A multi-tier architecture consisting of a macrocell overlaid with small cells, e.g., pico base station (BS), with provision of relays and device-to-device (D2D) communication is needed to satisfy the quality-of-service (QoS) requirements in a joint spectrum and energy efficient manner for the future Fifth generation (5G) networks. D2D communication enables the users located in close proximity to each other to communicate directly without going through the macro-cell, and hence, can be utilised to offload the traffic from the cellular infrastructure. This paper investigates the trade-off between Area Energy Efficiency (AEE) and Area Spectral Efficiency (ASE) in D2D-enabled uplink heterogeneous networks. The tradeoff is modelled as an optimization problem, in which each user wants to maximize its own ASE subject to its required AEE levels. Taking into consideration of the AEE requirement and maximum transmission power constraint, a distributed resource allocation approach is proposed to jointly optimize the mode selection, subcarrier and optimal power allocation by exploiting the properties of fractional programming. The relationship between the achievable AEE and ASE trade-off is investigated with different network parameters.
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