Page 65 - Antennas for Base Stations in Wireless Communications

38 Chapter Two

pattern as the tilt is increased, and the levels of nulls and sidelobes
remain almost unaffected for small tilt angles. The directivity of an
array falls with the cosine of the beamtilt—because the array appears
shorter when viewed from the position of the tilted beam maximum—
and also falls because the grating lobes above the horizontal increase
in level as the electrical tilt is increased, especially if the inter-element
spacing is electrically large.

2.2.1.5 Passive Intermodulation Products In digital mobile radio net-
works, base station antennas usually function as both transmitting
and receiving antennas, providing diversity on reception and air com-
∗
bining on transmission. Transmissions are usually made on more
than one frequency, and for orthogonal frequency-division multiplex-
ing (OFDM) and CDMA systems, a single channel may occupy a wide
bandwidth with multiple carriers or signals with noise-like spectra.
When multiple-frequency transmissions encounter any conductor or
joint with a nonlinear voltage/current relationship, the result will be
the generation of passive intermodulation (PIM) products. These are
spurious signals with frequencies related to those that give rise to them.
For two unmodulated carriers, intermodulation products are gener-
ated at frequencies given by:
Second order: f 2 − f , f + f 1
1
2
Third order: 2f − f , 2f − f , 2f + f , 2f + f 1
2
1
1
2
2
1
2
Fourth order: 3f − f , 2f − 2f …
1
2,
1
2
Fifth order: 3f 1 − 2f 2 , 2f 2 − 2f 1 , 4f 1 − f 2 , 4f 2 − f 1, …
Some of these frequencies are likely to fall in the receive frequency
band, and the situation becomes very complex when up to eight carriers
are fed into a single antenna. Severe problems are particularly likely if
an antenna operates in both low and high bands, where the second-order
product is likely to fall in-band.
Radiated PIMs may cause interference for other spectrum users—for
example another network sharing the same base station structure—or
they may fall in a receive band served by the same antenna. PIM levels
are usually specified for a two-carrier test, with a level of –153 dBc for a
carrier level of 2 × 43 dBm (20 W). Achieving this very low level, which
ensures the receiver is not significantly desensitized by locally generated
∗ A term used to indicate that different radio frequency channels associated with the same
radio system are transmitted from separate antennas rather than being combined into
a single transmitting antenna.