Analog multitone with interference suppression: Relieving the ADC bottleneck for wideband 60 GHz systems

Commercial exploitation of the large amounts of unlicensed spectrum available at 60 GHz 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. In this paper, 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. For efficient use of spectrum, we do not allow guard bands between adjacent subbands, hence the receiver signal processing must account for intercarrier interference (ICI) across subbands as well as intersymbol interference (ISI) within a subband due to channel dispersion. We illustrate our ideas for short-range (100-200 meters), highly directional, outdoor 60 GHz links, as might be employed for wireless backhaul. Given the large coherence bandwidth of the sparse multipath channels typical of such 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 × 4 elements. We investigate linear equalization strategies corresponding to different combinations of: (a) combining samples from both arrays/choosing the stronger array and (b) equalizing the subbands independently/jointly. We find that exploiting the spatial diversity completely by combining samples from both arrays is critical for combating fading and inter carrier interference.