Time Division Duplex Operation Research Articles

Improved TDD operation on Software-Defined Radio platforms towards future wireless standards

Software-Defined Radio (SDR) platforms are valuable for research and development activities or high-end systems that demand flexible wireless protocols. While low-latency digital baseband processing can be achieved using a dedicated processing unit, like an FPGA or hardware accelerator, its multi-purpose Radio Frequency (RF) front-end often poses a limitation. Zero Intermediate Frequency (ZIF) transceivers are favorable for SDR, however, even for Time Division Duplex (TDD) systems, these transceivers suffer from self-interference when the transmitting and receiving Local Oscillator (LO) is set to the same frequency. To achieve low self-interference, switching from receiving to transmitting mode is needed. However, the time this takes (turnaround time, TT) for configurable RF front-ends often violates the strict timing requirements of protocols like Wi-Fi and 5G, which require response times in the order of microseconds. In this work, we first evaluate the advantages and disadvantages of several methods to suppress self-interference of a ZIF transceiver. Next, a novel approach is proposed, which can reduce the TT to as low as 640ns using the widely used AD9361 configurable ZIF RF front-end, while the noise floor is at the same level as achieved by the conventional way of switching between transmit and receive mode. We have realized and validated this approach using openwifi — an open-source Wi-Fi implementation on SDR. As a result, the receiver sensitivity is improved by up to 17dB in the 2.4GHz band and 9.5dB in the 5GHz band, for over-the-air transmissions.

Design of 39-GHz Up- and Down-Conversion Mixers for 5G mmWave TDD Applications

This article presents fully integrated 39-GHz bidirectional up- and down-conversion mixers for 5G millimeter-wave (mmWave) applications. Fabricated in a 65-nm CMOS process with a 1.2-V supply voltage, the up- and down-conversion mixers consume 39 and 43 mW, respectively. For 5G time-division duplexing (TDD) operation, intermediate-frequency (IF)/local-oscillator (LO)/radio-frequency (RF) T/R switches are introduced. For better isolation and low insertion loss between the up- and down-conversion mixer, a series-shunt single-pole double-throw (SPDT) structure and an equivalent lumped λ/4 transmission line are proposed for IF and RF T/R switches, respectively. To realize compact area and wide bandwidth, a transformer-based matching network is adopted in this design. Targeting multi-channel phased array applications, the measurement result shows that the up-conversion mixer achieves a 2.5-dB peak conversion gain with 6.5 GHz 3-dB bandwidth. Including the insertion loss of the switch and IF signal routing, at the maximum gain of 36.5 GHz, the up-conversion mixer achieves an output 1-dB gain compression point (OP1dB) of 2.5 dBm. Furthermore, the down-conversion mixer achieves a 5-dB peak conversion gain with a 9.7-GHz 3-dB bandwidth.

Reliability-optimal pilot-assisted transmission for URLLC over non-reciprocal MISO channels: TDD or FDD?

In this paper, we focus on the pilot-assisted transmission design for downlink URLLC over non-reciprocal channels, in which the multi-antenna controller sends mission-critical data signals to a single-antenna actuator. In this system, the prior knowledge of downlink channel state information (CSI) is a prerequisite for reliable data transmission. Generally, the acquisition of downlink CSI is completed either via the uplink pilot measurement exploiting channel reciprocity and time-division duplex (TDD) operation, or via the downlink pilot measurement with quantized feedback and frequency division duplex (FDD) operation. Inspired by this, we aim to investigate how the degree of channel non-reciprocity impacts the transmission reliability of our URLLC system, and the superiority between the TDD mode and FDD mode in terms of transmission reliability maximization. To describe the degree of reliability loss, we derive the closed-form approximations on the transmission error probability of URLLC in TDD and FDD modes, via leveraging the Gauss-Hermite and Gauss-Chebyshev quadrature rules. Following by the theoretical approximations, we demonstrate how to determine the optimal training pilot length and quantized feedback duration that maximize the transmission reliability under given latency constraint. Through numerical results, we validate the accuracy of theoretical approximations derived in this paper, and obtain some meaningful conclusions.

