WO2015042803A1

WO2015042803A1 - Methods and apparatuses for secondary synchronization signal interference mitigation

Info

Publication number: WO2015042803A1
Application number: PCT/CN2013/084193
Authority: WO
Inventors: Hongmei Liu; Gang Wang; Lei Jiang; Ming Lei
Original assignee: Nec (China) Co., Ltd.
Priority date: 2013-09-25
Filing date: 2013-09-25
Publication date: 2015-04-02

Abstract

Methods and apparatuses for secondary synchronization signal interference mitigation are provided. According to embodiments of the present disclosure, one method comprises applying, at a base station, orthogonal cover codes to a secondary synchronization signal sequence to generate a synchronization signal code sequence. The method further comprises transmitting the generated synchronization signal code sequence to a user equipment communicating with the base station. The solutions according to the present disclosure can notably diminish the secondary synchronization signal interference without extra signaling cost and tight timing requirements, thereby giving cell recognition accuracy a big boost.

Description

METHODS AND APPARATUSES FOR SECONDARY SYNCHRONIZATION SIGNAL INTERFERENCE MITIGATION

FIELD OF THE INVENTION

[0001] Exemplary embodiments of the present disclosure generally relate to wireless communication techniques including the 3 GPP (the 3rd Generation Partnership Project) LTE (Long Term Evolution) technique. More particularly, exemplary embodiments of the present disclosure relate to methods and apparatuses for secondary synchronization signal interference mitigation.

BACKGROUND OF THE INVENTION

[0002] Various abbreviations that appear in the specification and/or in the drawing figures are defined as below:

CoMP Coordinated Multiple Point

CP Cyclic Prefix

CPvS Common Reference Signal

CSI-PvS Channel State Information Reference Signal

CSI-PvSRP Channel State Information Reference Signal Received Power

DL Downlink

DMPvS Demodulation Reference Signal

eNB evolved Node B

FDD Frequency Domain Division

LTE Long Term Evolution

OAM Operation And Maintenance

OCC Orthogonal Cover Code

PRACH Physical Random Access Channel

PSS Primary Synchronization Signal

RAN Radio Access Network

RE Resource Element

RB Resource Block

RRC Radio Resource Control

RSRP Reference Signal Received Power RSRQ Reference Signal Received Quality

SINR Signal to Interference and Noise Ratio

SNR Signal to Noise Ratio

SSS Secondary Synchronization Signal

TDD Time domain division

UE User equipment

UL Uplink

[0003] With dramatic increases of the traffic demands in the wireless communication, small cells with smaller transmission power and coverage areas are becoming more and more popular for providing large throughput in the hotspot area. As more and more small cells are densely deployed, cell capacity can be enlarged to a large extent while mutual interference between the small cells becomes more severe and complicated to deal with, particularly, the interference imposed on the SSS, which is referred to as SSS interference in the present disclosure.

[0004] As is known to those skilled in the art, PSS and SSS are used for UE to identify a specific cell, a TDD/FDD type, a CP length, as well as for time and frequency synchronization. The SSS interference is common in a wireless network, for example, a heterogeneous network consisting of the densely-deployed small cells and a macro cell. Since the SSS interference is detrimental to cell identification and timing synchronization, it should be mitigated as effectively as possible.

[0005] There have been proposed possible solutions to mitigate the SSS interference. One possible solution is disclosed in the patent application WO2013/040487, entitled "Extension Carrier Discovery for Carrier Aggregation," published on March 21, 2013. This patent application proposes allocating different time resources for small cells to mitigate the mutual PSS/SSS interference caused by the code domain division and also suggests applying new synchronization signals for small cells without mention of details about this newly designed synchronization signals. The drawback of this solution is that it incurs a large amount of signaling overhead since different cells are distinguished from each other in a temporal manner. In other words, to always allocate different time resources for different small cells leads to large resource consumption and decreases the spectrum efficiency.

[0006] Another possible solution is shown in a 3 GPP document designated Rl- 132084, entitled "RE Mapping for Small Cell Discovery Signal Based on Unused Res," filed on May 20, 2013. This document mainly proposes using the unused Res in the edge of the center six RBs for the small cell discovery. The drawbacks of this solution are that the unused REs are insufficient to carry the synchronization information and tight timing alignment and high synchronization accuracy are needed for synchronization between cells.

[0007] Therefore, there is a need in the art to provide methods and apparatuses for efficiently mitigating the SSS interference in the wireless network, for example, a dense small cell deployed network.

SUMMARY OF THE INVENTION [0008] It is the object of the embodiments of the present disclosure to address at least some of the above disadvantages and provide improved methods and apparatuses for SSS interference mitigation.

[0009] According to an aspect of the present disclosure, there is provided a method for SSS interference mitigation. The method comprises applying, at a BS, OCCs to an SSS sequence to generate a synchronization signal code sequence. The method further comprises transmitting the generated synchronization signal code sequence to a UE communicating with the BS.

[0010] In an embodiment, the method further comprises transmitting to the UE prior information about indexes of potential cells and OCCs to be used via a signaling message.

[0011] In another embodiment, wherein the OCCs include a first OCC and a second

OCC and the SSS sequence is an interleaved concatenation of a first sub-SSS sequence and a second sub-SSS sequence and the method further comprises applying the first OCC to the first sub-SSS sequence to generate a first sub-synchronization signal code sequence, applying the second OCC to the second sub-SSS sequence to generate a second sub-synchronization signal code sequence and concatenating the interleaved first and second sub-synchronization signal code sequences to generate the synchronization signal code sequence.

[0012] According to another aspect of the present disclosure, there is provided a method for SSS interference mitigation. The method comprises receiving, from a BS, a synchronization signal code sequence which is generated by applying OCCs to an SSS sequence. The method further comprises decoding the synchronization signal code sequence to determine a cell index.

