638 research outputs found
A Tight Bound for Probability of Error for Quantum Counting Based Multiuser Detection
Future wired and wireless communication systems will employ pure or combined
Code Division Multiple Access (CDMA) technique, such as in the European 3G
mobile UMTS or Power Line Telecommunication system, but also several 4G
proposal includes e.g. multi carrier (MC) CDMA. Former examinations carried out
the drawbacks of single user detectors (SUD), which are widely employed in
narrowband IS-95 CDMA systems, and forced to develop suitable multiuser
detection schemes to increase the efficiency against interference. However, at
this moment there are only suboptimal solutions available because of the rather
high complexity of optimal detectors. One of the possible receiver technologies
can be the quantum assisted computing devices which allows high level
parallelism in computation. The first commercial devices are estimated for the
next years, which meets the advert of 3G and 4G systems. In this paper we
analyze the error probability and give tight bounds in a static and dynamically
changing environment for a novel quantum computation based Quantum Multiuser
detection (QMUD) algorithm, employing quantum counting algorithm, which
provides optimal solution.Comment: presented at IEEE ISIT 2002, 7 pages, 2 figure
On multi-user EXIT chart analysis aided turbo-detected MBER beamforming designs
AbstractâThis paper studies the mutual information transfer characteristics of a novel iterative soft interference cancellation (SIC) aided beamforming receiver communicating over both additive white Gaussian noise (AWGN) and multipath slow fading channels. Based on the extrinsic information transfer (EXIT) chart technique, we investigate the convergence behavior of an iterative minimum bit error rate (MBER) multiuser detection (MUD) scheme as a function of both the system parameters and channel conditions in comparison to the SIC aided minimum mean square error (SIC-MMSE) MUD. Our simulation results show that the EXIT chart analysis is sufficiently accurate for the MBER MUD. Quantitatively, a two-antenna system was capable of supporting up to K=6 users at Eb/N0=3dB, even when their angular separation was relatively low, potentially below 20?. Index TermsâMinimum bit error rate, beamforming, multiuser detection, soft interference cancellation, iterative processing, EXIT chart
Space time transceiver design over multipath fading channels
Imperial Users onl
Antenna array calibration in wireless communications
Imperial Users onl
Scaling up MIMO: Opportunities and Challenges with Very Large Arrays
This paper surveys recent advances in the area of very large MIMO systems.
With very large MIMO, we think of systems that use antenna arrays with an
order of magnitude more elements than in systems being built today, say a
hundred antennas or more. Very large MIMO entails an unprecedented number of
antennas simultaneously serving a much smaller number of terminals. The
disparity in number emerges as a desirable operating condition and a practical
one as well. The number of terminals that can be simultaneously served is
limited, not by the number of antennas, but rather by our inability to acquire
channel-state information for an unlimited number of terminals. Larger numbers
of terminals can always be accommodated by combining very large MIMO technology
with conventional time- and frequency-division multiplexing via OFDM. Very
large MIMO arrays is a new research field both in communication theory,
propagation, and electronics and represents a paradigm shift in the way of
thinking both with regards to theory, systems and implementation. The ultimate
vision of very large MIMO systems is that the antenna array would consist of
small active antenna units, plugged into an (optical) fieldbus.Comment: Accepted for publication in the IEEE Signal Processing Magazine,
October 201
High Capacity CDMA and Collaborative Techniques
The thesis investigates new approaches to increase the user capacity and improve the error
performance of Code Division Multiple Access (CDMA) by employing adaptive interference cancellation
and collaborative spreading and space diversity techniques. Collaborative Coding Multiple
Access (CCMA) is also investigated as a separate technique and combined with CDMA. The
advantages and shortcomings of CDMA and CCMA are analysed and new techniques for both the
uplink and downlink are proposed and evaluated.
