24 May 2012
Vikas Durkha and Steven Chen
As mobile handheld devices continue to become essential parts of our lives, developers are inventing new applications every day to support our on-the-go lifestyles. Smartphones and tablets, for example, are increasingly finding use in media playback. Improving the consumption and transport of content, however, has proved to be one of the most challenging tasks. As a result, improved storage capacity and data-transfer rates are both crucial in mobile-system designs. Although the storage capacity of mobile devices doubles every year to keep up with increasing content sizes, data-transfer rates remain frustratingly low.
The emergence of USB 3.0
According to In-Stat, manufacturers in 2015 will ship more than 280 million mobile phones with USB 3.0 capability. USB 3.0 brings a tenfold bandwidth improvement over USB 2.0, achieving a raw throughput of 5 Gbps, or 600 Mbytes/sec—nearly 480 Mbytes/sec of actual data throughput after accounting for the 20% protocol overhead. Because USB 3.0 is data-agnostic, it can stream any type of content, including high-definition video. USB 3.0 has also raised the bar in power provisioning and management. It allows a device to draw as much as 900 mA from a PC during operation and still maintain plug-and-play capabilities. With several power-saving modes, such as idle, sleep, and suspend, USB 3.0 optimizes the operating life of battery-powered mobile devices.
With more than 6 billion devices enabled and nearly 100% penetration in mobile handheld devices, USB is now the most established interface. Because USB 3.0 is backward-compatible with USB 2.0, consumers should have no trouble understanding and using it.
USB 3.0 is equally familiar to both consumers and tablet designers. Two design philosophies are spearheading the shift toward mobile computing in the tablet form factor. Conventional-laptop manufacturers, including Asus, Acer, and Toshiba, opt for top-down miniaturization, whereas mobile-handset makers, including Apple, Samsung, and HTC, are taking a bottom-up approach. As USB 2.0 has held a firm presence in both fronts, tablet designers with a background in either laptops or handsets will find moving to USB 3.0 a straightforward process.
Although the standard A connector for USB 3.0 has the same dimensions as for USB 2.0, there are concerns with the size of the USB 3.0 microconnector for phones, tablets, and other handheld devices. The USB 3.0 microconnector has a unique configuration, where the USB 3.0 lines run alongside the USB 2.0 connector (Figure 1). This connector has a larger profile than that of the USB 2.0 connector, which causes a concern for mobile-device manufacturers striving for an ultracompact form factor. Manufacturers are wary of adding yet another bulky connector to their systems. The USB 3.0 microconnector has a total width of 12.85 mm (Figure 2).
In addition to a USB 2.0 microconnector, most high-definition handhelds today have an HDMI (high-definition-multimedia-interface) microconnector for streaming high-definition video. Manufacturers can, however, omit micro HDMI and instead stream high-definition video over an MHL (mobile high-definition link), a new proposed mobile video/audio interface for directly connecting mobile devices to high-definition TVs and external displays. Because MHL is connection-agnostic, USB 3.0 can instead carry its audiovisual signals, avoiding the need for another HDMI microconnector.
The USB 3.0 microconnector is smaller than USB and HDMI microconnectors together. The HDMI microconnector measures 6.4 mm long, and the USB 2.0 microconnector measures 7.8 mm long. Taking the separation between these two connectors into account, the result is a total connector length of about 15 to 16 mm, or about 25% larger than a USB 3.0 microconnector. Thus, mobile-device manufacturers can produce elegant designs using one multipurpose USB 3.0 microconnector that enables faster content data transfer, rapid battery charging, and high-definition video streaming over MHL.
Storage in most mobile devices employs either SD (secure-digital) card or eMMC (embedded-multimedia-card) flash technology. The SD 2.0 standard supports a maximum of 25 Mbytes/sec, whereas the eMMC 4.3 standard supports a maximum of 52 Mbytes/sec. Neither can effectively use the full USB 3.0 data pipeline. This underusage, however, changes with the new SD 3.0 standard that raises SD performance to 104 Mbytes/sec. Similarly, the latest eMMC 4.41 specifications will achieve a 104-Mbyte/sec upgrade. SD 3.0 cards are available, with micro-SD versions coming soon; eMMC 4.41-compliant flash devices are in full production.
USB 2.0 is barely supporting the current storage standards (Figure 3). With the UHS (ultra-high-speed) II- and UFS (universal-flash-storage)-based devices poised to hit the market by the end of 2012, USB 2.0 will soon become the rate-limiting link in the overall data-transfer pipeline. Mobile-system manufacturers must adopt USB 3.0 into their new designs to take full advantage of these storage improvements.
The amount of digital content on mobile devices will continue to increase with the proliferation of high-definition video-recording features. A 10-minute home movie can easily take up multiple gigabytes of storage. A common usage scenario today is for users to transfer these movies to a PC for editing, sharing, and playback. Although this scenario would be a considerable task for USB 2.0, USB 3.0 SuperSpeed offers instant gratification with less waiting.
