My personal feeling is that direct WiFi is not the best solution given that you need extended battery life and you are only transmitting minor amounts of data. Another aspect is that your system only appears to be transmitter ---> receiver. No bidirectional comms are needed (from what I can tell) and this will push-down the battery usage.

Given the range and the proposed operating frequency, the free space link loss will be: -

32.5dB + 20log(MHz) + 20log(kilometres) = 32.5dB + 68dB + (-16dB) = 84.5dB and with a transmit power of 18dBm, you would receive -66.5dBm - this is a big signal and even accounting for terrestrial problems it should be easy to receive. Having said that I'd consider going for a significantly lower frequency such as 900MHz - the link loss reduces from 84.5dB to 76dB meaning you can save on transmit power by nearly 10dB.

The amount of data you need to transmit is minor and could probably be achieved at 1kbps. A receiver that only has the bandwidth for receiving 1kbps will naturally have a much higher sensitivity than a WiFi receiver. Receiver sensitivity is governed by this empirical equation: -

Sensitivity = -154dBm + 10log(data rate)

At 11Mbps a WiFi receiver needs a signal that is -154dBm + 70.4dBm = =83.6dBm

At 1kbps, a receiver needs -154dBm + 30dBm = -124dBm i.e. it will be 40dB more sensitive!!

I'd consider going in with a proposal that uses the lowest frequency band and a reasonably low transmit power to achieve the objectives. Have a dedicated receiver wired to your network and forget WiFi.

My personal feeling is that direct WiFi is not the best solution given that you need extended battery life and you are only transmitting minor amounts of data. Another aspect is that your system only appears to be transmitter ---> receiver. No bidirectional comms are needed (from what I can tell) and this will push-down the battery usage.

Given the range and the proposed operating frequency, the free space link loss will be: -

32.5dB + 20log(MHz) + 20log(kilometres) = 32.5dB + 68dB + (-16dB) = 84.5dB and with a transmit power of 18dBm, you would receive -66.5dBm - this is a big signal and even accounting for terrestrial problems it should be easy to receive. Having said that I'd consider going for a significantly lower frequency such as 900MHz - the link loss reduces from 84.5dB to 76dB meaning you can save on transmit power by nearly 10dB.

The amount of data you need to transmit is minor and could probably be achieved at 1kbps. A receiver that only has the bandwidth for receiving 1kbps will naturally have a much higher sensitivity than a WiFi receiver. Receiver sensitivity is governed by this empirical equation: -

Sensitivity = -154dBm + 10log(data rate)

At 11Mbps a WiFi receiver needs a signal that is -154dBm + 70.4dBm = = -83.6dBm

At 1kbps, a receiver needs -154dBm + 30dBm = -124dBm i.e. it will be 40dB more sensitive!!

I'd consider going in with a proposal that uses the lowest frequency band and a reasonably low transmit power to achieve the objectives. Have a dedicated receiver wired to your network and forget WiFi.