Frequency Division Duplex (FDD) and Time Division Duplex (TDD) are among the most fundamental concepts in all mobile networks, including 4G LTE and 5G NR. Effective use of FDD and TDD in full-duplex and half-duplex modes ensures successful two-way communication.
4G LTE and 5G NR can use both FDD (Frequency Division Duplex) and TDD (Time Division Duplex) and operate in full and half-duplex modes. The FDD and TDD support makes 4G migration easier for 3G technologies on FDD or TDD. TDD enables 5G NR to adjust uplink and downlink resources as required.
What do FDD and TDD mean for mobile networks?
The way the communication works from the mobile phone to the network (uplink) and from the network to the phone (downlink) is determined by the duplex scheme being used. The two key duplex schemes used in mobile communications are Frequency Division Duplex – FDD and Time Division Duplex – TDD. Frequency Division Duplex or FDD is when a mobile network uses two separate frequency bands from the available frequency spectrum as dedicated uplink and downlink bands. Time Division Duplex or TDD is when a mobile network uses one frequency band both for uplink and downlink but separates the communication through different timeslots. I have written a dedicated post on the difference between FDD and TDD in mobile communications and the benefits for both.
What do half-duplex and full-duplex systems mean?
In telecommunications, the systems that can accommodate two-way communication simultaneously are called full-duplex systems, whereas the systems that can facilitate communication in only one direction at a time are called half-duplex systems. Both systems can enable two-way communication.
If you have ever used a walkie-talkie, you may have noticed that communication is done only in one direction at a time. For example, if you want to talk, you usually press the “push to talk” button and speak while the person on the other side listens, and the same can be done in the other direction if the other person wants to speak. This is a typical example of duplex communication, and since the communication is in one direction at a time, it is called half-duplex. If the communication is in both directions simultaneously, it is called full-duplex. Mobile phones and other telephone systems require that communication can take place in both directions simultaneously or at least almost the same time. As a result, mobile phones primarily use full-duplex techniques, but some of the key mobile communications technologies also use half-duplex schemes. There is a direct connection between the two handsets with walkie-talkies, allowing them to communicate directly. On the other hand, mobile phones work differently and first connect with the cellular network to connect with other phones. Duplex schemes play a fundamental role in the connection between the mobile network and the mobile phone.
Half-Duplex and Full-Duplex FDD and TDD in 4G LTE networks
4G LTE networks support FDD and TDD, and both of these duplex schemes in LTE use OFDMA for the downlink and SC-FDMA for the uplink. This approach allows LTE to be the primary cellular technology that allows all key third-generation (3G) technologies to migrate to 4G.
LTE networks provide a 4G migration path to all key 3G technologies, including CDMA2000, and as a result, they must support 4G migration from both FDD and TDD capable 3G networks. FDD – Frequency Division Duplex requires separate frequency bands for uplink and downlink communication where the two bands are paired together and separated by a guard band. TDD – Time Division Duplex uses the same frequency band for uplink and downlink communication. The uplink and downlink are separated in the time domain, i.e., transmitted at different time intervals. LTE also uses a half-duplex version of FDD in which the base station of the mobile network can send and receive simultaneously, but the mobile phone cannot do the same.
The TDD variant of LTE, also known as TD-LTE or LTE TDD, allows mobile operators currently on TDD-based 3G networks to migrate to LTE. TD-SCDMA is a typical example of such technologies used by one of the mobile operators in China for 3G services. TD-SCDMA networks can take the LTE TDD path to migrate to 4G. Since leading 3G technologies UMTS and CDMA2000 are based on the FDD duplex scheme, LTE FDD has been the 4G migration path for these technologies.
The downlink and uplink transmissions in LTE FDD are sent in 10 milliseconds (ms) radio frames. Each frame consists of 10 subframes of 1 ms duration. Each subframe is split into two timeslots of 0.5 ms. Half of the subframes are for uplink and half for downlink in both full and half-duplex.
The downlink and uplink transmission in LTE TDD uses a radio frame of 10 milliseconds (ms). The frame consists of 10 subframes of 1 ms duration divided into two halves, each with five subframes. The subframes can be either uplink or downlink, or special subframes.
