574 research outputs found
Collaborative modulation multiple access for single hop and multihop networks
While the bandwidth available for wireless networks is limited, the world has seen an unprecedented growth in the number of mobile subscribers and an ever increasing demand for high data rates. Therefore efficient utilisation of bandwidth to maximise link spectral efficiency and number of users that can be served simultaneously are primary goals in the design of wireless systems. To achieve these goals, in this thesis, a new non-orthogonal uplink multiple access scheme which combines the functionalities of adaptive modulation and multiple access called collaborative modulation multiple access (CMMA) is proposed. CMMA enables multiple users to access the network simultaneously and share the same bandwidth even when only a single receive antenna is available and in the presence of high channel correlation.
Instead of competing for resources, users in CMMA share resources collaboratively by employing unique modulation sets (UMS) that differ in phase, power, and/or mapping structure. These UMS are designed to insure that the received signal formed from the superposition of all usersâ signals belongs to a composite QAM constellation (CC) with a rate equal to the sum rate of all users. The CC and its constituent UMSs are designed centrally at the BS to remove ambiguity, maximize the minimum Euclidian distance (dmin) of the CC and insure a minimum BER performance is maintained. Users collaboratively precode their transmitted signal by performing truncated channel inversion and phase rotation using channel state information (CSI ) obtained from a periodic common pilot to insure that their combined signal at the BS belongs to the CC known at the BS which in turn performs a simple joint maximum likelihood detection without the need for CSI. The coherent addition of usersâ power enables CMMA to achieve high link spectral efficiency at any time without extra power or bandwidth but on the expense of graceful degradation in BER performance.
To improve the BER performance of CMMA while preserving its precoding and detection structure and without the need for pilot-aided channel estimation, a new selective diversity combining scheme called SC-CMMA is proposed. SC-CMMA optimises the overall group performance providing fairness and diversity gain for various users with different transmit powers and channel conditions by selecting a single antenna out of a group of L available antennas that minimises the total transmit power required for precoding at any one time.
A detailed study of capacity and BER performance of CMMA and SC-CMMA is carried out under different level of channel correlations which shows that both offer high capacity gain and resilience to channel correlation. SC-CMMA capacity even increase with high channel correlation between usersâ channels.
CMMA provides a practical solution for implementing the multiple access adder channel (MAAC) in fading environments hence a hybrid approach combining both collaborative coding and modulation referred to as H-CMMA is investigated. H-CMMA divides users into a number of subgroups where users within a subgroup are assigned the same modulation set and different multiple access codes. H-CMMA adjusts the dmin of the received CC by varying the number of subgroups which in turn varies the number of unique constellation points for the same number of users and average total power. Therefore H-CMMA can accommodate many users with different rates while flexibly managing the complexity, rate and BER performance depending on the SNR.
Next a new scheme combining CMMA with opportunistic scheduling using only partial CSI at the receiver called CMMA-OS is proposed to combine both the power gain of CMMA and the multiuser diversity gain that arises from usersâ channel independence. To avoid the complexity and excessive feedback associated with the dynamic update of the CC, the BS takes into account the independence of usersâ channels in the design of the CC and its constituent UMSs but both remain unchanged thereafter. However UMS are no longer associated with users, instead channel gainâs probability density function is divided into regions with identical probability and each UMS is associated with a specific region. This will simplify scheduling as users can initially chose their UMS based on their CSI and the BS will only need to resolve any collision when the channels of two or more users are located at the same region.
Finally a high rate cooperative communication scheme, called cooperative modulation (CM) is proposed for cooperative multiuser systems. CM combines the reliability of the cooperative diversity with the high spectral efficiency and multiple access capabilities of CMMA. CM maintains low feedback and high spectral efficiency by restricting relaying to a single route with the best overall channel. Two possible variations of CM are proposed depending on whether CSI available only at the users or just at the BS and the selected relay. The first is referred to Precode, Amplify, and Forward (PAF) while the second one is called Decode, Remap, and Forward (DMF). A new route selection algorithm for DMF based on maximising dmin of random CC is also proposed using a novel fast low-complexity multi-stage sphere based algorithm to calculate the dmin at the relay of random CC that is used for both relay selection and detection
Vehicle Communication using Secrecy Capacity
We address secure vehicle communication using secrecy capacity. In
particular, we research the relationship between secrecy capacity and various
types of parameters that determine secrecy capacity in the vehicular wireless
network. For example, we examine the relationship between vehicle speed and
secrecy capacity, the relationship between the response time and secrecy
capacity of an autonomous vehicle, and the relationship between transmission
power and secrecy capacity. In particular, the autonomous vehicle has set the
system modeling on the assumption that the speed of the vehicle is related to
the safety distance. We propose new vehicle communication to maintain a certain
level of secrecy capacity according to various parameters. As a result, we can
expect safer communication security of autonomous vehicles in 5G
communications.Comment: 17 Pages, 12 Figure
Principles of Physical Layer Security in Multiuser Wireless Networks: A Survey
This paper provides a comprehensive review of the domain of physical layer
security in multiuser wireless networks. The essential premise of
physical-layer security is to enable the exchange of confidential messages over
a wireless medium in the presence of unauthorized eavesdroppers without relying
on higher-layer encryption. This can be achieved primarily in two ways: without
the need for a secret key by intelligently designing transmit coding
strategies, or by exploiting the wireless communication medium to develop
secret keys over public channels. The survey begins with an overview of the
foundations dating back to the pioneering work of Shannon and Wyner on
information-theoretic security. We then describe the evolution of secure
transmission strategies from point-to-point channels to multiple-antenna
systems, followed by generalizations to multiuser broadcast, multiple-access,
interference, and relay networks. Secret-key generation and establishment
protocols based on physical layer mechanisms are subsequently covered.