Advancements in 5G New Radio TDD Cross Link Interference Mitigation

This article presents enhancements that further improve the performance of 5G New Radio time division duplex (TDD) operation and performance. The focus is on mechanisms to mitigate co-channel cross link interference (CLI) between neighboring cells, providing pointers also to recent advances in remote interference mitigation between cells separated by tens to hundreds of kilometers. A sophisticated framework where User Equipment's (UEs) are used as sensors detecting CLI problems is outlined, and the underlying design rationales are presented. Solutions for simple coordination of TDD radio frame configurations between network elements are described. Solutions based on simple reinforcement learning algorithms for the network to adjust the TDD radio frame configuration are proposed, and validated for both macro and indoor factory deployments, considering both ultra-reliable low latency communication (URLLC) and enhanced mobile broadband (eMBB) traffic. Results from extensive dynamic system-level simulations confirm that such solutions offer attractive benefits.

Performance Analysis of Cell-Free Massive MIMO Systems: A Stochastic Geometry Approach

Cell-free (CF) massive multiple-input-multiple-output (MIMO) has emerged as an alternative deployment for conventional cellular massive MIMO networks. As revealed by its name, this topology considers no cells, while a large number of multi-antenna access points (APs) serves simultaneously a smaller number of users over the same time/frequency resources through time-division duplex (TDD) operation. Prior works relied on the strong assumption (quite idealized) that the APs are uniformly distributed, and actually, this randomness was considered during the simulation and not in the analysis. However, in practice, ongoing and future networks become denser and increasingly irregular. Having this in mind, we consider that the AP locations are modeled by means of a Poisson point process (PPP) which is a more realistic model for the spatial randomness than a grid or uniform deployment. In particular, by virtue of stochastic geometry tools, we derive both the downlink coverage probability and achievable rate. Notably, this is the only work providing the coverage probability and shedding light on this aspect of CF massive MIMO systems. Focusing on the extraction of interesting insights, we consider small-cells (SCs) as a benchmark for comparison. Among the findings, CF massive MIMO systems achieve both higher coverage and rate with comparison to SCs due to the properties of favorable propagation, channel hardening, and interference suppression. Especially, we showed for both architectures that increasing the AP density results in a higher coverage which saturates after a certain value and increasing the number of users decreases the achievable rate but CF massive MIMO systems take advantage of the aforementioned properties, and thus, outperform SCs. In general, the performance gap between CF massive MIMO systems and SCs is enhanced by increasing the AP density. Another interesting observation concerns that a higher path-loss exponent decreases the rate while the users closer to the APs affect more the performance in terms of the rate.

Small-signal design consideration for two-dimensional change-over switch GaN MMICs

This paper presents small-signal design consideration for two-dimensional change-over switch GaN monolithic microwave integrated circuits (MMICs). For the design of two-dimensional change-over single pole single throw (SPST) switch circuit which electrically changes between low pass filter and high pass filter, the demands for design conditions are as follows: 1. two-dimensional switch with time/frequency domain. 2. The sufficient isolation in off-state at the signal frequencies for both low band and high band. Two-dimensional change-over single pole double throw (SPDT) switch provides a duplexer for frequency division duplex operation and an SPDT switch for time division duplex operation for low-cost the fifth generation (5G) front-ends (FEs). GaN two-dimensional change-over SPST and SPDT MMIC switches are successfully demonstrated by using 0.25 μm GaN foundry process for high power handling capability in 5G FEs. The demonstrated GaN MMIC switches shows almost the same two-dimensional switching characteristics as the designed ones.

