WO2010139095A1

WO2010139095A1 - Method of and base station for reducing peak - to - average power ratio for a multicarrier transmission

Info

Publication number: WO2010139095A1
Application number: PCT/CN2009/000630
Authority: WO
Inventors: Zhiheng Guo; Qingyu Miao
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2009-06-05
Filing date: 2009-06-05
Publication date: 2010-12-09
Also published as: CN102461279A

Abstract

A method (500) of and a BS (402) for reducing a PAPR for a multicarrier transmission in a cell in a TD-SCDMA system are provided. The method (500) comprises the steps of determining (502) an index of a basic midamble code of the cell and an index of each carrier of carriers used for the multicarrier transmission in the cell; for each carrier used for the multicarrier transmission in the cell, looking up (504) a phase shift in a table of phase shifts according to the index of the basic midamble code of the cell and the index of the carrier, and applying (506) the looked-up phase shift to the carrier. Each phase shift in the table of phase shifts has been optimized for PAPR reduction.

Description

METHOD OF AND BASE STATION FOR REDUCING PEAK-TO-AVERAGE POWER RATIO FOR A MULTICARRIER TRANSMISSION

TECHNICAL FIELD

The present invention relates generally to a Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) systemand, more particularly, to a reduction of a Peak-to-Average Power Ratio (PAPR) for a multicarrier transmission in a cell in the TD-SCDMA system.

BACKGROUND

TD-SCDMA is one of the 3rd Generation Partnership Project (3GPP) standards. A physical channel in a TD-SCDMA system is a burst, which is transmitted in a particular time slot within allocated radio frames. The duration of the burst is one time slot. Fig. 1 schematically shows a structure of a burst, which sequentially comprises a block of 352 chips for data symbols, a block of 144 chips for a midamble, a block of 352 chips for data symbols, and a block of 16 chips for a Guard Period (GP) . The midamble in the middle of the burst is used for channel estimation.

The use of a multicarrier technology in a TD-SCDMA system can make a Base station (BS) support more users and hence significantly reduce the number of BSs, leading to a reduction of a network operator's investment. Fig. 2 schematically shows a typical multicarrier transmission in a cell in a TD-SCDMA system,

Multiple carriers in the cell, including a primary carrier and secondary carriers, use cyclically shifted versions of one given basic midamble code. Multiple baseband signals each containing a cyclically shifted version of the given basic midamble code are modulated by the multiple carriers and then combined. The combined signal is subject to a Radio-Frequency (RF) modulation and then transmitted by antennas (not shown) .

Since the TD-SCDMA system is a synchronous system and the basic midamble codes for the multiple carriers are the same, a PAPR of the midamble field of the transmitted signal is very high compared with that of the data symbol field of the transmitted signal. The very high PAPR will cause the transmitted signal to enter a non-linear region and thus produce a signal distortion and a strong spectral leakage, resulting in a degradation of the system's performance.

Therefore, a PAPR reduction for a multicarrier transmission in a cell in a TD-SCDMA system is crucial. To reduce the PAPR, the prior art has proposed various solutions, such as block coding, clipping, etc. However, the implementation of the block coding is very complicated for a TD-SCDMA system. The clipping tends to cause a performance degradation of an Error Vector Magnitude (EVM) .

It has been noted that a Chinese patent application CN1953361A discloses a multicarrier transmission in a cell in a TD-SCDMA system in which phase shifting is used to reduce a PAPR, as schematically shown in Fig. 3. Multiple baseband signals each containing a cyclically shifted version of one given basic midamble code are first phase-shifted by corresponding phase shift factors. The phase-shifted signals are subject to a carrier modulation and a RF modulation, and then transmitted by antennas (not shown) .

However, the disclosed method has a number of disadvantages. For example, once the phase shift factors are determined, they will not vary as the basic midamble code varies. Hence, the disclosed method is not flexible in an actual implementation. Further, the introduction of phase shifts is at the baseband signals away from the finally transmitted signal, which will result in a less ideal effect in the PAPR reduction.

SUMMARY Therefore, it is an object of the present invention to address the above disadvantages by providing a method of and a BS for reducing a PAPR for a multicarrier transmission in a cell in a TD-SCDMA system.

