Many wireless communication systems dynamically change coding and modulation techniques in an effort to maximize data rates in light of changing channel conditions. In general, data rates decrease and the coding and modulation techniques are more robust as channel conditions degrade, and vice versa. Traditionally, coding and modulation schemes for transmission have been controlled based on channel quality estimates measured at a receiver and fed back to the transmitter. Unfortunately, channel quality often changes significantly in a short period of time, and the time necessary to estimate channel quality, feed these estimates back to the transmitter, and allow the transmitter to react, injects a delay in the feedback sufficient enough to make the channel quality estimates obsolete in many instances. For example, if a high quality channel quickly degrades to a significant extent, the transmitter may rely on the high channel quality estimates provided by the receiver and select a coding and modulation scheme substantially incompatible with the recently degraded channel. The result is lost data and the need for retransmission of all data.
With reference to FIG. 1, a graph is provided to illustrate the potential differences between actual channel quality and reported channel quality provided via feedback from the receiver. The delay associated with the feedback may result in significant differences between the actual and reported channel qualities, as noted above.
In addition to the unreliability of channel quality estimates fed back from a receiver due to delay, there is significant complexity associated with accurately measuring channel quality and providing channel quality estimates to the transmitter. When channel quality estimates are determined for the forward link from a base station to a mobile terminal, the reverse link design for providing channel quality estimates and feeding these estimates back to the base station adds complexity and cost to the design of the mobile terminal and the base station. Accordingly, there is a need to control modulation and coding for higher speed wireless communication networks while minimizing or eliminating the reliance upon channel quality estimates, which are measured by the receiver and fed back to the transmitter to control coding and modulation schemes.