Small cell network is the new trend for next generation mobile network design. One feasible model is using Relay stations (RS) as small cell providers to achieve extended coverage, lower cost, and higher network capacity. This paper studies Signal-aware relay station placement and power allocation problem in wireless relay networks with multiple base stations in the field. This problem consists of both subscriber coverage problem and relay power optimization problem, which have not been extensively studied together in previous works. This work takes into account physical constraints such as channel capacity, signal to noise ratio (SNR) requirement of subscribers, relay power cost and network topology. We set up a two-step goal that is firstly to find minimum number of RS in order to cover all the subscribers meeting each SNR requirement, and then to ensure communications built between any subscriber to a base station. In order to ensure each subscriber's SNR, transmission power of each RS should be adjustable. Thus, minimizing power cost of RSs is our goal in the second step. We divide the problem into two sub-problems, Lower-tier Coverage Relay Allocation (LCRA) problem and Upper-tier Connectivity Relay Allocation (UCRA) problem. For the LCRA problem, we present two approximation solutions based on minimum hitting set and maximum independent set. For the UCRA problem, an approximation algorithm and an optimal algorithm are proposed. At the end, an approximation solution for our original problem, which combines the approaches of the two sub-problems, is provided. Numerical results are presented to confirm the theoretical analysis of our schemes, and to show strong performances of our solutions.