In principle, this allows simultaneous uplink and downlink transmission over the entire frequency band. 12.3.3.3 Space division duplexing (SDD)įinally, SDD relies on the BS and mobile being equipped with multiple antennas and the creation of orthogonal spatial modes for uplink and downlink. The main drawbacks of TDD are latency (as information can only be sent when a channel becomes available) and the need for uplink synchronization (to account for differences in propagation time). In addition, channel state information estimated in the uplink can be used in the downlink, under the assumption of channel reciprocity. This approach enables asymmetric traffic and time-varying uplink and downlink demands. Time is divided up into short slots and some are designated for uplink while others are designated for downlink. TDD allows uplink and downlink to use the entire frequency spectrum, but in different time slots. In addition, the nodes must be equipped with dedicated filters, which may be costly. On the downside, the frequency bands are usually fixed by regulators, thus making FDD inflexible when uplink or downlink traffic requirements change. FDD creates a channel that is always available and thus does not incur any delay.
![uplink and downlink frequency in mobile communication uplink and downlink frequency in mobile communication](https://media.geeksforgeeks.org/wp-content/uploads/20190323190958/gmss.jpg)
The bands are typically separated by a large margin to avoid leakage. 12.3.3.1 Frequency division duplexing (FDD)įDD allows uplink and downlink transmission at the same time, but over different frequency bands. Eryilmaz, in Academic Press Library in Mobile and Wireless Communications, 2016 12.3.3 Example: DuplexingĪn example of orthogonal channels is duplexing, ie, the ability to provide separate channels for uplink and downlink in cellular networks (see Section 12.2.1), in frequency (FDD), time (TDD), and space (space division duplexing (SDD)).