Dr. Constantinos B. Papadias
The American College of Greece, Greece
Dr. Tharm Ratnarajah
The University of Edinburgh, UK
Dr. Dirk T.M. Slock
Wireless spectrum is a scarce commodity in today’s connected and data-hungry world, where demand for higher data rates is increasing exponentially on a daily basis. Beyond 5G cellular systems are venturing more and more into unlicensed spectrum, leading to increased coexistence of a multitude of wireless systems. To tackle this coexistence, efficient spectrum utilization techniques, such as spectrum sharing (SS) and full-duplex (FD) transmission have been considered. While SS can substantially increase the spectrum utilization efficiency by allowing licensed and unlicensed users to share the spectrum, FD radios have the potential to double the spectrum efficiency of current half-duplex links by transmitting and receiving at the same time and frequency resources. Furthermore, they allow simultaneous transmission and sensing, opening up avenues for new random-access schemes. The objective of this tutorial is to provide an overview of the following ingredients: 1) Key SS approaches (from cognitive radio to eLSA, CBRS, unlicensed access in 3GPP, etc.); 2) Enabling SS techniques (spectrum sensing, cooperative communications, antenna arrays, resource allocation, etc.); 3) New trends (FD, radar-communications, SS in mmWave, machine learning-based spectrum monitoring, etc.). This tutorial is based on (but goes beyond) our recent edited book: Spectrum Sharing: The Next Frontier in Wireless Networks.
Structure and content
This tutorial will attempt to showcase all the critical aspects of the spectrum sharing paradigm. Starting with a description of the spectrum sharing landscape as it looks today and a historical yet detailed review of the key technologies and their evolution in time, reaching up to mmWave frequencies, it then moves towards the most recent SS paradigms, such as LSA, eLSA, CBRS, and promising technology enablers, such as collaborative sensing and cooperative communication.
By offering the so-called spatial dimension, antenna arrays are an important enabler of spectrum sharing for all types of wireless systems. We review the basic attributes of antenna arrays that allows them to reuse efficiently the spectrum, handle the interference environment and even aid spectrum policy enforcement. Emphasis is placed on antenna array-based spectrum sharing that is applicable to the technologies in current wireless standards (such as the use of MIMO and CoMP in 4Gand massive MIMO in 5G) or have the potential of impacting spectrum sharing networks in the near future. We then go on to present, as promising approaches in this direction,a novel technique for antenna-array-aided spectrum sharing, based on coordinated linear precoding, as well as a new concept forspectrum sensing that relies on low complexity (parasitic) antenna arrays, for which over-the-air results are also presented.
The effectiveness of several spectrum sharing techniques, especially those used for horizontal sharing (i.e. between users of the same type, in the same band, at the same time), relies heavily on the accurate and timely knowledge of the involved user and interference channels. These techniques are often based on beamforming by multiple antennas, to separate users by exploiting the spatial dimension. The requirements for Channel State Knowledge at the Transmitter (CSIT) become crucial in mMIMO (massive Multiple Input Multiple Output) which is a key ingredient of 5G and is also well suited for LSA as it offers higher spatial resolution. However, mMIMO is hard to implement in FDD (frequency division duplex) systems, due to the complications imposed by the feedback channel. An alternative approach is to consider TDD (time division duplex) systems, where, in theory at least, the forward and reverse channels are equal, hence not requiring a feedback channel. We discuss the actual case in which reciprocity is only obtained after calibration of the radio frequency (RF) parts in the transmitter and receiver chains. An overview of the state of the art in reciprocity calibration techniques is first provided, with an emphasis on internal calibration usable in mMIMO. Then a number of promising reciprocity-based techniques are presented for the design of transmit precoders for spectrum sharing between incumbents and licensees. In particular, the concept of naive uplink / downlink duality is presented, which allows to reduce further the additional information exchange required for utility optimization or to deal with non-cooperative nodes.
Next we discuss the advantages of using full duplex (FD) in spectrum sharing technologies, such as cognitive radio (CR)and LSA. By transmitting in FD mode, a CR can simultaneously transmit and sense the transmission status of other nodes, which makes it suitable to combat numerous issues at the medium access control layer, such as hidden terminals, large delays, and congestion. Starting by introducing themotivation for using FD in CRs from the perspective of both the cellular systems (CS) and internet of things (IoT), we provide an overview of the design of FD transceivers and accordingly analyze the fundamental requirements for the co-implementation of the two technologies and the corresponding benefits obtained over transmission through traditional half duplex. Wethen postulate the necessary mathematical framework, including the optimization problems associated with both CS and IoT. Next, detailed steps for the conversion of the problems into tractable form are illustrated along with efficient transceiver design algorithms. Finally, comprehensive numerical results are provided to justify the use of FD in CRs and open problems in the field of CRs transmitting in FD mode will bepresented.
We will go on to provide an overview of recent progress in the area of communication and radar spectrum sharing (CRSS), which not only presents advantages in enabling the efficient usage of the spectrum, but also provides a new way to design novel systems that can benefit from the cooperation of radar and communications –thus introducing a new spectrum sharing paradigm. Starting by introducing the motivation forCRSS from both the civilian and military perspectives, we discuss the applicable scenarios and analyze the fundamental requirements for sharing the spectrum between the two systems. We then provide the general definitions and mathematical models, and further introduce the associated key performance metrics for radar and communication systems. As a step further, an overview of the state of the art for CRSS is provided, from the coexistence for individual radar and communication devices, to the design of thedual-functional system that enables simultaneous communication and remote sensing. Finally, the discussion is concluded by reviewing the open problems in the research field of CRSS.
An outline of the tutorial, including a tentative time scheduleis shown below:
Part I Overview of Spectrum Sharing (45 mins)
Regulation and standardization activities
- LSA (EU version)
- Pushed by CEPT, ETSI, 3GPP
- Two-tier model: incumbents, licensees
- Spectrum sensing is country-wide
- Incumbent protection through database
- SAS (USversion)
- Pushed by FCC, 3GPP, WinnForum
- Three-tier model: incumbents, PAL, GAA
- Spectrum sensing in reduced areas (e.g., census tracks of 4000 people)
- Interference mitigation across census tracts
- Sensing-based protection of incumbents
- LTE/WiFi coexistence -LTE-U & LTE-A (3GPP)
Part II Enabling Techniques & Tools (105 mins)
- Collaborative spectrum sensing
- Interference management
- Cooperative communications
- Spatial domain -antenna array
- Reciprocity-based beamforming
- Spectrum sharing in mmWave communications
- Communications and Radar coexistence
- Sensing and transmission based on full-duplex radio
- Full-Duplex communications
- Resource allocation
Part III Perspectives (30 mins)
- Machine learning-based spectrum monitoring
- Horizontalspectrum sharing (i.e., between the same type of users)
- Policy enforcement
- Business models