Novel Method to Improve Control Channel Reliability in LTE-Advanced Heterogeneous Network

In cellular systems, deploying a large number of antennas at the base station (BS), also called massive multiple-input multiple-output (MIMO), can offer a huge improvement in system throughput. This performance gain achieved through coherent beamforming highly depends on having accurate channel state information (CSI) at the BS. Initially, massive MIMO was considered promising only for time division duplexing (TDD) systems because the downlink training overhead in frequency division duplexing (FDD) systems could be large. To reduce the overhead in FDD systems, we propose efficient eigenspace training and precoding. In the downlink training, the prebeamforming matrix is designed based on the spatial fading correlation to probe the channel and minimize the mean squared error (MSE) in channel estimation. In data transmission, the precoding matrix is designed based on instantaneous CSI to manage the inter-user interference. Our algorithms achieve significant savings in the downlink training because the overhead is associated with the dimension of prebeamforming matrix and no longer limited by the number of BS antennas. Compared with joint spatial division and multiplexing, our algorithms offer much lower MSE and, under some conditions, higher sum rates.

Channel reciprocity is limited in frequency division duplex (FDD) system, and thus those channel reciprocity-based secret key extraction methods which are widely adopted in time division duplex (TDD) systems can hardly be implemented in FDD systems. In this paper, a novel secret key extraction method in OFDM-FDD system is proposed to achieve physical-layer security, by which the channel frequency response (CFR) in the uplink at the same time is estimated by the communication pair with the aid of a specially designed forwarding strategy based on confidential probing signal, and then the secret key is extracted from the CFR estimates. The effectiveness of the proposed method is verified by numerical simulations.

Spectrum and infrastructure sharing among multiple mobile network operators have recently drawn significant attention from the industry and the academia sectors. Recent advances in the area of Software Defined Networking (SDN) launch the design of innovative SDN-based solutions for resource sharing in multi-tier networks of macrocell and small-sized base stations, a.k.a. HetNet. In this paper, we present a novel SDN-based framework that enables efficient and elastic spectrum utilization among multiple operators in 3GPP LTE-A HetNet scenario. Assuming a multi-operator environment of Frequency Division Duplex (FDD) macrocells complemented by multi-tenant Time Division Duplex (TDD) pico cells, we present a SDN-based architecture that allows efficient resource sharing among the TDD and FDD systems in a dynamic way. A TDD frame re-configuration mechanism is also employed, to optimize the ratio of uplink and downlink slots in the TDD frame of picocells. System-level simulation results demonstrate that the combination of these two functional enhancements in the LTE-A HetNet, significantly reduces the application layer delay for both the FDD macrocell and TDD picocell systems, leading to highly efficient and dynamic spectrum sharing among multiple network operators.

3GPP standardization is evolving beyond 5G the New Radio air interface (NR-Uu) framework defined in 3GPP Release 15 (also known as 5G phase 1) to study the support of advanced vehicle to everything (V2X) services. The NR Sidelink (SL) being designed in 3GPP Release 16 (5G phase 2) to support advanced V2X services directly between vehicles and other road users is expected to finish by the end of 2019. The advanced V2X services primarily focuses on platooning, advanced driving, extended sensor and remote driving use-cases. Supporting an end-to-end latency of 3ms and high reliability up-to 99.999% requirement requires substantial improvements in the air-interface design. Long Term Evolution (4G LTE) V2X sidelink supports the delivery of Basic Safety Messages to nearby V2X User Equipments (UE, the mobile device), hence the sidelink physical layer and the higher protocol layers are designed to support only broadcast type traffic without Hybrid Automatic Repeat Request (HARQ) feedback. With the advanced V2X service requirement, NR sidelink design should study V2X message delivery for different cast types such as unicast, groupcast and broadcast type traffics that requires substantial changes starting in V2X Release 16 network architecture, physical and protocol layers. This paper provides insights on the current Release 16 NR sidelink design for V2X. The network architecture enhancement discusses supporting flow- based Quality of Service (QoS) model for NR sidelink and the newly introduced network entities such as Network Data Analytics Function (NWDAF) providing QoS analytic. The design of the NR sidelink physical layer based on the Cyclic- Prefix Orthogonal Frequency-Division Multiplexing (CPOFDM) waveform addresses the necessary improvements needed in the sidelink procedures for reliable packet transmissions in a certain communication range. The protocol layer introduces a new overlapping zone concept to study both about HARQ feedback in a communication range and the resource segregation.

