Patent Publication Number: US-7916710-B2

Title: Method, device, system and software product for alternative time division duplex frame structure optimization

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     Priority is claimed to U.S. Provisional Application 60/856,498 filed Nov. 2, 2006. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of wireless telecommunications. More particularly, the present invention pertains to time division duplex techniques. 
     BACKGROUND OF THE INVENTION 
     The telecommunications industry is in the process of developing a new generation of flexible and affordable communications that includes high-speed access while also supporting broadband services. Many features of the third generation mobile telecommunications system have already been established, but many other features have yet to be perfected. 
     One of the systems within the third generation of mobile communications is the Universal Mobile Telecommunications System (UMTS) delivers voice, data, multimedia, and wideband information to stationary as well as mobile customers. UMTS is designed to accommodate increased system capacity and data capability. Efficient use of the electromagnetic spectrum is vital in UMTS. It is known that spectrum efficiency can be attained using frequency division duplex (FDD) or using time division duplex (TDD) schemes. Space division duplex (SDD) is a third duplex transmission method used for wireless telecommunications. 
     As can be seen in  FIG. 1 , the UMTS architecture consists of user equipment  102  (UE), the UMTS Terrestrial Radio Access Network  104  (UTRAN), and the Core Network  126  (CN). The air interface between the UTRAN and the UE is called Uu, and the interface between the UTRAN and the Core Network is called Iu. 
     The UTRAN consists of a set of Radio Network Subsystems  128  (RNS), each of which has geographic coverage of a number of cells  110  (C), as can be seen in  FIG. 1 . The interface between the subsystems is called Iur. 
     Each Radio Network Subsystem  128  (RNS) includes a Radio Network Controller  112  (RNC) and at least one Node B  114 , each Node B having geographic coverage of at least one cell  110 . As can be seen from  FIG. 1 , the interface between an RNC  112  and a Node B  114  is called Iub, and the Iub is hard-wired rather than being an air interface. For any Node B  114  there is only one RNC  112 . A Node B  114  is responsible for radio transmission and reception to and from the UE  102  (Node B antennas can typically be seen atop towers or preferably at less visible locations). The RNC  112  has overall control of the logical resources of each Node B  114  within the RNS  128 , and the RNC  112  is also responsible for handover decisions which entail switching a call from one cell to another or between radio channels in the same cell. 
     The FDD method uses separate frequency bands for uplink and downlink transmissions over the Uu interface (i.e. over the air interface between UTRAN  104  and the User Equipment  102 ). In contrast, the TDD method allocates different time slots (compared to different frequencies) for these uplink and downlink communications. Generally, TDD is very flexible regarding the allocation of time slots, and therefore is very well-suited to applications that are asymmetric with respect to uplink and downlink data volume (e.g. web browsing entails a much higher downlink than uplink data volume). 
     The TDD method uses the same frequency band but alternates the transmission direction in time. Further details can be found in the book  WCDMA for UMTS; Radio Access for third Generation Mobile Communications , Third Edition, c. 2004, Edited by Harri Holma and Antti Toskala, and Chapter 13 titled “UTRA TDD Modes” is incorporated by reference herein. The term downlink or forward link refers to transmission from the base station (fixed network side) to the mobile terminal (user equipment), and the term uplink or reverse link refers to transmission from the mobile terminal to the base station. Since TDD uses the same frequency band but alternates the transmission direction in time, the frame structure of TDD needs to be carefully and smartly designed. 
     LTE, or Long Term Evolution (also known as 3.9G), refers to research and development involving the Third Generation Partnership Project (3GPP) aimed at identifying technologies and capabilities that can improve systems such as the UMTS. 3GPP is working on a standardization of LTE wherein both FDD and TDD duplex mode will be considered equally important. 
