Abstract:
Disclosed is an upstream access method in an Orthogonal Frequency Division Multiple Access (OFDMA)-based mobile communication system in which a random access method in an OFDMA network includes randomly selecting, by each subscriber device, one of a plurality uplink random access channels, if a random access request to the OFDMA network is transmitted by each subscriber device, accessing through the selected channel if a collision has not occurred in the selected channel, randomly selecting one of the uplink random access channels, if a collision has occurred in the selected channel, and if the number of retries for channel selection is less than a predetermined value, and determining whether a has collision has occurred, accessing through a finally selected channel, if it is determined that a collision has occurred; and randomly selecting one of the uplink random access channels if it is determined that a collision has occurred.

Description:
PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. §119 to an application entitled “Random Access Method in Multi-Cell OFDMA Networks” filed in the Korean Intellectual Property Office on Aug. 27, 2004 and assigned Serial No. 2004-68151, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to an upstream access method in an Orthogonal Frequency Division Multiple Access (OFDMA)-based mobile communication system, and in particular, to a random access method.  
         [0004]     2. Description of the Related Art  
         [0005]     In a next-generation mobile communication system, high speed and high quality data transmission will be required in order to support various multimedia services having an enhanced quality. Recently, an Orthogonal Frequency Division Multiple Access (OFDMA) technique, which is one of the techniques that meets the requirement for high-speed and high-quality data transmission, has been developed.  
         [0006]     A block diagram illustrating a typical OFDMA communication system is shown in  FIG. 1 . The OFDMA communication system has a multi-cell structure, comprised of a base station (BS)  11 , a network  100  connected to the BS  11  to access Internet protocol (IP) networks (not shown), and a plurality of mobile terminals (MTs)  12 - 1  and  12 - 2  assigned to each BS  11 .  
         [0007]     An Orthogonal Frequency Division Multiplexing (OFDM)/OFDMA technique is used for signal transmission/reception between the BS  11  and the MTs  12 - 1  and  12 - 2 .  
         [0008]     Moreover, when a plurality of subscriber devices (i.e., MTs) access the same BS, each subscriber device should use a random access channel of a random access method to initially access a network.  
         [0009]     In wireless communication networks such as Wideband Code Division Multiple Access (W-CDMA), also, a random access method of an ALOHA system is used for an MT to access a BS in uplink. In addition, in these wireless communication networks, the random access method is used for all subscriber devices to access channels shared by cells. Besides, for a voice service, the random access method is used by each subscriber to request an independent channel allocation.  
         [0010]     In next-generation mobile communication networks, which will include various data services, the random access method is well defined since there is a good possibility that it would be frequently used to apply a packet scheduling scheme to the uplink. However, sufficient studies of the random access method in next-generation mobile communication networks have not been performed as of yet.  
         [0011]     On example of a deficiency of the random access method is that conventional technologies related to the random access method consider only a single channel of a single cell.  
         [0012]     A flowchart illustrating a conventional random access method using a single channel is shown in  FIG. 2 . In the conventional random access method, if a random access request is transmitted by each subscriber device in step  201 , it is determined whether a collision has occurred in step  202 .  
         [0013]     As a result of the determination, if a collision has not occurred, the access is successfully achieved in step  204 . However, if a collision has occurred, each collided subscriber device backlogs its random access request and waits for a random time in step  203 , and attempts a new random access request in step  201 .  
         [0014]     An illustrative diagram of the conventional random access method using a single channel is shown in  FIG. 7 . As a common case, if a random access request “a” is transmitted by a subscriber device A, an uplink access can be achieved at a randomly requested slot  701 . However, if random access requests “a” and “b” are simultaneously transmitted by two subscriber devices A and B respectively, and a collision occurs at a requested slot  703 , the two subscriber devices A and B must then perform a backoff operation for random times  71  and  72  respectively. Thereafter, the two subscriber devices A and B must perform the uplink access at different slots  706  and  710 , respectively, using new random access requests.  
         [0015]     According to papers entitled “Throughput Analysis of Multichannel Slotted ALOHA Systems in Multiple Log-Normal and Rayleigh Interferers Environment” (VTC92) and “Multichannel ALOHA data Networks for Personal Communications Services” (Globecom 92), an access to multiple channels was assumed and analyzed. Thereafter, studies were made of combining coding techniques in multiple channels  
         [0016]     However, in an OFDMA system using a plurality of subcarriers, although throughput of cells can be raised, a frequency reuse rate must be considered. Unfortunately, studies considering the frequency reuse rate have not been carried out. Accordingly, a study considering the frequency reuse rate of a random access method in an OFDMA system is desirable.  
