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ANSL R&D Times

4.MAC processing

Figure 5 shows the frame format of WIPAS STEP 2. The modulation scheme is QPSK and/or 16QAM in the STEP 2 system. The 16QAM modulation is only used in payload fields for safety concerns pertaining to equipment control (QPSK is more robust against C/N+I degradation than is 16QAM). In the bandwidth allocation, an AP conducts polling, and WTs send the demand for bandwidth allocation to the AP by using the slot demand field, if they hold data to transmit. The AP dynamically allocates bandwidth to WTs corresponding to the WTs demands, and uses a round-robin algorithm for allocation processing.

To avoid collision of upward burst signals from any WT, the AP measures the round trip time of each WT that belongs to that AP using a Delay Measurement Frame (DMF) field, and then adjusts the transmission timing of the WTs.

The AP authenticates a WT when the WT initially connects to the AP, or the link between the AP and WT is re-established after a link fault to exclude fraudulent WTs. The MAC frame payloads are encrypted to protect from eavesdropping.

Fig. 5 Frame format


Fig. 5 Frame format