This paper analyzes the performance of linearly precoded time division duplex based multi-user massive MIMO downlink system under joint impacts of channel non-reciprocity (NRC) and imperfect channel state information. We consider a generic and realistic NRC model that accounts for transceiver frequency-response as well as mutual coupling mismatches at both user equipment (UE) and base station (BS) sides. The analysis covers two most prominent forms of linear precoding schemes, namely, zero-forcing (ZF) and maximum-ratio transmission (MRT), and assumes that only the statistical properties of the beamformed channel are used at the UE side to decode the received signal. Under the approximation of i.i.d. Gaussian channels, closed-form analytical expressions are derived for the effective signal to interference and noise ratios (SINRs) and the corresponding capacity lower bounds. The expressions show that, in moderate to high SNR, the additional interference caused by imperfect NRC calibration can degrade the performance of both precoders significantly. Moreover, ZF is shown to be more sensitive to NRC than MRT. Numerical evaluations with practical NRC levels indicate that this performance loss in the spectral efficiency can be as high as 42% for ZF, whereas it is typically less than 13% for MRT. It is also shown that due to the NRC, the asymptotic large-antenna performance of both precoders saturate to an identical finite level. The derived analytical expressions provide useful tools and valuable technical insight, e.g., into calculating the NRC calibration requirements in BSs and UEs for any given specific performance targets in terms of effective SINR or the system capacity bound.

In this paper, we propose a sequential hybrid beamforming design for multi-link transmission over mmwave frequency bands. As a starting point, a baseline data communication link is established via traditional analog beamforming at both the BS and UE. If an extra RF chain is available at the UE, it can continue to probe the propagation environment at the same frequencies. In case the environment is favorable and system resources allow, a secondary data communication link is established to enable multi-stream transmission. In principle, the secondary link could be served by the same BS and/or one or several other BS(s). To initialize the secondary data communication link, a parallel beam search scheme is proposed, which helps the UE/BS to find a suit-able beam pair with given optimization criteria without interrupting the baseline data communication. By applying the proposed two-step approach, hybrid beamforming becomes an add-on feature that can be easily switched on over an analog beamforming enabled system without interrupting its operation whenever system requires. Meanwhile, the information obtained by deploying the proposed parallel beam search scheme can also be used for deciding a back-up beam pair if signal blockage occurs to the baseline data communication link.

The effects of oscillator phase noises (PNs) on multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems are studied. It is shown that PNs of common oscillators at the transmitter and at the receiver have the same influence on the performance of (single-stream) beamforming MIMO-OFDM systems, yet different influences on spatial multiplexing MIMO-OFDM systems with singular value decomposition (SVD) based precoding/decoding. When each antenna is equipped with an independent oscillator, the PNs at the transmitter and at the receiver have different influences on beamforming MIMO-OFDM systems as well as spatial multiplexing MIMO-OFDM systems. Specifically, the PN effect on the transmitter (receiver) can be alleviated by having more transmit (receive) antennas for the case of independent oscillators. It is found that the independent oscillator case outperforms the common oscillator case in terms of error vector magnitude (EVM).

In this paper, we study the effects of effective channel non-reciprocity in coordinated TDD multi-cell MIMO network based on weighted sum rate (WSR) maximization. More specifically, we focus on UE transceiver non- reciprocity while the base stations (BS) are assumed to be perfectly calibrated, and both centralized and decentralized beamforming schemes are considered. In the centralized scheme, the cost function is constructed in a central controller using antenna specific UL pilots for channel estimation from all the connected BSs in the network. Then, even though the transceiver frequency response (FR) mismatches at the UE side corrupt the effective channel reciprocity, it is shown to have only a trivial impact on the WSR objective in such centralized case. However, when decentralized beamforming is deployed, the optimization is carried out in each BS and the corresponding cost function and optimization process depend on information acquired by over-the-air signaling between the BSs and all the users using precoded UL pilots. In this case, it is then shown that the transceiver FR mismatches at the UE side can cause severe performance degradation and even influence the convergence properties of the sum-rate optimization process. Further insight is provided for improving the performance by modifying the weight calculations in the optimization process and connecting users with good cell separations. Then a convergence-aware processing algorithm is also proposed to improve the performance of the decentralized scheme under UE transceiver non-reciprocity. Numerical experiments demonstrate that efficient processing algorithms for calibrating UE transceiver mismatches to be less than -30dB to -35dB are, in general, required in the decentralized system in order to achieve performance close to the ideal case without any RF imperfections.

