HK1232714B - Downlink synchronization channel and methods for cellular systems - Google Patents

Description

Downlink synchronization channel and method for cellular system

The present application is a divisional application of Chinese patent application No. 200780014079.1 (PCT/US2007/067137) filed on April 20, 2007 and entitled “Downlink Synchronization Channel and Method for Cellular System”.

Technical Field

The present invention is directed generally to wireless communications and, more particularly, to a transmitter and receiver, methods of operating the transmitter and receiver, and a cellular communication system employing the transmitter, receiver, and method.

Background Art

In cellular networks, such as those employing Orthogonal Frequency Division Multiplexing (OFDM) or Orthogonal Frequency Division Multiple Access (OFDMA), each cell employs a base station that communicates with user devices (such as cell phones, laptops, or PDAs) that are dynamically located within that cell. When a user device is first turned on, it must perform an initial cell search in order to connect to the cellular network. This involves a downlink synchronization process between the base station and the user device, in which the base station sends a synchronization signal to the user device. This synchronization signal is typically referred to as a synchronization preamble in IEEE 802.16e or a synchronization channel (SCH) in 3GPP WCDMA/HSDPA.

During the initial cell search, the user equipment establishes timing and frequency offset parameters. Timing involves knowing where to start sampling the synchronization frame and associated symbols. Frequency offset involves determining and controlling the mismatch between the base station oscillator and the local oscillator in the user equipment.

Depending on the quality of the local oscillator, the frequency offset can be large and require considerable search time and additional algorithms to adapt to. This effect is exacerbated if the user equipment is moving at car or train speeds. In addition to timing and frequency considerations, certain information specific to the initial cell must be acquired, such as the physical cell identity (cell ID). Because downlink synchronization involves several operations, the design and process of downlink synchronization should attempt to minimize receiver complexity and the time required for cell search. To help reduce complexity, the synchronization signal can be composed of two parts: a primary synchronization signal and a secondary synchronization signal. The primary synchronization signal is typically used for timing and frequency acquisition, while the secondary synchronization signal is typically used to acquire the cell ID and other cell-specific information. Unlike the secondary synchronization signal, the primary synchronization signal is typically the same for all cells. The primary synchronization signal carries the primary synchronization signal sequence. To ensure excellent performance, the primary synchronization signal is used to obtain the channel estimate necessary to decode the cell-specific information in the secondary synchronization signal via coherent detection.

As a mobile user equipment approaches a cell border between two adjacent cells, the user equipment performs a neighbor cell search in preparation for switching its activity from the initial cell to the neighboring cell. At this point, the user equipment receives information from two or more base stations. When base stations use a shared master signal sequence, this shared signal can create a mismatch between the channel traversed by the cell-specific transmission and the master signal transmitted for the user equipment. This mismatch is particularly severe for terminals at the cell edge, where each terminal receives two equally strong and overlapping channels from two important base stations. Another issue associated with the shared master synchronization sequence is a timing mismatch between the channel traversed by the master sequence and the cell-specific data transmission. In this case, the timing derived from the master sequence can lead to performance degradation when used to demodulate the cell-specific data transmission. This phenomenon occurs particularly in strictly synchronized networks, such as those used in the United States and Japan, and has become increasingly common for media with large cell radii. Furthermore, advanced cellular OFDM systems, such as 3GPP E-UTRA (Enhanced UMTS Terrestrial Radio Access) or Long-term Evolution (LTE), adapted for single-frequency networks (SFNs) used for enhanced multimedia broadcast and multicast systems (E-MBMS), rely heavily on network synchronization. However, this phenomenon also applies to initial cell searches and is particularly problematic for neighbor cell searches, where the operating signal-to-noise ratio (SNR) is quite low. This performance degradation results in longer cell search times, which can lead to a higher probability of signal interruption during handovers.

Therefore, there is a need in the art for an enhanced approach to perform initial and neighbor cell searches.

