ECE 445S Real-Time Digital Signal Processing Laboratory - Lecture 13 - Duplexing

Prof. Brian L. Evans

Announcements: Duplexing

5G Cellular communication systems either use time division duplex (TDD) or frequency division duplex (FDD) in the communication between a basestation and user equipment (smart phones). 5G Advanced, standardized in the first half of 2027, is considering the use of hybrid duplexing for 5G advanced. Full duplex will become a critical part of 6G to be able to provide low delay, high data rate, and high reliability at the same time.

Human Analogies

We can relate duplexing to human hearing of speech and audio signals. Normal human hearing is assumed. Credit goes to Mr. Keith Tinsley at Tektronix for this insight.

Time-division duplex (TDD): When the first person is speaking, the second person is listening, and vice-versa.

Advantage:: The same frequency band is used for each person (spectrally efficient)
Drawback: Each person takes twice as long to convey what they want to say (high latency).

Frequency-division duplex (FDD): Two audio signals are occurring at the same time, but in different frequency ranges. An example is a gong sound with principal frequencies 0-2000 Hz and a bird chirping with principal frequencies 2000-4000 Hz. Someone with normal human hearing could distinguish the two sounds.

Advantage: The number of frequency bands double (spectrally inefficient)
Drawback: No delay is incurred in conveying the content of both audio signals (low latency)

Hybrid duplexing: When the first person is speaking, the second person is not talking 90% of the time and talking at the same time in a louder voice 10% of the time. For the 10% of the time that the second person is talking, full duplex is occuring (see below).

Advantage: The same frequency band is used for each person's voice (spectrally efficient)
Drawback: Each person can convey a small amount of time-critical data immediately (low latency for 10% of the information) while the other is speaking and during the 10% of the time that both people are speaking, each person would have difficulty hearing the other (high interference).

Full duplex: Consider two people speaking who are 30 feet apart. Both would be speaking at the same time and at the same volume. Each person would hear their own voice much louder than that of the other. Each person would have to concentrate to try to ignore their own voice and pick up the voice of the other to be able to hear what the other person is saying.

Advantage: No delay is incurred in conveying content (low latency) and the same frequency band is used for each person (spectrally efficient)
Drawback:Each person would have difficulty hearing the other (high interference).

Cellular Communications

Time and frequency division duplexing. In TDD, the basestation transmits to user equipment in one timeslot, receives signals from user equipment in the next timeslot, transmits to user equipment in the next timeslot, etc. The user equipment would do the opposite. In FDD, the transmission from basestation to users occurs in one band and the transmission from users to the basestation occurs in the other. FDD has lower latency but with the tradeoff of needed two bands instead of one. The overhead in TDD is significant, and in practice, a basestation might only be able to transmit 20% of the time and not 50%.

Hybrid duplexing. In April 2025, Charlie Zhang (Samsung Research America; Dallas, TX) presented new work in his group in Dallas in xDD at the 6G@UT Wireless Summit at UT Austin. Consider a basestation in TDD mode that is transmitting to the users. xDD would allow a user equipment to transmit to the basestation using a narrow frequency band and short time duration at the same time but at a much higher transmit power. The user equipment transmission would be picked up by the user equipment receiver; the user equipment receiver could subtract out its uplink transmission. This is known as successive interference cancellation, which was used as far back as 2G CDMA systems in 1995.

Full duplexing. The basestation and user are both transmitting and receiving at the same time in the same frequency bands. This is a 2x gain in efficiency of the use of the spectrum. However, the primary drawback is that the basestation would receive its own transmission much more strongly than the transmission of the user (called the near-far problem) and the basestation would need to cancel out the receiving of its own transmission before it could reliably decode the user equipment transmission. Vice-versa for the user equipment. In practice, most the cancelling would occur in the analog/RF domain.

Last updated 10/30/25. Send comments to (Mailbox) bevans@ece.utexas.edu