Flexibility through wireless USB
01 June 2007
As the world moves toward a wireless future, the need to combine higher bandwidth and ubiquitous interoperability between disparate devices represents the next big challenge
With more than one billion USB ports shipped in 2006 and a growth rate of 25 per cent, wired USB is firmly established as the de-facto standard for connecting computers, peripherals, and other devices, but the flexibility of USB has thus far been limited by its reliance on physical cabling.
Next-generation Certified Wireless USB will leverage ultra-wideband (UWB) wireless technology to overcome these limitations. UWB makes use of unlicensed spectrum in the 3.1GHz to 10.6GHz range to deliver very high bandwidths over short distances (3m to 10m), with excellent power efficiency and low-noise characteristics.
Worldwide UWB
As UWB radios use spectrum currently occupied by other wireless services, the process to establish regulations for its use have been a controversial topic worldwide. However, initial UWB regulations have now been agreed in the US, Europe, Japan, and Korea. Other major regions, such as China, are expected to have regulations in place allowing UWB by the end of 2007.
European UWB regulations are very similar to Japan and Korea. Europe allows UWB to operate in two large spectrum band groups. The 6GHz to 8.5GHz band is available for use by UWB radios with a maximum transmit power limitation of - 41.3dBm/MHz (referred to as full power) and no other operational restrictions. The 4.2-4GHz band has a -41.3dBm/MHz limit until 2010 beyond which UWB radios are required to implement mitigation techniques to operate at full power. There are other frequency bands still under consideration for UWB operation and their allocation is contingent upon results from interference studies. While the European regulations are more restrictive than those in the US, they still enable a robust, high-performance radio link capable of 480Mbit/sec and ranges of 15m to 20m.
By standardising on UWB technology based on the WiMedia Alliance approach, the Certified Wireless USB
specification lays a solid foundation of interoperability that will eliminate the hassles of physical cabling while delivering speeds of up to 480 Mbit/sec.
Network connections
WiMedia offers a complete short-range wireless personal area network solution that has already been adopted by the USB Implementer’s Forum, the Bluetooth SIG, Ecma, and ETSI (European Telecommunications Standards Institute). This enables Certified Wireless USB to offer mobile users a complementary technology that seamlessly coexists with ubiquitous Wi- Fi networks, enabling seamless high-speed information flow between all of the users' local devices and enhancing the value of the broader network connection.
With existing USB and Bluetooth representing shipments of over 2billion ports per year and ongoing growth rates of 25 to 30 per cent, the shifting of even a fraction of this demand to high-speed wireless means that suppliers of UWB products will be facing explosive production ramp-up challenges. The movement toward highspeed UWB-based wireless solutions is likely to be constrained only by product developers' ability to achieve stringent performance, cost, power and reliability objectives.
Wireless silicon
The first ‘certified’ wireless USB silicon is expected to finally make it through interoperation testing and the certification process, and achieve ‘logo status’ by mid-year. This will unleash the first wave of products that will include dongles and hubs for retrofitting USB ports, laptop PCs, external storage devices, printers, digital cameras, and portable media players. In 2008 mobile handsets and portable consumer devices will enter the market. All will initially be based on the certified wireless USB protocol. By mid-2008, the first products based on the Bluetooth 3.0 UWB specification will emerge. With Bluetooth’s legacy in the handsets, it is likely that Bluetooth 3.0 will first find its way into the mobile market.
A third protocol, WiNet, has been developed by the WiMedia Alliance and is based on Internet Protocol. WiNet is meant to enable IP peer-to-peer connections and bridging to other IP-based networks such as Ethernet, HomePlug, MOCA, and others.
A major factor for wireless USB implementations will be minimising power consumption while maximising data transfer capabilities, or optimising MBytes per mW. This is particularly important in smaller mobile devices that must operate with tight power budgets to extend battery life.
Power budgeting
The multi-band orthogonal frequency division multiplexing (MB-OFDM) technology used for WiMedia ultra-wideband is inherently powerefficient. However, the selected chip-level implementation approach plays a critical role in achieving the low-power objectives needed to support widespread market adoption of wireless USB, especially for batterydependent portable devices.
There are three ways to look at power consumption for the target devices: the absolute power consumption, standby consumption, when the device is continually available in a ‘sleep’ or ‘off’ condition, and the power when the device is active and transferring data.
The first two modes can typically be calculated on a linear basis. In contrast, the third ‘active’ mode can exhibit a high degree of variability, depending on the amount and characteristics of the data. For example, bursts and peaks of data can present challenges in that higher power may be required for short periods but also can offer opportunities for smoothing out power usage, if the particular application’s transmission latency requirements are not too tight.
Radio functionality
WiMedia UWB was designed specifically to enable very fast transitions from ACTIVE to OFF and OFF to ACTIVE, in a microseconds. As shown in figure 1, this results in excellent power efficient usage of the radio function. For example, when using a 480Mbit/sec radio to transfer 10Mbit/sec of data, it can be asleep most of the time.
A comparison of various wireless and wired transmission methodologies, as shown in figure 3, illustrates that WiMedia-based ultrawideband is nearly three times more powerefficient than even 802.11n, the newest version of Wi-Fi. This enables the new generation of certified wireless USB to deliver Mbit/mW performance that is significantly better than any other wired or wireless standard, except for high-speed wired USB 2.0.
The overall objectives for certified wireless USB and Bluetooth 3.0 include standby or ‘sleep mode’ power consumption in the 2mW to 3mW range, combined with ‘active’ transmission power efficiency of 1Mbit/mW.
Single-chip CMOS
The aggregate levels of raw data are increasing with the growth in informationrich multimedia, and the need for low-latency is important for many applications, such as voice or video. Designers must ensure that chip-level implementation choices provide extra headroom for minimising power usage across all operating modes. To derive maximum benefit from the WiMedia standard's inherent flexibility and power efficiency, chip-level implementations need to intelligently integrate the entire wireless USB transceiver function to minimise power usage even during periods of peak data transmission.
The optimum approach for technology ramp-ups is to move quickly toward highlyintegrated CMOS-based, single-chip silicon solutions. Standardisation on all-CMOS solutions that integrate the complete MAC and PHY functionality on a single-chip enables system designers to both minimise power consumption in the near-term and to leverage proven CMOS semiconductor processes to optimise cost/performance throughout ramp-up and maturation.
Multi-chip implementations and/or the use of processes like SiGe, BiCMOS or GaAs might bring a difficult transition to bring down costs and reduce design complexity.
CMOS power
Multi-chip implementations have to ‘waste’ power for external buses and drivers. A single-chip CMOS-based design that combines the MAC/PHY radio along with all of the associated functionality, such as passives, filters, crystals, and other elements of the wireless interface offers a compact and power efficient alternative.
Wireless USB power efficiency will result from a combination of chip-level power efficiency, the inherent efficiency of the WiMedia ultra-wideband wireless standard and intelligent management of application level usage models. The ‘turn-on; transmit; turn-off’ model provides better efficiency than the ‘always-on’ model. Similarly, the capability to transmit higher amounts of data in a shorter period of time allows for maximum payload to be delivered during each period of active transmission and inevitably shortens the duration of higher power usage.
MARK BOWLES is co-founder, Staccato Communications
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