downlink

Downlink is the transmission path from a transmitter, such as a satellite or cellular base station, to a receiver, such as a ground station or user equipment. In network terminology, data flow is described as downlink when it travels from the infrastructure to the user, and as uplink when it travels from the user to the infrastructure.

In satellite communications, the downlink carries data from the satellite's transponder to an Earth station or directly to a user terminal. The uplink from the ground is received by the satellite, which then downlinks the processed signal. Downlink channels use designated frequency bands (for example, C-band, Ku-band, Ka-band) and are affected by propagation losses, weather, and antenna gain. Link-budget calculations determine achievable data rates, while latency is influenced by orbital height and processing delays.

In terrestrial mobile networks, the downlink is the data stream from the base station to the mobile device. Technologies such as LTE and 5G NR allocate downlink resources via a scheduler and use modulation and coding schemes and multiple access methods (for example OFDMA and MIMO) to deliver data. Downlink throughput depends on bandwidth, spectral efficiency, and radio conditions, while control information for scheduling and resource allocation is transmitted on dedicated channels.

Other uses and considerations: In broadcasting, downlink refers to satellite-downlinked television or radio content distributed to receivers. In positioning and navigation systems, downlink can denote data sent from satellites to receivers, such as system time, correction data, or auxiliary information.

See also: uplink, satellite communication, radio link, link budget.