Abstract:
In advanced wireless OFDMA communication systems, hierarchical synchronization is adopted to synchronize between a base station (BS) and a mobile station (MS). In a hierarchical synchronization architecture, primary advanced preamble (PA-Preamble) is used for coarse time domain synchronization while cell ID is detected using several accumulated secondary advanced preambles (SA-Preambles). Network entry latency can be reduced by adjusting the relative location of superframe header (SFH), PA-Preamble and SA-Preambles within a superframe. Three different synchronization channel (SCH) architectures are proposed to provide different tradeoffs between network entry latency and the robustness of SA-Preamble design and cell ID detection.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 from U.S. Provisional Application No. 61/163,934, entitled “Synchronous Scheme for Advanced Wireless OFDMA Communication Systems,” filed on Mar. 27, 2009; U.S. Provisional Application No. 61/168,638, entitled “Low Latency Synchronization Scheme for Advanced Wireless OFDMA Communication Systems,” filed on Apr. 13, 2009, the subject matter of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to cellular OFDM systems and, more particularly, to low latency network synchronization for base stations in advanced wireless OFDMA communication systems. 
     BACKGROUND 
     In wireless communication systems, such as defined by IEEE 802.16m specification, base stations and mobile stations communicate with each other by sending and receiving data carried in a series of superframes. Before a mobile station can access a base station, physical (PHY) layer synchronization and Media Access Control (MAC) layer synchronization are performed. Upon power-on, a mobile station first acquires downlink (DL) synchronization and adjusts its timing, frequency and power via synchronization channel (SCH) broadcasted by a serving base station. After DL synchronization, the mobile station acquires uplink (UL) PHY layer synchronization via ranging procedures and MAC layer synchronization via network entry procedures with the serving base station. 
     A synchronization channel (SCH) is a radio resource region within each superframe that is used for preamble transmission by base stations. A preamble is a predefined code sequence used to facilitate network synchronization. In general, preambles are designed to achieve the following goals: to provide a reliable reference signal in both time domain and frequency domain for network synchronization; to support channel estimation for data and superframe header (SFH) decoding; to provide DL channel quality indicator (CQI) measurement and receive signal strength indicator (RSSI) measurement; to reduce network entry latency; and to minimize communication overhead. Well-designed SCH structure is thus desirable to achieve those goals. 
     In current IEEE 802.16m wireless systems, a hierarchical two-stage synchronization scheme has been proposed. In a first stage, a primary advanced-preamble (PA-Preamble) is used to provide coarse timing synchronization. In a second stage, multiple secondary advanced-preambles (SA-Preambles) are used to provide fine timing synchronization and cell ID detection. Both PA-Preamble and SA-Preambles are time-domain multiplexed within one superframe. For example, each superframe has four frames, and one PA-Preamble and three SA-Preambles are distributed in each frame within one superframe. It remains a challenge to design a hierarchical SCH structure that reduces network latency and/or provides a desirable tradeoff between network entry latency and the robustness of cell ID detection. 
     SUMMARY 
     In advanced wireless OFDMA communication systems, hierarchical synchronization is adopted to provide synchronization between a base station-(BS) and a mobile station (MS). In a hierarchical synchronization architecture, primary advanced preamble (PA-Preamble) is used for coarse time domain synchronization while cell ID is detected using several accumulated secondary advanced preambles (SA-Preambles). The number of SA-Preambles being accumulated in time domain to provide faithful cell-ID information depends on network environment and the robustness of preamble sequence design, which in turn determines network entry latency. Well-designed synchronization architecture can reduce network entry latency without scarifying system performance. In general, network entry latency can be reduced by adjusting the relative location of superframe header (SFH), PA-Preamble and SA-Preambles within a superframe. 
     Three different synchronization channel (SCH) architectures are proposed to provide different tradeoffs between network entry latency and the robustness of SA-Preamble design and cell ID detection. In a first embodiment, a proposed S-P-S-S preamble allocation scheme provides robust cell ID detection with reduced network entry latency for various network environments. In a second embodiment, a proposed S-S-P-S preamble allocation scheme provides a good balance between high reliability of cell ID detection and low network entry latency. In a third embodiment, a proposed S-S-S-P preamble allocation scheme provides reasonable reliability of cell ID detection and lowest network entry latency for simple network configuration with good channel condition and robust SA-Preamble design. 
     Other embodiments and advantages are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cellular OFDMA system with downlink synchronization in accordance with one novel aspect. 
