vice C (attends to send) Carrier sensing Collision Random backoff = 14 Device A 13 12 11 10 9 8 7 6 5 Data A Random backoff = 10 Random backoff = 7 Device B 9876543210 Data B Random backoff = 16 Device C -DIFS Data C Signal Preamble symbol Data symbols Figure 8.5 Illustration of the asynchronous Wi-Fi channel access concept [30]. A generic IEEE 802.11 frame structure consists of a set of preambles (new and legacy pre- ambles), a signal field, and multiple data symbols. The preamble is a special waveform designed for signal identification, AGC adjustment, timing and frequency synchronization, and channel estimation. The preamble is particularly suited for activity detection in a chan- nel because waveform detection is 10-20 dB more sensitive than the energy-detection- based CCA. Furthermore, because the IEEE 802.11 frame structure does not have a fixed timing, the preamble detection is crucial for a receiver to synchronize to the incoming frame. The signal symbol following the preamble contains information that includes the modulation and coding scheme and the total number of octets for the following data sym- bols that carry a MAC PDU. Depending on the payload, the frame can be very short (e.g., ACK frame) or long in the case of data frame for user traffic. Each IEEE 802.11 MAC PDU contains a transmission duration field for informing the neighboring nodes of the Operation in Unlicensed and Shared Spectrum 859 medium occupancy time of the current burst. This is the amount of time that all nodes must wait, if they receive it. A local timer, called a network allocation vector (NAV), of a neigh- boring node is updated after the node reads the duration value from the ongoing transmis- sion. This node avoids medium access until the NAV counter expires. Using this virtual medium sensing mechanism, Wi-Fi utilizes a special clear-to-send (CTS)-to-self message to deal with the newer versions of Wi-Fi frames coexisting with a legacy node. CTS-to-self is a standard Wi-Fi CTS message except that it is addressed to the transmi