[0013] In an embodiment, the decoding the synchronization signal code sequence comprises decoding a first sub-SSS sequence number based on a first sub-synchronization signal code sequence, decoding a second sub-SSS sequence number based on a second sub-synchronization signal code sequence, wherein the synchronization signal code sequence is an interleaved concatenation of the first sub-synchronization signal code sequence and the second sub-synchronization signal code sequence and the SSS sequence is an interleaved concatenation of a first sub-SSS sequence and a second sub-SSS sequence, comparing the first sub-SSS sequence number with the second sub-SSS sequence number and determining timing information based on the comparing.

[0014] In another embodiment, the method further comprises receiving, from the BS, prior information about indexes of potential cells and OCCs to be used via a signaling message.

[0015] In a further embodiment, the decoding the first sub-SSS sequence number comprises determining the OCCs based on the prior information, obtaining an original sequence associated with the first sub-SSS sequence based on the decoded PSS sequence number and the OCC, and decoding the first sub-SSS sequence number based on cross-correlation operations and the prior information.

[0016] In an additional embodiment, the decoding the second sub-SSS sequence number comprises obtaining an original sequence associated with the second sub-SSS sequence based on the decoded PSS sequence number, the prior information and the first sub-SSS sequence number, and decoding the second sub-SSS sequence number based on cross-correlation operations and the prior information.

[0017] In yet another embodiment, the decoding the first sub-SSS sequence number comprises obtaining an original sequence associated with the first sub-SSS sequence based on the decoded PSS sequence number, and decoding the first sub-SSS sequence number based on cross-correlation operations and OCC trials.

[0018] In an embodiment, the decoding the second sub-SSS sequence number comprises obtaining an original sequence associated with the second sub-SSS sequence based on the decoded PSS sequence number and the first sub-SSS sequence number and decoding the second sub-SSS sequence number based on cross-correlation operations and the OCC trials.

[0019] According to an aspect of the present disclosure, there is provided an apparatus for SSS interference mitigation. The apparatus comprises an applying unit configured to apply, at a BS, OCCs to an SSS sequence to generate a synchronization signal code sequence. The apparatus further comprises a transmitting unit configured to transmit the generated synchronization signal code sequence to a UE communicating with the BS.

[0020] According to another aspect of the present disclosure, there is provided an apparatus for SSS interference mitigation. The apparatus comprises a receiving unit configured to receive, from a BS, a synchronization signal code sequence which is generated by applying OCCs to an SSS sequence. The apparatus further comprises a decoding unit configured to decode the synchronization signal code sequence to determine a cell index.

[0021] According to the methods and apparatuses and their extensions as discussed in the embodiments of the present disclosure, by virtue of the OCC applied to the SSS sequence generation, UEs are given the capability of differentiating two cell indexes with both the M code originated from the SSS sequence and the OCC code originated from the OCC sequence, which increases the difference between similar cell indexes and makes it more robust to confront the severe interference environment. Further, with the aid of the prior information relating to the OCC and potential cell indexes, the UEs are able to carry out suitable processing to mitigate the interference between different OCC codes to improve the detection probability under specific false alarm rate. As compared to the prior art solutions, for example, the two possible solutions as discussed previously, the solutions according to the present disclosure can notably diminish the SSS interference without extra signaling cost and tight timing requirements, thereby giving cell recognition accuracy a big boost.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The embodiments of the present disclosure that are presented in the sense of examples and their advantages are explained in more detail below with reference to the accompanying drawings, in which:

[0023] Fig. 1 is a schematic network architecture in which the embodiments of the present disclosure can be practiced;

[0024] Fig. 2 is a schematic diagram illustrating REs used for transmitting PSS/SSS sequences;

[0025] Fig. 3 is a flow chart exemplarily illustrating a method for SSS interference mitigation according to an embodiment of the present disclosure;

[0026] Fig. 4 is a flow chart exemplarily illustrating a method for SSS interference mitigation according to another embodiment of the present disclosure;

[0027] Fig. 5 is a schematic diagram illustrating how to generate synchronization signal code sequences associated with the SSS sequences in time slots 0 and 10 according to an embodiment of the present disclosure;

[0028] Fig. 6 is a flow chart exemplarily illustrating in detail a method for SSS interference mitigation according to an embodiment of the present disclosure;

[0029] Fig. 7 is a flow chart exemplarily illustrating in detail a method for SSS interference mitigation according to another embodiment of the present disclosure;

[0030] Fig. 8 is a flow chart exemplarily illustrating a method for SSS interference mitigation according to an embodiment of the present disclosure;

[0031] Fig. 9 is a flow chart exemplarily illustrating a method for SSS interference mitigation according to another embodiment of the present disclosure;

[0032] Fig. 10 is a schematic block diagram illustrating an apparatus for SSS interference mitigation according to an embodiment of the present disclosure;

[0033] Fig. 11 is a schematic block diagram illustrating an apparatus for SSS interference mitigation according to another embodiment of the present disclosure; and

[0034] Fig. 12 is a simulation diagram schematically illustrating differences of the probabilities for detecting small cells with the OCC according to the embodiments of the present disclosure and probabilities for detecting small cells without the OCC under the same number of small cells. DETAILED DESCRIPTION OF EMBODIMENTS

[0035] While the making and using of various embodiments of the present disclosure are discussed in detail below, it should be appreciated that the present disclosure provides many applicable inventive aspects that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the solutions of the present disclosure and do not delimit the scope thereof.

[0036] To facilitate the understanding of embodiments of the present disclosure, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present disclosure. Terms such as "a," "an" and "the" are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the present disclosure, but their usage does not delimit the scope of the present disclosure. For example, a BS in the present disclosure may refer to one of a NB, an eNB, a base transceiver station, a radio BS, and the like and thus they may be used interchangeably throughout the specification and claims as appropriate.

