FHSS Vs. DSSS Comparision

Frequency Hopping Spread Spectrum Technique

Some wireless local-area network products, such as Raytheon's RaylinkTM products, use
the frequency hopping method of spreading their signals. The range of available frequencies in
the ISM2 (Industrial Scientific Medical) band of 2.400 - 2.483 GHz is divided into a series of
1MHz channels up to 79 separate and distinct channels. Transmissions are sent over each of
these channels in what appears to be a random sequence (called a “pseudo-random sequence”)
such as channel 1, channel 32, channel 3, channel 56, etc. The radio switches frequencies many
times a second, transmitting on each channel for a fixed amount of time, then proceeding on to
the next channel in its sequence, covering all of the channels before repeating the sequence.
Without knowing how long to stay on each channel (the “dwell time”) and what the hopping
pattern is, it is impossible for a non-participating station to receive and decipher the data.
The frequency hopping physical layer has 22 hop patterns to chose from. The frequency
hopping physical layer is required to hop across the 2.4GHz ISM band covering 79 channels.
Each channel occupies 1Mhz of bandwidth and must hop at the minimum rate specified by the
FREQUENCY HOPPING VS. DIRECT SEQUENCE
regulatory bodies of the intended country. A minimum hop rate of 2.5 hops per second is
specified for the United States.
The use of different hopping patterns, dwell times, and/or number of channels is what
allows two disjoint wireless LANs to exist nearby one-another without causing interference and
without fear of data from one network being seen by the other.

Direct Sequence Spread Spectrum Technique

The DSSS physical layer uses an 11-bit Barker Sequence to spread the data before it is
transmitted. Each bit transmitted is modulated by the 11-bit sequence. This process spreads the
RF energy across a wider bandwidth than would be required to transmit the raw data. The
processing gain of the system is defined as 10x the log of the ratio of spreading rate (also know
as the chip rate) to the data. The receiver despreads the RF input to recover the original data. The
advantage of this technique is that it reduces the effect of narrowband sources of interference.
This sequence provides 10dB of processing gain that meets the minimum requirements for the
rules set forth by the FCC. The spreading architecture used in the direct sequence physical layer
is not to be confused with CDMA. All 802.11 compliant products utilize the same PN code and
therefore do not have a set of codes available as is required for CDMA operation.