With so much data to store, handset and tablet vendors are looking into employing RAID (redundant-array-of- independent-disks) technology to increase data throughput. Using a RAID-0 configuration, large data are striped and stored simultaneously in two SD or eMMC devices. RAID-0 works with USB 3.0 by effectively doubling the maximum throughput of storage devices and using more of USB 3.0’s available bandwidth. Today, the transfer of an 8-Gbyte movie over the fastest USB 2.0-enabled phone requires more than seven minutes at 18 Mbytes/sec. Upgrading to USB 3.0 broadens the data-transfer pipeline and allows data transfers to occur at peak current-storage limitations, meaning that an internal bridge supporting a USB 3.0 port and two storage devices in a RAID-0 configuration can operate at the maximum performance of the storage device. This rate is now approximately 150 to 200 Mbytes/sec. At this rate, the transfer time of an 8-Gbyte movie decreases to 41 sec (figures 4 and 5).
USB 3.0 can also help to reduce the manufacturing cost of devices. Many handheld-device vendors use USB during factory manufacture to preload apps, music, movies, and other content onto devices. Smartphone operating systems and preloaded content larger than 2 Gbytes allow manufacturers to preload phones with USB 3.0 at 10 times USB 2.0 speed, translating to a more efficient process and greater cost savings.
Sync and go
In the era of cloud computing, an abundance of cloud services, such as iCloud, Microsoft SkyDrive, and Dropbox, has emerged to allow users to wirelessly synchronize their data without going through a PC. The need for wired technology in mobile devices is up for debate. Wi-Fi is the most popular wireless standard today, with a 93% attachment rate on smartphones and tablets, according to IMS Research. Its untethered convenience has brought countless benefits to our daily life, including surfing the Web at a local Starbucks, sending e-mail just before catching a flight, or taking a picture on one device and having it appear on another through the cloud.
In its latest revision, Wi-Fi 802.11n supports MIMO (multiple-input/multiple-output), in which multiple antennas coherently resolve more data than a single antenna can. Wi-Fi 802.11n also provides a 40-MHz-wide channel for increased throughput and can operate in the less-congested 5-GHz ISM (industrial/scientific/medical) band. Although 802.11n’s theoretical bandwidth of 600 Mbps is still no match for the 5-Gbps bandwidth of USB 3.0, it is good enough for moving content between mobile devices and PCs.
Wi-Fi 802.11n’s benefits are diminished in mobile designs, however. The maximum 600-Mbps bandwidth typically occurs only under ideal operating conditions in which the 40-MHz-wide channel uses four spatial streams. Because each spatial stream requires an antenna and an ADC, it is not cost-effective for mobile-system vendors to implement all four sets. To be effective, each antenna also needs distant placement; the compact mobile form factor severely constrains this requirement. Furthermore, using a 40-MHz-wide channel is often impractical in the congested 2.4-GHz ISM band, which is already full of Bluetooth devices, microwave ovens, and other common RF devices. As a result, the practical Wi-Fi bandwidth on any mobile device decreases significantly.
Wi-Fi also lacks the battery-charging benefit of USB 3.0 devices. Digital content transfers wirelessly between mobile devices and PCs, concurrently draining the device’s battery unless you connect it to a wall charger or a PC using USB (Figure 6). Thus, mobile devices can never achieve full “cordless” synchronization using Wi-Fi unless the amount of transferred digital content is negligible. Wi-Fi is ideal for surfing the Web, sending e-mails, and listening to radios over the air; however, it is not the choice for large, fast content transfers. With USB 3.0’s versatile connectivity, high bandwidth, and battery-charging capabilities, it is likely to coexist with Wi-Fi on mobile devices for the foreseeable future.
Although USB 3.0 presents a promising technology for mobile devices, it must undoubtedly overcome hurdles to reach the same level of success as USB 2.0. Because USB 3.0 operates at a higher frequency, new challenges, such as signal integrity and cable length, arise. Designers can overcome these challenges, however, by following best-design practices.
Currently, only a few USB 3.0-enabled PCs exist; thus, mobile devices cannot connect to many PCs with USB 3.0, regardless of whether the mobile devices implement it. Now that Intel’s next-generation Ivy Bridge CPU with native USB 3.0 host support is out in the market, more PCs will come with standard USB 3.0 ports. In addition, early USB 3.0 adopters must develop their own USB 3.0 driver, which adds another layer of overhead and design complexity. Microsoft has announced robust USB 3.0-driver support in Windows 8 operating systems, and ubiquitous, seamless USB 3.0 support will be available throughout the market.
USB is still the most versatile connectivity option for everything mobile. USB 3.0 offers clear benefits, such as high bandwidth and battery charging, making it ideal for large data transfers and high-definition video streaming. USB 3.0 will employ the strong user awareness from USB 2.0 and develop a broad ecosystem of support from hardware to software. The time is right for mobile-system manufacturers to introduce SuperSpeed USB 3.0 in next-generation mobile devices.