The Holy Grail of software-defined radio (SDR) is flexible bandwidth combined with a flexible air-interface system. Such a combination could be tuned across a broad RF band that is capable of both time- and frequency-duplex operation. The technology that could enable this reality remains out of reach, however. This is particularly true when it comes to the needed RF components.

For SDR, one of the greatest challenges is the duplexing of transmit and receive signals. Time-duplex systems solve the transmit-to-receive interference problem by operating the receiver and the transmitter in different time slots. Only one of them is active at any instant in time. Simple switches can then be used to connect the receiver and transmitter to the antenna. Time-duplex systems are common in early TDMA systems, such as GSM and ANSI-136. In contrast, half-duplex systems are usually found in the public-service domain and wireless-LAN systems.

Most 3G wireless systems differ in that they require full-duplex operation. In this mode, both the transmitter and the receiver operate simultaneously on separate frequencies. The transmitter frequency must therefore be isolated from the receiver frequency. This task is handled by high-power filter structures, which are known as duplexers. But fully tunable duplexers are difficult to design and build. They suffer from higher insertion losses than their fixed-frequency cousins (e.g., a single-band cellular duplexer). Plus, they tend to be bulky and expensive.

The RF domain faces another obstacle in the development of radio-frequency power amplifiers. This sector demands power amplifiers that are capable of operating over broad bandwidths with a wide variety of power settings. They also should have high linear requirements. Although it is possible to build such power amplifiers, they are typically expensive and high in power consumption. These drawbacks become clear when they are compared to power amplifiers that are optimized for a specific air interface and designed to operate over narrower frequency bands.

Analog-to-digital converters (ADCs) form the last RF hurdle. ADCs must operate over the broad dynamic range. To cover the diverse air interfaces, they also need to function at high speeds. Consumer products within the wireless-/mobile-device category require integrated circuits (ICs) that are low in power consumption and cost, but high in performance. Currently, ADCs that meet such requirements—while being capable of SDR-type operation—are largely nonexistent.

Fortunately, today's world is much simpler than the software-defined-radio ideal. Governmental agencies designate frequency bands for specific uses. Standards bodies design wireless systems that are evolutionary within these allocations. As a result, the amount of flexibility that is needed from the RF interfaces is greatly narrowed.

Figure 1 illustrates a version of an SDR that applies to the commercial cellular market. In this design, frequency bands are expanded by augmenting the elements in both the duplexer design and the RF power amplifier. This power amplifier was chosen for the specific frequencies of operation. The programmable filters, which reside on the system's analog side, simplify the demands on the ADCs and digital-to-analog converters (DACs).

This design uses a finite number of fixed bands and bandwidths. Yet it provides for a fully programmable baseband processor. In addition, it implements a version of SDR that has considerable benefits in terms of development cost and flexibility.

This version is particularly applicable to consumer and public-service/emergency applications. These applications support a limited number of operating modes from a single user terminal. Frequency assignments do not change quickly. As a result, the main issue is how to augment capacity and improve functionality within existing allocations.

Beyond the RF challenges, the increasing diversity in air interfaces is placing much greater demands on a wireless system's digital-signal-processing elements. It doesn't matter if these elements are DSP-, FPGA-, or fixed-logic-based. Air interfaces like GSM, ANSI-136, and EDGE typically require high-resolution processing in the 16-to-20-b range. In contrast, CDMA interfaces typically work at lower resolutions of 6 to 8 b but at much higher processing rates. The CDMA interfaces also tend to include numerous single-bit operations. This diversity in both bit widths and operation types severely strains any architecture that comprises homogeneous elements. To get even more complicated, the systems that support programmable RF bandwidths need to be flexible in filtering. This characteristic imposes even greater loads on the signal processing.

Three basic IC options exist for the baseband portion of an SDR system: DSP-based, hybrid DSP/FPGA systems, and adaptive/reconfigurable architectures. Fixed logic-based systems are not included in this list. By definition, they are not compatible with software-defined radio.

The DSP-based systems are well known and supported by mature tools. Generally, however, they lack the processing power needed to handle complex systems. While DSPs have recently reached a performance level that is adequate for GSM, the more complex air interfaces, such as EDGE, cdma2000, and W-CDMA, are well beyond what can be handled by DSP-only systems. To meet the cost, power-consumption, and performance requirements of consumer products, 80% or more of the signal processing will typically be assigned to more efficient ASICs.

The design flow that is used for DSPs is represented in Figure 2. The advantages to this approach are the homogeneous nature of DSPs and the maturity of the toolchain. By basing the system design on high-level design languages like C, the toolchain considerably simplifies the design process.