Abstract:
A packet scheduling method using a cumulative distribution function. The scheduling method for use in a communication system including a plurality of MSs (Mobile Stations) and a central controller for assigning resources to individual MSs on the basis of a transmission rate associated with downlink channels fed back from the MSs, includes the steps of generating a uniform random variable of transmission rates of individual MSs; converting the uniform random variable into a scheduling priority; comparing the scheduling priority of each MS with each other; and assigning resources to a MS having the highest scheduling priority.

Description:
PRIORITY  
       [0001]     This application claims priority to an application entitled “PACKET SCHEDULING METHOD USING CUMULATIVE DISTRIBUTION FUNCTION”, filed in the Korean Intellectual Property Office on Dec. 5, 2003 and assigned Serial No. 2003-88188, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a wireless communication system, and more particularly to a packet scheduling method using a cumulative distribution function of a transmission rate for every terminal in a wireless communication system.  
         [0004]     2. Description of the Related Art  
         [0005]     With the increasing development of wireless communication technologies, a typical voice service and a variety of multimedia data services in the mobile communication field have continued to be enhanced and developed.  
         [0006]     In order to efficiently provide a multimedia service requiring a high-speed transmission rate in a mobile communication network, a variety of technologies are used, for example, an HDR (High Data Rate) for a CDMA (Code Division Multiple Access) scheme, and an HSDPA (High Speed Downlink Packet Access) scheme for the 3GPP (3rd Generation Partnership Project), etc., such that a variety of scheduling algorithms are to efficiently provide high-speed multimedia services and to maximize system capacity.  
         [0007]     Provided that a wireless channel acting as a time-varying channel transmits the data traffic, which is less sensitive to transmission delay as compared to voice traffic, only when good channel states are supplied to individual users, can the wireless channel acquire a high transmission rate. A representative example of the above technology is a CDMA  1 × EV/DO downlink data transmission scheme. The CDMA  1 × EV/DO system feeds its own highest transmission rate to a base station. The base station receives transmission rates from all the terminals, and carries out a packet scheduling operation based on the received transmission rates.  
         [0008]     The scheduling method having the highest average transmission rate is an MR (Maximum Rate) scheduling method for unconditionally selecting a terminal requesting the highest transmission rate from among a plurality of terminals. However, all of the users do not always use the same channel so that there arises a difference in average transmission rates of the users according to channel environments, resulting in an unequal application of the transmission rate scheduling. In order to solve the aforementioned problem, there has recently been proposed a variety of scheduling algorithms, for example, a PF (Proportional Fair) scheduling algorithm, and an Opportunistic Transmission (OT) scheduling algorithm, etc.  
         [0009]     The PF scheduling algorithm selects a user having a current feedback transmission rate greater than an average transmission rate of individual users, such that it can proportionally and fairly distribute system resources among the users. However, the PF scheduling algorithm assumes that the ratio of a current instantaneous transmission rate and an average transmission rate is equally distributed to all the users. Therefore, if the above assumption is not satisfied, a user having an average-high transmission rate can receive many more services than a user having an average-low transmission rate, such that users assigned to inferior channels may experience inconvenience while using their desired services. Furthermore, the PF scheduling algorithm has another disadvantage in that it is unable to supply different QoSs (Quality of Services) for different users.  
         [0010]     The opportunistic transmission scheduling algorithm determines a predetermined ratio indicative of the number of average service times of the individual users in advance, and maximizes the sum of the average transmission rates of individual users such that a ratio indicative of the number of average service times is satisfied. The opportunistic transmission scheduling algorithm can adjust only average service ratios of individual users, such that a user having a superior channel has an advantage over a user having an inferior channel in the same way as in the PF scheduling algorithm.  
