---
title: "MSK modulation and DSSS"
subject: "Wireless Communication"
chapter: 4
key: "semester_7/wireless_communication:msk-dsss"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/msk-dsss
site: BCT Engineering
---

# MSK modulation and DSSS

Wireless Communication · Chapter 4: Modulation and Demodulation in Mobile Communication

- Asked in: 2080 Chaitra, 2+5 marks
- Video: [Spread Spectrum Explained | Basics, Block Diagram, Working, and Significance](https://www.youtube.com/watch?v=p3KbOPEZ39E) — 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/msk-dsss
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-4

## Question

What is MSK modulation? Explain direct sequence spread spectrum with its necessary relevant diagram.

## Short answer

MSK is continuous-phase FSK with h=0.5 and tone spacing Rb/2, equivalent to OQPSK with half-sine shaping. DSSS multiplies each data bit by a fast PN code, spreading bandwidth to about 2Rc; the receiver despreads with a synchronized code and integrates over Tb. Processing gain is Tb/Tc.

## Answer

#### MSK modulation
**Minimum Shift Keying (MSK)** is a continuous phase binary FSK with modulation index **h = 0.5**, i.e. tone spacing Δf = R_b/2, the minimum spacing that keeps the two tones orthogonal. The phase changes linearly by ±π/2 per bit with no sudden jumps, so the envelope is constant. It can also be seen as OQPSK with half-sinusoidal pulse shaping. MSK has fast side-lobe roll-off and the same BER as BPSK.

#### Direct sequence spread spectrum (DSSS)
**DSSS** spreads the data over a wide band by multiplying each data bit with a fast **PN code** of chip period T_c ≪ T_b. Processing gain PG = T_b/T_c.

**Transmitter**
```text
m(t) --->(x)--->(x)---> s(t) = m(t)p(t)cos ωct
          ^      ^
    [PN gen p(t)] [cos ωct]
```
- Data (±1) × PN code (±1) gives a chip stream at rate R_c.
- The chip stream BPSK-modulates the carrier; the bandwidth becomes ≈ 2R_c.

**Receiver**
```text
r(t)->(x)--->(x)--->[∫ 0..Tb]->[Decision]-> m^(t)
       ^      ^
 [Local PN  ] [Carrier]
 [synchron. ] [recovery]
```
- Multiplying by the same synchronized PN code despreads the wanted signal (p² = 1).
- Interference is spread by the same step and mostly filtered out.
- Integrating over T_b and comparing with zero gives the data.

```text
 Before despread      After despread
   ____________          /\   <- data
  |  wideband  |    ____/  \____ noise
```

DSSS gives anti-jam capability, multipath resistance (RAKE) and CDMA multiple access; it is used in IS-95, WCDMA and IEEE 802.11b.

## Check yourself

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

1. What defines MSK?
   A. Continuous phase binary FSK with h = 0.5, i.e. tone spacing R_b/2
   B. FSK with h = 1 and phase jumps
   C. QPSK with a PN spreading code
   D. FSK whose carrier hops among channels
   Answer: A. h = 0.5 gives the minimum orthogonal spacing and a phase change of +/-pi/2 per bit with no jumps, so the envelope is constant. (Key point: MSK definition)

2. How is the transmitted DSSS signal formed?
   A. The data is FSK modulated and the carrier hops by PN code
   B. The data is multiplied by a fast PN code and the chip stream BPSK-modulates the carrier
   C. The data is passed through a Gaussian filter and integrated
   D. The data is split over orthogonal subcarriers
   Answer: B. s(t) = m(t)p(t)cos(wct); bandwidth becomes about 2R_c. (Key point: DSSS spreading)

3. Why does DSSS reject narrowband interference?
   A. The interference is cancelled because p(t)^2 = 1
   B. The interference is hopped away to another channel
   C. At the receiver the interference is multiplied by the PN code and spread, while the wanted signal is despread
   D. The integrator amplifies only the interference
   Answer: C. Despreading collapses the signal to the data bandwidth while interference is spread out and mostly filtered, giving gain equal to PG. (Key point: Interference rejection)

## Related in Wireless Communication

- [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
- [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
- [Spread spectrum and FHSS](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-fhss) — Wireless Communication, chapter 4
- [Direct sequence and frequency hopped SS](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adsss-fhss-techniques) — Wireless Communication, chapter 4
- [OFDM principle and spread spectrum types](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aofdm-spread-spectrum-types) — 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×
- [MSK modulation scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Amsk-modulation) — Communication Systems, chapter 7
- [DSSS bandwidth spreading](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adsss-bandwidth-spreading) — Data Communication, chapter 6