The 20th century was shaped by broadcast radio and TV. The 21st century will be shaped by two-way interactive wireless communications supporting portable personal information tools. One of the functions required in this paradigm is the ability to quickly change from one air interface standard (AIS) to another with a small, cost-effective radio with long battery life. Software-defined radio (SDR) is the enabling technology supporting this functionality.

The alternative to SDR is to have multiple separate radio chains — one for each mode and band. In personal portable devices (handsets, etc.) this results in larger, more expensive devices with shorter battery life. In basestations and access points, this results in what are commonly called "forklift upgrades" that require expensive physical replacement of remote components. For both, evolving AISes often require operation through a transition period of multiple devices.

Cognitive radio (CR) is the automatic reconfiguration of a radio device in response to changes in its environment. SDR does not require CR to operate and deliver significant benefits. Neither does CR require SDR to operate, but it can have a much greater positive impact if it is implemented on top of an SDR base. IS-95 (dual-mode second-generation AIS) handsets are an example of CR on top of a non-SDR base. The scheme required a mode change (modulation technique) but not a band change (frequency). An example of a CR application that is anticipated to operate on an SDR base is the proposed use of unused VHF TV channels for unlicensed use.

Both SDR and CR are evolutionary developments with deep roots in radio technology. In early systems, each function was implemented with discrete analog technology. This resulted in relatively large, expensive, high-power-consuming systems that were difficult to design, manufacture and manage or maintain in the field. The desire to reduce cost, size and power consumption while making devices easier to manage in the field has driven the technology evolution path we are still on today.

As digital technology entered the beginning of its period of rapid evolution, discrete analog components on complex printed-circuit boards were gradually replaced. First discrete digital logic components were used to implement the human interface, local control and protocol stack functions. With the appearance of the microprocessor, the discrete logic components were replaced with a microprocessor called a microcontroller and software. Gradually, spaghetti code controller implementations are being replaced with architected code. It is the continued evolution of the controller function that is materializing in CR.

Developments in advanced software, protocol and controller techniques both in devices and in the supporting infrastructure are currently pointing to opportunities for improving ease of use for the consumer, ease of operation for the network operator, ease of support for the subsystem and system vendor, and improved spectrum efficiency for the regulator.

After the basic controller functions were implemented in early microcontrollers, attention turned to low-speed signal processing. Analog discrete components were replaced with digital logic components. Then special mathematical functionality such as multiply-accumulate functions were added to microprocessors to create digital signal processors. Low-speed signal-processing functions were converted from discrete digital logic to DSPs and software.

After that, the high-speed signal-processing analog discrete components were replaced with digital logic components. The expectation was that the same process would continue and that the high-speed signal processing would be implemented by a microprocessor and software. However, a fundamental barrier was found: required DSP processor speeds exceeded the ability of single-stream instruction-set processors. Although processor speeds increased along the Moore's Law curve until recently starting to approach an asymptote, AIS evolution has driven the requirements for high-speed processing faster than processing speeds have increased. In the early 1990s, innovative solutions based on reconfigurable logic began to be developed. With these solutions to the high-speed signal-processing subsystem, the term SDR was coined. Differences in the requirement sets for mobiles and basestations produced different classes of solutions. Subsystems based on the first generation of innovation were scheduled to come to market this year. Management difficulties, technical problems resulting from high levels of complexity and escalating requirements from new AISes are delaying the arrival of these products.

When reconfigurable logic was introduced and SDR joined the alphabet, the dominant implementation architecture used for RF front ends was the superheterodyne, which was patented in 1915. The first alternative to the superhet that received widespread industry attention was the direct-conversion architecture, sometimes called homodyne, or zero IF. Direct conversion eliminates one of the two stages of up/down conversion in the superhet. That way it can eliminate approximately one-third of the parts.

Neither the superhet nor direct-conversion architecture can provide the flexibility previously obtained in the other subsystems (baseband and controller) through software-driven architectures. New innovative architectures are in development and coming to market soon. They promise small size, low part count, low power consumption, low cost and reconfiguration with software-like entities.

The other major component necessary for practical implementation of multimode, multiband systems is a multimode, multiband antenna. Here again, innovative architectures are required. Development of advanced antenna technology has until recently occurred primarily in the aerospace and defense sectors. One early successful effort to move innovative architectures developed in the military sector into the commercial market is SkyCross, which is providing a range of very small, highly efficient, embedded multimode, multiband antennas.

So today we have all the enabling technologies necessary to support SDR. Systems based on these enabling technologies are being fielded, with the military the early adopter.

In the commercial sector, the public-switched telephone network provides a lingua franca. If I have a mobile device using AIS X and you have one using AIS Y, and if we can both "see" our respective basestations, we can communicate in spite of the fact that our mobiles are incompatible. This communication is possible because each of our basestations can connect through the public-switched telephone network. The military can't do this, and to make things more critical, communicating entities are armed, so these communications limitations produce significant negative consequences.

For these reasons the military has been willing to accept larger, more expensive systems in order to field them as early as possible. Some farsighted individuals in the military and commercial sectors recognized this fact in the mid-1990s, seeing an opportunity for the commercial sector to gain from the military's early funding while the military would benefit from the later economies of scale developed by the commercial sector. This is one of the forces behind the creation of the SDR Forum.

Today, software-defined radio systems are coming into practical everyday use. Military products are widely deployed — for example, the military's Joint Tactical Radio program. SDR basestations are coming to market, both explicitly and not explicitly labeled as SDRs, and SDR handset announcements and advances for civil government applications are likely to arrive later next year.

As SDR comes to the forefront, the possible benefits to be achieved by adding CR capability on top of SDR are beginning to attract attention. In the past, the range of action available to CRs was limited by "hardwired" architectures. As SDR increases the range of variability, the potential benefits of CR increase dramatically.

Coupled with this increase in range of flexibility is an increase in the number of players in the entire wireless value chain. This leads to rapidly increasing complexity throughout the value chain and the life cycle.

One way to help manage this complexity under consideration in the SDR Forum is the creation of an industry-standard meta language that would describe the functionality of semiconductors, components, subsystems, systems and networks.

Such a language would improve the efficiency of the value chain and assist in life cycle management.

Mark Cummings mailto:markcummings@envia.comis managing director of enVia II (Atherton, Calif.) and chair of the Software Defined Radio (SDR) Forum.