SDR architecture | Software Defined Radio architecture benefits

This article describe SDR architecture, its elements and benefits. The architecture of software defined radio include software configured transmitter and receiver.

Introduction:
As the name suggests Software Defined Radio is the wireless communication system operating at radio frequency and configurable using software. In SDR, hardware components commonly available in analog counterpart are configured in software which is ported either on FPGA or DSP to be used in wireless chain. Following components are configured in SDR typically:
• mixers used in upconversion and downconversion
• Amplifiers
• Modulator and demodulator modules
• RF detector
• Filters (BPF, LPF) using FIR filter concept

Now-a-days using SDR concept, entire PHY layer and RF chain is implemented in software. SDR concept based designs are used in existing and next generation wireless technologies or standards such as WLAN, Mobile-WiMAX, 4G LTE, LTE-Advanced, 5G NR (New Radio) etc.

SDR architecture Transmitter part

The figure-1 depicts transmitter part of SDR architecture. As shown it consits of DSP, DUC (Digital Up Conversion), D/A converter, analog RF Up converter and power amplifier. The digital baseband part is coded in DSP which provides I/Q data as per different transmitter need. This is digitally up converted using DUC (Digital Up Conversion) with the use of digital LO (Local Oscillator) and digital mixer. The digital IF samples are converted to analog IF signal. This analog IF (Intermediate Frequency) is converted to analog RF (Radio Frequency) using RF up-converter. The RF signal is amplified before being transmitted over the air using appropriate antenna as per desired system operating frequency.

SDR architecture Receiver part

The figure-2 and figure-3 depicts analog radio receiver and SDR receiver block diagrams respectively. Let us understand difference between receivers of traditional radio and software defined radio architectures.

As shown in figure-2, RF signal is amplified using RF Amplifier. The amplified signal is later fed to the RF mixer for RF down conversion. This is done by beating input amplified RF signal with locally generated LO signal. IF signal is extracted and amplified at the output of RF mixer. Typically IF center frequencies of 455KHz and 10.7 MHz are used for AM and FM broadcasting receiver. This amplified IF signal is demodulated and passed to audio amplifier as shown. Envelope detector is used in Amplitude demodulator where as frequency disciminator is used in frequency demodulator. Here RF mixer does the major function to convert RF signal into IF signal.

As shown in figure-3, the first module is RF tuner. This RF tuner converts RF signal to amplified IF signal. It replaces first three modules (RF amplifier, mixer, IF amplifier) as mentioned in traditional analog receiver in figure-2.

Following modules are implemented after RF tuner module.

➨A/D converter: It converts analog IF into digital IF samples.
➨The digital samples are passed to the DDC (Digital Down Conversion) which converts digital IF samples into digital baseband samples (Referred as I/Q data). DDC consists of digital mixer, digital Local Oscillator (LO) and low pass FIR filter.
➨The digital baseband samples are passed to the DSP chip where algorithms have been ported which does the functions such as demodulation, decoding and any other tasks if required.
➨This digital implementation based architecture is referred as SDR or Software Defined Radio. Often FPGA is also used in place of DSP in this software defined radio architecture for fast signal processing algorithms.

The beauty to have software baseband processing chain on DSP/FPGA will help in correcting realtime baseband and RF related impairments present in I/Q data with the use of advanced algorithms. Typically algorithms such as DC offset correction, I/Q gain and phase imbalance correction, time, frequency and channel impairment correction are implemeted in SDR receiver.

Benefits of Software Defined Radio architecture

Following are the benefits of SDR architecture based system:
➨Easy to upgrade different versions of the software during evolution of the standards such as LTE or LTE-advanced. Hence product manufacturers meet requirement of time to market and consecutively client requirements.
➨Major savings into cost due to less time required compare to analog conventional system. This is because any changes required during testing can be easily done in software unlike conventional analog hardware counterpart.
➨It is also easy to experiment new ideas even while the system is operational.