         [0011]     For example, in the case of scheduling two users, it is assumed that a first user has an average transmission rate of 100, a second user has an average transmission rate of 20, the first and second users adapt the Gaussian Distribution, and the ratio of the number of average service times of individual users is 1:1. Provided that a feedback value of the first user is  121 , and a feedback value of the second user is  40 , the opportunistic transmission scheduling algorithm allows the first user to have a difference of 21 between a current transmission rate and an average transmission rate, and allows the second user to have a difference of 20 between a current transmission rate and an average transmission rate, such that the first user is selected by the opportunistic transmission scheduling algorithm. However, the first user receives a transmission rate greater than an average transmission rate by 21%, and the second user receives a transmission rate greater than an average transmission rate by 100%, such that the second user is unable to receive a desired service even though a superior channel environment generated at rare intervals is provided. Therefore, in order to provide the second user with a desired service, the channel state of the first user must be inferior to that of the second user, such that an average transmission rate for the second user is decreased. Provided that a third user having an average transmission rate of 10, instead of the first user, enters into competition with the second user, the second user has an advantage over the third user. In more detail, due to a relative relationship between a reference user&#39;s channel and another user&#39;s channel, a difference in average transmission rates of the users occurs. In conclusion, the users who participate in the competition have different average transmission rates. In this case, provided that different QoSs are assigned to transmission rates for every user, there is no solution to adjust such different QoSs.  
       SUMMARY OF THE INVENTION  
       [0012]     Therefore, the present invention has been made in view of at least the above problems, and it is an object of the present invention to provide a wireless packet scheduling method for numerically expressing a relationship between a current transmission rate an ideal transmission rate upon receiving feedback information from individual users indicating the number of times the ideal transmission rate occurs, comparing the numerical result of one user with those of other users, selecting one user who has the highest numerical result from among a plurality of users, and performing a scheduling process for the selected user having the highest numerical result.  
         [0013]     It is another object of the present invention to provide a wireless packet scheduling method for comparing current transmission rate information of a specific user with previous feedback transmission rate information, to absolutely compare the transmission rates of all users who have different transmission rate distributions, resulting in an improved fairness.  
         [0014]     In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a scheduling method for use in a communication system including a plurality of MSs (Mobile Stations) and a central controller for assigning resources to individual MSs on the basis of a transmission rate associated with downlink channels fed back from the MSs, comprising the steps of generating a uniform random variable of transmission rates for each MS; converting the uniform random variable into a scheduling priority; comparing the scheduling priorities of the MSs with each other; and assigning resources to a MS having the highest scheduling priority.  
         [0015]     Preferably, the uniform random variable is a cumulative distribution function of transmission rates of the MSs.  
         [0016]     In accordance with another aspect of the present invention, there is provided a scheduling method for use in a wireless communication system including k MSs (Mobile Stations) and a BS (Base Station) for selecting one MS to be assigned with an n-th timeslot on the basis of transmission rate information m k (n) associated with downlink channels fed back from an MS ‘k’, comprising the steps of: generating a uniform random variable U k (n) using transmission rate information received from an individual MS ‘k’; converting the uniform random variable U k (n) into a scheduling priority U k (n) 1/w     k   ; comparing the scheduling priority U k (n) 1/w     k    of each MS with each other; and assigning the n-th timeslot to a MS having the highest scheduling priority U k (n) 1/w     k    (where,  
         (     where   ,         ∑     k   =   1     K     ⁢     w   m       =   1       )     .       
 
