I am looking for literature for Comparison between 868 MHz, 915 MHz and 2.4 GHz wireless communication system. For example under what condition I should use 868 MHz or when I should not use 868 MHz. What are the advantages and disadvantages of this frequencies. I am interested only in terms of propagation effect not other. In environment like indoor/outdoor/vegetation/hill/industrial/ etc how each of these frequencies would behave?
In general, lower frequencies have lower free space path loss and therefor with the same power, same receiver sensitivity and same antenna gain will have longer range. However, as the frequency increases, antennas become more compact so if you are constructing a link with directional antennas higher frequencies can be advantageous.
Lower frequencies have lesser free space loss, and hence larger transmission range. However, they also have a longer delay spread, which calls for a larger required guard interval. For instance, 802.11af (operates in UHF bands) uses a guard interval of 4.5us in comparison to 802.11ac (operates in GHz band) which uses 800ns.
In an outdoor environment, with vegetation and weather effects, multipath is introduced as well, further increasing the delay spread.