[0037] The exemplary embodiments of the present disclosure provide solutions that aim to mitigate the SSS interference occurring in a wireless network, e.g., the heterogeneous network and thereby significantly improve recognition rate in relation to the potential cells in proximity to UEs. To this end, OCC is applied to the SSS sequence generation in various means to increase the detection capability under the low SINR. In an embodiment, specific OCC codes configured by the macro BS can help the UE to more effectively indentify the cells within the macro coverage. In another embodiment, a blind decoding or fallback mode can be adopted in the absence of the macro coverage. As compared with the prior art solutions, the OCC assisted SSS sequence generation is resource efficient and with less synchronization requirement in the frequency domain.

[0038] Embodiments of the present disclosure will be discussed in detail in the following with reference to accompanying drawings.

[0039] Fig. 1 is a schematic network architecture in which the embodiments of the present disclosure can be practiced. As illustrated in Fig. 1, the network architecture comprises a macro BS with a coverage area depicted by an ellipse and a plurality of small cells, which are densely deployed within the macro coverage area and provide relatively smaller coverage areas. The macro BS and small cells can be communicated with each other via an X2 interface, although only one X2 interface is showed in Fig. 1 for illustrative purposes. Also depicted in Fig. 1 is a UE within the coverage area of the small cell. The UE can communicate with the macro BS and small cells both in a dual connection mode and can communicate with the macro BS via the small cell in a single connection mode such that traffic can be offloaded by a backhaul link. It can be understood that the number of the small cells and the UE as shown is only for descriptive purposes and more small cells and the UEs could be present in the practical wireless communication scenario.

[0040] As previously discussed, in the heterogeneous network as depicted in Fig. 1, due to dense deployment of the small cells within the macro coverage, SSS interference could be engendered and would cause adverse effect on cell identification and synchronization. Therefore, the embodiments of the present disclosure propose applying the OCC to the SSS sequence to generate a new synchronization signal code sequence, and mapping the new synchronization signal code sequence to the REs used for carrying the SSS sequence. Upon reception and decoding of this new synchronization signal code sequence, the UE can distinguish two cells with similar indexes through M-code originated from the SSS sequence and OCC.

[0041] For a better understanding of the mapping operation as mentioned above, reference will be made to Fig. 2, which schematically illustrates REs used for transmitting synchronization information, such as PSS and SSS sequences, in a time slot 0 or 10. In the RE grid as illustrated in Fig. 2, the REs allocated to the time slot 0 or 10 for carrying the PSS and SSS are located in the middle portions of the RE grid and occupy two columns in the time domain, wherein the REs of the left column are used for transmission of the SSS and the REs of the right column are used for transmission of the PSS as shown by arrows, with a span of six RBs in the frequency domain. Consequently, except for 10 unused REs located at the upper and bottom portions of the left column, there are total of 62 REs that will be used for SSS transmission.

[0042] Fig. 3 is a flow chart exemplarily illustrating a method 300 for SSS interference mitigation according to an embodiment of the present disclosure. The method 300 discussed herein can be implemented at the macro or small cell as illustrated in Fig. 1. As illustrated in Fig. 3, at step S301, the method 300 applies, at a BS, OCCs to an SSS sequence to generate a synchronization signal code sequence. The details and example implementations of applying the OCCs to the SSS sequences are illustrated in Fig. 5, which will be discussed in detail later. Then the flow proceeds to step S302, at which the method 300 transmits the generated synchronization signal code sequence to a UE communicating with the BS.

[0043] Although not shown in Fig. 3, in an embodiment, the method 300 further comprises transmitting to the UE prior information about indexes of potential cells and OCCs to be used via a signaling message, e.g., an RRC message. By means of this prior information, the UE could be more effectively in decoding the generated synchronization signal code sequences.

[0044] In another embodiment, the OCCs include a first OCC (e.g., g₀ (j) as shown in Fig. 5)and a second OCC (e.g., g^j) as shown in Fig.5) and the SSS sequence is an interleaved concatenation of a first sub-SSS sequence (e.g., one shown in Fig.5) and a second sub-SSS sequence (e.g., one shown in Fig. 5). In this case, the method 300 further comprises applying the first OCC to the first sub-SSS sequence to generate a first sub-synchronization signal code sequence (e.g., SSC1 as shown in Fig. 5), applying the second OCC to the second sub-SSS sequence to generate a second sub-synchronization signal code sequence (e.g., SSC2 as shown in Fig. 5) and concatenating the interleaved first and second sub-synchronization signal code sequences to generate the synchronization signal code sequence. In other words, the resulting synchronization signal code sequence would be mapped to the REs such that the SSC1 would be mapped to the odd-numbered REs and the SSC2 would be mapped to the even-numbered REs in an interleaved manner.

[0045] It should be noted that in order to better carry out the solutions as proposed by the present disclosure, some control signaling could be introduced such that one of the following could be realized: 1) the OCC sequences of both macro and small cells can be configured by the OAM in a static way, 2) the macro cells can also inform the small cells located within its coverage area of the OCC configurations by the X2 signaling or other enhanced interface in backhaul adapting to the traffic variety in a dynamic way, 3) macro cells can provide the UE with the knowledge of small cell indexes and/or OCC indexes to help identification of the small cells.

[0046] Fig. 4 is a flow chart exemplarily illustrating a method 400 for SSS interference mitigation according to an embodiment of the present disclosure. The method 400 discussed herein can be implemented by the UE as illustrated in Fig. 1. As illustrated in Fig. 4, at step S401 , the method 400 receives, from a BS, a synchronization signal code sequence which is generated by applying OCCs to an SSS sequence. The applying of the OCCs to the SSS sequences can be implemented in a manner as illustrated in Fig. 5, which will be discussed in detail later. Then the flow proceeds to step S402, at which the method 400 decodes the synchronization signal code sequence to determine a cell index.