     3 GPP TR  25.814 , Physical Layer Aspects for Evolved Universal Terrestrial Access  ( UTRA ) ( Release  7), Version 7.0.0 (2006-6), is hereby incorporated by reference in its entirety. TR 25.814 defines two frame-structure options for LTE TDD; one of the two options is a frame structure compatible with a low chip rate (LCR) TDD, in order to accommodate coexistence with LCR-TDD. The LCR-TDD-compatible frame structure includes both data timeslots (TS) length TS 0 -TS 6 , and special timeslots position: downlink pilot timeslot (DwPTS), guard period (GP 1 ), and uplink pilot timeslot (UpPTS). 
     Some characteristics specific to the TDD system are as follows. In either downlink or uplink, the transmission is discontinuous, switching between transmission directions requires time. Thus a Guard Period (GP) is needed in order to counter the propagation delay of the inter-site-distance (ISD) so as to avoid base station to base station interference. Timing advance can be used to avoid mobile terminal to mobile terminal interference. There is a wide consensus that the guard period (GP) should be variable and flexible to satisfy different inter-site distance (ISD). Another characteristic specific to the TDD system is that a downlink and uplink synchronization channel should be previously known to a mobile terminal, and provide high accuracy performance. A further characteristic specific to the TDD system is that the channel reciprocity in TDD should be utilized, with appropriate frame structure design. Design of the TDD frame structure should at least take these issues into account, in order to achieve high and robust performance. 
     According to TR 25.814, chapter 6.2.1.1.1, an alternative frame structure is listed for LTE TDD in order to co-exist with LCR-TDD. Unfortunately, the alternative frame structure described in chapter 6.2.1.1.1 does not adequately take into account the issues described above, and therefore does not achieve high and robust performance. There are several reasons for this. First, the maximum inter-site distance (ISD) is upper-bounded by a fixed value of GP 1  (75 μs). Second, the coverage, performance and even functionality of a random access channel (RACH) could become degraded due to the short length for the RACH preamble, which is only 125 μs for TDD RACH compared with 1 ms for FDD RACH. Third, it is difficult and inefficient to utilize the non-synchronization-channel (i.e. non-SCH) and non-RACH subcarriers in DwPTS and UpPTS, respectively, or those subcarriers are in practice not even usable, so that the time domain occupancy efficiency is decreased for LTE TDD. 
     Thus, the alternative frame structure described in TR 25.814, chapter 6.2.1.1.1 has advantages and disadvantages. Among the disadvantages are the following four items. 
     First, the downlink synchronization channel (SCH) is always transmitted in the DwPTS of its central 72 subcarriers, according to R1-062786 CATT, “ SCH Structure and Cell Search Method for E - UTRA TDD system ”, Seoul, Korea, Oct. 9-13, 2006 which is hereby incorporated by reference in its entirety. Also incorporated by reference herein is R1-062785 CATT, Huawei, ZTE, RITT, “ Consideration on the non - synchronized random access procedure for EUTRA TDD ”, Seoul, Korea, Oct. 9-13, 2006. This means that, when operating at a higher channel bandwidth that corresponds to 72 subcarriers, the remaining subcarriers cannot be utilized for data transmission in any meaningful manner. 
     A second disadvantage is that the GP 1  is fixed to 75 μs. Third, the uplink synchronization channel (RACH) is transmitted in UpPTS of its one or multiple 72 (or 12 depending on the final conclusion in 3 GPP) subcarriers. And, fourth, the timeslot interval (TI) at the end of TS 0  is useless, because TS 0  and DwPTS are always for downlink transmission. 
     Among the advantages of the alternative frame structure described in TR 25.814, chapter 6.2.1.1.1 are the following. First, the relative position between SCH and GP 1  is fixed, and thus no extra signaling is needed to inform mobile terminals where the downlink-to-uplink switching point is. Second, the RACH is right after SCH and GP 1 , and the time distance between RACH and SCH is fixed and is the shortest possible, so that the channel reciprocity can be most efficiently utilized for initial non-synchronized RACH based on the latest channel information from SCH. Third, the exact timing enables co-existence with LCR-TDD. 
