The present invention is related to a wireless communication system. More particularly, the present invention is related to a synchronization channel (SCH) for evolved universal terrestrial radio access (E-UTRA) downlink transmissions and corresponding cell search procedures.
The long term evolution (LTE) of wideband code division multiple access (WCDMA) Third Generation Partnership Project (3GPP) cellular networks describes universal mobile telecommunications systems (UMTS) beyond 3GPP Release 7. LTE is also sometimes described by E-UTRA. In order to keep third generation (3G) technology competitive, both 3GPP and 3GPP2 are considering LTE, in which evolution of radio interface and network architecture is necessary.
Currently, orthogonal frequency division multiple access (OFDMA) is being considered for the downlink of E-UTRA. When a wireless transmit/receive unit (WTRU) is powered up, (i.e., activated), in an evolved universal terrestrial radio access network (E-UTRAN) where the downlink is OFDMA based, the WTRU must synchronize the frequency, frame timing and the fast Fourier transform (FFT) symbol timing with the (best) cell, and determine the cell identifier (ID). This process is called cell search.
The SCH and cell search process for OFDMA-based downlink are currently being studied in E-UTRA. It would be desirable to define a synchronization channel that is common for all cells in the system. Cell search procedures for E-UTRA preferably cause a small delay, result in satisfactory cell search performance, minimize system overload, and require low computational complexity.
Therefore, an appropriate synchronization channel and a corresponding cell search procedure for use in E-UTRA are desired.
In an OFDMA based system, a cell search method uses a primary synchronization channel (P-SCH) and optionally a secondary synchronization channel (S-SCH). Depending on the mapping scheme to each system transmission bandwidth, the P-SCH will use the same number of subcarriers for all possible bandwidths, or a different number of subcarriers according to the available P-SCH bandwidth centered within the system transmission bandwidth. A P-SCH symbol is transmitted at least one time during one radio frame. When several symbols are sent in one frame, then there can be either an equal time interval between symbols or an unequal time interval between symbols.
P-SCH symbols are processed to obtain initial detection of framing timing, orthogonal frequency division multiplexing (OFDM) symbol timing, cell ID, frequency offset and bandwidth. Optionally, a self check and correction of an OFDM symbol timing error is performed.
In one embodiment, polyphase codes with time reversal properties are preferably used to generate synchronization symbols. In an alternative embodiment, multiple synchronization channels are disclosed for enhancing cell search performance.
A more detailed understanding of the invention may be had from the following description of a preferred embodiment, given by way of example and to be understood in conjunction with the accompanying drawings wherein:
When referred to hereafter, the terminology “wireless transmit/receive unit (WTRU)” includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a computer, or any other type of user device capable of operating in a wireless environment.
When referred to hereafter, the terminology “base station” includes but is not limited to a Node-B, a site controller, an access point (AP), or any other type of interfacing device capable of operating in a wireless environment.
The present invention applies to the physical layer in a radio access communication network. Furthermore, the invention relates to the radio interface and the digital baseband subsystem of a wireless communication system.
The present invention is related to a synchronization channel and corresponding cell search procedures for E-UTRA. WTRUs process synchronization symbols to acquire frequency and time synchronization. A P-SCH enables at least the initial acquisition of symbol timing.
In a first embodiment of the present invention, it is possible that only one or more P-SCH symbols is transmitted. The P-SCH implicitly carries cell information such as cell ID. The WTRU can process P-SCH symbols to obtain OFDM symbol timing, frame timing, cell ID and other information. If the P-SCH is designed in such a way that the WTRU can detect the number of transmit antennas at the cell site, then the system does not have to transmit S-SCH symbols at all. Otherwise, one or more S-SCH symbols carrying information about a number of antennas will be transmitted.
Pseudorandom code sequences are preferably used to build synchronization symbols for P-SCH. The pseudorandom code sequences used by the present invention include, but are not limited to, generalized chirp-like (GCL), Zadoff-Chu, Frank, Golay, and Barker codes. A cell/sector specific code sequence will be used to implicitly carry cell ID information on the P-SCH or mitigate the intercell interference on the P-SCH.
Depending on the bandwidth of the cell, the number of points of DFT and IFFT may be different for different cell bandwidths. If the P-SCH is mapped to the central 1.25 MHZ and 5 MHZ portions of the system transmission bandwidth, regardless of the transmission bandwidth of the system as shown in
If the P-SCH is mapped to the central 1.25 MHZ and 5 MHZ portions of the system transmission bandwidths, as shown in
If the number of subcarriers used by the P-SCH is less than the number of available subcarriers, those subcarriers not used by the P-SCH will be set with zeros or carry user data.
Several possible frame formats are proposed by the present invention. Basically, the P-SCH symbol should be transmitted one or several times during one radio frame (of length 10 ms). If there are several P-SCH symbols in one radio frame, there can be equal or unequal intervals between P-SCH symbols. Compared to equal intervals, unequal intervals between P-SCH symbols may help the WTRU to better locate a frame boundary.
