130 research outputs found
A Survey of Physical Layer Security Techniques for 5G Wireless Networks and Challenges Ahead
Physical layer security which safeguards data confidentiality based on the
information-theoretic approaches has received significant research interest
recently. The key idea behind physical layer security is to utilize the
intrinsic randomness of the transmission channel to guarantee the security in
physical layer. The evolution towards 5G wireless communications poses new
challenges for physical layer security research. This paper provides a latest
survey of the physical layer security research on various promising 5G
technologies, including physical layer security coding, massive multiple-input
multiple-output, millimeter wave communications, heterogeneous networks,
non-orthogonal multiple access, full duplex technology, etc. Technical
challenges which remain unresolved at the time of writing are summarized and
the future trends of physical layer security in 5G and beyond are discussed.Comment: To appear in IEEE Journal on Selected Areas in Communication
INTERFERENCE MANAGEMENT IN LTE SYSTEM AND BEYOUND
The key challenges to high throughput in cellular wireless communication system are interference, mobility and bandwidth limitation. Mobility has never been a problem until recently, bandwidth has been constantly improved upon through the evolutions in cellular wireless communication system but interference has been a constant limitation to any improvement that may have resulted from such evolution. The fundamental challenge to a system designer or a researcher is how to achieve high data rate in motion (high speed) in a cellular system that is intrinsically interference-limited.
Multi-antenna is the solution to data on the move and the capacity of multi-antenna system has been demonstrated to increase proportionally with increase in the number of antennas at both transmitter and receiver for point-to-point communications and multi-user environment. However, the capacity gain in both uplink and downlink is limited in a multi-user environment like cellular system by interference, the number of antennas at the base station, complexity and space constraint particularly for a mobile terminal.
This challenge in the downlink provided the motivation to investigate successive interference cancellation (SIC) as an interference management tool LTE system and beyond. The Simulation revealed that ordered successive interference (OSIC) out performs non-ordered successive interference cancellation (NSIC) and the additional complexity is justified based on the associated gain in BER performance of OSIC. The major drawback of OSIC is that it is not efficient in network environment employing power control or power allocation. Additional interference management techniques will be required to fully manage the interference.fi=Opinnäytetyö kokotekstinä PDF-muodossa.|en=Thesis fulltext in PDF format.|sv=Lärdomsprov tillgängligt som fulltext i PDF-format
Relay assisted device-to-device communication with channel uncertainty
The gains of direct communication between user equipment in a network may not be fully realised due to the separation between the user equipment and due to the fading that the channel between these user equipment experiences. In order to fully realise the gains that direct (device-to-device) communication promises, idle user equipment can be exploited to serve as relays to enforce device-to-device communication. The availability of potential relay user equipment creates a problem: a way to select the relay user equipment. Moreover, unlike infrastructure relays, user equipment are carried around by people and these users are self-interested. Thus the problem of relay selection goes beyond choosing which device to assist in relayed communication but catering for user self-interest. Another problem in wireless communication is the unavailability of perfect channel state information. This reality creates uncertainty in the channel and so in designing selection algorithms, channel uncertainty awareness needs to be a consideration. Therefore the work in this thesis considers the design of relay user equipment selection algorithms that are not only device centric but that are relay user equipment centric. Furthermore, the designed algorithms are channel uncertainty aware. Firstly, a stable matching based relay user equipment selection algorithm is put forward for underlay device-to-device communication. A channel uncertainty aware approach is proposed to cater to imperfect channel state information at the devices. The algorithm is combined with a rate based mode selection algorithm. Next, to cater to the queue state at the relay user equipment, a cross-layer selection algorithm is proposed for a twoway decode and forward relay set up. The algorithm proposed employs deterministic uncertainty constraint in the interference channel, solving the selection algorithm in a heuristic fashion. Then a cluster head selection algorithm is proposed for device-to-device group communication constrained by channel uncertainty in the interference channel. The formulated rate maximization problem is solved for deterministic and probabilistic constraint scenarios, and the problem extended to a multiple-input single-out scenario for which robust beamforming was designed. Finally, relay utility and social distance based selection algorithms are proposed for full duplex decode and forward device-to-device communication set up. A worst-case approach is proposed for a full channel uncertainty scenario. The results from computer simulations indicate that the proposed algorithms offer spectral efficiency, fairness and energy efficiency gains. The results also showed clearly the deterioration in the performance of networks when perfect channel state information is assumed
Interference management techniques in large-scale wireless networks
In this thesis, advanced interference management techniques are designed and evaluated for large-scale
wireless networks with realistic assumptions, such as signal propagation loss, random node
distribution and non-instantaneous channel state information at the transmitter (CSIT).
