---
title: "Channelization code and IS-95 forward channels"
subject: "Wireless Communication"
chapter: 8
key: "semester_7/wireless_communication:channelization-code-is95-forward"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/channelization-code-is95-forward
site: BCT Engineering
---

# Channelization code and IS-95 forward channels

Wireless Communication · Chapter 8: Wireless Systems and Standards

- Asked in: 2073 Magh, 4 marks
- Video: [Frequency and Channel Specification of IS-95](https://www.youtube.com/watch?v=FR7svE1WbZg) — WIT Solapur - Professional Learning Community
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/channelization-code-is95-forward
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-8

## Question

What is channelization code? Explain briefly forward channels in cdma IS-95.

## Short answer

A channelization code is a spreading code separating same-source channels by orthogonality. In IS-95 forward, 64 Walsh codes create 64 channels: pilot on W0 for reference, sync on W32, paging on W1–W7, and traffic on the remaining 55.

## Answer

#### Channelization code
A channelization code is a spreading code used to **separate the different channels (users) transmitted from the same source** on one carrier. In IS-95 the forward link uses **64 orthogonal Walsh codes** of length 64 chips (1.2288 Mcps). Because Walsh codes are mutually orthogonal (zero cross-correlation when time-aligned), channels from one base station do not interfere with each other. Cells are separated by a different code – the short PN code offset – so channelization codes can be reused in every cell.

```text
 Walsh (order 4): W0 = + + + +
                  W1 = + - + -
                  W2 = + + - -
                  W3 = + - - +
 W1·W2 = (1 −1 −1 +1) sum = 0 → orthogonal
```

#### Forward channels in IS-95
The 64 Walsh codes give up to 64 logical channels on each 1.25 MHz forward carrier:

| Channel | Walsh code | Function |
|---|---|---|
| Pilot | W0 | Unmodulated reference for timing, coherent detection, handoff measurement |
| Sync | W32 | 1200 bps; system time, PN offset, paging channel rate |
| Paging | W1–W7 | 4800/9600 bps; paging, system parameters, channel assignment |
| Traffic | Remaining 55 | User data with power control subchannel (800 bps) |

Traffic data (9600 bps) is convolutionally coded (r = 1/2), interleaved, scrambled by the long code, Walsh-spread and then quadrature-spread by the I/Q short PN codes before QPSK transmission.

## Check yourself

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

1. What is a channelization code used for?
   A. To separate channels transmitted from the same source on one carrier
   B. To separate different base stations
   C. To encrypt the speech
   D. To correct errors
   Answer: A. Orthogonal Walsh codes separate channels from one BS; PN offsets separate cells. (Key point: Channelization code)

2. Why do Walsh channels from one base station not interfere?
   A. They are mutually orthogonal (zero cross-correlation when time-aligned)
   B. They use different carriers
   C. They use different power
   D. They are sent in different time slots
   Answer: A. Zero cross-correlation means the sum of products is zero. (Key point: Orthogonality)

3. Which Walsh code carries the pilot channel?
   A. W32
   B. W1
   C. W63
   D. W0
   Answer: D. W0 is the unmodulated pilot; W32 is the sync channel. (Key point: Pilot code)

4. How many traffic channels are available with the pilot, sync and 7 paging channels?
   A. 64
   B. 55
   C. 63
   D. 32
   Answer: B. 64 - 1 - 1 - 7 = 55. (Key point: Traffic channels)

## Related in Wireless Communication

- [Forward CDMA (IS-95) channel](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aforward-cdma-is95-channel) — Wireless Communication, chapter 8, asked 4×
- [Reverse CDMA (IS-95) channel](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Areverse-cdma-is95-channel) — Wireless Communication, chapter 8
- [Advantages and disadvantages of FDMA, TDMA, CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afdma-tdma-cdma-pros-cons) — Wireless Communication, chapter 7

## Related in other subjects

- [Forward error correction using Hamming code](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afec-hamming) — Data Communication, chapter 8