Interaction Between Adaptive Modulation and Scheduling for High-Rate Downlink Packet Transmission

This paper addresses adaptive modulation for downlink best effort packet data (“multiuser diversity”) in a system employing high-order modulation and multicode (MC) direct-sequence code-division multiple-access formats. In particular, it analyzes the interaction between adaptive modulation and scheduling under the MC constraint that the number of MCs is relatively small (e.g., for relay-based downlink data transmission) and that all MCs to a given user have the same modulation. For this, two scheduling schemes are considered—two-best user and selective relative best user—that provide a balance between throughput and fairness, while accounting for MC and power constraints, high-order modulation, and channel coding for a fine rate granularity. To highlight the tradeoff, it compares the throughput and fairness of the conventional pure opportunistic, relative best, and round robin schemes with the two schemes and develops an analytical framework for the purpose.