  It is predicted that demand in optical access networks will reach multi-Gb/s
per user. However, the limited performance of the direct detection receiver
technology currently used in the optical network units at the customers'
premises restricts data rates/user. Therefore, the concept of coherent-enabled
access networks has attracted attention in recent years, as this technology
offers high receiver sensitivity, inherent frequency selectivity, and linear
field detection enabling the full compensation of linear channel impairments.
However, the complexity of conventional (dual-polarisation digital) coherent
receivers has so far prevented their introduction into access networks. Thus,
to exploit the benefits of coherent technology in the ONUs, low complexity
coherent receivers, suitable for implementation in ONUs, are needed. In this
paper, the recently proposed low complexity coherent (i.e.,
polarisation-independent Alamouti-coding heterodyne) receiver is, for the first
time, compared in terms of its minimum receiver sensitivity with five
previously reported receiver designs, including a detailed discussion on their
advantages and limitations. It is shown that the Alamouti-coding based receiver
approach allows the lowest number of photons per bit (PPB) transmitted (with a
lower bound of 15.5 PPB in an ideal system simulations) whilst requiring the
lowest optical receiver hardware complexity. It also exhibits comparable
complexity to the currently deployed direct-detection receivers, which
typically require >1000 PPB. Finally, a comparison of experimentally achieved
receiver sensitivities and transmission distances using these receivers is
presented. The highest spectral efficiency and longest transmission distance at
the highest bit rate reported using the Alamouti-coding receiver, which is also
the only one, to date, to have been demonstrated in a full system bidirectional
transmission.
