Mobile WiMAX: Challenges and Opportunities
To enable roaming, mobile WiMAX devices will need to support multiple bands, which creates a variety of considerations for designers of such devices, as well as their component suppliers.
David Bailey, Texas Instruments
Although dozens of service providers worldwide have committed to or have already begun deploying mobile WiMAX (IEEE 802.16e), the technology will take several years before it achieves geographic coverage comparable to cellular. Even so, some consumers will immediately expect mobile WiMAX to be widely available — an expectation created by their experiences with cellular.
Service providers and vendors are responding to this expectation and are doing what it takes to speed up the deployment of mobile WiMAX. One recent example is
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Figure 1. TI’s TRF243x RF chipset at 5.8 GHz provides the entire RF and IF functionality of an RF front end in two highly integrated devices, saving components and costs. |
the partnership between Sprint Nextel and Clearwire, two companies that are building a joint mobile WiMAX network that by the end of 2008 will cover approximately 100 million people, or roughly one-third of the U.S. population. The total deployment is projected to be on the order of $2.5 to $3 billion. Another example is the WiMAX Forum’s™ Global Roaming Working Group, which is developing technical standards for roaming between WiMAX networks in order to assure the availability of global roaming services and to meet the time demands of the marketplace.
These efforts are important because greater geographic coverage inevitably creates a larger pool of potential customers. That in turn creates a snowball effect. With more people buying mobile WiMAX modems, handsets and other devices, equipment volumes will help push down device costs to the point that even more people — including price-sensitive demographics — can afford mobile WiMAX devices and services. WiMAX will realize the wireless community’s ultimate expectations, when consumer products such as WiMAX enabled digital cameras and gaming consoles, hit the market.
To enable roaming, mobile WiMAX devices will have to support multiple bands. Initially this means some combination of 2.3, 2.5, 3.5 and 5.8 GHz, with other bands possible, depending on future allocations. Future available spectrum could include the 700 MHz band. Some mobile WiMAX operators may eventually have networks in multiple neighboring countries, each with a different band for WiMAX, requiring multi-band devices. Regardless of the band combination, roaming creates a variety of considerations for designers of mobile WiMAX devices, as well as their component suppliers.
Balancing Cost and Performance
Mobile WiMAX has a variety of inherent design challenges:
•Low cost: Most service providers will position mobile WiMAX as a low-cost alternative to incumbent wired and wireless technologies, such as cellular, cable, and DSL. But operators such as Sprint Nextel have made it clear that they want to keep device subsidies to a minimum. This creates a challenge for device designers, which must hit operators’ aggressive retail price targets — such as sub-$200 customer premise equipment (CPE) — in order to grab a share of the mobile WiMAX market. In some cases, mobile WiMAX devices also must be able to compete with products from the highly commoditized Wi-Fi space. WiMAX handsets will also need to compete with the subsidized 3G devices and their economies of scale.
•High throughput: Many operators will market mobile WiMAX on the basis of its throughput and bandwidth-intensive services — such as video — that require those speeds. To meet the bandwidth requirements of consumers who are used to the broadband rates that cable and DSL offer, WiMAX will need to deliver throughput rates in the neighborhood of 1 to 10 Mb/s, but to meet the needs of enterprise customers WiMAX will need to deliver on the promise of speeds of up 72 Mb/s.
•Coverage: Some operators plan to use mobile WiMAX to bring traditional telecom services such as voice over WiMAX applications to sparsely populated, underserved areas in both developed and developing countries. Those plans require user devices and basestations that can maintain high data dates over long distances. The entire ecosystem will need to be built out, including backhaul services.
Basestation OEM’s will be challenged to provide cost constrained WiMAX equipment which meets the spectral requirements of the primary wavebands, 2.3, 2.5, 3.5 and 5.8 GHz, without requiring
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Figure 2. TI’s flexible TRF11xx chipset at 2.5 GHz and the TRF12xx RD chipset at 3.5 GHz supports a variety of system configurations for versatility and high performance in designing RF front ends for wireless broadband systems. |
significant variations in design implementation. Quadrature modulators, in direct conversion designs, which support wide bands of operation will enable a more flexible solution while reducing overall BOM cost by collapsing the transmit signal chain. Multi-mode terminals will require more costly chipsets while CPE designs may employ less costly pin compatible chipsets so as to enable ODMs to build variations on a common design to meet individual service provider frequency requirements and yet still meet WiMAX specifications as set forth by the WiMAX Forum™ Technical Working Group.
What’s Inside Counts
For some service providers, indoor coverage will be particularly important. For example, a cable operator might use its or a partner’s mobile WiMAX network as part of a quadruple-play strategy. Or a service provider that uses only mobile WiMAX might see indoor coverage as key to encouraging wireline displacement. The latter example isn’t unprecedented. Many cellular operators have spent the past several years improving their indoor coverage in order to create the perception among consumers that their cell phone is reliable enough to be their only phone.
As a result, some service providers are considering femtocells, which are base stations similar to Wi-Fi access points in terms of size and coverage. Designed to ensure seamless coverage inside homes, offices and other places that the macro network can’t adequately cover, femtocells can be standalone devices or integrated into other devices, such as cable set-top boxes.
Femtocells give service providers another tool for dealing with environmental variables that affect coverage. For example, the metallic window tinting that’s common in commercial construction can attenuate signals by 20 dB or more. Even the mobile WiMAX frequencies themselves can be an issue because they are quite high and thus may have inherent difficulty penetrating buildings. Femtocells provide an alternative to increasing power at macro cellular sites, which could produce interference elsewhere, or adding new macro cells, which are a significant additional cost and may not be possible due to factors such as zoning laws.
Mobile WiMAX femtocells have a variety of design considerations. For example, they may feature 2ࡨ Multiple Input Multiple Output (MIMO) antenna technology and have transmit output power (Pout) which allows for signals to penetrate walls. One way to implement these requirements is to simply utilize two 1ࡨ MIMO RF transceiver chipsets (2 direct conversion transceivers plus 2 PA devices). However, designers must be cognizant of LO phase alignment such that overall performance is not degraded. RF chipsets that can utilize a common LO source can easily compensate for any phase alignment issues that may occur due to PCB trace mismatch, or other electrical issues. RF chipsets which employ a super heterodyne architecture allow for narrow band filtering options in order to achieve low spurious and output noise, as well as achieve high immunity to adjacent channel interference. The ideal RF chipset also should feature a Pout of around 20 dBm to 30 dBm in order to provide solid, reliable performance in an indoor environment.
Reliable, consistently high performance is key for mobile WiMAX for several reasons. One is that many service providers plan to use mobile WiMAX to support voice services. So if the femtocell is susceptible to interference — such as from a satellite radio terrestrial repeater or C-band satellite in licensed bands, or from other unlicensed services at 5.8 GHz — then the service provider will have a difficult time convincing customers to replace their wireline phone service with Voice over IP (VoIP) over WiMAX.
For both voice and data, if the femtocell is unreliable, the user’s handset or other device may switch to the macro WiMAX network. That can have a ripple effect, with macro sites struggling to keep up with the onslaught of traffic that’s supposed to be carried by femtocells.
In the end, the un-tethered broadband services and vision of ubiquitous wirelessly connected devices that WiMAX promises to deliver, will cause designers, OEMs, and chipset providers to persevere and ultimately overcome all of these challenges so as to enable many endless possibilities.
About the Author
David T. Bailey is a product marketing engineer for High-Performance RF and Digital Radio products within the Communication and Wireless Infrastructure market segments of Texas Instruments. www.ti.com
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