---
title: "Cell footprint and interference tier"
subject: "Wireless Communication"
chapter: 2
key: "semester_7/wireless_communication:cell-footprint-interference-tier"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/cell-footprint-interference-tier
site: BCT Engineering
---

# Cell footprint and interference tier

Wireless Communication · Chapter 2: Cellular Mobile Communication Concepts

- Asked in: 2080 Chaitra, 1+4 marks
- Video: [Frequency Reuse Factor | Co-Channel Interference & Signal To Interference Ratio](https://www.youtube.com/watch?v=JfIIiecbEg8) — ECIT Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/cell-footprint-interference-tier
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-2

## Question

What is cell footprint? Explain interference tier in brief.

## Short answer

Cell footprint is the actual area where a base station signal is reliably received, modelled as a hexagon for planning. Co-channel cells form rings called tiers: first tier has six nearest cells at D=R√(3N), second tier twelve cells at √3D and 2D. Only the first tier is significant for interference.

## Answer

#### Cell footprint
The **cell footprint** is the actual geographical area over which a base station's signal is strong enough for reliable service (above the minimum usable level). The real footprint is irregular because of terrain, buildings and antenna patterns; it is modelled as a **hexagon** for planning, since hexagons tile the area without gaps or overlaps and approximate a circle closely.

#### Interference tier
Co-channel cells around a given cell are arranged in rings called **tiers**:

- **First tier:** the 6 nearest co-channel cells, each at distance D = R√(3N) from the centre cell.
- **Second tier:** the next ring of 12 co-channel cells, six at √3·D and six at 2D, and so on (the third tier has 18 cells, at about 2.6D to 3D).

```text
              (I)          (I)
                  \       /
         (I) ──── [ Serving ] ──── (I)     I = first-tier
                  /   cell  \           co-channel cell
              (I)          (I)          at distance D
```

Because received power falls as d⁻ⁿ (n = 2 to 4), each second-tier cell contributes only (1/√3)ⁿ to (1/2)ⁿ of a first-tier cell's interference (1/9 to 1/16 for n = 4). Therefore, in practice, only the **first tier** is considered:
```text
S/I = R⁻ⁿ / Σ(i=1..i₀) Dᵢ⁻ⁿ ≈ (D/R)ⁿ / i₀ = (√(3N))ⁿ / i₀
```
with i₀ = 6 for omni antennas, 2 for 120° sectoring and 1 for 60° sectoring (roughly). Example: N = 7, n = 4 gives S/I = 4.583⁴/6 = 73.5 ≈ 18.66 dB.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. The cell footprint is:
   A. The area of the hexagon used in planning
   B. The actual area where a base station's signal is strong enough for reliable service
   C. The area covered by the whole cluster
   D. The area where interference is zero
   Answer: B. The real footprint is irregular; the hexagon is only a model. (Key point: Cell footprint)

2. Why are hexagons used to model cells?
   A. They give the largest coverage possible
   B. Real footprints are exactly hexagonal
   C. They eliminate co-channel interference
   D. They tile the area without gaps or overlaps and approximate a circle closely
   Answer: D. Circles overlap or leave gaps; hexagons tile neatly. (Key point: Why hexagons)

3. The first tier of co-channel cells contains:
   A. 6 cells at distance D = R sqrt(3N)
   B. 12 cells at 2D
   C. 6 cells at 2D
   D. 18 cells at D
   Answer: A. First tier is the nearest ring of 6; the second has 12 at about 2D. (Key point: First tier)

4. Why is mainly the first tier considered for S/I?
   A. Second-tier cells use different frequencies
   B. Second-tier interference is larger
   C. Second-tier cells are about twice as far, so their interference is much smaller (1/16 each for n = 4)
   D. Power does not fall with distance
   Answer: C. Received power goes as d^-n, so farther tiers contribute little. (Key point: Only first tier used)

## Related in Wireless Communication

- [CCI/ACI, reuse ratio proof and channels per cell](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acci-aci-reuse-duplex-channels) — Wireless Communication, chapter 2