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Pervasive wireless connectivity has clearly catalyzed change in today's high technology businesses as well as in personal lives. When the first mobile phones were being developed back in the 1980s, few foresaw the impact that mobile communications would have on our daily lives. The growth in all types of mobile connectivity has exceeded expectations. On March 6, 1983 when Motorola introduced the world's first commercial portable cellular phone, the DynaTAC 8000X, none of us would have thought that by 2005 more than 80% of the population in Europe and in the United States would subscribe to a cellular service provider; more than 250 billion SMS messages would be sent in 2004; sales of mobile computing devices (laptops) would exceed those of desktops and that almost each laptop sold would have an 802.11 b/g adapter built in.

Although many protocols for wireless data have been proposed and implemented, WiFi (802.11) stands out. It has managed to cross a tipping point similar to the way cellular services became ubiquitous in the 1990s. The nationwide availability of hotspots has reached a point where the network effect has begun to drive WiFi's acceptance and inclusion in a range of mobile devices. Although hotspots are not yet as ubiquitous as many road warriors would like, it has become accessible enough that most users are confident that they can locate a hot spot quickly and easily.

In spite of its marketing success, the business model for WiFi services — offering speeds up to 54 Mbps within several hundred feet of an access point — has been difficult. Potential users have been turned off by roaming and coverage problems. Prices have also seemed high given the inconsistent coverage.

However, when considering how to best implement mobile data services, it would be incorrect to focus on just WiFi. What users want is seamless mobility. We realize that a single standard is incapable of providing global mobile data services. Engineering trade offs must be made to balance battery life, transmit power, sensitivity and throughput. Thus, in reality, WiFi is only one part of a successful global mobile data solution because of the inevitable need for true mobility provided by multiband multistandard devices.

When users consider which mobile data standard to choose, they intuitively trade off cost with range and throughput. Efforts to improve the WiFi standards have focused on just that. However, the wireless world doesn't only exist within the local area network (LAN), it can be compressed and stretched to match the geographic scale of business and personal lives. Users require laptop data connectivity in wide area networks (WANs) as well as in metropolitan area networks (MANs), LANs and personal area networks (PANs) as illustrated in Figure 1.

Many new emerging data protocols are on the horizon. WiMAX, which can transfer 70 Mbps over a distance of up to 30 miles to thousands of users from one base station, is undergoing numerous trials around the world, in countries such as the United Kingdom, the Philippines, Indonesia, Malaysia and in the United States.

In today's mobile world, it has become commonly accepted that mobile communication solutions have been the foundation for the productivity increases of the last decade. An overly simple approach to ubiquitous coverage is to choose one standard as a global standard; attempt global regulation; then rollout a dense infrastructure that provides seamless connectivity. This is seen as an idealistic vision. Most users intuitively accept that competition in heterogeneous mobile data markets will create a heterogeneous set of mobile data standards. And naturally, users and developers have asked themselves whether it would be possible to build a mobile data modem that could communicate using that set of heterogeneous mobile data technologies such as EVDO, TD-CDMA, HSDPA, OFDM, WiMAX, and WiBro (Table 1^[2]).

Many attempts have been made to address this problem and commercialize configurable mobile radios. Vanu Inc. has recently received FCC approval for its Anywave base station. This is the first FCC-approved software radio on the market. Although a successful SDR application, it is not a mobile radio. Other companies such as Icera, Sandbridge, Intel and Texas Instruments are developing software-defined radio (SDR) or SDR-like modem solutions for mobile radio.

But those software-defined modems are only one part of a mobile data solution; the next step in delivering true mobility is the successful development of a software-defined transceiver suitable for mobile applications.

The practical approach that today's transceiver vendors have defaulted to is providing multiple analog transceivers. They've built multichip modules, multidie packages, as well as multiple transceivers on die. Unfortunately, these approaches all come with limitations. They all require more die area for each additional band. Each additional transceiver draws additional power. Also, each additional transceiver may require an additional antenna and matching network. Designers are thus forced to make trade offs for cost, size, performance and power, yet the resulting designs are still inflexible, large, expensive and power hungry.

On the other extreme, an SDR takes a completely different approach to multiband yet still runs the same problems that the multitransceiver/multidie/multichip module approaches all do. The classical SDR architecture requires a high sampling rate, wide bandwidth and power-hungry analog-to-digital converter (ADC), as well as a high-performance, high-power digital drop receiver (DDR). For mobile applications, this creates unacceptable demands on the battery and so far has precluded its use in mobile devices.

