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In this paper, we propose and analyze novel and efficient cross-layer designs involving joint optimization of physical, link, and TCP layers in wireless. Particularly, we investigate the design of symbol mapping diversity (SMD) schemes using M-QAM at the physical layer for optimal goodput performance at the TCP layer. We present the design and TCP goodput analysis of two SMD schemes, one applying SMD at the TCP packet level, termed as full packet SMD (FP SMD), and the other applying SMD at the link layer (LL) packet level, termed as LL ARQ SMD. We show that although the LL ARQ SMD scheme offers good TCP goodput at high SNR, it performs poorer compared to the FP SMD scheme at low SNR because of increased delays incurred due to increased LL retransmissions at low SNR. We therefore propose and analyze a hybrid SMD scheme which adaptively switches modes (between LL ARQ SMD and FP SMD) based on measured LL packet error rate. We show that the hybrid SMD scheme combines the best TCP performance of both LL ARQ SMD and FP SMD under varying channel conditions.