Abstract:
A method, an apparatus and a computer program product for wireless communication, wherein the LTE HSPA carrier aggregation includes an improved HARQ feedback by optimizing the radio interface for the uplink direction.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to mobile communication networks. More specifically, the invention relates to the radio interface and the uplink feedback report between the user equipment and the network, wherein the network comprises multiple carriers. 
       BACKGROUND OF THE INVENTION 
       [0002]    3GPP, 3rd Generation Partnership Project, develops specifications for third generation mobile phone systems, and also from Release 8 (Rel-8) the next generation specifications often referred to as LTE, Long Term Evolution. Carrier aggregation is a proposed technology for the future mobile phone systems, in which multiple carriers are aggregated to increase the overall performance. For example the LTE and the HSPA (High Speed Packet Access) are used in a single system enabling the peak data rates of the two systems to be added together. The transition from UTRA (UMTS Terrestrial Radio Access) to E-UTRA (evolved UMTS Terrestrial Radio Access) will be gradual comprising several evolutionary steps. One scenario comprises HSDPA (High Speed Downlink Packet Access) and LTE, usually referred as LTE-HSPA CA. 
         [0003]    In one proposed arrangement all uplink transmissions would take place in the LTE system. The HARQ (Hybrid automatic repeat request) feedback and the ACK/NACK signals for the HSDPA transmissions in the downlink are transmitted in the uplink channel resources of the LTE. 
         [0004]    The radio frame length is 1 ms for the LTE and configurable between 10 ms or 2 ms for the HSPA. As a result, the overall frame lengths are different in uplink and downlink. Particularly the downlink data transmission and the uplink HARQ feedback are asymmetric. This leads to waste of channel resources, which is illustrated in  FIG. 1 . 
         [0005]    The time scale is represented at the bottom of the figure, each tick being equivalent to 1 ms. The downlink data transmission is represented in capital letters. A single data transmission unit in the HSPA takes 2 ms and in the LTE 1 ms. For each data unit exists a corresponding uplink resource in the LTE for HARQ feedback, wherein HARQ feedback provides the ACK/NACK information. Each acknowledgement is represented in lowercase letters. A hash sign (#) is here used to describe a NACK, requesting a retransmission. As there are four component carriers for data transmission and thus four HARQ processes, there are also four HARQ feedback symbols in each radio frame. For example HSPA component carrier A requires retransmission, arrow  2 ; whereas component carrier B is acknowledged, arrow  6 . As a result the component carrier A is retransmitted in the following downlink transmission, arrow  4 . 
         [0006]    The data in HSPA is transmitted only every 2 ms, which leads to every second acknowledgement resource being empty and thus unused resource. One solution would be repeating the feedback and increase the reliability of the HARQ feedback signaling. This solution would not lead to saving resources. 
       PURPOSE OF THE INVENTION 
       [0007]    The purpose of the invention is to propose a new method, an apparatus for wireless communication and a computer program product that optimizes the radio interface for the uplink direction from the user equipment to the network. 
       SUMMARY 
       [0008]    The invention discloses a method, comprising an apparatus for wireless communication connected to wireless network receiving a first downlink component carrier and a second downlink component carrier, the second component carrier comprising a shorter radio frame length than the first downlink component carrier; rearranging the order of the feedback symbols, wherein said feedback symbols correspond to the first downlink component carriers; sending uplink a feedback report comprising feedback symbols via an uplink carrier comprising the same frame length as the second downlink carrier. Examples of the first and the second downlink component carriers are HSPA and LTE. The uplink component carrier is the same carrier as the second downlink component carrier when the LTE system uses TDD, Time Division Duplex. In the case of LTE FDD, Frequency Division Duplex, downlink and uplink are transmitted in different frequencies; therefore the uplink and downlink component carriers share the LTE FDD system. The number of component carriers is not limited to two carriers. 
         [0009]    In one embodiment the feedback report is mapped in sequential order every second of the feedback symbols corresponding to the order of the first downlink component carriers, wherein said feedback symbols form first feedback symbols. In one embodiment the remaining feedback symbols corresponding to the first downlink component carriers are mapped after the first feedback symbols in sequential order. In other words all feedback symbols corresponding to the first downlink component carrier are divided into two portions by taking every second feedback symbol into one portion. Said portions are then arranged in sequential order. If the number of the first downlink component carriers is not even, the last feedback symbol is repeated in the feedback report. 
         [0010]    In one embodiment the feedback report is mapped into two portions, one to the beginning and one to the end. To the beginning are mapped the feedback symbols corresponding to the first portion of downlink component carriers; and to the end are mapped the feedback symbols corresponding to the second portion of downlink component carriers. For example all HSPA carriers are mapped before all LTE carriers. 
