What duplex-schemes 4G-LTE and 5G-NR networks use: FDD or TDD?

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, which allows them to communicate with each other directly. On the other hand, mobile phones work differently and connect with the cellular network first to connect with other phones. The connection between the mobile network and the mobile phone is where duplex schemes play a fundamental role. 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 called Frequency Division Duplex – FDD and Time Division Duplex – TDD.

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.

What is FDD in mobile telecom?

Frequency Division Duplex or FDD is a duplexing technique that uses separate frequency bands for uplink and downlink communication. FDD has been the most prevalent duplex technique for 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 spectrum is used for communicating from mobile phone to the network (uplink) and the other part for communicating from the network back to the mobile 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 TDD in mobile telecom?

Time Division Duplex or TDD is a duplexing technique that uses the same frequency band for uplink and downlink. 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 utilises separate timeslots or time intervals for uplink and downlink communication. While FDD has been the primary choice for most 2G and 3G networks including GSM and UMTS, 4G LTE and 5G NR networks support both FDD and TDD schemes. The 3G mobile networks that use TD-SCDMA (Time Division Synchronous Code Division Multiple Access) for the air interface also use TDD for duplexing.

What is the difference between LTE FDD and LTE TDD?

LTE networks provide a 4G upgrade path to all key 3G technologies, 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 both uplink and downlink communication where 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 transmit 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 who are currently using 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 LTE networks. Both TDD and FDD variants of LTE networks use a similar structure and employ OFDMA for the downlink and SC-FDMA for the uplink. This approach allows LTE to be the primary 4G technology that can provide a convergence route to all key 3G network technologies. Since most 3G technologies such as UMTS and CDMA2000 are based on FDD duplex scheme, LTE FDD has been the 4G migration path for them.

In LTE networks, the downlink and uplink transmissions are sent in radio frames of 10 milliseconds each. Each frame is then divided into 10 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 below.

Radio frame for LTE FDD
Radio frame structure for LTE FDD
Radio frame structure for LTE TDD
Radio frame structure for LTE TDD

Half of the subframes are reserved for uplink and half for downlink in both full-duplex and half-duplex FDD. 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 of 5 subframes each. 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 guard band to separate uplink and downlink communication. Special subframes include Downlink Pilot Timeslot (DwPTS), Uplink Pilot Timeslot (UpPTS) and Guard Period (GP).

Is 5G NR TDD or FDD?

The fifth generation of mobile networks, 5G, use a technology called New Radio for the air interface. Even though 5G NR networks have two modes of deployment including standalone and non-standalone, they are expected to co-exist with 4G LTE networks for a long time. The 3G networks will also be around for some time, so the 5G networks will need to work seamlessly with existing FDD and TDD networks (e.g. LTE, UMTS, CDMA2000, TD-SCDMA, etc.). 5G NR networks can operate in both paired (FDD) and unpaired (TDD) models using the same frame structure for both duplex schemes. As mentioned earlier, LTE is different in this regard as it employs two different frame types; type 1 for FDD and type2 for TDD. The basic radio frame structure of 5G NR is designed to support both half-duplex and full-duplex communication. FDD is full-duplex whereas TDD and half-duplex FDD are half-duplex systems. Since 5G NR networks can operate in the considerably higher frequency bands (both licensed and unlicensed) compared to earlier technologies, TDD can be very effective for some of the futuristic use cases of 5G. In order to deal with chaning data needs, the higher frequency bands can benefit from TDD by using dynamically changing uplink/downlink resource allocation as per 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 or shopping malls etc. That way, frequency interference is less of an issue because there are fewer base stations and devices to plan for. Look at this post if you want to find out which frequency bands are used by 5G NR.

Why do some mobile operators use TDD and others FDD?

In the grand scheme of things, 4G LTE and 5G NR support both FDD and TDD so that mobile operators with various technology needs can use a unified path for 4G and 5G migrations. But the reason why mobile operators ended up with either TDD or FDD in the first place (in the 3G era) can be justified through the advantages and disadvantages of each of these duplex schemes.

FDD is ideal for systems where the uplink and downlink (for upload and download) requirements are symmetric. As FDD offers a continuous flow of data in both uplink and downlink directions, it has a higher overall capacity to offer higher data throughput. The deployments with FDD scheme are also much simpler as compared to those with TDD. On the downside, it uses more spectrum as it requires two dedicated data streams continuously. So, whenever the data requirements are not symmetric, one of the communication links (uplink or downlink) can be under-utilised. As both communication links, uplink and downlink, require a portion of the frequency spectrum, it does not seem like the most efficient use of an expensive resource like frequency spectrum. Mobile devices that use FDD-based cellular technologies require a duplexer when using the uplink and downlink signals on the same antenna simultaneously. Duplexer can increase the noise level as well as the cost of the receiver.

TDD is ideal for systems where the uplink and downlink requirements vary considerably. In those systems, a mobile operator can benefit from a technology that allows them to have a more flexible approach to adjust the uplink/downlink as the data needs change. TDD utilises the available spectrum more efficiently and offers higher flexibility when the data demand changes, i.e., it allows operators to change the downlink/uplink ratio as per the changing data demand. The downside is that whenever TDD-based cellular networks are deployed over larger distances, a larger guard period (GP) is required to separate the uplink and the downlink, which consumes additional capacity. As a result, a mobile operator would require more base stations in TDD deployments over larger distances than FDD. Another challenge with TDD networks is the potential interference resulting from the lack of synchronisation between the serving cell and the neighbouring TDD cells. The time-synchronisation between the serving and neighbouring cells can make the TDD deployment more complex.

Here are some helpful downloads

Thank you for reading this post, I hope it helped you in developing a better understanding of cellular networks. But sometimes, we need some extra support especially when preparing for a new job, or studying a new topic, or maybe just 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 challenges given 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 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 product overview and product roadmap.

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