[5G Encyclopedia] How does 5G implement TDD?

The theme of this issue of 5G Encyclopedia is: How does 5G implement TDD? First, let’s look at some basic concepts, clarify the difference between TDD and FDD, and the necessity of implementing TDD.

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The main participants in mobile communications are base stations and terminals (most commonly mobile phones, which will be referred to as mobile phones in the following text). When the two parties communicate, there needs to be a clear sending and receiving relationship. Since the base station is high up, the path where the base station transmits and the mobile phone receives is called "downlink"; conversely, the path where the mobile phone transmits and the base station receives is called "uplink". This is shown by the arrows in the figure below.

The simultaneous operation of uplink and downlink is called "duplex", which is the most basic problem that must be solved in mobile communications. So how to use limited frequency to achieve duplex? The industry has always had two solutions: frequency division duplex (FDD) and time division duplex (TDD). The idea of ​​FDD is that uplink and downlink use two different spectrums, which is equivalent to two lanes. Uplink and downlink run in parallel on their own spectrum without interfering with each other.

The idea of ​​TDD is that I use the same spectrum for uplink and downlink. Although it is equivalent to only one lane, I let the uplink and downlink data use this channel at different times. The uplink sends data for a while, and the downlink sends data for a while, taking turns. Since the time taken for each uplink and downlink to send information is very short, people can't feel the interruption at all, so duplex is achieved.

In actual applications, FDD has been widely used in the 2G era because of its simplicity, easy implementation and reliable performance. The GSM and CDMA we are familiar with are both frequency division duplex.

In the 3G era, due to the rise of data services, the asymmetry of uplink and downlink demand began to emerge. Most people surf the Internet mainly for downloading, and the demand for uploading is very small, so the downlink traffic is often much larger than the uplink traffic. The FDD method uses the same bandwidth spectrum resources for uplink and downlink, which cannot be flexibly adjusted, which is obviously a waste of precious resources. In addition, as spectrum resources become increasingly scarce, it is increasingly difficult to find paired spectrum for uplink and downlink, so the disadvantages of FDD are becoming more and more obvious. TDD can flexibly configure the occupancy time of uplink and downlink. For example, for download services, the downlink time can be set to 80% and the uplink time can be set to 20%, so the spectrum utilization rate can be improved. And TDD uses the same spectrum for uplink and downlink, and does not need to be paired like FDD, so it is very convenient to use fragmented spectrum. In this way, TDD began to emerge in the 3G era, and TD-SCDMA technology was born; in the 4G era, TDD LTE has begun to challenge FDD LTE and has been widely used around the world; and in the 5G era, TDD duplex mode has become the absolute first choice. Let's take a look at how 5G implements TDD. First, let’s review the 5G frame structure, as shown in the figure below.

First, each wireless frame is 10 milliseconds long and contains 10 1-millisecond subframes. Each subframe contains a different number of time slots depending on the parameter set. The wider the subcarrier width, the more time slots there are (see the figure above for specific values), but the length of the subframe is always 1 millisecond. Each time slot, regardless of its length, contains 14 symbols, which is the minimum time unit for 5G wireless resource allocation. In order to use these symbols flexibly, 3GPP has defined 56 time slot formats and clarified the symbol combination method within the time slot.

In a time slot, there can be three types of symbols: downlink symbols (D), uplink symbols (U), and flexible symbols (F). Flexible symbols can be used for both uplink and downlink.

So how should these time slot formats be combined and used? 3GPP defines the frame format of TDD, which can be expressed by the following equation:

TDD frame format = several downlink time slots + 1 flexible time slot + several uplink time slots. There can be multiple downlink time slots, and all 14 symbols in each time slot are configured as downlink; there can also be multiple uplink time slots, and all 14 symbols in each time slot are configured as uplink; there is only one flexible time slot, and the downlink symbols and the ratio of flexible symbols to uplink symbols can be flexibly set, as long as they are defined in the table above. In summary, the frame structure of TDD is shown in the figure below.

Based on this definition, in order to meet different uplink and downlink performance requirements, the industry has the following three mainstream frame formats in the 5G transceiver band 3.5GHz, using a 30KHz subcarrier spacing: 2 milliseconds single cycle: 2 downlink time slots (D), 1 uplink time slot (U), and 1 flexible time slot (S) in each cycle.

2.5 ms single cycle: 3 downlink time slots (D), 1 uplink time slot (U), and 1 flexible time slot (S) in each cycle.

2.5 ms double cycle: Double cycle means two cycles with different configurations, which are combined into a large cycle, which contains 5 downlink time slots (D), 3 uplink time slots (U), and 2 flexible time slots (S).

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Among these three frame formats, for flexible time slots, it can be configured as: 10 downlink symbols + 2 flexible symbols + 2 uplink symbols. Two of the flexible symbols are used as isolation for conversion between uplink and downlink, and are not used for sending and receiving signals. Taking the uplink and downlink symbols in the flexible time slot into consideration, it can be calculated that the uplink resource proportion of a 2-ms single cycle is about 29%, the uplink resource proportion of a 2.5-ms single cycle is about 23%, and the uplink resource proportion of a 2.5-ms double cycle is about 33%. This leads to differences in uplink and downlink performance of several frame structures. As can be seen from the above time slot examples, 5G's implementation of TDD is very flexible. It no longer predefines the subframe ratio like TDD. The number of uplink and downlink time slots can be flexibly configured according to demand for applications with different 5G requirements.