Channel reciprocity can be used for providing sufficient key generation in time division duplex (TDD) system. However, in practice, its application is limited by the hardware fingerprint deviation (HFD) problem. In this paper, we propose a novel real-time transform that can cope with this problem in time- varying TDD channel without any calibration period or feedback loops. More specifically, a log-domain differential (LDD) transform is developed and the resulting performance is analyzed in terms of mean square error (MSE) between receptions at Alice and Bob and effective signal to error ratio (ESER). The analysis shows that the proposed transform can eliminate the impact of HFD, yet its performance is very sensitive to channel noise and moving speed. For this purpose, an enhanced version is proposed including an efficient noise reduction technique and the impact of mobility on parameter design is also analyzed. Numerical results show that the proposed LDD advanced transform provides performance comparable to the ideal case without HFD, and thus, can be used to form a simple, practical and flexible solution for secret key generation in time-varying TDD channel.

In this paper, we propose an efficient transceiver non-reciprocity estimation-compensation framework for precoded TDD multi-user MIMO-OFDM downlink transmission systems. General signal models are first developed to analyze the effects of transceiver non-reciprocity at both the base-station and user equipment sides. The analysis shows that transceiver non-reciprocity at the base-station causes inter-user interference and thus substantial performance degradation, while the impact of transceiver non-reciprocity at the user equipment side can be fairly easily handled with downlink detector processing. Next, an over-the-air (OTA) type pilot-based estimation algorithm is devised for efficient identification of base-station non-reciprocity parameters, which are then used in pre-compensating or precoding the multiuser data properly in the base-station. Compared to the existing work in the literature, the proposed approach does not require the use of feedback signaling or complicated channel matrix decomposition techniques for extracting the non-reciprocity parameters. The resulting link and system performance of the proposed estimation-compensation framework is then evaluated using extensive computer simulations in linear precoded TDD multiuser MIMO-OFDM system context. Based on the obtained results, the proposed estimation-compensation approach can provide a simple, practical and flexible solution to efficiently restore the channel reciprocity with reasonable calibration overhead.

This paper studies the impact of two implementation imperfections, namely, transceiver frequency-response non-reciprocity and antenna mutual coupling mismatches at the base-station and user equipment on the channel reciprocity assumption in TDD systems. Comprehensive signal models are first developed to analyze the joint effects of these two imperfections in the TDD multi-user MIMO-OFDM downlink transmission system context, covering both zero-forcing (ZF) and eigenbeamforming based transmitter processing. The corresponding performance degradation is then evaluated in terms of SINRs and maximum achievable sum-rate. The analysis shows that during downlink transmission, the transceiver non-reciprocity and antenna mutual coupling mismatches at the base-station introduce inter-user interference (IUI) and are the major causes for the resulting performance degradation. Implementation imperfections at user equipment side, in turn, only introduce inter-stream interference (ISI) that can be fairly easily suppressed in detector processing as part of effective precoded channels. In order to achieve throughputs close to the ideal case, transceiver frequency-response mismatches and antenna mutual coupling at the base-station side need to be extremely well calibrated, generally below 35-40dB in terms of relative mismatch levels.

OFDM and other multicarrier waveforms are in general very sensitive to RF non-idealities, such as phase noise and IQ imbalance, of transmitting and receiving devices. Extensive work has been carried out in the open literature in analyzing the performance of OFDM radio link under such RF impairments in terms of detection error rate and mostly concentrating on one impairment at a time. However, there is only very limited work on analytical investigations of mutual information and rate loss expressions, the heart of communication theory, as functions of RF impairment levels. In this article, we derive two closed-form mutual information expressions, in the form of infinite series representation, for an arbitrary subcarrier of a general OFDM radio link impaired with transceiver phase noise and IQ imbalance in frequency-selective Rayleigh distributed block-fading radio channel, covering both uncorrelated as well as fully correlated mirror subcarrier scenarios. We also show that the mutual information saturates to a finite value due to the inherent RF impairments even in the case that the symbol-to-noise ratio approaches infinity. Extensive comparisons with results obtained from full OFDM radio link simulations are also provided to illustrate and verify the accurate match between analytical and simulated mutual information behavior.

