A high-level optical access network based on coherent technology supporting ultra-dense Wavelength Division Multiplexing (WDM) is developed in the EU FP7 COCONUT project (COst-effective COherent ultra-dense-WDM-PON for lambda-To-the-user access networks). The COCONUT network should allow for seamless evolution from present PON architectures, and also support new emerging applications such as mobile back-haul and front-haul. Coherent techniques will hence allow serving a higher number of users, thanks to ultra-dense WDM, and reaching higher power budget, thanks to higher sensitivity. To this aim, COCONUT addresses realization of coherent transmitter / receivers exploiting low-cost components and simple electronics, so that the cost of typical line terminals would be affordable to the end-users. The paper presents an overview of the proposed enabling transmission techniques and of the first experimental results.