1. A method for communication, comprising:
in a receiver, receiving signals, which comprise one or more dedicated reference signals, that are transmitted from a transmitter over a communication channel in multiple blocks, wherein the signals in each block, including the dedicated reference signals, are transmitted on a respective group of subcarriers over a respective time interval and are precoded using a respective preceding scheme that maps the signals onto multiple antenna ports of the transmitter; identifying two or more of the blocks whose respective preceding schemes differ from one another by no more than a predefined distance; estimating one or more parameters of the communication channel over the dedicated reference signals comprised in the identified blocks; and decoding the signals based on the estimated parameters.', 'in a receiver, receiving signals, which comprise one or more dedicated reference signals, that are transmitted from a transmitter over a communication channel in multiple blocks, wherein the signals in each block, including the dedicated reference signals, are transmitted on a respective group of subcarriers over a respective time interval and are precoded using a respective preceding scheme that maps the signals onto multiple antenna ports of the transmitter;', 'identifying two or more of the blocks whose respective preceding schemes differ from one another by no more than a predefined distance;', 'estimating one or more parameters of the communication channel over the dedicated reference signals comprised in the identified blocks; and', 'decoding the signals based on the estimated parameters.

2. The method according to claim 1, wherein identifying the blocks comprises receiving from the transmitter signaling information indicative of the two or more blocks.

3. The method according to claim 2, wherein receiving the signaling information comprises receiving an indication of a time period during which the transmitter varies the preceding schemes by no more than the predefined distance.

4. The method according to claim 2, wherein receiving the signaling information comprises receiving an indication of a subset of the subcarriers over which the transmitter varies the preceding schemes by no more than the predefined distance.

5. The method according to claim 2, wherein receiving the signaling information comprises receiving a Boolean flag indicating whether the transmitter varies the preceding schemes by more than the predefined distance over the multiple blocks.

6. The method according to claim 1, wherein identifying the blocks comprises determining that the signals are transmitted from the transmitter to the receiver in more than one spatial layer.

7. The method according to claim 6, wherein identifying the blocks comprises determining that the signals are transmitted from the transmitter to the receiver in more than N spatial layers, N>2.

8. The method according to claim 1, wherein receiving the signals comprises receiving common reference signals that are precoded with a common preceding scheme that is not receiver-specific, and wherein estimating the parameters comprises assessing at least a first parameter of the communication channel based on the common reference signals, and subsequently assessing at least a second parameter of the communication channel based on the first parameter and on the dedicated reference signals comprised in the identified blocks.

9. A method for communication, comprising:
transmitting to a receiver signals, which comprise one or more dedicated reference signals, over a communication channel in multiple blocks, wherein the signals in each block, including the dedicated reference signals, are transmitted on a respective group of subcarriers over a respective time interval and are precoded using a respective preceding scheme that maps the signals onto multiple antenna ports of the transmitter; and transmitting to the receiver signaling information, which is indicative of two or more of the blocks whose respective preceding schemes differ from one another by no more than a predefined distance, so as to cause the receiver to estimate one or more parameters of the communication channel over the dedicated reference signals comprised in the identified blocks.', 'transmitting to a receiver signals, which comprise one or more dedicated reference signals, over a communication channel in multiple blocks, wherein the signals in each block, including the dedicated reference signals, are transmitted on a respective group of subcarriers over a respective time interval and are precoded using a respective preceding scheme that maps the signals onto multiple antenna ports of the transmitter; and', 'transmitting to the receiver signaling information, which is indicative of two or more of the blocks whose respective preceding schemes differ from one another by no more than a predefined distance, so as to cause the receiver to estimate one or more parameters of the communication channel over the dedicated reference signals comprised in the identified blocks

