Abstract:
In a cellular radio system, the number of TDM users per cell is limited. The cellular radio system can be a WCDMA system and in particular a WCDMA system employing an Enhanced Uplink (EUL). Other users in the cell are scheduled using CDM scheduling.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 U.S.C. §371 National Phase Entry Application from PCT/SE2009/050349, filed Apr. 3, 2009, and designating the United States. 
     TECHNICAL FIELD 
     The present invention relates to a method and a device for scheduling users in a cellular radio system. 
     BACKGROUND 
     Traditionally, Code Division Multiple Access (CDMA) systems such as Interim Standard-95 (IS-95) and Wideband Code Division Multiple Access (WCDMA) are designed to handle many low or medium bit rate users, creating a rather smooth and relatively slow changing interference. However, the current trend goes towards much more bursty traffic with high rate demands. Examples of application resulting in such behavior include for example World Wide Web (WWW) applications and peer-to-peer traffic. Such applications will generate highly varying data rates compared to speech and video services typically are associated with rather constant and moderate bit rates. 
     In WCDMA users are non-orthogonal to each other in the up link (UL) thereby generating interference between each other even within the same cell. Therefore the system has an upper interference limitation, where the cell noise can not be increased further if the system shall remain stable. This limits the maximum cell capacity. 
     One way to handle both the more bursty traffic and the lack of orthogonality is to use Time-division between the users instead of codes, and thereby increase the UL WCDMA efficiency, see  FIG. 1 . In  FIG. 1  the principle difference between normal code multiplexed scheduling and time division scheduling is schematically depicted for three users. As is apparent the use of a time division scheduling increases the possibility to transmit high data rates. 
     Time Division Multiple Access (TDMA) is a well-known method for divide the resources in cellular system. 
     The basis of a TDM scheme for WCDMA Enhanced Uplink (EUL) with a 2 ms Transmission Time Interval (TTI), is depicted in  FIG. 2   a . The TDM users are typically scheduled per Hybrid Automatic Repeat Request (HARQ) process. In EUL there are 8 HARQ processes for a 2 ms TTI. As a result, if one user is scheduled for HARQ process x, it typically is x+8 TTIs until the next transmission. To use the same HARQ process number is efficient since if user has to retransmit, it can automatically use the previous HARQ process number for the retransmission, without having to perform any scheduling. It should also be noted that TDM users can also use two (or more) continuous HARQ processes, as shown in  FIG. 2   b.    
     When CDM is used, All HARQ process in accordance with 3GPP specification TS 25.321 is activated for the UE, i.e. as long as the UE has a grant greater than zero, it can use any HARQ process to transmit the data, until a grant zero (i.e. not allowed to transmit any data) is received. For the 2 ms TTI it is also possible to use the “Per HARQ process”, see 3GPP specification TS 25.321, chapter 9.2.5.2.2. This can be used to achieve time division between users. Node-B schedule allocates only one user for each HARQ process, i.e. Node-B transmits an absolute grant to the UE which is valid for a specific HARQ process until a new grant is received. The valid HARQ process is decided by the CURRENT_HARQ_PROCESS_ID, see section 11.8.1.4 in 3GPP Specification TS 25.321. 
     Further, in  FIG. 3  a rough estimate of the cell capacity for different number of users per cell using theoretical calculations is shown. In  FIG. 3  it is assumed that the users transmit simultaneously, so that full buffers are assumed.  FIG. 3  shows the big difference in terms of capacity between one user transmitting such as in a Time Division Multiplexing scheme and one or more users transmitting such as in a Code Division Multiplexing (CDM) scheme. As can be seen in  FIG. 3 , the lack of orthogonality between users in the same cell quickly decreases the possible cell capacity. A cell with TDM scheduling will be able to maintain the high cell throughput for more than one user. The TDM capacity will also decrease with the number of users, due to more control signaling and less efficient TDM scheduling, but much more slowly than for a CDM system. The aim with TDM is to even with quite many high data rate user be able to maintain the cell capacity. 
     A problem arising when using TDM scheduling for a EUL WCDMA system, is that the 3GPP specification makes it difficult to handle too many TDM users efficiently. Thus, when many TDM users use the up-link simultaneously there will be an unacceptably long time between the transmission attempts for each user. A long time between the transmission attempts for each user will negatively impact the user experiences because it:
         gives bad throughput for TDM users—not able to reach high bit rates,   has a long ping time for first packet, and   is not acceptable for any time critical data.       