The trick is to come up with a transceiver architecture that is flexible enough to support a variety of signal bandwidths, modulation formats, signal levels and blocking specifications. As an example, cellular standards have low to medium bandwidths, but have very high dynamic range requirements and challenging blocker environments. WLAN standards, on the other hand, tend to operate in unlicensed spectrum and thus have lower power levels, less dynamic range, fewer blocker considerations, but have high signal bandwidths and high-order modulation (and thus require higher signal-to-noise ratio or SNR).

So one can either design multiple low noise amplifiers (LNAs), multiple baseband filters and several ADCs (which doesn't seem like an improvement over the multichip or multidie approach), or one can design the circuits to be configurable. The LNAs must be tunable over a wide bandwidth and support high linearity; the bandwidth, dynamic range and order of the baseband filters must be configurable; the ADC must adapt its power consumption to the dynamic range and bandwidth required. Finally, low-power digital processing is necessary to achieve the necessary decimation, downconversion, channel selectivity and gain/phase compensation.

On the transmitter side, one requires an architecture that supports narrow and wideband signals, constant and non-constant envelope modulation schemes, various output powers and covers a wide frequency tuning range. While polar modulators have proven efficient for narrowband signals, they have yet to be successful for wideband signals such as WiFi. Direct IQ modulators have significant flexibility as long as the digital-to-analog converter (DAC) sampling rates and filter bandwidth can be configurable. Again, highly configurable digital processing can be used for upsampling, digital upconversion, and even pulse shaping and modulation.

Since some data standards are full duplex while the WiFi and WiMAX protocols are half-duplex, the multiprotocol transceiver faces additional challenges. Whereas half-duplex designs typically share the synthesizer between transmit and receive, the full-duplex architecture requires that two local oscillators (LOs) be generated. To reduce power in the half-duplex mode of operation, a single LO can be used to drive the transmit and the receive mixers. However, this must be done in such a way that avoids leakage (from the transmit mixer LO to the receive mixer) when in full-duplex operation.

Based on the challenges, it's clear a parallel hardware architecture is not the economical solution. To enable this “Holy Grail” of true mobile multiband multistandard devices, an SDR-like solution is required. Only SDR can offer the adaptability, the low cost (programmability) and future proofing needed to convince operators, laptop designers and customers to accept SDR as a necessary part of the mobile world.

A traditional SDR solution places the ADC as close to the antenna as possible. For low-cost, low-power consumer devices, this has to be after the downconversion, either at baseband or at an intermediate frequency (IF). Once the entire signal band has been digitized with sufficient dynamic range to capture the maximum blocker levels as well as the minimum desired signals, the signal-processing algorithms can quite easily extract the signal of interest. One example was the Steinbrecher MiniCell base transceiver station, which digitized the 12.5 MHz of cellular bandwidth and extracted 30 kHz AMPS or TDMA channels using DSPs.

A multistandard SDR would require that the ADC dynamic range be variable (or designed for the worst case) and that the signal processing be programmable. This precludes the use of application-specific ICs (ASICs), and requires the use of field-programmable gate arrays (FPGAs) or DSP technology, both of which are expensive for consumer devices. For a 10 MHz band of interest and a 100 dB of dynamic range (-110 dBm to -30 dBm signals with 20 dB allocated for headroom, modulation crest factor and quantization noise), the ADC has to sample at greater than 20 Msamples/s with 17 effective number of bits (ENOB). Alternatively, one could also use an oversampling sigma-delta architecture with a 320 Msamples/s clock rate and reduce the ENOB to a more manageable 10 bits to 12 bits (depending on the order of noise shaping).

Both of these alternatives are about on the edge of current technology.^[1] (Figure 2.) As an example, an advanced ADC from TelASIC, the TC1411, claims to support 14 bits, 250 Msamples/s over a 75 MHz bandwidth. While this is sufficient for many SDR applications, it requires 1.9 W, which is too power hungry for mobile applications. As a consequence, the optimum transceiver design must include the channelization and AGC functions before the ADC for the foreseeable future.

BitWave's long experience with SDR has enabled it to develop a new solution to this complex problem. To convince the wireless industry to accept and adopt a new implementation of SDR, one must first step back and consider what the market requires. It's apparent that a low power solution is required. This led the decision to base all components on existing analog architectures. A software-defined transceiver must also be able to optimize itself for each unique protocol; i.e., be easily controlled with digital technology by the baseband through a simplified interface. Finally, the dynamic optimization required for each potential operating environment of the device led to innovation for individually controlling each RF, analog and mixed-signal blocks within the transceiver.

Hence, a Softransceiver technology was created with the preceding ideas in mind: it provides a way to increase flexibility while at the same time reducing cost, decreasing power consumption and increasing performance. (Figure 3).

