  Coordinated multipoint (CoMP) transmission and reception have been considered
in cellular networks for enabling larger coverage, improved rates, and
interference mitigation. To harness the gains of coordinated beamforming, fast
information exchange over a backhaul connecting the cooperating base stations
(BSs) is required. In practice, the bandwidth and delay limitations of the
backhaul may not be able to meet such stringent demands. These impairments
motivate the study of cooperative approaches based only on local channel state
information (CSI) and which require minimal or no information exchange between
the BSs. To this end, several distributed approaches are introduced for
coordinated beamforming (CB)-CoMP. The proposed methods rely on the channel
reciprocity and iterative spatially precoded over-the-air pilot signaling. We
elaborate how forward-backward (F-B) training facilitates distributed CB by
allowing BSs and user equipments (UEs) to iteratively optimize their respective
transmitters/receivers based on only locally measured CSI. The trade-off due to
the overhead from the F-B iterations is discussed. We also consider the
challenge of dynamic TDD where the UE-UE channel knowledge cannot be acquired
at the BSs by exploiting channel reciprocity. Finally, standardization
activities and practical requirements for enabling the proposed F-B training
schemes in 5G radio access are discussed.
