Comparision of Existing RF COmmunication Architectures
The existing systems can be classified as purely Analog, Digital-Analog Hybrid, and the upcoming completely Digital system. The report's inspiration has been taken from the existing de-facto Digital Analog Hybrids, more commonly known as the Digital Communication system.
The following discuss the advantages and disadvantages we can perceive relative to the Direct RF system under research.
Direct Conversion/Homodyne/Zero IF
Single-step conversion from baseband to RF. The following is the block diagram for the transmitter section.
Transmitter Section
Advantages:
DACs used are at low code rates.
Monolithic Integrated Circuits are available for this configuration.
Simple Filtering is needed.
Dis-advantages:
IQ imbalance is an issue while we have the variations in each of the components used.
DC Offset is often present that leads to LO leakage.
Flicker noise is again an issue to encounter at the baseband operations.
Receiver Section
Advantages:
No Image reject filters needed for the direct conversion systems
Monolithic Integrated Circuits are available for this configuration.
Dis-advantages:
IQ imbalance is again an issue while in the receive chain.
LO leakage is present due to the DC offsets of the components.
Flicker noise is again an issue to encounter at the baseband operations.
As these systems are non-linear, we have second-order distortions often added to the signal chain.
Super-heterodyne Architecture
Dual step conversion from the baseband to the IF level and then to the RF. The following is the block diagram for the transmitter section.
Transmitter Section
Advantages:
IQ mixer calibration is possible and IQ imbalance can be balanced.
DACs are operable at a relatively low frequencies.
Dis-advantages:
Multiple Los and mixers are needed for the operations.
Band Pass Filter is needed at the output to remove the image components.
Flicker noise is again an issue to encounter at the baseband operations.
Receiver Section
Advantages:
IQ mixer calibration is possible and IQ imbalance can be balanced.
Gain can be optimized at different stages for operations.
Leakage can suppressed by IF filters.
Dis-advantages:
High Q image reject filter needed at the input.
Increased part count, this often has economic and signal SNR issues that build up.
Flicker noise is again an issue to encounter at the baseband operations.
Super-heterodyne with Digital IF
In this architecture, we try to utilize the ever-growing speed of DAC systems. We push the IF generation stage into the digital domain and operate the RF systems alone. This is the standard many Software Defined Radios (SDRs) utilize.
Transmitter and Receiver Sections
Advantages:
Analog part counts are reduced
Flicker noise is no longer present as we perform baseband synthesis in the digital domain.
Direct digital synthesis that generates a baseband system needed is done in the digital domain, which removes the mixer impairments seen in the analog domain.
Dis-advantages:
High-speed DACs and ADCs needed for the digital IF generator.
These ADCs/DACs systems often have a limited operational range for the minimum sampling possible, demanding high-performance processor systems.
Direct RF Synthesizer
In this architecture, the DACs/ADCs operate at an RF coding/sampling rate to generate a transmissible/receivable waveform directly in the RF domain. The following is the transmitter block diagram.
Transmitter and Receiver Sections
Advantages:
No Analog RF impairments other than the front end BPF and the HPA.
Reduced part counts to probably only the DAC and the processor.
Vast bands and flexible RF ranges, using a single hardware platform.
Dis-advantages:
Ultra-high-speed DACs are needed for Direct RF, which also consume huge amounts of power.
These ADCs/DACs systems often have a limited operational range for the minimum sampling possible, demanding high-performance processor systems.
