Mobile WiMAX has emerged as a leading choice for 4G cellular technology, and now meets the requirements of new smart phones, Mobile Internet Devices (MIDs) and notebook PCs. It provides efficient broadband connectivity for multiple services such as data, VoIP and video streaming with carrier-class QoS, supporting the needs of mobile broadband now and for years to come.
Adding WiMAX connectivity to mobile products delivers broadband speeds greater than 1Mbit/s for all these services, while maintaining the form factor and battery life of existing 3G devices. Broadband speeds are opening a number of new categories of devices such as MIDs, ultramobile PCs (UMPCs) and netbooks. WiMAX subsystems packaged in self-contained modules simplify the integration of this wireless technology.
As nationwide WiMAX networks are built and costs continue to decline, even more possibilities become practical. Consumer products ranging from automobiles to appliances could benefit from the use of WiMAX ()Figure 1, below ). Several of the existing choices include notebooks, handsets, netbooks, UMPCs, and MIDs.
|Figure 1: A Mobile WiMAX module must include a WiMAX baseband PHY device, media access control (MAC) chip, RF devices and power amplifiers.|
The basic form factors for introducing Mobile WiMAX subscriber products were USB adapters and PC cards aimed primarily at notebook PC users. These products have worked well with the first WiMAX base stations in large cities and metro areas, where mobile users wanted broader access than Wi-Fi hotspots could provide.
Since the first USB adapters and PC cards, the mobile wireless market has shifted its focus to mobile handsets and other devices smaller than notebooks. Although there are no standard definitions for various types of mobile handsets, a couple of terms have gained acceptance in the industry: smart phones and VoIP handsets.
Smart phones support e-mail and Internet access in addition to the capability of a basic handset. By including multimode capability— 3G and WiMAX—these phones give customers seamless voice and data service as the WiMAX network is being built out.
At the other end of the handset spectrum is the low-cost VoIP handset, where traditional voice communication is the primary application. WiMAX is the only viable wireless technology for wide-area connectivity and in some cases, these handsets may include a few other basic functions such as messaging or very simple data transfers. The primary market for VoIP handsets is emerging countries, where the wired infrastructure is poor, and most people need voice service before any other application.
Netbooks are small notebooks with 7-inch to 10-inch LCD screens that provide mobile Internet and e-mail access. Typically, they use Windows or Linux OS and can support applications similar to those on notebooks at lower performance levels. Wi-Fi and other connectivity is standard today, and future netbooks will add WiMAX.
Developed by Microsoft and Intel, UMPCs are mini-tablet PCs with touchscreen displays measuring 4inches to 8inches. Most run Windows XP Tablet OS and cost less than $1,000. UMPCs are found in vertical markets such as medical and hospitality.
Introduced by Intel, this small portable device is bigger than a smart phone yet smaller than a netbook or UMPC. The goal is to offer the best mobile platform for accessing the Internet and e-mail as well as supporting multimedia applications. Although technically interesting, these devices have yet to reveal whether consumers are willing to carry a third device in addition to a smart phone and notebook PC.
Because consumers want a wide range of capabilities in mobile products, wireless handset designs are becoming increasingly complex. Devices must support a number of wireless options such as Bluetooth for headset connections, Wi-Fi for connectivity at home or hotspots and wide-area technologies such as 3G to connect to mobile networks. At the same time, consumers demand longer battery life without increased weight or size.
Since WiMAX is the first 4G technology on the market, handset manufacturers are scrambling to add this new capability. The way product designers integrate WiMAX will help determine whether a particular device achieves the mix of characteristics that proves to be “exactly right” in the marketplace.
Selecting the right WiMAX module
One way to ensure maximum versatility in a mobile product is to allow room for a wireless module with characteristics according to market requirements. By designing devices as a module, mobile device vendors can support a wide range of wireless technologies.
A self-contained module measuring approximately 20mm x 20mm can support all of the requirements for Mobile WiMAX certification and be added into new products. Such a module must meet a large number of technical requirements, including proper form factor, low power consumption, support for multiple frequency bands and high throughput. Also, the module must not affect the performance of other wireless devices within the mobile product.
As shown in Figure 1 above , a Mobile WiMAX module must include a WiMAX baseband PHY device, media access control (MAC) chip, RF devices and power amplifiers. A mobile product's host processor typically interfaces with the module via a standard such as SDIO, SPI and/or USB.
Using a module simplifies the design requirements for mobile product vendors, who can more or less ignore the implementation details of the WiMAX product and focus on their core strengths in developing the best product.
Typically, technologies that support much higher data rates also consume more power. Consumers like the higher performance but resist the sacrifice of battery life or the increased weight of heftier batteries. Thus, to ensure that consumers adopt WiMAX products quickly, device power requirements need to be similar to those of existing technologies—despite WiMAX's higher data rates.
