Pressure Mounts in Next-Gen Mobile Phone Designs
Bill Krenik and Mike Yonker, Texas Instruments
Apr 16, 2004 (6:00 AM)
Figure 1: In the near future, it will be commonplace for cell phones to incorporate a variety of interfaces to Bluetooth, UWB, 802.11, GPS, and even TV.
Several waves of rapid change are in store for cell phone handsets. Multi-mode smartphones, wireless PDAs and multimedia devices are encroaching on territory once dominated by the simple, voice-only, single-mode cell phone. A new wave of technologies including Bluetooth, 802.11 wireless LAN (WLAN) and global positioning services (GPS) are converging in mobile devices, pressuring handset designers to devise innovative solutions that meet the stringent design constraints of the wireless market including cost, form factor, performance and power consumption
As if these aforementioned design constraints of converging technologies weren't enough, another wave of technologies involving additional radio frequency (RF) capabilities is soon to follow. Some handsets on the market today already feature FM radio and analog television reception. In addition to these capabilities, an increasing number of handset designs will implement additional radio interfaces such as ultra wideband (UWB), digital TV and satellite radio (see Figure 1 below). As more radios are incorporated into single devices, design intricacies become increasingly complex.
Bringing new technologies into an existing design is difficult in its own right, but designers of handsets must also contend with the most demanding constraints of all those imposed by end consumers. Failing to meet or beat consumers' expectations for extended battery life, increased multimedia performance, sleek form factors and low cost would be disastrous. As well, time-to-market often trumps many of the designer's technical considerations.
So, what effects will these waves of converging technologies have on handset design? The first wave the integration of Bluetooth and WLAN into handsets offers an interesting prelude to the issues and design considerations that will be a part of the system-level integration projects of the future.
The First Wave
Bluetooth and WLAN pose an especially interesting integration challenge since both technologies employ the same frequency band the 2.4GHz to 2.4835GHz unlicensed Industrial Scientific Medical (ISM) band. Therefore, in addition to power, form factor and cost constraints, the handset designer must also face interoperability of the two standards, as shown in Figure 2.
Figure 2: Systems utilizing both Bluetooth and 802.11 must address co-existence issues.
Bluetooth uses a frequency hopping spread spectrum (FHSS) system in which the transmission band hops over 79 pre-defined 1 MHz channels. The hopping rate is roughly 1600 hops per second over a random pattern (note: adaptive frequency hopping will be deployed in the future). In this way, Bluetooth spreads energy over the entire band. However, since Bluetooth doesn't monitor the band before transmitting, it can easily interfere with other systems trying to use the same band. In this fashion, if 802.11 is transmitting or receiving when Bluetooth begins transmission, both air interfaces can fail to operate properly.
In contrast to Bluetooth, 802.11 does monitor its transmission band for other traffic before beginning to transmit. 802.11 employs direct frequency spread spectrum (DFSS) and orthogonal frequency division multiplexing (OFDM) air interfaces, and occupies roughly a quarter of the 83.5MHz ISM band. Since 802.11 will sense Bluetooth activity and not transmit if Bluetooth is active, WLAN service will be very seriously affected when Bluetooth interferes.
Fortunately, there are solutions to this interoperability challenge. Non-collaborative coexistence solutions include such techniques as adaptive fragmentation, which optimally adjusts packet sizes to minimize collisions, and adaptive frequency hopping, in which the Bluetooth interface selectively hops to channels where it can successfully transmit. Collaborative coexistence solutions involve communication between the 802.11 and Bluetooth interfaces in a handset and work to maximize the throughput of both interfaces while taking account of any quality of service (QoS) demands. Since Bluetooth is often used for voice communication, QoS is especially important, as the user will certainly notice if voice is interrupted.
Collaborative coexistence solutions can be both distributed, where each interface takes account of information from the other in determining when to transmit, or centralized, in which one of the interfaces assumes control and the second only transmits when it is signaled to do so. For example, the 802.11 media access controller (MAC) might assume control, taking account of any Bluetooth QoS demands and data transmission requirements, in addition to its own needs and optimally controlling both interfaces to make the best possible use of the transmission channel.
The Road to Digital Radios
Coexistence issues aside, form-factor limitations in modern handsets make the addition of new technologies that include a radio interface, such as Bluetooth and WLAN, very difficult. Normally a radio interface includes a transceiver function, power amplifier, and passive filtering and matching components. Consequently, the radio function is relatively large, making it very hard to meet form factor requirements in a small handset. Since integration is one of the most powerful tools available to semiconductor designers in reducing power and area, it is only nature to turn to integration to overcome the radio form factor challenge.
