---
title: "Spread spectrum disadvantages; CDMA implementation"
subject: "Wireless Communication"
chapter: 7
key: "semester_7/wireless_communication:spread-spectrum-disadvantages-cdma"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/spread-spectrum-disadvantages-cdma
site: BCT Engineering
---

# Spread spectrum disadvantages; CDMA implementation

Wireless Communication · Chapter 7: Multiple Access in Wireless Communication

- Asked in: 2077 Chaitra, 2+6 marks
- Video: [Spread Spectrum (Basics, Block Diagram, Working, Need & Types) Explained in Digital Communication](https://www.youtube.com/watch?v=xltpukBncs8) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/spread-spectrum-disadvantages-cdma
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7

## Question

What are the major disadvantages of spread spectrum system? Explain the implementation of CDMA system with appropriate example.

## Short answer

Spread spectrum is bandwidth inefficient for one user, has near-far problem, self-jamming, complex receivers, and strict synchronization. In DS-CDMA each user multiplies data by a unique high-rate code; the receiver correlates with the wanted code, recovering data while others remain noise. Example: orthogonal codes A=+1-1+1-1 and B=+1+1-1-1; A sends +1, B sends -1, channel sum 0 -2 +2 0; receiver A gives +1, receiver B gives -1.

## Answer

#### Disadvantages of spread spectrum systems
- **Bandwidth inefficient for a single user** – a signal of a few kHz is spread over MHz.
- **Near-far problem** – a nearby user's strong signal swamps far users; precise power control is needed.
- **Self-jamming** – codes are not perfectly orthogonal, so users interfere with each other.
- **Complex receivers** – code acquisition, tracking, RAKE receiver and high-speed chip processing.
- **Strict synchronisation** of PN codes is required.

#### Implementation of a CDMA system
In DS-CDMA every user transmits on the **same carrier at the same time**. Each user's data is multiplied by a unique high-rate PN (spreading) code. The receiver multiplies the received sum by the wanted user's code; only that user is despread back to narrowband, while all others stay as wideband noise.

```text
 Transmitter (user k)
 data d_k(t) --(x)--(x)--> sum of all users
               ^    ^         in the channel
             c_k(t) carrier
 Receiver for user 1
 r(t) --(x)--(x)--[Integrate over Tb]--> d1
          ^    ^
       carrier c_1(t)
```

Steps:
1. Data at bit rate R_b is multiplied by the code c_k(t), whose chip rate R_c >> R_b. Processing gain G_p = R_c / R_b.
2. The spread signal modulates the carrier (BPSK/QPSK) and is sent.
3. The receiver gets r(t) = Σ d_k(t)·c_k(t) + noise.
4. Multiplying by c_1(t): since c_1·c_1 = 1, user 1's data is recovered; cross-terms d_k·c_k·c_1 remain spread because the codes have low cross-correlation.
5. Integrating over one bit gives d_1; interference is reduced by G_p.

**Example (chip level)**: Let code of A = +1 −1 +1 −1, code of B = +1 +1 −1 −1 (orthogonal).
- A sends bit +1 → +1 −1 +1 −1; B sends bit −1 → −1 −1 +1 +1.
- Channel sum = 0 −2 +2 0.
- Receiver A: (0)(+1)+(−2)(−1)+(+2)(+1)+(0)(−1) = 4 → 4/4 = **+1** (A's bit).
- Receiver B: (0)(+1)+(−2)(+1)+(+2)(−1)+(0)(−1) = −4 → **−1** (B's bit).

Real systems (IS-95) use 1.2288 Mcps, Walsh codes for channel separation, PN short codes for cell separation, power control and a RAKE receiver.

## Check yourself

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

1. Which is a disadvantage of spread spectrum systems?
   A. Bandwidth inefficient for a single user
   B. Easily jammed
   C. No synchronisation needed
   D. Low-complexity receivers
   Answer: A. A few-kHz signal is spread over MHz; also near-far and complex receivers. (Key point: Disadvantages)

2. Why does multiplying by c1(t) recover user 1's data?
   A. c1.c1 = 1 gives data back while other users' terms stay spread
   B. All codes cancel
   C. The carrier cancels
   D. Noise is removed
   Answer: A. Only the wanted user is despread to narrowband; others remain wideband noise. (Key point: Despreading)

3. In the example, what is the correlation output at receiver B?
   A. -4, giving -1
   B. +4, giving +1
   C. 0
   D. -2
   Answer: A. Sum = (0)(1)+(-2)(1)+(2)(-1)+(0)(-1) = -4; divided by 4 gives -1. (Key point: Example)

4. What is processing gain Gp?
   A. Rc / Rb, the chip rate over bit rate
   B. Rb / Rc
   C. Rc x Rb
   D. Rc - Rb
   Answer: A. Interference is reduced by Gp = Rc/Rb. (Key point: Processing gain)

## Related in Wireless Communication

- [CDMA features and hybrid spread spectrum](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acdma-features-hybrid-ss) — Wireless Communication, chapter 7
- [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
- [Direct sequence spread spectrum (DSSS)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adsss-block-diagram) — Wireless Communication, chapter 4
- [Spread spectrum and DSSS transmitter/receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-dsss-tx-rx) — Wireless Communication, chapter 4
- [Spread spectrum and FHSS](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-fhss) — Wireless Communication, chapter 4
- [MSK, GMSK and advantages of spread spectrum](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amsk-gmsk-spread-spectrum-advantages) — Wireless Communication, chapter 4

## Related in other subjects

- [Spread spectrum: FHSS and DSSS](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-fhss-dsss) — Data Communication, chapter 6, asked 4×
- [Spread spectrum frequency hopping in Bluetooth](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Afreq-hopping-bt) — Instrumentation II, chapter 5, asked 2×
- [Digital vs analog spread spectrum](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-digital-vs-analog) — Data Communication, chapter 6
- [Short note: Spread spectrum](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-short-note) — Data Communication, chapter 6