     SUMMARY OF THE INVENTION 
     The present invention provides a modification to the existing alternative TDD frame structure, in such a way as to correct the above-described problems in the existing frame structure, while the advantages described above may be maintained. This invention is not just limited to the context of 3GPP, but rather has potential applications in other present and future contexts and systems. 
     Specifically, the invention formulates a special new timeslot TS 0 *. This timeslot merges the existing TS 0  slot with the downlink pilot timeslot (DwPTS). 
     In one embodiment of the present invention, there is also a new special timeslot TS 1 *. This timeslot merges the existing TS 1  and uplink pilot timeslot (UpPTS). As shown in  FIG. 1 , user equipment  102  is configured to put the UpPTS in TS 1 . 
     The new special timeslot structure is characterized by the following properties. A guard period (GP) is first reserved for 50 μs (the current GP 1  value). If larger inter-site distance (ISD) is required, then more symbols from the end of TS 0  can be flexibly allocated to the guard period, symbol by symbol. Some principles of TDD operation are illustrated in  FIG. 2 , wherein the guard period separates TS 0  from TS 1 , instead of separating pilot timeslots. 
     A synchronization channel (SCH), which may consist of primary and secondary SCHs, is transmitted at a fixed location inside timeslot TS 0 *, e.g. 2 nd  and 3 rd  symbol of TS 0 *, as opposed to the current fixed location in DwPTS. This enables the user equipment (UE) to detect the start of a TDD frame, based on the SCH decoding. 
     The random access channel (RACH), including both non-synchronized and synchronized, may be transmitted in TS 1 * (0.825 ms). This yields a similar performance as LTE FDD and LTE TDD with generic frame structure in a macro cell environment. A further property of the new special timeslot structure is that all possible resources in both time and frequency domains can be utilized. 
     As an alternative way of implementing this invention, the timeslot structure of LCR-TDD remains unchanged, but the mapping of physical channels onto these timeslots is modified accordingly. For example, the LTE non-synchronized RACH may be mapped to cross the original timeslot TS 1  and UpPTS. Also, the shared data channel may be mapped to cross these two adjacent timeslots. 
     The same principle can additionally or alternatively be applied to the two downlink timeslots TS 0  and DwPTS. Thus, one physical channel may be mapped to more than one timeslot, for example the more than one timeslot being the original timeslot TS 0  and the DwPTS. 
     Advantages of this invention include the ability to support variable ISD deployment. Also, as mentioned, all possible resources in both time and frequency domain are utilized. This is accomplished while maintaining many, if not all, benefits of the existing frame structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a UTRAN system. 
         FIG. 2  shows principles of TDD operation. 
         FIG. 3  shows an LTE TDD alternative frame structure 
         FIG. 4  shows a refined LTE TDD alternative frame structure 
         FIG. 5  is a flow chart illustrating an embodiment of the present invention. 
         FIG. 6  is a block diagram illustrating a further embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the present invention will now be described. This is merely to illustrate one way of implementing the invention, without limiting the scope or coverage of what is described elsewhere in this application. 
     A frame structure compatible with a low chip rate (LCR) TDD, in order to accommodate coexistence with LCR-TDD, is shown in present  FIG. 3 . Timeslots  0 - 6  are separated by timeslot intervals  30 . Also, downlink pilot timeslot  306 , guard period  307 , and uplink pilot timeslot  308  are located between timeslot  0  and timeslot  1 . A switching point  305  from downlink to uplink (DUSP) occurs several timeslots before a switching point  310  from uplink to downlink. The present invention either maintains  FIG. 3 , while mapping physical channels onto these timeslots in a modified manner, or includes a special new timeslot TS 0 * which merges the existing TS 0  slot with the downlink pilot timeslot (DwPTS). 
