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Reduce power consumption in UWB chips

Posted: 28 Apr 2008 ?? ?Print Version ?Bookmark and Share

Keywords:Ultrawideband technology? UWB? USB 2.0? Wireless USB? Bluetooth?

By Gadi Shor

Ultrawideband (UWB) technology is a wireless radio technology for short-range, high-bandwidth communication at very low energy levels that uses a large portion of the radio spectrum. It is considered the most efficient technology employed today in terms of joules per bit. Wireless USB, also known as Certified Wireless USB, is based on the UWB common radio platform from the WiMedia Alliance. Because it combines the speed and ease of use of USB 2.0 with the convenience of wireless technology, it holds great promise for a range of applications, including high-speed digital media transfer between devices in personal and home area networks.

One especially sweet spot for UWB and Wireless USB is in mobile, battery-operated devices requiring high bandwidth and low power consumption. In fact, according to a recent ABI research study, shipments of UWB-enabled devices will grow from virtually nil today to more than 400 million in 2013. This growth stems from UWB's application as a wireless enabler for mobile devices such as cellular phones, digital still cameras (DSCs), notebook computers and PMPs. Such devices store large amounts of data in the form of photos, music and video, and therefore require the ability to move content from one device to another, including to home PCs or televisions.

Design for low-power
Although UWB makes wireless data transfer viable, a number of obstacles make its use challenging, including its small size and throughput. One key obstacle involves minimizing battery power consumption while still providing high throughput. Effectively addressing this challenge requires today's UWB chip designers to adopt special design techniques and to take full advantage of Wireless USB's outstanding power efficiency.

Minimized power consumption and maximized throughput are critical requirements for today's battery-operated mobile devices because of these devices' usage scenarios. Imagine, for example, a consumer who uses a camcorder throughout the day. At the end of the day, upon returning home, the consumer finds that the device has only 10 percent of its battery power left, making it impossible to transfer its contents to another device. During typical usage, this is exactly the type of capability that today's consumer demands.

If the industry is to continue progressing toward the next phase of UWB/Wireless USB in mobile devices, this concern must be addressed. The problem, of course, is that UWB chip design poses a number of interesting technical challenges, not the least of which is that of slashing chip prices by migration to a single-chip CMOS solution affordable to average consumers. This single-chip implementation must include the radio, PHY, WiMedia MAC layer, Wireless USB layer and all relevant interfaces. Because of the low power consumption requirement of mobile devices, realizing this low-cost design has become an even greater priority.

Designing UWB solutions
To successfully design a single-chip, CMOS-based UWB solution in support of emerging mobile devices and their low power consumption requirement, designers must adopt special design techniques to deal with the challenges that exist in several key chip development areas:

Radio. The implementation of a low-power, high-frequency CMOS radio at 3 to 10 GHz requires that special techniques be used to reduce power consumption while still retaining the high-frequency, wide-bandwidth design's sensitivity and linearity.

Hopping local oscillator. The hopping local oscillator is a unique on-chip element that must cover the entire 3-10GHz range. It must also be able to change its frequency by 1GHz within a few nanoseconds. Implementing this element while supporting the low-power requirement is extremely challenging and must therefore be carefully considered during the design process.

Viterbi decoder and FFT. In the UWB chip, the FFT/IFFT block and the Viterbi decoder work at a few hundred MHz, making them difficult to implement. Because of this, the correct architecture of those blocks is vital to low power consumption.

Complete MAC, conversion layers and security engine. The higher layers of the UWB chip must be able to process several hundred Mbit/s of data in real-time. Only efficient design can ensure these blocks' low power consumption.

Low leakage power. The UWB chip design must have low leakage power. Unfortunately, because of the amount of elements in the design, and because the elements must run very fast during active mode, this is hard to achieve. However, proper architecture and process selection can help keep this value low.

Power management. Aside from all of a chip's individual blocks, its power management architecture must be correctly defined to allow transitioning between active and low-power modes.