         FIG. 2  illustrates a flow chart of a method of downlink synchronization in accordance with one novel aspect. 
         FIG. 3  illustrates a first SCH architecture that is used in a hierarchical synchronization scheme in an IEEE 802.16m system. 
         FIG. 4  illustrates a second SCH architecture that is used in a hierarchical synchronization scheme in an IEEE 802.16m system. 
         FIG. 5  illustrates a third SCH architecture that is used in a hierarchical synchronization scheme in an IEEE 802.16m system. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 1  illustrates a cellular OFDMA system  10  with hierarchical downlink synchronization in accordance with one novel aspect. Cellular OFDMA system  10  comprises base station BS 11  and mobile stations MS 12 , MS 13  and MS 14 . Base station BS 11  comprises a radio frequency (RF) transmitter and receiver module  15 , a synchronization channel (SCH) allocation module  16 , a processor  17 , a storage device  18 , and an antenna  19  that is coupled to RF transmitter and receiver  15 . Upon power-on, multiple mobile stations MS 12 , MS 13  and MS 14  receive radio signals broadcasted from base station BS 11 . Before the mobile stations can access BS 11  and communicate data, downlink (DL) physical layer synchronization is required. The mobile stations make timing, frequency, and power adjustments via synchronization channel (SCH) monitoring and tracking during DL synchronization. 
     In the example of cellular OFDMA system  10 , base station BS 11  and mobile stations MS 12 , MS 13 , and MS 14  communicate with each other by sending and receiving data carried via radio resource blocks in superframe structure. Each superframe (e.g., SU 0 , SU 1 , or SU 2 ) contains four frames (e.g., F 0 , F 1 , F 2 , and F 3 ). A synchronization channel (SCH) is a radio resource region allocated within each superframe that is used to carry preambles to facilitate DL synchronization. In general, a SCH comprises one or multiple predefined preamble code sequences transmitted from the base station. Based on the received preamble code sequences, the mobile stations are able to obtain reliable reference signals in both time and frequency domain for DL synchronization. In addition, the mobile stations are able to perform channel estimation for data or superframe header (SFH) decoding. 
     As illustrated in  FIG. 1 , a hierarchical two-stage synchronization scheme is used for DL synchronization. In a first synchronization stage, a primary advanced-preamble (PA-Preamble) is used to provide coarse timing synchronization. PA-Preamble also carries partial system information, e.g., carrier type and carrier bandwidth. PA-Preamble is usually common among base stations in the same frequency spectrum. Typically, frequency reuse  1  is applied for PA-Preamble transmission. In a second synchronization stage, multiple secondary advanced-preambles (SA-Preambles) are used to provide fine timing synchronization and cell ID detection. SA-Preamble is usually different among various base stations. Typically, frequency reuse  3  is applied for SA-Preamble transmission. Both PA-Preamble and multiple SA-Preambles are time-domain multiplexed (TDM) within one superframe. 
     In one novel aspect, SCH allocation module  16  of BS 11  allocates PA-Preamble and multiple SA-Preambles such that desirable tradeoff between network latency and the robustness of cell ID detection is achieved. In the example of  FIG. 1 , one PA-Preamble is allocated in the first OFDM symbol of the second frame (i.e., F 1 ) within each superframe, while each of the three SA-Preambles is allocated in the first OFDM symbol of the corresponding first, third, and fourth frame (i.e., F 0 , F 2 , and F 3 ) within each superframe respectively. Each superframe also contains a superframe header (SFH), which carries system information and is typically located in the second OFDM symbol of the first frame. By allocating the PA-Preamble after the SFH, mobile stations are able to decode the SFH information in a time duration that is less than the superframe length (i.e., 20 ms). 
       FIG. 2  illustrates a flow char of a method of hierarchical DL synchronization in a cellular OFDMA system in accordance with one novel aspect. During DL synchronization, a PA-Preamble and multiple SA preambles are broadcasted to a mobile station via an allocated SCH in each superframe by a base station. In step  21 , the base station first allocates one PA-Preamble as the first OFDM symbol in one of the frames within the superframe. The frame that carries the PA-preamble is denoted as a PA frame. The PA frame must not be the first frame (i.e., not frame F 0 ) along time domain within the superframe. In step  22 , the base station allocates one SA-Preamble in each of the remaining frames within the superframe. Each SA-Preamble is also allocated at the first OFDM symbol in each frame. However, no SA-Preamble is allocated in the PA frame. Finally, in step  23 , the base station transmits the superframe with both PA-Preamble and SA-Preambles to the mobile station. The superframe also contains a SFH that carries system information and is located at the second OFDM symbol in the first frame (i.e., frame F 0 ) along time domain within the superframe. 
