Wireless Communications with Low Resolution ADCs
Millimeter wave (mmWave) is a technology that can provide high bandwidth communication links in cellular systems. As mmWave uses larger bandwidths, the corresponding sampling rate of the analog-to-digital converter (ADC) scales up. Unfortunately, high speed, high precision (e.g., 8-12 bits) ADCs are costly and power-hungry for portable devices. As shown in the figure, at rates above 100 megasamples per second, ADC power consumption increases quadratically with sampling frequency. In addition, there is also a lot of interests to use low resolution ADCs for the massive MIMO receiver where a large number of ADCs are needed.
A possible solution is to use special ADC structures like a time interleaved ADC (TI-ADC) architecture where a number of low-speed, high-precision ADCs operate in parallel. The main challenge of the TI-ADC is the mismatch among the sub-ADCs in gain, timing and voltage offset which can cause error floors in receiver performance. An alternative solution is to live with ultra low precision ADCs (1-3 bits). The main advantage of this architecture is the ADCs can be implemented with very low power consumption and hardware cost. The architecture also simplifies the overall complexity of the circuit for example automatic gain control may not be required.
The use of few- and especially 1-bit ADCs radically changes both the theory and practice of communication. The nonlinear quantization function renders the analyses of communication systems much more complicated and usually the exact closed-form results can not be found. Our research focuses on the performance limits and system design with low resolution ADCs, for example, the capacity analysis, detector design, channel estimation algorithm and limited feedback method. First, we investigated the channel capacity of one-bit quantized MIMO channel with CSIT and CSIR. Second, we implemented near maximum-likelihood (nML) detectors using convex optimization techniques for frequency flat fading massive MIMO systems. Third, we present an algorithm estimating the mmWave channel with few-bit ADCs. Last, the limited feedback method, through which the estimated CSI is fed back to the transmitter, was proposed.