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.
As a general rule, a communication system that sends and transmits simultaneously is called a full-duplex system. In contrast, a system that transmits and receives at different time intervals is called a half-duplex system. FDD offers two-way simultaneous communication and is, therefore, a full-duplex system. TDD offers concurrent two-way communication and can emulate the experience of full-duplex systems even though, technically, it is half-duplex. Both technologies are used in modern cellular networks, including 4G LTE and 5G NR.
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.
Frequency Division Duplex or FDD is a duplexing technique that employs separate frequency bands for uplink and downlink communication. 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.
For example, the primary frequency band for the original GSM networks is from 890 MHz to 960 MHz. In this band, the first part from 890 MHz to 915 MHz is reserved for uplink communication, and the second part from 935 MHz to 960 MHz is dedicated to downlink communication. A 20 MHz band (from 915 MHz to 935 MHz) is used as a guard band between these two bands.
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.
Time Division Duplex or 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 utilises 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.
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.
When is Frequency Division Duplex (FDD) a better choice?
Frequency Division Duplex (FDD) is ideal for communication systems where the uplink and downlink requirements are symmetric. Since FDD offers a continuous flow of data in both uplink and downlink directions, it has a higher overall capacity to deliver higher data throughput.
The deployments with the FDD scheme are also much simpler than those with TDD. On the downside, it uses more spectrum as it continuously requires two dedicated data streams. So, whenever the data requirements are not symmetric, one of the communication links (uplink or downlink) can be under-utilised. Both communication links, uplink and downlink, require a portion of the frequency spectrum, so 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 simultaneously using the uplink and downlink signals on the same antenna. Duplexer can increase the noise level as well as the cost of the receiver.
When is Time Division Duplex (TDD) a better choice?
Time Division Duplex (TDD) is ideal for communication systems where the uplink and downlink requirements are asymmetric, i.e. requirements change. With TDD, a mobile operator can dynamically adjust the uplink and downlink capacity depending on the customer demand for upload vs download.
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 adjacent cells can make the TDD deployment more complex.
Conclusion
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 utilisation | 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 synchronisation | Time synchronisation is not an issue with FDD | TDD systems require time synchronisation between the serving cell and the neighbouring cells to avoid interference |
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.