     In one novel aspect, the above-described SCH allocation scheme for hierarchical DL synchronization is able to reduce network entry latency without scarifying system performance as compared to other traditional SCH allocation schemes. Network entry latency of a mobile station is defined as the delay between the time of receiving a first PA-Preamble and the time of starting data communication. In a traditional preamble allocation scheme, a PA-Preamble is always allocated together with a SFH. In a four-frame superframe structure, for example, both PA-Preamble and SFH are allocated in the first frame, while three SA-Preambles are allocated in the next three frames along the time domain within the superframe. Based on such traditional preamble allocation scheme, network entry latency is always equal to at least one superframe length (i.e., 20 ms). This is because the mobile station is only able to start data communication after decoding SFH information successfully. Furthermore, the mobile station is only able to decode the SFH information after detecting complete cell ID information carried in SA-Preambles. As a result, after receiving a first PA-Preamble and one SFH in the first frame of the current superframe, the mobile station always accumulate three SA-Preambles for complete cell ID detection before receiving another SFH in the next superframe. Consequently, the mobile station must wait at least four frames to decode the SFH information in the next superframe and then start data communication. Therefore, in this traditional preamble allocation scheme, network entry latency is always equal to at least one superframe length (i.e., 20 ms). 
     On the other hand, with more details described below, under the novel SCH allocation scheme described above with respect to  FIG. 2 , network entry latency may be reduced without scarifying system performance. Furthermore, in many network environments, using three accumulated SA-Preambles for cell ID detection is not necessary. Depending on the transmission channel condition and the preamble sequence design, successful cell ID detection may be obtained using less than three SA-Preambles. A novel PA-Preamble and SA-Preamble allocation thus can further reduce network entry latency. Reference will now be made in detail to different embodiments of the proposed SCH allocation scheme, examples of which are illustrated in the accompanying drawings. 
       FIG. 3  illustrates a first embodiment of an SCH architecture that is used in a hierarchical synchronization scheme in an IEEE 802.16m system. This first embodiment is also referred to as an S-P-S-S preamble allocation scheme.) In the S-P-S-S allocation scheme, each PA-Preamble is allocated in the first OFDM symbol of the second frame (F 1 ) within each superframe, while three SA-Preambles are allocated in the first OFDM symbol of the first, third and fourth frames respectively within each superframe. As illustrated in  FIG. 3 , at time t 0 , the mobile station receives a first PA-preamble, which is located in the first OFDM symbol in F 1  of a current superframe SU 0 . The mobile station performs OFDM symbol synchronization and acquires subframe SA-Preamble offset. At time t 1 , the mobile station receives a first SA-Preamble, which is located in the first OFDM symbol of F 2  in SU 0 . The mobile station then starts to seek cell ID by detecting the received SA-Preamble sequence. 
     Successful cell ID detection depends on network configuration and network condition. The mobile station may or may not be able to detect cell ID information using one received SA-Preamble. At time t 2 , the mobile station receives a second SA-Preamble, which is located in the first OFDM symbol of F 3  in SU 0 . The mobile station seeks cell ID again if cell ID detection was failed at time t 1 . At time t 3 , the mobile station receives a third SA-Preamble, which is located in the first OFDM symbol of F 0  in the next superframe SU 1 . The mobile station seeks cell ID again if cell ID detection was failed at both time t 1  and time t 2 . Under normal network condition, cell ID detection is most likely to be successful at this time after three accumulated SA-Preambles. Immediately after time t 3 , the mobile station also receives another SFH located in the second OFDM symbol of F 0  in SU 1 . Therefore, at time t 3 , the mobile station can successfully decode the SFH information received in SU 1  with faithfully detected complete cell ID information. The mobile station thus is able to start data communication with the base station at time t 3 . As a result, in this S-P-S-S SCH allocation scheme, network entry latency is reduced to 15 ms (3×5 ms). It can be seen that because the PA-Preamble is allocated in the second frame in each superframe, three consecutive SA-Preambles are still accumulated for cell ID detection before decoding the SFH information. By allocating the PA-Preamble in the second frame in each superframe, the third accumulated SA-Preamble is allocated immediately before the SFH in each superframe. As a result, the same robustness of cell ID detection is achieved in this S-P-S-S allocation scheme as compared to traditional allocation scheme while network entry latency is reduced. 
