Ever increasing number of cellular users and their high data requirements necessitates need for improvement in the present heterogeneous cellular networks (HetNet). Non-orthogonal multiple access (NOMA) has proven its superiority for the 5th generation networks. This paper proposes a mathematical model for an improved HetNet with macro base station (MBS) and femto base station (FBS) tier. The FBS tier is equipped to support NOMA and carrier sensing for its transmissions. Carrier sensing prevents base stations within a certain range of the transmitter from transmitting, and hence, aids in reducing the interference. Offloading is performed for load balancing in HetNet, where the macro users (MU) from congested MBS tier are offloaded to the FBS tier. The FBS tier pairs the offloaded MU (OMU) with an appropriate pairing user (PU) to perform NOMA. The performance of the OMU is studied under different channel conditions with respect to the available PU at the FBS and some useful observations are drawn. A decrease in outage probability by $\text{74.04\%}$ for cell center user (CCU) and $\text{48.65\%}$ for cell edge user (CEU) is observed for low-density FBS. The outage probability decreases by $\text{99.60\%}$ , for both the CCU and CEU, for high-density FBS using the proposed carrier sensing in NOMA. The results are validated using simulations.