Diversity

Diversity, as it applies to an antenna system, refers to a system used in
wireless communication as a method for comparing signal fading in the
environment. Diversity gain is based on the gain that over what fading
would have taken place in the event that a diversity technique was not
used. In the case of a two branch diversity system, if the received signal into
both antennas is not of an equal signal strength, then there cannot be any
diversity gain. This is an interesting point considering most link budget calculations
incorporate diversity gain as a positive attribute. The only way
diversity gain can be incorporated into a link budget is if a fade margin is
included in the link budget and the diversity scheme chosen attempts to
improve or reduce the fade margin that is included there.
There are several types of diversity that need to be accounted for in both
the legacy systems as well as 2.5G and 3G platforms. When discussing
diversity, the concept for a radio engineer is usually focused on the receive
path, uplink from the mobile to the base station.With the introduction of
2.5G and 3G platforms, transmit diversity has been introduced but is
implemented in a fashion where the subscriber does not need a second
antenna.

The type of antenna diversity used can and is often augmented with
another type of diversity that is accomplished at the radio level:

■ Spacial
■ Horizontal
■ Vertical
■ Polarization
■ Frequency
■ Time
■ Angle
For most, 2G systems and 2.5G and 3G systems use two antennas separated
by a physical distance, that is horizontal. Some 2.5G platforms like
iDEN utilize a three branch diversity receive scheme but they are the
exception, and the usual method is to deploy only two antennas per sector
for diversity reception.

The spacing is associated with the antennas located in the same sector is
normally a design requirement that is stipulated from RF engineering.
Diversity spacing is a physical separation between the receive antennas that is needed to ensure that the proper fade margin protection is designed
into the system. As mentioned earlier horizontal space diversity is the most
common type of diversity scheme that is used in wireless communication
systems.

The following is a brief rule of thumb used to determine the required horizontal
diversity requirements for a site and is shown in Figure 11-1.
n
h/d
11 where h
height (ft)
d
distance between antennas (ft)
The equation used was derived for cellular systems operating in the 800
MHz band but has been successfully applied for the other wireless bands in
1800 and 1900.
With the introduction of both CDMA2000 as well as Universal Mobile
Telecommunications Service (UMTS), the application for transmit diversity
needs to be factored into the antenna design. Two different transmit diversity schemes are possible with these platforms. The use of two different
transmit diversity schemes is driven by the practical issue of antenna
installation concerns. The two transmit diversity schemes are STD and
OTD. STD is space transmit diversity while OTD is orthogonal transmit
diversity. The preferred transmit diversity scheme, when implemented, is
the STD method.

What follows is a simplified diagram for both the STD and OTD transmit
diversity schemes. Figure 11-2 shows the STD transmit diversity scheme
for a single channel when there are two antennas available on a sector. The
two antennas could also be separate ports on a X-pole antenna. The important
issue is that when integrating a second carrier, either more antennas
need to be added or additional transmit (Tx) combing losses will ensue.
Figure 11-3 shows a configuration recommended for the rest of the network
that involves using OTD transmit diversity. In examining the differences
between Figure 11-2 and 11-3, one immediate observation is that a
second carrier is introduced with the same amount of physical antennas.
One immediate observation with the use of Tx diversity for the new radio
platforms is the issue with what happened to the legacy systems. That will
be covered shortly. 438