The difference between FDD and TDD in Microwave Transmission
Microwave links typically use Frequency-division duplexing (FDD) which is a method for establishing a full-duplex communications link that uses two different radio frequencies for transmitter and receiver operation. The transmit direction and receive direction frequencies are separated by a defined frequency offset.
Advantages of FDD
In the microwave realm, the primary advantages of this approach are:
- The full data capacity is always available in each direction because the send and receive functions are separated;
- It offers very low latency since transmit and receive functions operate simultaneously and continuously;
- It can be used in licensed and license-exempt bands;
- Most licensed bands worldwide are based on FDD; and
- Due to regulatory restrictions, FDD radios used in licensed bands are coordinated and protected from interference, though not immune to it.
Disadvantages to FDD
The primary disadvantages of the FDD approach to microwave communication are:
- Complex to install. Any given path requires the availability of a pair of frequencies; if either frequency in the pair is unavailable, then it may not be possible to deploy the system in that band;
- Radios require pre-configured channel pairs, making sparing complex;
- Any traffic allocation other than a 50:50 split between transmit and receive yields inefficient use of one of the two paired frequencies, lowering spectral efficiency; and
- Collocation of multiple radios is difficult.
TDD compared with FDD
Time-division duplexing (TDD) is a method for emulating full-duplex communication over a half-duplex communication link. The transmitter and receiver both use the same frequency but transmit and receive traffic is switched in time. The primary advantages of this approach as it applies to microwave communication are:
- It is more spectrum friendly, allowing the use of only a single frequency for operation and dramatically increasing spectrum utilization, especially in license-exempt or narrow-bandwidth frequency bands ;
- It allows for the variable allocation of throughput between the transmit and receive directions, making it well suited to applications with asymmetric traffic requirements, such as video surveillance, broadcast and Internet browsing;
- Radios can be tuned for operation anywhere in a band and can be used at either end of the link. As a consequence, only a single spare is required to serve both ends of a link.
Disadvantages of TDD
The primary disadvantages of traditional TDD approaches to microwave communications are:
- The switch from transmit to receive incurs a delay that causes traditional TDD systems to have greater inherent latency than FDD systems;
- Traditional TDD approaches yield poor TDM performance due to latency;
- For symmetric traffic (50:50), TDD is less spectrally efficient than FDD, due to the switching time between transmit and receive; and
- Multiple co-located radios may interfere with one another unless they are synchronized.