Software-defined radio (SDR) used to be rare and exotic. But today, most modern radios use SDR’s architecture and techniques. Each year with continuing advances in ICs and other technologies, SDR becomes more capable and widespread. In fact, new techniques like cognitive radio (CR) are making SDR more useful and beneficial to wireless communications.
SDR Defined
SDR uses software to perform some of the signal processing in a receiver and transmitter. For example, a traditional receiver using the ubiquitous superheterodyne architecture performs all signal processing with basic electronic circuits (Fig. 1a). The superheterodyne downconverts the input signal to an intermediate frequency (IF) for demodulation and other processing.
Early SDR receivers (Fig. 1b) replaced the demodulator with an analog-to-digital converter (ADC) after the IF stage and performed the demodulation and some filtering in a digital signal processor (DSP). Today, because ADCs sample faster, DSPs can handle more functions.
To make DSPs work, the amplitude and phase of the signals both must be known. This has led to an architecture that divides the received signal into two paths, one producing an in-phase (I) signal and a 90° shifted quadrature (Q) signal. A basic carrier signal has the form:
V = Ac cos(2πfct + ?)
Where fc is the carrier frequency, ? is the phase, and Ac is the carrier amplitude. Any of these may be varied for modulation. For demodulation in the digital domain, a single signal is insufficient for existing algorithms. Therefore, the modulated signal is converted into the I and Q signals:
V = I(t) cos(2πfct) + Q(t) sin(2πfct)
With the quadrature signals, any variations in amplitude, frequency, or phase can be detected and used in a demodulation or other process.
Figure 2 shows a modern I/Q SDR receiver. A low-noise amplifier (LNA) usually boosts the input signal from the antenna before it is applied to the two mixers. The mixers develop the I and Q signals. Both receive a local oscillator (LO) signal from the phase-locked loop (PLL) frequency synthesizer. Note the 90° shift between the LO signals to the two mixers.
The LO frequency is set to the signal frequency so the difference signal from the mixers is zero without modulation. With modulation, the difference is the baseband or original modulating signal. This architecture is called direct conversion or zero IF.
After the baseband signals have been filtered in low pass filters to eliminate the sum components at the output of the mixers, the signals are digitized in a pair of ADCs. The digital baseband signals are then processed with digital downconverters (DDCs) to lower the sampling rate, making them more compatible with the digital signal processing circuits. The digital signal processing circuits then use both the I and Q signals to perform the demodulation, equalization, and additional filtering as the application demands.
In a modern SDR transmitter, the DSP modulator divides the data to be transmitted into I and Q signals and feeds them to digital upconverters (DUCs) that boost their sample rate (Fig. 3). The I and Q signals are next sent to digital-to-analog converters (DACs) that produce the final baseband signal. The signals are then low pass filtered and sent to the mixers that upconvert the signal to the final transmitted frequency. The signal is finally sent to a power amplifier before being applied to the antenna.
All modern SDR transceivers use some basic variation of the receiver and transmitter circuits shown here. Of course as ADCs and DACs get faster, the digital processing moves closer to the antenna. The ultimate receiver then becomes simply a filter at the antenna to limit the bandwidth and a LNA before a fast ADC (Fig. 4). Then, the DSP performs all other processing like demodulation and filtering. Commercially available amateur radio and shortwave receivers covering up to 30 MHz already use this advanced architecture.
Many functions are now performed digitally:
- Filtering (low pass, high pass, band pass, and band reject)
- Modulation (AM, FM, PM, FSK, BPSK, QPSK, QAM, OFDM, etc.)
- Demodulation
- Equalization
- Compression
- Decompression
- Spectrum analysis
- Predistortion
New modulation methods and related procedures are generally known as waveforms. By changing waveform software, a radio for one application like FM voice could be reprogrammed for high-speed data on a different frequency with a different protocol.
The advantages of SDRs lie in the greater simplicity of the hardware. Standard RF circuits are reduced to a minimum, keeping the cost of ICs low. DSP software improves operation with functions (like filters) that provide better performance than equivalent analog circuits. Digital signal processing also can compensate for some failing of RF components.
Furthermore, the flexibility of reprogramming allows defects to be fixed, new features to be added, upgraded operations to be included, and performance to be improved. An SDR of flexible design can be quickly changed with software to accommodate new modulation methods, new protocols, and other major adjustments that would ordinarily require new hardware.
The downsides of SDR include software complexity, development costs and time, limited frequency range for some applications, and generally higher power consumption.