In mobile communications, terminologies like FDD and TDD have existed from the beginning, and they are fundamental to the network design of any cellular technology. This post looks at the basic concept of FDD (Frequency Division Duplex) and TDD (Time Division Duplex) and their benefits for mobile operators.
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 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 the 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 Time Division Duplex – TDD?
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.
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.
Benefits of FDD
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 the 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.
Benefits of TDD
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.
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.
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 some extra support, especially when preparing for a new job, 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 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.