This paper considers the multi-user multiple-input and multiple-output (MU-MIMO) downlink system where each user is equipped with multiple receive antennas. Due to the inevitable channel imperfection, the system performance may degrade significantly. A robust signal-to-leakage-and-noise ratio (SLNR)-based beamforming is developed to provide robustness against channel uncertainties, which maximizes the expectation of the signal power and maintains a low probability of the severe leakage. Two types of errors are taken into account, that is, the estimation Gaussian error in time division duplexing (TDD) system and the quantization error in frequency division duplexing (FDD) system. In order to solve the underlying problem efficiently, the probabilistic constraint is transformed into a convex one by using Markov's inequality, which places an upper bound of the robust scheme based on channel statis-tics. Furthermore, the proposed design is applicable to general error distributions, rather than restricted to Gaussian errors or quantization errors. Simulation results show that the proposed approach with probabilistic constraint can achieve high SLNR performance and prevent a pessimistic result by considering the leakage power proportionally.

Multiple transmit antennas can be employed in a downlink channel to serve multiple users under the assumption of perfect channel state information (CSI) of all users at the base station. In practice, however, the channel knowledge available at the base station is not perfect. For instance, in case of a time division duplex (TDD) system, the transmit CSI is the estimated uplink channel. On the other hand, in a frequency division duplex (FDD) system, feedback from the users is required to obtain transmit CSI. In this case, we assume the transmit CSI consists of a quantized version of the normalized estimated channel vector and the statistics of the channel norm of each user. In this work we consider the transceiver design of the downlink of a multiuser MISO system based on the minimum mean square error (MMSE) criterion with imperfect CSI as discussed above.

This paper proposes a Layer 1 (L1) control channel (CC) multiplexing scheme using in-band full duplex (FD) with synchronous orthogonal sequence assignment in the downlink and uplink to shorten the round trip time (RTT) due to hybrid automatic repeat request (HARQ). We describe the proposed L1 CC multiplexing scheme employing in-band FD associated with a cyclic-shifted Zadoff-Chu sequence and block spreading for synchronous orthogonal sequence assignments. We show the supported coverage area where the L1 CC multiplexing using in-band FD is available for various subcarrier spacings (SCSs). Finally, we show that the proposed L1 CC multiplexing scheme using in-band FD reduces the RTT due to HARQ compared to that for half duplex (HD) time division duplex (TDD) based CC multiplexing for various SCSs and slot assignments of data channels in HD TDD.

Multi-user multiple-input multiple-output (MU-MIMO) system operating in frequency division duplex (FDD) mode has an advantage over that operating in time division duplex (TDD) mode in achieving a lower return time latency as both the uplink and downlink data streaming can occur simultaneously. However, unlike TDD, the reciprocity of the uplink and downlink channels does not hold and hence the system typically needs to rely on channel feedback which reduces the uplink channel capacity. Reducing the channel feedback while maintaining the channel estimation accuracy is a critical factor for a successful implementation of MU-MIMO FDD. In addition, when combined with orthogonal frequency division multiple access (OFDMA), channel feedback needs to occur independently for different sub-channels assigned to different user terminals. This paper proposes the use of discrete cosine transformation (DCT) based channel compression for the purpose of MU-MIMO-OFDMA channel feedback and shows its advantage over conventional discrete Fourier transform (DFT) based method. Compared to ideal channel feedback without channel estimation or compression errors, the proposed method achieves within 1 dB loss in uncoded bit error performance for a MU-MIMO-OFDMA system using 32 antennas at the central access point supporting 16 user terminals each with a single antenna.

