One of the things all technical disciplines excel at is creating terminology that can trip up those who are not accustomed to speaking the language every day. Take the title of this article for example. These three words sound similar and are definitely inter-related, but they are not inter-changeable.
Duplex is used to refer to a communications system that allows the transmission of two signals – e.g. a question and an answer. This is in contrast to Simplex, which permits only one-way transmission – e.g. a radio broadcast. Duplex is essential for an exchange of data such as in a conversation or systems like networking protocols.
Now we can have Half-Duplex, which allows one to communicate via taking-turns (e.g. a walkie-talkie, or yodeling from a hillside). Alice talks for a bit and says ‘over’, and then it is Bobs turn on the radio. Then there is Full-Duplex, which is what we are used to in real life. Both parties can transmit and receive at the same time. (This says nothing about Bobs ability to listen to Alice while he is talking, but in theory at least they can both talk and both listen at the same time and we get full-duplex (analog in this case) communication.)
To achieve Full Duplex in digital communications there are three approaches one can take:
Frequency Domain Duplex (FDD) means transmitting at one frequency and receiving at another. The signals are separated from one another by filters into separate channels – commonly these filters are combined into one component called a Diplexer (more on this later). A disadvantage of the FDD approach is that we take the Channel Bandwidth we are allocated and chop it into pieces – one band for Uplink, one band for Dowlink, and a band in-between (a guard-band) that separates Uplink from Downlink and allows the system to ‘resolve’ the two signals as separate.
Time Domain Duplex (TDD) means taking turns really fast. It is similar to how usually our computers pretend to be doing things in parallel but in reality, they are doing things in series very (very) fast. TDD is still regarded as Full Duplex because with digital communications sender and receiver are nearly always unaware that they are taking turns to send bursts of data back and forth. TDD allows us to avoid chopping up the Channel Bandwidth (although bandpass filters are still needed to protect distinct TDD channels from interference), but still eats into our Channel Capacity by insisting that there are Inter Frame Gaps (guard bands in time) to prevent the two sides of the conversation from running into each other.
Then there is Full Duplex. It’s a bit weird because the term gets used again as a means to achieve Full Duplex – but in this context it refers to a system that allows transmit and receive at the same time and at the same frequency (so achieving Full Duplex but not through the use of TDD or FDD). A true Full Duplex system would allow one to maximize the Channel Capacity but building such systems can prove complex, so TDD and FDD have historically been preferred approaches.
In terms of signal management structures and components required FDD requires Diplexers, TDD utilizes fast switching alongside Band Pass Filters and Full Duplex can be implemented with devices like Duplexers or Circulators.
Diplexers are 3-port RF devices, which allow the use of two signal paths on the same transmission line (such as an antenna). This is achieved through filters that separate the frequencies of interest, allowing signals at two different frequencies to be sent and received from the same antenna. High performance Diplexers rely on the selectivity and attenuation of the filter structures and their performance over temperature.
Duplexers are also a 3-port RF devices, and their function is to separate the transmit and receive signals from the antenna into two different signal paths based on the direction of travel (e.g. if the antenna is port A, then ‘forward’ signals move from B to A, and ‘backward’ signals move from A to C). The transmit and receive signals can operate at the same frequency, enabling true Full Duplex communication on a single antenna.
So, we have:
- Duplex,which leads to Full Duplex.
- Under Full Duplex we have FDD for which we need Diplexers.
- We also have true Full Duplex achieved by devices like Duplexers.
- Duplex = ‘Two Way’
- Diplexer = ‘By Frequency’
- Duplexer = ‘By Direction’
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