Method and apparatus for supporting multiple reference signals in OFDMA communication systems

Abstract

Methods and apparatuses are described for a Node B to transmit reference signals (RS) from multiple antennas to enable User Equipments (UEs) to perform demodulation of received information signals and to estimate channel quality Indication (CQI) metrics. To minimize overhead and enable backward compatible operation with legacy systems, RS from a first set of Node B antennas are transmitted in every transmission time interval and substantially over the whole operating BandWidth (BW). RS from a second set of Node B antennas serving for CQI estimation are periodically transmitted, substantially over the whole operating BW, with transmission period informed to UEs through broadcast signaling by the Node B and starting transmission sub-frame determined from the identity of the cell served by the Node B. RS from the second set of antennas, and new RS from the first set of antennas, serving for demodulation of information signals have substantially the same BW as the information signals which can be smaller than the operating BW.

Description

690 is transmitted at most in the first N OFDM symbols and that the system should support legacy UEs configured to receive signal transmission from at most four Node B antennas, RS5 and RS6 should not exist in the PDCCH region (first N OFDM symbols) because this may require the PDCCH to extend to the first N+1 OFDM symbols in order to maintain the same capabilities for control signaling. Then, legacy UEs may not be able to successfully receive the PDCCH. Additionally, puncturing sub-carriers where the PDCCH is transmitted in order to insert additional RS may cause significant degradation in the PDCCH reception reliability. Unlike the PDSCH 695, the PDCCH does not typically benefit from the application of Hybrid Automatic Repeat reQuest (HARQ) and requires better reception reliability than the PDSCH.

The present invention takes into consideration that RS from additional Node B antennas, beyond the ones supported for legacy UEs, are always placed outside the PDCCH region. Note however that PDCCH transmission from all Node B antennas may still apply for UEs supporting reception of signals transmitted from all Node B antennas.

Continuing from FIG. 6, FIG. 7 illustrates the introduction of reference signals RS7 760 and RS8 770 which are transmitted from Node B antenna ports 7 and 8, respectively, in addition to the RS1 731, RS2 732, RS3 733, RS4 734, RS5+RS6+RS7+RS8 740, and RS5−RS6+RS7−RS8 750 which are respectively transmitted from Node B antennas 1 through 6. Unlike the RS from the four Node B transmission antennas which are orthogonally multiplexed either by occupying different sub-carriers 720 (Frequency Division Multiplexing (FDM)) or different OFDM symbols 710 (Time Division Multiplexing (TDM)), or both, RS5, RS6, RS7, and RS8 are multiplexed in the same sub-carriers and the same OFDM symbols through Code Division Multiplexing (CDM). With CDM, Walsh-Hadamard (WH) codes apply to the RS in two consecutive OFDM symbols and in two consecutive sub-carriers having RS transmission. The WH codes are:

RS5: {1, 1} in the time domain and {1, 1} in the frequency domain;

RS6: {1, 1} in the time domain and {1, −1} in the frequency domain;

RS7: {1, −1} in the time domain and {1, 1} in the frequency domain; and

RS8: {1, −1} in the time domain and {1, −1} in the frequency domain.

At the UE receiver, the reverse operations are performed to remove the covering of WH codes. For example, if the {1, 1} WH code is applied at the Node B transmitter, the UE receiver needs to sum (average) the RS from two consecutive locations in time or frequency while if the {1, −1} WH code is applied at the Node B transmitter, the UE receiver needs to sum (average) the RS from two consecutive locations in time or frequency after having reversed the sign of the RS value in the second location. A requirement for successfully applying CDM is that the response of the channel medium remains practically the same within two consecutive locations (in time or frequency) so that orthogonality is maintained in the received signal.

S₁₁ and S₁₂ denote the received signal on odd and even RS sub-carriers, respectively, in the first OFDM symbol with RS transmission, and S₂₁ and S₂₂ denote the received signal on odd and even RS sub-carriers, respectively, in the second OFDM symbol with RS transmission. Ignoring normalization factors, the respective channel estimates for the signals transmitted Node B antennas 5 through 8 in each OFDM symbol at sub-carriers at or between odd and even RS sub-carriers could be obtained as:

Channel Estimate for Antenna 5: S₁₁+S₁₂+S₂₁+S₂₂;

Channel Estimate for Antenna 6: S₁₁−S₁₂+S₂₁−S₂₂;

Channel Estimate for Antenna 7: S₁₁+S₁₂−S₂₁−S₂₂; and

Channel Estimate for Antenna 8: S₁₁−S₁₂−S₂₁+S₂₂.