Multiple access interference (MAI) problem in the uplink of CDMA is investigated first. The
practical issues of multiuser detection (MUD) techniques are reviewed and a novel blind adaptive
approach to interference cancellation (IC) is proposed. It exploits the constant modulus (CM)
property of digital signals to blindly suppress interference during the despreading process and obtain
amplitude estimation with minimum mean squared error for use in cancellation stages. Two
new blind adaptive receiver designs employing successive and parallel interference cancellation
architectures using the CM algorithm (CMA) referred to as âCMA-SICâ and âBA-PICâ, respectively,
are presented. These techniques have shown to offer near single user performance for large
number of users. It is shown to increase the user capacity by approximately two fold compared
with conventional IC receivers. The spectral efficiency analysis of the techniques based on output
signal-to interference-and-noise ratio (SINR) also shows significant gain in data rate. Furthermore,
an effective and low complexity blind adaptive subcarrier combining (BASC) technique using a
simple gradient descent based algorithm is proposed for Multicarrier-CDMA. It suppresses MAI
without any knowledge of channel amplitudes and allows large number of users compared with
equal gain and maximum ratio combining techniques normally used in practice.
New user collaborative schemes are proposed and analysed theoretically and by simulations
in different channel conditions to achieve spatial diversity for uplink of CCMA and CDMA. First,
a simple transmitter diversity and its equivalent user collaborative diversity techniques for CCMA
are designed and analysed. Next, a new user collaborative scheme with successive interference
cancellation for uplink of CDMA referred to as collaborative SIC (C-SIC) is investigated to reduce
MAI and achieve improved diversity. To further improve the performance of C-SIC under high
system loading conditions, Collaborative Blind Adaptive SIC (C-BASIC) scheme is proposed.
It is shown to minimize the residual MAI, leading to improved user capacity and a more robust
system. It is known that collaborative diversity schemes incur loss in throughput due to the need of
orthogonal time/frequency slots for relaying sourceâs data. To address this problem, finally a novel
near-unity-rate scheme also referred to as bandwidth efficient collaborative diversity (BECD) is proposed and evaluated for CDMA. Under this scheme, pairs of users share a single spreading sequence to exchange and forward their data employing a simple superposition or space-time
encoding methods. At the receiver collaborative joint detection is performed to separate each
paired usersâ data. It is shown that the scheme can achieve full diversity gain at no extra bandwidth
as inter-user channel SNR becomes high.
A novel approach of âUser Collaborationâ is introduced to increase the user capacity of CDMA
for both the downlink and uplink. First, collaborative group spreading technique for the downlink
of overloaded CDMA system is introduced. It allows the sharing of the same single spreading
sequence for more than one user belonging to the same group. This technique is referred to as
Collaborative Spreading CDMA downlink (CS-CDMA-DL). In this technique T-user collaborative
coding is used for each group to form a composite codeword signal of the users and then a
single orthogonal sequence is used for the group. At each userâs receiver, decoding of composite
codeword is carried out to extract the userâs own information while maintaining a high SINR performance.
To improve the bit error performance of CS-CDMA-DL in Rayleigh fading conditions,
Collaborative Space-time Spreading (C-STS) technique is proposed by combining the collaborative
coding multiple access and space-time coding principles. A new scheme for uplink of CDMA
using the âUser Collaborationâ approach, referred to as CS-CDMA-UL is presented next. When
usersâ channels are independent (uncorrelated), significantly higher user capacity can be achieved
by grouping multiple users to share the same spreading sequence and performing MUD on per
group basis followed by a low complexity ML decoding at the receiver. This approach has shown
to support much higher number of users than the available sequences while also maintaining the
low receiver complexity. For improved performance under highly correlated channel conditions,
T-user collaborative coding is also investigated within the CS-CDMA-UL system
Near-capacity MIMOs using iterative detection
