We undertake a fundamental investigation of diversity and equalization over highly directional links in the millimeter (mm) wave band. Our focus is on outdoor links using the 60 GHz unlicensed spectrum. Such links could be used to form wireless mesh networks with multiGigabit data rates. The use of directional transmission and reception contributes to a few strong paths which fall within the transmit and receive antenna beamwidths. We find that such sparse multipath channels require new design approaches relative to design of conventional wireless links in rich scattering environments.

We investigate spatial and frequency diversity for such highly directional links for which the number of dominant paths is significantly smaller than for the scattering environments seen by omnidirectional links. While fading can still be severe, we observe that the channel statistics are very different from classical Rayleigh or Rician fading models, and are characterized by the variations in a small number of parameters characterizing the propagation geometry. While our findings are quite general, our specific focus is on modeling outdoor millimeter wave lamppost-to-lamppost links in an urban environment (e.g., for multiGigabit mesh networks using 60 GHz unlicensed spectrum). We show that it is possible to design quasi-deterministic diversity strategies such that geometric configurations which result in destructive interference are unlikely. The rules of thumb regarding antenna spacing and bandwidth differ significantly from those for standard rich scattering models, and outage probabilities of the order of 10-4 or less can be obtained with small link margins (a few dB) relative to a line-of-sight (LoS) link.

Commercial exploitation of such multiGigabit mesh network at 60 GHz band requires that we take advantage of the low-cost digital signal processing (DSP) made available by Moore's law. A key bottleneck, however, is the cost and power consumption of high-precision analog-to-digital converters (ADCs) at the multiGigabit rates of interest in this band. This makes it difficult, for example, to apply traditional DSP-based approaches to channel dispersion compensation such as time domain equalization or Orthogonal Frequency Division Multiplexing (OFDM), since these are predicated on the availability of full-rate, high-precision samples. We investigate the use of analog multitone for sidestepping the ADC bottleneck: transmissions are split into a number of subbands, each of which can be separately sampled at the receiver using a lower rate ADC. Given the large coherence bandwidth of the sparse multipath channels typical of such highly directional outdoor 60 GHz links that we consider, reliable performance requires spatial diversity, in addition to the beamforming required to close the link. We therefore consider one transmit and two receive antenna arrays, each with 4 x 4 elements. We find that exploiting spatial diversity completely by combining samples from both arrays is critical for combating fading and inter carrier interference.