  Information freshness, characterized by age of information (AoI), is
important for sensor applications involving timely status updates. In many
cases, the wireless signals from one sensor can be received by multiple access
points (APs). This paper investigates the average AoI for cooperative APs, in
which they can share information through a wired backbone network. We first
study a basic backbone-assisted COoperative AP (Co-AP) system where APs share
only decoded packets. Experimental results on software-defined radios (SDR)
indicate that Co-AP significantly improves the average AoI performance over a
single-AP system. Next, we investigate an improved Co-AP system, called
Soft-Co-AP. In addition to sharing decoded packets, Soft-Co-AP shares and
collects soft information of packets that the APs fail to decode for further
joint decoding. A critical issue in Soft-Co-AP is determining the number of
quantization bits that represent the soft information (each soft bit) shared
over the backbone. While more quantization bits per soft bit improves the joint
decoding performance, it leads to higher backbone delay. We experimentally
study the average AoI of Soft-Co-AP by evaluating the tradeoff between the
backbone delay and the number of quantization bits. SDR experiments show that
when the number of sensors is large, Soft-Co-AP further reduces the average AoI
by 12% compared with Co-AP. Interestingly, good average AoI performance is
usually achieved when the number of quantization bits per soft bit is neither
too large nor too small.
