-80dB is a very weak signal - you certainly won’t get good performance at that signal level. See the table on page 7 on the following document:
http://www.adtechglobal.com/Data/Sites/1/marketing/juniperwhitepaperwlancoverageorcapacity.pdf
Assuming a single stream (single antenna) adaptor, which most small USB adaptors and 20Mhz channels (most AP’s don’t use 40Mhz channels on 2.4Ghz) are you’ll only be able to use MCS mode 1, which is a wireless rate of 6.5Mbps, in real world performance you would get well under 2Mbps from this in best case conditions.
To get maximum performance you really need a signal of -65dBm or better. (A larger negative number is a weaker signal so -65 is stronger than -80)