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Deploying fixed relay stations (RSs) in traditional mobile cellular networks is widely recognized as a promising technology to improve cell coverage, user throughput, and system capacity, to save the transmit power of a mobile station (MS) in the uplink, and to provide a low-cost deployment for coverage extension. One crucial step toward developing such a relay-assisted cellular network is to fully evaluate its performance from both theoretical and practical viewpoints. In the literature, however, the system has only been evaluated for very limited system configurations: with a fixed number of RSs and locations and/or a fixed frequency-reuse pattern. This paper aims to investigate the downlink performance limits of a general relay-assisted network with optimized system parameters in a multicell environment. A genetic algorithm (GA) is proposed for the joint optimization of system parameters, including the number of RSs and their locations, frequency-reuse pattern, path selection, and resource allocation to maximize the system spectral efficiency (SE). Two types of quality of end-user experience (QoE), i.e., 1) fixed-bandwidth allocation (FBA) and 2) fixed-throughput allocation (FTA), are investigated along with two path-selection methods, i.e., 1) SE based and 2) signal-to-interference-plus-noise ratio (SINR) based. Numerical results show that significant improvement on system performance can be achieved with the optimized system parameters.