Patent Document ID: 9226252
Application ID: 13909031
Patent Flag: 1

Claim One:
1. A computer-implemented recursive time synchronization protocol method for a clustered wireless sensor network (WSN), comprising the steps of: electing a single reference node i for offset and drift compensation, the electing being limited to nodes that are clusterheads; calculating a time read by a logical clock of the i th node of the clustered WSN, the calculation utilizing the relation: 
 C i ( t )=α i +β i t, where α i is the clock offset deviation from ideal time at the reference time t=0 and β i is the frequency of the clock; re-synchronizing the clock within a time T≦δ/2ρ, where δ is an offset and ρ is a skew rate; estimating the offset and a drift of the clock, thereby re-synchronizing the clock; initiating a time synchronization request, the time synchronization request being initiated by any initiating node in the clustered WSN, wherein the step of initiating the time synchronization request comprises the steps of: initiating the time synchronization request from the initiating node at the initiating node's local time T1 and transmitting the time synchronization request to a first intermediate node; receiving the time synchronization request at the first intermediate node at the first intermediate node's local time T2; storing identification data and local times T1 and T2 in computer readable memory, the local times T1 and T2 being respectively stored as variables T1 old and T2old; and forwarding the time synchronization request from the first intermediate node to a second intermediate node at the first intermediate nodes local time T1; recursively forwarding the time synchronization request from the second intermediate node to the reference node, thereby establishing a request-path, the reference node receiving the forwarded time synchronization request at the reference node's local time T2; using a REP (reply to time synchronization) message to forward the time T back to the initiating node via the request-path; and compensating the time T with a propagation delay time at each hop in the request-path, thereby time synchronizing all clustered WSN nodes in the request-path, wherein the step of compensating the time T comprises the steps of: transmitting the REP message from the reference node to the second intermediate node at the reference node's local time T3; receiving the REP message at the second intermediate node at the second intermediate node's local time T4; calculating a first propagation delay, d1, as d ⁢ ⁢ 1 = ( T ⁢ ⁢ 2 - T ⁢ ⁢ 1 ) + ( T ⁢ ⁢ 4 - T ⁢ ⁢ 3 ) 2 ; updating the time T as T=T+d1; updating the time T by adding an elapsed time since the second intermediate node's local time T4 to T; forwarding the REP message from the second intermediate node to the first intermediate node at the first intermediate node's local time T3; calculating a second propagation delay, d2, as d ⁢ ⁢ 2 = ( T ⁢ ⁢ 2 - T ⁢ ⁢ 1 ) + ( T ⁢ ⁢ 4 - T ⁢ ⁢ 3 ) 2 ; updating the time T as T=T+d2; updating the time T by adding an elapsed time since the first intermediate node's local time T3 to T; and recursively forwarding the REP message from the first intermediate node to the initiating node.