# Data Gathering Techniques in WSN: A Cross-Layer View
## 4 Medium Access Control (MAC)
### MAC Protocols for Other Setups  
Next, we review some MAC protocols and MAC adaptations for various setups, such as multi-channel, multi-radio, busy-tone utilization, or different from the duty-cycle approach. DURI-MAC [91] adopts the traditional busy-tone scheme and allocates a sub-channel for control such that while receiving data on the data channel, a busy signal is transmitted on the control channel, which notifies neighboring nodes of the ongoing transmission, and therefore helps avoid interference from hidden terminals. EM-MAC [92] utilizes multiple orthogonal radio channels and allows devices to dynamically select the channels for their transmissions based on the channel conditions they sense without the utilization of a control channel. Accordingly, EM-MAC can avoid using channels that are currently heavily loaded, inferior due to interference, or jammed. Typically, the traffic load sent by a node is spatially and temporally variable. Different nodes need to send different traffic loads due to their tasks, topological location, and the amount of traffic they need to relay. Furthermore, the same node can experience different loads at different time intervals due to events or requests triggering different traffic loads. Accordingly, several studies have explored an adaptive duty-cycle approach. For example, [PERSON] and [PERSON] [93] have developed a reinforcement-learning-based self-adaptive sleep/wake-up scheduling approach. In the proposed method, each node (device) divides the time into time-slots, which are not necessarily synchronized between neighboring nodes. Each node decides whether to sleep or wake up in each time slot, and while awake, it decides whether to listen or transmit. The decision is based on its current situation and its estimation of its neighbors' situations and is attained via Q-learning. [PERSON] et al. [94] devise an alternative approach to duty-cycle, which utilizes two radios: the primary radio transceiver and an additional wake-up radio (WuR). The wake-up radio is a low-power receiver triggered by an external event and can turn on the main transceiver when required. [PERSON] et al. [95] provide a detailed characterization of a specific WuR, the SubCarrier Modulation WuR (SCM-WuR), through physical experiments and measurements, evaluating it for different performance metrics and comparing it to other wake-up radio-based systems. The authors of [95] model and simulate their own designed WuR hardware platform, which is compared to four widely employed MAC protocols for WSN under three real-world network deployments [96]. [PERSON] et al. [97] further design and prototype a very-low-power-consumption (<1.3 \(\upmu\)W), high-sensitivity (up to \(-55\) dBm), fast-reactivity (wake-up time of 130 \(\upmu\)s), and selective-addressing wake-up receiver (WRx) and describe its integration to a wireless sensor node. The authors leverage their WRx and present ALBA-WUR, a cross-layer solution for data gathering in wireless sensing systems. Similar to duty-cycled MAC protocols, wake-up radio-based protocols also distinguish between transmitter-initiated WuR (TI-WuR) protocols in which the transmitter wakes up its potential receivers (e.g., [98; 99]) vs. receiver-initiated WuR (RI-WuR) that adopt the RI-MAC paradigm such that when a receiving node is ready to collect data, it wakes up all the nodes in its neighborhood by broadcasting a wake-up call (e.g., [100; 101]). An energy-harvesting-based MAC protocol for cognitive radio networks (CRNs) is suggested by [PERSON] et al. [102], in which the secondary users (SUs) utilize the transmissions by primary users (PUs) to harvest energy. Accordingly, the suggested protocol interlaces SUs' data transmissions within these PUs' transmission holes. The proposed energy-harvesting/data-transmission schedule considers the imbalance between the small amount of energy collected per PU's transmission and the energy required by an SU data transmission.