In the orthogonal frequency division multiple access(OFDMA)wireless broadband networks

the efficiency of random access can be improved by distributing contention onto multiple channels.However

due to the multichannel property

the traditional systems based on request-to-send(RTS)control frame have to encounter the idleness of subchannel

the extension of RTS transmission time and the complex synchronization of uplink communication.To solve these problems

in our scheme

the users send signals on random subcarrier to request transmission opportunity.The access point senses these request signals

and determines which user is permitted to access channel according to the busy state of the subcarriers.The theoretical analysis and simulation results both show that our scheme outperforms the traditional RTS-based schemes in term of the throughput.