[0047] In an embodiment, the decoding of the synchronization signal code sequence comprises decoding a first sub-SSS sequence number based on a first sub-synchronization signal code sequence (e.g., SSC1), decoding a second sub-SSS sequence number based on a second sub-synchronization signal code sequence (e.g., SSC2), wherein the synchronization signal code sequence is an interleaved concatenation of the first sub-synchronization signal code sequence and the second sub-synchronization signal code sequence and the SSS sequence is an interleaved concatenation of a first sub-SSS sequence and a second sub-SSS sequence. The decoding of the synchronization signal code sequence further comprises comparing the first sub-SSS sequence number with the second sub-SSS sequence number, and determining timing information based on the comparing.

[0048] In an embodiment, the method 400 further comprises receiving, from the BS, prior information about indexes of potential cells and OCCs to be used via a signaling message, for example, an RRC message. In this case, the decoding of the first sub-SSS sequence number (e.g., ⁰ or ¹ ) comprises determining the OCCs based on the prior information, obtaining an original sequence (e.g., so° (n) for the slot 0 or s? (n) for the slot 10) associated with the first sub-SSS sequence based on the decoded PSS sequence number and the OCC, and decoding the first sub-SSS sequence number based on cross-correlation operations and the prior information. Correspondingly, the decoding of the second sub-SSS sequence number (e.g., ¹ or ^m° ) comprises obtaining an original sequence (e.g., ¹ ^ ' for the slot 0 or ⁰ ^ ' for the slot 10) associated with the second sub-SSS sequence based on the decoded PSS sequence number, the prior information and the first sub-SSS sequence number, and decoding the second sub-SSS sequence number based on cross-correlation operations and the prior information.

[0049] In the absence of the prior information, in an embodiment, the decoding of the first sub-SSS sequence number comprises obtaining an original sequence (e.g., so [ n) for the slot 0 or

(«) for the slot 10) associated with the first sub-SSS sequence based on the decoded PSS sequence number, and decoding the first sub-SSS sequence number based on cross-correlation operations and OCC trials. Correspondingly, the decoding of the second sub-SSS sequence number comprises obtaining an original sequence (e.g., ¹ ^ ' for the slot 0 or ⁰ ^ ' for the slot 10) associated with the second sub-SSS sequence based on the decoded PSS sequence number and the first sub-SSS sequence number, and decoding the second sub-SSS sequence number based on cross-correlation operations and the OCC trials.

[0050] It should be noted that the decoding process as described above is just a simple explanation and other advanced soft decision reception methods or joint detection methods can also be utilized under the combination of OCC and a small cell index to identify a specific cell. Also, to identify the small cell index more accurately, more SSS samples may be needed to recover the correct OCC to improve the detection reliability. Further, time diversity of the SSS sequences can be obtained with suitable weighting vector configurations (e.g., reversing the coefficients related to '- in the OCC). Additionally, legacy UEs cannot get the OCC by macro configuration or bind decoding and therefore all '+1 OCC should be adopted for the cells which are mainly for service coverage rather than hotspot performance enhancement.

[0051] Fig. 5 is a schematic diagram illustrating how to generate synchronization signal code sequences associated with the SSS sequences in the time slots 0 and 10 according to

(^mo) ( \ an embodiment of the present disclosure. As illustrated, the two sequences ° ^w and

^(m\) ( ~(

s\ ⁿ⁾ _m pig 5 _are defined as two different cyclic shifts of the m-sequence ^{s w} according to

"°^ ( ) = ί ((« + »¾) mod 31)

s _j ^m' ' («) = ?((« + »¾_[) mod 31) where s (i) = 1 - 2x(i) , 0 < i < 30 , is defined by

x(J + 5) = (x( + 2) + x( ))mod 2, 0 < < 25 with initial conditions x(0) = 0, x(l) = 0, x(2) = 0, x(3) = 0, x(4) = 1 .

[0052] The two scrambling sequences and ^c^ depend on the PSS and are defined by two different cyclic shifts of the m-sequence ^c according to c₀ („) = c ((„ + g⁾ ) mod 31)

(«) = c ((« + N + 3) mod 31) where N$ e {0,1,2} is the physical-layer identity within the physical-layer cell identity group N$ and c (i) = 1 - 2x(i) , 0 < i < 30 , is defined by

x( + 5) = (x(J + 3) + x( ))mod2, 0 < J≤ 25 with initial conditions x(0) = 0, x(l) = 0, x(2) = 0, x(3) = 0, x(4) = 1 .

(^mo⁾ ( z^^m^ (n)

[0053] The scrambling sequences ^{Ζ w} and ¹ ^ ' are defined by a cyclic shift of the m-sequence ^z according to

_Zl ^{(mo )} («) = z ((« + (m₀ mod 8)) mod 31)

= z {{n + {m_l mod 8)) mod 31) where m₀ < m_l , 0≤ m₀ < 29 and !≤??¾≤ 30 , and z (i) = l - 2x(i) , 0 < i < 30 , is defined by

x(J + 5) = (x(J + 4) + x(J + 2) + x( + 1) + x( ))mod 2, 0 < ϊ < 25 with initial conditions x(0) = 0, x(l) = 0, x(2) = 0, x(3) = 0, x(4) = 1 .

[0054] As is known to those skilled in the art and also as illustrated in Fig. 5, in the time slot 0, the SSS sequence can be generated by an interleaved concatenation of two length-31 binary sequences

(n)

(i.e., the second sub-SSS sequence).

