Duplexing and its types

Duplexing is the technique that allows a communication link to send and receive at the same time (two-way communication), separating the forward (base to mobile) and reverse (mobile to base) channels either in frequency or in time.

A simplex system is one-way (paging), half duplex allows two-way but one direction at a time (walkie-talkie), and full duplex allows both directions at once (telephone). Duplexing methods give full or near full duplex operation.

1. Frequency Division Duplexing (FDD)

• Two separate frequency bands are used: one for the forward channel and one for the reverse channel, separated by a fixed duplex spacing.
• A duplexer filter in the handset and base station lets one antenna transmit and receive together.
• Example: GSM-900 uplink 890–915 MHz, downlink 935–960 MHz, spacing 45 MHz.

 Freq ↑  Downlink (forward) ███████████
       |      45 MHz spacing
       |  Uplink (reverse)   ███████████
       +------------------------------ time

• Advantages: true simultaneous transmission, no timing guard needed, suited to symmetric voice traffic.
• Disadvantages: needs paired spectrum and a duplexer; fixed split cannot adapt to asymmetric data.

2. Time Division Duplexing (TDD)

• A single frequency is used; time is divided into slots for forward and reverse transmission, switched fast enough that the user sees it as full duplex.
• Guard time is needed between transmit and receive.
• Example: DECT cordless phones, TD-SCDMA, LTE-TDD, WiMAX, 5G NR TDD bands.

 Freq ↑  [DL][UL][DL][UL][DL][UL]...  one carrier
       +------------------------------ time

• Advantages: unpaired spectrum, flexible DL/UL ratio for asymmetric data traffic, same channel in both directions helps beamforming.
• Disadvantages: needs strict time synchronisation, guard time reduces efficiency, limited range due to propagation delay.