Patent Publication Number: US-6909707-B2

Title: Method and apparatus for pseudo-random noise offset reuse in a multi-sector CDMA system

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to communication systems, and, more particularly, to a method and apparatus that achieves reuse of pseudo-random noise indices or offsets in a multi-sector communications systems such as a CDMA system. 
     BACKGROUND OF THE INVENTION 
     Typically in wireless communication systems such as that illustrated in  FIG. 1 , a base station  102  that transmits to and receives from mobile units  104  is used to create what is termed a cell. In CDMA wireless systems, in particular, optimization is a critical consideration that has significant impact on the performance and capacity of a CDMA system. Particularly in both 2G and 3G CDMA systems, the need to enhance the capacity of the systems is increasing. One method for optimizing capacity in a CDMA cell is to minimize areas suffering from pilot pollution. Typically a base station  102  has a number of antennas, each of which is used to form a respective sector within a CDMA cell. Each CDMA sector sends its own pilot channel requiring its own corresponding pseudo-random noise (PN) offset. If a mobile unit  104  is in a location within the CDMA cell where numerous pilot channels are received with relatively equal signal strength, pilot pollution likely will result. This pollution is detrimental because it may cause dropped calls and decreased capacity. Thus, the more sectors per cell may result in increased pilot pollution. 
     As the number of sectors per site or cell increases, however, so does the capacity of the system as those illustrated by the table in FIG.  2 . As is shown, the increase in capacity from 1 to 12 sectors, for example, is over six times gain and the Erlangs for the site or cell increases from approximately 21 to 134. As mentioned previously, however, the increase in the number of sectors may increase degradation of the CDMA performance due to pilot pollution. A further problem is that more sectors require more antennas, power amplifiers and cables thereby increasing the cost of the system. As an example, a 12 sector system would require 24 receive antennas (i.e., 12 sectors×2 for receive diversity) and 24 transmit antennas (12 sectors×2 for transmit diversity). 
     Hence, there is a need to increase the capacity of CDMA systems by increasing the number of sectors while decreasing pilot pollution and reducing the cost outlay for system hardware. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages of the disclosed system and method will be apparent to those of ordinary skill in the art in view of the detailed description of preferred embodiments that is made with reference to the drawings, a brief description of which is provided below. 
         FIG. 1  is a diagram illustrating components of a CDMA cell. 
         FIG. 2  is a table illustrating the increase capacity of a CDMA cell as the number of sectors is increased. 
         FIG. 3  illustrates a CDMA cell having multiple sectors constructed in accordance with the teachings of the present invention. 
         FIG. 4  illustrates an example of a base station antenna configuration constructed in accordance with the teachings of the present invention. 
         FIG. 5  illustrates a block diagram of the reverse link receiver portion of the system at a base station in accordance with the teachings of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to enhance the capacity of a CDMA system, a method and apparatus are disclosed that include reuse or sharing of pseudo-random noise (PN) offsets within a cell of a wireless communication system such as a multi-sector CDMA system. Reuse of PN offsets affords a system where less PN offsets are needed. The disclosed method and apparatus further employ adaptive antenna arrays or a number of fixed narrow beams to form the sector areas within the CDMA cell. Moreover, the disclosed method and apparatus detect the existence of multi-path links between a base station and a mobile unit that are sharing or reusing common PN offsets. A forward link data channel is then established between both sectors, which provides desirable transmit diversity. 
       FIG. 3  illustrates an exemplary sector pattern of a CDMA cell in accordance with the teachings of the invention. A base station  202  includes a number of transmit and receive antennas (not shown) that are used to establish multiple sectors that emanate in beam patterns from the base station  202 . The particular pattern illustrated in  FIG. 3  utilizes a 12 beam pattern corresponding to a 12 sector system. The antenna arrays are set to form 12 sector beams  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  218 ,  220 ,  222 ,  224  and  226  with each sector covering approximately 30° of the 360° coverage area. In the particular system shown, four PN offsets are used with each offset being repeated three times within the system. As will be appreciated by those skilled in the art, other configurations may be envisioned such as the use of three PN offsets repeated four times or six PN offsets repeated two times. Irrespective of the particular configuration that is to be used, the same PN offset should not be used in adjacent sectors and at least one PN offset should be used more than once at a given site. 
     As illustrated in  FIG. 3 , four PN offsets labeled A-D are utilized in the predetermined distance or angle and occurs in a repeating sequence of the PN offset sectors (e.g., A, B, C, D). For example, in the present disclosed system utilizing four PN offsets, beam sectors  204  and  212  that share the PN offset A are separated by 120°. Thus, for the present disclosed system the spatial degree of separation for sectors sharing the same PN offset is 120°. As an alternate example, if the system utilized three PN offsets being repeated four times, the spatial separation between sectors sharing the same PN index would be 90°. 
     Each of the PN offsets A-D has associated with its own pool of Walsh codes, such that the entire system has four times the number of Walsh codes in its pool as that of a single sector. Thus, all of the identical PN offsets are kept orthogonal to each other, which prevents cross-sector cross-talk. Additionally, pilot pollution in a mobile unit  203  is limited by the use of relatively narrow beams, thereby limiting the area that is covered by multiple PN offsets. 
