ISPs and private companies are rapidly installing IEEE 802.11-based wireless point-to-multipoint networks to provide fixed wireless connectivity to businesses and homes in areas that are not served well by DSL, ISDN, or cable modems. An issue, however, is that wireless ISPs and private wireless metropolitan solutions operating in the same area often interfere with each other. This degrades throughput and causes occasional disruptions in service, which aggravates wireless ISP owners and their customers.
What's the problem?
The primary cause of RF interference between wireless MANs is that most implementations utilize 802.11 products operating in the same 2.4 GHz frequency band. Because the FCC designates the 2.4 GHz band as license-free (at least in the United States), competing users of the technology are not required to coordinate transmissions. For example, an ISP using frequency hopping spread spectrum (FHSS) can operate with any of the 802.11-defined hopping sequences, and ISPs using direct sequence spread spectrum (DSSS) can select any of the authorized DSSS channels. As a result, the lack of coordination among competitors leads to simultaneous use of the frequency band and damaging interference.
DSSS is the basis of the popular 802.11b standard, and most wireless ISPs install systems with 802.11b-compliant products. DSSS doesn't operate well, however, in the presence of FHSS implementations. A FHSS system transmits signals uniformly over the entire 2.4 GHz band (83.5 MHz) by rapidly hopping over different frequencies. On the other hand, DSSS continuously transmits over approximately one-third of the band (22 MHz). This means that DSSS will overlap with roughly one-third of an FHSS transmission, but FHSS overlaps with the entire DSSS signal. This causes an FHSS system to impart greater damage in terms of RF interference to one based on DSSS. If a competitor starts blasting signals from a nearby FHSS network, then a DSSS (i.e., 802.11b) system will generally fall to its knees.
How can wireless ISPs live happily together? In some areas, such as Anguilla in the West Indies, companies can obtain special licenses from local governments to have exclusive use of the 2.4 GHz band. This is rare in other areas such as the United States and Europe, however, and most wireless ISPs must be prepared to deal with possible consequences of the uncontrolled frequency space.
If you have an existing system experiencing interference, try tuning the transmitters to different hopping sequences (for FHSS) or operating channels (for DSSS). If that doesn't solve the problem, then you could try redesigning the layout of the system to avoid the source of interference. If possible, try relocating your central transmission site and repeaters to higher ground in order to improve the signal power received by your customers.
As part of planning the implementation of a wireless ISP or private wireless metropolitan network, you'd better consider the risks of interference. Is it likely that someone else will install a wireless metropolitan network within the same area? If a "yesanswer to this question is even a remote possibilitythen ensure you have enough of the optimum transmitter sites in area so that future competitors will as little impact possible.
To significantly reduce interference, you should also consider the use of the newer 5.8 GHz, 802.11a point-to-multipoint products that will become available toward the end of 2002. In addition to higher performance (54 Mbps using OFDM), 802.11a operates in the roomier 5.8 GHz band and offers the use of 12 noninterfering channels in the same area. This provides much more capacity than 802.11b implementations, which minimizes the potential for interference among wireless systems.
Stay tuned! Next time, we'll discuss ways to support higher performance requirements of densely populated end-user environments.