Weakly-coupled Space-Division-Multiplex (SDM) fiber is a promising candidate transmission medium for the remote true-time-delay (TTD) beamforming system. However, the signal-to-crosstalk beating interference (SCBI) can result in severe phase difference (or time delay) variation between TTD channels, which deteriorates the beam direction stability. Here, we develop a model to illustrate the phase variation problem when employing the coherent light source. To obtain a stable beam direction, we propose using an ASE source instead of a coherent source as the optical carrier. When utilizing the ASE source, the signal-to-noise ratio (SNR) of the remote RF signals is also thoroughly derived. It indicates that the RF signal with high SNR can be achieved via increasing the ASE source bandwidth and pre-compensating the fiber link dispersion. We carry out a proof-of-concept experiment of the remote beamforming system with the ASE source at the end. The variation of the phase difference between different channels is less than 3°. The SNR of the RF signal is over 110 dB with a 1 Hz bandwidth. These results pave the way for exploring the SDM fiber in the remote beamforming system.