TDD Pattern
In modern wireless communication systems like 5G, the efficient utilization of available resources is essential to ensure reliable and high-speed data transmission. One key aspect of achieving this efficiency is the concept of Time Division Duplexing (TDD). TDD is a fundamental technique that enables simultaneous uplink (UL) and downlink (DL) communication on the same frequency band, albeit at different time intervals. This method plays a crucial role in optimizing the capacity and performance of 5G networks.
TDD offers several advantages in 5G networks:
- Full-Duplex Potential: While it's challenging for a single radio to simultaneously transmit and receive on the same frequency, TDD overcomes this challenge by allocating separate time slots for transmission and reception.
- Flexibility: TDD's dynamic adaptation allows network operators to optimize resource allocation based on varying demands This flexibility is crucial for meeting diverse application requirements.
- Efficiency: By efficiently utilizing the same frequency band for both UL and DL communication, TDD optimizes the overall network capacity and data rates.
LTE uses a set of fixed TDD patterns, defined by the 3GPP standard, while NR provides a flexible way of configuring the DL and UL resources. The parameters used to define a custom TDD configuration are:
- DL-UL transmission periodicity in milliseconds (ms).
- Reference subcarrier spacing to calculate the number of slots in the DL-UL pattern.
- Number of consecutive full DL slots at the beginning of each DL-UL pattern.
- Number of consecutive DL symbols in the beginning of the slot following the last full DL slot.
- Number of consecutive full UL slots at the end of each DL-UL pattern.
- Number of consecutive UL symbols in the end of the slot preceding the first full UL slot.
- Number of guard period (GP) symbols.
GP is the time between Downlink and Uplink transmission. Its purpose is to avoid interference within a cell and ensure coexistence among cells by compensating for propagation delays. GP is not required between Uplink and Downlink, as there is less chance of collision because of the base station timing advance feature. The figure below is an example of the resulting TDD DL-UL pattern based on these parameters. This DL-UL pattern repeats itself in the timeline.
Let us do the calculations for the example in the figure above. First of all, the periodicity of the TDD pattern is noticeable as 5 ms. The values that could be used for the periodicity according to the standard are 0.5, 0.625, 1, 1.25, 2.5, 5, and 10 ms. This means each pattern is defined within a frame (10ms) and the number of times the pattern is repeated within a frame is given in the table below.
| Periodicity (ms) | Number of patterns per frame |
|---|---|
| 0.5 | 20 |
| 0.625 | 16 |
| 1 | 10 |
| 1.25 | 8 |
| 2.5 | 4 |
| 5 | 2 |
| 10 | 1 |
Now we need to calculate the slot duration to figure out how many slots could fit in the time period defined by the pattern. The slot duration depends on the reference subcarrier spacing which has a value of 15 kHz in the example of the figure above. To properly calculate the slot duration, we first define an intermediate variable called the numerology . The numerology of the system is defined by the logarithm of the ratio of subcarrier spacing in kHz as over 15kHz. The table below shows the numerology values for different subcarrier spacings. LTE only supports the numerology value of zero.
| Subcarrier spacing (kHz) | SCS divided by 15kHz | Numerology () | Slot duration | Slots per frame |
|---|---|---|---|---|
| 15 | 1 | 0 | 1 ms | 10 |
| 30 | 2 | 1 | 0.5 ms | 20 |
| 60 | 4 | 2 | 0.25 ms | 40 |
| 120 | 8 | 3 | 125 s | 80 |
| 240 | 16 | 4 | 62.5 s | 160 |
| 480 | 32 | 5 | 31.25 s | 320 |
| 960 | 64 | 6 | 15.625 s | 640 |
So in our example of the figure above, the slot duration is 1ms and hence the number of slots in the pattern of 5ms is 5. From this 5 slots, 3 are used for DL, 1 for UL, and 1 for the flexible DL/UL. In the flexible DL/UL slot, the number of DL symbols is 7 and the number of UL symbols is 5 with 2 guard symbols. The total number of symbols in each slot is 14, unless you use the extended Cyclic Prefix (CP), only applicable for the SCS of 60kHz, which has 12 symbols per slot.
The guard symbols are needed because of the signal propagation. The DL signal takes some time to reach (propagate to) UE, hence we need some additional time from the end of DL signal. Otherwise, the UE may transmit UL signal before it completes the reception of DL signals and as a result there would be interference between DL and UL signal. The guard period is not needed for UL to DL because DL and UL are always properly aligned thanks to timing advance where the gNB sends timing advance to make UL signal perfectly aligned in time domain. The suggested GPs for the different cell sizes (propagation delays) are as the table below.
| Cell size (km) | Guard Period (GP) |
|---|---|
| < 10.7 | 2 symbols |
| 10.7 - 21.4 | 4 symbols |
| 21.4 - 32.1 | 6 symbols |
For other examples of the TDD pattern, checkout this ShareTechnote page.