Performance Investigation of Peak Shrinking and Interpolating the PAPR Reduction Technique for LTE-Advance and 5G Signals

Orthogonal frequency division multiplexing (OFDM) has become an indispensable part of waveform generation in wideband digital communication since its first appearance in digital audio broadcasting (DAB) in Europe in 1980s, and it is indeed in use. As has been seen, the OFDM based waveforms work well with time division duplex operation in new radio (NR) systems in 5G systems, supporting delay-sensitive applications, high spectral efficiency, massive multiple input multiple output (MIMO) compatibility, and ever-larger bandwidth signals, which has demonstrated successful commercial implementation for 5G downlinks and uplinks up to 256-QAM modulation schemes. However, the OFDM waveforms suffer from high peak to average power ratio (PAPR), which is not desired by system designers as they want RF power amplifiers (PAs) to operate with high efficiency. Although NR offers some options for maintaining the efficiency and spectral demand, such as cyclic prefix based (CP-OFDM), and discrete Fourier transform spread based (DFT-S-OFDM) schemes, which have limiting effects on PAPR, the PAPR is still as high as 13 dB. This value increases when the bandwidth is increased. Moreover, in LTE-Advance and 5G systems, in order to increase the bandwidth, and data-rate, carrier aggregation technology is used which increases the PAPR the same way that bandwidth increment does; therefore, it is essential to employ PAPR reduction in signal processing stage before passing the signal to PA. In this paper, we investigate the performance of an innovative peak shrinking and interpolation (PSI) technique for reducing peak to average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) based signals at waveform generation stage. The main idea behind the PSI technique is to extract high peaks, scale them down, and interpolate them back into the signal. It is shown that PSI technique is a possible candidate for reducing PAPR without compromising on computational complexity, compatible for existing and future telecommunication systems such as 4G, 5G, and beyond. In this paper, the PSI technique is tested with variety of signals in terms of inverse fast Fourier transform (IFFT) length, type of the signal modulation, and applications. Additional work has been carried out to compare the proposed technique with other promising PAPR reduction techniques. This paper further validates the PSI technique through experimental measurement with a power amplifier (PA) test bench and achieves an adjacent channel power ratio (ACPR) of less than –55 dBc. Results showed improvement in output power of PA versus given input power, and furthermore, the error vector magnitude (EVM) of less than 1 % was achieved when comparing of the signal after and before modification by the PSI technique.

Pilot Decontamination Based on Superimposed Pilots Assisted by Time-Multiplexed Pilots in Massive MIMO Networks

Pilot contamination has been recognized as one of the main limiting factors in massive multiple-input multiple-output (MIMO) since it damages the acquisition of accurate channel state information (CSI) at base stations. In this work, we propose a novel pilot decontamination approach in a massive MIMO network where the time division duplex operation is assumed. In our proposed approach, user equipments are allowed to use superimposed (SP) pilots in combination with time-multiplexed (TM) pilots to estimate the CSI prior to downlink data transmission. Specifically, the contaminated channel estimates obtained by TM pilots are used to reduce the amount of interference caused by transmitting pilots together with uplink data, which is a typical problem in SP pilots-based channel estimation. Through numerical results, we show that the interference that emerges from transmitting pilots with data is substantially reduced by our proposed approach. This reduction, in combination with the reduced overhead in TM pilots transmission, leads to an improved spectral efficiency achieved in massive MIMO networks.

A Comparison Between Measured and Computed Assessments of the RF Exposure Compliance Boundary of an In-Situ Radio Base Station Massive MIMO Antenna