Summary of the Invention

To address the aforementioned shortcomings in the art, the present invention provides a base station transmitter for use with OFDM and OFDMA communication systems. The base station transmitter includes a synchronization unit configured to provide a cellular downlink synchronization signal having a primary portion and a secondary portion, wherein the primary portion utilizes a respective one of a plurality of different synchronization signals (or primary synchronization sequences) assigned to different transmitting cells, and the secondary portion provides cell-specific information. The base station transmitter also includes a transmitting unit configured to transmit the cell downlink synchronization signal. In one embodiment, a set of primary synchronization codes is used to represent partial cell ID information, thereby reducing the amount of cell ID information required to be carried by the secondary portion. In another embodiment, the set of primary synchronization codes does not carry any cell ID information.

The present invention also provides a user equipment receiver for use with an OFDM or OFDMA communication system. In one embodiment, the user equipment receiver includes a receiving unit configured to receive a cellular downlink synchronization signal having a primary portion and a secondary portion, wherein the secondary portion provides cell-specific parameters. The user equipment receiver also includes a processing unit configured to identify and extract the secondary portion.

In another embodiment, the method includes providing a cellular downlink synchronization signal having a primary synchronization signal and a secondary synchronization signal, wherein the primary portion utilizes a corresponding one of a plurality of different primary synchronization signals (or primary synchronization sequences) assigned to adjacent transmitting cells. The method also includes further providing cell-specific information in the secondary portion and transmitting the cellular downlink synchronization signal.

The present invention also provides a method of operating a user equipment receiver for use with an OFDM or OFDMA communication system. The method comprises receiving a cellular downlink synchronization signal having a primary portion and a secondary portion, wherein the secondary portion provides cell-specific parameters, identifying and extracting the secondary portion.

In another aspect, the present invention also provides a cellular communication system. The cellular communication system includes a manager that assigns a set of primary synchronization sequences to a plurality of cells and a central cellular transmitter that provides a cellular downlink synchronization signal using one of the primary synchronization sequences. The cellular communication system also includes a plurality of neighboring cellular transmitters that provide cellular downlink synchronization signals having corresponding primary synchronization sequences that are distinguishable from the one primary synchronization sequence.

The foregoing summarizes preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention below. Additional features of the present invention will be described below, which form the subject matter of the claims of the present invention. Those skilled in the art will appreciate that they may readily use the concepts and specific embodiments disclosed herein as a basis for designing or modifying other structures for carrying out the same purposes as the present invention. Those skilled in the art will also recognize that such equivalent constructions do not depart from the spirit and scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is made to the following description taken in conjunction with the accompanying drawings, in which:

FIG1 is a diagram illustrating an embodiment of a cellular network constructed according to the principles of the present invention;

FIG2 shows an exemplary primary synchronization sequence allocation for multiple cells (3 primary sequences) of a cell site comprising 3 cells (sectors);

FIG3 shows an exemplary primary synchronization sequence allocation for multiple cells (7 primary sequences) of a cell site comprising 3 cells (sectors);

FIG4 shows an exemplary primary synchronization sequence allocation for multiple cells (3 primary sequences) of a cell site comprising 6 cells (sectors);

FIG5 illustrates receiver operation assuming the use of three primary synchronization sequence timing and primary synchronization signal detection;

FIG6 illustrates an exemplary 2-step cell search process utilizing multiple primary synchronization sequences;

FIG7 illustrates an exemplary time-domain method for increasing the number of primary synchronization signal implementations by employing sub-frame-level migration;

FIG8 illustrates an exemplary time domain method for increasing the number of primary synchronization signal realizations by employing symbol layer migration;

FIG9 illustrates a flow chart of an embodiment of a method of operating a base station transmitter according to the principles of the present invention;

FIG10 illustrates a flow chart of an alternative embodiment of a method of operating a base station transmitter according to the principles of the present invention;

FIG11 shows a flow chart of an embodiment of a method for operating a user equipment receiver according to the principles of the present invention; and

FIG12 shows a flow chart of an alternative embodiment of a method of executing a user equipment receiver in accordance with the principles of the present invention.

DETAILED DESCRIPTION

Reference is now made to FIG. 1 , which illustrates an embodiment of a cellular network, generally designated 100, constructed in accordance with the principles of the present invention. Cellular network 100 comprises a cellular mesh having a central cell site and six peripheral first-tier cell sites. The central site employs a central base station BS1, while the peripheral first-tier cell sites employ first-tier base stations BS2-BS7, as shown. Cellular network 100 also includes user equipment (UE) located at the central site. Note that a cell site may consist of one or more cells. A cell is typically associated with a sector. Therefore, we use the terms "cell" and "sector" interchangeably. A "site" represents a collection of cells/sectors associated with the same base station.

