Abstract:
A method for reducing cross cell interference in a wireless time division duplex communication system using code division multiple access, the system having at least one user equipment (UE) and a base station (BS) is disclosed. The method begins by measuring an interference level of each timeslot at the BS. A timeslot is eliminated for additional uplink communication if the measured interference level exceeds a first threshold. UEs in nearby cells that are large interferers are identified and their downlink timeslot usage is gathered. A timeslot is eliminated for uplink communication for a large interferer UE that uses the timeslot for downlink communication.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/593,908, filed Aug. 24, 2012, which is a continuation of U.S. patent application Ser. No. 12/243,622, filed Oct. 1, 2008, which issued as U.S. Pat. No. 8,265,560 on Sep. 11, 2012, which is a continuation of U.S. patent application Ser. No. 11/099,325, filed Apr. 5, 2005, which issued as U.S. Pat. No. 7,450,905, on Nov. 11, 2008, which is a continuation of U.S. patent application Ser. No. 10/427,174, filed May 1, 2003, which issued as U.S. Pat. No. 6,882,849, on Apr. 19, 2005, which is a continuation of U.S. patent application Ser. No. 10/003,487, filed Nov. 1, 2001, which issued as U.S. Pat. No. 6,591,109 on Jul. 8. 2003, which claims the benefit of U.S. Provisional Application No. 60/313,336 filed Aug. 17, 2001, which are incorporated by reference as if fully set forth herein. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates to wireless time division duplex (TDD) communication systems using code division multiple access (CDMA). In particular, the invention relates to reducing cross cell user equipment interference in such systems. 
     BACKGROUND 
       FIG. 1  is an illustration of a wireless TDD/CDMA communication system  10 . The communication system  10  has base stations  12   1  to  12   n  ( 12 ) which communicate with user equipments (UEs)  14   1  to  14   n  ( 14 ). Each base station  12  has an associated operational area or cell. The base station communicates with UEs  14  in its cell. 
     In CDMA communication systems, multiple communications are sent over the same frequency spectrum. These communications are distinguished by their channelization codes. To more efficiently use the frequency spectrum, TDD/CDMA communication systems use repeating frames divided into timeslots, such as fifteen timeslots, for communication. In TDD, each cell&#39;s timeslots are used solely for either the uplink or downlink at a time. A communication sent in such a system has one or multiple associated code(s) or timeslot(s) assigned to it. The use of one code in one timeslot with spreading factor of sixteen is referred to as a resource unit. 
     Cross cell interference is a problem in such systems as illustrated in  FIG. 2 . If two different cell&#39;s UEs  14  are close to each other, their uplink transmissions interfere with the other UE&#39;s downlink transmissions in the same timeslot. As shown in  FIG. 2 , UE  14   1  uplink transmission U 1  interferes with UE  14   2  downlink transmission D 2 . Likewise, UE  14   2  uplink transmission U 2  interferes with UE  14   1  downlink transmission D 1 . Although the effective isotropic radiant power (EIRP) of UEs  14  is much less that base stations  12 , the close proximity of the UEs  14  results in the unacceptable interference. This problem is exacerbated when adding new users or user services. Although a cell&#39;s base station and UE  14  may make timeslot interference measurements, such as interference signal code power (ISCP), to assure its new transmissions will not see unacceptable interference, other cells&#39; users may end up experiencing unacceptable interference due to the new transmission. As a result, existing calls may be dropped or unacceptable quality of service (QOS) may occur. 
     Accordingly, it is desirable to reduce cross cell interference. 
     SUMMARY 
     A method for reducing cross cell interference in a wireless time division duplex communication system using code division multiple access, the system having at least one user equipment (UE) and a base station (BS) is disclosed. The method begins by measuring an interference level of each timeslot at the BS. A timeslot is eliminated for additional uplink communication if the measured interference level exceeds a first threshold. An interference level of each timeslot is measured at the UE, and the timeslot is eliminated for downlink communication for the UE if the measured interference level exceeds a second threshold. UEs in nearby cells that are large interferers are identified and their downlink timeslot usage is gathered. A timeslot is eliminated for uplink communication for a large interferer UE that uses the timeslot for downlink communication. 