     In one embodiment of the present invention, a new special timeslot TS 1 * is formulated which merges the existing TS 1  and uplink pilot timeslot (UpPTS). This modified frame structure is illustrated in  FIG. 4  (with the asterisks removed). Only guard period  407  is located between timeslot  0  and timeslot  1 . A switching point  405  indicates a shift from downlink to uplink (DUSP). 
     Several concepts included in the present invention will now be briefly described, without in any way limiting what may ultimately be claimed in this application. 
     The invention includes a first concept which is a method comprising: forming a time division duplex frame structure; including (e.g. mapping or merging) a downlink pilot timeslot within an initial data timeslot of the frame structure; and, including an uplink pilot timeslot in a further data timeslot of the frame structure, wherein a guard period is located between the initial data timeslot and the further data timeslot. 
     The invention includes a second concept which is the method of the first concept, further comprising allocating symbols from the initial data timeslot to the guard period to provide a larger inter-site distance. 
     The invention includes a third concept which is the method of the first concept, further comprising transmitting a synchronization channel at a fixed location within the initial data timeslot. 
     The invention includes a fourth concept which is the method of the third concept, wherein the synchronization channel comprises a primary synchronization channel and a secondary synchronization channel. 
     The invention includes a fifth concept which is the method of the third concept, wherein the fixed location is at the second and third symbols of the initial data timeslot. 
     The invention includes a sixth concept which is the method of the first concept, further comprising transmitting a random access channel in the further data timeslot. 
     The invention includes a seventh concept which is the method of the sixth concept, wherein the random access channel includes both a non-synchronized and a synchronized channel. 
     The invention includes an eighth concept which is the method of the first concept, wherein the first data timeslot and the further data timeslot are two of only six data timeslots within the frame structure. 
     The invention includes a ninth concept which is a software product configured to perform the method of any of the first thru eighth concepts. 
     The invention includes a tenth concept which is an apparatus comprising: means for forming a time division duplex frame structure; means for including a downlink pilot timeslot within an initial data timeslot of the frame structure; and, means for including an uplink pilot timeslot in a further data timeslot of the frame structure, wherein a guard period is located between the initial data timeslot and the further data timeslot. 
     The invention includes an eleventh concept which is the apparatus of the tenth concept, further comprising means for allocating symbols from the initial data timeslot to the guard period to provide a larger inter-site distance. 
     The invention includes a twelfth concept which is the apparatus of the tenth concept, further comprising means for transmitting a synchronization channel at a fixed location within the initial data timeslot. 
     The invention includes a thirteenth concept which is the apparatus of the twelfth concept, wherein the synchronization channel comprises a primary synchronization channel and a secondary synchronization channel. 
     The invention includes a fourteenth concept which is the apparatus of the twelfth concept, wherein the fixed location comprises the second and third symbols of the initial data timeslot. 
     The invention includes a fifteenth concept which is the apparatus of the tenth concept, further comprising means for transmitting a random access channel in the further data timeslot. 
     The invention includes a sixteenth concept which is the apparatus of the fifteenth concept, wherein the random access channel comprises both a non-synchronized and a synchronized channel. 
     The invention includes a seventeenth concept which is the apparatus of the tenth concept, wherein the first data timeslot and the further data timeslot are two of only six data timeslots within the frame structure. 
     The invention includes an eighteenth concept which is any combination of the tenth thru seventeenth concepts. 
     The invention includes a nineteenth concept which is a software product comprising a computer readable medium having executable codes embedded therein; the codes, when executed, adapted to carry out the functions of: forming a time division duplex frame structure; including a downlink pilot timeslot within an initial data timeslot of the frame structure; and, including an uplink pilot timeslot in a further data timeslot of the frame structure, wherein a guard period is located between the initial data timeslot and the further data timeslot. 
     The invention includes a twentieth concept which is the software product of the nineteenth concept, wherein the functions further comprise allocating symbols from the initial data timeslot to the guard period to provide a larger inter-site distance. 
