ing D2D-Coop P-1 + P-3 will be larger than the direct path P-1. It should be noted that the direct path in Case-1, i.e., the path between the femtoBS and CU 1, has a good channel condition, since EU 1 is a center user, which can be utilized efficiently. However, the direct path in Case-2, i.e., the path between the femtoBS and EU 1, has poor channel condition, since EU 1 is a cell edge user. Hence, D2D-Coop is preferred over the direct path. P. Swami and V. Bhatia 5.6.2 Some Useful Probabilities This section highlights probabilities useful in the calculation of the overall system performance. 5.6.2.1 Offloading Probability Offloading plays a vital role in HetNet to decide whether a macroU will be offloaded from the macroBS tier to the femtoBS tier or will be served by the macroBS tier itself. Such decision-making is based on biased received power rule such that the macroU is served by the tier from which the long-term biased received power is higher [34]. Biasing is done to make the offloading favorable to one particular tier. For instance, if the system requirements are such that the offloading to the femtoBS tier needs to be increased due to severe congestion at the macroBS tier, the bias factor of the femtoBS tier can be chosen to be high. The closed form expression for the offloading probability is expressed in (5.2) as follows [30, 37] (5.2) where Bm and B f are the bias factor for macroBS and femtoBS tier, respectively. E(n,x) evaluates the exponential integral as E(n, x) = foo e-xt/tn dt, and (x) = e-tt-1 dt is the complete gamma function. 5.6.2.2 NOMA Compatibility Probability For NOMA application channel gain condition plays a crucial role. Based on the channel gain condition, the offloaded user can either be treated as a CU or EU at the femtoBS tier. The NOMA compatibility probability defines whether the offloaded macroU is served as a CU or EU by the femtoBS tier with respect to the available pairing user. Let us assume index k to denote the offloaded macroU and index n for the available pairing user. 