Coexistence research for LTE systems is indispensable to ensure that the time division duplex (TDD) Long Term evolution (LTE) system can coexist with the frequency division duplex (FDD) LTE system operating at the adjacent frequency band within the same geographical area. Previous studies on this topic only focus on the throughput loss of the victim system from the data channel viewpoint. In this paper, the performance of control channels (CCHs) in the typical coexistence scenario is investigated, since the performance of communication systems is largely dependent on the quality of CCHs. The CCHs' interferences are analyzed in detail, including the coexistence issue for CCHs, Co-Channel Interference (CCI) analysis, and Adjacent Channel Interference (ACI) on CCHs. The simulation results indicate that the ACI generated by the Base Station (BS) of collocated LTE system degrades the performance of the uplink (UL) CCHs of the victim LTE system significantly.

Cell-free system where a group of base stations (BSs) cooperatively serves users has received much attention as a promising technology for the future wireless systems. In order to maximize the cooperation gain in the cell-free systems, acquisition of downlink channel state information (CSI) at the BSs is crucial. While this task is relatively easy for the time division duplexing (TDD) systems due to the channel reciprocity, it is not easy for the frequency division duplexing (FDD) systems due to the CSI feedback overhead. This issue is even more pronounced in the cell-free systems since the user needs to feed back the CSIs of multiple BSs. In this paper, we propose a novel feedback reduction technique for the FDD-based cell-free systems. Key feature of the proposed technique is to choose a few dominating paths and then feed back the path gain information (PGI) of the chosen paths. By exploiting the property that the angles of departure (AoDs) are quite similar in the uplink and downlink channels (this property is referred to as angle reciprocity), the BSs obtain the AoDs directly from the uplink pilot signal. From the extensive simulations, we observe that the proposed technique can achieve more than 60% reduction in feedback overhead over the conventional CSI feedback scheme.

In this paper, a comparison study of various UMTS data transmission techniques is performed. An analysis of the system in the presence of geographically non-uniform multi-service traffic profiles is done for three transmission techniques: Frequency Division Duplex (FDD), Time Division Duplex (TDD), and High Speed Downlink Packet Access (HSDPA). Several combinations were selected: FDD, TDD, FDD combined with HSDPA, FDD combined with TDD, and FDD combined with TDD and HSDPA. The TDD mode is the technique with better results, but the introduction of HSDPA is clearly beneficial for data services, as expected. The distribution of the services by the three techniques is the best scenario, namely for some traffic profiles. For example, for a threshold of 128 kbps (for HSDPA services) and 50 % of the power reserved to HSDPA, it is possible to get 1.4 GB of carried traffic.

Long Term Evolution (LTE) Networks are being planned to be upgraded to LTE Advanced along with increasing availability of Release 10 User Equipment (UEs) and Spectrum Availability for Carrier Aggregation. LTE Advanced brings a major step increase in Down Link and Uplink Throughputs and related spectral efficiencies. This paper explains the evolution of Frequency Division Duplex (FDD) throughputs for different Channel Bandwidths. The maximum theoretical physical layer throughputs and expected actual application layer throughputs after removing the necessary overheads by different broadcast, signaling, synchronization & random access channels have been explained with an impact analysis thereof. Throughput gains for upcoming LTE Advanced deployments have been computed for Frame Structure Type 1 (FDD) for Channel Bandwidth of 1.4 MHz, 3MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz along with Carrier Aggregation of 2 & 5 Component Carriers. Impact of increasing Signaling Channels, retransmissions & code rate on throughputs has been explained so that the computations can be extended to any deployment scenario.