In the first part of the thesis, the Maddah-Ali and Tse (MAT) scheme for the 2-user and 2-antenna
base station (BS) broadcast channel (BC) is generalised and optimised using the probabilistic-constrained
optimisation approach. With consideration of the unknown channel entries, the
proposed optimisation approach guarantees a high probability that the interference leakage power
is below a certain threshold in the presence of minimum interference leakage receivers. The
desired signal detectability is maximised at the same time and the closed-form solution for the
receiving matrices is provided. Afterwards, the proposed optimisation approach is extended to the
3-user BC with 2-antenna BS. Simulation results show substantial sum rate gain over the MAT
scheme, especially with a large spatial correlation at the receiver side.
In the second part, the MAT scheme is extended to the time-correlated channels in three scenarios,
in which degrees of freedom (DoF) regions as well as achievability schemes are studied: 1) 2-user
interference channel (IC) using imperfect current and imperfect delayed CSIT; 2) K-user BC with
K-antenna BS using imperfect current and perfect delayed CSIT; 3) 3-user BC with 2-antenna
BS using imperfect current and perfect delayed CSIT. Notably, the consistency of the proposed
DoF regions with the MAT scheme and the ZF beamforming schemes using perfect current CSIT
consents to the optimality of the proposed achievability schemes.
In the third part, the performance of the ZF receiver is evaluated in Poisson distributed wireless
networks. Simple static networks as well as dynamic networks are studied. For the static network,
transmission capacity is derived whereby the receiver can eliminate interference from nearby
transmitters. It is shown that more spatial receive degrees of freedom (SRDoF) should be allocated
to decode the desired symbol in the presence of low transmitter intensity. For the dynamic network,
in which the data traffic is modelled by queueing theory, interference alignment (IA) beamforming
is considered and implemented sequentially. Interestingly, transmitting one data stream achieves
the highest area spectrum efficiency.
Finally, a distance-dependent IA beamforming scheme is designed for a generic 2-tier
heterogeneous wireless network. Second-tier transmitters partially align their interferences to
the dominant cross-tier interference overheard by the receivers in the same cluster. Essentially,
the proposed IA scheme compromises between enhancing the signal-to-interference ratio and
increasing the multiplexing gain. It is shown that acquiring accurate distance knowledge brings
insignificant throughput gain compared to statistical distance knowledge. Simulation results
validate the derived expressions of success probabilities as well as throughput, and show that the
distance-dependent IA scheme significantly outperforms the traditional IA scheme in the presence
of path-loss effect
Interference Management in 5G Reverse TDD HetNets with Wireless Backhaul: A Large System Analysis
This work analyzes a heterogeneous network (HetNet), which comprises a macro
base station (BS) equipped with a large number of antennas and an overlaid
dense tier of small cell access points (SCAs) using a wireless backhaul for
data traffic. The static and low mobility user equipment terminals (UEs) are
associated with the SCAs while those with medium-to-high mobility are served by
the macro BS. A reverse time division duplexing (TDD) protocol is used by the
two tiers, which allows the BS to locally estimate both the intra-tier and
inter-tier channels. This knowledge is then used at the BS either in the uplink
(UL) or in the downlink (DL) to simultaneously serve the macro UEs (MUEs) and
to provide the wireless backhaul to SCAs. A geographical separation of
co-channel SCAs is proposed to limit the interference coming from the UL
signals of MUEs. A concatenated linear precoding technique employing either
zero-forcing (ZF) or regularized ZF is used at the BS to simultaneously serve
MUEs and SCAs in DL while nulling interference toward those SCAs in UL. We
evaluate and characterize the performance of the system through the power
consumption of UL and DL transmissions under the assumption that target rates
must be satisfied and imperfect channel state information is available for
MUEs. The analysis is conducted in the asymptotic regime where the number of BS
antennas and the network size (MUEs and SCAs) grow large with fixed ratios.
Results from large system analysis are used to provide concise formulae for the
asymptotic UL and DL transmit powers and precoding vectors under the above
assumptions. Numerical results are used to validate the analysis in different
settings and to make comparisons with alternative network architectures.Comment: 14 pages, 12 figures. To appear IEEE J. Select. Areas Commun. --
Special Issue on HetNet
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