[0055] To further distinguish different cell indexes, the embodiments of the present disclosure introduce OCC, which may consist of g₀ (j) and g^j) , wherein

J ⁼ [ⁿ _siot + 20 * « _frame /lO , n_slot \s the slot number in a frame, starting from 0, and n_frame \s the frame number, starting from 0, g₀ is for the first length-31 binary sequences and chosen from the total set and g_l is for the second length-31 binary sequences and chosen from the total set.

[0056] Based on the arrangement of time slots 0 and 10, the g₀ (7) and g_l ( j) according to the embodiments of the present disclosure may take a form of the following:

[0059] Therefore, in the time slot 0 as illustrated in the left side of Fig. 5, the synchronization signal code sequence according to the embodiments of the present disclosure can be generated by an interleaved concatenation of two length-31 binary sequences s₀ ^(mo) (n)c₀ (n) g₀ (j) (i.e., SSC1 as illustrated and also referred to as the first sub-synchronization signal code sequence for the slot 0) and s^ in)^ («) z₁ ^(m°⁾ [n) g_l (y)

(i.e.,SSC2 as illustrated and also referred to as the second sub-synchronization signal code sequence for the slot 0). Likewise, in the time slot 10 as illustrated in the right side of Fig. 5, the synchronization signal code sequence according to the embodiments of the present disclosure can be generated by an interleaved concatenation of two length-31 binary sequences s_l ^{(m, )} (n)c₀ n) g₀ j) (i.e., SSC1 as illustrated and also referred to as the first sub-synchronization signal code sequence for the slot 10) and s^ in)^ [n) z ^{mi )} [n) g_l (y)

(i.e.,SSC2 as illustrated and also referred to as the second sub-synchronization signal code sequence for the slot 10). It should be noted that the number of OCCs can be increased with a larger window size. Further, to avoid unnecessarily obscuring the solutions of the present disclosure, details regarding PSS and SSS sequences are omitted herein and can be found in the 3GPP TS 36.211 , which is incorporated herein by reference in its entirety.

[0060] Fig. 6 is a flow chart exemplarily illustrating in detail a method 600 for SSS interference mitigation according to an embodiment of the present disclosure. The method 600 discussed herein can be implemented by the UE. At step S601 , the UE tries to decode PSS with prior information of cell indexes. At step S602, it is determined whether the decoding is successful. If it is successful, then frequency synchronization, 5ms interval timing, and a PSS sequence number can be obtained. Otherwise, the flow loops back to step S601 and the UE keeps on decoding until a successful PSS sequence number is obtained. During the steps S603 to S612, the UE tries to decode the SSS sequence applied with OCCs as shown in Fig. 5.

[0061] At step S603, the UE decodes and gets the length-31 upper sequence, that is, the first sub-synchronization signal code sequence. Then, at step S604, by means of the OCC configured by the Macro BS, the UE recovers the sequence without the applied OCC. After that, with the decoded PSS sequence number, the UE may get the original sequence so° [n) for the slot 0 or s? [n) for the slot 10 at step S605. [0062] Upon obtaining the original sequence, the flow proceeds to step S606, at which the UE decodes and gets the first sub-SSS sequence number m₀ or m_l by cross-correlation comparison as well as the prior information of cell indexes configured by the macro BS. For example, with respect to the comparison of the length-31 original sequence so (ft) and a possible length-31 sequence ^{0 K} ' , the cross-correlation result is the absolute value of the accumulation of multiple digit-wise multiplying results. A larger cross-correlation result means stronger correlation between these two sequences.

[0063] At step S607, the UE gets the length-31 lower sequence, i.e., the second sub-synchronization signal code sequence. On the basis of the prior OCC configuration, decoded PSS sequence number and just decoded ⁰ or ¹ , the UE may get the original sequence ¹ ^ ' for the slot 0 or ⁰ ^ > for the slot 10 at step S608. Next, at step S609, the

UE decodes and gets the second sub-SSS sequence number ¹ or ⁰ by cross-correlation comparison as well as the prior information of cell indexes configured by the macro BS. For example, with respect to all the possible cell indexes provided by the prior information, the UE obtains every possible length-31 sequence ^w associated with a specific cell index and compares the length-31 original sequence ^ ' and a possible length-31 sequence ^w . The cross-correlation result is the absolute value of the accumulation of multiple digit-wise multiplying results. The cell index with largest cross-correlation result is the decoded cell index.

[0064] At step S610, the UE compares the two sub-SSS sequence numbers decoded from the upper and lower sequences, i.e., the first and second sub-synchronization signal code sequences. At step S611 , if the sequence number of the upper sequence is smaller than that of the lower sequence, then it is determined that the timing is slot 0; otherwise the timing is slot 10, just as shown in Fig. 5. At step S612, the UE judges the FDD/TDD type and CP length with accurate frequency and time synchronization. After that, with the decoded PSS and SSS, the UE detects indexes of possible cells and reports the detected results to the serving BS at step S613.

[0065] The forgoing has discussed details about how to decode the SSS sequence applied with the OCC with the aid of prior information configured and signaled by the macro BS. With this prior information, the UE may be capable of decoding the synchronization signal code sequence in a timely fashion. The following will discuss blind decoding of both the cell indexes and OCCs by the UE without prior information from the macro BS, with reference to Fig. 7.

[0066] Fig. 7 is a flow chart exemplarily illustrating in detail a method 700 for SSS interference mitigation according to another embodiment of the present disclosure. At step S701 , the UE tries to decode the PSS sequence. At step S702, it is determined whether the decoding is successful. If it is successful, then frequency synchronization, 5ms interval timing, and a PSS sequence number can be obtained. Otherwise, the flow loops back to step S701 and the UE keeps on decoding until a successful PSS sequence number is obtained. During the steps S703 to S711 , the UE tries to decode the SSS sequence applied with OCCs as shown in Fig. 5.

