Wimax standards and Security The Wimax

FIGURE 8.3
Placement of logical channels in the frame. The first transmission opportunity of every network configuration frame is reserved for MSH-NENT messages (basic channel); the other transmission opportunities in the frame are reserved for MSH-NCFG messages (broadcast channel). All transmission opportunities in the control subframe of the scheduling frames are reserved for the broadcast channel. However, the last MSH-DCSH-NUM control transmission opportunities are reserved for MSH-DSCH messages.

election scheduling. In distributed election scheduling, each transmitter sharing the channel broadcasts the range of opportunities it considers for transmissions. The transmitters whose ranges of transmission opportunities overlap with their two-hop neighbor’s ranges perform a distributed election procedure for each transmission opportunity. The election algorithm guar- antees that each transmission opportunity has only one winner, so that the transmissions in the channel are collision-free. We first describe how trans- mission opportunities are related to actual OFDM symbols and then describe how 802.16 distributes the ranges of transmission opportunities and elects winners of each transmission opportunity.

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The network configuration broadcast channel is located in the control subframe of every network configuration frame (Figure 8.3). The trans- mission opportunities can be viewed on their own axis if we ignore all of the OFDM symbols not used by the channel (Figure 8.4). Given the index of a transmission opportunity in the channel, CurrentTxOpp, the frame in which the transmission should take place can be found by divid- ing CurrentTxOpp by the number of network configuration transmission opportunities in each frame, MSH-CTRL-LEN 1, and then multiplying by the number of frames between successive network configuration frames. The index of the starting OFDM symbol for the transmission can be found by subtracting the number of transmission opportunities before the start of the frame and then multiplying by 7 to account for the length of each transmission opportunity.
The distributed scheduling broadcast channel is located in the last

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7 MSH-DSCH-NUM OFDM symbols of the control scheduling subframes, after the centralized scheduling messages (Figure 8.3). MSH-DSCH-NUM is a network parameter indicating the number of transmission opportunities in the control subframe allocated to distributed scheduling messages. As with

Network configuration

Centralized scheduling

Centralized scheduling

Network configuration

Frames

MSG-NCFG
transmission opportunities

7  (MSH-CTRL-LEN-1)

OFDM symbols