 Preferably, the uniform random variable U k (n) is a cumulative distribution function F R     k   (r) of a transmission rate R k (n) corresponding to the transmission rate information m k (n). Preferably, the cumulative distribution function F R     k   (r) is updated using a probability density function associated with the transmission rate. Preferably, the step of updating the cumulative distribution function F R     k   (r) further includes the step of, if the probability density function is set to p k,m ≡Pr(R k (n)=r k,m ) and the cumulative distribution function is set to  
           q     k   ,   m       ≡       ∑     i   =   1     m     ⁢     p     k   ,   i           ,       
 
 updating the probability density function using an equation denoted by p k,m ←λp k,m +(1−λ)1 m=m     k     (n)  (where, λ is 0&lt;λ&lt;1, and 1 A  is an indicator which is set to ‘1’ when the condition ‘A’ is satisfied or is set to ‘0’ when the condition ‘A’ is not satisfied); and updating the cumulative distribution function using an equation  
         q     k   ,   m       ←       ∑     i   =   1     m     ⁢       p     k   ,   i       .           
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0018]      FIG. 1  is a diagram illustrating a wireless access network for use with a wireless packet scheduling method in accordance with a preferred embodiment of the present invention;  
         [0019]      FIG. 2  is a flow chart illustrating a wireless packet scheduling method in accordance with a preferred embodiment of the present invention;  
         [0020]      FIG. 3  is a graph comparing an inventive scheduling method, an MR scheduling method, and an opportunistic transmission scheduling method in consideration of scheduling fairness in accordance with a preferred embodiment of the present invention; and  
         [0021]      FIG. 4  is a graph comparing an inventive scheduling method and an opportunistic transmission scheduling method in consideration of other terminals&#39; distribution effects associated with an average service reception quantity in accordance with a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     Now, preferred embodiments of the present invention will be described in detail with reference to the annexed drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted when it may obscure the subject matter of the present invention.  
         [0023]     The present invention relates to a wireless packet scheduling method for determining a scheduling priority (also called a scheduling metric factor) using statistical characteristics of channel information fed back from a terminal to a base station, and improves the fairness of a current wireless channel scheduling algorithm, so that it can satisfy different QoSs of a plurality of users.  
         [0024]      FIG. 1  is a diagram illustrating a wireless access network for use with a wireless packet scheduling method in accordance with a preferred embodiment of the present invention.  
         [0025]     Referring to  FIG. 1 , a base station BS  10  provides individual user equipment denoted by MS (Mobile Station)  23  and MS  25  with data services over a wireless channel  30 . The BS  10  includes buffers  13  and  15  for receiving/storing packet data to be transmitted to the MS  23  and MS  25 . The scheduler  17  transmits data stored in the buffers  13  and  15  to corresponding MS  23  and MS  25  in descending priority order. The scheduler  17  assigns scheduling priorities (i.e., scheduling metric factors) to individual MSs using channel state information periodically fed back from the MS  23  and MS  25 .  
         [0026]     Individual MS  23  and MS  25  contained in the aforementioned wireless access network measure a downlink channel, calculate a maximum rate (MR) supportable from the downlink channel, and feed back the calculated MR to the BS  10 .  
         [0027]     The scheduler  17  of the BS  10  extracts the MR information from feedback information received from individual MS  23  and MS  25 , estimates a channel distribution (i.e. a histogram) associated with MS ‘k’ (k=1, 2, . . . , K; where K is the total number of MSs), and generates a cumulative distribution function (cdf) F R     k   (r) (where, k=1, 2, . . . , K). The scheduler  17  acquires priority of a timeslot ‘n’ of individual user ‘k’ using the cumulative distribution function (cdf). Priority k*(n) of a user ‘k’ in the n-th timeslot is calculated by the following Equation 1:  
                 k   *     ⁡     (   n   )       =     arg   ⁢           ⁢       max   k     ⁢       [       F     R   k       ⁡     (       R   k     ⁡     (   n   )       )       ]       1   /     w   k                     (   1   )             
 
         [0028]     With reference to Equation 1, R k (n) is a maximum rate (MR) capable of being transmitted to the MS ‘k’ over the timeslot ‘n’, and w k  indicative of a weight factor assigned to the MS ‘k’ is determined by  
           ∑     k   =   1     K     ⁢     w   k       =   1.       
 
         [0029]     The scheduler  17  compares priorities k*(n) associated with MS k as calculated by Equation 1, so that it assigns the n-th timeslot to an MS having the highest priority.  
         [0030]      FIG. 2  is a flow chart illustrating a wireless packet scheduling method in accordance with a preferred embodiment of the present invention.  
         [0031]     The scheduling method of the present invention assumes that K MSs compete with each other to receive their resources, and an MR of MS ‘k’ is determined to be R k (n)ε{r k,1 ,r k,2  , . . . , r k,M }, r k,1 &lt; . . . &lt;r k,M . Provided that r k,m     k   (n) is determined to be an MR, the MS ‘k’ feeds back a transmission rate index m k (n)ε{1, 2, . . . M} of the n-th timeslot to the BS. A probability density function (pdf) associated with the transmission rate of the MS ‘k’ is p k,m ≡Pr(R k (n)=r k,m ), and a cumulative distribution function (cdf)  
         q     k   ,   m       ≡       ∑     i   =   1     m     ⁢     p     k   ,   i             
 
 (where k=1, 2, . . . , K, and m=1, 2, . . . , M, where M is a data rate index or a modulation scheme index.). For the convenience of description, q k,0  is zero. 
 