[0067] At step S703, the UE decodes and gets the length-31 upper sequence, that is, the first sub-synchronization signal code sequence. Then, at step S704, with the decoded PSS sequence number, the UE gets the original sequence so" (n) for the slot 0 or 5Ί ' («) for the slot 10. Upon obtaining the original sequence, the flow proceeds to step S705, at which the UE decodes and gets the first sub-SSS sequence number m₀ or w_t by cross-correlation comparison and OCC trials. Regarding the cross-correlation comparison and OCC trials, for all possible cell indexes (ranging from 0 to 503), the UE obtains every possible length-31 sequence

^{Si w} associated with a specific cell index and compares the length-31 original sequence

^ ' and a possible length-31 sequence ^w . The cross-correlation result is the absolute value of the accumulation of multiple digit-wise multiplying results. The cell index with largest cross-correlation result is the decoded cell index. The cross-correlation phase is the phase of the accumulation of multiple digit- wise multiplying results (ranging from -180 to +180). The positive cross-correlation phase means the OCC value is +1 ; otherwise, OCC value is -1.

[0068] At step S706, the UE gets the length-31 lower sequence, i.e., the second sub-synchronization signal code sequence. On the basis of the decoded PSS sequence number and just decoded ^m° or ^Ml , the UE gets the original sequence ¹ ^ ' or ⁰ ^ ' at step S707.

Next, at step S708, the UE decodes and gets the second sub-SSS sequence number ⁰ or ¹ by cross-correlation comparison and the OCC trials.

[0069] At step S709, the UE compares the two sub-SSS sequence numbers decoded from the upper and lower sequences, i.e., the first and second sub-synchronization signal code sequences. If the sequence number of the upper sequence is smaller than that of the lower sequence, then it is determined at step S710 that the timing is slot 0; otherwise, it is determined that the timing is slot 10, just as shown in Fig. 5. At step S711, the UE judges the FDD/TDD type and CP length with accurate frequency and time synchronization. After that, with the decoded PSS and SSS, the UE detects indexes of possible cells and reports the detected results to the serving BS at step S712.

[0070] It should be noted that the particulars and details regarding how to decode the PSS sequence and SSS sequence and obtain the sequence number are not discussed herein at length to avoid unnecessarily obscuring the essence of the solutions of the present disclosure. A person skilled in the art can understand and use any suitable means or algorithms to complete the above decoding without departing the spirit and scope of the present disclosure.

[0071] Fig. 8 is a flow chart exemplarily illustrating a method 800 for SSS interference mitigation according to an embodiment of the present disclosure. As can be known from Fig. 8, some key interaction operations between the macro BS and the UE are illustrated. At the outset, the macro BS configures the UE with the prior information, such as cell indexes and related OCCs, at step S801. Upon receipt of the synchronization signal code sequence as applied with the OCC, the UE identifies, at step S802, the timing and cell indexes based on this prior information. This may involve the operations as discussed at steps S601 to S612 with reference to Fig. 6. After that, at step S803, the UE reports the cell index to the serving BS or cell and then may measure the SRP/ S Q for cell association.

[0072] Fig. 9 is a flow chart exemplarily illustrating a method 900 for SSS interference mitigation according to another embodiment of the present disclosure. As can be seen from Fig. 9, some steps as performed by the UE in identifying the cell index are illustrated. At the beginning, the UE blindly decodes the cell index without any prior information from the macro BS at step S901. This may involve the operations as discussed at steps S703 to S710 with reference to Fig. 7. At step S902, it is determined whether the decoded sequence is the opposite of possible sequence. If this is the case, then at step S903, the UE changes the decoded sequence, i.e., performs complementary operations. Otherwise, the flow proceeds directly to step S904, at which the UE judges the decoded sequence and identifies the cell index.

[0073] It should be noted that operations as illustrated in Figs. 8 and 9 are descriptive of some pertinent steps in forming the synchronization signal code sequence at the BS side and decoding the synchronization signal code sequence at the UE side and details thus are not discussed in detail herein for simplicity.

[0074] Fig. 10 is a schematic block diagram illustrating an apparatus 1000 for SSS interference mitigation according to an embodiment of the present disclosure. As illustrated in Fig. 10, the apparatus 1000 comprises an applying unit 1001 and a transmitting unit 1002, wherein the applying unit 1001 is configured to apply, at a BS, OCCs to an SSS sequence to generate a synchronization signal code sequence, and the transmitting unit 1002 is configured to transmit the generated synchronization signal code sequence to a UE communicating with the BS.

[0075] In an embodiment, the transmitting unit 1002 is further configured to transmit to the UE prior information about indexes of potential cells and OCCs to be used via a signaling message.

[0076] In another embodiment, the OCCs include a first OCC and a second OCC and the SSS sequence is an interleaved concatenation of a first sub-SSS sequence and a second sub-SSS sequence, and the applying unit 1001 is configured to apply the first OCC to the first sub-SSS sequence to generate a first sub-synchronization signal code sequence, apply the second OCC to the second sub-SSS sequence to generate a second sub-synchronization signal code sequence and concatenate the interleaved first and second sub-synchronization signal code sequences to generate the synchronization signal code sequence.

[0077] Fig. 11 is a schematic block diagram illustrating an apparatus 1100 for SSS interference mitigation according to another embodiment of the present disclosure. As illustrated in Fig. 11, the apparatus 1100 comprises a receiving unit 1101 and a decoding unit 1102, wherein the receiving unit 1101 is configured to receive, from a BS, a synchronization signal code sequence which is generated by applying OCCs to an SSS sequence and the decoding unit 1102 is configured to decode the synchronization signal code sequence to determine a cell index.