     Another problem is related to the fact that some TDM users are not utilizing the bandwidth available in a TDM scheme. Examples of such applications are Voice over IP (VoIP) and chat applications such as chat clients, email, presence etc. In  FIG. 4 , the problem is illustrated. As can been seen in  FIG. 4  the user  2  is not fully utilizing the bandwidth provided in the TDM scheme. 
     There is a constant desire to improve the utilization bandwidth in radio communication. Hence there exists a need to improve the use of bandwidth in a cellular radio system, in particular a WCDMA radio system. 
     SUMMARY 
     It is an object of the present invention to improve the bandwidth utilization in radio communication. 
     This object and others are obtained by the method and device as set out in the appended claims. 
     Thus, in accordance with the present invention a limit to the number of TDM users per cell is provided in a cellular radio system, such as WCDMA system and in particular a WCDMA system employing an Enhanced Uplink (EUL). Other users in the cell are scheduled using CDM scheduling. 
     In accordance with one embodiment a method is provided in accordance with which users are moved from a CDM mode to a TDM mode if a user activity measurement is higher than a threshold value for a certain period of time. 
     In accordance with one embodiment a method is provided in accordance with which users are moved from a TDM mode to a CDM mode if a user activity measurement is lower than a threshold value for a certain period of time. 
     Hereby an efficient method for utilizing available bandwidth is obtained whereby users can be efficiently scheduled in both a TDM mode and a CDM mode simultaneously. 
     The invention also extends to a device and a node in a cellular radio system adapted to perform the method as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawings, in which: 
         FIG. 1  is a view illustrating CDMA transmission and TDM transmission, 
         FIGS. 2   a  and  2   b  is a view illustrating different scheduling of TDM users, 
         FIG. 3  is a view illustrating the utilization of bandwidth for different number of users in a cell, 
         FIG. 4  is a view illustrating use of bandwidth for different types of applications, 
         FIG. 5  is a flowchart illustrating procedural steps performed when scheduling users in a cellular radio system, 
         FIG. 6  is a scheduler adapted to schedule users, and 
         FIG. 7  is a view illustrating division of HARQ process numbers into TDM slots or CDM slots. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the present invention a scheduler is provided, which is adapted to schedule users both in a CDM scheme and in a TDM scheme in response to available radio resources in a cell of a cellular radio system. 
     In  FIG. 5  an exemplary flow chart illustrating logical steps performed in a scheduler for scheduling users in a cell is shown. The cellular radio system in which the scheduler schedules users can for example be a WCDMA system. 
     First in a step  501  it is checked if each user activity measurement is higher than a threshold value for a certain period of time. If the user activity is higher than the threshold value, the user is moved from a CDM mode to a TDM mode, in a step  503 . Next in a step  505 , it is checked if a user activity measurement is lower than a threshold value for a certain period of time. If the activity is lower than the threshold value users are moved from a TDM mode to a CDM mode in a step  507 . The procedure can then be repeated by returning to step  501 . 
     In  FIG. 6  an exemplary scheduler  600  is depicted. The scheduler  600  comprises a module  601  for providing measurements. The module  601  can in accordance with one embodiment comprise means for generating measurements within the scheduler. In accordance with another embodiment the module  601  is adapted to receive an external feed of measurements from another unit within the radio system. The scheduler  600  further comprises a user moving module  603  connected to the measurement module  601  adapted to move users of a cellular radio system between a CDM mode to a TDM mode. In particular the module  603  is adapted to move users between a CDM mode to a TDM mode in response to measurements provided by the measurement module  601 . 
     Below different exemplary methods for measuring the activity of TDM and CDM users are described. These measurements can then be used to decide if a user should be assigned to TDM mode or CDM mode. 
     In order to measure the activity measurement to decide if a CDM user shall be moved to TDM mode, one or more of the following parameters for monitor each UE can be monitored:
         given grants, or   estimate the rate used (in node B), or   estimated received power in node B,   a combination of above measurement       

     In accordance with one embodiment an averaging window can be implemented, including periods of no transmission, to get the average activity. 
     In order to measure the activity measurement to decide if a TDM user shall be moved to CDM mode, one or more of the following parameters for monitor each UE can be monitored:
         estimate the rate used (in node B), or   estimated received power in node B, or   a combination of above measurement       

     The given grant can also be measured for the TDM users, but it can be less efficient, since it may be that the TDM scheduling assigns a higher grant to a TDM user that it is actually using. 
     In accordance with one embodiment, threshold decides if the CDM UE activity is high enough to be in TDM mode. The threshold can be fixed, but may also be a variable threshold. A variable threshold can for example be set up as a function of the TDM load in a cell. In accordance with one embodiment the variable threshold can be implemented as:
         High TDM load→high activity needed to be in TDM mode   Medium TDM load→medium activity needed to be in TDM mode   Low TDM load→all EUL users allowed in TDM mode       

     Another threshold can be set to decide when a TDM users&#39; activity is low enough to be switched to the CDM mode. The threshold can be a constant, for example zero, or a variable threshold can be used such a s function of the TDM/CDM load as described above 
     In accordance with another exemplary embodiment, the TDM slot resources are divided among the TDM users based on their activity. For example, if TDM user i has two times as high activity compared to TDM user j, TDM user I will get two times more TDM slots. The number of TDM slots a TDMN user can achieve can be expressed more general as: 
     
       
         
           
             
               TDM_slot 
               i 
             
             = 
             
               ⌈ 
               
                 8 
                 · 
                 
                   
                     activity 
                     i 
                   
                   
                     ∑ 
                     activity 
                   
                 
               
               ⌉ 
             
           
         
       
     
     In accordance with another exemplary embodiment, the HARQ process numbers (“TDM slots”) are divided into TDM slots or normal WCDMA (CDM) slots. This approach is depicted in  FIG. 7 . When the activity of a TDM user is below an activity threshold, it is assigned to the CDM slots. There can be more than one user assigned to these CDM slots, i.e. more than one user can be scheduled simultaneously at these CDM slots. If a user exceeds an activity threshold, it can instead be assigned to one of the TDM slots. For each TDM slot, there can only be assigned one single user, i.e. one user at a time can be scheduled at that TDM slot (HARQ process number). 
     By using the method a device as described herein it is possible to achieve a more efficient use of TDM resources. For example, it is possible to achieve high TDM user peak bit rates. It is further possible to obtain a higher total capacity. Further, in a scenario with high TDM load and many TDM users it is possible to switch the users with lowest activity to normal CDM scheduling in order to retain the peak bit rate for the remaining TDM users.