Cable-Free USB fits the bill for handsets Transferring large amounts of data to a handset can be accomplished by Cable-Free USB, one of the flavors of UWB.
By Martin Rofheart, Freescale Semiconductor

Mobile handsets are quickly becoming the ubiquitous multimedia device, capable of everything from snapping high-resolution photos to streaming audio and video. Handsets can use USB, one of the most popular connection technologies on the market today, to transfer multimedia files. USB has traditionally required a cable, which takes away the key feature of cell phones: mobility.

Next-generation handsets are looking to leverage a new wireless connection for this connectivity: Ultra-Wideband (UWB) technology. This high-rate wireless protocol lets consumers download or transfer MP3 files, photos, video, and data without a cable. In addition, UWB can work with the existing USB 1.1 and 2.0 standards to achieve a Cable-Free USB solution. With an instant plug-and-play wireless connection between any USB 2.0 certified devices, such as cell phones, digital cameras, MP3 players, digital camcorders, large-screen displays, printers, set-top boxes, and PCs, UWB brings a new freedom to mobile applications where low power consumption is critical.

One of the most useful and prevalent connection methods that exists today is USB. With all those connections, it’s understandable that there will be many wires. Every one of those wires represents a limitation on the consumer. One example is that the physical connection must be matched between devices. There are several physical connector types for USB. As USB devices becomes more popular, it’s desirable to reduce the wiring to allow better device placement and reduce clutter and complexity.

One obvious solution to reducing the wires is to use wireless connections. Cable-Free USB takes advantage of the vast number of USB devices already used today to enable a true wireless USB connection. This solution employs a wireless technology called Direct Sequence Ultra-Wideband (DS-UWB) to transmit the USB signals. The USB serial data stream is converted directly into UWB radio signals. The devices don’t need to know if the USB is transported through a cable or through a wireless signal.

The nature of UWB radio technology is to allow high data rates at low transmission power, with the added benefit of requiring little system power consumption. The ideal rule of thumb for power consumption is 1 Mbit of transmission rate for 1 mW of system consumption.

To achieve a true plug-and-play experience for the consumer, it’s imperative that the design achieves a true USB interface to the high-performance DS-UWB radio system. There are several advantages to strictly adhering to the USB 2.0 interface using the Cable-Free USB implementation, including:

• A Cable-Free USB system can connect to a USB-enabled system without needing any hardware, software, or driver updates as it’s based on the USB 2.0 standard.
• A Cable-Free USB system can be placed on a mini-card form factor and use USB connections directly, without modification to the host system.

Cable-Free USB meets the following criteria to implement a high-performance wireless USB solution:

• Compatibility with USB 2.0 standard, with full backward compatibility
• Security at the same level as wired USB
• A user experience equivalent to wired systems

When implementing USB over a wireless channel, factors to consider include:

• Switching between RF transmit and receive modes
• Varying characteristics of wireless communications channels
• The impact of lost and retransmitted packets
• Additional error correction and detection capability
• Addition of wireless security algorithms

UWB implementations must enable standard USB hosts, hubs and devices to be used with wireless channels at wired performance levels. Furthermore, users can mix and match different devices with different speeds. Just like standard USB, devices with different speed and transfer-type attributes can be attached to and detached from the system at random.

Direct-sequence UWB
UWB is a wireless wideband technology that transmits a low-power signal over a wide swath of radio spectrum. Unlike conventional radio systems that operate within a relatively narrow bandwidth, such as Bluetooth or Wi-Fi, UWB operates across a wide frequency spectrum by transmitting a series of narrow, low-power pulses. The theoretical limit for UWB transmission rate is around 4 Gbits/s.

The combination of broader spectrum, lower power and pulsed data means that UWB causes less interference than conventional narrowband radio solutions, and delivers wire-like performance. This suits UWB for a number of wireless multimedia applications that require transmitting large amounts of data at high speeds. UWB provides high data rates at distances up to around 20 m. Suitable applications include high-definition video and CD-quality sound.