[0078] In an embodiment, the decoding unit 1102 is configured to decode a first sub-SSS sequence number based on a first sub-synchronization signal code sequence and decode a second sub-SSS sequence number based on a second sub-synchronization signal code sequence, wherein the synchronization signal code sequence is an interleaved concatenation of the first sub-synchronization signal code sequence and the second sub-synchronization signal code sequence and the SSS sequence is an interleaved concatenation of a first sub-SSS sequence and a second sub-SSS sequence. The decoding unit 1102 is further configured to compare the first sub-SSS sequence number with the second sub-SSS sequence number, and determine timing information based on the comparing.

[0079] In another embodiment, the receiving unit 1101 is further configured to receive, from the BS, prior information about indexes of potential cells and OCCs to be used via a signaling message. [0080] In a further embodiment, the decoding unit 1102 is configured to determine the OCCs based on the prior information, obtain an original sequence associated with the first sub-SSS sequence based on the decoded PSS sequence number and the OCC, and decode the first sub-SSS sequence number based on cross-correlation operations and the prior information.

[0081] In yet another embodiment, the decoding unit 1102 is configured to obtain an original sequence associated with the second sub-SSS sequence based on the decoded PSS sequence number, the prior information and the first sub-SSS sequence number, and decode the second sub-SSS sequence number based on cross-correlation operations and the prior information.

[0082] In an embodiment, the decoding unit 1102 is configured to obtain an original sequence associated with the first sub-SSS sequence based on the decoded PSS sequence number and decode the first sub-SSS sequence number based on cross-correlation operations and OCC trials.

[0083] In another embodiment, the decoding unit 1102 is configured to obtain an original sequence associated with the second sub-SSS sequence based on the decoded PSS sequence number and the first sub-SSS sequence number and decode the second sub-SSS sequence number based on cross-correlation operations and the OCC trials.

[0084] It should be noted that the apparatuses 1000 and 1100 can be implemented in any suitable manners, such as in software, hardware, firmware, or any combination thereof. Further, the apparatus 1000 can be implemented in or at the BS and the apparatus 1100 can be implemented in the UE.

[0085] Fig. 12 is a simulation diagram schematically illustrating differences of the probabilities for detecting small cells with the OCC according to the embodiments of the present disclosure and probabilities for detecting small cells without the OCC under the same number of small cells.

[0086] Below is a table that illustrates simulation parameter arrangements.

Detection method Remove the interference with different OCC code before sequence correlation comparison

UE distribution 2/3 within the cluster area, 1/3 within the macro coverage; 80% indoor, 20% outdoor

UE noise figure 9 dB

Detection SIN Threshold -6dB

Transmission power Macro:46dBm

Pico:30dBm

OCC code g0 = (-l,-l,+l,+l);

gl = (-l,-l,+ l,+l)

[0087] As seen from Fig. 12, according to the Cumulative Distribution Function (CDF) as depicted at a vertical axis, under the same number of the detected small cells, the solutions using the OCC according to the embodiments of the present disclosure have a higher probability to detect such number of the small cells than the prior art solution without using the OCC. For example, for nine detected small cells, the solution with the OCC may have a probability of (100%-42%=68%) to detect nine small cells (as shown by the solid line) while the solution without the OCC may have a probability of (100%-72%=28%) to detect nine small cells (as shown by the dotted line). It is apparent that the solutions according to the present disclosure can detect more small cells than the existing solution under the same network environment.

[0088] The techniques described herein may be implemented by various means so that an apparatus implementing one or more functions of a corresponding mobile entity described with an embodiment comprises not only prior art means, but also means for implementing the one or more functions of a corresponding apparatus described with an embodiment and it may comprise separate means for each separate function, or means may be configured to perform two or more functions. For example, these techniques may be implemented in hardware (one or more apparatuses), firmware (one or more apparatuses), software (one or more modules), or combinations thereof. For a firmware or software, implementation can be through modules (e.g., procedures, functions, and so on) that perform the functions described herein.

[0089] Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these embodiments of the invention pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the embodiments of the invention are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

WHAT IS CLAIMED IS:

1. A method for secondary synchronization signal interference mitigation, the method comprising:

applying, at a base station, orthogonal cover codes to a secondary synchronization signal sequence to generate a synchronization signal code sequence; and

transmitting the generated synchronization signal code sequence to a user equipment communicating with the base station.

2. The method according to claim 1, further comprising:

transmitting to the user equipment prior information about indexes of potential cells and orthogonal cover codes to be used via a signaling message.

3. The method according to claim 1, wherein the orthogonal cover codes include a first orthogonal cover code and a second orthogonal cover code and the secondary synchronization signal sequence is an interleaved concatenation of a first sub-secondary synchronization signal sequence and a second sub-secondary synchronization signal sequence, and the method comprises:

applying the first orthogonal cover code to the first sub-secondary synchronization signal sequence to generate a first sub-synchronization signal code sequence;

applying the second orthogonal cover code to the second sub-secondary synchronization signal sequence to generate a second sub-synchronization signal code sequence; and

concatenating the interleaved first and second sub-synchronization signal code sequences to generate the synchronization signal code sequence.

4. A method for secondary synchronization signal interference mitigation, the method comprising:

receiving, from a base station, a synchronization signal code sequence which is generated by applying orthogonal cover codes to a secondary synchronization signal sequence; and

decoding the synchronization signal code sequence to determine a cell index.