In this thesis, Multiple-Input Multiple-Output (MIMO) techniques designed for transmission over narrowband Rayleigh fading channels are investigated. Specifically, in order to providea diversity gain while eliminating the complexity of MIMO channel estimation, a Differential Space-Time Spreading (DSTS) scheme is designed that employs non-coherent detection. Additionally, in order to maximise the coding advantage of DSTS, it is combined with Sphere Packing (SP) modulation. The related capacity analysis shows that the DSTS-SP scheme exhibits a higher capacity than its counterpart dispensing with SP. Furthermore, in order to attain additional performance gains, the DSTS system invokes iterative detection, where the outer code is constituted by a Recursive Systematic Convolutional (RSC) code, while the inner code is a SP demapper in one of the prototype systems investigated, while the other scheme employs a Unity Rate Code (URC) as its inner code in order to eliminate the error floor exhibited by the system dispensing with URC. EXIT charts are used to analyse the convergence behaviour of the iteratively detected schemes and a novel technique is proposed for computing the maximum achievable rate of the system based on EXIT charts. Explicitly, the four-antenna-aided DSTSSP system employing no URC precoding attains a coding gain of 12 dB at a BER of 10-5 and performs within 1.82 dB from the maximum achievable rate limit. By contrast, the URC aidedprecoded system operates within 0.92 dB from the same limit.On the other hand, in order to maximise the DSTS systemâs throughput, an adaptive DSTSSP scheme is proposed that exploits the advantages of differential encoding, iterative decoding as well as SP modulation. The achievable integrity and bit rate enhancements of the system are determined by the following factors: the specific MIMO configuration used for transmitting data from the four antennas, the spreading factor used and the RSC encoderâs code rate.Additionally, multi-functional MIMO techniques are designed to provide diversity gains, multiplexing gains and beamforming gains by combining the benefits of space-time codes, VBLASTand beamforming. First, a system employing Nt=4 transmit Antenna Arrays (AA) with LAA number of elements per AA and Nr=4 receive antennas is proposed, which is referred to as a Layered Steered Space-Time Code (LSSTC). Three iteratively detected near-capacity LSSTC-SP receiver structures are proposed, which differ in the number of inner iterations employed between the inner decoder and the SP demapper as well as in the choice of the outer code, which is either an RSC code or an Irregular Convolutional Code (IrCC). The three systems are capable of operating within 0.9, 0.4 and 0.6 dB from the maximum achievable rate limit of the system. A comparison between the three iteratively-detected schemes reveals that a carefully designed two-stage iterative detection scheme is capable of operating sufficiently close to capacity at a lower complexity, when compared to a three-stage system employing a RSC or a two-stage system using an IrCC as an outer code. On the other hand, in order to allow the LSSTC scheme to employ less receive antennas than transmit antennas, while still accommodating multiple users, a Layered Steered Space-Time Spreading (LSSTS) scheme is proposed that combines the benefits of space-time spreading, V-BLAST, beamforming and generalised MC DS-CDMA. Furthermore, iteratively detected LSSTS schemes are presented and an LLR post-processing technique is proposed in order to improve the attainable performance of the iteratively detected LSSTS system.Finally, a distributed turbo coding scheme is proposed that combines the benefits of turbo coding and cooperative communication, where iterative detection is employed by exchanging extrinsic information between the decoders of different single-antenna-aided users. Specifically, the effect of the errors induced in the first phase of cooperation, where the two users exchange their data, on the performance of the uplink in studied, while considering different fading channel characteristics
Concept of Cryptographic Operations Based on Code Division Multiple Access
