  In this paper, we consider a relative received link power (RRLP)-based
coordinated multi-point (CoMP) joint transmission (JT) in the multi-tier
ultra-dense networks (UDN). In this CoMP scheme, we identify the cooperating
base stations (BSs) by comparing the average received link power (ARLP) of the
neighbouring BSs with respect to the BS having the strongest ARLP (i.e., the
main link BS) to a user. To analyze the performance of this CoMP scheme in the
downlink multi-tier UDN, we first approximate the received signal power
distribution, and derive the coverage probability using stochastic geometry.
After revisiting the area spectral efficiency (ASE) to make it more suitable
for CoMP transmission in UDN, we also analyze the ASE and the network energy
efficiency (NEE). Using simulations, we validate the derived coverage
probability, and investigate the CoMP performance in multi-tier UDN. Our
simulations show that the RRLP-based CoMP scheme can outperform the fixed
number of strongest BS-based CoMP scheme in the high BS density regime. Our
study of the NEE performance reveals that not only the RRLP-based CoMP scheme
is more efficient than conventional non-CoMP transmission scenario, but also
its NEE performance improves with the average number of cooperating BSs.
