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FIGURE 8.4
The MSH-NCFG transmission opportunities are mapped from OFDM symbols in the control subframe to the logical transmission opportunities on the MSH-NCFG axis. On the MSH-NCFG axis, the transmissions are indexed as a continuous set of integers starting with 0. In this example, MSH-CTRL-LEN = 6 and SchedulingFrames = 2.
7 × MSH-CTRL-LEN
OFDM symbols
MSH–DSCH
transmission opportunities
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Network configuration
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Centralized scheduling
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Centralized scheduling
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Network configuration
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Frames
MSG-CSCH, MSH-CSCF
transmission opportunities
7 × MSH-CTRL-LEN
OFDM symbols
FIGURE 8.5
The MSH-CSCH, MSH-CSCF and MSH-DSCH transmission opportunities are mapped to two different transmission opportunity axes. On the MSH-CSCH, MSH-CSCF axis, the transmission opportunities are assigned with the tree scheduling algorithm. On the MSH-DSCH axis, the transmission opportunities are assigned with the distributed election algorithm. In this example, MSH-CTRL-LEN = 6, SchedulingFrames = 2.
the MSH-NCFG channel, the transmission opportunities in the distributed scheduling channel can be viewed on their own axis (Figure 8.5). Given the index of a transmission opportunity in the channel, CurrentTxOpp, the frame in which the transmission should take place can be found by taking a modulus with respect to SchedulingFrames and adding 1 to account for the network configuration frame. The index of the starting OFDM symbol can be found by subtracting the number of transmission opportunities before the start of the frame and then adding the number of OFDM symbols used for the centralized scheduling channel.
In both the network configuration and distributed scheduling broadcast channels, transmission opportunities are assigned with the use of a dis- tributed election algorithm. The distributed election algorithm specified in the 802.16 mesh standard works in two parts. First, the nodes exchange the range of transmission opportunities they consider for transmission. Second, the nodes contending for the same transmission opportunity perform an election to decide who should transmit during the conflicting transmis- sion opportunity. The election procedure uses a combination of the conflicting transmission opportunity index and each of the conflicting node identifiers to create a unique, pseudorandom, 16-bit hash value. The node with the highest 16-bit hash value for the transmission opportunity wins the election.
For the election procedure to be deterministic, all nodes must have the same view of which transmission opportunities are in dispute. The informa- tion about available transmission opportunities is distributed in a two-hop neighborhood of every node. Each node transmits a range of transmission opportunities it considers for transmission in terms of lower and upper bounds. The nodes also rebroadcast the ranges of transmission opportuni- ties of their one-hop neighbors, so that the transmission opportunities are known throughout the two-hop neighborhood of the nodes. Since the size of control packets is limited, the 802.16 mesh standard specifies that the range of contended transmission opportunities should be compressed into a 3-bit maximum value, Mx, and a 5-bit hold-off exponent, He. Given the encoding for the range, the minimum number of transmission opportunities before the next transmission by a node is calculated with
MinNextXmtTime = 2He+4 + Mx × 2He (8.2) and the maximum number of transmission opportunities is calculated with
MaxNextXmtTime = 2He+4 + (Mx + 1) × 2He (8.3)
Potential transmission conflicts can be found since all nodes broadcast their Mx and He values as well as rebroadcast all of their immediate neighbor’s Mx and He values. Given the ranges of potential transmissions for their two-hop neighborhood, nodes can check if their choice of next transmission time in the channel conflicts with any transmissions with
2Hei +4 + Mxi × 2Hei ≤ NextOpportunity ≤ 2Hei +4 +(Mxi + 1) × 2Hei (8.4) where Mxi and Hei are associated with the two-hop neighbor i, and
NextOpportunity is the opportunity the node is considering for its next transmission. To avoid collisions with neighbors whose Mxi and Hei are not known, the nodes assume that those neighbors transmit all the time.
Performance of the distributed election scheme is analyzed in Ref. 12. In that work, the authors derive an analytical expression for the average time required to access the distributed election channel. The authors use a partial
802.16 mesh simulator to measure the distributed election access times and compare the simulations to theoretical results. The simulations show that the theoretical model is fairly accurate. The paper also points out that the expected time to access the channel depends on the He value, so flows requiring quicker access to the channel should use smaller values of He.
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