ing Channel Initial Ranging Channel Periodic Ranging Channel (Non-synchronized MS) (Non-synchronized MS) (Synchronized MS) 6 OFDM Symbols = 1 Uplink Subframe FIGURE 10-16 Ranging channel structure in the time-domain [2] In the IEEE 802.16-2009 standard, the initial ranging channel is located in the first three OFDM symbols of the uplink subframe (TDD mode), and occupies six adjacent UL-PUSC subchannels, as shown in Figure 10-1 and Figure 9-20. The first OFDM symbol of the ranging sub-channels is formed in the same way as other OFDM symbols, i.e., performing an IFFT operation on the initial ranging code Table 10-20 Initial Ranging Channel Parameters and Coverage [2] Maximum Format Physical Resource Coverage IEEE Std 802. .16-2009 144 sub-carriers X 12 km 3 OFDM symbols IEEE 802. 16m Format 0 3.5Tg - Tu 1 sub-band X 1 subframe 18 km IEEE 802. 16m Format 1 3.5Tg + 7Tu u 1 sub-band X 3 subframe 100 km CHAPTER 10 The IEEE 802. 16m Physical Layer (Part II) and then appending the cyclic prefix to the beginning of the symbol. The cyclic prefix for the second OFDM symbol is a copy of the first few samples of the symbol and is appended to the end of the OFDM symbol (as shown in the bottom of Figure 10-16) to ensure phase continuity at the symbol boundary. The virtual extension of the ranging opportunity would increase the probability of success for the mobile station making simultaneous random access. In order to perform the initial ranging, a ranging code is repeated twice and transmitted over two consecutive OFDM symbols, as described earlier. The ranging codes in the IEEE 802.16-2009 standard are PN sequences of length 144, and are chosen from a set of 256 codes. Note that the set of pseudo-random codes are divided into four subsets, where one subset is dedicated to initial ranging, periodic ranging, handover ranging, and bandwidth request [1]. The MS randomly selects one of the initial ranging codes, modulates it using BPSK, and sends it to the BS during the initial ranging opportunity over the allocated sub-channels a