As of today, the fifth generation (5G) mobile communication system has been rolled out in many countries and the number of 5G subscribers already reaches a very large scale. It is time for academia and industry to shift their attention towards the next generation. At this crossroad, an overview of the current state of the art and a vision of future communications are definitely of interest. This article thus aims to provide a comprehensive survey to draw a picture of the sixth generation (6G) system in terms of drivers, use cases, usage scenarios, requirements, key performance indicators (KPIs), architecture, and enabling technologies. First, we attempt to answer the question of "Is there any need for 6G?" by shedding light on its key driving factors, in which we predict the explosive growth of mobile traffic until 2030, and envision potential use cases and usage scenarios. Second, the technical requirements of 6G are discussed and compared with those of 5G with respect to a set of KPIs in a quantitative manner. Third, the state-of-the-art 6G research efforts and activities from representative institutions and countries are summarized, and a tentative roadmap of definition, specification, standardization, and regulation is projected. Then, we identify a dozen of potential technologies and introduce their principles, advantages, challenges, and open research issues. Finally, the conclusions are drawn to paint a picture of "What 6G may look like?". This survey is intended to serve as an enlightening guideline to spur interests and further investigations for subsequent research and development of 6G communications systems.

The Road Towards 6G: A Comprehensive Survey

Two of the major second-generation standards, GSM and TDMA/136, have built the foundation to offer a common global radio access for data services. Through use of a common physical layer, EDGE, both standards will have the same evolutionary path toward providing third-generation services. EDGE is currently subject to standardization in TIA TR45.3 and ETSI SMG, a process which will be finalized at the end of 1999. Compared to the existing data services in GSM and TDMA/136, EDGE will provide significantly higher user bit rates and spectral efficiency. EDGE can be introduced in these systems in a smooth way, using existing frequency plans of already deployed networks. This article gives the rationale behind the development of the EDGE concept, presents the EDGE technology, and addresses performance by means of system simulations.

EDGE: enhanced data rates for GSM and TDMA/136 evolution

This paper presents the wide-band code-division multiple-access (WCDMA) radio interface chosen by the ETSI as the basic radio-access technology for the universal mobile telecommunications system (UMTS). A detailed description of the physical layer of ETSI WCDMA is given together with an overview UMTS of the higher layers of the WCDMA radio interface. Finally, the WCDMA performance, based on results from the ETSI evaluation of UMTS radio interface candidates, is presented.

WCDMA-the radio interface for future mobile multimedia communications

A linear multiuser receiver for a particular user in a code-division multiple-access (CDMA) network gains potential benefits from knowledge of the channels of all users in the system. In fast multipath fading environments we cannot assume that the channel estimates are perfect and the inevitable channel estimation errors will limit this potential gain. We study the impact of channel estimation errors on the performance of linear multiuser receivers, as well as the channel estimation problem itself. Of particular interest are the scalability properties of the channel and data estimation algorithms: what happens to the performance as the system bandwidth and the number of users (and hence channels to estimate) grows? Our main results involve asymptotic expressions for the signal-to-interference ratio of linear multiuser receivers in the limit of large processing gain, with the number of users divided by the processing gain held constant. We employ a random model for the spreading sequences and the limiting signal-to-interference ratio expressions are independent of the actual signature sequences, depending only on the system loading and the channel statistics: background noise power, energy profile of resolvable multipaths, and channel coherence time. The effect of channel uncertainty on the performance of multiuser receivers is succinctly captured by the notion of effective interference.

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Large system performance of linear multiuser receivers in multipath fading channels

The design tradeoffs of turbo-coded burst-by-burst adaptive orthogonal frequency division multiplex (OFDM) wideband transceivers are analyzed. We demonstrate that upon aiming for a higher throughput a higher proportion of low-quality OFDM subcarriers has to be used for the transmission of inherently vulnerable high-order modem modes, transmitting several bits per subcarrier. Upon invoking turbo coding and adjusting the modem mode switching regime near-error-free performance can be achieved at the cost of a reduced throughput. Various blind modem mode detection techniques have also been investigated and the most complex channel coding trellis-based detection algorithm was found to be the most powerful. Last, the design tradeoffs of spectral pre-equalization have been explored and quantified. We concluded that AOFDM provides a convenient framework for adjusting the required target integrity and throughput both with and without turbo channel coding.

/pdf/adaptive-modulation-techniques-for-duplex-ofdm-transmission-35bmgsr1my.pdf

Adaptive modulation techniques for duplex OFDM transmission

The UMTS terrestrial radio access is based on wideband 4.096 Mchip/s DS-CDMA technology. UTRA will be connected to an evolved GSM core network for both circuit and packet services. A merger between ETSI/Europe and ARIB/Japan based on W-CDMA, a GSM core network, and a common frequency allocation according to the ITU Recommendation of 2 GHz makes a global IMT-2000 standard feasible. UTRA based on W-CDMA fully supports the UMTS/IMT-2000 requirements (e.g., support of 384 kb/s for wide-area coverage and 2 Mb/s for local coverage). Furthermore, the air interface has flexible support of mixed services, variable-rate services, and an efficient packet mode. Key W-CDMA features also include improved basic capacity/coverage performance compared to second-generation systems, full support of adaptive antenna arrays, support of hierarchical cell structures with interfrequency handover, and support of asynchronous inter-base-station operation. There have been no constraints due to the strong requirements for backward compatibility with second-generation systems. This has facilitated a high degree of flexibility and a future-proof air interface. Extensive evaluations by means of simulations and field trials have been carried out by a number of companies, and full system tests are ongoing. Consequently, W-CDMA technology can now be regarded as a mature technology, ready to provide the basis for UMTS/IMT-2000.

http://paginas.fe.up.pt/~mricardo/03_04/tsc/artigos/dahl.pdf

E. Dahlman

Papers

UMTS/IMT-2000 based on wideband CDMA