We investigate transceiver design and digital signal processing for
spatially multiplexed transmission over multimode fibers. In
conventional architectures, the full spatial domain of the transmission
fiber has to be detected and processed such that the modal walk-off and
mixture can be estimated and equalized. These architectures scale poorly
with the number of modes supported, besides the sparsity of the fiber
transfer matrix is not fully exploited. Instead, here we aim to employ
selective mode vector launch and detection in order to minimize the
number of optical front-ends required. In this case, an ideal basis for
multiplexing is offered by principal modes, that to first order are
frequency independent. We show that such mode vector basis can be used
for full baud rate transmission over inter-data center distances despite
limited coherence bandwidth and vulnerability to environmental-induced
drift of the optical channel. It is shown that crosstalk at the receiver
front-end can be significantly suppressed, critically reducing the
number of coherent receiver front-ends to that of spatial tributaries
aimed for data transmission -- as opposed to the total number of fiber
modes. Residual crosstalk can still be experienced due to
environmental-induced channel drift and loss of orthogonality in
presence of mode dependent loss. Multiple-input single-output digital
signal processing is shown to be effective in this case, with the
required equalizer array size scaling sub-linearly with the number of
tributaries. A multimode fiber with 156 spatial and polarization modes
and optimized for low modal dispersion is considered.