Wireless USB Cuts the Cord for Multipoint-to-Point Systems
New 2.4 GHz technology brings simple and affordable wireless connectivity to multipoint-to-point systems.By David Wright
Ubiquitous wireless technologies are evolving, at breakneck speeds, to offer alternatives to most, if not, eventually, all traditional hard-wired transmission systems. One of these promising and emerging technologies is wireless USB.
Figure 1. Cypress WirelessUSB LS block diagram.
What's Available at a Glance
Today, along with wireless USB there are a number of other wireless options available each suited to a different type of traffic. Existing low-cost RF transceivers typically use the 27 MHz, 433 MHz, 868 MHz, or 900 MHz frequency ranges. In the 2.4 GHz and 5.4 GHz frequency ranges, there are options such as various renditions of 802.XX and Bluetooth.
Crossing the three spectrums is ZigBee (868 MHz, 915 MHz, and 2.4 GHz). For non-network, multipoint-to-point applications, there is wireless USB in the 2.4 GHz range (See Table 1). Another alternative is infrared.
The IEEE 802.11 is most suitable for data networks. It has an established foothold in the market and has proven itself viable. Its strengths, however, are weakness in low bandwidth applications. 802.11 was designed to serve a multimedia/data network. It carries a great deal of overhead in order to manage features such as quality of service or routing. It requires both baseband and stack processing. The cost of an 802.11 radio is simply too much for low-bandwidth applications such as sensors or HID peripherals. As a high-level aggregator and bridge to the control network, however, it is a promising technology.
Bluetooth's niche is the PAN. It supports multimedia and data traffic, but on a more limited basis than 802.11 does. Unfortunately, it still carries a relatively high overhead for these features. Bluetooth is a multilayer protocol requiring approximately 15 Mips and 128 KB running on a 16-bit MCU. One of the Bluetooth industry's mantras has been "$5 per node". So far, this $5 cost is the OEM cost for the radio and may not include stack code, the cost of external memory, and the price of the MCU.
Both 900 MHz and 868 MHz transceivers offer a low-cost approach to low-bandwidth wireless nodes, costing under $3 in volume. Unfortunately there are differing standards for the US and Europe, meaning that you have to create distinct product lines for each of these markets (compared to the universal acceptance of the 2.4 GHz unregulated ISM band). Infrared, the technology of choice for remote controls, costs less than $1.
Figure 2. Wireless development options: increased complexity extends time-to-market.
Infrared's Achilles heel, however, is its 3 meter LOS limitation and unidirectional signal. Imagine hanging a new flat panel television on the wall but having to leave open the cabinet housing the speaker amplifier and set-top box just so you can adjust the volume and change the channel remotely.
ZigBee is an up-and-coming standard gaining momentum. Its protocol stack overhead is more reasonable than Bluetooth, requiring approximately 10 Mips and 4 to 32Kb on an 8-bit processor, making it appropriate for home and industrial automation networks. However, ZigBee is still trying to get market traction, and there is no native support for it in the PC arena.
Supporting data rates from 62.5 Kbps to 235 Kbps (with 10 to 20 Mbps availability targeted for mid-2004) over a range of 10 meters (and up to 50 meters down the road), wireless USB (See Figure 1) strips away layers of protocol complexity to optimize throughput and power consumption for multipoint-to-point systems such as sensor arrays, PC peripherals, and remotes. The name, wireless USB, refers to the optional USB back-end of the radio link. Coupled with a USB device, a wireless USB aggregation point communicates with other wireless USB devices over the 2.4 GHz band and bridges data to the wired network over standard USB. Given USB's strong BIOS and driver support in the PC and enterprise network markets, it is fairly straightforward to connect a wireless USB device to the data network. Nodes only incur the cost of the 2.4 GHz radio, keeping node costs down.
2.4 GHz for Multipoint-to-Point Systems
When it comes to point-to-point and multipoint-to-point wireless systems, 2.4 GHz offers several advantages for both designers and end-users. For device designers, the benefits include very low latency, high-throughput and worldwide availability. Offering very high bandwidth and up to 79 useable channels, 2.4 GHz also enables co-location of multiple independent networks in the same physical space.