Other averaging methods preserving and restoring orthogonality may also apply. For example, the channel estimate at an even RS sub-carrier may incorporate both odd RS sub-carriers at each side of the even RS sub-carrier and vice versa.

With the use of CDM to transmit the RS from Node B antennas 5 through 8 in FIG. 7, the respective received RS SINR is decreased by a factor of 2 relative to the SINR obtained for the RS transmitted from Node B antennas 3 and 4, and by a factor of 4 relative to the SINR obtained for the RS transmitted from Node B antennas 1 and 2, assuming the same transmission power for all RS. This is because for the RS from Node B antennas 5 through 8, four RS share the same sub-carrier while the RS from Node B antennas 3 and 4 has no such sharing and the RS from Node B antennas 1 and 2 is transmitted in twice as many sub-carriers. This SINR reduction may be less than the previous factors if RS in different cells of a communication system do not always occupy the same sub-carriers.

The reduction in the received SINR for the RS from Node B antennas 5 through 8 is offset by the savings in time-frequency resources. Typically, PDSCH transmission using all eight Node B antennas is targeted to relatively high SINR UEs with low velocities for which channel estimation is highly accurate and a small loss in RS SINR does not lead to noticeably degraded PDSCH reception reliability. Additionally, legacy UEs capable of supporting reception of signals transmitted from a maximum of four Node B antennas are not affected by the transmission of RS from Node B antennas 5 through 8. The legacy UEs can assume PDSCH transmission in the sub-carriers where the RS from Node B antennas 5 through 8 are actually transmitted with the only ramification being a small degradation in the PDSCH reception reliability which the Node B scheduler can consider in advance when selecting the modulation and coding scheme. Moreover, as PDSCH benefits from HARQ, the overall impact on system throughput is negligible while no change in the receiver processing is needed for the legacy UEs.

Consequently, the RS transmission structure in FIG. 7 can support eight Node B antennas with a total overhead of about 19% without affecting the functionality of legacy UE receivers which are assumed to be configured for receiving signals transmitted from at most the four Node B antennas.

The RS transmission from Node B antennas 5 through 8 in FIG. 7 spanned the entire operating BW. This is typically appropriate when the RS is a Common RS (CRS) that can be received from all UEs. The second object of the invention considers that Node B antennas 5 through 8 transmit a mixture of CRS and UE-Dedicated RS (DRS). As it is subsequently analyzed, this can provide another mechanism for controlling the respective RS overhead.

FIG. 8 illustrates the concept of DRS from Node B antennas 5 through 8 (this can obviously be extended to DRS from Node B antennas 1 through 4). A reference UE capable of receiving a signal transmitted from all eight Node B antennas is scheduled to receive PDSCH in the sub-set 830 of sub-carriers 820 during the portion of OFDM symbols 810 allocated to PDSCH transmission in a sub-frame. The CRS from Node B antennas 1 through 4, namely RS1 841, RS2 842, RS3 843, and RS4 844, remain unchanged. The RS from Node B antennas 5 through 8, namely RS5, RS6, RS7, and RS8, are multiplexed in the same sub-carriers and OFDM symbols through CDM as described in FIG. 7. In particular, in the odd sub-carriers of the first OFDM symbol having RS transmission from Node B antennas 5 through 8, RS5+RS6+RS7+RS8 850 is transmitted while in the even sub-carriers, RS5−RS6+RS7−RS8 860 is transmitted. In the odd sub-carriers of the second OFDM symbol having RS transmission from Node B antennas 5 through 8, RS5+RS6-RS7-RS8 870 is transmitted while in the even sub-carriers, RS5−RS6−RS7+RS8 880 is transmitted. Compared to FIG. 7, the additional RS overhead from Node B antennas 5 through 8 in FIG. 8 is smaller and the PDSCH reception from legacy UEs remains entirely unaffected.

