Patent ID: 8477826

Claim:
A wireless communication system comprising: a portable device configured to spread one of a first signal and a second signal by a first spreading code and to transmit a wireless signal that is a respective one of a spread first signal and a spread second signal; and an in-vehicle device configured to receive the wireless signal and to despread a received wireless signal by a second spreading code identical to the first spreading code to wirelessly communicate with the portable device by a spread spectrum method, wherein: the in-vehicle device transmits a synchronization signal to the portable device to inform the portable device of a reference period determined by the in-vehicle device; when the portable device receives the synchronization signal while the portable device is not transmitting the spread second signal, the portable device changes the first spreading code synchronously with the synchronization signal, spreads the first signal by the changed first spreading code, and transmits the spread first signal; when the portable device receives an operation of a button, the portable device stops transmitting the spread first signal, spreads the second signal by the first spreading code, and transmits the spread second signal; when the in-vehicle device receives the wireless signal after transmitting the synchronization signal, the in-vehicle device performs a first synchronous acquisition process of searching for a synchronous acquisition point for a search period; the synchronous acquisition point is a point where a correlation value in despreading becomes a maximum value; the search period is determined based on a variation range in a delay time from when the in-vehicle device transmits the synchronization signal to when the portable device transmits the spread first signal; the search period is from a search starting point to a search ending point and is shorter than the reference period; when the in-vehicle device fails in the first synchronous acquisition process, the in-vehicle device performs a second synchronous acquisition process of searching for the synchronous acquisition point for a residual period; the residual period starts at the search ending point and ends at an ending point of the reference period when a starting point of the reference period is set at the search starting point; when the in-vehicle device succeeds in one of the first synchronous acquisition process and the second synchronous acquisition process, the in-vehicle device changes the second spreading code synchronously with the received wireless signal, and despreads the received wireless signal by a changed second spreading code; and when the in-vehicle device fails in the second synchronous acquisition process, the in-vehicle device searches for the synchronous acquisition point in period that starts after the residual period ends and has a length same as the reference period.