---
title: "CDMA vs LTE system architecture"
subject: "Wireless Communication"
chapter: 8
key: "semester_7/wireless_communication:cdma-vs-lte-architecture"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/cdma-vs-lte-architecture
site: BCT Engineering
---

# CDMA vs LTE system architecture

Wireless Communication · Chapter 8: Wireless Systems and Standards

- Asked in: 2074 Magh, 3+5 marks
- Video: [Wireless Basics - GSM, CDMA, and LTE](https://www.youtube.com/watch?v=SBYXzNTnZA8) — Matt Diehl
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/cdma-vs-lte-architecture
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-8

## Question

Compare system architecture of CDMA with LTE. Mention function of entities in the architecture.

## Short answer

CDMA is hierarchical and circuit-switched with BTS, BSC, MSC, HLR/VLR and PDSN for packet data. LTE is flat all-IP: eNodeB handles radio control, MME does mobility and authentication, S-GW/P-GW route user packets, HSS stores subscriber data, and PCRF manages policy.

## Answer

#### Comparison of CDMA and LTE architectures
CDMA (IS-95/CDMA2000) is a 2G/3G **hierarchical, circuit-switched** architecture; LTE (4G) is a **flat, all-IP, packet-switched** architecture.

```text
 CDMA (CDMA2000 1x)
 [MS]~~[BTS]--[BSC/PCF]--[MSC]--PSTN
                  |        |-[HLR/AuC] [VLR]
                  +--[PDSN]--IP net (AAA, HA)

 LTE (EPS)
 [UE]~~[eNodeB]==X2==[eNodeB]
          |S1-MME   |S1-U
        [MME]----[S-GW]--S5--[P-GW]--Internet
          |                    |
        [HSS]               [PCRF]
```

| Point | CDMA | LTE |
|---|---|---|
| Structure | Hierarchical (BTS–BSC–MSC) | Flat (eNodeB – EPC) |
| Switching | Circuit voice; packet via PDSN | All-IP packets (voice as VoLTE) |
| Radio controller | Separate BSC | Built into eNodeB |
| Air interface | DS-CDMA, 1.25 MHz | OFDMA down, SC-FDMA up, 1.4–20 MHz |
| Handoff | Soft handoff | Hard handoff over X2 |
| Latency | High | Low (< 10 ms radio) |

#### Functions of CDMA entities
- **MS**: user terminal; does spreading, RAKE reception and power control.
- **BTS**: radio transceivers, spreading/despreading, pilot transmission.
- **BSC**: radio resource management, soft handoff (frame selection/distribution), vocoding, power control.
- **MSC**: call switching, mobility, link to PSTN; **HLR/VLR/AuC** store subscriber data, location and keys.
- **PCF/PDSN** (CDMA2000): packet control and gateway to IP networks; **AAA** for authentication/billing and **HA** for Mobile IP.

#### Functions of LTE entities
- **UE**: user equipment with USIM.
- **eNodeB**: the only radio node; radio resource control, scheduling, header compression, ciphering, handoff decisions; eNodeBs linked by **X2**.
- **MME (Mobility Management Entity)**: control plane – attach, authentication with HSS, tracking area updates, paging, bearer setup, S-GW selection.
- **S-GW (Serving Gateway)**: routes user packets; mobility anchor during handoff between eNodeBs.
- **P-GW (PDN Gateway)**: connects to external IP networks; IP address allocation, policy enforcement, charging.
- **HSS (Home Subscriber Server)**: subscriber database (like HLR + AuC).
- **PCRF**: policy and charging rules (QoS).

## Check yourself

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

1. How do CDMA and LTE architectures differ?
   A. Both are flat all-IP
   B. CDMA is all-IP; LTE circuit switched
   C. LTE has a separate BSC
   D. CDMA is hierarchical (BTS-BSC-MSC); LTE is flat all-IP (eNodeB-EPC)
   Answer: D. LTE merges radio control into eNodeB. (Key point: Structure)

2. What does the P-GW do?
   A. Authenticates users with Ki
   B. Does handoff decisions
   C. Does speech transcoding
   D. Connects to external IP networks, assigns IP address, enforces policy and charging
   Answer: D. It is the exit point to the Internet. (Key point: P-GW)

3. What air interface does LTE use compared with IS-95?
   A. DS-CDMA both ways
   B. TDMA/FDMA
   C. OFDMA down and SC-FDMA up; IS-95 uses DS-CDMA
   D. FHSS
   Answer: C. SC-FDMA lowers PAPR for the mobile. (Key point: Air interface)

## Related in Wireless Communication

- [GSM and CDMA standards; GSM architecture](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-cdma-standard-architecture) — Wireless Communication, chapter 8
- [Spread spectrum multiple access; FDMA vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-ma-fdma-cdma) — Wireless Communication, chapter 7
- [Multiple access; FDMA vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amultiple-access-fdma-vs-cdma) — Wireless Communication, chapter 7
- [CDMA vs TDMA; FHSS multiple access](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acdma-advantages-fhss-types) — Wireless Communication, chapter 7
- [GSM frame structure; GSM vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-frame-vs-cdma) — Wireless Communication, chapter 8
- [GSM system architecture](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-architecture) — Wireless Communication, chapter 8, asked 4×

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