How does DSSS differ from FHSS?

DSSS multiplies the data signal by a PN code with a chip rate much higher than the data rate, spreading the signal continuously across the entire bandwidth simultaneously. The receiver correlates the received signal with a synchronized copy of the PN code to despread, concentrating the desired signal while spreading any narrowband interference. DSSS provides precise timing (used for GPS ranging) and high processing gain. FHSS rapidly changes the carrier frequency according to a pseudo-random hopping pattern, spending a short dwell time on each frequency. The signal occupies only a narrow bandwidth at any instant but hops across the full spread bandwidth over time. FHSS is more robust against partial-band jamming (the hopper avoids jammed frequencies) and is simpler to implement. Bluetooth uses FHSS with 79 channels and 1600 hops per second. Military systems often use hybrid DSSS plus FHSS for maximum robustness.

How does CDMA use spread spectrum for multiple access?

CDMA assigns each user a unique spreading code (Walsh code for downlink orthogonal separation, long PN code for uplink scrambling). All users transmit simultaneously on the same frequency band. At the receiver, correlating with a specific user's code despreads that user's signal while leaving other users' signals spread (appearing as wideband noise). The capacity is limited by the total interference from all other users: each additional user raises the noise floor by a small amount. The system capacity is approximately N = PG / (Eb/N0_required times voice activity times frequency reuse), which for IS-95 gave approximately 20 to 40 users per sector per carrier (1.25 MHz). WCDMA (3G) used 3.84 Mchip/s spreading with variable spreading factors (4 to 512) to support data rates from 15 kbps to 2 Mbps. Power control is critical: without it, a strong near user overwhelms weak far users (near-far problem), requiring fast closed-loop power control at 1500 Hz update rate.

Wideband Modulation

Spread Spectrum

Q: What is processing gain?

Processing gain (PG) is the fundamental advantage of spread spectrum: it measures how much the receiver can suppress interference by despreading. PG = BW_spread / BW_data = 10*log10(R_chip / R_data) in dB. For GPS C/A code: chip rate = 1.023 Mchip/s, data rate = 50 bps, PG = 10*log10(1.023e6 / 50) = 43 dB. This means a GPS receiver can operate with the signal 43 dB below the noise floor and still decode the data. For IS-95 CDMA: chip rate = 1.2288 Mchip/s, data rate = 9.6 kbps, PG = 10*log10(1.2288e6 / 9600) = 21 dB. The processing gain also determines the jamming margin: JM = PG minus system losses minus required SNR. Higher processing gain allows operation in more hostile interference environments.

/spred spek-trum/

Deliberately spreads signal bandwidth far beyond minimum for interference rejection, LPI, and multiple access. DSSS: multiply by PN code at chip rate >> data rate, processing gain PG = 10log(R_chip/R_data). FHSS: hop carrier across BW using pseudo-random sequence. GPS C/A: PG = 43 dB (operates below noise floor). IS-95 CDMA: 21 dB PG, Walsh codes for user separation. Bluetooth: FHSS, 79 channels, 1600 hops/s.

DSSS PG: 10-43 dB

FHSS: Frequency hopping

CDMA: Code division

Understanding Spread Spectrum

Spread spectrum was originally developed for military communications, motivated by the need to resist enemy jamming and avoid detection. By spreading the signal power across a wide bandwidth, the spectral density drops below the noise floor, making the signal invisible to conventional receivers and resistant to narrowband interference. The receiver, knowing the spreading code, can despread the signal and recover the data.

Commercial applications of spread spectrum are everywhere today. GPS uses DSSS Gold codes for satellite ranging. 3G cellular (CDMA/WCDMA) used DSSS for multiple access. WiFi 802.11b used DSSS with Barker codes. Bluetooth uses FHSS with 79 channels. Even 4G/5G OFDM systems use scrambling codes that provide some spread-spectrum-like properties for interference averaging across cells.

Spread Spectrum Equations

Processing gain:
PG = 10 log₁₀(R_chip/R_data) dB
= 10 log₁₀(BW_ss/BW_data) dB
GPS: 10log(1.023M/50) = 43 dB
IS-95: 10log(1.2288M/9.6k) = 21 dB

Jamming margin:
JM = PG − L_sys − (E_b/N₀)_req
PG=43, L=3, Eb/N0=10:
JM = 43−3−10 = 30 dB

CDMA capacity:
N ≈ PG / ((E_b/N₀) × v × f)
v = voice activity (0.4-0.6)
f = frequency reuse (0.6-0.85)
IS-95: ~20-40 users/sector/carrier

Spread Spectrum System Comparison

System	Type	Chip Rate	PG	BW	Application
GPS C/A	DSSS	1.023 Mc/s	43 dB	2 MHz	Navigation
IS-95	DSSS	1.2288 Mc/s	21 dB	1.25 MHz	2G CDMA
WCDMA	DSSS	3.84 Mc/s	25 dB	5 MHz	3G cellular
Bluetooth	FHSS	N/A	~17 dB	80 MHz	Short-range
802.11b	DSSS	11 Mc/s	10 dB	22 MHz	WiFi (legacy)

Common Questions

Frequently Asked Questions

What is processing gain?

PG = 10log(R_chip/R_data). Measures interference suppression from despreading. GPS: 43 dB (signal 43 dB below noise, still decodable). IS-95: 21 dB. Also determines jamming margin: JM = PG - system losses - required Eb/N0. Higher PG = more hostile environment tolerance. Fundamental spread spectrum advantage.

DSSS vs. FHSS?

DSSS: spreads continuously across entire BW simultaneously. PN code correlation despreads desired, spreads interference. Precise timing (GPS ranging). FHSS: hops carrier pseudo-randomly, narrow BW each instant, covers full BW over time. More robust vs. partial-band jamming (avoids jammed channels). Bluetooth: 79 ch, 1600 hops/s. Military: hybrid DSSS+FHSS for max robustness.

How does CDMA work?

Each user gets unique code (Walsh downlink, long PN uplink). All transmit simultaneously on same frequency. Receiver correlates with specific code to despread one user; others remain as noise. Capacity: N ≈ PG/(Eb/N0 × voice activity × reuse). IS-95: 20-40 users/sector. Near-far problem requires fast power control (1500 Hz). WCDMA: 3.84 Mc/s, variable spreading factors.

Related Terms

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