference by ensuring each mobile transmits at the minimum power needed. Without it, near mobiles drown out far mobiles (near-far problem) and system capacity collapses.
Control Loops
| Loop | Rate | Mechanism |
|---|---|---|
| Reverse inner | 800 Hz (per PCG) | Eb/Nt measurement → sigma-delta quantizer → binary PCB |
| Reverse outer | Per frame | CRC pass/fail → sliding window FER → IIR filter → adjust target Eb/Nt |
| Forward outer | On PMRM | Mobile pilot strength reports → per-channel gain scaling |
| Open loop | Access probes | Initial power from received signal strength (IS-95 formula) |
Reverse Inner Loop
Operates once per Power Control Group (PCG = 1.25 ms):
| Step | Action |
|---|---|
| 1 | Measure Eb/Nt from received signal |
| 2 | Compare against target set by outer loop |
| 3 | Quantize error through sigma-delta modulator |
| 4 | Transmit binary Power Control Bit (PCB) on forward traffic channel |
| 5 | Mobile adjusts TX power up/down by step size |
The sigma-delta quantizer provides finer effective control than simple thresholding.
Reverse Outer Loop
Adjusts the inner loop’s Eb/Nt target based on actual frame error performance:
| Step | Action |
|---|---|
| 1 | Track CRC pass/fail in sliding window |
| 2 | Estimate current Frame Error Rate (FER) |
| 3 | Filter through IIR for stability |
| 4 | FER too high → raise target. FER too low → lower target |
Forward Outer Loop
| Step | Action |
|---|---|
| 1 | Mobile measures pilot strength, sends PMRM |
| 2 | BSC parses PMRM, adjusts per-traffic-channel gain |
Open Loop
For access probes before closed-loop is established:
| Parameter | Purpose |
|---|---|
NOM_PWR | Nominal power offset |
INIT_PWR | Initial power offset |
PWR_STEP | Power increment per probe attempt |
{ "bts": { "access": { "nom_pwr": 0, "init_pwr": 0, "pwr_step": 3 } } }