Advanced antenna technology in the form of massive multiple input multiple output (mMIMO) antennas has been deployed in fourth generation (4G) long term evolution (LTE) mobile networks and will be a standard feature in fifth generation (5G) networks. A characteristic of mMIMO is beamforming in which a large array of antenna elements is used to concentrate energy in specific narrow directions to improve data throughput. Assessing the electromagnetic field (EMF) compliance boundary around a beamforming capable mMIMO antenna using a traditional approach that assumes maximum power is simultaneously transmitted in every possible direction and the radio base station (RBS) is fully loaded (100% traffic utilization) may produce very conservative results. In this study we report on measurements of the 6-minute time-averaged EMF levels in front of a mMIMO antenna operating in a 4G LTE time division duplex (TDD) network and compare results with conservatively computed EMF levels. The RBS serves users scattered throughout a dense urban environment characterized by low rise commercial/industrial buildings. When scaled to 100% RBS traffic loading and taking TDD operation into account, the highest measured EMF level was 7.3% to 16.1% of the ICNIRP occupational reference level compared to a conservative computed level of 79.3%. This represents a far-field reduction in the occupational compliance boundary of between 2.2 and 3.3 times and is consistent with other statistically based theoretical studies. An additional reduction in the compliance boundary can be expected due to actual RBS traffic loading often being much less than 100%.

Pilot Power Control for Cell-Free Massive MIMO

In this correspondence paper, we consider a cell-free massive Multiple-Input Multiple-Output (MIMO) system where access points (APs) serve a much smaller number of users under the time-division duplex operation. The APs first estimate the channels via the uplink training phase. Then, these channel estimates are used to detect desired symbols in the uplink and precode the transmit symbols in the downlink. Nonorthogonality of pilot sequences and AP selection (e.g., received-power-based selection or largest-large-scale-fading based selection schemes) are taken into account. To reduce the effect of pilot contamination, we propose a pilot power control design, which chooses the pilot power control coefficients to minimize the mean-squared error of the channel estimation. This is achieved via the sequential convex approximation method. By using pilot power control in training phase, the system performance is considerably improved. In addition, we derive closed-form expressions for the uplink and downlink achievable rates with arbitrary power data/pilot control coefficients and any AP selection schemes.

Distributed Interference Alignment for Multi-Antenna Cellular Networks With Dynamic Time Division Duplex

In this letter, we propose a distributed interference alignment (DIA) technique in a two-cell multiple-input multiple-output (MIMO) network with dynamic time division duplex (TDD) mode under local channel state information assumption. The dynamic TDD operation introduces new types of interference, such as downlink-to-uplink [base station to base station (BS–BS)] and uplink-to-downlink [mobile station to mobile station (MS–MS)] interferences. Under the assumption of rank-deficient BS–BS MIMO interference channel, we mathematically characterize the achievable degrees-of-freedom of the proposed DIA technique for a given number of antennas at BS/MS. Through extensive computer simulations, it is shown that the proposed DIA technique outperforms the conventional techniques in terms of sum-rate at the cost of signaling overheads for the BS–BS and MS–MS channel overhearing.

On the Total Energy Efficiency of Cell-Free Massive MIMO

We consider the cell-free massive multiple-input multiple-output (MIMO) downlink, where a very large number of distributed multiple-antenna access points (APs) serve many single-antenna users in the same time-frequency resource. A simple (distributed) conjugate beamforming scheme is applied at each AP via the use of local channel state information (CSI). This CSI is acquired through time-division duplex operation and the reception of uplink training signals transmitted by the users. We derive a closed-form expression for the spectral efficiency taking into account the effects of channel estimation errors and power control. This closed-form result enables us to analyze the effects of backhaul power consumption, the number of APs, and the number of antennas per AP on the total energy efficiency, as well as, to design an optimal power allocation algorithm. The optimal power allocation algorithm aims at maximizing the total energy efficiency, subject to a per-user spectral efficiency constraint and a per-AP power constraint. Compared with the equal power control, our proposed power allocation scheme can double the total energy efficiency. Furthermore, we propose AP selections schemes, in which each user chooses a subset of APs, to reduce the power consumption caused by the backhaul links. With our proposed AP selection schemes, the total energy efficiency increases significantly, especially for large numbers of APs. Moreover, under a requirement of good quality-of-service for all users, cell-free massive MIMO outperforms the colocated counterpart in terms of energy efficiency.

