Time and frequency synchronization

1. A method of transmitting reference signals for synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) system, said method comprising:

transmitting a first channel state information (CSI) reference signal in a first portion of each of a plurality of regularly spaced time periods in accordance with a first configuration of transmission resources, wherein the first configuration of transmission resources defines a first allocation of time and frequency resources for the first CSI reference signal;

transmitting a second CSI reference signal in a second portion of each of one or more of the plurality of regularly spaced time periods in accordance with a second configuration of transmission resources, wherein the second configuration of transmission resources defines a second allocation of time and frequency resources for the second CSI reference signal, and

wherein transmitting the first CSI reference signal in accordance with the first configuration and transmitting the second CSI reference signals in accordance with the second configuration comprises transmitting the first and second CSI reference signals from the same antenna port in both the first portion and the second portion of at least a first one of the plurality of regularly spaced time periods,

wherein the same antenna port is used for transmission of the first and second CSI reference signals to ensure a user terminal in the OFDM system can utilize the first and second CSI reference signals for synchronization assistance.

2. The method of claim 1 wherein:

transmitting the first CSI reference signal in accordance with the first configuration comprises periodically transmitting the first CSI reference signal in the first portion of two or more time periods with a first repetition frequency; and

transmitting the second CSI reference signal in accordance with the second configuration comprises periodically transmitting the second CSI reference signal in the second portion of two or more of the time periods with a second repetition frequency different from the first repetition frequency.

3. The method of claim 1 wherein:

the first configuration of transmission resources supports transmission of CSI reference signals from a first set of antenna ports;

the second configuration of transmission resources supports transmission of CSI reference signals from a second set of antenna ports; and

the first and second sets of antenna ports include at least one common antenna port for transmission of the first and second CSI reference signals.

4. The method of claim 1 further comprising transmitting configuration information for the first and second CSI reference signals to the user terminal on a downlink broadcast channel.

5. The method of claim 1 , wherein each of the plurality of regularly spaced time periods comprises at least 14 OFDM symbols.

6. The method of claim 1 , wherein each of the plurality of regularly spaced time periods has a duration of 1 millisecond.

7. A base station in an Orthogonal Frequency Division Multiplexing (OFDM) system, said base station comprising:

a transceiver circuit configured to communicate with a user terminal over a wireless communicate channel; and

a control circuit providing reference signals for transmission to the user terminal via the transceiver circuit, said control circuit being configured to:

transmit a first channel state information (CSI) reference signal in a first portion of each of one or more regularly spaced time periods;

transmit a second CSI reference signal in a second portion of each of one or more of the same regularly spaced time periods; and

transmit the first and second CSI reference signals from the same antenna port to ensure the user terminal in the OFDM system can utilize the first and second CSI reference signals for synchronization assistance.

8. The base station of claim 7 wherein the control circuit is further configured to:

periodically transmit the first CSI reference signal in the first portion of each of the one or more regularly spaced time periods with a first repetition frequency; and

periodically transmit the second CSI reference signal in the second portion of each of the one or more regularly spaced time periods with a second repetition frequency different from the first repetition frequency.

9. The base station of claim 7 wherein the control circuit is further configured to:

transmit a first set of CSI reference signals in the first portion from a first set of antenna ports;

transmit a second set of CSI reference signals in the second portion from a second set of antenna ports; and

wherein the first and second sets of antenna ports include at least one common antenna port for transmission of the first and second CSI reference signals.

10. The base station of claim 7 wherein the control circuit is further configured to transmit configuration information for the first and second CSI reference signals to the user terminal on a downlink broadcast channel.

11. A method of synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) system, said method comprising:

receiving a first channel state information (CSI) reference signal in a first portion of each of a plurality of regularly spaced time periods in accordance with a first configuration of transmission resources, wherein the first configuration of transmission resources defines a first allocation of time and frequency resources for the first CSI reference signal;

receiving a second CSI reference signal in a second portion of each of one or more of the plurality of regularly spaced time periods in accordance with a second configuration of transmission resources difference from the first configuration, wherein the second configuration of transmission resources defines a second allocation of time and frequency resources for the second CSI reference signal;

wherein receiving the first CSI reference signal in accordance with the first configuration and receiving the second CSI reference signal in accordance with the second configuration comprises receiving CSI reference signals transmitted on the same antenna port during both the first portion and the second portion of at least a first time period of the plurality of regularly spaced time periods; and

synchronizing with the OFDM system using the first and second CSI reference signals received during the first time period.

12. The method of claim 11 wherein:

receiving the first CSI reference signal in accordance with the first configuration of transmission resources comprises periodically receiving the first CSI reference signal in the first portion of each of the one or more regularly spaced time periods with a first repetition frequency; and

receiving the second CSI reference signal in accordance with the second configuration of transmission resources comprises periodically receiving the second CSI reference signal in the second portion of each of the one or more regularly spaced time periods with a second repetition frequency different from the first repetition frequency.

13. The method of claim 11 wherein:

the first configuration of transmission resources supports transmission of CSI reference signals from a first set of antenna ports;

the second configuration of transmission resources supports transmission of CSI reference signals from a second set of antenna ports; and

the first and second sets of antenna ports include at least one common antenna port for receiving of the first and second CSI reference signals.

14. The method of claim 11 further comprising receiving configuration information for the first and second CSI reference signals on a downlink broadcast channel.

15. A user terminal in an Orthogonal Frequency Division Multiplexing (OFDM) system, said user terminal comprising:

a transceiver circuit configured to communicate with a base station over a wireless communicate channel; and

a control circuit for synchronizing with the OFDM system, said control circuit being configured to:

receive a first channel state information (CSI) reference signal in a first portion of each of a plurality of regularly spaced time periods in accordance with a first configuration of transmission resources, wherein the first configuration of transmission resources defines a first allocation of time and frequency resources for the first CSI reference signal;

receive a second CSI reference signal in a second portion of each of one or more of the plurality of regularly spaced time periods in accordance with a second configuration of transmission resources difference from the first configuration, wherein the second configuration of transmission resources defines a second allocation of time and frequency resources for the second CSI reference signal;

receive the first and second CSI reference signals on the same antenna port in at least one of the plurality of regularly spaced time periods; and

synchronize with the OFDM system using the first and second CSI reference signals received on the same antenna port in the at least one time period.

16. The user terminal of claim 15 wherein:

the first configuration of transmission resources supports transmission of CSI reference signals from a first set of antenna ports;

the second configuration of transmission resources supports transmission of CSI reference signals from a second set of antenna ports; and

the first and second sets of antenna ports include at least one common antenna port for receiving of the first and second CSI reference signals.

17. The user terminal of claim 15 wherein the first set of antenna ports has a larger number of antenna ports than the second set.

18. The user terminal of claim 15 wherein the control circuit is further configured to receive configuration information for the first and second CSI reference signals on a downlink broadcast channel.

19. The user terminal of claim 15 , wherein each of the plurality of time periods comprises at least 14 OFDM symbols.

20. The user terminal of claim 15 , wherein each of the plurality of regularly spaced time periods has a duration of 1 millisecond.