How does DSSS despreading work?

The receiver multiplies the received signal by a locally generated copy of the PN code, synchronized in time and frequency. For the desired signal, this multiplication removes the spreading (since PN times PN = 1), concentrating the signal energy back into the original data bandwidth. For any interfering signal that is not correlated with the PN code, the multiplication spreads the interference across the full spread bandwidth. After bandpass filtering to the data bandwidth, the interference power is reduced by the processing gain PG = BW_spread/BW_data. This is why GPS can operate with signals 20 to 43 dB below the noise floor: the despreading process pulls the signal above the noise.

What PN codes are used?

Barker codes are short (up to 13 chips) with perfect autocorrelation sidelobes of 1 or 0, used in 802.11b (11-chip Barker). Gold codes are formed by XOR of two maximum-length sequences, providing good cross-correlation properties for CDMA (used in GPS C/A code, 1023 chips). Walsh codes are perfectly orthogonal (zero cross-correlation when synchronized), used for CDMA downlink channel separation (IS-95 uses 64-chip Walsh, WCDMA uses variable-length Walsh for different data rates). Kasami codes provide lower cross-correlation than Gold for systems with fewer users. M-sequences (maximum length) have period 2^n minus 1 and near-ideal autocorrelation but limited cross-correlation.

What limits DSSS performance?

Near-far problem: a strong interfering DSSS user can overwhelm a weak desired user even with processing gain, because the interferer power may exceed the PG advantage. Solution: fast closed-loop power control (IS-95 uses 800 Hz, WCDMA uses 1500 Hz). Multipath: if reflected signals arrive more than one chip period late, they appear as uncorrelated interference and are suppressed by PG. If within one chip period, they cause inter-chip interference. RAKE receivers exploit resolvable multipath by combining delayed copies. Synchronization: the receiver must acquire and track the PN code timing to within a fraction of a chip period. For GPS (chip = 977 ns), acquisition can take seconds scanning all possible code phases. For IS-95 (chip = 814 ns), the base station transmits a pilot channel for fast acquisition.

Modulation Technique

DSSS

/dee-ess-ess-ess/ — Direct Sequence Spread Spectrum

Multiplies data by PN code at chip rate >> data rate, spreading BW by processing gain PG = 10log(R_chip/R_data). Despreading: correlate with synchronized PN code; desired signal concentrates, interference spreads. GPS C/A: 1.023 Mc/s Gold code, PG=43 dB (signal below noise). IS-95: 1.2288 Mc/s, Walsh/PN, PG=21 dB. 802.11b: Barker/CCK. Anti-jam, LPI, multipath RAKE, CDMA multiple access.

GPS PG: 43 dB

IS-95: 21 dB

Codes: Gold/Walsh/Barker

Understanding DSSS

DSSS is the most widely deployed spread spectrum technique, forming the basis of GPS navigation, 2G/3G CDMA cellular, and early WiFi. The core principle is elegant: multiply the narrowband data signal by a wideband pseudo-random code, spreading the signal's spectral density below the noise floor. Only a receiver with the identical code can reverse the process and recover the data, providing inherent security and interference immunity.

The processing gain, equal to the ratio of spread bandwidth to data bandwidth, quantifies the system's advantage. GPS achieves 43 dB of processing gain, meaning the satellite signals can be received at power levels 43 dB below the thermal noise floor. This enables the tiny signals from 20,000 km altitude to be reliably decoded by a smartphone antenna.

DSSS Equations

Processing gain:
G_p = 10log(BW_spread/R_data) dB
= 10log(chip rate / data rate)

Jamming margin:
M_j = G_p − SNR_req − L_sys

Near-far ratio:
NFR = P_near/P_far (must be < G_p)

DSSS System Comparison

Standard	Chip rate	G_p	BW	Application
802.11b	11 Mchip/s	11 dB	22 MHz	Legacy WiFi
GPS L1 C/A	1.023 Mchip/s	43 dB	2.046 MHz	Navigation
IS-95 CDMA	1.2288 Mchip/s	21 dB	1.25 MHz	2G cellular
WCDMA	3.84 Mchip/s	25 dB	5 MHz	3G cellular
GPS P(Y)	10.23 Mchip/s	53 dB	20.46 MHz	Military GPS

Common Questions

Frequently Asked Questions

How does despreading work?

Receiver multiplies by synchronized PN copy. Desired: PN×PN=1, concentrates to data BW. Interference: multiplied by uncorrelated PN, spreads across full BW. After filtering: interference reduced by PG. GPS: signal 43 dB below noise, despreading recovers it. Requires precise code timing synchronization (<1 chip).

What codes are used?

Barker (11 chips): perfect autocorrelation, 802.11b. Gold (1023): good cross-correlation, GPS C/A. Walsh (64/256): perfectly orthogonal when synchronized, IS-95/WCDMA downlink. Kasami: low cross-correlation, fewer users. M-sequences (2^n−1): near-ideal autocorrelation, limited cross-correlation.

Performance limits?

Near-far: strong user overwhelms weak even with PG. Solution: fast power control (1500 Hz WCDMA). Multipath: >1 chip delay = suppressed by PG. <1 chip = inter-chip interference. RAKE receiver combines resolvable paths. Synchronization: must acquire PN timing (GPS: seconds scanning all code phases). Pilot channel aids fast acquisition.

Related Terms

← DPD Dynamic Range →

RF Systems

Request a Quote

Need DSSS system design, CDMA analysis, or GPS receiver development? Contact our team.

Get in Touch