     A system for reducing cross cell interference in a wireless time division duplex communication system using code division multiple access includes a user equipment (UE), a base station (BS), and a Node B. The UE includes an interference measurement device for measuring interference in a timeslot, a transmitter, and a receiver. The BS includes an interference measurement device for measuring interference in a timeslot, a transmitter, and a receiver. The Node B includes a resource allocation device configured to receive interference measurement values from the UE and the BS; eliminate timeslots for communication where the measured interference exceeds a first threshold; identify UEs in nearby cells that are large interferers; gather downlink timeslot usage for large interferer UEs; and eliminate a timeslot for uplink communication for a large interferer UE that uses the timeslot for downlink communication. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a wireless TDD/CDMA communication system. 
         FIG. 2  is an illustration of cross interference between UEs. 
         FIG. 3  is a flow chart for UE cross cell interference reduction. 
         FIG. 4  is a flow chart for determining potentially interfered UEs. 
         FIG. 5  is an illustration of neighboring cell UE usage. 
         FIG. 6  is an illustration of large interfering UE timeslot usage. 
         FIG. 7  is an illustration of available UE timeslots. 
         FIG. 8  is a simplified UE cross cell interference reduction system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although the UE cross cell interference reduction is explained in the context of unsectorized cells, the approach is extendable to any UE operating area division, such as sectors of a cell. In such an extension, each operating area, such as a sector, is treated as a separate cell in the analysis. 
       FIG. 3  is a flow chart for UE cross cell interference reduction. For each cell, the cell&#39;s base station  12  measures the interference level in each timeslot, such as by using ISCP, step  22 . The measured interference in each timeslot is compared to a threshold. If the measured interference in a timeslot exceeds the threshold, that timeslot is eliminated as a timeslot for any additional uplink communications in that cell, step  23 . The threshold level is typically set by the system operator. 
     Each UE  14  measures the interference level in each timeslot, such as by ISCP, step  24 . To determine available downlink timeslots for a particular UE  14 , the measured interference in each timeslot is compared to a threshold. The threshold level is typically set by the system operator. If the measured interference exceeds the threshold, that timeslot is eliminated for the downlink for that particular UE  14 , step  25 . 
     Another concern is whether a particular UE&#39;s new uplink transmissions will interfere with another cell&#39;s UE downlink transmission. In TDD, UEs  14  in the same cell do not transmit on uplink and downlink in the same timeslot. Since the transmissions are new, other cells&#39; UEs  14  cannot measure the resulting interference levels until the new transmissions begin. These new transmissions may result in a drop of a user or unacceptable QOS for existing users. 
     Determining other nearby cells&#39; UEs  14  which may interfere with a particular UE  14  is per the flow chart of  FIG. 4 . Each neighboring cell&#39;s UE uplink timeslot usage is gathered, step  29 . This usage is typically stored at the radio network controller (RNC)  42  and/or at the Node-B  46  (see  FIG. 8 ). Only the UE usage of nearby cells or, alternately, only adjacent cells are used. Further cells&#39; UEs  14  are too far away to suffer interference from the particular UE  14 . An example of nearby UE uplink usage is shown in  FIG. 5 . Each UE  14  is represented by a different letter, “B” to “L”. The particular UE  14  is an unshown letter “A”. 
     Using the particular UE&#39;s timeslot interference measurements, the timeslots are categorized into either a large or small interference category, step  30 . The small or large interference determination is performed such as by a threshold test. The threshold is typically set by the system operator. All nearby cell UEs  14  transmitting uplink communications in timeslots having a small interference are considered too far away to suffer interference from the particular UE&#39;s uplink communications, step  31 . All the other UEs are considered to be potentially interfered with by this UE&#39;s uplink communications, step  32 . 
     To illustrate using the example of  FIG. 5 , UE A has nearby UEs B-L. Uplink timeslots are indicated with a “U”. Out of the eight uplink timeslots (slots S 1 , S 3 , S 5 , S 7 , S 9 , S 11 , S 13 , S 15 ), three slots have large interferences (slots S 1 , S 3  and S 7 ) and five have small interferences (slots S 5 , S 9 , S 11 , S 13  and S 15 ). The UEs  14  transmitting in small interference uplink slots are UE C, D, F, G, H, I, J, K and L and in large interference uplink slots are UE B, D, F and H. Although UE D and F have an uplink transmission in a large interference cell, they also have an uplink transmission in a small interference cell. As a result, UE D and F are not considered the interfering UEs  14  in the large interference timeslots. In this example, UE B and H are determined to be the interfering UEs. 
