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 slot of a plurality of subframes 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 slot of one or more of the plurality of subframes 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 in both the first slot and the second slot of at least a first subframe of the plurality of subframes,

wherein the same antenna port is used for transmission of the first and second CSI reference signals if the first and second CSI reference signals are for synchronization assistance and different antenna ports are used for transmission if the first and second CSI reference signals are not 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 slot of two or more subframes 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 slot of two or more of the subframes 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 3 , wherein the first set of antenna ports has a larger number of antenna ports than the second set, and wherein the repetition frequency of the first CSI reference signal is shorter than the second CSI reference signal.

5. 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.

6. 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 slot of a plurality of subframes 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 slot of one or more of the plurality of subframes in accordance with a second configuration of transmission resources different 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 during both the first slot and the second slot of at least a first subframe of the plurality of subframes; and synchronizing with the OFDM system using the first and second CSI reference signals received during the first subframe if the first and second CSI reference signals are received on the same antenna port during both the first slot and the second slot of the at least first subframe.

7. The method of claim 6 , 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 slot of the two or more subframes 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 slot of two or more of the subframes with a second repetition frequency different from the first repetition frequency.

8. The method of claim 6 , 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.

9. The method of claim 8 , wherein the first set of antenna ports has a larger number of antenna ports than the second set, and wherein the repetition frequency of the first set of CSI reference signals is shorter than the second set of CSI reference signals.

10. The method of claim 8 , further comprising receiving configuration information for the first and second CSI reference signals on a downlink broadcast channel.

11. 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 slot of a plurality of subframes 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 slot of one or more of the plurality of subframes in accordance with a second configuration of transmission resources different 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; and 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 during both the first slot and the second slot of at least a first subframe of the plurality of subframes; synchronize with the OFDM system using the first and second CSI reference signals if the first and second CSI reference signals are received on the same antenna port during both the first slot and the second slot of the at least first subframe.

12. The user terminal of claim 11 , wherein the control circuit is further configured to:

periodically receive the first CSI reference signal in the first slot with a first repetition frequency; and

periodically receive the second CSI reference signal in the second slot with a second repetition frequency different from the first repetition frequency.

13. The user terminal of claim 11 , wherein the control circuit is further configured to:

receive a first set of CSI reference signals in the first slot of two or more subframes from a first set of antenna ports; and

receive the second CSI reference signal in the second slot of two or more of the same subframes with a second repetition frequency different from the first repetition frequency.

14. The user terminal of claim 11 , wherein the first set of antenna ports has a larger number of antenna ports than the second set, and wherein the repetition frequency of the first CSI reference signal is shorter than the second CSI reference signal.

15. The user terminal of claim 11 , wherein the control circuit is further configured to receive configuration information for the first and second CSI reference signals on a downlink broadcast channel.