         [0011]    In one embodiment the wireless network comprises a wireless network element assigning an SCell index to the component carrier. The apparatus for wireless communication is rearranging feedback symbols in the order of SCell indexes of the corresponding component carriers. SCell index is also abbreviated as SCellIndex, relating to secondary cell index value. One example of SCell index is defined in the document 3GPP TS 36.331 version 10.1.0 Release 10. 
         [0012]    In one embodiment the apparatus for wireless communication is configured to operate as part of a user equipment. Examples of a user equipment are a mobile phone, a mobile computing device such as PDA, a laptop computer, an USB stick—basically any mobile device with wireless connectivity to a communication network. 
         [0013]    The invention discloses also an apparatus for wireless communication, comprising at least one processor configured to receive a first downlink component carrier and a second downlink component carrier, the second component carrier comprising a shorter radio frame length than the first downlink component carrier; rearrange the order of the feedback symbols, wherein said feedback symbols correspond to the first downlink component carriers; and send uplink a feedback report comprising feedback symbols via an uplink carrier comprising the same frame length as the second downlink carrier. 
         [0014]    In one embodiment the apparatus is configured to map to the feedback report in sequential order every second of the feedback symbols corresponding to the order of the first downlink component carriers, wherein said feedback symbols form first feedback symbols. In one embodiment the apparatus is configured to map to the feedback report after the first feedback symbols, in sequential order the remaining feedback symbols corresponding to the first downlink component carriers. In one embodiment the apparatus is configured to repeat the last feedback symbol in the feedback report if the number of the first downlink component carriers is not even. 
         [0015]    In one embodiment the apparatus is configured to map to the feedback report to the beginning of the feedback report the feedback symbols corresponding to the first portion of downlink component carriers; and to the end of the feedback report the feedback symbols corresponding to the second portion of downlink component carriers. In one embodiment the apparatus is configured to rearrange feedback symbols in the order of SCell indexes of the corresponding component carriers. In one embodiment the apparatus is configured to operate as part of a user equipment. 
         [0016]    The invention discloses also a computer program product comprising a computer-readable medium bearing computer program code embodied therein for use with a computer, the computer program code comprising receiving a first downlink component carrier and a second downlink component carrier, the second component carrier comprising a shorter radio frame length than the first downlink component carrier; rearranging the order of the feedback symbols, wherein said feedback symbols correspond to the first downlink component carriers; and sending uplink a feedback report comprising feedback symbols via an uplink carrier comprising the same frame length as the second downlink carrier. 
         [0017]    The invention improves the radio interface by freeing the resources to other usage. The feedback is optimized by using the available slots in an effective manner. The HARQ feedback resources are efficiently used without unnecessarily unused symbols. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings: 
           [0019]      FIG. 1  is a block diagram of an example embodiment according to prior art, 
           [0020]      FIG. 2  is a block diagram the present invention illustrating the user equipment and network elements and applied network technologies, and 
           [0021]      FIG. 3  is a block diagram illustrating the functionality of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
         [0023]      FIG. 2  is a block diagram illustrating an apparatus  100  according to an embodiment connected to a mobile communication network. The apparatus  100  comprises at least one controller  110 , such as a processor, a memory  120  and a communication interface  130 . In one embodiment the apparatus is a computer chip. Stored in the memory  120  are computer instructions which are adapted to be executed on the processor  110 . The communication interface  130  is adapted to receive and send information to and from the processor  110 . The apparatus  100  is commonly referred as user equipment. 
         [0024]    The base station  150  is adapted to be part of a cellular radio access network such as E-UTRAN applying WCDMA technology or similar networks suitable for high speed data transmission. Such networks are often also referred to as 4G or LTE. In this example the cellular radio access network supports carrier aggregation comprising LTE and HSPA. The base station  150  illustrated in  FIG. 2  symbolizes all relevant network elements required to carry out the functionality of the wireless network. 
         [0025]    The user equipment  100  is connected to the wireless network  150  by two carriers, LTE and HSPA. Transmission originating from the base station  150  to the user equipment  100  is called downlink transmission, whereas the data traffic from the user equipment  100  to the base station  150  is called uplink transmission. The downlink carriers to the user equipment are LTE DL and HSPA DL, for the uplink carrier only LTE UL is used. The number of HARQ feedback resources that are needed for HSPA HARQ feedback is one half of the number of the HSPA component carriers. The result is rounded up in case of odd number of carriers. 
         [0026]    Hybrid automatic repeat request, HARQ, is a combination of forward error-correcting coding and error detection using the ARQ error-control method. HARQ is used both uplink and downlink in high speed data transmission technologies such as HSDPA, HSUPA and HSPA, UMTS, he IEEE 802.16-2005 standard for mobile broadband wireless access, also known as “mobile WiMAX” and 3GPP Long Term Evolution, LTE. 
