A data-optimized communication system provides support for legacy access terminals, such as access terminals operating under the 1xEV-DO standard. The system also supports multiple-input multiple-output (MIMO) communications with access terminals configured to support MIMO. A number of MIMO spatial signatures are predefined or negotiated. MIMO rank and an identifier of the spatial signature determined at the MIMO access terminal are sent on the I-Branch and Q-Branch of the existing 1xEV-DO structure. In one embodiment, 1-bit rank and 3-bit spatial signature are transmitted through the I-Branch as a 4-bit symbol, replacing the data rate control (DRC) channel. In another embodiment, a 4-bit spatial signature is transmitted through the I-Branch, also replacing the DRC channel. A 2-bit rank is transmitted together with a 4-bit DRC channel through the Q-Branch. The 2-bit rank and the DRC channel can be distinguished because they are sent using different orthogonal codeword Walsh covers.