Supplementary Downlink (SDL) and Supplementary Uplink (SUL) are the modes, that allow for additional capacity, either allowing only uplink or downlink capabilities.
As the name suggests, Supplementary bands are used as additional spectrum frequency bands to enhance the link capacity.
Supplementary Downlink Bands (SDL)
FDD bands are paired and dedicated for uplink and downlink purposes. In addition to these bands, 3GPP has added support for a few additional bands that are defined to use only in the downlink direction. These are known as Supplementary Down Link bands. SDL bands can be used as secondary bands to carry out the downlink user planes.
SDL was the feature proposed by Qualcomm R&D’s HSPA+ program for enhancing downlink capacity in Release 9 (and later on releases). Idea was to enhance the downlink capacity by utilizing the unused additional carriers with original paired downlink & uplink carriers.
5G Frequency Spectrum allocated for Supplementary Downlink (SDL) by 3GPP:
| NR Operating Band | UL(Uplink) Operating Band (MHz) | DL (Downlink) Operating Band (MHz) | Duplex Mode |
| n29 | N/A | 717 – 728 | SDL |
| n67 | N/A | 738 – 758 | SDL |
| n75 | N/A | 1432 – 1517 | SDL |
| n76 | N/A | 1427 – 1432 | SDL |
Supplementary Uplink Bands (SUL)
Usually, Cell coverage for uplink is less than the downlink. To reduce the performance degradation, very low-frequency bands are used additionally with the original uplink frequency bands.
For time being when channel conditions are poor and specific network-defined criteria met, these additional SUL frequency bands are used and get reverted back once the network is in good condition.
There are already defined SUL bands which are n80, n81, n82, n83, n84, n86.
SUL is mainly used for coverage expansion in high-frequency range use cases.
5G Frequency Spectrum allocated for Supplementary Uplink (SUL) by 3GPP:
| NR Operating Band | UL(Uplink) Operating Band (MHz) | DL (Downlink) Operating Band (MHz) | Duplex Mode |
| n80 | 1710 – 1785 | N/A | SUL |
| n81 | 880 – 915 | N/A | SUL |
| n82 | 832 – 862 | N/A | SUL |
| n83 | 703 – 748 | N/A | SUL |
| n84 | 1920 – 1980 | N/A | SUL |
| n86 | 1710 – 1780 | N/A | SUL |
| n89 | 824 – 849 | N/A | SUL |
| n95 | 2010 – 2025 | N/A | SUL |
| n97 | 2300 – 2400 | N/A | SUL |
| n98 | 1880 – 1920 | N/A | SUL |
| n99 | 1626.5 – 1660.5(Uplink restricted to 1627.5–1637.5 MHz and 1646.5–1656.5 MHz) | N/A | SUL |