Software radio is a buzzword that's been around for many years, with deep roots in the military. These were "be all and do all" receivers, the workhorses of military intelligence.1,2 As the cold war melted, software-radio enthusiasts found a new home for their technology: cellular-radio applications.3 This article reviews the concept, architecture, technology challenges, and economics of the software-defined radio.
Historically, the relatively low number of prevalent standards, as well as the state of the art and high cost of key components, have limited the benefits and use of software radios. The second generation of wireless systems has offered a variety of different modulation formats and multiple-access technologies to be covered by a single radio.
Dual-mode operation and compatibility requirements with analog systems make this task even more challenging. The main goal of current developments in dual-mode (and eventually triple-mode) transceivers, which cover drastically different data rates and modulation formats, is often reduction of cost, power, and size.4
Defining Software Radio
The most literal translation of software radio would be a radio where signals on the antenna, or perhaps at an intermediate frequency, are digitized with a high-performance analog-to-digital converter (ADC) and sent to a terminal (computer, mobile phone, etc.). Once digitized and inside the terminal, code would be used to select an RF channel and demodulate the signal (Fig. 1a). While this is a worthy goal, it's only now becoming practical for specific applications.
A more reasonable name for this desired technology would be a "digital reprogrammable radio." (Note that digital receivers can be designed to receive digitally modulated signals, as well as analog, or FM, signals.) As with a software radio, an ADC is used to digitize the signal at the antenna or at an intermediate frequency. Instead of processing the digitized data solely in software, however, a variety of flexible, reconfigurable ASICs and general-purpose digital signal processors (DSPs) are used to reduce system power dissipation, size, and cost (Fig. 1b). These ASICs are programmable and can be adjusted for different channel characteristics and modulation schemes. These implementations, which include ASICs or field-programmable gate arrays (FPGAs), are more economical than fully flexible DSP implementations.5
A practical definition of software radio includes radios with a set of predefined hardware modules (such as ASICs or FPGAs). These modules must be selectable through software as common hardware for several different systems.
These modules provide multirate signal-processing functions (like decimators and interpolators), digital down/up conversion capabilities, and filter programmability via RAM-coefficient, finite-impulse-response (FIR) filters. This approach enables the efficient realization of transceiver functions in terms of power consumption, minimal component count, and compactness. In effect, the filters and demodulation that would have run on the terminal have been generalized and committed to silicon with programmable characteristics.
Thus, a software radio that's practical for today is one where selected functions have been committed to silicon. But, enough flexibility must be retained in order to reconfigure it for a variety of different standards.
What Technology Is Needed?
In recent years, there's been significant improvement in critical technologies such as low-noise amplifiers (LNAs), mixers, data conversion, and DSPs. Only now do these enhancements make "software radios" possible.
Whether sampled at the antenna or at an intermediate frequency (IF), the signal must still be sampled with an analog-to-digital converter (ADC). In the case of the military archetype (Fig. 1a, again), the usual specification was for a 16-bit, 1-GHz sampler. Needless to say, if such a converter ever existed, it was quite expensive. Although advances have been made in RF-bandpass sigma-delta converters, a much more practical solution is to sample at an IF frequency.
A key breakthrough in the commercialization of software radios has been to limit the bandwidth of the receiver. The personal communications services (PCS) and cellular industries have done this through the licenses granted to operators (typically under 15 MHz per operator). Technically, this means that as long as the band of interest has bounds, images and other spurious signals can be managed and placed out of band. When applied to software radio, a defined bandwidth means that a system's sample rate and dynamic range can be reasonably limited.
This means that the IFs able to be selected also can be directly sampled with current ADC technology. Five years ago, data converters required that the RF signal be converted to baseband. Present technology allows the sampling of IF signals up to 250 MHz. An added benefit of IF sampling is that one or more downconvert stages can be eliminated. This results in very small receiver designs and, thereby, reduced cost.
Figure 2a shows a baseband-sampling receiver. This is a triple downconvert to near baseband with analog channel filtering. The final downconvert incorporates an IQ separation feeding separate baseband ADCs. The ADC datastream goes to the DSP, where demodulation is done in software. In addition to the RF band-select filter on the front end of this receiver, a channel-select filter is implemented in the analog domain.
Although the software could be altered to support a different air interface, the channel characteristics cannot be changed since the bandwidth of the analog filters is fixed. Certainly, different analog filters could be switched in and out. But often, these filters are quite expensive and add complexity.
Figure 2b depicts a similar IF sampling receiver. In this case, a single analog mixer is used to downconvert to a convenient IF where the signal is digitized. I and Q separation is done digitally in the receive-signal-processor (RSP) chip, along with channel-filter and data-rate selection. In this instance, the DSP is only used for demodulation and the receiver is fully programmable. Both the channel characteristics and demodulation methodology can be changed. In a narrow sense, software radios as presented here are "future proof" in that they permit incremental channel or standards changes with little or no impact on the hardware. Since this architecture lends itself well to IF sampling, such receivers are both smaller and cheaper.6