We consider the downlink of a cellular network supporting data traffic. In addition to the direct traffic from the base-station, each user is equipped with the same type of 802.11-like WLAN or WPAN interface that is used to relay packets to further users and hence to improve the performance of the overall network. We are interested in analyzing what are the design guidelines for such networks and how much capacity improvements can the additional relay layer bring, in comparison to cellular networks. We consider a realistic dynamic setting where users randomly initiate downloads and leave the system upon transfer completion. A first objective is to provide a scheduling/relay strategy that maximizes the network capacity, which is the traffic in bit/s/cell that the network can support. We find that, regardless of the spatial traffic distribution, when the cell approaches saturation (the number of active users is very large), the capacity-achieving strategy divides the cell into two areas: one closer to the base-station where the relay layer is always saturated and some nodes receive traffic through both direct and relay links, and the further one where the relay is never saturated and the direct traffic does not exist. We further show that it is approximately optimal to use fixed link lengths, and we derive this length. We give a simple algorithm to calculate the cell capacity. The obtained capacity is shown to be independent of the cell size (unlike in traditional cellular networks), and it is 20%-60% higher than already proposed relay architectures when the number of users is large. Finally, we provide guidelines for future protocol design.