Cell-phone radio components
BitWave’s Softransceiver
The deluge of new wireless consumer and industry applications continues to fuel the unmitigated growth of the wireless market. As consumers become increasingly addicted to these rapid-fire apps, they also become more vocal and demanding when expectations are not met. Pressure then begins to mount on carriers and device vendors to try and meet these expectations with a broad selection of multimode devices.
On top of that, delivering a ubiquitous service becomes complicated by the expanding list of voice and data protocols, as well as broadcast entertainment, which must be supported by a mobile device. Today, the list includes WCDMA, HSDPA, GSM, CDMA, EV-DO, Wi-Fi, WiMAX, DVB-H, MediaFLO, Bluetooth, and UWB, among others. Whereby in the past, carriers deployed a single protocol on a single band, they now must support dual-, tri- or quad-band solutions.
FRAGMENTED MARKETS
Today’s design and development teams face a couple of tough major challenges from the start. They must produce new solutions quickly, optimised to perform in a given market. And they must redesign the radio and form factor to support deployment in the fragmented global wireless market.
To achieve seamless connectivity, vendors must integrate more radios into a device. That takes up valuable space in the handset and increases overall costs. Therefore, wireless device manufacturers are forced to make hard choices between the applications, bands, and protocols their products will support.
For example, a single mobile phone may come to market as a tri-band, a quad-band or dual-mode handset. Each combination will present its own engineering challenges, requiring a team to redesign the layout of the hardware for that particular version. It’s not clear whether the incremental improvements made between successive generations of devices will be enough to allow device vendors to match the pace of growth in protocols and spectrum with the pace of new device development.
MEETING SYSTEM SPECIFICATION
Today’s radios integrate three main components or modules: the baseband modem, the transceiver, and the front-end module (FEM). The basic block diagram of a wireless device is shown in Figure 1. The FEM, which consists of filters and a power amplifier, connects the transceiver to the antenna and provides the necessary filtering and amplification required to meet the system specification.
The transceiver converts the received, amplified, and filtered RF signal to modulated I/Q signals, which are demodulated by the baseband. Each transceiver is optimised for the particular frequency, bandwidth, data-rate, and modulation scheme used by the application. In general, today’s devices require multiple transceivers to support the multiple applications deployed on each device.
The baseband modem converts the transceiver’s modulated I/Q signals into digital data, which it can process and act upon. The baseband runs call control algorithms and sets up any input/output to the device user (i.e., voice, video, data).
A typical multimode phone must operate over the required number of frequency bands and support current protocols for voice and data. This doesn’t include additional functionality for short-range wireless connectivity (Bluetooth), location (GPS), and entertainment (broadcast video).
Semiconductor technology has enabled the successful implementation of new bands. Baseband modems, which contain all digital circuitry, leveraged advances in process node to absorb more and more functionality into the same die. Moreover, the low incremental cost for added digital processing power makes it possible to design flexible digital processors that can implement multiple protocols in the same die.
THE RF BOTTLENECK
Still, RF functionality remains a bottleneck. Due to the sensitivity of analogue and RF circuitry to parasitics and interference, designing the RF and analogue portions of a radio are challenging. Not surprisingly, successful designs are considered the province of a relatively small group of experienced engineers.
Faced with greater numbers of protocols and an increase in licensed spectrum, device vendors as well as RF IC Integrated Device Manufacturers (IDMs) must deliver more products faster to the market. Unfortunately, the long lead time associated with new RF products makes it difficult to predict future integration preferences. IDMs are forced to find other means to deliver the solutions quickly, but without the tools or architectures to enable a reduced time to market, IDMs will become ever more challenged to keep up the pace of development.
Programmable radio architectures can meet this demand. Basebands that support multiple protocols have already been developed. What’s still needed, though, is a programmable RF architecture. To be competitive with existing solutions and offer new value, it should meet commercial benchmarks for cost, power, and size.
MEETING THE POWER BUDGETS
The trick is to come up with a flexible-enough architecture to support a wide variety of signal bandwidths, modulation formats, signal levels, and blocking specifications, and yet it still meets the power budgets for portable devices.
As an example, cellular standards have low to medium bandwidths, but have very high dynamic range requirements due to challenging blocker environments. Data standards such as WiMAX have high signal bandwidths and high order modulation.
A programmable architecture must quickly reconfigure its operating characteristics in real time under software control. It should shift the centre frequency, modify the bandwidth and sampling rate, and change the linearity and noise figure of a transceiver channel in real time.
For instance, a programmable transceiver (Fig. 2) could replace the many fixed transceivers now found in a typical cell phone or data modem. And since a programmable transceiver could be optimised post-fab, the design of a programmable transceiver needn’t account for the worst-case process tolerance stack up. In other words, a programmable transceiver can be assumed to be always operating at its most efficient and may, therefore, significantly reduce its size and power consumption. It will also be more flexible when compared to a multi-transceiver-on-die solution.
Since each standard supported by a programmable transceiver has an optimal architecture for implementation, a programmable transceiver should also let the designer choose between multiple architectures. For example, most GSM standards can be more easily implemented in a low IF architecture, while a zero IF architecture is a better option for other standards such as Wi-Fi or WiMAX. A programmable transceiver should allow the selected standard to be demodulated with whichever works best.
NEW PARADIGM
Programmable radio solutions can offer benefits in the three P’s of price, power, and performance. Programmability can also foster a new paradigm in the creation of a supply chain, the implementation of an application set, or in carrier network planning. But to achieve this, designers must select an architecture that delivers on those promises while continuing to meet commercial goals for the three P’s.
Many believe that the next phase in radio’s evolution involves cognitive radios that can analyse their environment (from an RF perspective and a network perspective) as well as modify their transmission characteristics so as to efficiently optimise the available spectrum. Truly reconfigurable architectures offer a means to improving upon current multimode solutions while simultaneously providing a path toward this goal.
Wireless devices and/or networks that can choose which bands and protocols to communicate over will ultimately lead to the next giant step forward in low-cost, efficient, and effective communications over the wireless spectrum.