Intermodulation distortion in switched multibeam antennas for cellular radio systems

Abstract

The performance of switched multibeam antennas for cellular radio systems is investigated when the transmit amplifier is of a multicarrier (MCPA) type. The MCPA nonlinearity generates intermodulation distortion in the transmitted signal that is spatially filtered by the array radiation pattern. Hence, the intermodulation distortion becomes direction-dependent, and the direction of the generated intermodulation products of any order is derived. It is shown by using Monte Carlo simulations how the downlink carrier to interference ratio depends on the frequency reuse distance and on the MCPA linearity measured as its noise power ratio (NPR). For reuse factor one systems, which is made possible by use of multibeam antennas, the linear co-channel interference dominates the interference. On the contrary, for a reuse factor seven system it is shown that a weak nonlinearity directly gives a degradation in carrier to interference ratio (CIR) due to intermodulation distortion. However the reuse factor seven system has a large CIR overhead due to the increase in CIR using a multibeam antenna, and the outage probability is thereby only slightly affected.

Effects of nonlinear distortion on switched multibeam FDMA systems

“…a larger variability, and a deviation from the 10 log 10 (N ) expression is increased. Note that in the N = 8, M = 5 case, the probability that a mobile is not in the direction of an IMP main beam in the empty center frequency channel is from (21),…”

“…The radiation pattern provided by the beam-forming network will generate IMD that, in general, are radiated in directions different from the principal beam directions [19], [20], [21]. These IMD lobes have also been called "phantom lobes" [22] as they are not generated purposely, but is created by the nonlinearity.…”