Software-defined radio (SDR) represents the future for some wireless technologies,
but it's still a work in progress. Like any technology, it's followed an evolutionary
path as components and practices continue to improve. Its adoption is more widespread,
too, as chips track Moore's law and as better software comes along.
SDR is a moving target. Its current "reality" is relatively small today, but its definition and place in the industry continues to evolve. Cell phone basestations and military radios are just the onset of an SDR movement that will grow as needs and opportunities are identified and as electronic and software technology permit.
WHAT IS SDR?
In the beginning of any new technology, definitions are a bit fuzzy until we
see real products and applications. While the term SDR is still subject to interpretation,
though, the industry is gradually settling on some concrete definitions. Early
on, SDR was broadly defined as "any radio that used software to perform modulation
and demodulation." Using that definition, a huge population of existing radios
like cell phones, basestations, and wireless local-area networks (WLANs) qualifies
as SDR.
In any case, SDRs are digital radios that attempt to complete as much of the signal processing—digitally—on both the transmit and receive sides of a wireless application. Most times, this involves a programmed processor, such as a general-purpose processor (e.g., an embedded controller or digital signal processor).
The SDR Forum, an organization dedicated to advancing the development and deployment of SDR, has a formal definition of SDR that includes five tiers:
- Tier 0: A digital hardware radio that cannot be altered
- Tier 1: Software-controlled radio (SCR); software can change some functions
like power level and interconnects, but not modulation or frequency of operation
- Tier 2: Software control of modulation, wide/narrow band, security, waveform
generation and detection, but mostly frequency constrained
- Tier 3: Ideal software radio (ISR); elimination of any downconversion or
upconversion in reception or transmission; full programmability
- Tier 4: Ultimate software radio (USR): fully programmable but able to support
a broad range of frequencies and functions concurrently (two-way, GPS, video,
smartcard, satellite, etc.).
Even these definitions are subject to interpretation, and the SDR Forum admits that it's updating and revising them. Nevertheless, they offer the big picture. In general, the end point is to get SDR to the point where it's completely flexible in terms of defined operational standards and independence of operating frequency.
Cell phones and basestations qualify under the Tier 0 and 1 definitions. So do some WLANs. However, real SDR isn't widely used yet—except in the military, where R&D teams have made great strides in creating an effective, and reasonably sized and priced, SDR.
The military's Joint Tactical Radio System (JTRS) project's goal is to build a group of compatible radios that operate from 2 MHz to 2 GHz with complete frequency agility. They also must be able to adapt to any modulation or protocol. Prototypes were built and tested, but final units haven't reached widespread adoption to this point.
An ideal SDR digitally codes and modulates the data that's going to be communicated
in a baseband processor before transmitting it (Fig.
1). Next, the SDR sends the data to a digital-to-analog converter (DAC)
and then to a power amplifier (PA), where it finally reaches the antenna.
On the receive side, the signal picked up by the antenna is fed to a low-noise
amplifier (LNA), where it's boosted to a level that can be handled by an analog-to-digital
converter (ADC). The digital output of the ADC is then processed, as necessary,
in a baseband processor to recover the originally transmitted signal.
While it's possible to realize an ideal SDR at low frequencies, most wireless
activity is above the VHF and UHF ranges and well into the microwave region.
That's why most of today's SDRs use mixers in the front end to perform analog
upconversion and downconversion (Fig.
2).