The phenomenal growth seen in wireless markets is driven by the expanding variety of applications targeting both consumers and industry. Coupled with the ubiquity of these applications is the increased addiction to them. Thus, when expectations fall short, consumers become more demanding. Carriers and device vendors are constantly challenged to meet these expectations for a broad selection of multimode devices.
Meeting customer demand with a universal service offering is complicated by the growing 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, UWB, and many others. Previously, carriers deployed a single protocol on a single band; now they must support dual-, tri-, or quad-band solutions.
Today’s design teams not only have to produce new solutions quickly, optimised to perform in a given market, but they must also redesign the radio and form factor to support deployment in the fragmented global wireless market.
To achieve seamless connectivity, vendors must add more radios into a device, taking up valuable space in the handset and increasing overall costs. As a result, wireless device manufacturers are forced to make hard choices between the applications, bands, and protocols supported by their products.
For example, a single mobile phone may come to market as a tri-band, quad-band, or dual-mode handset. Each combination will present its own engineering challenges and require a team to redesign the layout of the hardware for that particular version. It’s not clear if the incremental improvements accomplished between successive generations of devices will be enough to allow device vendors to keep pace with new protocols and spectrum growth.
Radio Technology Today
Today’s radios consist of three main components or modules: the baseband modem, the transceiver, and the front-end module (FEM). Figure 1 illustrates the basic block diagram of a wireless device. The FEM, which consists of filters and a power amplifier, connects the transceiver to the antenna and provides the necessary filtering and amplification that’s required to meet the system’s specification.
The transceiver converts the received, amplified, and filtered RF signal to modulated I/Q signals, which are then 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 modulated I/Q signals received from the transceiver into digital data, which it can process and act on. 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 and protocols. Baseband modems, which contain all-digital circuitry, have leveraged advances in the process node to absorb more and more functionality into the same die. Moreover, the low incremental cost for additional digital processing power enables the design of flexible digital processors that can implement multiple protocols in the same die.
The RF Bottleneck
Still, a bottleneck remains when it comes to RF functionality. Due to the sensitivity of analogue and RF circuitry to parasitics and interference, the design of the RF and analogue portions of a radio is quite a challenge. Successful designs are considered the province of a small group of experienced engineers.
With the explosion of protocols and increased licensed spectrum, device vendors as well as RF IC integrated device manufacturers (IDMs) must deliver more products within a shorter timeframe to the market. Unfortunately, the long lead time associated with new RF products makes it difficult to predict future integration preferences. Consequently, IDMs are forced to find other means to deliver the solutions quickly. However, without the tools or architectures to reduce time-to-market, the challenge deepens for IDMs to keep up the pace of development.
One answer to this challenge comes in the form of programmable radio architectures. Basebands that support multiple protocols have already been developed, but there’s still the need for a programmable RF architecture. It should meet commercial benchmarks for cost, power, and size if it’s to be competitive with existing solutions and offer new value.
The trick is to come up with a flexible architecture that can support a wide variety of signal bandwidths, modulation formats, signal levels, and blocking specifications, yet meets the portable-device power budgets. For instance, cellular standards have low to medium bandwidths, but very high dynamic range requirements due to challenging blocker environments. And data standards such as WiMax have high signal bandwidths and high order modulation.
A programmable architecture must quickly reconfigure its operating characteristics 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.
One such programmable transceiver could replace the many fixed transceivers now found in a typical cell phone or data modem (Figure 2). Also, 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 offer the option of selecting between multiple architectures. For example, most GSM standards can be more easily implemented in a Low IF architecture, while implementation of other standards such as Wi-Fi or WiMax is more simply handled with a Zero IF architecture. A programmable transceiver should make it possible for the selected standard to be demodulated with whichever method works best.
Programmable radio solutions can offer benefits in price, power, and performance. Programmability can enable a new paradigm in the creation of a supply chain, the implementation of an application set, or in carrier network planning. To achieve this goal, however, designers must select an architecture that’s able to deliver on those promises while continuing to meet commercial goals for price, power, and performance.
Many believe that the next phase in radio’s evolution is cognitive radios. These would be able to analyse their environment (from an RF and a network perspective) and then modify their transmission characteristics so that the available spectrum is used as efficiently as possible. Truly reconfigurable architectures offer a means to improving on current multimode solutions while simultaneously providing a path toward this goal.