       SUMMARY OF THE INVENTION  
       [0017]     An object of the present invention is to provide a random access method in a multi-cell OFDMA network which can be efficiently used considering a frequency reuse rate in a network system having OFDMA-based multiple cells.  
         [0018]     Another object of the present invention is to provide a random access method in a multi-cell OFDMA network which enables effective collision avoidance by obtaining an optimal value of the number of access retries for resource reuse of random access channels in a network system having OFDMA-based multiple cells.  
         [0019]     According to one aspect of the present invention, there is provided a random access method in an Orthogonal Frequency Division Multiple Access (OFDMA) network, the method including the steps of (1) if a random access request to the OFDMA network is issued by each subscriber device, randomly selecting, by each subscriber device, one of uplink random access channels; (2) if no collision occurs in the selected channel, accessing through the selected channel; (3) if a collision occurs in the selected channel, and if the number of retries for channel selection is less than a predetermined value, randomly selecting one of the uplink random access channels and determining whether a collision occurs; (4) if no collision occurs as a result of the determination of step (3), accessing through a finally selected channel; and (5) if the collision occurs as the result of the determination of step (3), jumping to step (3). 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0021]      FIG. 1  is a schematic structural diagram of a typical OFDMA communication system;  
         [0022]      FIG. 2  is a flowchart illustrating a conventional random access method using a single channel;  
         [0023]      FIG. 3  is a flowchart illustrating a random access method in a multi-cell OFDMA network in which a reuse rate of multiple channels is considered according to a preferred embodiment of the present invention;  
         [0024]      FIG. 4  is a flowchart illustrating a process of setting a retry number m applied to the random access method in the multi-cell OFDMA network according to the preferred embodiment of the present invention;  
         [0025]      FIGS. 5A  to  5 C are illustrative diagrams of experimental results performed in a single cell based on the random access method in the multi-cell OFDMA network according to the preferred embodiment of the present invention;  
         [0026]      FIGS. 6A and 6B  are illustrative diagrams of experimental results performed in multiple cells based on the random access method in the multi-cell OFDMA network according to the preferred embodiment of the present invention;  
         [0027]      FIG. 7  is an illustrative diagram of the conventional random access method using a single channel; and  
         [0028]      FIG. 8  is an illustrative diagram of the random access method in which a reuse rate of multiple channels is considered according to the preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]     Preferred embodiments of the present invention will be described herein below with reference to the accompanying 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, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.  
         [0030]     In the preferred embodiment of the present invention, it is assumed that the number of uplink random access channels of all cells is N.  
         [0031]     A flowchart illustrating a random access method in a multi-cell OFDMA network in which a reuse rate of multiple channels is considered according to a preferred embodiment of the present invention is shown in  FIG. 3 .  
         [0032]     Referring to  FIG. 3 , according to the random access method of the present invention, a random access request retry number is set to 0 in step  300 . A random access request is then transmitted by each subscriber device (i.e., an MT) in step  301 , one of multiple channels is randomly selected by each MT, in step  302 . That is, as a result of multiple channels as opposed to a single channel, if two or more random access requests collide, because they were sent using the same time slot on the same channel, then each of the random access requests can be allocated to each of a plurality of random channels, thereby maximizing the probability of avoiding a collision by selecting and transmitting using the random channels, without requiring the operation of backlogging as is taught by the prior art.  
         [0033]     Moreover, because multiple channels are randomly selected, the likelihood of a collision is reduced.  
         [0034]     After a random channel is selected in step  302 , it is determined whether a collision has occurred in the selected channel in step  303 , and if a collision has not occurred, the access is successfully achieved in step  307 .  
         [0035]     As a result of the determination in step  303 , if the collision occurs, it is determined whether the number of retries of each collided subscriber device is greater than a predetermined threshold “m” in step  304 . If the number of retries is less than the predetermined threshold m, a retry number is increased ( FIG. 3  illustrates that “0” is given for an initial random access request retry.) in step  305 , and thereafter the random channel selection procedure is repeated in step  302 .  
         [0036]     As a result of the determination in step  304 , if the number of retries of each collided subscriber device is greater than the predetermined threshold m, the random access request of each corresponding subscriber device is backlogged for a random time in step  306 , and a new random access request is transmitted in step  300 .  