To minimize power consumption, mobile WiMAX devices have two power saving modes: sleep and idle. Power savings is achieved by turning off parts of the mobile device when it is not actively transmitting or receiving any data.
In sleep mode, the mobile device is turned off for periods defined by the base station. Handover is supported in this mode as the mobile device can scan for other base stations. Idle mode can save even more power than sleep mode because the mobile device can be completely turned off and not registered to a base station nor does it have to handle handover.
The way these modes are implemented has a great impact on overall power consumption. Specifically, the speed and exact timing of the transitions among the modes has a major influence on power consumption. In addition to these modes, SoC designers can take advantage of many techniques for reducing power consumption in both active and sleep/idle modes.
Because all these techniques are interrelated and depend on the timing and throughput demands of data traffic, it is difficult to generalize about the specific power consumption levels users can expect. The real test of performance happens when running specific real-world applications.
For existing 3G smart phone handsets, talk times typically average 3hrs to 6hrs, and Internet access times average 5hrs to 6hrs. For widespread adoption, WiMAX devices must offer similar performance with the same kinds of batteries.
Achieving this goal is not as difficult as it may seem, because higher data rates can actually reduce overall power consumption if the device transitions rapidly enough in and out of idle/standby modes. By transmitting more data in a shorter time, WiMAX devices can spend less time in active mode and more time in powerdown modes that consume a small fraction of active-mode power. So long as a user does not run applications that constantly demand full throughput, WiMAX devices can work within existing battery lifetimes.
A wide variety of national and international radio-spectrum regulations make a patchwork of frequencies available for unlicensed technologies such as WiMAX. So far in the evolution of Mobile WiMAX, three different bands seem to be the most popular. Ideally, to support seamless worldwide roaming, Mobile WiMAX devices need to support all of these frequencies. The most popular Mobile WiMAX frequency bands are 2.3-2.4GHz, 2.5-2.7GHz, and 3.4- 3.6GHz.
Products designed for one of these bands will only operate within specific countries, which may be appropriate for the lowest- cost handsets. However, extremely compact radio modules are available to cover all three bands at a reasonable cost for users who want unlimited roaming.
Since many mobile devices integrate multiple wireless technologies, designers must ensure that the radios do not interfere with one another. Bluetooth provides short-range communications for headsets, while Wi-Fi connects to hotspots and home networks. Both operate in the 2.4GHz frequency band, yet work simultaneously because semiconductor vendors have arranged methods for coexistence that mitigate interference.
As described earlier, WiMAX devices usually operate at frequencies that do not overlap the 2.4-2.5GHz frequency band of Bluetooth and Wi-Fi devices. Since the frequency bands for all three technologies are fairly close, however, and the power output of WiMAX is higher, the combination creates a good chance for interference.
Standards bodies for these wireless technologies have not resolved the interference issue, so the task is left to the developers of specific implementations. Since Wi-Fi and Bluetooth are established wireless technologies, the sensible approach is for WiMAX semiconductor manufacturers to develop and support techniques that prevent WiMAX products from causing interference. Evaluating a vendor's ability to work alongside other wireless technologies is thus an important aspect of choosing a WiMAX chipset.
The wide variety of mobile devices has led to a corresponding variety of operating systems. To support these platforms and help drive WiMAX acceptance, semiconductor vendors need to provide device drivers that suit the widest possible range of operating systems. These drivers are tailored to suit the interfaces established by each OS company.
The easy choices for PCs are Microsoft Windows Vista and its predecessors. Linux comes in a distant second, but is making some headway in smaller devices such as netbooks. For mobile handsets, the main choices include Symbian, iPhone OS, RIM Blackberry and Windows Mobile.
Together, these operating systems claim more than 90 percent of the handset market. Other entrants that look promising include Google's Android and Palm's Web OS. In addition to getting support for the widest possible variety of operating systems, it is important to have this support tested and even certified to ensure that products operate as expected.
Second-generation Mobile WiMAX chipsets were launched, following the lead of the Mobile WiMAX modules that complies fully with the IEEE 802.16 standard and WiMAX Forum certification profiles. The module works with a scalable OFDMA PHY and operates in time division duplex mode. Capable of supporting frequency bands around 3.5-, 5-, 7-, 10- and 20MHz, the module also has options to support multiple frequency bands in the 2.3-, 2.5- and/or 3.5GHz ranges.
Second-generation Mobile WiMAX chipsets are optimized for smart phones and PDAs and con- figured for a total module-based solution. Such chipset target low-power operation in several ways. Because the baseband chip is implemented in 65nm low leakage process technology and implements extensive powergating technology and other low-power design strategies, the chip consumes ultralow-power in both idle and standby modes. The chipsets include a dedicated power-management IC for highly efficient control of power usage at the system level.
Dean Chang is senior manager in the Application Specific Standard Products Group at tFujitsu Microelectronics America.