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Recent interest in wide-band multi-giga-bit-per second wireless communications over mm-wave bands has created both new opportunities and design challenges. The realization of such technologies including multi-giga-samples-per second data conversion and digital signal processing systems is extremely challenging. In this brief, we propose a fully analog QAM demodulator as a step towards eliminating the power hungry and ultra-high speed digital components. The proposed low-complexity, low-overhead solution is shown to be robust against analog processing errors. 

We introduce and investigate the opportunities of multi-antenna communication schemes whose training and feedback stages are interleaved and mutually interacting. Specifically, unlike the traditional schemes where the transmitter first trains all of its antennas at once and then receives a single feedback message, we consider a scenario where the transmitter instead trains its antennas one by one and receives feedback information immediately after training each one of its antennas. The feedback message may ask the transmitter to train another antenna; or, it may terminate the feedback/training phase and provide the quantized codeword (e.g., a beamforming vector) to be utilized for data transmission. As a specific application, we consider a multiple-input single-output system with t transmit antennas, a short-term power constraint P, and target data rate ρ. We show that for any t, the same outage probability as a system with perfect transmitter and receiver channel state information can be achieved with a feedback rate of R1 bits per channel state and via training R2 transmit antennas on average, where R1 and R2 are independent of t, and depend only on ρ and P. In addition, we design variable-rate quantizers for channel coefficients to further minimize the feedback rate of our scheme.