In wireless keyboards and mice, the upgrade to 2.4 GHz means greater range and reliability. Users don't have to position the receiver in "just the right spot" or avoid large metal items to get a wireless desktop working. It is also possible that users will experience longer battery life, especially in keyboards, since the available bandwidth allows input devices to be in sleep mode most of the time.
An offshoot advantage of being able to operate in an extended range means keyboards and pointing devices may finally start appearing in advanced set top boxes for cable and dish systems. Consumers will welcome this as the user interface for these systems grows more complex with each generation. 2.4 GHz could potentially become a catalyst for the next jump in television and computer convergence.
For applications like video game controllers, 2.4 GHz wireless freedom will help remove the clutter of cables and the interference with cordless telephones typically associated with 900 MHz game controllers. Game controllers based on 2.4 GHz technology are already on the market today, and they have gotten excellent reviews in gaming magazines. More are scheduled to arrive on store shelves this summer.
Choosing the Right Radio
Hardware and system complexity are major issues that drive system costs. Many wireless solutions (2.4 GHz and otherwise) come in module form, which tends to drive up costs. But all or most of the components for the RF subsystem are integrated into the module, making the designer's job much easier.
In order to make 2.4 GHz an attractive play, designers have had to figure out how to develop a single 2.4 GHz radio IC that had only a few passive external components and readily available antenna designs. Their goal was to make RF as simple as laying out standard digital systems, without the additional module mark-ups. Bluetooth has overcome some of these obstacles, but unless you are buying in million unit quantities, the cost of Bluetooth technology is still above the $10 per unit mark for a wireless system.
Using a raw radio interface requires HID manufacturers to implement a baseband in the microcontroller, where more full-featured 2.4 GHz radio ICs provide a transparent serial interface, with integrated bit slicing and correlation features. The amount of work and time that can be saved by using a device with these functions integrated into the radio IC is significant. In addition, there are other functions that must either be accounted for in the wireless solution or designed into the HID system. These functions affect how the wireless system behaves on the air, how it interacts with other nodes, and what it does when it encounters errors or interferers.
The Power of Wireless
One of the primary challenges for designers of wireless systems is power consumption. In the PC and gaming worlds, users don't want to have to change out batteries in the middle of an intense session. For industrial applications, it isn't cost effective to have to hire someone whose job is to simply change out batteries in sensors all day long.
Since most radios can drop power consumption to almost negligible levels during sleep times, the key factor to keeping power consumption down and increasing battery life is to limit transmissions, both in terms of length and frequency. The more complex a protocol, the more control data that needs to be sent.
The frequency of the transmission is also a major consideration. In some cases, data doesn't need to be collected more than once every few seconds. However, depending on the wireless technology deployed, it may require more frequent transmissions as a matter of protocol.
For example, Bluetooth devices need to synchronize the network on a regular basis to "discover" new devices. In applications where devices regularly enter and leave the network, such as a data network at a coffee shop, "discovery" is a required and vital function. In a low-cost multipoint-to-point system, servicing the protocol can sometimes be responsible for consuming more power than the data it transmits.
Another way to increase battery life is to design transmit-only nodes. Since the radio cannot receive, it does not burn power having to check for signals on a regular basis. It only consumes power when it transmits. Of course, this comes at the cost of not being able to send messages to a node. However, in those cases where little value would be gained by two-way communication just what might you tell a keyboard? the power savings and extended battery life are worth the exchange. Note that it is also possible to design a device using a transceiver as a transmit-only device. Occasionally it could transmit a "ready to receive" message, at which time it listens for any messages. Doing so enables the support for a two-way channel without burning power continuously.
