A downlink cellular system in which multiple relays assist the transmission of the base station is considered. Cooperation strategies based on superposition coding are derived for this network. Superposition coding is attained by sending each message to one or more relays while satisfying the rate constraints of the base station-to-relay links. The chain graph representation of achievable schemes is used to maximize the network throughput over the set of feasible transmission strategies based on superposition coding. Rate advantages as compared to the non-cooperative scenario are obtained under varying relay positions and available power at the base station and relay nodes.