Although, from a system capacity perspective, max-C/I scheduling is beneficial, this scheduling principle will not be fair in all situations. Where R i is the instantaneous data rate for user i. Hence, in contrast to the traditional view that rapid variations in the radio-link quality is an undesirable effect that has to be combated, the possibility of channel-dependent scheduling implies that rapid variations are in fact potentially beneficial and should be exploited. The gain obtained by transmitting to users with favorable radio-link conditions is commonly known as multi-user diversity the multi-user diversity gains are larger, the larger the channel variations and the larger the number of users in a cell. This translates into a high system capacity. Since the radio conditions for the different radio links within a cell typically vary independently, at each point in time there is almost always a radio link whose channel quality is near its peak and supporting a correspondingly high data rate. This scheduling strategy is an example of channel-dependent scheduling known as max-C/I (or maximum rate) scheduling. Rate control is more efficient compared to power control, which adjusts the transmission power to follow the channel variations while keeping the data rate constant. Together with rate control, this implies that the highest data rate is achieved for a given transmit power or, in other words, for a given interference to other cells, the highest link utilization is achieved. In this case, the utilization of the radio resources is maximized if, at each time instant, all resources are assigned to the user with the best instantaneous channel condition. However, there are some basic scheduling strategies in the literature, useful to illustrate the principles.įor the purpose of illustrating the principles, consider time-domain-only scheduling with a single user being scheduled at a time and all users having an infinite amount of data to transmit. What is standardized is the supporting functions for scheduling such as transmission of scheduling grants, quality-of-service mechanisms, and various feedback information, for example channel-state reports and buffer-status reports. Different vendors may choose different strategies in various scenarios to match the user needs. The scheduling strategy in LTE is not standardized but is a base-station-implementation issue-and an important one as the scheduler is a key element in LTE and to a large extent defines the overall behavior.
#DOWNLINK UPLINK PRO#
Johan Sköld, in 4G LTE-Advanced Pro and The Road to 5G (Third Edition), 2016 9.1 Scheduling Strategies In practice, however, SDD is rarely used as leakage from transmit to receive antennas can lead to dominant interference. 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. 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)).
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