Nonlinearities in array antennas

Introducing antenna arrays at the basestation site increases the size of the basestations by multiplication of the number of radio modems, amplifiers and so on. Simply, if a basestation is operating on M frequency channels, it needs M radio modems and M amplifiers if the conventional single carrier amplifier (SCPA) technology is applied. And by introducing an N element array antenna, the number of amplifiers increases to NM .

Thus, it is highly desirable to integrate the hardware used at the base station site to reduce cost, power dissipation and the space a basestation site occupies. One proposed solution is to combine the signals prior to amplification and co-amplify them in a Multi-Carrier Power Amplifier (MCPA). Hereby, the number of amplifiers is reduced to N and no need for bulky filter combiners are needed. When several constant-envelope signals are combined as when using the MCPA, the envelope of the composite signal becomes non-constant and the amplifier cannot be operated in the high-efficient nonlinear region of the amplifier due to the generation of intermodulation distortion. The amplifier must thus be linearized to increase its efficiency.

The linearisation of MCPA:s has been the subject of considerable research effort and promising results exists, although the technology is still expensive. The combining of several independent signals give rise to a large peak-to-mean ratio of the MCPA input signal. Even if it is possible to construct a MCPA which meets the stringent linearity requirements, its peak power requirement is much greater than the individual power of the M carriers. For example, a 16 channel basestation running 10 Watts per channel, the peak power in the MCPA could be as high as 2.56 kW. The amplifier has to be biased with a large back-off from the saturating point to avoid that the peak power rating of the MCPA is exceeded. Consequently, a large back-off implies low power added efficiency (PAE), so most of the DC power is lost as heat.
 

 

The effect of the nonideal MCPA:s

By introducing MCPA:s in adaptive antenna array systems, and assuming that they are not perfectly linearized, three effects can be observed:

• Intermodulation distortion power will be emitted from the basestation. As opposed to a conventional (one-antenna) basestation, the array antenna makes the intermodulation distortion to be dependent on the azimuthal angle. Thus in some direction the intermodulation products from some or all antennas adds coherently and an mobile on that frequency and in that particular direction will experience a increased interference level. This has previously been studied by Litva for receiving arrays, where the concept phantom-lobes were introduced.

• The AM/AM and AM/PM conversion in the MCPA will make the transmitting antenna branches unmatched, due to phase shifts and gain compression. Thus the resulting radiation pattern will be distorted, and an intended ``null'' in the radiation pattern towards a known co-channel user will be shifted and the interference for that particular user will increase.

• Due to the large amplitude variations (crest factor) of the combined signal on the input of the MCPA, the peak-to-mean ratio of the input signal is often limited prior to amplification so the peak power rating of the amplifier can be reduced. This leads to considerable benefits in terms of power and cost savings in the design of the MCPA. The limiter will also generate intermodulation products, that will be transmitted in some direction, dependent on the weight settings for that particular beam. And consequently, the signal to interference level for the mobiles will decrease.
  

For further reading, simulation results and conclusions, see the conference paper presented at Wireless 99 i Munich