An Approach to Achieve Zero Turnaround Time in TDD Operation on SDR Front-End

Thanks to the digitization and softwarization of radio communication, the development cycle of new radio technologies can be significantly accelerated by prototyping on software-defined radio (SDR) platforms. However, a slow turnaround time (TT) of the front-end of an SDR for switching from receiving mode to transmitting mode or vice versa, are jeopardizing the prototyping of wireless protocols, standards, or systems with stringent latency requirements. In this paper, a novel solution called BaseBand processing unit operating in Half Duplex mode and analog Radio Frequency front-end operating in Full Duplex mode, BBHD-RFFD, is presented to reduce the TT on SDR. A prototype is realized on the widely adopted AD9361 radio frequency frontend to prove the validity of the proposed solution. Experiments unveil that for any type of application, the TT in time division duplex (TDD) operation mode can be reduced to zero by the BBHD-RFFD approach, with negligible impact on the communication system in terms of receiver sensitivity. The impact is measured for an in-house IEEE 802.15.4 compliant transceiver. When compared against the conventional TDD approach, only a 7.5-dB degradation is observed with the BBHD-RFFD approach. The measured sensitivity of −91 dBm is still well above the minimum level (i.e., −85 dBm at 2.4 GHz) defined by the IEEE 802.15.4 standard.

Massive MIMO Performance—TDD Versus FDD: What Do Measurements Say?

Downlink beamforming in Massive multiple-input and multiple-output (MIMO) either relies on uplink pilot measurements-exploiting reciprocity and time-division duplexing operation, or on the use of a predetermined grid of beams with user equipments reporting their preferred beams, mostly in frequency-division duplexing operation. Massive MIMO in its originally conceived form uses the first strategy, with uplink pilots, whereas there is currently significant commercial interest in the second, grid-of-beams. It has been analytically shown that with isotropic scattering (independent Rayleigh fading) the first approach outperforms the second. Nevertheless, there remains controversy regarding their relative performance in practical channels. In this contribution, the performances of these two strategies are compared using measured channel data at 2.6 GHz.

Online compressive diagnosis of massive MIMO calibration state

With hundreds of antennas at one single base station (BS), it is critical to take full advantage of time-division duplexing (TDD) channel reciprocity to learn the downlink (DL) channel state information (CSI) from the uplink (UL) channel measurements at the BS. However, due to different radio frequency branches for transmitting and receiving, calibrating the antenna array at the BS is a necessary and prerequisite task to effect this reciprocity between the end-to-end DL and UL channels. This calibration procedure becomes extremely challenging in massive multiple-input multiple-output (MIMO) since all the existing calibration approaches do not scale well with the size of the antenna array. To release all the potentials promised by massive MIMO, we rely on compressive sensing theory to develop an efficient and robust way to diagnose the massive antenna array and further restore the full calibration at the BS without service interruption. The diagnosis only requires CSI feedback in the amount of O(log N) instead of O(N) as required by those conventional calibration schemes. Extensive simulations demonstrate our approach can maintain the whole large scale antenna system in a calibrated state with only limited amount of feedback overhead. The proposed novel scheme thus allows us to make full use of the channel reciprocity in massive MIMO under TDD operation.

Cooperative TDD reciprocity calibration with diversity combining for distributed large-scale MIMO systems

Thanks to channel reciprocity, the time-division duplexing operation is typically used for distributed large-scale multiple-input multiple-output systems. With the knowledge of uplink channel state, APs (Access Points) perform the downlink precoding to cooperatively communicate with multiple UEs (User Equipments). Unfortunately, the channel reciprocity is often jeopardized by the non-symmetric transceiver RF (Radio Frequency) circuits at both sides of the link. By excluding UEs from the calibration procedure, the TLS (Total Least Squares) method is extended to the case of partial calibration, where only the APs are involved to exchange calibration signals to compensate for RF mismatches. Nevertheless, channel fluctuations between APs significantly degrade system performance. Therefore, we propose a scheme to obtain the diversity gain, by performing a calibration with the combination of the calibration signals of different channel coherent times. Simulation results confirm the effective performance and robustness of our scheme.

