Abstract:

For cognitive wireless networks, due to the long time delay characteristics of channel frequency,both OFDM symbol time and slot time are big,which makes the SIFS increase in the CSMA/CA.In addition,with the improvement of the data rate on physical layer in wireless local area network,the number of data bits carried by each OFDM symbol increase.To assess the physical long extension performance of DCF under the environment of high speed model,we obtain the CSMA/CA system throughput expression based on the twodimensional Markov model. Mathematical analysis shows that the throughput is mainly determined by the node number of the network,packet length,the number bits carried by OFDM,the slot time and OFDM symbol time.Theoretical calculation and simulation results show that as the data bits of OFDM increase,the slot time and the OFDM symbols are longer. And the throughput performance of CSMA/CA is also degraded.

Key words: OFDM;CSMA/CA;throughput;delay characteristics