Diversity Combining of Signals With Different Modulation Levels in Cooperative Relay Networks

In digital cooperative relaying, signals from the source-destination and relay-destination links are combined at the destination to achieve spatial diversity. These signals do not necessarily belong to the same modulation scheme due to the varying channel qualities of the two links. In this paper, the authors present novel and low complexity schemes for diversity combining of signals with different modulation levels. They start by developing the optimum solution as a Maximum Likelihood Detector (MLD). Due to its high complexity, they propose two other receiver structures that they refer to as Soft-Bit Maximum Likelihood Detector (SBMLD) and Soft-Bit Maximum Ratio Combiner (SBMRC).