Eradication of pilot contamination and zero forcing precoding in the multi‐cell TDD massive MIMO systems

In massive multiple-input multiple-output (M-MIMO) systems, the base station (BS) estimates the downlink and uplink channels using the uplink pilot training in conjunction with channel reciprocity property of time division duplex (TDD) operation. However, this channel estimation is contaminated due to the use of non-orthogonal pilot sequences for the uplink training in neighbouring cells that results in inter-cell interference. This study proposes an effective pilot contamination elimination scheme along with zero forcing precoding for multi-cell TDD M-MIMO systems. In the proposed scheme, each BS is assigned with a specific orthogonal variable spreading factor (OVSF) code row and a set of Zadoff–Chu (ZC) sequences are used for uplink pilot training. Then, the set of ZC sequences is multiplied element-wise at each BS with its OVSF code row to generate the orthogonality among the pilot sequences across the neighbouring cells. The proposed method can promise uncontaminated channel estimation and interference free downlink transmission without requiring the knowledge of the channels’ second-order statistics and without increasing the training overhead by a factor equal to the number of interfering cells. The simulation results authenticate the effectiveness and superiority of the proposed scheme over other schemes.

Energy Efficiency in Cell-Free Massive MIMO with Zero-Forcing Precoding Design

We consider the downlink of a cell-free massive multiple-input multiple-output (MIMO) network where numerous distributed access points (APs) serve a smaller number of users under time division duplex operation. An important issue in deploying cell-free networks is high power consumption, which is proportional to the number of APs. This issue has raised the question as to their suitability for green communications in terms of the total energy efficiency (bits/Joule). To tackle this, we develop a novel low-complexity power control technique with zero-forcing precoding design to maximize the energy efficiency of cell-free massive MIMO taking into account the backhaul power consumption and the imperfect channel state information.

Active Three-Way Circulator Using Transistor Feedback Network

This letter presents an active three-way circulator with both theoretical analyzes and experimental verifications. The conventional active circulators have their limitations on power handling and noise figure (NF), however, this proposed structure maintains both performances, which is formed by three 2-port transistor feedback networks (TFNs). This TFN is formed by an FET together with a feedback inductor. Experimental results show that this circulator achieves the isolation more than 35 dB, the insertion of around 2 dB, and the return losses better than 10 dB at each port at 1.8 GHz. The output power is linearly proportional to the power operation and the NF is around 5 dB. This compact structure, therefore, is suitable for use in the transmit/receive front end with a sufficient isolation, power handling, and NF for time division duplex operation.

Novel Detection Scheme for LSAS Using Power Allocation in Multi User Scenario with LTE-A and MMB Channels

Massive MIMO (also known as the "Large-Scale Antenna System") enables a significant reduction of latency on the air interface with the use of a large excess of service-antennas over active terminals and time division duplex operation. For large-scale MIMO, several technical issues need to be addressed (e.g., pilot pattern design and low-antenna power transmission design) and theoretically addressed (e.g., channel estimation and power allocation schemes). In this paper, we analyze the ergodic spectral efficiency upper bound of a large-scale MIMO, and the key technologies including channel uplink detection. We also present new approaches for detection and power allocation. Assuming arbitrary antenna correlation and user distributions, we derive approximations of achievable rates with linear detection techniques, namely zero forcing, maximum ratio combining, minimum mean squared error (MMSE) and eigen-value decomposition power allocation (EVD-PA). While the approximations are tight in the large system limit with an infinitely large number of antennas and user terminals, they also match our simulations for realistic system dimensions. We further show that a simple EVD-PA detection scheme can achieve the same performance as MMSE with one order of magnitude fewer antennas in both uncorrelated and correlated fading channels. Our simulation results show that our proposal is a better detection scheme than the conventional scheme for LSAS. Also, we used two channel environment channels for further analysis of our algorithm: the Long Term Evolution Advanced channel and the Millimeter wave Mobile Broadband channel.