As the previous edition of EWT was being netcasted, I was on the phone having an interesting conversation with Byron Young, VP of marketing and sales for Adaptix based in Seattle, WA. Adaptix is an equipment manufacturer specializing in software-defined base stations and client boxes for the deployment of fixed and mobile WiMAX networks. This is a relatively young company with roots reaching back to 2000, but it is staffed with engineers that offer decades of experience in tone and subcarrier modulation technologies such as xDSL and Wi-Fi. They believe their position in the future of WiMAX deployment is strong with a foundation of intellectual property, innovation and experience.

In addition to the usual corporate overview, Byron and I discussed WiMAX markets and the sophisticated technology that is making the global rollout of WiMAX possible. Topics included available spectrum by region, rollout hot spots, mobile WiMAX topography and some interesting technical details surrounding orthogonal frequency division multiple access (OFDMA), the air interface, a.k.a. physical medium means of communication.

The technical discussion was of particular interest to me and fits well with those topics that I usually tackle in this space. Consequently, I’ll spend a little time here sharing some of the details with you.

He characterized the mobile WiMAX infrastructure as a cellular system, but one with many more cells each smaller in size and power levels than that of cell phone systems. Most WiMAX cell sizes will range from sub-km to around 5 km--larger radii in rural areas with line of sight, and definitely shorter in urban areas with mostly NLOS performance and tons of multipath and other interference sources. Operators will naturally be desirous of deployments that can provide average bit rates per user in the 1 Mbps+ range in order to support emerging services such as mobile IPTV, music/video downloads, etc. Thus, cell sizes must be small, mostly below 2 km to 3 km in cities in order to connect at these speeds.

Instead of tall, obvious towers and antenna systems fed by large expensive base stations, mobile WiMAX installations are far more modest with ‘any-structure’ placement (poles, top of buildings, inside buildings, bridges, etc.) and small 3-inch rack-mounted base stations--that’s not to say that WiMAX will not, in some cases, share towers and rack space with cell phone installations.

Byron described Adaptix’s mobile WiMAX base stations and CPEs as software-defined MAC and PHY radios that are extremely agile to dynamically address user needs as they enter the network. That means channel characteristics adjust dynamically based on user bandwidth and quality of service (QoS) needs and on the quality of the base-to-mobile connection. This is where it gets even more fascinating.

Mobile WiMAX systems use OFDMA, an air interface method that uses a wideband super channel, which is composed of a large number of narrow subcarriers divided into groups forming subchannels. When a user enters the network, a subchannel is dynamically defined to meet the needs of the user. I was interested to learn that the number of subcarriers contained in each subchannel is not a fixed value--it is determined according to the user’s bandwidth requirements, which means the number of subcarriers vary according to the transported content, whether it is streaming music, streaming video, voice or time-indifferent data. This smart dynamic spectrum allocation optimizes the number of users at any given time.

What is more, subcarriers are each dynamically and optimally modulated using the highest degree of quadrature amplitude modulation (QAM) that each subcarrier can tolerate under the prevailing subchannel conditions. This is often referred to as bit loading because the subcarrier is modulated to form a phase and voltage analog symbol that contains (represents) a combination of data bits. For example, 64 QAM loads the subcarrier with 6 bits to form 64 unique analog symbols. Higher bit loading means more data transported per subcarrier with each symbol interval. Streaming video requires more subcarriers and the highest level of bit loading for each subcarrier as compared to the needs for generic Web surfing.

Each of Adaptix’s base stations can handle traffic for a couple hundred users at the same time. Even though subchannels are dynamically adjusted to accommodate the user’s capacity needs, as discussed above, a maximum bit rate is established by the network service provider in a range up to 5.8 Mbps for downlink and up to 1.2 Mbps uplink for each user--maximums are determined by the equipment, which is designed to meet mobile market needs. Still, the dynamic nature of each subchannel enables better user access and greater numbers of simultaneous users.

Adaptix has fixed-location WiMAX client boxes that look like dual-antenna Wi-Fi boxes. With this company stretching into mobile WiMAX markets, you can look forward to upcoming product announcements for higher capacity base stations and for card interface and embedded solutions to equip notebooks, smart phones and other on-the-go appliances.

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