         [0037]     In the above description, collisions are an indicative of a subscriber device&#39;s failure to access a random access channel because of a collision of its random access request with another subscriber station&#39;s random access message.  
         [0038]     An illustrative diagram of the random access method using a reuse rate of multiple channels according to the preferred embodiment of the present invention is shown in  FIG. 8 . Random access channels according to the preferred embodiment of the present invention include five channels  81 ,  82 ,  83 ,  84  and  85  including time slots  811  to  820 ,  821  to  830 ,  831  to  840 ,  841  to  850  and  851  to  860  on the same time axis, respectively.  
         [0039]     If random access requests are transmitted by subscriber devices A, B and C, channels are randomly selected by the corresponding subscriber devices. That is, according to the illustrative diagram, the channel  81  is selected for subscriber device A, the channel  84  is selected for subscriber device B, and the channel  85  is selected for subscriber device C. Accordingly, uplink accesses of the subscriber devices can be achieved at randomly requested slots  811 ,  841  and  851 , respectively.  
         [0040]     However, when more than two subscriber devices issue random access requests and select random channels, a situation in which two subscriber devices D and M simultaneously select the same slot (e.g., slot  823 ) may occur. Likewise, when a plurality of subscriber devices simultaneously select the same slot of the same channel, unlike the prior art teaching of backlogging (i.e., waiting for random times, and again attempt random access requests), each collided subscriber device determines a next slot by randomly selecting a channel again. As a result, the collided subscriber devices (e.g. D and M) can successfully achieve uplink accessing slots  814  and  844 , respectively.  
         [0041]     In a case where a collision occurs between two subscriber devices I and J at slot  836 , a random channel selection is performed, and if a collision again occurs at the slot  847 , it is determined whether the number of random channel selections exceeds a predetermined retry number m, and if the number of random channel selections does not exceed the predetermined retry number m, a procedure in which each collided subscriber device (e.g., I and J) randomly selects a channel is repeated. As a result, the subscriber devices which collided at slots  836  and  847  successfully achieve uplink accesses at the slots  818  and  838 .  
         [0042]     Although subscriber devices I and J are shown to be repeatedly colliding, the second (i.e. repeated) collision can be between either I and/or J and other subscriber stations, in which case each subscriber device which has collided performs similar operations.  
         [0043]     In a case where the number of random channel selections exceeds the predetermined retry number m because collisions repeatedly occur, each subscriber device backlogs, waits for a random time, and attempts a new random access request.  
         [0044]     In the preferred embodiment of the present invention, it is assumed that the number of uplink random access channels of all cells is N. In addition, it is assumed that a reuse rate of the uplink random access channels of all cells is 1. That is, all subscriber devices perform transmission by randomly selecting one of the N channels in the uplink.  
         [0045]     A value of the predetermined retry number m used in the preferred embodiment of the present invention is set by estimating a packet arrival rate of a network in a system to which the present invention is applied.  
         [0046]     A flowchart illustrating a process of setting the retry number m applied to the random access method in the multi-cell OFDMA network according to the preferred embodiment of the present invention is shown in  FIG. 4 . A value of the retry number m is set by estimating a packet total arrival rate R in a system to which the present invention is applied in step  401 . The estimation of the total arrival rate R is performed by summing up a total number of random accesses attempted by all subscriber devices in one superframe and may be also performed by obtaining a mean value of a plurality of superframes.  
         [0047]     The value of m is determined as 1 to n according to greatness or smallness between the total arrival rate R and R 1 , R 2 , . . . , R n−1 , which are values pre-calculated by analyses or experiments based on the total channel number N, in steps  402  to  408 .  
         [0048]     The values of R 1 , R 2 , . . . , R n−1  are assigned as values pre-calculated by analyses or experiments based on the total channel number N. The value of n can be arbitrarily set by the system.  
         [0049]     Illustrative diagrams of experimental results performed in a single cell based on the random access method in the multi-cell OFDMA network according to the preferred embodiment of the present invention are shown in  FIGS. 5A and 5C .  
         [0050]     An illustrative diagram of throughputs in a single cell to which the random access method in the multi-cell OFDMA network according to the preferred embodiment of the present invention is applied is shown in  FIG. 5A . An illustrative diagram of collision probabilities in a single cell to which the random access method in the multi-cell OFDMA network according to the preferred embodiment of the present invention is applied is shown in  FIG. 5B .  