According to one aspect of the invention, there is provided a method of reducing a PAPR for a multicarrier transmission in a cell in a TD-SCDMA system, where the number of carriers in the cell, N, is greater than or equal to two. The method comprises the steps of determining an index of a basic midamble code of the cell and an index of each carrier of the N carriers used for the multicarrier transmission in the cell; for each carrier used for the multicarrier transmission in the cell, looking up a phase shift in the table of phase shifts according to the index of the basic midamble code and the index of the each carrier, and applying the looked-up phase shift to the carrier, wherein each phase shift in the table of phase shifts has been optimized for PAPR reduction.

In an embodiment of the method, the table of phase shifts is stored in a BS included in the TD-SCDMA system when manufacturing/configuring the BS at a factory.

In an embodiment of the method, the table of phase shifts is dynamically generated in a BS included in the TD-SCDMA system.

In an embodiment of the method, the table of phase shifts varies according to the number of carriers in a cell and/or the frequency separation between adjacent carriers.

In an embodiment of the method, the optimization of each phase shift in the table of phase shifts is performed in a digital intermediate frequency domain. Preferably, the basic midamble codes are converted into digital intermediate frequency signals by Root-Raised Cosine (RRC) filters.

In an embodiment of the method, all basic midamble codes in a group allocated to a cell are ordered in the table of phase shifts in terms of their optimized PAPRs.

In an embodiment of the method, a basic midamble code with the lowest optimized PAPR in a group allocated to a cell is selected as a basic midamble code specific to the cell during network planning .

In an embodiment of the method, applying the looked-up phase shift to the carrier comprises multiplying the carrier with the looked-up phase shift.

In an embodiment of the method, the number of all available basic midamble codes in the TD-SCDMA system is 128.

According to another aspect of the invention, there is provided a BS for reducing a PAPR for a multicarrier transmission in a cell in a TD-SCDMA system, where the number of carriers in the cell, N, is greater than or equal to two. The BS comprises one or more processing circuits configured to determine an index of a basic midamble code of the cell and an index of each carrier of the ISl carriers used for the multicarrier transmission in the cell; for each carrier used for the multicarrier transmission in the cell, look up a phase shift in a table of phase shifts according to the index of the basic midamble code of the cell and the index of the carrier, wherein each phase shift in the table of phase shifts has been optimized for PAPR reduction, and apply the looked-up phase shift to the carrier.

In an embodiment of the BS, the table of phase shifts is stored in the BS when manufacturing/configuring the BS at a factory.

In an embodiment of the BS, the table of phase shifts is dynamically generated in the BS.

In an embodiment of the BS, the table of phase shifts varies according to the number of carriers in a cell and/or the frequency separation between adjacent carriers.

In an embodiment of the BS, the optimization of each phase shift inthe table of phase shifts is performed in a digital intermediate frequency domain. Preferably, the basic midamble codes are converted into digital intermediate frequency signals by RRC filters .

In an embodiment of the BS, all basic midamble codes in a group allocated to a cell are ordered in the table of phase shifts in terms of their optimized PAPRs.

In an embodiment of the BS, a basic midamble code with the lowest optimized PAPR in a group allocated to a cell is selected as a basic midamble code specific to the cell during network planning .

In an embodiment of the BS, the one or more processing circuits are configured to apply the looked-up phase shift to the carrier by multiplying the carrier with the looked-up phase shift.

In an embodiment of the BS, the number of all available basic midamble codes in the TD-SCDMA system is 128.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the accompanying drawings, in which:

Fig. 1 schematically shows a structure of a burst in a TD-SCDMA system;

Fig. 2 schematically shows a typical multicarrier transmission in a cell in a TD-SCDMA system;

Fig. 3 schematically shows a multicarrier transmission in a cell in a TD-SCDMA system in which phase shifting is used to reduce a PAPR;

Fig. 4 is^' a schematic block diagram of a BS for reducing a PAPR for a multicarrier transmission in a cell in a TD-SCDMA system in accordance with an embodiment of the present invention;

Fig. 5 schematically shows a flow chart illustrating a method of reducing a PAPR for a multicarrier transmission in a cell in a TD-SCDMA system in accordance with an embodiment of the present invention;

Fig. 6 schematically shows a multicarrier transmission in a cell in a TD-SCDMA system in accordance with an exemplary embodiment of the present invention; and

Fig. 7 schematically shows a simulation result of non-optimized and optimized PAPRs for 128 basic midamble codes with 9 carriers .

