Motivated by International Mobile Telecommunications (IMT)-2030, sixth generation (6G) mobile networks in 3GPP (third-generation partner project) commenced with a workshop in March 2025, attracting more than 200 contributions and 700 in-person attendances. While the details of 6G study and the eventual 6G specifications in 3GPP are yet to be developed, there is strong motivation in 3GPP to focus on the fundamental values that 6G may bring, most notably in terms of improving user experience and the overall network operation, particularly with respect to reducing the total cost of ownership (TCO). It is evident that there is a strong need to streamline and simplify the 6G specifications for efficient standardization, implementation, and commercial deployments. For the services also accommodated by the fifth generation (5G) networks, 6G is expected to provide meaningful enhancements. That is, instead of simply pushing for even higher envelopes such as peak data rates, more emphasis will be on improved coverage (especially at the cell edge and for certain data rates or services), and energy efficiency (for both end devices and networks) using the existing and new spectrum. Moreover, 6G is expected to support new services, with artificial intelligence (AI) and sensing as two primary examples. In this article, we provide a tutorial on the key technologies driving fundamental enhancements for the standardization of 6G and beyond, covering the necessary co-existence between 5G and 6G for smooth migration, AI-native radio access network (RAN), multiple-input-multiple-out (MIMO), sustainable operation with increased energy efficiency, coverage enhancements involving both terrestrial and non-terrestrial networks (NTN), integrated sensing and communication (ISAC), diverse device types including the low-end back-scattering based ambient internet of things (IoT) devices, and native support of NTN. We conclude this article by pointing out several key challenges and opportunities towards standardization of 6G and beyond.

Since the start of 5G New Radio (NR) work in the 3rd Generation Partnership Project (3GPP) in early 2016, tremendous progress has been made in both standardization and commercial deployments. The first 5G NR release (Release 15) laid out a solid foundation in accommodating a diverse set of services, a wide range of spectra, and a variety of deployment scenarios, while being forward compatible. Expansion to vertical domain services [e.g., vehicle to everything (V2X), non-terrestrial networks (NTN)] was introduced in Release 16. Such an expansion was further accelerated in Release 17, with the standardization work being completed despite the extreme challenges due to COVID-19.

Since the start of 5G work in 3GPP in early 2016, tremendous progress has been made in both standardization and commercial deployments. 3GPP is now entering the second phase of 5G standardization, known as5G-Advanced, built on the 5G baseline in 3GPP Releases 15, 16, and 17. 3GPP Release 18, the start of 5G-Advanced, includes a diverse set of features that cover both device and network evolutions, providing balanced mobile broadband evolution and further vertical domain expansion and accommodating both immediate and long-term commercial needs. 5G-Advanced will significantly expand 5G capabilities, address many new use cases, transform connectivity experiences, and serve as an essential step in developing mobile communications towards 6G. This paper provides a comprehensive overview of the 3GPP 5G-Advanced development, introducing the prominent state-of-the-art technologies investigated in 3GPP and identifying key evolution directions for future research and standardization.

The fifth generation (5G) new radio (NR) of mobile communications is designed to support two major class of services with vastly heterogeneous requirements: ultra-reliable low-latency communication (URLLC) and enhanced mobile broadband (eMBB). To enable the coexistence of URLLC and eMBB in the same radio spectrum in a cost-effective manner while guaranteeing the latency and reliability performance of URLLC, 5G NR has developed an innovative preemption and superposition framework which allows a dynamic multiplexing of the two services in an interleaved manner. In this tutorial paper, we review the key technological innovations in this framework in NR Release 15 and 16, namely, preemption indication and enhanced power control, and explain the design principles underlying these technologies. System-level and link-level simulations are provided to illustrate the performance benefits of these designs.

Tactile internet is defined as a network that offers a response to a physical process or an object in perceived real time. The development of new radio (NR) and long-term evolution (LTE) technologies with tailored high-reliability and low-latency design is expected to reliably transmit data in milliseconds, leading to the promising realization of tactile internet with applications in areas such as factory automation, education, gaming, and healthcare. In this paper, from a physical layer and medium-access control (PHY/MAC) perspective, we discuss the systems design of ultrareliable and low-latency communications (URLLC) in NR and LTE technologies, both belonging to the fifth-generation (5G) wireless technologies. Motivated by the safety and privacy requirements of tactile internet, we also outline the 5G security landscape, major categories of attackers, and potential countermeasures. Finally, we provide a case study of factory automation as an example of the proposed 5G-based tactile internet.

This paper studies the physical-layer benefits of low latency operation in long-term evolution (LTE) networks. Latency reduction can be achieved by reducing the transmission time interval (TTI) from 1ms to the duration of only a few orthogonal frequency-division multiplexing symbols. The TTI shortening potentially enables faster link adaptation, thereby enhancing system performance. However, enabling low latency operation in a backward compatible manner requires a careful design and performance characterization. This paper conducts a link-level performance analysis of a low latency LTE network in both downlink and uplink with different transmission schemes and under various operating regimes. Our results reveal that a low latency LTE network can provide reasonable link-level performance improvements as compared to a legacy LTE network.

