Symposium Surveys The State Of The Broadband Art

Wireless networking for cars was among the topics discussed at last week's confab. Although the technology would eliminate most, if not all, of the wires needed in a typical car, questions remain about whether the wireless nets would deliver the same performance and reliability as wired subsystems.

Princeton, N.J. -- Wireless broadband was front and center at last week's Sarnoff Symposium here. Three technical presentations discussed state-of-the-art research in wireless for automotive networks, broadband access and free-space optics.

Researchers at HRL Laboratories and General Motors R&D proposed the buildout of intravehicle wireless automotive sensor networks based on the IEEE 802.15.4 standard to ease communications between the sensors or switches and control units located within a vehicle.

Today, wide-area sensor networks in which vehicles are themselves the sensor nodes are a more common sensor network application. The intervehicle nets are used for asset tracking and communication of traffic, weather and road conditions.

In-vehicle wireless sensor networks would enable the elimination of most, if not all, of the 50 kg of wiring harnesses used in a typical production vehicle. But questions remain about whether the wireless nets would deliver the same performance and reliability as wired subsystems.

In a typical modern vehicle, the most-demanding sensor will require a latency of less than 1 milliseccond, with throughput of 12 kbits/s, said researchers at General Motors. Further, the network will need to support about 15 such sensors. The least-demanding sensor subsystemwill require 20 sensors with a latency of approximately 50 ms and data throughput of 5 bits/s.

Because of the variety of sensing applications within a vehicle, the GM researchers concluded that a one-size- fits-all wireless network solution would be problematic. And in characterizing the propagation environment within a vehicle, they found that fading and path-loss characteristics of the intravehicle wireless channel would greatly affect design choices. A heterogeneous network needs to be architected to meet the requirements of both inter- and intravehicle comms, the researchers said.

For outside the vehicle, researchers from Intel Corp. (Santa Clara, Calif.) demonstrated that orthogonal frequency-division multiple access is superior to TDMA and CDMA for both fixed and mobile broadband wireless data network access. OFDMA's advantages include scalability, uplink orthogonality and the ability to take advantage of the frequency selectivity of the channel. The time and frequency resources can be organized into subchannels for allocation to individual users. With little modification to the air interface, the Intel researchers said, OFDMA systems can be deployed in various frequency band intervals to address the need for a range of spectrum allocation and usage model requirements.

Researchers from the broadband and mobile networking department at NEC Laboratories America evaluated WiMax as a protocol for use over free-space optical media, analyzing the performance of optical wireless communication systems with multi-subcarrier orthogonal frequency-division multiplexing as set out in the IEEE 802.16-2004 WiMAX standard.

Free-space optical (FSO) technology is recognized as an important broadband access means in next-generation commercial and military networks. Unencumbered by spectrum regulations and capable of multigigabit data rates, FSO offers excellent security and quick, inexpensive setup. Vulnerability to weather effects and strict line-of-sight pointing requirements limit link lengths to a few kilometers, but commercial products have neared 99.999 percent availability over this range. Various multiple-input, multiple-output (MIMO) techniques are under investigation to improve FSO performance.

Thus, for short-range terrestrial applications such as interoffice links in urban areas, where fiber is either unavailable or too costly and time-consuming to install, FSO can be a reliable, cost-effective solution, according to NEC.

The NEC researchers claim that the protocol used in the IEEE 802.16d-2004 standard, upon which the WiMax specification is based, can be applied in FSO applications as an inexpensive, secure, short-range wireless transport for WiMax traffic and, thus, can help to shape the architecture of the distribution network. One application might be in a densely populated urban area, where the physical landscape results in a highly dispersive RF multipath channel.

To carry OFDM traffic over an optical wireless link, however, multiple-subcarrier modulation (MSM) needs to be used, the NEC researchers said. They presented a case for FSO transmission of 802.16-2004 OFDM traffic via MSM and reported on the resultant effects of adaptive modulation, atmospheric turbulence and receiver noise on system performance.

The analysis indicated that smaller constellation sizes are better suited for the FSO MSM channel.