Frequency Hopping vs. Direct Sequence Spread Spectrum Systems:

Even with the ratification of an international standard (IEEE 802.11) for wireless networking, it is important to recognize that there are important design differences in RF systems that can have a measurable impact on performance, scaleability, and the potential for interoperability. In this regard, there are two basic signal formats defined for use in the 2.4 GHz band by the IEEE 802.11 international standard: Frequency Hopping Spread Spectrum (FHSS) and Direct Se-quence Spread Spectrum (DSSS). The following discussion highlights the benefits of FHSS in Wireless Local Area Networks (WLAN) relative to DSSS.

What is the Difference Between FHSS and DSSS?
FHSS distributes the signal energy of a wireless net-work over a much wider frequency bandwidth than that of a single signal at a fixed frequency. It does this by spending a fraction of the total time at any given frequency, and hops with a predictable pseu-do-randomness to other frequencies. In the United States, the IEEE 802.11 standard uses 79 hop channels with a data rate of 1 to 2 Mbps. Higher data rates are currently under review by the IEEE 802.11 committee.

With DSSS, the signal energy is spread by converting each bit into many smaller chips that are sent out at a much higher rate. In the 802.11 standard, the DSSS format uses 11 chips-per-bit with a single de-fined chipping sequence. Because of limitations of the 2.4GHz band allocated by the FCC in the USA and ETSI in Europe, this provides a maximum of three frequency channels to work with.

How Does an FHSS Network Operate?
In a given network, all FHSS radios will hop to the predetermined next channel at the same time and continue communications where it left off. The en-tire sequence and hop times are predetermined. Thus, once a radio joins a network-which typically takes less than 1/100th of a second-it can stay syn-chronized even when turned off for seconds or min-utes for power conservation. FHSS radios keep track of all Access Points within range to determine the best Access Point to use. This translates to seam-less roaming and handoff.

How Well Does FHSS Perform in a Wireless Environment?
Wireless LANs encounter a significantly tougher medium than their related wired cousins with many sources of interference and degradation. The prima-ry interference sources are microwave ovens, of which there are an estimated 80 million in the Unit-ed States, which emit RF energy in the 2.4GHz band. Interference sources also include other wire-less devices including our own systems. Because FHSS radios spend only a short time at each fre-quency, the interference is typically absent after hopping to the next frequency. Unlike a DSSS radio that is stationary at a pre-selected frequency, an FHSS radio cannot be blocked by a single interferer.

The primary source of signal degradation is multi--path or reflections of signals bouncing off of walls and other objects. This causes some channels to be attenuated relative to other channels. The hopping of the FHSS format inherently provides a solution to this problem by hopping to a different frequency which is not attenuated. Because of the short nature of typical multi-path, the DSSS format is not capable of overcoming this effect with the factor of 11 spreading. Both formats can achieve some improve-ment through the use of dual-diversity antennas.

How Well Does FHSS Perform in Multi-ple Cell Environments?
Different FHSS networks use different hopping se-quences. By law, these networks are not synchro-nized and will eventually share the same frequency for a short period of time. By selecting optimized hopping sequences, any two IEEE 802.11 networks will not share the same frequency more than 1/79th of the time. Furthermore, even during that short pe-riod, there is a protocol that allows networks to avoid interfering with each other. Thus, there can be many FHSS networks in the same area with accept-able performance.

In a large installation where the maximum distance is too great for all radios to communicate with each other, there will be multiple Access Points that provide coordination and wired-LAN connectivity within a cellular arrangement. These cells are over-lapped to ensure continuous coverage. A single ter-minal may be within range of up to three access points, but may also see the lower power signals from other access points. In this common scenario, the availability of 79 different channels is a distinct advantage. This is in contrast to the maximum of three DSSS networks that can overlap without constantly inter-fering with each other. The DSSS spreading factor of 11 is insufficient to provide true DSSS benefits which typically require spreading factors greater than 1000.

Which Format Is More Scaleable?
Unlike DSSS, there is no frequency overlap between FHSS cells. Consequently, as additional FHSS cells are incorporated into a Radio Frequency network, overall network capacity increases. With DSSS, the addition of more cells tends to reduce network capacity and data throughput.

With The Ratification of 802.11, Which Format Provides More Potential For Interoperability?
The IEEE 802.11 FHSS format is based entirely on mature, proven technology and available parts. There are numerous commercially available systems that are very similar to the 802.11 format and are currently in compliance testing. In addi-tion, the overwhelming interest of radio manufactur-ers in the 802.11 meetings is on FHSS. Since there is no compatibility between FHSS and DSSS, com-patibility between 80 to 90 percent of the radios or 10 to 20 percent will be a significant factor in choos-ing a format. Accordingly, it is reasonable to conclude that FHSS systems will afford the most opportunities for interoperability.

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