--- title: "Forward CDMA (IS-95) channel" subject: "Wireless Communication" chapter: 8 key: "semester_7/wireless_communication:forward-cdma-is95-channel" source: https://www.bctengineer.com/q/semester_7/wireless_communication/forward-cdma-is95-channel site: BCT Engineering --- # Forward CDMA (IS-95) channel Wireless Communication · Chapter 8: Wireless Systems and Standards - Asked in: 2082 Baisakh, 6 marks; 2080 Bhadra, 4 marks; 2080 Chaitra, 6 marks; 2078 Chaitra, 4 marks (asked 4 times) - Video: [Forward CDMA Channel](https://www.youtube.com/watch?v=05SAbRoBhi0) — 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/forward-cdma-is95-channel - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-8 ## Question Describe the forward CDMA (IS-95) channel with suitable figure. ## Short answer IS-95 forward channel uses a 1.25 MHz band with 64 orthogonal Walsh codes. The pilot (W0) provides timing and phase reference; sync (W32) sends system info; up to 7 paging channels (W1–W7) alert mobiles. Traffic channels apply convolutional coding, interleaving, long-code scrambling for privacy, and power control subchannel punctured into the stream. ## Answer The forward (downlink) channel of IS-95 carries signals from the base station to the mobiles on a **1.25 MHz carrier at 1.2288 Mcps**. It is divided into **64 logical channels** using 64-chip **Walsh codes** (orthogonal), so the downlink users do not interfere with each other within a cell. Each base station is told apart by the time offset of its short PN code. | Walsh code | Channel | Use | |---|---|---| | W0 | Pilot | Unmodulated reference: timing, phase, signal strength for handoff | | W32 | Sync | 1200 bps; system time, PN offset, paging rate | | W1–W7 | Paging (up to 7) | 9600/4800 bps; pages, channel assignment, system info | | Remaining (up to 55) | Forward traffic | User speech/data + power control bits | #### Forward traffic channel processing ```text speech 9600 bps (rate set 1, also 4800/2400/1200) | [Conv. encoder r=1/2, K=9] -> 19.2 ksps | [Symbol repetition] (for lower rates) -> 19.2 ksps | [Block interleaver, 20 ms] | (XOR)<--[Long code gen]->[Decimator 64:1] (user mask) | | [MUX] <--power ctrl bits 800 bps (decimated long code | picks position) (XOR)<-- Walsh code Wn (64 chips) -> 1.2288 Mcps | +--> (x) I short PN -> [Baseband filter] -> cos wct +--> (x) Q short PN -> [Baseband filter] -> sin wct | (+) -> QPSK RF out ``` #### Steps 1. **Vocoder**: variable-rate speech coder (QCELP) gives 9600, 4800, 2400 or 1200 bps in 20 ms frames. 2. **Convolutional coding**: rate 1/2, constraint length 9 → 19.2 ksps; lower rates are repeated to keep 19.2 ksps. 3. **Block interleaving** over 20 ms to fight burst errors. 4. **Data scrambling**: XOR with the user's long PN code (2⁴² − 1), decimated to 19.2 ksps; gives privacy. 5. **Power control subchannel**: power control bits (800 bps) are punctured into the stream; they tell the mobile to raise or lower power by 1 dB. 6. **Walsh spreading**: each symbol is multiplied by 64-chip Walsh code → 1.2288 Mcps; gives orthogonal channelisation. 7. **Quadrature spreading**: the same chips are spread by I and Q short PN codes (2¹⁵ length, base-station specific offset). 8. **Filtering and QPSK modulation** onto the carrier; all channels are summed and transmitted. Pilot channel: all-zero data spread by W0 – it lets the mobile acquire timing and do coherent detection and RAKE combining. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How are the 64 logical channels of the IS-95 forward link separated? A. By different frequencies B. By time slots C. By 64-chip orthogonal Walsh codes D. By long code masks Answer: C. Walsh codes give orthogonal channelisation; base stations differ by short PN offset. (Key point: Forward channel basics) 2. What is the pilot channel? A. An unmodulated reference (W0) used for timing, phase and signal strength B. A voice channel on W32 C. A paging channel D. A channel carrying SMS Answer: A. Pilot is all-zero data spread by W0, enabling acquisition, coherent detection and handoff measurements. (Key point: Pilot channel) 3. What does the forward traffic encoder do to 9600 bps speech? A. Rate 1/2, K=9 coding gives 19.2 ksps B. Rate 1/3 gives 28.8 ksps C. No coding is applied D. Rate 1/2 gives 4.8 ksps Answer: A. Rate 1/2 coding doubles 9600 bps to 19.2 ksps; lower rates are repeated to keep 19.2 ksps. (Key point: Convolutional coding) 4. What do the 800 bps power control bits tell the mobile? A. Change Walsh code B. Switch to sync channel C. Change frame rate D. Raise or lower its transmit power by 1 dB Answer: D. They are punctured into the stream as the closed-loop power control subchannel. (Key point: Power control bits) 5. What is the purpose of XOR with the decimated long PN code? A. Spreading to 1.2288 Mcps B. Privacy through data scrambling C. Error correction D. Base station identification Answer: B. The user's long code scrambles data at 19.2 ksps for privacy; spreading is by Walsh codes. (Key point: Long code scrambling) ## Related in Wireless Communication - [Reverse CDMA (IS-95) channel](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Areverse-cdma-is95-channel) — Wireless Communication, chapter 8 - [Channelization code and IS-95 forward channels](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Achannelization-code-is95-forward) — Wireless Communication, chapter 8 - [Forward/reverse channel and 5G versus 4G](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aforward-reverse-5g-vs-4g) — Wireless Communication, chapter 1, asked 2× - [Forward and reverse channel](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aforward-reverse-channel) — Wireless Communication, chapter 1 - [Forward/reverse channel and features of 5G](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aforward-reverse-5g-features) — Wireless Communication, chapter 1 ## Related in other subjects - [Slope change 1 in 95 to 1 in 1420 with jump](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Aslope-change-95-to-1420-jump-height-or-spillway) — Hydraulics, chapter 9