         [0051]     The horizontal axis of  FIG. 5A  indicates an arrival rate, and the vertical axis indicates a throughput. The horizontal axis of  FIG. 5B  indicates an arrival rate, and the vertical axis indicates a collision probability.  FIGS. 5A and 5B  show Poisson distribution charts in which total packets have arrival rates of 0 to 10 in a state of seven random access channels having different subcarriers in a single cell to which the random access method in a multi-cell OFDMA network according to the preferred embodiment of the present invention is applied.  
         [0052]     Referring to  FIGS. 5A and 5B , A indicates a case of static selection according to the conventional method in which every subscriber device can access only one random access channel assigned to each subscriber device, while B to E indicate cases of dynamic selection according to the preferred embodiment of the present invention in which every subscriber can randomly select one of random access channels.  
         [0053]     Upon analysis of  FIGS. 5A and 5B , it can also be recognized that the dynamic selection shows better performance than the static selection when m=1. It can also be recognized that the collision probability increases along with an increase of the m value while points indicating an optimal throughput separately exist based on the arrival rate.  
         [0054]     That is, a case of m=4 has the best throughput at the arrival rate of 0 to 3.6, a case of m=3 has the best throughput at the arrival rate of 3.6 to 4.3, a case of m=2 has the best throughput at the arrival rate of 4.3 to 6, and a case of m=1 has the best throughput at the arrival rate of over 6.  
         [0055]      FIG. 5C  is a comparative diagram of success probabilities based on the number of transmission trials in a single cell to which the random access method in the multi-cell OFDMA network according to the preferred embodiment of the present invention is applied. The horizontal axis of  FIG. 5C  indicates the number of transmission trials, and the vertical axis indicates a success probability.  
         [0056]      FIG. 5C  shows a comparison result of the success probabilities based on the number of transmission trials when a pack arrival rate is 2.625, and it can be recognized that the distribution changes according to a change of the m value.  
         [0057]      FIGS. 6A and 6B  are illustrative diagrams of experimental results performed in multiple cells based on the random access method in the multi-cell OFDMA network according to the preferred embodiment of the present invention.  
         [0058]      FIG. 6A  is an illustrative diagram of throughputs in multiple cells to which the random access method in the multi-cell OFDMA network according to the preferred embodiment of the present invention is applied.  FIG. 6B  is an illustrative diagram of collision probabilities in multiple cells to which the random access method in the multi-cell OFDMA network according to the preferred embodiment of the present invention is applied.  
         [0059]     The horizontal axis of  FIG. 6A  indicates an arrival rate, and the vertical axis indicates a throughput. The horizontal axis of  FIG. 6B  indicates an arrival rate, and the vertical axis indicates a collision probability.  FIGS. 6A and 6B  show Poisson distribution charts in which total packets have arrival rates of 0 to 10 in a state of seven random access channels having different subcarriers in multiple cells to which the random access method in the multi-cell OFDMA network according to the preferred embodiment of the present invention is applied.  
         [0060]     Referring to  FIGS. 6A and 6B , A indicates a case of static frequency reuse according to a case where one channel is assigned to each cell, i.e., a frequency reuse rate is 7, while B indicates a case of differential frequency reuse according to a case where three frequencies are allocated to every cell so that one frequency overlaps in two arbitrary cells. Also, C to E indicate cases of dynamic frequency reuse according to the preferred embodiment of the present invention in which every call can use seven frequencies.  
         [0061]     Upon analysis of  FIGS. 6A and 6B , it can be recognized that the suggested method, the dynamic frequency reuse, has the best performance. That is, like the experiments in the single cell, the collision probability increases along with an increase of value of m, while points indicating an optimal throughput separately exist based on the arrival rate. That is, a case of m=3 is optimal at the arrival rate of under 2, a case of m=2 is optimal at the arrival rate of 2 to 3, and a case of m=1 is optimal at the arrival rate of over 3.  
         [0062]     As described above, according to an embodiment of the present invention, a system can use the same random access channels without separately distributing the random access channels to cells, by setting a resource reuse rate of the random access channels to 1.  
         [0063]     In addition, in a case of a random access failure, a quick access can be achieved by changing a channel based on a link usage rate without waiting for an arbitrary time.  
         [0064]     The method of the present invention can be embodied as a program and stored in computer-readable media (e.g., CD-ROMs, random access memories (RAMs), floppy disks, hard disks, optical magnetic disks, etc.)  
         [0065]     While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.