Antenna Diversity only applies to the receive side. A transmitter is typically a isotopic (Omni-directional) and diversity in transmit has no function. Yes, there are patch antennas that are also known as semi-directional (usually in a 90 or 180 degree pattern of radiation) and people seems to forget about the transmit and receive RF pattern that causes the majority of crash’s. In the receiver diversity there is a proprietary process from the manufacture in selecting the antenna to receive with in which signal levels, SNR, interference, data retry rates among others are constantly being evaluated. The best way to envision what is going on is, next time your in a car listening to the radio, when you pull up to a light or a truck pulls next to you and the radio signal fades out. If you move forwards slightly (about 1 car length), the radio signal improves. That is diversity. Typically the antennas are one wave length apart.
A popular misbelief is that having a high gain antenna makes the world better. The majority of times a high gain antenna can cause more problems than they solve, specially when used wrong. Antennas are passive devices. To get gain, one must take energy from somewhere. I.E. physics 101. Think of the RF radiation pattern (Omni-directional antenna) as being in the shape of a basketball. To get gain, the shape needs to be changed, i.e. squished to being more like a donut. You took energy and reshaped it to get a gain. Besides increasing gain of the antenna to operate further, it also causes the receive gain to ‘hear’ further, “RF noise” that is not desired and is the biggest cause of problems. In my profession as an RF Engineer, I constantly find reducing gain of an antenna makes a bigger difference then increasing. Antennas technology are definitely not an exact science. It’s more theory than anything.