I/Q signal processing based communication systems suffer from analog front-end (FE) imperfections such as in-phase and quadrature-phase (I/Q) imbalance and carrier frequency offset (CFO). These impairments are commonly encountered in all practical implementations, and severely degrade the obtainable link performance. Moreover, orthogonal frequency division multiplexing (OFDM)-based systems are particularly sensitive to radio frequency (RF) impairments. In this paper, we analyze the impact of transmitter and receiver I/Q imbalance together with channel distortion and CFO error on an ideal transmit signal, and propose low-complexity DSP algorithms and compensation structure for coping with such imperfections. Based on our proposed estimation/compensation structure, we are able to decouple the impairments and process them individually with rather low-complexity. More specifically, we first apply a blind algorithm for receiver I/Q imbalance compensation, followed by an efficient time domain CFO estimator and compensator. The transmitter I/Q imbalance and channel are then equalized jointly, in the frequency domain, with maximum-likelihood (ML) or zero-forcing (ZF) schemes, respectively. The applied algorithms are either blind working without aid of any training symbol or use only one OFDM symbol for impairments estimation, providing an efficient alternative solution with reduced complexity. The computer simulation results indicate a close to ideal performance of ZF scheme, and suggest that additional performance improvement due to frequency diversity can be obtained when ML estimation technique is employed.

This article addresses the signal distortion caused by receiver phase noise (PN) on OFDM waveforms in direct-conversion radio receivers. A closed-form solution for the observed signal-to-interference-plus-noise ratio (SINR) is derived, describing the level of intercarrier interference (ICI) stemming from PN. Compared to existing literature, the analysis is valid for arbitrary oscillator spectral shape, the only assumption being that reasonably small phase noise values are observed. The analysis results can be used to derive practical circuit-level oscillator design criteria in terms of the allowable PN spectral density. The applicability and validity of the derived analysis are verified with extensive computer simulations.

This paper studies the impact of one important radio transceiver impairment, namely the I/Q imbalance, on the link performance of spatial multiplexing MIMO-OFDM systems. A closed-form solution for the effective signal-to-interference-plus-noise-ratio (SINR) at the input of the receiver detection stage due to frequency-selective transmitter and receiver I/Q imbalances is derived, taking also the effects of fading multipath radio channel and additive noise into account. As will be shown, the derived SINR can be directly mapped to the achievable detection error rate at high SNR regime, yielding a valuable analytical tool for radio transceiver designers to analyze the imbalance effects at link-level without any actual data simulations. The analytical outcomes are also verified using extensive computer simulations backing up the theoretical studies. In general, the obtained results indicate that I/Q imbalance can easily become a limiting factor to the achievable link performance in future spatial multiplexing MIMO-OFDM systems, and thus should be carefully mitigated using proper digital and/or analog signal processing.

This paper presents a pilot-based compensation algorithm for mitigation of frequency-selective I/Q imbalances in direct-conversion OFDM transmitters. By deploying a feedback loop from RF to baseband, together with a properly-designed pilot signal structure, the I/Q imbalance properties of the transmitter are efficiently estimated in a subcarrier-wise manner. Based on the obtained I/Q imbalance knowledge, the imbalance effects on the actual transmit waveform are then mitigated by baseband pre-distortion acting on the mirror-subcarrier signals. The compensation performance of the proposed structure is analyzed using extensive computer simulations, indicating that very high image rejection ratios can be achieved in practical system set-ups with reasonable pilot signal lengths.

The combination of orthogonal frequency division multiplexing (OFDM) and multiple-input multiple-output (MIMO) techniques has been widely considered as the most promising approach for building future wireless transmission systems. In such systems, the limited implementation resources for the radio parts cause big restrictions on the size, cost, and quality of the individual radio transceivers. This implies that there are several imperfections at the analog radio front-end stages, which in turn can severely limit the overall system performance. One good example is the so called I/Q imbalance problem related to the amplitude and phase matching of the transceivers I and Q chains. This paper studies the performance of space-time coded (STC)-OFDM systems under I/Q imbalance. As a practical example, a 2 × 1 STC-OFDM system is examined in detail and a closed-form solution for the resulting signal-to-interference ratio (SIR) due to I/Q imbalance at the output of the receiver combining stage is derived. The analytical outcomes are verified using extensive computer simulations, and can easily be extended to multi-antenna receiver cases as well. In general, the obtained results indicate that I/Q imbalance can easily become a limiting factor in practical STC-OFDM systems and should be carefully mitigated using proper digital and/or analog signal processing.