Improving capacity in MIMO systems with asynchronous PAM

Consider a multiple input multiple output (MIMO) system, where each transmit antenna uses pulse amplitude modulation (PAM), possibly with an antenna dependent delay introduced deliberately at the transmitter. We show that under commonly used transmitter spectral mask constraints, the channel has unbounded degrees of freedom. While synchronous PAM exploits only a fixed number of these degrees of freedom, asynchronism exploits them to improve capacity. For most cases of interest, two distinct shifts attain most of the capacity improvements. Spectrum analysis suggests that asynchronism with optimal water-filling based schemes are useful for improving capacity only in directional links, while uniform power allocation leads to capacity improvement without any extra spectral leakage.