An alternative structure for the DRS transmission from Node B antennas 5 through 8 is illustrated in FIG. 9. The same structure applies for the DRS from Node B antennas 1 through 4 (not shown for brevity). The respective DRS overhead is doubled relative to the DRS overhead in FIG. 8 but there is no constraint for the channel medium response to effectively remain the same between consecutive sub-carriers or OFDM symbols with RS transmission as required for the successful application of CDM. Similarly to FIG. 8, a reference UE capable of receiving a signal transmitted from all eight Node B antennas is scheduled to receive PDSCH in the sub-set 930 of sub-carriers 920 during the portion of OFDM symbols 910 allocated to PDSCH transmission in a sub-frame. The CRS from Node B antennas 1 through 4, namely RS1 941, RS2 942, RS3 943, and RS4 944, remain unchanged. The RS from Node B antennas 5 through 8, namely RS5 950, RS6 960, RS7 970, and RS8 980, are multiplexed in different sub-carriers or different OFDM symbols using FDM/TDM.

It should be noted that although in all the described RS structures the separation of sub-carriers and OFDM symbols with RS transmission from Node B antennas 5 through 8 are shown to be the same as the ones for the RS transmission from Node B antennas 1 through 4, this is only an exemplary embodiment. The separation of RS sub-carriers and OFDM symbols with RS transmission can generally be different between Node B antennas 5 through 8 and Node B antennas 1 through 4. It is also possible for the RS structure from Node B antennas 5 through 8 to be configurable. For example, in channels with small frequency selectivity, CDM may apply as in FIG. 7 or FIG. 8, while in channels with large frequency selectivity, FDM/TDM may apply as in FIG. 6 or FIG. 9. The multiplexing method may be blindly determined by the UEs having a reception capability of signals transmitted from eight Node B antennas or it can be signaled using 1 bit in a broadcast channel from the serving Node B.

Although having a DRS transmitted from Node B antennas 5 through 8 is sufficient for PDSCH reception by a UE, this cannot apply for PDCCH transmission which typically needs to be frequency diverse and not located only in a sub-set of contiguous sub-carriers, and cannot apply for CQI estimation enabling scheduling from Node B antennas 5 through 8. To address the first issue, an embodiment of the invention considers that a Node B having eight antennas uses only four of these antennas for PDCCH transmission (for example, by combining pairs from eight antennas) while the Node B can use all eight antennas for PDSCH transmission.

To address the second issue, another embodiment of the present invention considers that a CRS is also transmitted from Node B antennas 5 through 8 to at least enable UEs to obtain a CQI estimate from antennas 5 through 8. This CQI estimate can then be provided by UEs to the serving Node B through the uplink communication channel in order for the Node B to perform scheduling of PDSCH transmissions to UEs from Node B antennas 5 through 8 using the appropriate parameters, such as the set of sub-carriers and the modulation and coding scheme, for each scheduled UE. As this CRS transmitted from Node B antennas 5 through 8 is intended to primarily serve for CQI estimation, and not for channel estimation to perform PDSCH demodulation in each sub-frame, the CRS does not need to be transmitted in every sub-frame, thereby avoiding significantly increasing the total RS overhead. Considering that PDSCH transmissions from Node B antennas 5 through 8 are primarily intended for UEs with low or medium velocities, the CQI variations in time are slow and the CRS transmission from Node B antennas 5 through 8 does not need to be frequent. Naturally, in sub-frames where CRS from Node B antennas 5 through 8 is transmitted, it can also be used in PDSCH reception and possibly in the reception of control channels if a method involving all Node B transmission antennas in the respective sub-frames is used for their transmission.

FIG. 10 further illustrates an exemplary CRS transmission from Node B antennas 5 through 8. This CRS transmission is assumed to be once every 5 sub-frames. The sub-frame structure consists of OFDM symbols 1010 in the time domain and sub-carriers 1020 in the frequency domain as it was previously described. The CRS from Node B antennas 1 through 4, namely RS1 1031, RS2 1032, RS3 1033, and RS4 1034, is transmitted in all sub-frames. The CRS from Node B antennas 5 through 8, namely RS5 1045, RS6 1046, RS7 1047, and RS8 1048, are transmitted only in sub-frame 4 1054 and sub-frame 9 1059. DRS transmission from Node B antennas 5 through 8 is not shown for simplicity.

To minimize the CRS overhead from Node B antennas 5 through 8, an exemplary embodiment of the invention considers that each of these CRS is transmitted in only one OFDM symbol. Otherwise, the same structure with the CRS from Node B antennas 1 through 4 is maintained to allow for similar processing at a UE receiver. Nevertheless, the CRS from each of the Node B antennas 5 through 8 may be transmitted in two OFDM symbols or CDM can be used for the transmission of RS5, RS6, RS7, and RS8 as described in FIG. 7. Moreover, the CRS from all Node B antennas 5 through 8 are transmitted in one sub-frame to enable UEs to monitor only the respective sub-frames, thereby enabling UE power savings, or assist in the reception of specific control channels transmitted in such sub-frame.