There are two primary flavors of UWB: DS-UWB and Multiband orthagonal frequency division multiplexing (ODFM). DS-UWB technology has been shown to operate at lower power than ODFM technology. Recently, it was demonstrated in a Samsung UWB-enabled cell phone. The DS-UWB radio has been shown to co-exist with cell phone radios, with no interference. DS-UWB technology can also be implemented in a single-chip solution. This is a critical issue for handset makers, because phone form factors are shrinking while functionality is increasing, meaning that board space is at a premium.

Bluetooth has made progress as a data-transfer technology for handsets. However, Bluetooth’s data rate is not as high as UWB, making it a less attractive candidate for high-data-rate multimedia applications. The Bluetooth Special Interest Group (SIG) has announced that it’s adopting UWB radio technology. Even the Enhanced Data Rate (EDR) version of Bluetooth can only operate at speeds up to 3 Mbits/s. This is fine for applications such as headsets, but not enough for multimedia. For example, streaming high-definition video can require anywhere from 9 Mbits/s (Windows Media Format) to more than 20 Mbits/s (MPEG2). UWB also operates with less power than Bluetooth.

Engineered for low power
For decades, classic radio design has involved a fundamental trade-off between data rate, cost of the implementation at the semiconductor level, and the power consumed by the total solution. Using classic radio design techniques to increase data rate exacts a penalty in the form of increased cost and power. This is primarily because of the increased signal processing needed to achieve the higher data rates. The handheld and mobile markets are unique because they simultaneously require high data rates for media transfer, low cost for broad consumer adoption, and low power consumption for embedding into battery-powered handheld appliances.

Additionally, networks that support multiple streams of digital video and/or audio require even more bandwidth. Devices such as digital displays (plasma or DLP), PDAs, and MP3 players all use data formats that range from hundreds of kilobits per second (i.e., MP3 at 320 kbits/s) to tens of megabits per second (i.e., DVDs at 10 Mbits/s for MPEG2 and moving to 25 Mbits/s for MPEG2 HD).

DS-UWB modulation schemes
Almost all wireless systems can trade off between operating range and transmitted data rate. All other factors being equal, more energy in each transmitted bit will result in longer operating range, but this also allows fewer bits to be sent for the same average transmit power. For example, if a system needs to double the data rate, then half as much energy (i.e., 3 dB less) is available for each data bit. This lower energy-per-bit would result in about a 30% operating range reduction (assuming 1/R2 propagation loss).

However, when the system scales to higher data rates, it sometimes must to change to a different modulation scheme that has different baseline energy-per-bit requirements. For example, if a system is bandwidth-constrained, it may have to switch from quadrature phase-shift keying (QPSK, 2 bits/symbol) to 16-ary quadrature amplitude modulation (16-QAM, 4 bits/symbol) to achieve twice the data rate in the same signal bandwidth. In this case, there’s still the same 3 dB lower transmit energy-per-bit (because of 2X the data rate), but there’s an additional range penalty because 16-QAM requires 4 dB higher energy-per-bit then QPSK to achieve the same bit error rate performance. Thus, instead of a 30% range reduction, there would be a 56% reduction if the system shifts to 16-QAM. Similarly, 16-PSK results in an even higher power efficiency penalty (about 10 dB) and would lead to over 75% range reduction to achieve the higher data rate.

To avoid these scaling problems, DS-UWB has been designed to support extremely high data rates without resorting to less power efficient modulation. DS-UWB is based on spread spectrum techniques that use sequences of UWB pulses modulated by data bits using binary phase-shift keying (BPSK). Because of the high pulse rate and signal bandwidth (1326 MHz), DS-UWB doesn’t need to use higher order modulation to reach extremely high date rates—it naturally scales to provide over 1000 Mbits/s while still using simple and efficient BPSK.

In addition to the power-efficiency considerations, scaling a system to higher data rates using high-order modulation will also increase complexity. In general, supporting 16-QAM in a receiver would require higher precision analog-to-digital converters, higher precision internal processing, and more stringent amplifier requirements. These factors lead to both higher cost and power consumption. DS-UWB avoids these problems by using a single-carrier, wide-bandwidth signal that scales to high data rates without the need for high-order modulation and the associated complexity increases.