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RF 101 Ultra Low Power Bluetooth Design

Mon, 02/02/2004 - 7:49am

Bluetooth solutions are, by definition, low cost, low power and have small form factor. But is this really the case? And if so, does an ultra-low power design make sense as well? By Jürgen Mayer-Zintel

In a perfect world, developers working in the Bluetooth world have the perfect marriage of low-power, low-cost and small form factor. However in our imperfect world, reality is that most fear that they will have to sacrifice the power consumption objective so as not to endanger the other design goals.

In this article, the concept of a low-power Bluetooth solution will be discussed. It presents several considerations that the designer must contemplate. Among them are:

• For which applications does ultra low power make design sense?

• What does the engineer have to sacrifice, assuming today's technology?

• What does the engineer and the end user or consumer gain with a low-power application (this point will be elaborated on with the development of a Bluetooth headset)?

What is Ultra Low-Power Bluetooth?

Firstly, we need to define what Ultra low power means. Ultra low power consumption is generally defined by criteria such as low current consumption and, low supply voltage — i.e. low power consumption.

In most designs, current consumption during normal operation is of primary importance. However, the standby and peak current consumption of an application are of similar importance. The amount of power the application demands determines the battery capacity, while standby and peak power demand usually determines the battery technology.

It is generally accepted that the lower the supply voltage the better design it is. Therefore, it is better to talk about, and compare, the different applications and solutions based on their "energy consumption" during the different operation modes. In the end , all parameters must be well balanced in an ultra low power design as they determine the battery requirement.

A Quick BT Overview

Bluetooth is a wireless communication standard for personal area networking with many features. These features include worldwide operation, security, low cost, ad-hoc networking and so on. Most of the characteristics are already known from other wireless standards like HomeRF and WLAN. However, when taking a close look at Bluetooth, there is a feature not always common to other wireless standards. That feature is Interoperability.

Unlike other wireless standards, the Bluetooth wireless specification includes both link layer and application layer definitions for the product. All Bluetooth-enabled devices can talk to each other, if both devices serve the same application layer specification — Bluetooth Profiles. This means, simply, that, products from different manufacturers will be guaranteed to communicate with each other, making Interoperability a reality.

A typical Bluetooth user model may be where a portable application or portable peripheral is connected to a communication node. Today, such nodes are typically laptops or cell phones. Extrapolating this thesis, two types of application can be readily be defined: communication nodes and portable peripherals.

Applications and Power

Communication nodes have virtually unlimited power budgets. Therefore, the battery size is determined by the overall application and not just by the Bluetooth functionality. Peripherals and applications like Bluetooth enabled printers and household appliances are the most typical examples of communications nodes.

In some communications nodes, even though they are battery powered, Bluetooth power consumption isn't as critical since the power budget available for a Bluetooth subsystem is significantly less critical than the core application. In such a case, the core application is the major delimiter that defines the battery size. Some examples are:

Laptops - Already have huge batteries due, mostly, to power hungry displays. Therefore add-ons requires neither low voltage nor low current consumption.

USB dongles - The USB port can be used to reverse power the Bluetooth solution, the USB port is able to deliver far about 1.5A/5V thus neither low voltage nor low current consumption is required.

Cell phones - The RF power amplifier is still the major determining factor for energy consumption and hence, battery technology. The cell phone will be rechargeable for the foreseeable future. And since transmission power is still the determining power factor, there is no need (though it would be nice) to incorporate ultra low power design. In addition low current consumption would be nice but low voltage is not really required since typical battery voltage is still around 3.6 V.

Portable Peripherals

In contrast BT-based portable peripherals use small BT profiles, and the battery technology and capacity are almost always defined by the power consumption of the BT unit. Any reduction in BT power consumption can be directly seen in the overall power budget.

Therefore, portable peripherals, such as the ones listed here, benefit significantly from a dedicated ultra low power design:

CF/SD/MS/SM, etc. interface cards - These cards typically interface to small portable equipment with small power packs. Typical devices are PDAs and digital cameras. Here a current consumption of > 150 mA @ 3 V simply can not be handled by the batteries typically found in the target application. Therefore low energy and specifically, low current consumption is a must. Low voltage would be nice to have but is not really required since typical battery voltage is 3V.

Headsets - It is generally understood that the battery is the size determining factor, even in today's available Bluetooth headset solutions. This means that ultra low energy consumption is a must, low voltage (single cell system: 1.0 V to 1.5 V) as well as low power consumption is vital for the complete application.

HID device - These include keyboards and mice, typically. Again, no questions, it is also generally understands that a battery lifetime of six months or more for a wireless mouse is mandatory. On top of that there is pressure to keep the battery as small as possible. For a keyboard it could be less critical as the battery might be slightly bigger to compensate for higher energy consumption.

What Does the Designer Have to Sacrifice Assuming Today's Technology?

Dealing with different partners doesn't ease the design issues.

To get the lowest power solution, quite often devices and tools from different proprietary sources must be combined. Interoperability issues often arise when hardware and software from different companies are commingled.

To ease this transition, low power design include the BT protocol stack. Furthermore, using an ARM7 controller can eliminate the issue of the application developer having to sign a contract to port the BT protocol stack to the used platform. The approach seen today where companies deliver only BT protocol stack software, is insufficient for a complete low power design. This software is often generic and far from being an optimized solution to reduce the required memory space to the minimum.