Central base station BS1 includes a base station transmitter 105 having a synchronization unit 106 and a transmitting unit 107. In one embodiment, synchronization unit 106 is configured to provide a cellular downlink synchronization signal having a primary portion and a secondary portion. The secondary portion provides information specific to the central cell or point (i.e., "cell-specific"). Transmitting unit 107 is configured to transmit the cell downlink synchronization signal to user equipment (UE). Synchronization unit 106 is also configured to provide a cellular downlink synchronization signal having a primary portion and a secondary portion. The primary portion uses one of N (N>1) different primary synchronization signals (primary synchronization sequences), each of which is assigned to the cells shown in FIG. As before, the secondary portion provides information specific to the central cell, and transmitting unit 107 transmits the cellular downlink synchronization signal to user equipment (UE).

The user equipment UE comprises a user equipment receiver 110 having a receiving unit 111 and a processing unit 112. The receiving unit 111 is configured to receive a primary part and a secondary part of a cellular downlink signal from the base station transmitter 105. The processing unit 112 is configured to identify and extract the secondary part, which may provide cell-specific parameters for the center cell.

Providing a primary part and a secondary part of the downlink synchronization signal allows timing and frequency offset issues to be resolved before determining the cell-specific information. This reduces the complexity of the initial cell search and the switching mode of the user equipment UE. In addition to the cell ID, the cell-specific information may also include other parameters, such as frame timing information and antenna configuration indicators. The cell-specific information embedded in the secondary part may be partial or full information. For example, the cell ID-related information may be a complete physical cell ID or a cell ID group indicator. Another example is the exact number of base station transmit antennas or a 1-bit indicator that indicates whether the base station uses one or more transmit antennas. When only partial information is transmitted in the secondary part, the complete information should be parsed using some other method. For example, the number of these transmit antennas can be signaled in a broadcast channel, which is a wideband channel that is demodulated by the user equipment after the cell search process is completed. The partial indicator of the number of these transmit antennas can also be used as a transmission diversity indicator for the broadcast channel.

One possible application of N different primary synchronization sequences is to carry some partial cell-specific information, such as a partial cell ID. In this case, the network should use a fixed number of primary synchronization sequences (n = N). The physical cell ID information is then split into a primary synchronization signal and a secondary synchronization signal. Note that if the physical cell ID is an incompletely encoded synchronization signal (a combination of the primary and secondary synchronization signals), the complete cell ID should be acquired through some other method, such as a cell-specific downlink pilot or reference signal. Obviously, if the physical cell ID is a fully encoded synchronization signal, the complete cell ID can be acquired via the synchronization signal. That is, if there are M different cell IDs divided into M/L groups of cell IDs, the secondary synchronization signal indicates the cell ID group (one of the M/L possibilities), while the primary synchronization signal specifies the cell ID within the cell ID group (one of the L possibilities). In this case, the downlink reference signal can be used to verify the acquired cell ID.

Alternatively, a set of N different primary synchronization sequences can be used whenever necessary to simply avoid inter-channel mismatches experienced by the primary and secondary synchronization signals. That is, the network is configured to utilize a variable number of primary synchronization sequences (1=n=N). For example, in an asynchronous network, a cell-common primary synchronization sequence selected from a set of N sequences can be used. In a strictly synchronous network, all or a subset of the N sequences can be used, depending on the cell structure. In this case, the primary synchronization signal is not used to carry any cell-specific information, such as a partial cell ID, but this cell-specific information may or may not be a function of the cell ID. Therefore, the secondary synchronization signal can carry all or part of the cell ID. If the secondary synchronization signal carries a partial cell ID, this means that a device other than the synchronization signal should be used to collect the remaining cell ID information. An example is detection via a cell-specific downlink reference signal or pilot. Otherwise, the downlink reference signal can be used to verify the collected cell ID.