Multi-User MIMO (MU-MIMO) has attracted much attention due to its significant advantage of increasing the utilization ratio of wireless channels. However, Frequency-Division Duplex (FDD) systems are vulnerable to eavesdropping, since the explicit CSI feedback can be manipulated. In this paper, we show that Time-Division Duplex (TDD) systems are insecure as well. In particular, we show that it is possible to eavesdrop on other users' downloads by tuning training sequences. In order to defend MU-MIMO against such threats, we propose a secure CSI estimation scheme, which can provide correct estimates of CSI when adversarial users are in presence. We prove that our scheme is secure against training sequence based eavesdropping attack. We have implemented our scheme for TDD MU-MIMO systems and performed a series of experiments. Results demonstrate that our secure CSI estimation scheme is highly effective in protecting TDD MIMO networks against eavesdropping attack. Furthermore, we extend our scheme to support massive MU-MIMO networks, with a carefully redesigned uplink protocol and optimized power allocation to achieve higher spectral efficiency. To be more practical, we also take mismatch channel issue into our consideration. An enhancement scheme is proposed and we show that our scheme with enhancement is secure and correct under mismatch channel.

Reconfigurable intelligent surface (RIS) has recently emerged as a promising technology for wireless communications, which intelligently controls the phase shift of each unit cell to form desired beams. Most prior works on RIS focus on a single frequency band, and thus for time-division duplexing (TDD) systems, the same phase shifts can be applied to both uplink and downlink. However, for the frequency-division duplexing (FDD) mode, if the same phase shifts are applied in both uplink and downlink, the directions of the uplink RIS beams will not be aligned with those of the downlink, which will in turn cause performance degradation. To address this issue, in this paper, we investigate the practical RIS design and optimization for FDD systems. By representing the reflection coefficients of RIS with the equivalent circuit model which includes the resistor, inductor and tunable capacitor, we first provide the guidelines on the circuit design to realize 2π phase control over the two frequency bands of the FDD system. In addition, we propose a low-resolution RIS configuration scheme with two tunable modes corresponding to two capacitances, and we formulate a max-min signal-to-noise ratio (SNR) problem to maximize the minimum SNR of uplink and downlink. To solve the non-convex problem, we propose an alternating optimization algorithm to obtain a suboptimal solution. Simulation results show that our proposed RIS design outperforms those benchmarks which design the circuits by only optimizing uplink or downlink.

Massive MIMO is a powerful multiuser/multiantenna technology that exploits a very large number of antennas at the base station side and the knowledge of the channel matrix between base station antennas and multiple users in order to achieve large beamforming and multiplexing gain. Classical massive MIMO exploits Time-Division Duplexing (TDD) and channel reciprocity, such that the channel matrix can be learned at the base station from the incoming uplink pilot signals sent by the users. However, the large majority of cellular networks deployed today make use of Frequency Division Duplexing (FDD) where channel reciprocity does not hold and explicit downlink probing and uplink CSI feedback are required in order to achieve some spatial multiplexing gain. Unfortunately, the overhead incurred by explicit probing and feedback is very large in massive MIMO, since the channels are high-dimensional random vectors. In this paper, we present a new approach to achieve very competitive tradeoff between spatial multiplexing gain and probing/feedback overhead in FDD massive MIMO. Our approach is based on two novel concepts: 1) an efficient and mathematically rigorous technique to extrapolate the channel covariance matrix from the uplink to the downlink, such that the second order statistics of each downlink channel can be accurately learned for free from uplink pilots; 2) a novel “sparsifying precoding” approach, that introduces sparsity in the channel in a controlled form, such that for any assigned overhead (i.e., downlink pilot dimension) it is possible to set an optimal sparsity level for which the “effective” channels after sparsification can be estimated at the base station with low mean-square error. We compare our method with that of the state-of-the-art compressed sensing (CS) based method. Our results show that the proposed method is much more robust than compressed sensing methods, since it is able to “shape the channel sparsity” as desired, instead of being at the mercy of nature (i.e., at the mercy of the natural sparsity induced by the nronaaation environment).