Corresponding reference characters indicate corresponding components throughout the several views of the drawings.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of the practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims .

Throughout the description and claims of this specification, the terminology "BS" includes, but is not limited to, a base station, a Node-B, an evolved Node-B (eNode-B), or any other type of device with radio transmission/reception capabilities for providing a radio coverage in a part of a TD-SCDMA system.

Fig. 4 is a schematic block diagram of a BS 402 for reducing a PAPR for a multicarrier transmission in a cell in a TD-SCDMA system in accordance with an embodiment of the present invention . N carriers are used for a multicarrier transmission in the cell in the TD-SCDMA system where N is greater than or equal to two. The BS 402 comprises one or more processing circuits 404.

The one or more processing circuits 404 are configured to determine an index of a basic midamble code of the cell and an index of each carrier of the N carriers used for the multicarrier transmission in the cell. The one or more processing circuits 404 are further configured to, for each carrier used for the multicarrier transmission in the cell, look up a phase shift in a table of phase shifts according to the index of the basic midamble code and the index of the each carrier, and apply the looked-up phase shift to the carrier, for example, multiply the carrier with the looked-up phase shift. Each phase shift in the table of phase shifts has been optimized for PAPR reduction.

It should be understood that the one or more processing circuits 404 may comprise hardware, firmware, software, or any combination thereof. In at least one embodiment, the one or more processing circuits 404 includes one or more general or special purpose microprocessor and/or digital signal processor that are programmed to carry out operations corresponding to the method steps as discussed below. Such instructions may be embodied as one or more computer programs comprising stored program instructions in a storage element (e.g., memory) .

Referring to Fig. 5, there is schematically shown a flow chart illustrating a method 500 of reducing a PAPR for a multicarrier transmission in a cell in a TD-SCDMA system in accordance with an embodiment of the present invention. N carriers are used for the multicarrier transmission in the cell where N is greater than or equal to two. It should be understood that the method is not necessarily limited to the illustrated sequence, some steps may be omitted as desired, and some steps may be performed together or otherwise in an interrelated fashion.

The method begins with step 502 in which a BS included in the TD-SCDMA system determines an index of a basic midamble code of the cell and an index of each carrier of the N carriers used for the multicarrier transmission in the cell when the cell is set up.

Then in step 504, for each carrier used for the multicarrier transmission in the cell, the BS looks up a phase shift in a table of phase shifts according to the index of the basic midamble code of the cell and the index of the carrier. Each phase shift in the table of phase shifts has been optimized for PAPR reduction.

An example of how to obtain the table of phase shifts optimized for the PAPR reduction is given below in conjunction with Fig. 6, which schematically shows a multicarrier transmission in a cell in the TD-SCDMA system in accordance with an exemplary embodiment of the present invention.

In general, there are 128 basic midamble codes in a TD-SCDMA system, which are divided into 32 groups. Each group comprises four basic midamble codes and is allocated to one cell. The one cell will use one of the four basic midamble codes as a pilot for channel estimation for both downlink and uplink. Since the total number of basic midamble codes in the TD-SCDMA system is only 128, it is possible to generate a table of optimized phase shifts for all the basic midamble codes in the case of a multicarrier transmission in a cell.

In this exemplary embodiment of the present invention, the midamble code is oversampled (or interpolated) by a Root-Raised

Cosine (RRC) filter which is implemented by a Digital UpConverter (DUC) in hardware and the oversampled signal is called a digital intermediate frequency signal (with respect to the baseband signal) .