         [0032]     Referring to  FIG. 2 , according to the wireless packet scheduling method of the present invention, an MS ‘k’ feeds back a transmission rate index m k (n) to the BS at step S 21 . Upon receiving the transmission rate index m k (n) at step S 21 , the scheduler of the BS generates a uniform random variable U k (n) at intervals of [q k,m     k(n)−1   , q k,m     k(n)   ] at step S 22 , and converts the uniform random variable U k (n) into scheduling metric information U k (n) 1/w     k    associated with the MS ‘k’ at step S 23 . In this case, the scheduling metric factor U k (n) 1/w     k    can also be considered to be scheduling priority information.  
         [0033]     The scheduler acquires scheduling priority information of individual MSs, compares the scheduling priority information of the individual MSs, and selects a MS having the highest priority from among the MSs at step S 24 . In this case, a maximum rate (MR) can be represented by the following Equation 2:  
                 k   *     ⁡     (   n   )       =     arg   ⁢           ⁢       max   k     ⁢         U   k     ⁡     (   n   )         1   /     w   k                     (   2   )             
 
         [0034]     If the MS having the highest priority is selected, the BS assigns the n-th timeslot to the MS k*(n) having the highest priority, and transmits data to the MS k*(n) at step S 25 . A probability density function (pdf) p k,m  and a cumulative distribution function q k,m  of a corresponding MS can be represented by the following Equations 3 and 4, respectively:  
               p     k   ,   m       ←       λ   ⁢           ⁢     p     k   ,   m         +       (     1   -   λ     )     ⁢     1     m   =     m   k                     (   3   )                 q     k   ,   m       ←       ∑     i   =   1     m     ⁢     p     k   ,   i                 (   4   )             
 
         [0035]     With reference to Equations 3 and 4, λ is 0&lt;λ&lt;1, and 1 A  is an indicator, where A represents the condition m=m k (n). The value of 1 A  is ‘1’ when the condition ‘A’ is satisfied, i.e. when m equals m k (n), and is ‘0’ when the condition ‘A’ is not satisfied.  
         [0036]      FIG. 3  is a graph comparing an inventive scheduling method, an MR scheduling method, and an opportunistic transmission scheduling method in consideration of scheduling fairness in accordance with a preferred embodiment of the present invention.  
         [0037]     The aforementioned simulation for comparing performances of the individual scheduling methods is provided at a predetermined condition in which the number K of overall MSs is 5, average transmission rates of individual users are each set to ‘m=10’, standard deviations σ k  of an individual user k (where k=1, 2, 3, 4, and 5) is determined to be 1.2, 1.4, 1.6, 1.8, and 2.0, respectively.  
         [0038]     Referring, to  FIG. 3 , the MR scheduling method shows unequal allocation of transmission times of individual MSs, such that an MS having a larger variation occupies a longer transmission time. On the other hand, the opportunistic transmission method and the inventive scheduling method indicate that individual MSs occupy almost the same transmission time.  
         [0039]      FIG. 4  is a graph comparing an inventive scheduling method and an opportunistic transmission scheduling method in consideration of other terminals&#39; distribution effects associated with an average service reception quantity in accordance with a preferred embodiment of the present invention. In this simulation of  FIG. 4 , a standard deviation of the fifth user is divided into two values 1 and 2 on the condition that distributions of other users are maintained in such a way that the graph of  FIG. 4  is provided.  
         [0040]     As can be seen from  FIG. 4 , the opportunistic transmission algorithm changes an average service reception quantity of each MS according to a standard deviation variation of the fifth MS (i.e., a fifth MS&#39;s distribution). Therefore, the higher the standard deviation of the fifth terminal, the lower the average service reception quantity of a specific MS. Furthermore, the opportunistic transmission algorithm is unable to maintain the ratio of a scheduling gain and a standard deviation.  
         [0041]     On the other hand, the scheduling method of the present invention can control the first to fourth MSs to maintain the same average server reception quantity, irrespective of the standard deviation variation of the fifth MS.  
         [0042]     As can be seen from the comparison simulations of  FIGS. 3 and 4 , the scheduling method of the present invention has a performance superior to that of the opportunistic transmission method in association with fairness or scheduling gain field.  
         [0043]     As apparent from the above description, the scheduling method of the present invention selects an MS to be scheduled on the basis of a reception rate distribution of a specific MS, irrespective of probability distributions of other MSs, so that it can predict in advance the average service reception rates of individual users. Although several MSs have non-identical channels, individual MSs are operated as if their channels were identical with channels of other MSs. Although individual MSs have different requirements, the scheduling method of the present invention can provide individual MSs with effective services. Furthermore, the scheduling method of the present invention can easily increase an average transmission rate of a user having an inferior channel until reaching a predetermined transmission rate.  
         [0044]     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.