In either embodiment, primary synchronization sequence planning can be used to improve the performance of the synchronization network. This type of planning is beneficial because N is kept small, minimizing terminal complexity growth and potential degradation in timing estimation accuracy. The distribution of the N primary synchronization sequences across cells can vary depending on how the physical cell IDs are allocated between the primary and secondary synchronization signals. However, for the second embodiment, random distribution of the N primary sequences is not excluded.

An exemplary planning/allocation of N primary synchronization sequences for a three-sector hexagonal point is shown in embodiment 201 of FIG. 2 , where N = 3. Here, within a three-sector point, each cell within the same point is assigned one of the three available sequences. This process is then repeated at multiple points. This embodiment is particularly suitable when the primary synchronization signal carries partial cell ID information, where L = 3 (three cell IDs within each cell ID group). Specifically, there is a one-to-one relationship between the three cell IDs within the cell ID group transmitted in the secondary synchronization signal. While this embodiment reduces the load on the secondary synchronization signal in carrying cell ID information, channel mismatches can occur at sector boundaries within the same point. This is because the cell ID group information carried by the secondary synchronization signal is point-specific, whereas the primary synchronization signal is cell-specific. However, note that mismatches due to multipath combining across base stations are mitigated. Furthermore, this embodiment is suitable when the primary synchronization sequence/signal is associated with the cell ID information transmitted in the secondary synchronization signal. This is true whether the master synchronization signal carries cell ID information (in which case some of the cell ID information is redundant) or not. The illustration in 201 of Figure 2 shows only three cells. This distribution pattern is repeated for the entire network, as is typical for a network consisting of a large number of cells.

Another exemplary planning/allocation of N primary synchronization sequences for a 3-sector hexagonal point is shown in embodiment 202 of FIG. 2 , where N=3. This embodiment describes the use of point-specific primary synchronization sequences, where the same sequence is used for different sectors within the same point. This is particularly suitable when the primary synchronization signal does not carry any cell ID information, the secondary synchronization signal is point-specific (i.e., the cell ID group is point-specific), and the remaining cell IDs are assumed to be resolved via cell-specific downlink reference signals or pilots. In this case, the multipath channel seen by the primary and secondary synchronization signals is the same. The illustration of 202 in FIG. 2 shows only three cell points. This allocation pattern is repeated throughout the network, as is typical for a network consisting of a large number of cell points.

FIG3 illustrates an exemplary extension of the previous embodiment to the case where N = 7. While channel mismatch due to multipath combining across first-layer nodes of the primary synchronization signal can be avoided when N = 3, multipath combining across second-layer nodes occurs. Multipath combining across first and second-layer nodes can be avoided when N = 7. FIG3 illustrates only seven nodes. This distribution pattern is repeated throughout the network, as in a typical network consisting of a large number of nodes.

An exemplary plan/assignment of N primary synchronization sequences for a 6-sector hexagonal cell is shown in FIG4 , where N=3. In this case, the three primary synchronization sequences are simply distributed across sectors/cells, preventing any adjacent cells associated with different base stations from using the same primary synchronization sequence. The illustration of 201 in FIG4 shows only three cells. This allocation pattern is repeated throughout the network, as is typical for a network consisting of a large number of cells.

The above embodiments serve as examples of how multiple master synchronization sequences can be used to facilitate faster cell searches. Other variations are also possible, as will be apparent to those skilled in the art.

The proposed solution suggests a receiver implementation as shown in FIG5 , where the timing and master sequence index are jointly detected. Here, the “index” is simply a sequence indicator (index n represents the nth master sequence, where n=1, 2, …, N). That is, the received signal is correlated with each of the N candidate sequences (for illustrative purposes, N=3 in FIG5 ). After comparing the N correlation profiles, the peak indicates the correct timing and master synchronization sequence index. Further optimization of the receiver operation is still possible to reduce receiver complexity. By exploiting the inherent structure of the sequence, correlation can be performed more efficiently. For example, when the sequence is a binary value {+1, -1} in the time domain, only real addition is required. Symbol correlation can also be performed when the received signal is correlated with the sequence symbols. Moreover, subsets of the N sequences may share certain common terms or structures that can be used to further reduce complexity.

