ALLPCB
Overview
A duplexer is a main component in heterodyne duplex radios and repeaters. Its function is to isolate transmit and receive signals so that both can operate simultaneously without interfering with each other. A duplexer is typically built from two bandpass filters tuned to different frequencies to prevent the local transmitter signal from reaching the receiver.
Main Functions
- Enable two-way communication: A duplexer separates transmit and receive signals and prevents mutual interference, allowing transmit and receive paths to operate simultaneously without affecting one another.
- Improve spectrum efficiency: When spectrum is limited or constrained, a duplexer can enable two-way communication within a single frequency band, improving effective spectrum utilization and supporting more devices.
- Reduce equipment count and complexity: By combining transmit and receive functions, a duplexer can reduce the number of separate devices required for two-way communication.
- Improve link quality: Proper duplexing reduces mutual interference between transmit and receive equipment, preventing signal leakage and improving reliability and stability.
Time Division Duplexing (TDD)
Time Division Duplexing separates transmit and receive operations in time so they do not overlap. For example, the system transmits first, then switches to receive. TDD is commonly used in radar systems, which transmit a pulse and then listen for the echo. Duplexers for TDD are usually implemented with a circulator or a switch.
Frequency Division Duplexing (FDD)
Frequency Division Duplexing assigns different frequency bands to the transmit and receive links, allowing simultaneous transmit and receive operations. For example, mobile phones and base stations use different uplink and downlink frequency ranges, so both directions can operate at the same time. Duplexers for FDD are typically formed from two sets of filters.
Key Differences Between TDD and FDD
- Principle: TDD separates transmit and receive in time on the same frequency band, alternating time slots for each direction. FDD separates them in frequency, allocating distinct frequency ranges for transmit and receive.
- Spectrum use: TDD uses the same frequency band for both directions and allocates time dynamically, making spectrum usage flexible. FDD uses dedicated frequency bands for each direction, allowing simultaneous transmit and receive but resulting in fixed spectrum allocation.
- Synchronization: TDD requires strict time synchronization between transmit and receive endpoints to ensure proper time-slot switching. FDD does not require this level of time synchronization because directions operate on different frequencies.
- Scalability: TDD can dynamically allocate time slots among users or links, offering flexibility in resource allocation. FDD requires separate frequency allocations for each direction, which can limit scalability when spectrum resources are scarce.
The choice between TDD and FDD depends on application requirements and technical constraints. TDD is suitable when flexible time-based allocation within the same band is needed, while FDD is appropriate for simultaneous transmit and receive operation without strict time synchronization.