In 4G LTE networks, both FDD and TDD, the transmissions are sent in radio frames of 10 milliseconds. Each frame is then divided into ten subframes of 1-millisecond duration. Finally, each subframe is split into two timeslots, each with a duration of 0.5 milliseconds. This is where the TDD and FDD variants of LTE use a slightly different approach. There are two types of frame structures in LTE; type 1 used for FDD and type 2 for TDD, as shown in the diagrams above.
In FDD, half of the subframes are reserved for uplink and half for downlink in both full-duplex and half-duplex. The uplink and downlink bands are separated in the frequency domain using a guard band. In TDD, each radio frame consists of two half-frames and each half-frame consists of five subframes. Subframes can be either uplink or downlink, or special subframes. Special subframes are used when switching from downlink transmission to uplink transmission. This is where the Guard Period (GP) is found, which is the TDD equivalent of a guard band to separate uplink and downlink communication. Special subframes include Downlink Pilot Timeslot (DwPTS), Uplink Pilot Timeslot (UpPTS) and Guard Period (GP).
Half-Duplex and Full-Duplex FDD and TDD in 5G networks
5G NR can operate in both FDD (paired) and TDD (unpaired) using the same radio frame structure for both duplex schemes. LTE employs two distinct frame types; type 1 for FDD and type2 for TDD. The basic radio frame structure of 5G is designed to support both half-duplex and full-duplex communication.
The fifth generation of mobile networks, 5G, uses the New Radio (NR) technology for the air interface. 5G NR networks work closely with existing LTE networks and have two modes of deployment, standalone and non-standalone. 5G NR and 4G LTE are expected to co-exist for a long time.
FDD is full-duplex, whereas TDD and the half-duplex version of FDD are half-duplex systems. While TDD does not technically enable simultaneous communication in both directions (hence half-duplex), it enables concurrent two-way communication that emulates a full-duplex experience. As a result, you will likely come across documentation suggesting that both TDD and FDD are full-duplex.
In order to deal with changing data needs, the higher frequency bands can benefit from TDD by dynamically changing uplink/downlink resource allocation depending on the customer needs. It is also more pragmatic to use TDD for higher frequency bands because those bands are mainly beneficial for deployments in smaller areas such as factories, shopping malls, etc. That way, frequency interference is less of an issue because fewer base stations and devices are needed to plan for. Since 5G NR networks can operate in considerably higher frequency bands (both licensed and unlicensed) than earlier technologies, TDD can be very effective for some of the futuristic use cases of 5G.
Conclusion
4G LTE networks employ both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) to provide a backwards-compatible 4G migration path to all key 3G technologies. 5G NR networks also support both FDD and TDD. Since most futuristic use cases of 5G NR operate at higher frequency bands, TDD is expected to help by offering the flexibility to dynamically adjust downlink/uplink network resources depending on the data needs. Both 4G and 5G support full-duplex and half-duplex modes.
Here are some helpful downloads
Thank you for reading this post. I hope it helped you in developing a better understanding of cellular networks. Sometimes, we need extra support, especially when preparing for a new job, studying a new topic, or buying a new phone. Whatever you are trying to do, here are some downloads that can help you:
Students & fresh graduates: If you are just starting, the complexity of the cellular industry can be a bit overwhelming. But don’t worry, I have created this FREE ebook so you can familiarise yourself with the basics like 3G, 4G etc. As a next step, check out the latest edition of the same ebook with more details on 4G & 5G networks with diagrams. You can then read Mobile Networks Made Easy, which explains the network nodes, e.g., BTS, MSC, GGSN etc.
Professionals: If you are an experienced professional but new to mobile communications, it may seem hard to compete with someone who has a decade of experience in the cellular industry. But not everyone who works in this industry is always up to date on the bigger picture and the challenges considering how quickly the industry evolves. The bigger picture comes from experience, which is why I’ve carefully put together a few slides to get you started in no time. So if you work in sales, marketing, product, project or any other area of business where you need a high-level view, Introduction to Mobile Communications can give you a quick start. Also, here are some templates to help you prepare your own slides on the product overview and product roadmap.