Depending on the master sequence design, the N master synchronization sequences can be defined in the time or frequency domain. While defining sequences in the frequency domain is more natural for OFDM/OFDMA-based systems, defining sequences in the time domain offers better correlation properties and reduces receiver complexity. Sequences are typically chosen to have good autocorrelation and cross-correlation properties. Some examples are constant amplitude zero autocorrelation (CAZAC), Zadoff-Chu sequences, Golay sequences, and Walsh-Hadamard sequences.

An exemplary cell search process using multiple primary synchronization sequences is illustrated in Figure 6. It is assumed that the secondary synchronization signal carries a cell ID group, and the primary synchronization signal indicates the cell IDs within the group. Step 1 601 utilizes the primary synchronization signal and consists of symbol timing and primary sequence index detection, as described in the previous paragraph. The primary sequence index corresponds to a cell ID within the cell ID group. For the initial cell search, frequency offset is also estimated. Frequency offset estimation is not necessary for neighbor cell searches, as base stations within a network are typically frequency synchronized. To determine the success of step 1, a test criterion is used. If step 1 is successful, the receiver proceeds to step 2. Otherwise, step 1 is repeated, and averaging/accumulating multiple events can be used to improve the probability of success. Step 2 602 utilizes the secondary synchronization signal and attempts to acquire the cell ID group. In this example, frame timing and a transmit diversity indicator are also detected in this second cell search step. Note that the cell search process differs depending on the precise structure and type of information carried by the synchronization signal.

As mentioned above, the number of primary synchronization sequences should be kept to a minimum primarily to minimize the impact on terminal complexity. However, it is often desirable to increase the number of possible primary synchronization signal implementations because it provides flexibility in planning and deployment, thereby avoiding the impact of channel mismatch in asynchronous networks. To multiply the number of primary synchronization signal implementations, frequency domain or time domain methods can be used. The frequency domain method allocates part of the synchronization signal bandwidth to specific primary synchronization signal implementations, similar to frequency reuse. However, this also leads to increased complexity. On the other hand, the time domain method is equivalent to changing the position of the primary synchronization signal within a radio frame across different cells. Different cells can be (multiple) cells within the same point (intra-point cell) or associated with different points (inter-point cell). For N' different time shifts and N sequences, a total of NxN' synchronization signal implementations are available. A subset or all of these implementations can be used in the network. The time domain method does not increase terminal complexity because it is transparent to the terminal.

Regarding the time domain approach, two different embodiments are possible. A first embodiment is shown in FIG7 , in which sub-frame-level migration is used to increase the number of primary synchronization signal implementations. As an example, assume that 1 radio frame consists of 10 sub-frames and 2 sub-frames are used to carry the synchronization signal. N’=2, 3 and 5 are shown in the figure. A second embodiment is shown in FIG8 , in which symbol layer migration is used to increase the number of primary synchronization signal implementations. In this case, the sub-frame carrying the synchronization signal is fixed, but the position of the synchronization signal within the sub-frame is changed. An exemplary case of a total of 5 migrations is given in FIG8 , in which the primary synchronization signal and the secondary synchronization signal are adjacent to each other. However, another possible embodiment is to apply a circular shift of the primary synchronization sequence.

Referring now to FIG. 9 , a flow chart illustrating one embodiment of a method for operating a base station transmitter in accordance with the principles of the present invention, generally designated 900, is shown. The method begins at step 901. Then, at step 902, a cellular downlink synchronization signal having a primary portion and a secondary portion is provided. The primary portion utilizes a corresponding one of a plurality of different primary synchronization signals assigned to adjacent transmitting cells. At step 903, cellular-specific information is further provided in the secondary portion. Cell-specific parameters include at least cell identity information. Other cell-specific parameters may include radio frame timing and antenna configuration indicators. The cellular downlink synchronization signal is transmitted at step 904, and method 900 concludes at step 905.

Referring now to FIG. 10 , a flow chart illustrating an alternative embodiment of a method for operating a base station transmitter in accordance with the principles of the present invention, generally designated 1000, is shown. Method 1000 begins at step 1001. Then, at step 1002, a cellular downlink synchronization signal having a primary portion and a secondary portion is provided. The primary portion utilizes a corresponding one of a plurality of different primary synchronization signals assigned to adjacent transmitting cells. Furthermore, the selection of the primary synchronization signal indicates partial cellular identity information. At step 1003, the remaining cell identity information and certain other cell-specific parameters, such as radio frame timing and antenna configuration indicators, are further provided in the secondary portion. The cellular downlink synchronization signal is transmitted at step 1004, and method 1000 concludes at step 1005.

