13-09-2012, 03:52 PM
Frequency Hopping Spread Spectrum (FHSS) vs. Direct Sequence Spread Spectrum (DSSS) in the Broadband Wireless Access and WLAN Arenas
FHSS_vs_DSSS.doc (Size: 242 KB / Downloads: 99)
Scope
In 1997 IEEE defined the 802.11 Wireless LAN (WLAN) standard, intended to allow wireless connection of work-stations to their "base" LAN. The original standard targeted the case in which both the workstation and the LAN are owned by the same entity, providing in fact a wireless extension to an existing, wired LAN.
While this WLAN application represents a limited niche in the market, the technology on which it is based started to be used on large scale for a new application, that of providing Broadband Wireless Access (BWA) to public networks. We are still connecting work-stations to "base" LAN, but this time the "base" LAN is owned by a service provider (ISP, ITSP, ILEC, CLEC), while the workstation is owned by a subscriber.
This white paper explains the principles of the radio technologies used in WLAN and BWA applications as well as the advantages and disadvantages of each one of them.
Executive Summary
WLANs may be implemented using optical or radio technologies for the transmission of the signals through the air and both are defined in the IEEE 802.11 standard, ratified on June 26,1997.
The radio technology on which WLANs are based is known as Spread Spectrum modulation and has its origins in the military. Among the advantages of Spread Spectrum technologies, one can mention the inherent transmission security, resistance to interference from other radio sources, redundancy, resistance to multipath and fading effects, etc. As a result, Spread Spectrum systems can coexist with other radio systems, without being disturbed by their presence and without disturbing their activity. The immediate effect of this elegant behavior is that Spread Spectrum systems may be operated without the need for license, and that made the Spread Spectrum modulation to be the chosen technology for license-free WLAN and BWA operation. However, as mentioned above, spread spectrum technologies have many other advantages, making them an excellent option for the operation of systems in licensed bands, too.
Time and frequency diversity
Both DSSS and FHSS retransmit lost packets, until the receiving part acknowledges correct reception. A packet could be lost because of noises or multipath effects.
This capability of a system to repeat unsuccessful transmissions at later moments in time is known as "time diversity".
DSSS systems use time diversity, but the problem is that they retransmit on the same 22 MHz sub-band! If the noise is still there or if the topography of the site did not change, and as a result the multipath effects will be again present, the transmission could be again unsuccessful!
The multipath effects are a function of frequency. For same topography, some frequencies encounter multipath effects, while others do not.
FHSS systems use "time diversity" (they retransmit lost packets at later moments in time) but they also use "frequency diversity" (packets are retransmitted on different frequencies / hops). Even if some hops (frequencies) encounter multipath effects or noises, others will not, and the FHSS system will succeed in executing its transmission.
Conclusions
DSSS provides 11 Mbps capacity links, but it is a very sensitive technology (collocation, multipath, near/far, Bluetooth). The most limiting factor, multipath, may be minimized by using the technology in point to point applications.
FHSS provides only 3 Mbps capacity links, but is a very robust technology, with excellent behavior in harsh environment characterized by large areas of coverage, multiple collocated cells, noises, multipath, Bluetoooth presence, etc. The technology allows easy cellular point to multi point deployment, providing excellent reliability.