Digital Front-End Development for ALMA

Improvement of ALMA observatory requires increase of bandwidth, sensitivity and number of antennas. All these improvements will meet a bottleneck if ALMA digital system is not addressed. Currently a 2 bit digitizer supporting <8 GHz bandwidth per polarization is employed limiting correlation efficiency at 88%. Implementation of ALMA downconverter substantially limits line searches compared to sensitivity limit and it's bandwidth is not extendable. We have initiated study on possible implementation of ALMA digital front-end system which includes modular digitizer, digital processing at antenna and TCP/IP optical fiber data transfer. We report on requirements, trade-offs and possible implementation of modular digital front-end and analyze its impact on sensitivity of current ALMA system as well as future one. We review technology and performance available in near future. Significant performance improvement can be realized at moderate cost.