What are FDD and TDD in Cellular networks?

FDD and TDD are two fundamental concepts in mobile communications and other telecom systems. Together they represent the duplexing schemes in mobile networks that deal with two-way communication.

FDD (Frequency Division Duplex) is a technique in mobile networks that uses separate frequency bands for uplink and downlink communication; TDD (Time Division Duplex) is a technique that uses the same frequency band for uplink and downlink but only communicates in one direction at a time.

What is Frequency Division Duplex – FDD?

Frequency Division Duplex or Duplexing (FDD) is a communication technique where uplink (phone to network) and downlink (network to phone) communications are sent on separate frequency bands. A guard band separates uplink and downlink frequency bands to avoid interference.

FDD has been the most prevalent duplex technique in mobile communications, and the majority of 2G, 3G and 4G networks like GSM, UMTS and LTE have adopted it as their primary duplex scheme.

In FDD communication, the available frequency spectrum is split into two parts. One part of the frequency spectrum is reserved for communicating from the mobile phone to the radio network (uplink).

In contrast, the other part of the spectrum is used for communicating from the mobile radio network to the phone (downlink).

FDD communication requires that some part of the overall frequency spectrum be used as a guard band so that the uplink and downlink frequency bands have a clear separation to avoid any potential interference.

What is Time Division Duplex – TDD?

Time Division Duplex or Duplexing (TDD) is a communication technique where uplink (phone to network) and downlink (network to phone) communications are sent on the same frequency band at different time slots. Uplink and downlink communication are separated by guard times (period) to avoid overlaps.

TDD is a duplexing technique that employs a single frequency band for uplink and downlink, but these two communications occur at different timeslots.

At first, this may seem like a slightly confusing concept if we solely look at it from a frequency interference perspective. However, the answer is hidden in the time-division aspect of the duplexing technique that utilizes separate timeslots or time intervals for uplink and downlink communication.

While FDD has been the primary choice for most 2G and 3G networks, both TDD and FDD schemes can be used in 4G and 5G networks. The 3G mobile networks that use TD-SCDMA (Time Division Synchronous Code Division Multiple Access) for the air interface also use TDD for duplexing.

FDD and TDD are duplex schemes that allow mobile networks to communicate in the uplink (phone to the network) and downlink (network to the phone) directions. FDD – Frequency Division Duplex uses two separate frequency bands for the uplink and downlink. On the other hand, TDD or Time Division Duplex uses the same frequency band for both uplink and downlink but communicates only in one direction at a time.

Feature	FDD	TDD
Spectrum utilization	Less efficient because separate bands used for uplink and downlink	More efficient because it uses the same frequency band for uplink and downlink
Cellular coverage	More extended range because guard time (period) is not an issue in FDD	Shorter range because guard time (period) is proportional to the range
Network investment	Fewer base stations are required because of broader coverage	More base stations are needed due to smaller coverage
Phone hardware – duplexers	A duplexer is required to use the same antenna for uplink and downlink signals simultaneously.	Uplink and downlink are at different time slots
Uplink and downlink needs	Symmetric	Asymmetric
Uplink and downlink capacity allocation	Uplink and Downlink bands are reserved and cannot be changed on the fly	Since the same band is used for uplink and downlink, the capacity for uplink/downlink can be dynamically adjusted
Guard bands/period	Requires guard bands between uplink and downlink	Requires guard times for uplink and downlink separation
Time synchronization	Time synchronization is not an issue with FDD	TDD systems require time synchronization between the serving cell and the neighboring cells to avoid interference