In this paper we investigate the feasibility of using microwave frequencies for fixed non-line-of-sight wireless backhauling connecting small-cell radio base stations with an aggregation node in an outdoor urban environment, i.e. a typical heterogeneous network scenario. We study system level simulations for a point-to-point system where the wave propagation is based on diffraction over rooftops. We further investigate the effects of carrier frequency, interference, antenna height, rain, and tolerance to antenna alignment errors. It is found that the higher frequencies offer not only larger bandwidths but also higher antenna gains which would ideally work to their advantage. However, these advantages may be lost when taking antenna alignment errors and rain into account. Different frequencies simply have their different trade-offs.