       FIG. 4  illustrates a second embodiment of an SCH architecture that is used in a hierarchical synchronization scheme in an IEEE 802.16m system. This second embodiment is also referred to as an S-S-P-S preamble allocation scheme. In the S-S-P-S allocation scheme, each PA-Preamble is allocated in the first OFDM symbol of the third frame (F 2 ) within each superframe, while three SA-Preambles are allocated in the first OFDM symbol of the first, second and fourth frames respectively within each superframe. As illustrated in  FIG. 4 , at time t 0 , the mobile station receives a first PA-preamble, which is located in the first OFDM symbol in F 2  of a current superframe SU 0 . The mobile station performs OFDM symbol synchronization and acquires subframe SA-Preamble offset. At time t 1 , the mobile station receives a first SA-Preamble, which is located in the first OFDM symbol of F 2  in SU 0 . The mobile station then starts to seek cell ID by detecting the received SA-Preamble sequence. At time t 2 , the mobile station receives a second SA-Preamble, which is located in the first OFDM symbol of F 0  in the next superframe SU 1 . The mobile station seeks cell ID again if cell ID detection was failed at time t 1 . 
     Under good network condition, cell ID detection is most likely to be successful at this time. Therefore, at time t 2 , the mobile station can successfully decode the SFH information received in SU 1  with faithfully detected complete cell ID information. The mobile station thus is able to start data communication with the base station at time t 2 . As a result, in this S-S-P-S SCH allocation scheme, network entry latency is reduced to 10 ms (2×5 ms). Because only two consecutive SA-Preambles are accumulated for cell ID detection, this S-S-P-S SCH allocation scheme has larger probability of cell ID detection failure as compared to the traditional preamble allocation scheme. However, in a network with good channel condition, S-S-P-S provides good tradeoff between low network entry latency and cell ID detection reliability. 
       FIG. 5  illustrates a third embodiment of an SCH architecture that is used in a hierarchical synchronization scheme in an IEEE 802.16m system. This third embodiment is also referred to as an S-S-S-P preamble allocation scheme. In the S-S-S-P allocation scheme, each PA-Preamble is allocated in the first OFDM symbol of the fourth frame (F 3 ) within each superframe, while three SA-Preambles are allocated in the first OFDM symbol of the first, second and third frames respectively within each superframe. As illustrated in  FIG. 5 , at time t 0 , the mobile station receives a first PA-preamble, which is located in the first OFDM symbol in F 3  of a current superframe SU 0 . The mobile station performs OFDM symbol synchronization and acquires subframe SA-Preamble offset. At time t 1 , the mobile station receives a first SA-Preamble, which is located in the first OFDM symbol of F 0  in the next superframe SU 1 . The mobile station starts to seek cell ID by detecting the received SA-Preamble sequence. 
     In a simple network configuration and good network channel condition with robust SA-Preamble design, the mobile station may able to perform cell ID detection using a single SA-Preamble. Therefore, at time t 1 , the mobile station can successfully decode the SFH information received in SU 1  with faithfully detected complete cell ID information. The mobile station thus is able to start data communication with the base station at time t 1 . As a result, in this S-S-S-P SCH allocation scheme, network entry latency is reduced to five mille seconds. Because only one SA-Preamble is accumulated for cell ID detection, this S-S-S-P SCH allocation scheme has much larger probability of cell ID detection failure as compared to the traditional preamble allocation scheme. However, in a simple network configuration with excellent channel condition and robust SA-Preamble design, S-S-S-P provides minimum network entry latency with reasonable cell ID detection reliability. 
     With any of the preamble allocation schemes, it is possible that cell ID detection would fail before receiving the next SFH in the first round. According to traditional P-S-S-S, if cell ID detection fails in the first round, then cell ID detection is continually performed in the next superframe. This results in increasing network entry latency to 40 mille seconds. Similarly, first round cell ID detection failure would result in increasing network entry latency to 35 ms, 30 ms and 25 ms for S-P-S-S, S-S-P-S and S-S-S-P allocation schemes respectively. Overall, the proposed S-P-S-S preamble allocation scheme provides robust cell ID detection with reduced network entry latency. In addition, the proposed S-S-P-S preamble allocation schemes provide a good balance between high reliability of cell ID detection and low network entry latency, and the proposed S-S-S-P preamble allocation scheme provides minimum network entry latency with reasonable cell ID detection reliability for simple network configuration with good channel condition and robust SA-Preamble design. 
     Although the present invention has been described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.