The sub-frames having CRS transmission from Node B antennas 5 through 8 can be either pre-determined or signaled by the serving Node B using a broadcast channel. In the former case, CRS transmission can be pre-determined, for example, that every fifth sub-frame contains CRS transmission from Node B antenna ports 5 through 8 (at predetermined time-frequency locations). The exact sub-frames with CRS transmission from Node B antennas 5 through 8 may also be pre-determined, such as sub-frame 0 and sub-frame 4, or may simply have a predetermined offset with the first sub-frame depending on the cell identity (Cell-ID). For example, for a first Cell-ID the first sub-frame is sub-frame 0 while for a second Cell-ID the first sub-frame is sub-frame 3. This further assumes that UEs obtain the Cell-ID after initial synchronization with their serving cell.

With broadcast signaling of the sub-frames where the Node B transmits the CRS from antennas 5 through 8, several such configurations can be supported, for example, depending on the system load. If the cell primarily serves legacy UEs supporting RS transmission from only Node B antennas 1 through 4, no sub-frames may contain CRS transmission from Node B antennas 5 through 8. If the cell primarily serves UEs supporting RS transmission from all eight Node B antennas, all sub-frames may contain CRS transmission from Node B antennas 5 through 8. Naturally, intermediate configurations can also be supported. Table 1 outlines possible configurations of sub-frames with CRS transmission from Node B antennas 5 through 8 assuming that 3 bits are included in a broadcast channel to specify the configuration.

TABLE 1

Broadcasted 3-bit Field Specifying Sub-Frames with CRS
Transmission from Antennas 5 through 8.
                  Sub-Frame Configuration with CRS
Broadcasted Value Transmission from Antennas 5 through 8

000               No sub-frame
001               One every 60 sub-frames
010               One every 20 sub-frames
011               One every 10 sub-frames
100               One every 5 sub-frames
101               One every 3 sub-frames
110               One every 2 sub-frames
111               All sub-frames

The starting sub-frame may always be the same, for example, the first sub-frame every 60 sub-frames, or may depend on the Cell-ID as previously described. As legacy UEs may not be able to interpret the broadcasted field specifying the sub-frames with CRS transmission from Node B antennas 5 through 8, this field may be in a broadcast channel that is received only by UEs capable of receiving this CRS transmission.

While the present invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.

Claims

1. A method for transmitting first and second sets of reference signals (RSs) from a set of Node B antennas, the set of Node B antennas also transmitting control data signals in a physical downlink Control channel (PDCCH) and information data signals in a physical downlink Shared channel (pdsch) over a transmission time interval having transmission symbols, the PDCCH being located in different transmission symbols than the pdsch, the method comprising:

transmitting the first set of RSs, from the set of Node B antennas, using frequency division multiplexing and time division multiplexing, in both PDCCH transmission symbols and pdsch transmission symbols of consecutive transmission time intervals; and

periodically transmitting the second set of RSs, from the set of Node B antennas, using code division multiplexing in a time domain and in a frequency domain, in pdsch transmission symbols of the transmission time interval,

wherein a transmission period is received from the Node B.

2. The method of claim 1, wherein the first set of RSs is used for demodulation of the control data signals or for demodulation of the information data signals and for obtaining channel quality estimates, and the second set of RSs is used for obtaining the channel quality estimates.

3. The method claim 1, wherein the second set of RSs is used for obtaining a channel quality estimate.

4. The method of claim 1, further comprising:

transmitting the first set of RSs from the set of Node B antennas over an entire operating bandwidth; and

transmitting the second set of RSs from the set of Node B antennas over a portion of the operating bandwidth that is less than the entire operating bandwidth.

5. A method for transmitting first and second sets of reference signals (RSs) from a set of Node B antennas, over a transmission time interval in a set of transmission time intervals and over an entire operating bandwidth in a cell, the method comprising:

transmitting the first set of RSs from the set of Node B antennas in all sub-frames in consecutive transmisson time intervals using frequency division multiplexing and time division multiplexing; and

periodically transmitting the second set of RSs from the set of Node B antennas in one sub-frame among a set of sub-frames in transmission time intervals using code division multiplexing in a time domain and in a frequency domain, a number of the sub-frames in the set being greater than 1,

wherein a sub-frame comprises a plurality of symbols, and

wherein a transmission period is received from the Node B.

