  Mobile edge caching can effectively reduce service delay but may introduce
information staleness, calling for timely content refreshing. However, content
refreshing consumes additional transmission resources and may degrade the delay
performance of mobile systems. In this work, we propose a freshness-aware
refreshing scheme to balance the service delay and content freshness measured
by Age of Information (AoI). Specifically, the cached content items will be
refreshed to the up-to-date version upon user requests if the AoI exceeds a
certain threshold (named as refreshing window). The average AoI and service
delay are derived in closed forms approximately, which reveals an AoI-delay
tradeoff relationship with respect to the refreshing window. In addition, the
refreshing window is optimized to minimize the average delay while meeting the
AoI requirements, and the results indicate to set a smaller refreshing window
for the popular content items. Extensive simulations are conducted on the
OMNeT++ platform to validate the analytical results. The results indicate that
the proposed scheme can restrain frequent refreshing as the request arrival
rate increases, whereby the average delay can be reduced by around 80% while
maintaining the AoI below one second in heavily-loaded scenarios.
