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 . in the preferred embodiment of the present invention , it is assumed that the number of uplink random access channels of all cells is n . 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 fig3 . referring to fig3 , 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 . moreover , because multiple channels are randomly selected , the likelihood of a collision is reduced . 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 . 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 ( fig3 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 . 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 . 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 . 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 fig8 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 fig4 . 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 . 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 . 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 . 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 fig5 a and 5c . 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 fig5 a . 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 fig5 b . the horizontal axis of fig5 a indicates an arrival rate , and the vertical axis indicates a throughput . the horizontal axis of fig5 b indicates an arrival rate , and the vertical axis indicates a collision probability . fig5 a 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 . referring to fig5 a 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 . upon analysis of fig5 a 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 . 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 . fig5 c 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 fig5 c indicates the number of transmission trials , and the vertical axis indicates a success probability . fig5 c 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 . fig6 a 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 . fig6 a 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 . fig6 b 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 . the horizontal axis of fig6 a indicates an arrival rate , and the vertical axis indicates a throughput . the horizontal axis of fig6 b indicates an arrival rate , and the vertical axis indicates a collision probability . fig6 a 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 . referring to fig6 a 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 . upon analysis of fig6 a 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 . 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 . 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 . 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 .) 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 .