| 摘要 |
<p>Method to perform link adaptation at the radio interfaces of an enhanced packet data cellular network handling several Modulation and Coding Schemes (MCS) for maximizing data throughput. In a preliminary off-line step the system behaviour, in terms of net throughput of the various available MCSs, is simulated for different C/l conditions. From the simulation two sets of tables are obtained, each table including upgrade and downgrade thresholds expressed in terms of Block Error Rate (BLER). Thresholds correspond to switching points from an MCs to the two available MCSs having the immediate less or more protection. The two sets of tables are referred to higher or lower diversity RF environments and are further specialized for taking into account EGPRS type II hybrid ARQ, namely Incremental Redundancy (IR). During transmission the transmitted blocks are checked for FEC and the results are sent to the network. The network continuously updates BLER using exponential smoothing. In order to achieve the correct time response, in spite of that RLC blocks can be received or not, a reliability filter is provided whose output is used to decide the weight between the new and old measurements to make the BLER filter impulse response exponentially decreasing with time. The IR efficiency is tested for each incoming block and an indicative variable IR status is filtered using the same approach used for BLER. Each actual threshold of BLER to be used in link adaptation is obtained by a linear interpolation between the tabulated threshold without IR and with perfect IR, both weighed with filtered IR status. Filtered BLER is then compared with said interpolated thresholds for testing the incoming of a MCS switching condition. Power control pursues the goal of maintaining constant QoS peak throughput per time slot.</p> |
