The effects of sub-channel packet routing and adaptive modulation (AM) are discussed in this book for MIMO linking system under differing channel conditions. The challenges is to exploit time-varying imbalances between logical sub-channels by different cross-layer routing and modulation strategies to improve overall goodput. MIMO sub-channels are modelled using OMNeT++ to form a single protocol link and then extended into the final interesting quad- parallel independent link model (Quad-PILM) to explore the effect of channel BER imbalance by implementing frame-by-frame sub-channel re-routing and retry mechanism with the aim of improving overall goodput. As a final model, a BER-directed adaptive modulation switching scheme is evaluated. The explanation in this book demonstrates that even quite simple switching heuristics and packet re-routing mechanisms can provide an overall improvement in a common real-world situation where MIMO sub-channels exhibit slightly unequal error rates for durations of one or more transmission frames.