     In this simplified example, there was no ambiguous information. However, ambiguous information may exist. For instance, if UE H also had an uplink transmission in a small interference cell, such as slot S 9 , the information is ambiguous. UE H would be considered both a large interferer in slot S 7  (being the only uplink user) and a small interferer in slot S 9 . In a conservative implementation, UE H could be deemed a large interferer. In a more aggressive implementation, UE H could be deemed a small interferer. There may be an unaccounted for interferer or interference source in that timeslot (slot S 7 ). 
     Another situation where ambiguous information may occur is where multiple potential large interferers transmit uplink communications in the same timeslots. To illustrate, UE H may also transmit in the uplink in slots S 1  and S 3 . As a result, UE B may or may not be a large interferer. UE H may be the only large interferer. In this case, UE B is still deemed a large interferer to be conservative. 
     After the large interferer UEs  14  are determined, step  26  ( FIG. 3 ), those UEs&#39; downlink timeslot usage is gathered, such as in  FIG. 6 , step  27 . For all the timeslots that the large interferers use for the downlink, that timeslot is eliminated for the uplink for that UE, as shown by an “X”, step  28 . As a result, a table such as in  FIG. 7  is produced. The table indicates which timeslots are available to the particular UE  14 . The available timeslots are blank and the non-available have an “X”. Timeslots are assigned to the particular UE by selecting from the non-eliminated timeslots. 
       FIG. 8  illustrates a simplified system implementation for cross cell UE interference reduction. The RNC  42  has a resource allocation device  44 . The resource allocation device  44  allocates the resources, such as code and timeslot assignments, for the cells. The resource allocation device  44  has an associated memory  45  for storing information, such as UE code and timeslot assignments, interference measurements and UE timeslot availability lists. Depending on the type of system, the computational component of cross cell UE interference reduction may be performed by the RNC resource allocation device  44 , the Node-B resource allocation device  48  or shared between the two. Typically, performing the computation at the Node-B  46  allows for faster updates. 
     The Node-B  46  communicates with the radio network controller  42 . The Node-B  46  has a resource allocation device  48  and an associated memory  49 . The resource allocation device  48  allocates resources to that Node-B′s users. The resource allocation device memory  49  stores information, such as the Node-B′s UE timeslot and code assignments, interference measurements and UE timeslot availability lists. 
     The Node-B  46  typically communicates with a group of base stations  12 . The base station  12  has a channel code and timeslot controller  54 . The channel code and timeslot controller  54  controls the timeslots and channel codes assigned to user communications as directed by the Node-B  46  and RNC  42 . A modulation and spreading device  56  processes data to be transmitted to the users. The data is processed to be time multiplexed with a channel code as directed by the channel and timeslot controller  54 . A transmitter  52  formats the processed data for transfer over the radio interface  78 . The resulting signal passes through an isolator or switch  58  and is radiated by antenna or antenna array  60 . 
     Signals are received by the base station  12  using the antenna or antenna array  60 . The received signals pass through the isolator or switch  58  to a receiver  50 . The receiver  50  processes the received signals with channel codes in the timeslots directed by the channel code and the timeslot controller  54  to recover the received user data. The base station  12  also has an interference measurement device  74 . The interference measurement device  74  measures the timeslot interference levels. 
     The UE  14  receives signals over the radio interface  78  using its antenna or antenna 1  array  62 . The received signals pass though an isolator or switch  64  to a receiver  68  to recover the received data for the user as directed by the channel code and timeslot controller  70 . The channel code and timeslot controller  70  sends the channel code and timeslot information to the receiver  68  and UE modulation and spreading device  72 . The controller  70  also retrieves the code and timeslot assignments signaled by the base station  12 . 
     A UE interference measurement device  76  measures the interference levels in the timeslots. The modulation and spreading device  72  processes user data with the channel codes and timeslots as directed by the UE controller  70 . The processed data is formatted for transmission over the air interface  78  by the transmitter  66 . The resulting signal passes through the isolator or switch  64  and is radiated by the antenna or antenna array  62 .