The oversampled midamble code will have 144 X r samples where r is the oversampling rate. It is assumed that a signal after multicarrier combination in the midamble field is given by

where N is the number of carriers in a cell, v is an index of

a basic midamble code ofthe cell, aisanindexofthe multicarrier

signal, r is the oversampling rate, and

where is the oversampled i-th samples in the midamble field

for one carrier and the v-th basic midamble code,

is the phase shift for the n-th carrier, and f_n is the frequency of

the n-th carrier.

To reduce the PAPR of the multicarrier signal, the phase shift for each carrier is chosen carefully to minimize the PAPR of the oversampled signal after multicarrier combination. The following computation is one possible way to find an optimized phase shift for each carrier. It is assumed that the phase shift vector is given by

where N is the number of carriers in a cell, v is an index of

a basic midamble code of the cell, and a is an index of the phase

shift vector. If

can be any value, the number of possible will be infinite. To reduce the complexity of computation

while achieving a relatively optimal result, it is assumed that

_and

_{where And}

then

, The typical number of L is 2, 4 or 8.

The PAPR of

will be

Then the optimized phase shift will be

Thus, the optimized phase shift for each carrier is

where n is an index of the carrier, JV is the number of carriers in the cell, and v is an index of a basic midamble code of the cell.

In this way, the optimized phase shift

can be calculated by simulation, and hence the table of the optimized phase shifts can be obtained. Table 1 is an example of a portion of the table of optimized phase shifts for 6 carriers in a cell. Table 1

It should be noted that the table of optimized phase shifts obtained through the above-mentioned computation generally varies according to the number of carriers in a cell and/or the frequency separation between adjacent carriers. For example, values of phase shifts in the table obtained for 6 carriers in a cell are generally different from those of phase shifts in the table obtained for 9 carriers in a cell . Further, the frequency separation between adjacent carriers also affects values of phase shifts. In the TD-SCDMA system, the frequency separation between adjacent bands is normally about 1.6MHz since the chip rate is 1.28MHz and the bandwidth is 1.6MHz. In some cases, however, the frequency separation between adjacent carriers may be 1.8MHz , and hence the values of phase shifts can be different.

It should also be noted that the obtained table of optimized phase shifts can be stored in the BS when manufacturing/configuring the BS at a factory. Alternatively, the table of optimized phase shifts can be dynamically generated in the BS.

It can be seen from the above that the optimization of the phase shifts in the table may be performed in a digital intermediate frequency domain. Since the digital intermediate frequency signal is closer to the finally transmitted signal than the baseband signal, a better effect in PAPR reduction can be achieved .

Further, after the optimized phase shifts are obtained by simulation, in the case of N carriers, each basic midamble code has an optimized PAPR, which varies from a basic midamble code to a basic midamble code . Fig. 7 schematically shows a simulation result of non-optimized and optimized PAPRs for 128 basic midamble codes with 9 carriers. Obviously, four basic midamble codes belonging to the same group allocated to a cell have different optimized PAPRs. Hence, the four basic midamble codes can be ordered in the table in terms of their optimized PAPRs. Further, a basic midamble code with the lowest optimized PAPR in the group can be selected as a basic midamble code specific to the cell during network planning.

Referring back to Fig. 5, finally in step 506, for each carrier used fortherαulticarrier transmission inthecell, the BS applies the looked-up phase shift to the carrier, for example, multiplies the carrier with the looked-up phase shift for use in the multicarrier transmission in the cell.

With phase shifting of each carrier, the PAPR of the multicarrier can be greatly reduced. This means that for the same Power Amplifier (PA) the maximum average radiation power can be increased, and a radio coverage can be increased. For the same maximum average radiation power, a PA with the narrower linear range can be used and hence the cost can be saved.

Moreover, the present invention is flexible in an actual implementation due to the use of the table of phase shifts, which makes the present invention easily applicable to various BSs utilizing different multicarrier transmissions without the need for hardware change .

Throughout the description and claims of this specification, the words "comprise", "include", and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

It will be understood that the foregoing description of the embodiments of the invention has been presented for purposes of illustration and description. This description is not exhaustive and does not limit the claimed invention to the precise forms disclosed. Modifications and variations are possible in light of the above description or may be acquired from practicing the invention. The claims and their equivalents define the scope of the invention.