6. The method of claim 5, wherein the second set of RSs is used for obtaining a channel quality estimate.

7. The method of claim 5, wherein the one sub-frame in the set of sub-frames for the second set of RSs is determined from a cell identity.

8. The method of claim 5, wherein the Node B communicates with a first category and a second category of User Equipments (UEs) and the second category of UEs interprets the broadcast signaling from the Node B.

9. The method of claim 5 wherein the second set of RSs is used for obtaining channel quality estimates.

10. The method of claim 5, further comprising:

combining at the Node B the first set of RSs or the second set of RSs for demodulation of information control signals; and

transmitting the first set of RSs and the second set or RSs separately for demodulation of information data signals.

11. The method of claim 10, wherein a number of RSs used for the demodulation of the information data signals is eight, and a number of RSs used for the demodulation of the information control signals is four.

12. The method of claim 5, wherein the number of the sub-frames in the set is transmitted to a user equipment.

13. An apparatus for transmitting first and second sets of reference signals (RSs) from a set of Node B antennas, the set of Node B antennas also transmitting control data signals in a physical downlink Control channel (PDCCH) and information data signals in a physical downlink Shared channel (pdsch) over a transmission time interval having transmission symbols, the PDCCH being located in different transmission symbols than the pdsch, the apparatus comprising:

a first transmitter for transmitting the first set of RSs, from the set of Node B antennas, using frequency division multiplexing and time division multiplexing, in both PDCCH transmission symbols and pdsch transmission symbols of consecutive transmission time intervals; and

a second transmitter for periodically transmitting the second set of RSs, from the set of Node B antennas, using code division multiplexing in a time domain and in a frequency domain, in pdsch transmission symbols of the transmission time interval,

wherein a transmission period is received from the Node B.

14. The apparatus of claim 13, wherein the first set of RSs is used for demodulation of the control data signals or for demodulation of the information data signals and for obtaining channel quality estimates, and the second set of RSs is used for obtaining the channel quality estimates.

15. An apparatus for transmitting first and second sets of reference signals (RSs) from a set of Node B antennas, over a transmission time interval in a set of transmission time intervals and over an entire operating bandwidth in a cell, the apparatus comprising:

a first transmitter for transmitting the first set of RSs from the set of Node B antennas in all sub-frames in consecutive transmission time intervals using frequency division multiplexing and time division multiplexing; and

a second transmitter for periodically transmitting the second set of RSs from the set of Node B antennas in one sub-frame among a set of sub-frames in transmission time intervals using code division multiplexing in a time domain and in a frequency domain, a number of the sub-frames in the set being greater than 1,

wherein a sub-frame comprises a plurality of symbols, and

wherein a transmission period is received from the Node B.

16. The apparatus of claim 15, wherein the number of the sub-frames in the set is transmitted to a user equipment.

17. The apparatus of claim 15, wherein the second set of RSs is used for obtaining a channel quality estimate.

18. The apparatus of claim 15, wherein the one sub-frame in the set of sub-frames for the second set of RSs is determined from a cell identity.

19. The apparatus of claim 15, wherein a Node B communicates with a first category and a second category of User Equipments (UEs) and the second category of UEs interprets the broadcast signaling from the Node B.

20. The apparatus of claim 15, further comprising:

combining at a Node B the first set of RSs or the second set of RSs for demodulation of information control signals; and

transmitting the first set of RSs and the second set of RSs separately for demodulation of information data signals.

21. The apparatus of claim 20, wherein a number of RSs used for the demodulation of the information data signals is eight, and a number of RSs used for the demodulation of the information control signals is four.

22. A method for receiving reference signals (RSs) in a wireless communication system, from a set of base station antennas, the set of base station antennas transmitting control data signals in a physical downlink Control channel (PDCCH) and information data signals in a physical downlink Shared channel (pdsch) over a sub-frame having transmission symbols, the PDCCH being located in different transmission symbols than the pdsch, the method comprising:

receiving, by a terminal, a first set of RSs using frequency division multiplexing and time division multiplexing, in PDCCH symbols and pdsch symbols of consecutive sub-frames; and

periodically receiving, by the terminal, a second set of RSs using code division multiplexing in a time domain and in a frequency domain, in pdsch symbols of a sub-frame based on a transmission period of the second set of RSs,

wherein the second set of RSs is used for obtaining a channel quality estimate, and

wherein the transmission period of the second set of RSs is received from the base station.

