Data transfer speeds are important to most, although the cost in terms of power consumed must also be considered. Communications systems designers normally choose a single technology to transfer and control the flow of data. For example, ultrawideband (UWB) technology offers high data rates, but has some trade-offs. For example, the time needed for connection to another UWB device can be considerably longer than other wireless communications formats, such as Bluetooth. UWB systems use very low power-per-bit in transmission mode, but if a UWB communications device has to search to find its communications partner, that savings is lost. A hybrid system, using Bluetooth technology as the device control manager, makes UWB more efficient and enables its use in a broader range of consumer electronic devices.

Information transfer, in the form of documents, music, news, and other data, is vital to the modern workplace and lifestyle. Time is often critical in making a transfer, but the power expended to make that transfer is also important. To understand the advantages of a combination of Bluetooth and UWB technologies, it is useful to go back to a basic principle: why use wireless communications at all? Wireless, as a technology, is usually two orders of magnitude more difficult than using wires. Wireless technology is better suited for devices that move around or are too far apart to make wires practical.

Five classes of wireless-communications devices offer decreasing areas of coverage:

• Wireless Global Area Networks (WGANs)
• Wireless Regional Area Networks (WRANs)
• Wireless Metropolitan Area Networks (WMANs)
• Wireless Local Area Networks (WLANs)
• Wireless Personal Area Networks (WPANs)

Figure 1 presents a simple graphical representation of these wireless-communications classes. In approximate terms, WGAN and WRAN are about two orders of magnitude greater distance, with the WMAN and WLAN about one order of magnitude—arriving at about a 10 meter range for WPANs. WGAN technology uses satellite communications and has the potential for covering vast areas, for example in the manner of satellite-television and Global Positioning System (GPS) systems. WRANs are terrestrially based systems and include traditional broadcast radio and television, plus the communications technology being defined by IEEE Project 802.22. WMANs are the domain of mobile telephony, but also have an IEEE activity, Project 802.16. Although other, less successful, examples of WLANs exist, IEEE 802.11a/b/g/h is perhaps the best-known example.

The IEEE also defines WPANs with various subtypes of Project 802.15, including a version of the Bluetooth technology (802.15.1), a multi-media-oriented type (802.15.3), and a low data rate (802.15.4). Two UWB technologies fought to a stalemate to become the physical layer (PHY) for 802.15.3: a direct-sequence spread-spectrum (DSSS) version promoted by the UWB Forum (www.uwbforum.org), and a multiband OFDM version promoted by WiMedia (www.wimedia.org)¹. Many of these technologies share an overall structure, which, for example, makes it possible for synergy between Bluetooth and UWB technologies.

With more that a billion units in the field at the end of 2006, Bluetooth technology is far and away the most successful WPAN technology. The Bluetooth Special Interest Group selected WiMedia’s version of UWB to boost performance. This boost will enable Highly Mobile Devices (HMDs) to benefit from the UWB’s better than 400 Mb/s data rate, which in turn will enable new classes of Bluetooth applications.

To encourage mobility, an effective HMD must be portable and easy to carry or transport. This also implies low power consumption to support a small battery. The critical issue for an HMD that utilizes a WPAN is minimizing power consumption. Mobility is both an advantage and a potential problem in the WPAN environment. Data transfers without wires is convenient but, with an effective range of 10 m, a wireless connection can be easily lost with just a little movement. Would it make more sense to increase the range of an HMD device?

Farther and faster are not always better for wireless applications. For some applications, the reach of the system is critical. For example, early UWB radios were used to communicate with submerged submarines. And in some applications, such as point-to-point microwave radio systems, high speed is critical without critical dependence on power consumption. But for most applications, trade-offs must be considered.

Digital-communication throughputs are traditionally measured in the number of bits transmitted per second. With wireless communications, especially consumer-oriented versions, there is literally another dimension to be considered. Data transmissions through a wire occur on a point-to-point basis and essentially represent a one-dimensional (1D) operation, a vector. Without the wire medium, air serves as the transport medium in a wireless system in three-dimensional (3D) space. Figure 2 is a representation of this 3D attribute. The black line represents the wired data path and the red and blue spheres represent nodes. The green portion is the volume where wireless radios can communicate.

When the population of wireless-communication devices is sparse, increasing the reach of the devices only has an effect on power consumption. In a consumer device, a larger problem occurs when there are many devices in range of each other. Unlike most wired systems, the presence of collocated communication paths greatly impacts data throughput. The larger the spheres of communications, the more coexistence problems exist, which result in retransmissions, causing an even greater impact on the battery life for a mobile device.

Coexistence is defined in the IEEE 802.19 Technical Activity Group as “the ability of one system to perform a task in a given shared environment where other systems have an ability to perform their tasks and may or may not be using the same set of rules.”

If minimizing power is a key to the success of an HMD, then using the technology with the lowest instantaneous power budget would make the most sense. Unfortunately, a system’s power requirements change under various conditions. Bluetooth technology’s approximate average of 11 mA of consumption far surpasses the performance of the 500 mA UWB radio. However, there are many more factors involved in the total power cost of moving data. The table approximates the most important of those factors. (It should be noted that the actual power consumption of the UWB radio is not currently available without non-disclosure agreements in place. The power-consumption figure cited above is approximate and based on first-generation UWB systems. The power consumption cited for Bluetooth is from a commercially available single-chip radio.)