Claims

1. Amethod (500) of reducing a Peak-to-Average Power Ratio (PAPR) for a multicarrier transmission in a cell in a Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) system where the number of carriers in the cell, N, is greater than or equal to two, the method comprising the steps of:

determining (502) an index of a basic midamble code of the cell and an index of each carrier of the N carriers used for the multicarrier transmission in the cell;

for each carrier used for the multicarrier transmission in the cell :

looking up (504) a phase shift in a table of phase shifts according to the index of the basic midamble code of the cell and the index of the carrier, wherein each phase shift in the table of phase shifts has been optimized for PAPR reduction; and

applying (506) the looked-up phase shift to the carrier.

2. The method (500) according to claim 1, wherein the table of phase shifts is stored in a Base Station (BS) included in the TD-SCDMA system when manufacturing/configuring the BS at a factory .

3. The method (500) according to claim 1, wherein the table of phase shifts is dynamically generated in a Base Station (BS) included in the TD-SCDMA system.

4. The method (500) according to any of claims 1 to 3, wherein the table of phase shifts varies according to the number of carriers in a cell and/or the frequency separation between adjacent carriers.

5. The method (500) according to any of claims 1 to 4, wherein the optimization of each phase shift in the table of phase shifts is performed in a digital intermediate frequency domain.

6. The method (500) according to claim 5, wherein the basic midamble codes are converted into digital intermediate frequency signals by Root-Raised Cosine (RRC) filters.

7. The method (500) according to any of claims 1 to 6, wherein all basicmidamble codes in a group allocated to a cell are ordered in the table of phase shifts in terms of their optimized PAPRs.

8. The method (500) according to any of claims 1 to β, wherein a basic midamble code with the lowest optimized PAPR in a group allocated to a cell is selected as a basic midamble code specific to the cell during network planning.

9. The method (500) according to any of claims 1 to 8, wherein applying (506) the looked-up phase shift to the carrier comprises multiplying the carrier with the looked-up phase shift.

10. The method (500) according to any of claims 1 to 9, wherein the number of all available basic midamble codes in the TD-SCDMA system is 128.

11. A Base Station (BS) (402) for reducing a Peak-to-Average Power Ratio (PAPR) for a multicarrier transmission in a cell in a Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) system where the number of carriers in the cell, N, is greater than or equal to two, the BS comprising one or more processing circuits (404) configured to:

determine (502) an index of a basic midamble code of the cell and an index of each carrier of the N carriers used for the multicarrier transmission in the cell;

for each carrier used for the multicarrier transmission in the cell: look up (504) a phase shift in a table of phase shifts according to the index of the basic midamble code of the cell and the index of the carrier, wherein each phase shift in the table of phase shifts has been optimized for PAPR reduction; and

apply (506) the looked-up phase shift to the carrier.

12. The BS (402) according to claim 9, wherein the table of phase shifts is stored in the BS when manufacturing/configuring the BS at a factory.

13. The BS (402) according to claim 9, wherein the table of phase shifts is dynamically generated in the BS.

14. The BS (402) according to any of claims 11 to 13, wherein the table of phase shifts varies according to the number of carriers in a cell and/or the frequency separation between adjacent carriers.

15. The BS (402) according to any of claims 11 to 14, wherein the optimization of each phase shift in the table of phase shifts is performed in a digital intermediate frequency domain.

16. The BS (402) according to claim 15 , wherein the basic midamble codes are converted into digital intermediate frequency signals by Root-Raised Cosine (RRC) filters.

17. The BS (402) according to any of claims 11 to 16, wherein all basic midamble codes in a group allocated to a cell are ordered in the table of phase shifts in terms of their optimized PAPRs.

18. The BS (402) according to any of claims 11 to 16, wherein a basic midamble code with the lowest optimized PAPR in a group allocated to a cell is selected as a basic midamble code specific to the cell during network planning.

19. The BS (402) according to any of claims 11 to 18, wherein the one or more processing circuits (404) are configured to apply (506) the looked-up phase shift to the carrier by multiplying the carrier with the looked-up phase shift.

20. The BS (402) according to any of claims 11 to 19, wherein the number of all available basic midamble codes in the TD-SCDMA system is 128.