Feedback Constraints for Adaptive Transmission

In this article, the feedback constraints that limit the potential of AMC and MIMO systems for FDD systems are presented. It has been shown that while adaptive transmission efficiently utilizes the available bandwidth and transmit power, feedback constraints could be a significant impediment. Channel prediction can be employed to compensate the impact of feedback delay but its effectiveness would depend on the accuracy of the channel model. It is also showed that feedback errors could cause outage regions where AMC is not feasible and could also impact the benefits of multiuser scheduling. Limited feedback is a major issue for MIMO-AMC systems because channel state information grows with the rank of the MIMO system, the number of transmission modes, and the number of users for broadcast systems. Since feedback transmission consumes resources that could otherwise be used for data transmission on the reverse channel, feedback design is extremely important especially for power-limited devices such as mobile stations. Finally, some open problems are outlined that are of interest to adaptive transmission systems