Inter-cell interference is a major issue in OFDMA networks, due to the increasing density of Low Power Nodes (LPN) used to offload the macro base stations. Coordination between these nodes also called Coordination MultiPoint (CoMP), is identified as a promising solution to improve the signal quality and the achievable throughput while ensuring spectral efficiency over the network. Joint Transmission (JT) mode of CoMP consists to jointly transmit the useful signal from more than one BS, typically the best serving one, and one or several other base stations. In dense networks, the performance evaluation and the analysis of JT CoMP approach become a hard task which needs lot of time and huge resources to conduct simulations. In this paper, we present a new mathematical framework based on spatial fluid modeling which reduces the analysis complexity and provides a macroscopic evaluation of the performance, quite faithful to those obtained using Monte Carlo simulations. The key idea is to consider a continuum of nodes rather than a fixed finite number, and to derive the mean impact of a density of nodes in a certain region of the network. The closed-form formulas of the downlink interference factor are defined for three scenarios depending on the number of coordinated nodes. Then, they are used to evaluate the signal quality improvement, particularly at the cell edge.