         [0027]      FIGS. 1 and 3  illustrate the framework using just two component carriers for each system, being an example of a wider set of possible configurations. The number of component carriers is not limited in either HSDPA or LTE, it may be different in HSDPA and LTE. Also the number of uplink HARQ resources is increased according to the number of downlink carriers. For example in  FIG. 1 , one parallel set of resources for each carrier for both systems and in  FIG. 3 , one for each LTE carrier plus a number being the half the number of HSDPA carriers, rounded up in case of odd number of HSDPA carriers. 
         [0028]    Every second piece of resources is mapped with a 1-ms delay. This is illustrated in  FIG. 3 , where the same HARQ process is carried out with only three HARQ feedback symbols in each radio frame. The data A is negatively acknowledged (NACK) in the first opportunity; arrows 8 and 10; and the data B is acknowledged (ACK) 1 ms later; arrow  12 . Similar functionality applies with G and H. 
         [0029]    The optimization according to the present invention is particularly effective with even number of HSPA component carriers. If the number of carriers is odd, one of the resources may be left unused. The method is further improved by repeating the last feedback symbol. 
         [0030]    The LTE HARQ feedback symbols are easy to allocate, because each feedback symbol has a fixed delay after the transmission of the data. This delay is 4 ms in the present LTE specifications. 
         [0031]    Each component carrier in currently proposed carrier aggregation technology has a unique SCell index, i.e. no two carriers have the same index. In one embodiment component carriers are organized by SCellIndex. To further simplify the mapping procedure, component carriers are organized so that all the LTE component carriers are first and the HSPA carriers follow. In one embodiment the network allocates the component carrier indices so that all LTE carriers have lower indices than any HSPA carrier. 
         [0032]    Rules for organizing the HARQ feedback symbols in each radio frame can be listed as follows, where the components carriers are mapped to the feedback symbols: 
         [0033]    1. Mapping the LTE carriers to the first feedback symbols in each radio frame so that the carrier with the lowest index is mapped to the first feedback symbol and the rest of the feedback symbols in the ascending order. 
         [0034]    2. Mapping the first, third, fifth, etc. HSPA carrier to the first, second, third, etc. feedback symbol after the LTE symbols in radio frames with the same delay as the LTE feedback symbols. 
         [0035]    3. Mapping the second, fourth, sixth, . . . HSPA carrier to the first, second, third, . . . feedback symbol after the LTE symbols in radio frames having 1 ms longer delay than the LTE feedback symbols. 
         [0036]    4. If the number of HSPA carriers is odd, the last feedback symbol is repeated with an extra 1-ms delay. 
         [0037]    The delay of the first HSPA HARQ feedback may be different from the LTE feedback delay. The wording of the rules above must then be changed accordingly by adding an extra delay to rules 2 and 3. 
         [0038]    Said rules cause the HARQ feedback symbols to be mapped into the feedback report. To the beginning of the feedback report are mapped the feedback symbols corresponding to the first portion of downlink component carriers; and to the end of the feedback report the feedback symbols corresponding to the second portion of downlink component carriers. As an example for two HSDPA carriers the portion consists of all downlink component carriers. For more than two downlink component carriers the portion may be divided into several feedback reports. 
         [0039]    Embodiments of the present invention may be implemented in software, hardware, application logic or a combination of software, hardware and application logic. In an example embodiment, the application logic, software or instruction set is maintained on any one of various conventional computer-readable media. In the context of this document, a “computer-readable medium” may be any media or means that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer. A computer-readable medium may comprise a computer-readable storage medium that may be any media or means that can contain or store the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer. The exemplary embodiments can store information relating to various processes described herein. This information can be stored in one or more memories, such as a hard disk, optical disk, magneto-optical disk, RAM, and the like. One or more databases can store the information used to implement the exemplary embodiments of the present inventions. The databases can be organized using data structures (e.g., records, tables, arrays, fields, graphs, trees, lists, and the like) included in one or more memories or storage devices listed herein. The processes described with respect to the exemplary embodiments can include appropriate data structures for storing data collected and/or generated by the processes of the devices and subsystems of the exemplary embodiments in one or more databases. 
         [0040]    All or a portion of the exemplary embodiments can be conveniently implemented using one or more general purpose processors, microprocessors, digital signal processors, micro-controllers, and the like, programmed according to the teachings of the exemplary embodiments of the present inventions, as will be appreciated by those skilled in the computer and/or software art(s). Appropriate software can be readily prepared by programmers of ordinary skill based on the teachings of the exemplary embodiments, as will be appreciated by those skilled in the software art. In addition, the exemplary embodiments can be implemented by the preparation of application-specific integrated circuits or by interconnecting an appropriate network of conventional component circuits, as will be appreciated by those skilled in the electrical art(s). Thus, the exemplary embodiments are not limited to any specific combination of hardware and/or software. 
         [0041]    If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. 
         [0042]    Furthermore, if desired, one or more of the above-described functions may be optional or may be combined. Although various aspects of the invention are set out in the independent claims, other aspects of the invention comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims. 
         [0043]    It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.