The Softransceiver can effectively move its operating characteristics in real time by software commands. It can shift the center frequency, modify the bandwidth and sampling rate, and change the linearity and noise figure of a transceiver channel in real time. Thus, one programmable transceiver can replace the many fixed transceivers now found in a typical cell phone or data modem. This reconfigurable transceiver technology also dramatically reduces the size and power of the transceiver chip and increases flexibility when compared with a multitransceiver on die solution. The Softransceiver stands apart from other multiband solutions in that it is competitive in performance even against single standard transceivers.

One of the ways in which the Softransceiver compresses the required area is through a programmable LO. Each standard that the Softransceiver supports has an optimal architecture for implementation. For example, most GSM standards can be more easily implemented in a low IF (LIF) architecture while other standards such as WiFi or WiMAX may be more easily implemented with a zero IF (ZIF) architecture. The Softransceiver allows the selected standard to be demodulated with whichever architecture works best.

In an analog architecture such as that shown in Figure 4, implementing both LIF and ZIF is possible, however, it requires a lot more analog die area since the designer needs to provide a second analog stage.

In Figure 5, the combination of a widely programmable LO and the inclusion of a digital downconverter allows for the complete elimination of the second analog stage. In this manner, the Softransceiver achieves the desired combination of low area and flexibility needed.

The digital downconverter required for a LIF architecture can be implemented in < 5k gates. Compared to the complex digital filtering, this is very small and has little effect on the overall chip area when implemented in a digital CMOS process. Figure 6 shows the receive chain of the Softransceiver in its final form.

Furthermore, it has been recognized that if one changes the transceiver's operating characteristics in real time, one can dynamically re-optimize the transceiver for its environment. Thus, the transceiver could be optimized in real time for performance, battery life or many other operating parameters. It was also found that if one can control the transceivers' reconfiguration with software, one can offer carriers, OEMs, distributors and cell phone designers valuable choices with a single chip.

The dynamically reconfigurable components required to implement the software-defined Softransceiver chip have been proven. BitWave has developed and demonstrated in silicon its digitally programmable RF and analog technology with components that include low-noise amplifiers (Figure 7), voltage-controlled oscillators, mixers and synthesizers. All components are programmable over the frequency range, bandwidth, linearity and dynamic range required to meet the different WAN, LAN and PAN standards. Each component used significantly less die area with much higher power efficiency than silicon components found in existing transceiver solutions. A low power, low die area, flexible and dynamically programmable solution such as the Softransceiver is precisely what is required to enable laptops and other wireless data devices with true anytime, anywhere connectivity.

Mobile data solutions are an integral part of our global communications chain. While WiFi will continue to grow, other emerging technologies will continually evolve and be implemented to cover gaps in coverage. True global mobile data connectivity will only be achieved when users can seamlessly roam from one network to the next using the same flexible mobile data device.

While many different approaches have been taken over the years, neither of the two primary approaches — parallel hardware architecture and SDR (supported by high bandwidth ADCs) — has been successful in enabling a truly mobile data modem. Both parallel hardware architectures as well as SDR have required designers to make undesired trade offs between flexibility, power, performance and cost.

The Softransceiver approach offers a new solution for low-cost flexible mobile data modems. The innovative reconfigurable analog components together with the programmable digital processing elements have been integrated into a complete transceiver solution that offers handset designers, carriers and users alike an opportunity to benefit from true mobile data.

1. Kester, Analog Dialogue 39-06, June (2005).

2. “Emerging 4G Broadband Standards,” Lehman Bros., May 26, 2005 and BitWave research.

John A. Kilpatrick is director of systems engineering at BitWave Semiconductor Inc., Lowell, Mass. He has more than 30 years of experience in wireless communications. Kilpatrick received his BA in Applied Mathematics from Gordon College and his MSSE from the University of Lowell.

Russell J. Cyr is CMO and co-founder of BitWave Semiconductor. A 20-year veteran of the wireless communications industry, Cyr has published numerous articles and has been granted several patents in radio technology. He received his BSEE and MSEE from the University of Massachusetts and his MBA from Rivier College.

Erik L.Org is marketing manager for BitWave Semiconductor Inc. He has more than 10 years of experience with wireless markets and technology. He earned an MBA from Columbia Business School, as well as a BS in Electrical Power Engineering from Rensselaer Polytechnic Institute.

Geoffrey Dawe is CTO and co-founder of BitWave Semiconductor. Dawe is a 21-year veteran of the RFIC industry. He is an author/co-author of more than 35 publications in the field of RFIC technology. He has one issued patent and several others pending. Dawe received a BSEE from Norwich University.