Approaches for secrecy based on channel coding design are then examined, along
with a description of inter-disciplinary approaches based on game theory and
stochastic geometry. The associated problem of physical-layer message
authentication is also introduced briefly. The survey concludes with
observations on potential research directions in this area.Comment: 23 pages, 10 figures, 303 refs. arXiv admin note: text overlap with
arXiv:1303.1609 by other authors. IEEE Communications Surveys and Tutorials,
201
Reconfigurable Intelligent Surface-Empowered MIMO Systems
Reconfigurable intelligent surface (RIS)-assisted communications appear as a
promising candidate for future wireless systems due to its attractive
advantages in terms of implementation cost and end-to-end system performance.
In this paper, two new multiple-input multiple-output (MIMO) system designs
using RISs are presented to enhance the performance and boost the spectral
efficiency of state-of-the-art MIMO communication systems. Vertical Bell Labs
layered space-time (VBLAST) and Alamouti's schemes have been considered in this
study and RIS-based simple transceiver architectures are proposed. For the
VBLAST-based new system, an RIS is used to enhance the performance of the
nulling and canceling-based sub-optimal detection procedure as well as to
noticeably boost the spectral efficiency by conveying extra bits through the
adjustment of the phases of the RIS elements. In addition, RIS elements have
been utilized in order to redesign Alamouti's scheme with a single radio
frequency (RF) signal generator at the transmitter side and to enhance its bit
error rate (BER) performance. Monte Carlo simulations are provided to show the
effectiveness of our system designs and it has been shown that they outperform
the reference schemes in terms of BER performance and spectral efficiency.Comment: To appear in IEEE SYSTEMS JOURNAL, 9 pages, 6 figures, and 1 tabl
Downlink Training in Cell-Free Massive MIMO: A Blessing in Disguise
Cell-free Massive MIMO (multiple-input multiple-output) refers to a
distributed Massive MIMO system where all the access points (APs) cooperate to
coherently serve all the user equipments (UEs), suppress inter-cell
interference and mitigate the multiuser interference. Recent works demonstrated
that, unlike co-located Massive MIMO, the \textit{channel hardening} is, in
general, less pronounced in cell-free Massive MIMO, thus there is much to
benefit from estimating the downlink channel. In this study, we investigate the
gain introduced by the downlink beamforming training, extending the previously
proposed analysis to non-orthogonal uplink and downlink pilots. Assuming
single-antenna APs, conjugate beamforming and independent Rayleigh fading
channel, we derive a closed-form expression for the per-user achievable
downlink rate that addresses channel estimation errors and pilot contamination
both at the AP and UE side. The performance evaluation includes max-min
fairness power control, greedy pilot assignment methods, and a comparison
between achievable rates obtained from different capacity-bounding techniques.
Numerical results show that downlink beamforming training, although increases
pilot overhead and introduces additional pilot contamination, improves
significantly the achievable downlink rate. Even for large number of APs, it is
not fully efficient for the UE relying on the statistical channel state
information for data decoding.Comment: Published in IEEE Transactions on Wireless Communications on August
14, 2019. {\copyright} 2019 IEEE. Personal use of this material is permitted.
Permission from IEEE must be obtained for all other use
Joint signal detection and channel estimation in rank-deficient MIMO systems
L'Ă©volution de la prospĂšre famille des standards 802.11 a encouragĂ© le dĂ©veloppement des technologies appliquĂ©es aux rĂ©seaux locaux sans fil (WLANs). Pour faire face Ă la toujours croissante nĂ©cessitĂ© de rendre possible les communications Ă trĂšs haut dĂ©bit, les systĂšmes Ă antennes multiples (MIMO) sont une solution viable. Ils ont l'avantage d'accroĂźtre le dĂ©bit de transmission sans avoir recours Ă plus de puissance ou de largeur de bande. Cependant, l'industrie hĂ©site encore Ă augmenter le nombre d'antennes des portables et des accĂ©soires sans fil. De plus, Ă l'intĂ©rieur des bĂątiments, la dĂ©ficience de rang de la matrice de canal peut se produire dĂ» Ă la nature de la dispersion des parcours de propagation, ce phĂ©nomĂšne est aussi occasionnĂ© Ă l'extĂ©rieur par de longues distances de transmission. Ce projet est motivĂ© par les raisons dĂ©crites antĂ©rieurement, il se veut un Ă©tude sur la viabilitĂ© des transcepteurs sans fil Ă large bande capables de rĂ©gulariser la dĂ©ficience de rang du canal sans fil. On vise le dĂ©veloppement des techniques capables de sĂ©parer M signaux co-canal, mĂȘme avec une seule antenne et Ă faire une estimation prĂ©cise du canal. Les solutions dĂ©crites dans ce document cherchent Ă surmonter les difficultĂ©s posĂ©es par le medium aux transcepteurs sans fil Ă large bande. Le rĂ©sultat de cette Ă©tude est un algorithme transcepteur appropriĂ© aux systĂšmes MIMO Ă rang dĂ©ficient
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