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Cirronet transceivers boost signal range, base station capabilities

Posted: 16 Sep 2005 ?? ?Print Version ?Bookmark and Share

Keywords:cirronet? transceiver? wit910? wit2450?

Transceiver cuts costs

Cutting costs and boosting range are two attractive benefits for transceivers used in wireless networking systems. Cirronet has designed its latest transceiver products to do exactly that for both 900MHz and 2.4GHz applications.

Featuring "store and forward" capability, these transceivers can also be used in lieu of repeaters in star network topologies with long-range needs or in those with many natural or architectural obstructions.

The modules are designated the WIT910 for 900MHz, with a 172.8kbps data rate and the WIT2450 for 2.4GHz, with a 460.8kbps data rate. When used as a repeater, this rate is reduced by 50 percent, but only for the first store-and-forward action.

So, what were the major design challenges for these transceivers? "These radios are frequency-hopping, spread-spectrum designs, so the biggest challenge was keeping all the listening and sending functions synchronized with the hop, given that we have a very low-power microprocessor," said Tim Cutler, Cirronet's VP of marketing. "We overcame that with clever buffer management techniques and were able to take advantage of the sophisticated network features inherent in the modules."

A store-and-forward transceiver is unique in that it has multiple functions, those associated with a traditional transceiver as well as those traditionally associated with a repeater. There are necessary throughput performance tradeoffs for this increased functionality. "Of course, you must trade off throughput, but the challenge is to minimize that." observes Cutler. "We designed our transceivers so that when you have multiple store/forward repeaters in a row, you only pay the throughput penalty once."

Some competitive products offering store-and-forward capability are not module based, and Cirronet feels its module product offers designers an advantage. "Designers can integrate the module into their device rather than purchasing a $1,000 standalone box to get the capability," notes Cutler.

For designers, this product does a nice job of addressing range and obstructions with a single unit. For instance, the 900MHz module is designed to achieve a 20 mile range with omni-directional antennas. In effect, Cirronet's approach can actually change the way a base station's range is defined. For instance, instead of from the base station to the most remote radio, range can now be significantly extended by the radios themselves. In addition, these modules can be used in an end device, transmitting on to a radio that is not in direct contact with the base station.

The new modules are built using the company's existing frequency-hopping, spread-spectrum radio technology, which has been optimized to address issues of reliability, including immunity to jamming and multipath fading. The modules use a 24-bit cyclical redundancy check (CRC) and automatic retransmit request (ARQ) along with a 3K buffer to reduce errors in communications.

We can look for Cirronet to continue working in this area, and the design team is committed to finding ways to drive down product costs. For instance, by taking advantage of some of the latest RFICs, the team was able to drive down the cost of the WIT2450 by 50 percent from the previous generation.

For designers curious about adding store-and-forward transceivers, Cirronet offers a developer's kit and free tech support to get you started.

- Janine Love

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