An Opposing Position on Mobile Broadband
Vol 21 No 4
Anyone familiar with the technologically advanced mobile command centers that our biggest refineries and chemical plants have put into service knows that live video feeds, infrared thermal imaging and split-second utilization of information databases are reality, not science fiction. Much of this new technology depends on providing high bandwidth communications in emergency situations.
Cohda Wireless has staked out a position on providing mobile broadband for public safety services that puts it in direct opposition to some industry giants in the communications field. At the center of the controversy is the Institute of Electrical and Electronics Engineers (IEEE) 802.11 wireless standards that specify an "over-the-air" interface between a wireless client and a base station or access point, as well as among wireless clients.
According to Martin Suter, CEO for Cohda Wireless, competing companies hope that municipalities, public sector agencies and other potential customers overlook one key fact: 802.11 was neither designed for mobility nor for outdoor deployments, especially not in cities where harsh multipath is a major issue leading to device failure.
"Off the shelf 802.11 today is a wireless LAN technology," Suter said. "When you try to connect it at speed or outdoors you're making it perform unnatural acts. By that I mean those are two deployment scenarios for which it was never intended and for which it is poorly designed."
As Suter explained in a recent article in Wireless Week, the ease with which people have become accustomed to using 802.11 in their homes and offices has misled them into believing it is an attractive option for public safety users. However, to overcome the physical layer (PHY) limitations of the IEEE standard requires the saturation of an area with 802.11 devices such as repeaters and routers at great expense.
"802.11 was designed for indoor propagation in homes and offices," Suter said. "Some minor reflection of energy off walls and other things can be dealt with. It is fundamentally different when you're outdoors and have buildings, billboards and vehicles. It is a much harsher RF environment."
Much of the 802.11 standard depends on OFDM (Orthogonal Frequency Division Multiplexing) as a carrier technology.
"Wireless transmission is basically transmitted energy that is modulated at the transmitter and demodulated at the receiver," Suter said. "In an ideal world, the receiver recognizes the energy in a coherent way that tells it the energy is something it should care about. To do this, OFDM uses a cyclic prefix that acts like a packet header for the information being transmitted."
However, if that transmitted energy arrives from multiple points due to outdoor reflections, problems creep in, Suter said.
"Imagine trying to interpret a conversation in which the same words are arriving at your ear at different points in time," Suter said. "Try and make coherent sense out of that. It's impossible unless you have some way of filtering the transmission to compensate for the echoes and reflections."
The industry is trying to deal with the problem by extending the cyclic prefix in new standards like 802.16, however there are no plans to support public safety bands in the near term. Cohda has taken a different approach involving new patent pending techniques that resolves the problem while maintaining interoperability with the 802.11 standard, Suter said.
"What we do involves far greater receiver sensitivity, plus channel estimation that alters the receiver to accommodate the changes due to reflection and mobility," Suter said. "Channel estimation is very difficult but is fundamental to what we do."
In February 2002, the Federal Communications Commission (FCC) adopted rules dedicating 50 MHz of spectrum in the 4.9 GHz band for use in support of public safety. Cohda's approach allows public safety users to take advantage of 802.11 standards at 4.9 GHz while delivering high speed mobility and a much more robust radio signal, Suter said.
"An important decision to public safety is between proprietary systems that may have capabilities that are compelling," Suter said. "One particular capability of importance is high speed mobility. Clearly that requirement is different from the average municipal wireless consumer who is not likely to require broadband data conductivity in their vehicle while traveling at speed.
Making the licensed spectrum at 4.9 GHz more available to public safety goes a long way towards addressing the many issues of interference, Suter said.