     The invention includes a twenty-first concept which is the software product of the nineteenth concept, wherein the functions further comprise transmitting a synchronization channel at a fixed location within the initial data timeslot. 
     The invention includes a twenty-second concept which the software product of the twenty-first concept, wherein the synchronization channel comprises a primary synchronization channel and a secondary synchronization channel. 
     The invention includes a twenty-third concept which is a network element comprising: a processor module configured to form a time division duplex frame structure having a downlink pilot timeslot within an initial data timeslot of the frame structure; and, a further processor module configured to include a guard period located between the initial data timeslot and a further data timeslot. 
     The invention includes a twenty-fourth concept which is the network element of the twenty-third concept, further comprising an allocation module configured to allocate symbols from the initial data timeslot to the guard period in order to provide a larger inter-site distance. 
     The invention includes a twenty-fifth concept which is the network element of the twenty-third concept, further comprising a memory module configured to store and supply the data for the initial data timeslot and the further data timeslot. 
     The invention includes a twenty-sixth concept which is a mobile device comprising: a processor module configured to form a time division duplex frame structure having an initial data timeslot, and also having an uplink pilot timeslot located within a further data timeslot of the frame structure; and, a further processor module configured to include a guard period located between the initial data timeslot and the further data timeslot. 
     The invention includes a twenty-seventh concept which is the mobile device of the twenty-sixth concept, further comprising an allocation module configured to allocate symbols from the initial data timeslot to the guard period in order to provide a larger inter-site distance. 
     The invention includes a twenty-eighth concept which is the mobile device of the twenty-sixth concept, further comprising a memory module configured to store and supply the data for the initial data timeslot and the further data timeslot. 
     The invention includes a twenty-ninth concept which is an apparatus comprising: means for forming a time division duplex frame structure having a downlink pilot timeslot within an initial data timeslot of the frame structure; and, means for including a guard period located between the initial data timeslot and a further data timeslot. 
     The invention includes a thirtieth concept which is the apparatus of the twenty-ninth concept, further comprising means for allocating symbols from the initial data timeslot to the guard period in order to provide a larger inter-site distance. 
     The invention includes a thirty-first concept which is an apparatus comprising: means for forming a time division duplex frame structure having an initial data timeslot, and also having an uplink pilot timeslot located within a further data timeslot of the frame structure; and, means for including a guard period located between the initial data timeslot and the further data timeslot. 
     The invention includes a thirty-second concept which is the apparatus of the thirty-first concept, further comprising means for allocating symbols from the initial data timeslot to the guard period in order to provide a larger inter-site distance. 
     The invention includes a thirty-third concept which is any of the first thru thirty-second concepts, wherein the frame structure remains unchanged, but mapping of physical channels onto timeslots is modified. 
     The invention includes a thirty-fourth concept which is a method comprising: forming a time division duplex frame structure; mapping a downlink pilot timeslot within an initial data timeslot of the frame structure; and, mapping an uplink pilot timeslot in a further data timeslot of the frame structure, wherein a guard period is located between the initial data timeslot and the further data timeslot. 
     The invention includes a thirty-fifth concept which is the method of the thirty-fourth concept, further comprising allocating symbols from the initial data timeslot to the guard period to provide a larger inter-site distance. 
     The invention includes a thirty-sixth concept which is the thirty-third or thirty-fourth concept, wherein a non-synchronized random access channel is mapped to cross the further data timeslot and the uplink pilot timeslot. 
     The invention includes a thirty-seventh concept which is the thirty-third or thirty-fourth concept, wherein a synchronization channel is mapped to cross the initial data timeslot and the downlink pilot timeslot. 
     For example, the LTE non-synchronized RACH may be mapped to cross the original timeslot TS 1  and UpPTS. Also, the shared data channel may be mapped to cross these two adjacent timeslots. The same principle can be applied to the two downlink timeslots TS 0  and DwPTS. 