10. A communication apparatus, comprising:
one or more receive antennas; and a receiver, which is configured to receive via the receive antennas, signals comprising one or more dedicated reference signals, that are transmitted from a transmitter over a communication channel in multiple blocks, wherein the signals in each block, including the dedicated reference signals, are transmitted on a respective group of subcarriers over a respective time interval and are precoded using a respective preceding scheme that maps the signals onto multiple antenna ports of the transmitter, to identify two or more of the blocks whose respective preceding schemes differ from one another by no more than a predefined distance, to estimate one or more parameters of the communication channel over the dedicated reference signals comprised in the identified blocks, and to decode the signals based on the estimated parameters.', 'one or more receive antennas; and', 'a receiver, which is configured to receive via the receive antennas, signals comprising one or more dedicated reference signals, that are transmitted from a transmitter over a communication channel in multiple blocks, wherein the signals in each block, including the dedicated reference signals, are transmitted on a respective group of subcarriers over a respective time interval and are precoded using a respective preceding scheme that maps the signals onto multiple antenna ports of the transmitter, to identify two or more of the blocks whose respective preceding schemes differ from one another by no more than a predefined distance, to estimate one or more parameters of the communication channel over the dedicated reference signals comprised in the identified blocks, and to decode the signals based on the estimated parameters

11. The apparatus according to claim 10, wherein the receiver is configured to receive from the transmitter signaling information indicative of the two or more blocks, and to identify the two or more blocks responsively to the signaling information

12. The apparatus according to claim 11, wherein the receiver is configured to receive in the signaling information an indication of a time period during which the transmitter varies the preceding schemes by no more than the predefined distance, and to identify the two or more blocks responsively to the indication

13. The apparatus according to claim 11, wherein the receiver is configured to receive in the signaling information an indication of a subset of the subcarriers over which the transmitter varies the preceding schemes by no more than the predefined distance, and to identify the two or more blocks responsively to the indication

14. The apparatus according to claim 11, wherein the receiver is configured to receive in the signaling information a Boolean flag indicating whether the transmitter varies the preceding schemes by more than the predefined distance over the multiple blocks, and to identify the two or more blocks responsively to the Boolean flag

15. The apparatus according to claim 10, wherein the receiver is configured to identify the blocks by determining that the signals are transmitted from the transmitter to the receiver in more than one spatial layer.16. The apparatus according to claim 15, wherein the receiver is configured to identify the blocks by determining that the signals are transmitted from the transmitter to the receiver in more than N spatial layers, N>2.17. The apparatus according to claim 10, wherein the receiver is configured to receive common reference signals that are precoded with a common preceding scheme that is not receiver-specific, to assess at least a first parameter of the communication channel based on the common reference signals, and to subsequently assess at least a second parameter of the communication channel based on the first parameter and on the dedicated reference signals comprised in the identified blocks.18. The apparatus according to claim 10, wherein the receiver is comprised in a wireless communication terminal.19. A communication apparatus, comprising:
multiple antenna ports; and a transmitter, which is configured to transmit over a communication channel to a receiver signals comprising one or more dedicated reference signals, wherein the signals are transmitted in multiple blocks, wherein the signals in each block, including the dedicated reference signals, are transmitted on a respective group of subcarriers over a respective time interval and are precoded using a respective preceding scheme that maps the signals onto the multiple antenna ports, and wherein the transmitter is further configured to transmit to the receiver signaling information, which is indicative of two or more of the blocks whose respective preceding schemes differ from one another by no more than a predefined distance, so as to cause the receiver to estimate one or more parameters of the communication channel over the dedicated reference signals comprised in the identified blocks.', 'multiple antenna ports; and', 'a transmitter, which is configured to transmit over a communication channel to a receiver signals comprising one or more dedicated reference signals, wherein the signals are transmitted in multiple blocks, wherein the signals in each block, including the dedicated reference signals, are transmitted on a respective group of subcarriers over a respective time interval and are precoded using a respective preceding scheme that maps the signals onto the multiple antenna ports, and wherein the transmitter is further configured to transmit to the receiver signaling information, which is indicative of two or more of the blocks whose respective preceding schemes differ from one another by no more than a predefined distance, so as to cause the receiver to estimate one or more parameters of the communication channel over the dedicated reference signals comprised in the identified blocks.']