Referring now to FIG. 11 , there is shown a flow chart of an embodiment of a method for operating a user equipment receiver, generally designated 1100, performed in accordance with the principles of the present invention. Method 110 begins at step 1101 and proceeds to step 1102 by receiving a cellular downlink synchronization signal having a primary portion and a secondary portion. The primary portion utilizes one of a plurality of different primary synchronization signals assigned to adjacent transmitting cells. The primary portion of the cellular downlink synchronization signal is used to perform timing acquisition and detect the index of the primary synchronization sequence in step 1103. Frequency offset estimation and correction are also performed. Timing acquisition is performed by correlating the primary portion with a corresponding one of a plurality of copies of the plurality of different primary synchronization signals. Once timing and frequency lock is established in step 1103, the secondary portion is identified and extracted in step 1104. In step 1105, the secondary portion is then used to provide cell-specific parameters. The cell-specific parameters are determined by demodulating and decoding the secondary portion of the cell downlink synchronization signal. The cell-specific parameters include at least cell identity information. Method 110 concludes in step 1106.

Referring now to FIG. 12 , there is shown a flow chart of another embodiment of a method of operating a user equipment receiver, generally designated 1200, according to the principles of the present invention. Steps 1201, 1202, 1203, 1204, and 1206 are identical to their counterparts in embodiment 1100 shown in FIG. 11 . The only difference is step 1205, where the cell identity information is determined from the primary and secondary parts. The partial cell identity information is obtained from the primary part via a detected primary synchronization sequence index.

The method disclosed in the invention can be used in any cellular communication system using any modulation or multiple access technology, such as OFDM/OFDMA, CDMA or TDMA. The solution is also valid for any multiplexing scheme, such as frequency division duplexing (FDD) and time division duplexing (TDD).

Although the methods disclosed herein have been described and illustrated with reference to specific steps performed in a specific order, it is understood that these steps may be combined, subdivided, or rearranged to form equivalent methods without departing from the teachings of the present invention. Therefore, unless otherwise specified herein, the order or grouping of steps does not limit the present invention.

Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the invention in its broadest spirit and scope.

Claims (9)

1. A method of operating a base station transmitter, comprising: providing a downlink synchronization signal having a primary portion and a secondary portion, wherein the primary portion adopts a respective one of a plurality of different primary signals, wherein the plurality of different primary signals are associated with a plurality of different primary synchronization sequences and are obtained by performing a subframe-level migration or a symbol-level migration on each of the plurality of different primary synchronization sequences; further providing cell specific information in the secondary portion, wherein the cell specific information comprises at least cell identity information and a radio frame timing indicator, wherein the plurality of different primary signals correspond to part of the cell identity information and the secondary portion carries remaining cell identity information; and The downlink synchronization signal is transmitted. 2. The method of claim 1, wherein the cell-specific information also includes an antenna configuration indicator. 3. The method of claim 1, wherein the plurality of different primary signals are assigned to different transmission cells. The method of claim 1 , wherein the number of different primary signals is three. 5. A base station transmitter, comprising: a synchronization unit configured to provide a downlink synchronization signal having a primary part and a secondary part, wherein the primary part adopts a respective one of a plurality of different primary signals and the secondary part provides cell-specific information, wherein the cell-specific information comprises at least cell identity information and a radio frame timing time indicator, wherein the plurality of different primary signals correspond to partial cell identity information and the secondary part carries remaining cell identity information; and a transmitting unit configured to transmit the downlink synchronization signal, The multiple different primary signals are associated with multiple different primary synchronization sequences and are obtained by performing subframe-level migration or symbol-level migration on each of the multiple different primary synchronization sequences. 6. The transmitter of claim 5, wherein the cell-specific information also includes an antenna configuration indicator. 7. The transmitter of claim 5, wherein the base station transmitter uses Orthogonal Frequency Division Multiplexing (OFDM). 8. The transmitter of claim 5, wherein the plurality of different primary signals are allocated to different transmission cells. 9. The transmitter of claim 5, wherein the number of different primary signals is three.