     An additional feature of the disclosed method and apparatus include reallocation of system resources when a multi-path exists. For example, there is a finite probability that the mobile  203  shown in  FIG. 3  is covered by sector  224  using PN offset C will see a multi-path ray from sector  216  as the pilot channel therefrom is reflected off some object  228 . Similarly, the mobile  203  may see a multi-path ray from sector  208  also utilizing the PN offset C or multiple multi-path rays from both sectors  208  and  216 . In conventional CDMA systems, if these sectors were utilizing different PN offsets, the system would go into what is termed “softer” handoff where the mobile  203  as it is traveling will receive a data channel from two or more sectors as it travels and eventually will be completely handed off from one sector to the other sector. 
     In the presently disclosed method and apparatus, however, the same PN offset that is reused in the two or more sectors may present a problem because the mobile unit  203  will not be able to differentiate between sectors  224 ,  208  and  216 , for example. Accordingly, the mobile  203  may try to combine the data channels. Because sectors  208  and  216  do not transmit the same Walsh codes that are assigned to sector  224 , however, the data channels from sectors  208  and  216  cannot be combined with the data channel from sector  224 . In order to overcome this problem, the method and apparatus constructed in accordance with the teachings of the present invention utilize reverse link spatial information. That is, once a mobile unit  203  is identified at the base station  202  on the reverse link, the base  202  will scan through the entire 360° that it covers and will characterize the multi-path manifold of the mobile unit  203 . 
     As a means to perform the reverse link scan, the method and apparatus constructed according to the teachings of the present invention preferably utilize an adaptive antenna array. An example of the topology of such an array is illustrated in  FIG. 4 , which shows a top view of the adaptive antenna array  400 . Within the array  400  are three individual antenna arrays  402 A,  402 B and  402 C whose faces are disposed at angles of approximately 60° with respect to one another, thereby forming a triangular arrangement. Each array  402  is used to cover 120° of the CDMA cell. Additionally, each one of the arrays  402  includes four specific adaptive antennas  404  that are directed by an antenna beam steering control to cover a particular 30° sector. Thus, the full 360° of the CDMA cell are covered by the 12 individual adaptive antennas  404 . It is noted that the configuration illustrated in  FIG. 4  is merely exemplary and other antenna topologies may be utilized, such as four adaptive arrays having three adaptive antennas each or simply 12 individual antenna arranged in a circular configuration to effect the exemplary pattern of FIG.  3 . 
     It is noted that the adaptive antenna array illustrated in  FIG. 4  is utilized for both the reverse link or forward link of the CDMA site. Accordingly, only one physical adaptive antenna array is required for the forward link or transmission side of the base station and the reverse link or receive portion of the base station. Additionally, redundancy may be introduced to the system by adding additional antenna arrays for the forward and reverse links. However, the method and apparatus disclosed according to the teachings of the invention eliminate the need for redundant antennas, which are typically used in the conventional art for transmit and receive diversity, since the use of multi-path connections achieves transmit diversity without the need for additional redundant antennas. 
     As multi-path connections to the mobile unit  203  shown in  FIG. 3  is characterized, the base station  202  may utilize this information to determine if a significant multi-path link exists from either sectors  208  or  216 , continuing with the previously described example. The forward link transmitter of the base  202  is then configured to transmit the sector  224  data onto one or both of sectors  208  and  216  to this specific mobile unit  203 . Hence, the effect of this multi-path transmission is a even “softer” handoff diversity in the mobile unit  203  than is known previously in the conventional art. 
       FIG. 5  illustrates a block diagram of the mechanism used in the reverse link portion of the base station  202  to achieve scanning and characterization of the multi-path manifold for a CDMA cell. In particular, a plurality of antennas A 1 , A 2  . . . A n  for receiving transmitted signals on the reverse link are connected to respective receivers RCVR 1 -RCVRn ( 501   1 - 501   n ). The variable “n” represents the number of sectors within the CDMA cell. In the example of  FIG. 3 , this “n” number of sectors is equal to 12. Once the information is received by the receivers  501 , it is fed to searchers SRCHR 1 -SRCHRn ( 502   1 - 502   n ) corresponding to each receiver  501 . The searchers  502  are used to accomplish scanning to determine which corresponding sectors may be covering a particular mobile user. The information from the searcher blocks  502  is passed to a resource management decision logic  504 , which is used to characterize the multi-path manifold and decide if data should be transmitted to the mobile user from multiple sectors to establish multi-path links. Once this determination is arrived at by the resource management decision logic  504 , a control signal is passed to the forward link transmitter  505  for execution of the particular multi-path transmission.  FIG. 5  also includes an optional antenna beam steering control  503  that is used to direct the receivers  501  and their respective antennas to cover their corresponding sector areas. However, it is noted that if single antennas are used instead of an adaptive antenna array, the antenna beam steering control  503  is not necessary. 
     As will be appreciated by those skilled in the art, the disclosed method and apparatus reduces the number of PN offsets utilized by reusing or sharing PN offsets among two or more sectors in a CDMA cell. Additionally, by utilizing an adaptive antenna array, determination of multi-path links serves to maximize the capacity of a base site, which is particularly useful for high data rate users. Moreover, the disclosed method and apparatus afford an increase in site capacity to approximately 135 erlangs while maintaining the antenna complexity of a six sector level. A six sector system requires 12 antenna elements, 6 for the main branch and 6 for the diversity branch. The exemplary disclosed method utilizes the same number of antennas to provide a significant capacity enhancement (i.e., 12 sectors). 
     The foregoing description has been presented for the purposes of illustration and description. It is not intended to be exhausted or to limit the teaching of the invention to the exemplary embodiments disclosed. Many modifications and variations are possible in light of the above teachings and it is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.