In this paper we quantify 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 8 downlink system performance for a macro cell hexagonal grid scenario. The system performance is analyzed for a closed loop Single User Multi-Input Multi Output (SU-MIMO) mode and compared with Single Input Multiple Output (SIMO) and Multi-User (MU) MIMO modes, for a full buffer scenario and static users. In addition to the full buffer scenario, traffic models are considered for SIMO mode to evaluate impact of partial loading and handover. Voice over Internet Protocol (VoIP) system performance is quantified for static users. Mobility simulations are performed for Video Telephony (VT) users and compared to the static case.

This paper focuses on energy-efficient packet transmission with individual packet delay constraints. The solution presented herein is a generalization of Uysal-Biyikoglu (2002), which considered energy-efficient transmissions for a group of M packets subject to a single transmission deadline. First, the optimal offline scheduler (vis-À-vis total transmission energy) for packet transmissions with individual packet delay constraints is developed. It is shown that when packet inter-arrival times are independent and identically distributed (i.i.d.), the optimal transmission durations of packet $m$ and packet M-m+1, m ∈ [1,...,M, M ≥ 1, are identically distributed. This symmetry property leads to a simple and exact solution of the average packet delay for any i.i.d. inter-arrival times under the optimal offline scheduling. In addition, the packet delay performance for the single transmission deadline model is analyzed and shown to grow monotonically with $M$ and at a rate proportional to √M. A heuristic online scheduler, which assumes no future arrival information, is also studied and shown to achieve a comparable energy performance to the optimal offline scheduler in a wide range of scenarios. The flexible energy and delay tradeoff provided by the individual delay constraint model is further illustrated via simulations.

This paper focuses on energy-efficient packet transmission with individual packet delay constraints. The optimal offline scheduler (vis-a-vis total transmission energy), assuming information of all packet arrivals before scheduling, was developed by Zafer, et al. (2005) and Chen et al. (2006). This paper shows that when packet inter-arrival times are identically and independently distributed (Ltd.), the resulting optimal transmission durations of packets m and M - m +1, m epsiv [1, .. ., M], M ges 1, are identically distributed. This symmetry property leads to a simple and exact solution of the average packet delay under the optimal offline schedule. Two heuristic online scheduling algorithms, which assume no future arrival information, are then studied. These online schedulers are compared with the optimal offline scheduler in terms of delay and energy performance via analysis and simulations. While both online schedulers are inherently inferior, one online scheduler is shown to achieve a comparable energy performance to the optimal offline scheduler in a wide range of scenarios.

This paper focuses on delay-constrained energy-efficient packet transmission over a static multihop link. Optimal offline scheduling (vis-à-vis total transmission energy), assuming information of all packet arrivals before scheduling, is derived. The optimal offline schedule relies on a simple delay budget allocation scheme, which allocates the delay budget to the first hop (from source to the first relaying node) as much as possible. All the relaying nodes simply perform buffer-clearing during any transmission opportunities. The total transmission energy and average packet delay are analyzed and characterized. It is demonstrated that energy savings via multihopping are possible, but depend heavily on factors such as multihop resource orthogonalization mode, delay constraints, and SNR operating regimes.

Abstract — This paper focuses on energy-efficient packet transmission with individual packet delay constraints. The optimal offline scheduler (vis-à-vis total transmission energy), assuming information of all packet arrivals before scheduling, was developed by Zafer, et al. (2005) and Chen et al. (2006). This paper shows that when packet inter-arrival times are identically and independently distributed (i.i.d.), the resulting optimal transmission durations of packets m and M − m + 1, m ∈ [1, · · · , M], M ≥ 1, are identically distributed. This symmetry property leads to a simple and exact solution of the average packet delay under the optimal offline schedule. Two heuristic online scheduling algorithms, which assume no future arrival information, are then studied. These online schedulers are compared with the optimal offline scheduler in terms of delay and energy performance via analysis and simulations. While both online schedulers are inherently inferior, one online scheduler is shown to achieve a comparable energy performance to the optimal offline scheduler in a wide range of scenarios. I.

This paper investigates proactive packet dropping to achieve transmission energy savings. Such a scheme can be employed for applications which can tolerate a small fraction of packet losses. For a group of packets subject to a single transmission deadline, the optimal dropping scheme (vis-` a-vis total transmission energy) is derived. For packets subject to individual delay constraints, the optimal scheme depends on the energy function and packet sizes. Thus, asymptotically optimal dropping schemes, i. e., when packet size grows large, are pursued. The asymptotically optimal dropping scheme for a single dropped packet is obtained. For dropping more than one packet, two suboptimal, recursive schemes are proposed. These schemes achieve performance very close to the asymptotically optimal schemes as determined by an exhaustive search. Additionally, two performance bounds are derived. It is observed via simulations that significant energy savings are possible via intelligent packet dropping schemes.