Adding Intelligence to Power Management
Intelligent power management can significantly increase battery life. This said, it is desirable to implement a mechanism for handling that 2- to 10-year event when a battery begins to fail. Adding components to each node to self-monitor battery strength is one alternative, but this increases the cost of each node. A more efficient mechanism is to employ RSSI. With RSSI, the receiver measures the transmit power of each node. If the signal strength from a node begins to degrade, the receiver can send a message up through the system requesting a battery change.
Another important consideration for any wireless system is how robust it is to noise and interference from collocated traffic, such as 802.11, Bluetooth, and microwave ovens. The DSSS/CDMA code scheme of Wireless USB LS (low speed) promotes coexistence with other wireless traffic. Additionally, the impedance matching L/C network provides rejection of more powerful out-of-band signals such as cellular phones and 900 MHz cordless phones, performing as a bandpass filter in this regard. The antenna response to out-of-band signals is also low, improving reception. Additionally, the LNA and mixer are AC coupled at a very high frequency, providing rejection of lower frequencies, such as AM radio and television.
In proximity with Bluetooth, for example, the incidents of collision are less than 1.5 percent of "on-air" time. When a collision does occur, the transmitter needs to retry the affected packets. Since the latency of LS is less than 4 ms, this delay is below the perception of humans, which is more than sufficient given the applications for which LS is suited. In the case of 802.11, noticeable degradation occurs in primarily one instance: when the receiver is within 1 inch of an 802.11 access point (not a node). In many cases, moving the wireless USB receiver is all it takes to resolve the problem. For applications where the USB port is situated next to the 802.11 access point, such as in a laptop computer, you can move the receiver the necessary distance away using a standard USB cable.
In terms of interfering with itself, wireless USB uses 49 CDMA codes supported across 82 one MHz channels. This yields a theoretical spectral capacity of 4018 channels, which presents a negligible matrix for collisions.
Filling the Multipoint-to-point System Niche
There are several options for building a wireless communications system, and each has its place depending upon the range and bandwidth needs of individual nodes and aggregation points. As it stands, there are no standards for simple nodes, such as wireless mice or sensors, as there is little gained in taking on the overhead of a standard protocol and the challenges of certification to enable interoperability in applications where node and aggregation points generally ship together.
2.4 GHz technology has proven a viable solution for point-to-point and multipoint-to-point applications by providing high bandwidth and enabling co-location. Many 2.4 GHz technologies, however, are too complex and expensive for the simple devices they serve. Where interoperability may be required for these devices, a simple bridge to the control network can be used. For this, 802.11 and USB as standards serve well.
Wireless USB, which requires few external components (less than 10 resistors and capacitors for a mouse) and no external Flash, suits low-cost multipoint-to-point systems. Preconfigured code connects the radio with an external USB device, and an example low-overhead wireless protocol is available as a template. A wireless USB node can send more than 1 Gb of data, including protocol overhead, using a set of AA batteries. Currently sampling, wireless USB-LS, available as transceiver or transmit-only, starts at less than $2 in volume, and is expected to move below $1 over the next two years. Wireless USB-LR (long range) reaching 50 meters, with industrial temperature range, is targeted to be available in 2004.
Editor's Note: Wireless USB is discussed in general in this article. However, it is based upon a trademarked WirelessUSB, which is a development of Cypress Semiconductor.
About the Author
David Wright is a systems engineer in the Design Engineering Department at Cypress Semiconductor, focused on Cypress' WirelessUSB product architectures. Mr. Wright joined Cypress in 1998 from Saitek, a world-leading manufacturer of video game peripherals, based in the UK. He attended Southampton University in England, where he earned a masters degree in Engineering.
Glossary of Acronyms Used in This Article
BIOS - Basic In/Out System
DSSS/CDMA - Dynamic Sequencing Spread Spectrum/Code-Division Multiple Access
HID - Human Interface Device
IC - Integrated Circuit
ISM - Industrial, Scientific, Medical
LNA - Low-noise Amplifier
LOS - Line of Site
MCU - Multipoint Control Unit
PAN - Personal Area Networks
RSSI - Received Signal Strength Indicator
USB - Universal Serial Bus