Upstream pilot structure in point to multipoint orthogonal frequency division multiplexing communication system

A central access network unit comprising a processor configured to assign a plurality of upstream training blocks from an upstream OFDM symbol to a plurality of downstream network units, wherein the OFDM symbol comprises a plurality of pilot subcarriers equally spaced across an upstream RF spectrum in a pre-determined time interval, and wherein each upstream training block comprises a different subset of the pilot subcarriers that are non-consecutive and situated across the upstream RF spectrum, and generate one or more messages comprising assignments of the upstream training blocks, and a transmitter coupled to the processor and configured to transmit the messages to the plurality of downstream network units via a network, wherein the messages instruct at least one of the plurality of downstream network units to transmit a modulated pre-determined sequence at the pilot subcarriers corresponding to the upstream training block assigned to the downstream network unit.