  Small cell networks have recently been proposed as an important evolution
path for the next-generation cellular networks. However, with more and more
irregularly deployed base stations (BSs), it is becoming increasingly difficult
to quantify the achievable network throughput or energy efficiency. In this
paper, we develop an analytical framework for downlink performance evaluation
of small cell networks, based on a random spatial network model, where BSs and
users are modeled as two independent spatial Poisson point processes. A new
simple expression of the outage probability is derived, which is analytically
tractable and is especially useful with multi-antenna transmissions. This new
result is then applied to evaluate the network throughput and energy
efficiency. It is analytically shown that deploying more BSs or more BS
antennas can always increase the network throughput, but the performance gain
critically depends on the BS-user density ratio and the number of BS antennas.
On the other hand, increasing the BS density or the number of transmit antennas
will first increase and then decrease the energy efficiency if different
components of BS power consumption satisfy certain conditions, and the optimal
BS density and the optimal number of BS antennas can be found. Otherwise, the
energy efficiency will always decrease. Simulation results shall demonstrate
that our conclusions based on the random network model are general and also
hold in a regular grid-based model.
