  Pilot-aided channel estimation is nowadays a standard component in each
wireless receiver enabling coherent transmission of complex-valued
constellations, only affected by noise and interference. Whenever these
disturbances are sufficiently small and long data frames are used, high data
rates can be achieved and the resource overhead due to the pilots vanishes
asymptotically. On the other, it is expected that for the next generation of
mobile networks not only data rate is in the main focus but also low latency,
short and sporadic messages, massive connectivity, distributed & adhoc
processing and robustness with respect to asynchronism. Therefore a review of
several well-established principles in communication has been started already.
A particular implication when using complex-valued pilots is that these values
have to be known at the receiver and therefore these resources can not be used
simultaneously for user data. For an OFDM-like multicarrier scheme this means
that pilot tones (usually placed equidistantly according to the Nyquist
theorem) are allocated with globally known amplitudes and phases to reconstruct
the channel impulse response. Phases are designed and allocated globally which
is in contrast to a distributed infrastructure. In this work we present
therefore a new phaseless pilot scheme where only pilot amplitudes need to be
known at the receiver, i.e., phases are available again and can be used for
various other purposes. The idea is based on a phase retrieval result for
symmetrized and zero-padded magnitude Fourier measurements obtained by two of
the authors. The phases on the pilot tones can now be used to carry additional
user-specific data or compensate for other signal characteristics, like the
PAPR.