It is a given that numerous users connect to a single satellite transponder on a daily basis in order to communicate while discussing any satellite-based technology. As a result, they must all share the resources that are available without compromising the privacy of one another. Thus, the multiple access strategy is employed to achieve this. This paper focuses on code division multiple access, which does not require time slots or frequency slots to be shared across numerous users. The primary source of communication problems is multipath fading; and if the signal undergoes any multipath fading, then the total signal may be distorted. This work presents improved correlation features of the current Walsh code through one simple yet powerful algorithm. Here, a simulation-based method is used to evaluate performance. Utilizing power delay profiles in several mobile radio propagation channels, measurement-based channel models for indoor, outdoor, suburban, and urban environments are derived. The number of taps and tap gains are then estimated using statistics on the path loss characteristics. Since the source, output is known it is compared with a delayed version of the decision device output to obtain an empirical basis for the error rate. The suggested codeâs performance is then compared to a few existing orthogonal and semi- orthogonal codes using a variety of performance criteria, and the conclusion is that this proposal is superior
Design and implementation of a downlink MC-CDMA receiver
Cette thĂšse prĂ©sente une Ă©tude d'un systĂšme complet de transmission en liaison descendante utilisant la technologie multi-porteuse avec l'accĂšs multiple par division de code (Multi-Carrier Code Division Multiple Access, MC-CDMA). L'Ă©tude inclut la synchronisation et l'estimation du canal pour un systĂšme MC-CDMA en liaison descendante ainsi que l'implĂ©mentation sur puce FPGA d'un rĂ©cepteur MC-CDMA en liaison descendante en bande de base. Le MC-CDMA est une combinaison de la technique de multiplexage par frĂ©quence orthogonale (Orthogonal Frequency Division Multiplexing, OFDM) et de l'accĂšs multiple par rĂ©partition de code (CDMA), et ce dans le but d'intĂ©grer les deux technologies. Le systĂšme MC-CDMA est conçu pour fonctionner Ă l'intĂ©rieur de la contrainte d'une bande de frĂ©quence de 5 MHz pour les modĂšles de canaux intĂ©rieur/extĂ©rieur pĂ©destre et vĂ©hiculaire tel que dĂ©crit par le "Third Genaration Partnership Project" (3GPP). La composante OFDM du systĂšme MC-CDMA a Ă©tĂ© simulĂ©e en utilisant le logiciel MATLAB dans le but d'obtenir des paramĂštres de base. Des codes orthogonaux Ă facteur d'Ă©talement variable (OVSF) de longueur 8 ont Ă©tĂ© choisis comme codes d'Ă©talement pour notre systĂšme MC-CDMA. Ceci permet de supporter des taux de transmission maximum jusquĂ 20.6 Mbps et 22.875 Mbps (donnĂ©es non codĂ©es, pleine charge de 8 utilisateurs) pour les canaux intĂ©rieur/extĂ©rieur pĂ©destre et vĂ©hiculaire, respectivement. Une Ă©tude analytique des expressions de taux d'erreur binaire pour le MC-CDMA dans un canal multivoies de Rayleigh a Ă©tĂ© rĂ©alisĂ©e dans le but d'Ă©valuer rapidement et de façon prĂ©cise les performances. Des techniques d'estimation de canal basĂ©es sur les dĂ©cisions antĂ©rieures ont Ă©tĂ© Ă©tudiĂ©es afin d'amĂ©liorer encore plus les performances de taux d'erreur binaire du systĂšme MC-CDMA en liaison descendante. L'estimateur de canal basĂ© sur les dĂ©cisions antĂ©rieures et utilisant le critĂšre de l'erreur quadratique minimale linĂ©aire avec une matrice' de corrĂ©lation du canal de taille 64 x 64 a Ă©tĂ© choisi comme Ă©tant un bon compromis entre la performance et la complexitĂ© pour une implementation sur puce FPGA. Une nouvelle sĂ©quence d'apprentissage a Ă©tĂ© conçue pour le rĂ©cepteur dans la configuration intĂ©rieur/extĂ©rieur pĂ©destre dans le but d'estimer de façon grossiĂšre le temps de synchronisation et le dĂ©calage frĂ©quentiel fractionnaire de la porteuse dans le domaine du temps. Les estimations fines du temps de synchronisation et du dĂ©calage frĂ©quentiel de la porteuse ont Ă©tĂ© effectuĂ©s dans le domaine des frĂ©quences Ă l'aide de sous-porteuses pilotes. Un rĂ©cepteur en liaison descendante MC-CDMA complet pour le canal intĂ©rieur /extĂ©rieur pĂ©destre avec les synchronisations en temps et en frĂ©quence en boucle fermĂ©e a Ă©tĂ© simulĂ© avant de procĂ©der Ă l'implĂ©mentation matĂ©rielle. Le rĂ©cepteur en liaison descendante en bande de base pour le canal intĂ©rieur/extĂ©rieur pĂ©destre a Ă©tĂ© implĂ©mentĂ© sur un systĂšme de dĂ©veloppement fabriquĂ© par la compagnie Nallatech et utilisant le circuit XtremeDSP de Xilinx. Un transmetteur compatible avec le systĂšme de rĂ©ception a Ă©galement Ă©tĂ© rĂ©alisĂ©. Des tests fonctionnels du rĂ©cepteur ont Ă©tĂ© effectuĂ©s dans un environnement sans fil statique de laboratoire. Un environnement de test plus dynamique, incluant la mobilitĂ© du transmetteur, du rĂ©cepteur ou des Ă©lĂ©ments dispersifs, aurait Ă©tĂ© souhaitable, mais n'a pu ĂȘtre rĂ©alisĂ© Ă©tant donnĂ© les difficultĂ©s logistiques inhĂ©rentes. Les taux d'erreur binaire mesurĂ©s avec diffĂ©rents nombres d'usagers actifs et diffĂ©rentes modulations sont proches des simulations sur ordinateurs pour un canal avec bruit blanc gaussien additif
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