Page 40 - Antennas for Base Stations in Wireless Communications

Fundamentals of Antennas 13

when determining the gain. High-gain antennas have the advantage of
longer range and better signal quality but must be aimed carefully in a
particular direction. Low-gain antennas have shorter range, but the ori-
entation of the antenna is inconsequential. For example, a dish antenna
on a spacecraft is a high-gain device (must be pointed at the planet to be
effective) whereas a typical wireless fidelity (WiFi) antenna in a laptop
computer is low-gain (as long as the base station is within range, the
antenna can be in any orientation in space). Improving horizontal range
at the expense of reception above or below the antenna makes sense.

1.1.7 Intermodulation
Generally, an antenna is considered a passive linear device. However,
when such a device is excited by high enough power, it acts slightly as a
nonlinear device. The nonlinearity is normally caused by metal-to-metal
joints and nonlinear materials in the antenna structure. Therefore, when
signals with multiple frequencies are fed into nonlinear devices, inter-
modulation product terms whose frequencies are different to those of
the input signal are generated. A typical passive intermodulation signal
level is from –180 to –120 dBc (dBc relative to carrier power). 7–8
An antenna’s intermodulation degrades a wireless system’s perfor-
mance if the system has the following features:
■ High transmitted power is adopted.
■ The system is equipped with high receiver sensitivity.
■ One antenna is used for both transmitting and receiving.
■ Signals at more than one frequency are transmitted.

Base stations normally have this entire feature set. Base stations,
therefore, suffer from passive intermodulation (PIM). High-power sig-
nals excite the antenna of the base station, and intermodulation com-
ponents cause back reflection to the receiver due to the antenna’s PIM.
Since the receiver is highly sensitive and is able to sense very weak
signals, the intermodulation signals cause interference. The problem
becomes worse if the intermodulation term falls inside the receiving
band because the interference cannot be removed by filtering. For exam-
ple, for the P-GSM-900 system whose downlink band is from 935 MHz
to 960 MHz and whose uplink band is from 890 MHz to 915 MHz,
the 3rd order intermodulation term at the base station side may be
2 × 935 − 960 = 910 MHz, which falls inside the uplink band. On the
other hand, the PIM problem is not that serious at a client terminal,
such as a cell phone, a personal digital assistant (PDA), or a laptop
with wireless capability, and is normally ignored. On the client side,