A method for channel estimation in a multiple-input-multiple-output (MIMO) system, the MIMO system including a central node and a plurality of communication terminals, the central node including an antenna array, the method comprising: transmitting, by the central node to the plurality of communication terminals simultaneously on separate, respective downlinks of the MIMO system, a long-duration downlink pilot signal carrying a first set of pilot sequences of a first duration, the first duration being equal to or greater than a quantity of antennas in the antenna array; receiving, at the central node, a long-duration uplink pilot signal as a plurality of long-duration signals transmitted substantially simultaneously by the plurality of communication terminals on separate, respective uplinks of the MIMO system, each long-duration signal of the plurality of long-duration signals being a retransmission of the long-duration downlink pilot signal by a separate communication terminal of the plurality of communication terminals, each long-duration signal carrying the first set of pilot sequences; correlating, at the central node, the received long-duration uplink pilot signal with at least one pilot sequence in the first set of pilot sequences to generate a pilot signal matrix; transmitting, by the central node to at least one communication terminal of the plurality of communication terminals on at least one respective downlink of the MIMO system, a short-duration downlink pilot signal carrying a second set of pilot sequences of a second duration, the second duration being less than or equal to the quantity of antennas in the antenna array; receiving, at the central node, a short-duration uplink pilot signal carrying the second set of pilot sequences transmitted by the at least one communication terminal on at least one respective uplink of the MIMO system, the received short-duration uplink pilot signal being a retransmission of the short-duration downlink pilot signal; and estimating an uplink channel between the central node and the at least one communication terminal based on the pilot signal matrix and the received short-duration uplink pilot signal. transmitting, by the central node to the plurality of communication terminals simultaneously on separate, respective downlinks of the MIMO system, a long-duration downlink pilot signal carrying a first set of pilot sequences of a first duration, the first duration being equal to or greater than a quantity of antennas in the antenna array; receiving, at the central node, a long-duration uplink pilot signal as a plurality of long-duration signals transmitted substantially simultaneously by the plurality of communication terminals on separate, respective uplinks of the MIMO system, each long-duration signal of the plurality of long-duration signals being a retransmission of the long-duration downlink pilot signal by a separate communication terminal of the plurality of communication terminals, each long-duration signal carrying the first set of pilot sequences; correlating, at the central node, the received long-duration uplink pilot signal with at least one pilot sequence in the first set of pilot sequences to generate a pilot signal matrix; transmitting, by the central node to at least one communication terminal of the plurality of communication terminals on at least one respective downlink of the MIMO system, a short-duration downlink pilot signal carrying a second set of pilot sequences of a second duration, the second duration being less than or equal to the quantity of antennas in the antenna array; receiving, at the central node, a short-duration uplink pilot signal carrying the second set of pilot sequences transmitted by the at least one communication terminal on at least one respective uplink of the MIMO system, the received short-duration uplink pilot signal being a retransmission of the short-duration downlink pilot signal; and estimating an uplink channel between the central node and the at least one communication terminal based on the pilot signal matrix and the received short-duration uplink pilot signal.