23. The method of claim 22, wherein the first set of RSs is used for demodulation of the control data signals or for demodulation of the information data signals and for obtaining channel quality estimates.

24. The method of claim 22, wherein the first set of RSs are received over an entire operating bandwidth, and

wherein the second set of RSs are received over a portion of the operating bandwidth that is less than the entire operating bandwidth.

25. A terminal for receiving reference signals (RSs) in a wireless communication system, from a set of base station antennas, the set of base station antennas transmitting control data signals in a physical downlink Control channel (PDCCH) and information data signals in a physical downlink Shared channel (pdsch) over a sub-frame having transmission symbols, the PDCCH being located in different transmission symbols than the pdsch, the terminal comprising:

a first receiver for receiving a first set of RSs using frequency division multiplexing and time division multiplexing, in PDCCH symbols and pdsch symbols of consecutive sub-frames; and

a second receiver for periodically receiving a second set of RSs using code division multiplexing in a time domain and in a frequency domain, in pdsch symbols of a sub-frame based on a transmission period of the second set of RSs,

wherein the second set of RSs is used for obtaining a channel quality estimate, and

wherein the transmission period of the second set of RSs is received from the base station.

26. The terminal of claim 25, wherein the first set of RSs is used for demodulation of the control data signals or for demodulation of the information data signals and for obtaining channel quality estimates.

27. The terminal of claim 25, wherein the first receiver receives the first set of RSs over an entire operating bandwidth, and

wherein the second receiver receives the second set of RSs over a portion of the operating bandwidth that is less than the entire operating bandwidth.

28. A method for transmitting reference signals (RSs) in a wireless communication system, from a set of base station antennas, the set of base station antennas transmitting control data signals in a physical downlink Control channel (PDCCH) and information data signals in a physical downlink Shared channel (pdsch) over a sub-frame having transmission symbols, the PDCCH being located in different transmission symbols than the pdsch, the method comprising:

transmitting, by the base station, a first set of RSs using frequency division multiplexing and time division multiplexing, in PDCCH symbols and pdsch symbols of consecutive sub-frames; and

periodically transmitting, by the base station, a second set of RSs using code division multiplexing in a time domain and in a frequency domain, in pdsch symbols of a sub-frame based on a transmission period of the second set of RSs,

wherein the second set of RSs is used for obtaining a channel quality estimate, and

wherein the transmission period of the second set of RSs is transmitted to a terminal.

29. The method of claim 28, wherein the first set of RSs is used for demodulation of the control data signals or for demodulation of the information data signals and for obtaining channel quality estimates.

30. The method of claim 28, wherein the first set of RSs are transmitted over an entire operating bandwidth, and

wherein the second set of RSs are transmitted over a portion of the operating bandwidth that is less than the entire operating bandwidth.

31. A base station for transmitting reference signals (RSs) in a wireless communication system, from a set of base station antennas, the set of base station antennas transmitting control data signals in a physical downlink Control channel (PDCCH) and information data signals in a physical downlink Shared channel (pdsch) over a sub-frame having transmission symbols, the PDCCH being located in different transmission symbols than the pdsch, the base station comprising:

a first transmitter for transmitting a first set of RSs using frequency division multiplexing and time division multiplexing, in PDCCH symbols and pdsch symbols of consecutive sub-frames; and

a second transmitter for periodically transmitting a second set of RSs using code division multiplexing in a time domain and in a frequency domain, in pdsch symbols of a sub-frame based on a transmission period of the second set of RSs,

wherein the second set of RSs is used for obtaining a channel quality estimate, and

wherein the transmission period of the second set of RSs is transmitted to a terminal.

32. The base station of claim 31, wherein the first set of RSs is used for demodulation of the control data signals or for demodulation of the information data signals and for obtaining channel quality estimates.

33. The base station of claim 31, wherein the first transmitter transmits the first set of RSs over an entire operating bandwidth, and

wherein the second transmitter transmits the second set of RSs over a portion of the operating bandwidth that is less than the entire operating bandwidth.