Frequency 2ghz / bgn, Protocol: 802.11, antenna gain: 15dbi, txpower: all fixes rated 23dbm.
The use is to distribute to wireless equipment: cell phones, laptops, tablets.
If you don't fear the "radio police", then set antenna gain to low value, such as 0 or 3 dBi.
Try to set txpower=card-rates
... according to official specifications, all groove models are able to transmit at power higher than 23 dBm at low datarates, so by setting txpower to 23dBm you're giving up some coverage. OTOH you're instructing to transmit at higher power than the device is capable at high data rates. If you "unleash" tx power by setting antenna-gain lower, the card might try to transmit higher data rates with power higher than output PA is capable of thus transmitted signal will get distorted and you'll never be able to achieve highest data rates. This problem is not (yet) present due to lower TX power used currently (see next paragraph).
Rationale for change of antenna-gain: there are country specific rules about maximum allowed EIRP. EIRP includes antenna gain and if antenna gain is high, actual EIRP might be higher than allowed. To avoid violation of rules, device needs to know antenna gain (antenna being passive element can not tell that to transmitter so you have to enter it by hand) to lower maximum used TX power so that EIRP remains under legal limit. And that lowered power will probably not be visible in any of diagnostic print-outs as it falls into category of dynamic power control.
Hence if you don't fear somebody from some law-enforcement agency might come by, you can set antenna-gain to some lower value and you'll actually start to enjoy benefits of your high-gain antenna. I advise you to set antenna gain to a value which will give your just the coverage you want, not any lower. This way you will both receive less radiated energy into your head/feet when you pass near by to the antenna and you will cause less interference to your neighbours (in case you drink a beer or two with them from time to time).