     The embodiments described above can be implemented using a general purpose or specific-use computer system, with standard operating system software conforming to the method described herein. The software is designed to drive the operation of the particular hardware of the system, and will be compatible with other system components and I/O controllers. The computer system of this embodiment includes a CPU processor, comprising a single processing unit, multiple processing units capable of parallel operation, or the CPU can be distributed across one or more processing units in one or more locations, e.g., on a client and server. A memory may comprise any known type of data storage and/or transmission media, including magnetic media, optical media, random access memory (RAM), read-only memory (ROM), a data cache, a data object, etc. Moreover, similar to the CPU, the memory may reside at a single physical location, comprising one or more types of data storage, or be distributed across a plurality of physical systems in various forms. 
     A person skilled in the art will understand that it is possible to merge the two-TDD frame structure into a one-frame structure, while keeping the idea of “special timeslots” (e.g. DwPTS, GP and UpPTS) in the newly formed TDD frame structure. The idea of combining DwPTS with the preceding DL timeslot/subframe, as one longer slot for one data packet transmission or physical channel, would then remain a useful innovation. Likewise, it would also remain useful to combine the UpPTS with the following UL timeslot/subframe, as one longer slot for one data packet transmission or physical channel. A physical channel can then be mapped into more than one timeslot, e.g. DL channel is mapped to subframe #0 plus DwPTS, and UL channel is mapped to UpPTS plus subframe #2. 
     A person skilled in the art will also understand that the present invention can address a problem scenario that is not limited to the frame structure described in 3 GPP TR  25.814 , Physical Layer Aspects for Evolved Universal Terrestrial Access  ( UTRA ) ( Release  7), Version 7.0.0 (2006-6). Instead, the invention can address a more general scenario, including in the context of Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). Likewise, the number of timeslots or subframes per frame need not be limited to only TS 0 -TS 6 , and instead can generally be any numbers depending on the frame length (e.g. it may be 10 ms or 20 ms frame) and timeslot/subframe length (e.g. they can be 0.5 ms or 1 ms), resulting in different numbers of timeslots or subframes per radio frame. 
     A method  500  according to an embodiment of the present invention is shown in  FIG. 5 . A TDD frame structure is formed  505 . A downlink pilot timeslot (DwPTS) is included  520  (e.g. via merging or mapping) within an initial data timeslot. An uplink pilot timeslot (UpPTS) is also included  525  in a further data timeslot. A guard period is located  550  between the initial data timeslot and the further data timeslot. And, symbols are allocated  555  from the initial data time slot and or from the further data timeslot to the guard period, so as to support a larger inter-site distance. 
     Referring now to the simplified block diagram of  FIG. 6 , an apparatus  600  is shown according to an embodiment of the present invention. The first processor module  610  creates a TDD frame structure. The second processor module  620  puts the DwPTS in the initial data timeslot TS 0 * (e.g. via merging or mapping). The third processor module  630  puts the UpPTS in a further data timeslot TS 1 *, with a guard period (GP) located between the TS 0 * and the TS 1 * (although the guard period may or may not be immediately adjacent to TS 0 * or TS 1 *). It is noted that the parts of the apparatus  600  can be located at a single location, or at different locations within a telecommunications network; for example, the second processor module may be located at a network element and/or the like, whereas the third processor module may be located at a mobile device and/or the like. 
     It is to be understood that the present figures, and the accompanying narrative discussions of best mode embodiments, do not purport to be completely rigorous treatments of the method, system, mobile device, network element, and software product under consideration. A person skilled in the art will understand that the steps and signals of the present application represent general cause-and-effect relationships that do not exclude intermediate interactions of various types, and will further understand that the various steps and structures described in this application can be implemented by a variety of different sequences and configurations, using various different combinations of hardware and software which need not be further detailed herein.