Jihua Zhou Yubo Yang Di Pang Xin Jin Jinglin Shi Zhongcheng Li Contention region allocation optimization in ieee 802.16 ofdma systems. 197-200 2007 MSWiM http://doi.acm.org/10.1145/1298126.1298161 conf/mswim/2007 db/conf/mswim/mswim2007.html#ZhouYPJSL07