## 8 Cross Layer Optimization
The cross-layer system also provides receiver-centric scheduling based on time hopping sequence of impulse radio of Ultra-Wide-Band MAC and physical layer that allows multiple parallel transmissions, prevents collisions at the receiver node (by using unique TH sequence for each receiver) and saves energy by avoiding idle listening and wasteful transmissions (by turning on exactly on the incoming transmission). This is done by scheduling scheme where each sensor node is responsible to schedule the data packets from its children nodes (closer to sink has higher priority to schedule first) by sending scheduling packets contain different time hopping sequence for each one of them, and also dynamic channel coding to adapt to interference. Another cross layer design for WMSNs is implemented in [81] where an extension of a routing protocol is presented along with path priority scheduling algorithm for efficient communication of real-time video over WMSNs. The proposed routing protocol is an extension of DGR, Directed Geographical Routing, for constructing multiple disjoint paths in order to enlarge the aggregate bandwidth, facilitate load balancing, and guarantee packet delivery. Using hop-by-hop deviation angle adjustment method, a path can be established using any initial deviation angle specified at the source node, and then other disjoint paths are constructed by changing the value of the deviation angle. To meet the delay constraint of video frames, a path priority scheduling algorithm is used that specifies the number of paths used and assign video sub-streams according to the status of the paths. Using this scheduling algorithm, the weight of each path will be calculated based on the estimated available bandwidth, path delay, and path energy level. Then, by using path weight along with packet priority, shorter delay paths will be used for time-constrained packets while other paths are used for balancing energy and bandwidth usage for other traffic. In case the required bandwidth is larger than the aggregate bandwidth, least priority packets will be dropped. But if the available bandwidth is still not enough, intermediate nodes should decide whether to forward these packets or drop them if they cannot meet their deadline and inform the source node to use either multiple frame selecting or intra-frame refreshing. In [82], a context-aware cross-layer optimized multi-path multi-priority (MPMP) transmission scheme is proposed in which a multipath routing is used in the routing layer in conjunction with a context-aware multipath selection algorithm in the transport layer. TPGF, Two-Phase geographic Greedy Forwarding, routing that is described in the Routing Section is used to explore maximum number of node-disjoint routing paths while CAMS, Context-Aware Multipath Selection algorithm, is used to choose the maximum number of paths from all found node-disjoint paths for maximizing the delivery of the important data to the sink and guaranteeing end-to-end transmission delay. CAMS algorithm selects the proper routing paths that are suitable for each type of multimedia content based on two types of priority: end-to-end transmission delay based priority for constraint real-time video communication, and context-aware multimedia based priority (image vs. audio) that depends on the importance of multimedia stream which can reflect precisely the event in WMSN. In [83], a cross layer design is considered for data gathering in WMSN where an adaptive scheme called (RRA) is used to dynamically adjust the \"transmission Radius and data generation Rate Adjustment\". Based on the result that the transmission radius of sensor nodes and the data generation rate of sensor nodes are two critical factors in affecting the data gathering performance, the proposed scheme aims to address this research challenge by taking into account the interaction among physical layer, routing layer, and transport layer.