US 6975877

Patent Overview — US 6975877 B1

Title: System and method for synchronizing clock dividers in a wireless network
Patent number: US 6975877 B1
Application number: US09/982,591
Inventors: Alex Dergun; Ian Sayers
Assignee: Cisco Technology, Inc. (original and current assignee listed)
Priority/filing date: 2001-10-17
Grant/publication date: 2005-12-13
Status listed: Expired – Fee Related, adjusted expiration 2023-12-03
Source: https://patents.google.com/patent/US6975877/en

Purpose. In wireless networks, base stations must stay synchronized so handoffs between cells are not discernable to mobile stations. GPS receivers can supply a timing reference, but GPS reception is not always possible—especially indoors or in private wireless systems—and GPS receivers are cost-prohibitive for low-traffic private networks. The patent therefore addresses synchronizing base stations down to internal frame and burst clock signals without relying on external sources.

Plain-language scope. A wireless network's base stations are partitioned into partially overlapping clusters (e.g., cluster A and cluster B). Each base station has a local clock unit with a clock divider that derives local clock signals (frame and burst clocks) from a received master clock signal, and the divider can reset itself upon receiving a sync pulse. A base station that belongs to two adjacent clusters propagates the sync pulse from one cluster to the next, extending propagation until all base stations receive it. Because the sync pulse travels over the same radio interface used for mobile communications, propagation is contemplated at low-usage hours (late night/early morning) and on a once-per-day basis, though on-demand or other periodicities are contemplated. The description states this can synchronize frame and burst clocks to within 70 nanoseconds, well inside a desired 900-nanosecond error threshold.

What the excerpt does not establish. It does not include the full claims, detailed figures, or the complete description; does not detail the referenced copending application Ser. No. 09/310,566; and does not state litigation, ownership transfers, validity, or prior-art conclusions. Legal status is an assumption, not a legal conclusion.