DSSS vs FHSS: Understanding the Differences in Spread Spectrum

This page clarifies the differences between DSSS (Direct Sequence Spread Spectrum) and FHSS (Frequency Hopping Spread Spectrum). Both technologies are commonly used in 802.11 WLAN and 802.15 WPAN networks.

DSSS: Direct Sequence Spread Spectrum

In DSSS, information bits are modulated using PN (Pseudonoise) codes, also referred to as “chips.” These PN codes have a shorter duration compared to the information bits themselves. This process results in the transmitted signal occupying a larger bandwidth than the original user information. DSSS is the modulation technique used in IEEE 802.11-based WLAN products. In DSSS systems, the entire system bandwidth is available to each user at all times. Figure 1 (above) shows a block diagram of a DSSS transmitter and receiver. PRS stands for Pseudo-Random Sequence.

FHSS: Frequency Hopping Spread Spectrum

In FHSS, the RF carrier frequency is changed according to a Pseudo-random sequence (PRS or PN sequence). This PN sequence is known to both the transmitter and the receiver, enabling the demodulation/decoding of the information. The RF frequency remains constant during one “chip” duration. Based on the relationship between the hopping rate and the information bit rate, there are two types of FHSS:

• Fast Hopped FHSS: Hopping occurs at a rate faster than the message (information) bit rate.
• Slow Hopped FHSS: Hopping occurs at a rate slower than the information bit rate.

Figure 2 (above) shows a block diagram of an FHSS transmitter and receiver.

DSSS vs. FHSS: Key Differences

Figure 3 (above) illustrates the spectrum usage of DSSS and FHSS. Here’s a summary of the key differences:

• Error Characteristics: FHSS systems are prone to bursty errors due to frequency-selective fading, as they rely on varying the RF carrier frequency.

• Interference and Fading: In DSSS, information bits are spread across both frequency and time planes, which minimizes the impact of interference and fading. Consequently, DSSS systems are also prone to errors, but the errors are generally at a lower level compared to FHSS systems. FHSS produces stronger, more bursty errors.

• Capacity: DSSS can deliver a capacity of up to 11 Mbps, while FHSS typically supports up to 3 Mbps.

• Robustness: DSSS is a more sensitive technology, whereas FHSS is generally considered more robust. This difference is observed in harsh environments with large coverage areas, noise, collocated cells, multi-path propagation, and the presence of Bluetooth frequency waves.

• Application: DSSS is ideal for point-to-point applications, while FHSS is well-suited for point-to-multipoint deployments, delivering excellent performance in those scenarios.