Wireless Propagator: A Wireless Repeater to Switch Transition? -- Part 4

Metro Wi-Fi deployments are expensive. Although coverage over capacity remains a viable goal in this early stage of the market, anything an operator can do to reduce the number of nodes will drive those installation costs down. Cohda, Go Networks and Wavion each offers technologies to address those requirements.

August 30, 2006

5 Min Read
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In my last three columns I discussed the challenges of dense wireless deployments and some of the ways they can be addressed. In the first column, I focused on channel design and selection. In the second column, I looked at the poly-radio APs (access points) from Meru and Xirrus that facilitate dense deployments by dramatically increasing the number of radios available for clients. In the third column, I examined some non-metro Wi-Fi antenna and radio technologies more applicable to the consumer space, with some possible enterprise applications. In this fourth and final column, I'll touch on what some radio and receiver vendors that target the metro Wi-Fi space are doing to decrease deployment costs, enhance coverage and increase throughput and link stability.

More metro Wi-Fi projects are in planning and trial phases than deployed in the United States. But those actually in production, such as Chaska, Minn., St. Cloud, Fla. and Mountain View, Calif., were installed with approximately 16, 20 and 33 Tropos nodes per square mile, respectively. It's important to note that this list is ordered from the oldest to the most recent deployment. Although it's perhaps risky to tease out a pattern from such a short list as this, apparently cities are installing more nodes per square mile than before.

A few months ago, the Chicago Tribune wrote a story on the Chaska deployment. According to a former member of that project, performance and coverage issues plagued the system until more nodes were brought in to boost link stability and fill the gaps. The node count was bumped up from 16 to almost 23 mesh nodes per square mile. In St. Cloud, the news was the lack of good indoor coverage. Mountain View may have unique terrain that requires an even higher density, but its high watermark count of 33 nodes per square mile may be indicative that cities have learned from past coverage and service problems and are now designing denser deployments.

Beyond these specific examples, the generally accepted number is roughly 25 nodes per square mile to provide the necessary 95-percent outdoor 802.11 coverage and 85 percent of indoor rooms that have an outside-facing wall. With nodes at roughly $3,000 a pop, $75,000 per square mile adds up quickly. Even if you apply volume discounts to the mesh hardware, there's still wireless backhaul equipment, pole rental and installation. Currently, with low penetration and standard access rates that border sub-DSL speeds, metro Wi-Fi deployments will be able to focus on minimum coverage requirements for quite some time. In fact, if the link budget (that is, the difference in signal strength between what is transmitted and what is received) can be increased, then lower densities will likely be architected to drive down deployment costs.

Link budgets are not automatically symmetrical in nature. FCC regulations state that access points or nodes in certain configurations are allowed to transmit at significantly higher levels than the clients (downstream). This means that even if the clients can interpret the transmitted signal, the nodes--even with a well-developed receiver and good sensitivity--may not be able to demodulate the response (upstream). Because the majority of consumer traffic is downstream, optimizing that aspect of the link is valuable; higher link connection rates result in a better subscriber experience and a more efficient use of the air time, which in turn increases overall capacity. Of course, if the access point or node can't hear the return signal, it's all for naught.Cohda Wireless addresses the currently asymmetric nature of metro Wi-Fi installations. One of the challenges with OFDM technology in outdoor settings is that long distances and multiple objects introduce long delay spreads between symbols (a discrete potion of a wireless signal). That is, one symbol may come in 'long' after the next. Cohda uses a receiver technology that processes the whole signal, even for delay spreads 10 times longer than normal. All this leads to an approximately 10 dB gain in link budget. Because of its ability to deal with long and changing delay spreads, Cohda's receiver technology also robustly supports mobility--even if the client moves quickly and the delay spreads change rapidly. According to Cohda, if its technology is on both ends, the metro Wi-Fi solution should require two-thirds fewer nodes, but if it's only on the mesh node, a more realistic proposition is about 20 percent to 25 percent fewer nodes. A graphic on Cohda's marketing material documents one-third of the nodes but twice the coverage in comparison to a leading enterprise wireless AP.

Go Networks also targets the metro Wi-Fi market but performs adaptive beam-forming, the act of sending out a signal with different phase and amplitude weightings. Although the signal may be disjointed on the transmitting side, the client receives a coherent signal. In a similar manner, the received signal can be more easily demodulated over multiple antennas and radios, despite the multipath (the different paths that signals can travel because of signal spreading and subsequent reflections). Go Networks' micro-base station uses three 120-degree sectorized antennas. The company claims that its xRF technology allows a 50-percent reduction in the number of nodes while still increasing link quality, throughput and coverage.

Wavion is the last vendor. Using six radios and an equal number of omni-directional antennas in a fixed configuration, the company also uses beam-forming to top out at FCC-restricted EIRP levels. Wavion just released its WS410, an outdoor AP, but the company will target the mesh market through OEM partners. According to Wavion's chief scientist, the company's technology delivers up to a 10 dB gain in both directions, which extends coverage as well as raises link rates and indoor coverage. This promises lower deployment costs because only one-third to one-fourth of the number of nodes is required for comparable service. Future releases of the software will support SDMA (Space Division Multiple Access), which is the ability to send multiple signals to different users simultaneously. This, too, will increase capacity.

Although metro Wi-Fi networks remain a coverage rather than a capacity play, these antenna and receiver technologies will lower the cost for municipal operators to enter more towns and cities where the financial model was not that attractive the first time around. And any developments along these lines, while not so obviously applicable to WLAN products in the carpeted enterprise market at this stage, can only lead to further innovations in link stability and performance for all types of wireless connections.

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