A low cost and low power BT solution requires a BT upper protocol stack code generator that automatically generates optimized upper protocol stack software. This tool enables the engineer to specify the application environment e.g. type of host controller, peripherals and required profiles, and the tool generates code dedicated to this specific application. It is obvious that this tool should support several hardware platforms to allow the developers to choose the best fitting solution.

Size Beyond Cost: One of the Most Critical System Parameters

And last but not least, the level of integration presently available is not high enough. The driving factor to move towards higher integrated solutions is not only cost reduction; it is also the reduction in size and ease-of-use.

The shorter product life cycles directly translate to a much shorter product development time. Engineers have to use readily available, qualified components in their designs, especially if the building block is for add-on functionality like BT and doesn't belong to the company's core competence. This reduces the project risk as well as the time-to-market of a product.

One way to sidestep this is the usage of state-of-the art packaging technology like LTCC and HTCC. Based on such technologies, even critical passive components can be integrated to get closer and closer to the optimum of a real PNP, easy-to-use BT module.

What Does the Designer and End User Gain?

Unarguably, today's market driver is mobile communication. The headset market has already increased as a result of the growth of the mobile phone market, health issues (electro-smog) and official regulations. Furthermore worldwide legislation is pushing to ban the use of handheld mobile phones while driving a car. This bodes well for expanding BT equipped vehicles Currently, automakers BMW, DaimlerChrysler, Saab, Citroën and other are a few of the car manufacturers who have announced their entrance into the BT vehicular domain.

Some visionaries don't think that in-vehicle communication will determine the long-term success or failure of BT. However with little doubt, it is a significant hurdle that was laid in BT path, which BT has to pass. If BT is successful in-car, it will have a positive impact on the overall demand for other BT consumer applications.

Besides other in-vehicle entertainment devices (joysticks, headphones) the headset will be one of the most seen applications in the car.

Headsets will be marketed for use with cellular phones, PC and PDA mobile equipment. Today there is a fairly decent selection of BT headsets on the market, typically priced from about $100.00 (Figure 1, (a-c) shows a sampling of current offerings). However, the success of these products will be depend on the manufacturers' ability to meet several critical milestones.

First, the general consensus is that the price at the consumer level needs to be down around $50.00. This seems to be the sweet spot the customer will pay for a wireless headset function.

Second the consumer needs to be made aware of the safety and versatility benefits of using BT technology, compared to other wireless standards. This includes awareness of the BT wireless headsets, such as superior voice quality, reduced RF interference issues and, last but not least cost.

Another market constraint is definitely the comfort factor that customers are looking for when investing in a BT-based headsets. Besides the audio quality and the housing, the battery lifetime is becoming more and more important.

Presently, there are no BT-based headsets on the market which allows a significantly longer talk time than about four hours using a reasonable battery form factor.

Bluetooth and Power Today

When discussing wireless headsets and low power, a significant factor, both positive and negative is present battery technology. Battery technology becomes more of an issue with chatty cell phone users than occasional users. Typically, one can define three types of cell-phone user levels:

Low talker - typically less that 1/2 hour per day.

Mid talker - typically around one hour per day.

High talker - typically longer that 1 1/2 hours per day.

One area the industry has begun to pay attention is the primary battery. Currently, with an acceptable battery replace cycle of one month for disposable batteries, one can define the following talk and standby times:

Low talker talk time: 15 hours, standby time: 200 hours.

Mid talker talk time: 30 hours, standby time: 400 hours.

High talker talk time: 45 hours, standby time: 600 hours.

As soon as the power consumption of a headset is reduced to get 30 hours of talk time (mid. talker) and 400 hours of standby out of a disposable battery, rechargeable batteries can be replace with primary batteries. This has the potential of significantly lowering the cost of a headset as cheaper battery technology can be used and no recharger will have to be provided.

For that to be practical, the power consumption of a BT headset, during full operation (HV3), must be in the order of 15 mW. Today's solutions of 25 mW @ 1.8 V are getting close but are not yet the ultimate solutions.

Conclusion

This BT headset discussion shows that, for some applications, the firm implementation of an ultra low power BT design will not only reduce the overall cost, it makes the designs more attractive for the end consumer and it makes any new "Portable Peripheral" applications more feasible. Latest generation BT modules have become remarkably compact (See Figure 2). Now, it's a matter of economies of scale, functionality and suitability to task — all of which are on the center of major BT players radar screens.

Jürgen started his engineering career with Texas Instruments as a board level design engineer. He joined XEMICS in 2000 as a Marketing Manager for wireless products, responsible for product strategy, product definition and technology support. He received his MSEE degree from the Technical University of Munich, published several articles and holds 4 patents in the wireless and analogue domain.

Glossary

BT — Bluetooth
CF — Compact Flash
HID — Human Interface Devices
HTCC — High Temperature Co-fired Ceramic
LTCC — Low Temperature Co-fired Ceramic
MS — Memory Stick
PDA — Personal Digital Assistant
PNP — Plug-and-Play
SD — Secure Digital
SM — Smart Media
USB — Universal Serial Bus
WLAN — Wireless Local Area Network
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