This paper considers the application of distributed transmit diversity (TD) to digital broadcast networks. Distributed TD allows spatial encoding across transmit sectors (cell-sites or satellites) each with a single antenna. We focus on Alamouti space-frequency (SF) codes in particular and consider terrestrial systems based on cellular type networks. System coverage results are presented for a 1xEV-DO broadcast network based on a comprehensive evaluation methodology developed in 3GPP2 for system modeling and orthogonal frequency division modulation (OFDM). The results show that distributed transmit diversity can deliver significant coverage gains. For example, given a bandwidth of 1.25 MHz, a data rate of 1.2 Mbps and a minimum bit-error rate (BER) requirement of 0.1%, the broadcast coverage increases from roughly 40% in the absence of distributed TD to 70% with the Alamouti linear combiner and to 82% with a zero-forcing (ZF) spatial equalizer

An improved correlation matrix estimation scheme for blind adaptive MMSE receivers for DS-CDMA communications was proposed by Chen and Mitra (see IEEE Journal on Selected Areas on Communications, vol.19, p.1531-43, 2001). Blind adaptive MMSE receivers with the estimation scheme not only maintain the merit of robustness against fading, but achieve performance comparable to the training-sequence based adaptive MMSE receivers. This paper presents an enhanced version of the improved correlation matrix estimation scheme for blind adaptive MMSE detection, which results in a better convergence and tracking property and noticeably improved performance than the original scheme of Chen and Mitra. Theoretical analysis is performed for the flat fading case which predicts the advantages of the new scheme. Detailed computer simulations are also carried out to verify the performance of blind adaptive MMSE receivers with the new estimation scheme.

This correspondence shows the equivalence of three previously proposed reduced-rank detection schemes for direct-sequence code-division multiple-access (DS-CDMA) communication systems. The auxiliary vector filtering (AVF) algorithm is simplified through a key observation on the construction of the auxiliary vectors. After simplification, it is shown that the AVF algorithm is equivalent to the multistage Wiener filtering (MWF) algorithm of Honig and Goldstein (2002). Furthermore, these schemes can be shown to be equivalent to the multistage linear receiver scheme based on the Cayley-Hamilton (CH) theorem when the minimum mean-square error (MMSE) criterion is applied to the reduced dimensional space of the received signal.

In this paper, an improved correlation matrix estimation scheme for blind adaptive MMSE receivers is provided. The new scheme takes advantage of the fact that the desired linear receiver can be expressed as a function of the interference correlation matrix only, rather than the total data correlation matrix. A theoretical analysis is performed for the flat fading case which predicts that the new estimation scheme will result in significant performance improvement. Blind adaptive MMSE receivers with the new estimation scheme appear to achieve performance comparable to the training-sequence based adaptive MMSE receivers. Detailed computer simulations for the fast multipath fading environment verify that the proposed scheme yields strong performance gain over previous methods.

Blind adaptive minimum mean-squared errors (MMSE) receivers for multiuser direct-sequence code-division multiple access (DS-CDMA) systems that assume knowledge of the steering vector, i.e., the cross-correlation between the desired output and the input signal, are known for their robustness against channel fading as they do not attempt to explicitly track the channel of the user of interest. However, these receivers often have higher excess mean squared error and, hence, poorer performance than training-sequence based adaptive MMSE receivers. In this paper, an improved correlation matrix estimation scheme for blind adaptive MMSE receivers is provided. The new scheme takes advantage of the fact that the desired linear receiver can be expressed as a function of the interference correlation matrix only, rather than the total data correlation matrix. A theoretical analysis is performed for the flat fading case which predicts that the new estimation scheme will result in significant performance improvement. Blind adaptive MMSE receivers with the new estimation scheme appear to achieve performance comparable to the training-sequence based adaptive MMSE receivers. Detailed computer simulations for the fast multipath fading environment verify that the proposed scheme yields strong performance gains over previous methods.

Two previously proposed training sequence optimization techniques for channel estimation are compared. One method is based on a frequency domain based channel estimation method (FD) and the other is based on a time domain channel estimation technique (TD). The FD method produces a lower complexity search strategy but does not always result in the optimal training sequences in terms of the mean-squared channel estimation error. A proof of the superiority of the TD method over the FD method is presented in this paper. Based on the proof an alternative search criterion is proposed, which generally performs better than the FD method while still enjoying the low search complexity.

Two previously proposed training sequence optimization techniques for channel estimation are compared. One method is based on a frequency-domain (FD) based channel estimation method and the other is based on a time-domain (TD) channel estimation technique. The FD method produces a lower complexity search strategy, but does not always result in the optimal training sequences in terms of the mean-squared channel estimation error. A proof of the superiority of the TD method over the FD method is presented. Based on the proof, an alternative search criterion is proposed, which, in general, provides equivalent or better performance than the FD method while still enjoying the low search complexity.