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1. A Comparison of Hybrid Beamforming and Digital Beamforming with Low-Resolution ADCs for Multiple Users and Imperfect CSI

   https://doi.org/10.1109/JSTSP.2018.2813973  

   Roth, Kilian; Pirzadeh, Hessam; Swindlehurst, A. Lee; Nossek, Josef A. (March 2018, IEEE Journal of Selected Topics in Signal Processing)

   For 5G it will be important to leverage the available millimeter wave spectrum. To achieve an approximately omni- directional coverage with a similar effective antenna aperture compared to state-of-the-art cellular systems, an antenna array is required at both the mobile and basestation. Due to the large bandwidth and inefficient amplifiers available in CMOS for mmWave, the analog front-end of the receiver with a large number of antennas becomes especially power hungry. Two main solutions exist to reduce the power consumption: hybrid beam forming and digital beam forming with low resolution Analog to Digital Converters (ADCs). In this work we compare the spectral and energy efficiency of both systems under practical system constraints. We consider the effects of channel estimation, transmitter impairments and multiple simultaneous users for a wideband multipath model. Our power consumption model considers components reported in literature at 60 GHz. In contrast to many other works we also consider the correlation of the quantization error, and generalize the modeling of it to non- uniform quantizers and different quantizers at each antenna. The result shows that as the Signal to Noise Ratio (SNR) gets larger the ADC resolution achieving the optimal energy efficiency gets also larger. The energy efficiency peaks for 5 bit resolution at high SNR, since due to other limiting factors the achievable rate almost saturates at this resolution. We also show that in the multi- user scenario digital beamforming is in any case more energy efficient than hybrid beamforming. In addition we show that if mixed ADC resolutions are used we can achieve any desired trade-off between power consumption and rate close to those achieved with only one ADC resolution. 

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2. An SNR-Enhanced 8-Ary (SNRE-8) Modulation Technique for Wireline Transceivers Using Pulse Width, Position, and Amplitude Modulation

   https://doi.org/10.1109/JSSC.2024.3361499  

   Megahed, Mohamed; Chun, Yusang; Wang, Zhiping; Anand, Tejasvi (August 2024, IEEE Journal of Solid-State Circuits)

   This article presents a novel eight-ary modulation technique with improved signal-to-noise ratio (SNR) compared to conventional pulse amplitude modulation 8 (PAM-8). The proposed SNR-enhanced 8-ary (SNRE-8) scheme modulates pulse width, position, and amplitude to improve the SNR. The proposed SNRE-8 modulation leverages the wireline channel loss to perform the modulation. Digital decoding of mutually exclusive eyes generated by the proposed SNRE-8 modulation further improves the eye margin at the receiver. A 27-Gb/s transceiver is implemented in a 65-nm CMOS process employing the proposed modulation. A PAM-8 transmitter is implemented on the same chip for comparison purposes. Compared to the PAM-8 modulation, the proposed SNRE-8 modulation shows an average SNR improvement of 10.6 dB at the near-end eye at the cost of 6.6% eye width reduction. With the aid of a time-domain feed-forward equalizer (FFE) and a continuous-time linear equalizer (CTLE), the proposed SNRE-8 transceiver achieves a bit error rate (BER) of 10−8 on a 9-dB loss channel with an energy efficiency of 5.39 pJ/bit. 

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3. Keyless Covert Communication in the Presence of Non-causal Channel State Information

   Zivari-Fard, H.; Bloch, M.; Nosratinia, A. (August 2019, IEEE Information Theory Workshop)

   We consider the problem of covert communication over a state-dependent channel, for which the transmitter and the legitimate receiver have non-causal access to the channel state information. Covert communication with respect to an adversary, referred to as the “warden,” is one in which the distribution induced during communication at the channel output observed by the warden is identical to the output distribution conditioned on an inactive channel-input symbol. Covert communication involves fooling an adversary in part by a proliferation of codebooks; for reliable decoding at the legitimate receiver the codebook uncertainty is removed via a shared secret key that is unavailable to the warden. Unlike earlier work in state-dependent covert communication, we do not assume the availability of a shared key at the transmitter and legitimate receiver. Rather, a shared randomness is extracted at the transmitter and the receiver from the channel state, in a manner that keeps the shared randomness secret from the warden despite the influence of the channel state on the warden’s output. An inner bound on the covert capacity, in the absence of an externally provided secret key, is derived. 

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4. Interleaving Channel Estimation and Limited Feedback for Point-to-Point Systems with a Large Number of Transmit Antennas

   https://doi.org/10.1109/TWC.2018.2864204  

   Koyuncu, Erdem; Zou, Xun; Jafarkhani, Hamid (January 2019, IEEE Transactions on Wireless Communications)

   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. 

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5. Neural Network-based Channel Estimation for 2x2 and 4x4 MIMO Communication in Noisy Channels

   Yang, Kanghyeok; Vuran, Mehmet C.; Scott, Stephen; Guo, Fujuan; Ahn, Changbum R. (May 2018, International Balkan Conference on Communications and Networking)

   With increasing needs of fast and reliable commu- nication between devices, wireless communication techniques are rapidly evolving to meet such needs. Multiple input and output (MIMO) systems are one of the key techniques that utilize multiple antennas for high-throughput and reliable communication. However, increasing the number of antennas in communication also adds to the complexity of channel esti- mation, which is essential to accurately decode the transmitted data. Therefore, development of accurate and efficient channel estimation methods is necessary. We report the performance of machine learning-based channel estimation approaches to enhance channel estimation performance in high-noise envi- ronments. More specifically, bit error rate (BER) performance of 2 × 2 and 4 × 4 MIMO communication systems with space- time block coding model (STBC) and two neural network-based channel estimation algorithms is analyzed. Most significantly, the results demonstrate that a generalized regression neural network (GRNN) model matches BER results of a known-channel communication for 4 × 4 MIMO with 8-bit pilots, when trained in a specific signal to noise ratio (SNR) regime. Moreover, up to 9dB improvement in signal-to-noise ratio (SNR) for a target BER is observed, compared to least square (LS) channel estimation, especially when the model is trained in the low SNR regime. A deep artificial neural network (Deep ANN) model shows worse BER performance compared to LS in all tested environments. These preliminary results present an opportunity for achieving better performance in channel estimation through GRNN and highlight further research topics for deployment in the wild. 

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