An exemplary frame format of P-SCH symbols with equal time intervals is shown in
An exemplary frame format of P-SCH symbols with unequal time intervals is shown in
The proposed cell search method includes processing one or more P-SCH symbols and, optionally, one or more S-SCH symbols, to obtain frame timing, OFDM symbol timing, cell ID, frequency offset, bandwidth and the like. A self-check procedure is performed, and any existing OFDM symbol timing errors are corrected.
An example of initial detection of frame timing, OFDM symbol timing and other information is performed by a process 600 shown in
In accordance with another embodiment of the present invention, a WTRU can process one or more P-SCH symbols to obtain OFDM symbol timing, frame timing and other information. In this embodiment, the P-SCH does not carry cell information, such as the cell ID. Therefore, the WTRU needs to process the S-SCH symbols to obtain information such as cell ID.
Pseudorandom code sequences are used to build synchronization symbols for the P-SCH. The pseudorandom code sequences may be Zadoff-Chu codes, Golay codes, Barker codes and the like. A common code sequence will be used for all cells/sectors.
Depending on the bandwidth of the cell, the number of points of DFT and IFFT may be different. If P-SCH is mapped to the central 1.25 MHz of the system transmission bandwidth as shown in
If P-SCH is mapped to the central 1.25 MHz and 5.0 MHz of the system transmission bandwidth as shown in
If the number of subcarriers used by the P-SCH is less than the number of available subcarriers, subcarriers not used by the P-SCH will be put zeros or carry user data.
Several possible methods of P-SCH symbol mapping within a frame for the second embodiment are proposed. Basically, the P-SCH symbol should be transmitted one or several times during one radio frame, (of length 10 ms), and the S-SCH symbol may be transmitted, (optional, depending on the conditions described earlier), one or several times during one radio frame. The number of P-SCH and S-SCH symbols may not be the same. S-SCH symbol(s) should be transmitted after P-SCH symbol(s). If there are several P-SCH symbols in one radio frame, there can be equal or unequal intervals between P-SCH symbols. Compared to equal intervals, unequal intervals between P-SCH symbols may help the WTRU to better locate a frame boundary. Although P-SCH symbols are placed in the first OFDM symbol of a sub-frame in
The cell search method according to the second embodiment of the present invention will now be described. The P-SCH symbol is processed first to obtain initial OFDM symbol timing and frame timing in the same way as the first embodiment. The difference is that cell ID information cannot be obtained by processing of P-SCH symbol. The OFDM symbol timing obtained above may have errors. The proposed P-SCH symbol structure allows self-check and correction of timing errors in the same manner as previously described.
The present invention may be implemented in a WTRU, base station, network or system, at the physical layer (radio/digital baseband), as a digital signal processor (DSP) or application specific integrated circuit (ASIC). The present invention is applicable to 3GPP long term evolution (LTE) based communication air interfaces. Although the present invention has been described in reference to evolved UTRA or LTE, the method can also be readily applied to any OFDMA based system.
In accordance with another embodiment of the present invention, synchronization symbol(s) that implicitly carry information of cell/sector ID (or cell/sector group index) are utilized. Potentially, pseudorandom code sequences with zero auto correlation, (for example, GCL code, Zadoff-Chu code, Polyphase code and the like), are used to build synchronization symbols. Alternatively, cell-specific codes can be used to implicitly carry information such as cell/sector ID. In the frequency domain, the synchronization sequence, (i.e., code sequence), is mapped to equal-spaced subcarriers. The preferred distance between subcarriers used by one synchronization symbol is four subcarriers. That is, if a subcarrier s is used by the SCH, then subcarriers s+4, s+8, and so on, are used as well. Therefore, for one synchronization symbol there are four non-overlapping subcarrier mapping patterns, namely 1, 2, 3 and 4, respectively.
Referring to
In another embodiment shown in
There are several possible formats of time reversal. For the first and second blocks, the synchronization sequence A contained in one block has the following property:
A(k)=±A(2Np+1−k), k=1, 2, . . . , Np, Equation (1)
or
A(k)=±(A(2Np+1−k)*, k=1, 2, . . . , Np, Equation (2)
where ( )* is the conjugate operator. Similarly for the third and fourth blocks, the synchronization sequence A contained in one block has the following property:
A(k)=±A(4Np+1−k), k=2Np+1, 2Np+2, . . . , 3Np, Equation (3)
A(k)=±(A(4Np+1−k))*, k=2Np+1, 2Np+2, . . . , 3Np. Equation (4)
Both synchronization symbol formats in
Depending on the bandwidth of the cell, the number of subcarriers used by a synchronization symbol may be the same or different for different cell bandwidths. For example, a synchronization symbol is mapped to the central 1.25 MHz of the bandwidth regardless of the transmission bandwidth of the system, as shown in
K synchronization symbols should be transmitted per radio frame (10 msec), where K is a design parameter whose value is preferably larger than one in order to obtain good cell search performance in a reasonably short time. Those K synchronization symbols can be transmitted concatenated or separated in time. When synchronization symbols are transmitted separated in time, equal-distance between symbols is preferred, making it easier for the receiver to combine the received synchronization symbols.
