Wimax standards and Security The Wimax

TDMA Framing and Transmission Timing

OFDM symbols are grouped into TDMAframes of equal length and the frames are repeated over time (Figure 8.2). OFDM symbols in each frame are divided


Frame

Frames

Frame transmission opportunities

Control
Control subframe

Data subframe

Data

OFDM
symbols
transmission

transmission

FIGURE 8.2
802.16 Time Division Multiple Access (TDMA). 802.16 uses OFDM to achieve TDMA. OFDM symbols are grouped into frames of fixed duration. Frames are logically divided into the control subframe and the data subframe. In the control subframe, transmission opportunities are 7 OFDM symbols long. The length of transmission opportunities in the data subframe depends on the number of OFDM symbols in the frame. In this example, the length of transmission opportunities in the data frame is 2 OFDM symbols.

into two subframes. The first part of the frame is the control subframe, used to transmit 802.16 control packets. The second part of the frame is the data sub- frame, used to transmit data packets. There are two types of control subframes and the whole network alternates between them. The first type of control subframe is the scheduling subframe in which nodes transmit scheduling messages. The second type of control subframe is the network configuration subframe in which nodes broadcast network configuration packets contain- ing topology information, network provisioning information, and network management messages. The network configuration subframes occur period- ically with the period indicated with the parameter SchedulingFrames. SchedulingFrames is a network parameter transmitted in the network configuration subframe.

The management of OFDM symbols is simplified by grouping them into
transmission opportunities. In the control subframe, the symbols are grouped into transmission opportunities with a fixed length of 7 OFDM symbols. Four of the symbols are used to transmit information at the lowest bitrate, while the other three are used as guard symbols (Figure 8.2). There are a total of MSH-CTRL-LEN transmission opportunities in each control sub- frame, where MSH-CTRL-LEN is a network parameter transmitted in the network configuration subframe. In the data subframe, the symbols are grouped into transmission opportunities whose length depends on the num- ber of OFDM symbols in the frame. For example, in Figure 8.2, the data transmission is 3 transmission opportunities long, corresponding to 6 OFDM

symbols. The size of data transmission opportunities is found by dividing the number of data symbols in the frame by 256 and taking the integer part of the result:

, ,=
DataTxOppSize ^{Nf − 7 × MSH-CTRL-LEN} (8.1)
256
where N_f is the number of OFDM symbols in the frame and

×
7 MSH-CTRL-LEN is the number of OFDM symbols in the control sub- frame. The reason for limiting the number of transmission opportunities in the data subframe to 256 is that fields referring to transmission opportunities in 802.116 scheduling packets are 8-bits long.
Transmission opportunities are assigned to logical channels. There are three types of logical channels: basic, broadcast, and data. The basic channel is used for ranging and network entry packets, the broadcast channel is used to trans- mit mesh control packets, and the data channels are used for data packets and some 802.16 control packets. The basic channel is allocated in the control sub- frame. Some of slots for the broadcast channels are in the control subframe and some are in the data subframe. All data channel slots are located in the data subframe. The basic channel and the data channels are unicast since only one node is supposed to process transmissions from the channel, while messages in the broadcast channel are intended for all first-hop neighbors of a node.
The channels are closely related to the types of packets transmitted in them; we summarize the relationship between the mesh control packet types and channel types in Table 8.2. The basic channel is used by nodes enter- ing the network to transmit the network entry MSH-NENT packets. Broadcast channels are used to transmit MSH-NCFG, network configuration messages, and MSH-CSCF, MSH-CSCH, and MSH-DSCH scheduling messages. There are three types of broadcast channels depending on how transmission opportu- nities in the channel are shared. There are two reliable broadcast channels that use coordinated transmissions to prevent collisions. The first uses dis- tributed election-based scheduling for MSH-NCFG and MSH-DSCH messages. The second uses tree-based scheduling for MSH-CSCH and MSH-CSCF mes- sages. Optionally, MSH-DSCH messages can also be transmitted in the unused