Power-efficient Wireless USB
Despite these challenges and the special design techniques they necessitate, UWB (and, consequently, Wireless USB) provides an ideal solution for emerging battery-operated mobile applications in cases in which low power consumption is critical. Wireless USB has the advantage of speed over other wireless technologies, as it is able to effectively send at 480Mbit/s over distances up to 3m and at 110Mbit/s up to 10m. Even more critical, the UWB protocol has a well defined superframe structure that has specific time slots.

Bluetooth and 802.11 would consume 18 percent and 7.6 percent of their batteries, respectively, to transfer 4Gbytes of data. In contrast, UWB would consume 0.75 percent of its battery.

As defined by the WiMedia Alliance, a superframe is 65,536?s and has a period that is divided into 256 media allocation slots (MAS), each one 256?s long. It allows all devices that can hear one another to share a common clock accurate up to a few microseconds even when taking into account drift over multiple superframes, a capability that allows devices to announce when they will be transmitting and when they will be listening. This feature also makes it easy to deduce when the medium will not be in use.

This superframe structure allows the UWB protocol to support multiple low-power modes (apart from the obvious "standby" mode found in other wireless protocols and the "hibernate" mode found in Wireless USB) that can be used during data transmission. In Wireless USB, these modes are labeled "ready," "standby" and "sleep" and allow both the host and device to sleep in a synchronized way for short periods during data transmission. As a result, the average power consumption of UWB is significantly lower than that of other wireless technologies.

By its very nature, then, UWB is very low power. This is not because active power (in a cell phone, the power supporting talk time or data transfer) is on, although it is fairly high compared with other standard technologies. Instead, it stems from the fact that UWB works so fast that its total power energy consumption is significantly lower than that of a technology such as Bluetooth.

Other wireless technologies
If today's power-consumption design challenges can be overcome, the benefits for emerging mobile devices can be significant, especially in light of UWB's power efficiency. To fully understand this benefit, consider this true-to-life scenario of a digital still camera with a 4Gbyte memory card and a dedicated lithium battery providing 3W-hr, used throughout the day as content (such as photos and video) fills the memory card until just 5 percent of its battery, or 150mWh, remains.

In such a situation, it is possible to examine how the DSC would behave using three technologies: UWB, 802.11 and Bluetooth.

UWB. Assume for UWB a 100Mbit/s average data transfer rate. Note that this assumes a 400Mbit/s PHY rate active half the time. Assume an average power consumption of 250mW during active mode.

Bluetooth and 802.11 technologies would consume 18 percent and 7.6 percent of their batteries, respectively, to transfer 4Gbytes of data. In contrast, UWB would consume a mere 0.75 percent of its battery.

Transmitting 4Gbytes of data over the air would take UWB 328s (4096/12.5), or 5.5mins. Based on a 250mW average power consumption, the data transfer would consume 22.7mWh, less than 0.75 percent of the camera's battery.

802.11. For a Wi-Fi implementation, assume a 20Mbit/s average data transfer rate and an average 500mW active-mode power consumption.

Transmitting 4Gbytes of data using 802.11 would take 1638 seconds (4096/2.5), or 27mins. Based on a 500mW average power consumption, the data transfer would consume 228mWh, or approximately 7.6 percent of the camera's battery.

Bluetooth. For a Bluetooth implementation, assume a 1Mbit/s data transfer rate and an average of 60mW of active-mode power consumption.

Transmitting 4Gbytes of data with Bluetooth would take 32,768s (4096/0.125), or 9hrs. Based on 60mW average power consumption, the data transfer would consume 546mWh, or roughly 18 percent of the camera's battery.

The results of this usage scenario played out with the three different technologies makes it is clear to see that UWB consumes significantly less power.

By effectively addressing the challenges in these areas and capitalizing on UWB/Wireless USB's inherently low power use, designers can successfully reduce power consumption and maximize throughput, both of which are essential to driving the next phase of UWB/Wireless USB in mobile devices.

About the author
Gadi Shor
, chief technology officer at Wisair, has more than 18 years of hands-on experience in researching, simulating and implementing wireless communication systems. Shor holds MSc and BSc degrees in electrical engineering from Tel Aviv University.

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