5. The method according to claim 4, wherein the decoding the synchronization signal code sequence comprises:

decoding a first sub-secondary synchronization signal sequence number based on a first sub-synchronization signal code sequence;

decoding a second sub-secondary synchronization signal sequence number based on a second sub-synchronization signal code sequence, wherein the synchronization signal code sequence is an interleaved concatenation of the first sub-synchronization signal code sequence and the second sub-synchronization signal code sequence and the secondary synchronization signal sequence is an interleaved concatenation of a first sub-secondary synchronization signal sequence and a second sub-secondary synchronization signal sequence;

comparing the first sub-secondary synchronization signal sequence number with the second sub-secondary synchronization signal sequence number; and

determining timing information based on the comparing.

6. The method according to claim 5, further comprising:

receiving, from the base station, prior information about indexes of potential cells and orthogonal cover codes to be used via a signaling message.

7. The method according to claim 6, wherein the decoding the first sub-secondary synchronization signal sequence number comprises:

determining the orthogonal cover codes based on the prior information;

obtaining an original sequence associated with the first sub-secondary synchronization signal sequence based on the decoded primary synchronization signal sequence number and the orthogonal cover code; and

decoding the first sub-secondary synchronization signal sequence number based on cross-correlation operations and the prior information.

8. The method according to claim 7, wherein the decoding the second sub-secondary synchronization signal sequence number comprises:

obtaining an original sequence associated with the second sub-secondary synchronization signal sequence based on the decoded primary synchronization signal sequence number, the prior information and the first sub-secondary synchronization signal sequence number; and

decoding the second sub-secondary synchronization signal sequence number based on cross-correlation operations and the prior information.

9. The method according to claim 5, wherein the decoding the first sub-secondary synchronization signal sequence number comprises:

obtaining an original sequence associated with the first sub-secondary synchronization signal sequence based on the decoded primary synchronization signal sequence number; and decoding the first sub-secondary synchronization signal sequence number based on cross-correlation operations and orthogonal cover code trials.

10. The method according to claim 9, wherein the decoding the second sub-secondary synchronization signal sequence number comprises:

obtaining an original sequence associated with the second sub-secondary synchronization signal sequence based on the decoded primary synchronization signal sequence number and the first sub-secondary synchronization signal sequence number; and

decoding the second sub-secondary synchronization signal sequence number based on cross-correlation operations and the orthogonal cover code trials.

11. An apparatus for secondary synchronization signal interference mitigation, the apparatus comprising:

an applying unit configured to apply, at a base station, orthogonal cover codes to a secondary synchronization signal sequence to generate a synchronization signal code sequence; and

a transmitting unit configured to transmit the generated synchronization signal code sequence to a user equipment communicating with the base station.

12. The apparatus according to claim 11, wherein the transmitting unit is further configured to transmit to the user equipment prior information about indexes of potential cells and orthogonal cover codes to be used via a signaling message.

13. The apparatus according to claim 11, wherein the orthogonal cover codes include a first orthogonal cover code and a second orthogonal cover code and the secondary synchronization signal sequence is an interleaved concatenation of a first sub-secondary synchronization signal sequence and a second sub-secondary synchronization signal sequence, and the applying unit is configured to:

apply the first orthogonal cover code to the first sub-secondary synchronization signal sequence to generate a first sub-synchronization signal code sequence;

apply the second orthogonal cover code to the second sub-secondary synchronization signal sequence to generate a second sub-synchronization signal code sequence; and

concatenate the interleaved first and second sub-synchronization signal code sequences to generate the synchronization signal code sequence.

14. An apparatus for secondary synchronization signal interference mitigation, the apparatus comprising:

a receiving unit configured to receive, from a base station, a synchronization signal code sequence which is generated by applying orthogonal cover codes to a secondary synchronization signal sequence; and

a decoding unit configured to decode the synchronization signal code sequence to determine a cell index.

15. The apparatus according to claim 14, wherein the decoding unit is configured to:

decode a first sub-secondary synchronization signal sequence number based on a first sub-synchronization signal code sequence; decode a second sub-secondary synchronization signal sequence number based on a second sub-synchronization signal code sequence, wherein the synchronization signal code sequence is an interleaved concatenation of the first sub-synchronization signal code sequence and the second sub-synchronization signal code sequence and the secondary synchronization signal sequence is an interleaved concatenation of a first sub-secondary synchronization signal sequence and a second sub-secondary synchronization signal sequence;

compare the first sub-secondary synchronization signal sequence number with the second sub-secondary synchronization signal sequence number; and

determine timing information based on the comparing.

16. The apparatus according to claim 15, wherein the receiving unit is further configured to receive, from the base station, prior information about indexes of potential cells and orthogonal cover codes to be used via a signaling message.

17. The apparatus according to claim 16, wherein the decoding unit is configured to:

determine the orthogonal cover codes based on the prior information;

obtain an original sequence associated with the first sub-secondary synchronization signal sequence based on the decoded primary synchronization signal sequence number and the orthogonal cover code; and

decode the first sub-secondary synchronization signal sequence number based on cross-correlation operations and the prior information.

18. The apparatus according to claim 17, wherein the decoding unit is configured to:

obtain an original sequence associated with the second sub-secondary synchronization signal sequence based on the decoded primary synchronization signal sequence number, the prior information and the first sub-secondary synchronization signal sequence number; and

decode the second sub-secondary synchronization signal sequence number based on cross-correlation operations and the prior information.

19. The apparatus according to claim 15, wherein the decoding unit is configured to:

obtain an original sequence associated with the first sub-secondary synchronization signal sequence based on the decoded primary synchronization signal sequence number; and

decode the first sub-secondary synchronization signal sequence number based on cross-correlation operations and orthogonal cover code trials.

20. The apparatus according to claim 19, wherein the decoding unit is configured to:

obtain an original sequence associated with the second sub-secondary synchronization signal sequence based on the decoded primary synchronization signal sequence number and the first sub-secondary synchronization signal sequence number; and decode the second sub-secondary synchronization signal sequence number based on cross-correlation operations and the orthogonal cover code trials.