Of course, several alternatives exist for radio integration in a handset including multiple air interfaces. It might seem most straightforward to simply integrate all the radios together into a single radio IC and put all baseband processing functions in another. In this way, both the radio and baseband functions can be in an optimized process technology and very little new technology development is needed.
However, this approach suffers serious drawbacks. Clearly, there is no possibility to mix and match handset options in a modular fashion since every handset would have to include all incorporated air interfaces. Additionally, lack of processing capability on the radio IC would mean that aggressive control of the RF function by the processing logic would be lost, leading to lower yields and performance enabled by other possible options.
A more attractive approach is to simply integrate the RF and baseband functions of each air interface on a separate IC in deep submicron CMOS. In this way, each function is fully self-contained, allowing phone models to be developed with any number of desired radio functions to be included. Such a modular approach at the system level is especially attractive when many phone models must be designed under tight development time constraints. Additionally, this approach offers the benefit that the RF function is moved into the realm of deep submicron CMOS processing.
As opposed to designing the radio in a BiCMOS or SiGe technology, building the radio in advanced CMOS ensures that the radio function will be implemented in the most aggressively scaled technology the semiconductor industry has to offer. In addition to the other benefits of CMOS, this move to the most advanced CMOS processes available ensures that the radio will offer the lowest possible power consumption level and very low cost. Finally, tight coupling of the radio function with the baseband processing elements allows the radio to be automatically tuned, calibrated and tested by the logic functions. Higher production yields and better performance result.
Of course, integration of the radio function into CMOS with the baseband processing functions does require that a new radio architecture be implemented. Figure 3, shows a digital RF processor (DRP) that is suitable for CMOS integration. While the direct conversion receivers used in many of today's handset designs have served the industry well and literally billions of them have been shipped, their requirement for many stages of analog processing and high performance passive elements makes them unattractive for implementation in advanced digital CMOS. In contrast, the DRP receiver makes extensive use of sampled data processing and digital techniques, leveraging the very high logic density and fast clocking capability of advanced CMOS.
Figure 3: Digital technology can greatly simplify RF processing while reducing the cost and power consumption of transmitting and receiving information wirelessly.
Assuming that the requisite technical advancements and higher levels of integration are achieved so that future waves of technologies do indeed converge on handsets, new designs still will face the inevitable market-driven tests of cost, performance, board space, power consumption and time-to-market. In addition, a converged handset with multiple functions sharing certain resources only accentuates the importance of other prickly issues such as certification and security.
It is not difficult to imagine a usage scenario for a converged handset where three technologiescellular, Bluetooth, and 802.11 are active at the same time, and where Bluetooth and 802.11 are sharing the resources of a single RF transceiver. For example, a cellular phone call could be directed to a handset while the user is listening to music that is streaming through the handset's WLAN interface before it is transferred via Bluetooth to a headset.
Of course, system certification is always a critical development hurdle to overcome. Type approval of a handset's cellular interface is complex and involves extensive testing. The process becomes even more complicated when multiple air interfaces are included in a single handset. This necessitates the simultaneous fulfillment of multiple certification standards promulgated by multiple worldwide standards agencies. Over and above the cellular certification process, WLAN and Bluetooth have their own requirements for interoperability testing and for meeting certain regulatory requirements.
Even though a high level of silicon integration will not decrease the amount of testing required for certification, reducing the system's complexity gives manufacturers greater confidence that serious problems will not occur during the certification process. For example, semiconductor vendors can provide example layouts for the small number of external devices that would accompany an integrated solution. Since an integrated solution would reduce the form factor layout of the system, these external devices could be easily adopted with only minor changes to the design. Consequently, high levels of silicon integration ease system integration and make the path through system certification simpler and more predictable.
Security is always a concern in today's world, and the wireless handset is no exception. As more technologies converge on handsets, the number of access points into the system increases, raising the vulnerability of handsets to attack. It is incumbent upon handset developers to design security measures into their architectures and to take advantage of the security features of wireless chipsets.
Modular architectures lend themselves to strict security measures because each module can establish its own security perimeter. Breaking into one module would not necessarily compromise the security of the entire system. At the same time, integrated security features like hardware-based secure bootloaders and sophisticated cryptographic engines are very effective at keeping hackers, viruses, worms and other types of malevolent code at bay.
As the handset market continues its rapid pace of growth and innovation, handset designers will be challenged to control the convergence process instead of being controlled by it. That will mean careful architectural planning, close collaboration with component suppliers and creative innovation.
About the Authors
Bill Krenik is the wireless advanced architecture manager at Texas Instruments. He received a Ph.D. in Electrical Engineering from the University of Texas at Dallas and can be reached at firstname.lastname@example.org.
Mike Yonker is chief technologist for the wireless computing group. He received a bachelor's degree in computing engineering from the Georgia Institute of Technology and can be reached at email@example.com.
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