If the synchronization channel in accordance with the embodiments of the present invention as described above cannot carry all the information a WTRU needs for synchronization, then an S-SCH may be required. Where an S-SCH is required, a fixed timing should exist between the P-SCH and the S-SCH.
Where both a P-SCH and an S-SCH are utilized, subcarrier mapping patterns Mi(p) is applied to the ith synchronization symbol of cell p. It should be noted that it is possible that Mi(p)=Mj(p) for i≠j. In another embodiment of the invention, for each synchronization symbol, different (non-overlapping) subcarrier mapping patterns are used at neighboring cells/sectors. That is, for cells p and q (p≠q) and each synchronization symbol i, Mi(p)≠Mi(q). In this way, interference of synchronization symbols from neighboring cells/sectors may be reduced, which in turn improves cell search performance. An example of this embodiment is shown in
In yet another embodiment, it is possible that all synchronization symbols in one frame use the same subcarrier mapping pattern. One example is shown in
Let Ci(p) denote the code used in the ith synchronization symbol of cell/sector p. It should be noted that it is possible that Ci(p) =Cj(p) for i≠j. Since more than one synchronization symbol (i.e. K>1) is transmitted per radio frame, combined code indices (and potentially mapping patterns as well) are used to implicitly carry cell/sector ID information. In this way, the number of cell/sector IDs that can be represented by the synchronization symbols is increased remarkably.
The cell/sector ID of cell/sector p can be mapped to the combination of code indices used in the K synchronization symbols. As depicted by Equation (5) below:
Cell_IDp=f(C1(p), C2(p), . . . , CK(p)). Equation (5)
Alternatively, the cell/sector ID of cell/sector p can be mapped to the combination of code indices and mapping patterns used in the K synchronization symbols. As depicted by Equation (6) below:
Cell_IDp=f(C1(p), C2(p), . . . , CK(p), M1(p), M2(p), . . . , MK(p)). Equation (6)
In this way, a large number of cell/sector indices can be supported by the synchronization channel. For example, seventy-six (76) subcarriers in the center can be used for purposes of synchronization and K=2 synchronization symbols are transmitted per radio frame. Since an equal-spaced subcarrier mapping with distance of four subcarriers is used, pseudorandom codes with length of 19 will be used for synchronization symbols. The number of cell/sector indices that can be supported is 361 if the cell/sector ID of cell/sector p is mapped to the combination of code indices used in the two synchronization symbols. For the K>2 case, cell/sector ID can be mapped to the combination of code indices in a similar way.
Where an S-SCH is used, S-SCHs of different sectors are preferably transmitted on different subcarriers to avoid, (or mitigate), the intercell interference on S-SCHs. For each sector, equal-distant subcarriers are preferably used for the S-SCH. The distance is preferably equal to the number of sectors. For example, the distance between subcarriers used for the S-SCH is three in a cell site with three sectors. Alternatively, a pre-defined mapping between subcarrier positions of S-SCH and cell/sector ID, (or just the code index used by P-SCH symbols), may be used. Hence, once the WTRU detects the cell/sector ID, it knows the subcarriers' positions to receive the S-SCH.
The present invention may be implemented in a UE, a base station, and generally in a wireless communication network or system comprising both a WTRU and a base station. The present invention may also be implemented in an application specific integrated circuit (ASIC), or a digital signal processor.
Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention. The methods or flow charts provided in the present invention may be implemented in a computer program, software, or firmware tangibly embodied in a computer-readable storage medium for execution by a general purpose computer or a processor. Examples of computer-readable storage mediums include a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs).
Suitable processors include, by way of example, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) circuits, any other type of integrated circuit (IC), and/or a state machine.
A processor in association with software may be used to implement a radio frequency transceiver for use in a wireless transmit receive unit (WTRU), user equipment (UE), terminal, base station, radio network controller, or any host computer. The WTRU may be used in conjunction with modules, implemented in hardware and/or software, such as a camera, a video camera module, a videophone, a speakerphone, a vibration device, a speaker, a microphone, a television transceiver, a hands free headset, a keyboard, a Bluetooth® module, a frequency modulated (FM) radio unit, a liquid crystal display (LCD) display unit, an organic light-emitting diode (OLED) display unit, a digital music player, a media player, a video game player module, an Internet browser, and/or any wireless local area network (WLAN) module.
CROSS REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Application No. 60/752,317 filed Dec. 21, 2005 and U.S. Provisional Application No. 60/765,421 filed Feb. 3, 2006, which are incorporated by reference as if fully set forth.
Number | Date | Country | |
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60752317 | Dec 2005 | US | |
60765421 | Feb 2006 | US |