---
title: "Proper vs improper handoff; cellular concept N = 7"
subject: "Wireless Communication"
chapter: 2
key: "semester_7/wireless_communication:handoff-situation-cellular-n7"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/handoff-situation-cellular-n7
site: BCT Engineering
---

# Proper vs improper handoff; cellular concept N = 7

Wireless Communication · Chapter 2: Cellular Mobile Communication Concepts

- Asked in: 2076 Bhadra, 4+3 marks
- Video: [What is handoff? Handoff Strategies](https://www.youtube.com/watch?v=ktnBPBOZY4E) — 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/handoff-situation-cellular-n7
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-2

## Question

Compare and contrast between improper handoff situation and proper handoff situation. Explain cellular concept for N = 7.

## Short answer

An improper handoff drops the call because the signal falls below the minimum usable level before transfer. A proper handoff completes between the handoff threshold and minimum level. For N=7, cluster uses i=2, j=1, reuse ratio √21=4.58, giving S/I about 18.7 dB, meeting AMPS requirements.

## Answer

#### Improper vs proper handoff
A handoff starts when the signal from the serving base station falls to the **handoff threshold** Pr,handoff, which is kept above the **minimum usable level** Pr,min by a margin Δ.

```text
 Signal  BS1                          BS2
   │ \                                /
   │   \ A: Pr,handoff reached     /
   │     \                  ____/
   │.......\ B: Pr,min ....../...........
   │  (improper: call drops at B)
   │  (proper: transferred to BS2 at C, before B)
   └───────────────────────────────── distance
```

| Point | Improper handoff | Proper handoff |
|---|---|---|
| What happens | Signal falls below Pr,min before transfer | Transfer completes between Pr,handoff and Pr,min |
| Result | Call is dropped | Call continues smoothly |
| Typical causes | Δ too small, slow MSC processing, no free channel in new cell | Adequate Δ, fast processing, free channel available |
| User experience | Disconnection | No noticeable break |

Too large a Δ is also bad: it gives unnecessary handoffs and loads the MSC.

#### Cellular concept for N = 7
In the cellular concept, the service area is divided into small hexagonal cells, each served by a low-power base station. The total S channels are divided among N = 7 cells forming a **cluster**, so each cell gets S/7 channels. The cluster is repeated across the area (frequency reuse).

```text
        ___
    ___/ 2 \___
   / 7 \___/ 3 \
   \___/ 1 \___/
   / 6 \___/ 4 \
   \___/ 5 \___/
       \___/
```
- N = 7 is obtained with i = 2, j = 1 (N = i² + ij + j² = 4 + 2 + 1).
- Reuse ratio Q = √(3×7) = 4.58, so co-channel cells are 4.58R apart.
- With n = 4 and six interferers, S/I = 4.58⁴/6 ≈ 73.5 = 18.7 dB, which meets the 18 dB needed by AMPS. That is why N = 7 is the classic choice.
- Frequency reuse factor = 1/7.

## Check yourself

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

1. Improper handoff leads to:
   A. The call continues
   B. Signal falling below Pr,min before transfer, so the call drops
   C. A new channel being allocated in the old cell
   D. Reduced interference
   Answer: B. A proper handoff completes between Pr,handoff and Pr,min. (Key point: Improper handoff result)

2. For N = 7, the reuse ratio Q is:
   A. 4.58
   B. 7
   C. 3
   D. 2.65
   Answer: A. Q = sqrt(21) = 4.58, from i = 2, j = 1. (Key point: N=7 reuse ratio)

3. Why is N = 7 the classic choice for AMPS?
   A. It gives the lowest possible S/I
   B. It has the most channels per cell
   C. With n = 4 and 6 interferers S/I = 18.7 dB, meeting the 18 dB requirement
   D. It needs no frequency reuse
   Answer: C. N = 4 would give only 13.8 dB. (Key point: Why N = 7)

## Related in Wireless Communication

- [Proper and improper handoff](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aproper-improper-handoff) — Wireless Communication, chapter 2
- [2G vs 3G, prioritized handoff, cell dragging](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3A2g-vs-3g-prioritized-handoff) — Wireless Communication, chapter 1
- [Numerical: cellular vs non-cellular voice channels](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acellular-vs-noncellular-channels) — Wireless Communication, chapter 2

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