11 research outputs found
SOME PROPERTIES OF M-SEQUENCES OVER FINITE FIELD F p
ABSTRACT M-Sequences over F p , when p is odd prime, are compatible with binary M-Sequences by orthogonal property but their even period gives them some other properties that are not exist in binary M-Sequences. This research shows or clarifies some of these properties that: the set of cyclic permutations of elements one non zero period is not closed under the addition, the matrix of these permutations is symmetric for the second diagonal, the sum of any two rows, one of them is translated by index equal to half of the periodis equal to zero sequences, the repetitions of the non-zero elements in one period are equal and sum of the squares of all entries in any row or any column by mod p is equal to zero
Quasi-orthogonal sequences for code-division multiple-access systems
In this paper the notion of quasi-orthogonal sequence (QOS as a means of increasing the number of channels in synchronous code-division multiple-access (CDMA) systems that employ Walsh sequences for spreading information signals and separating channels is introduced. It is shown that a QOS sequence may be regarded as a class of bent (almost bent) functions possessing, in addition, a certain window property. Such sequences while increasing system capacity, minimize interference to the existing set of Walsh sequences. The window property gives the system the ability to handle variable data rates, A general procedure of constructing QOS's from,well-known families of binary sequences with good correlation, including the Kasami and Gold sequence families, as well as from the binary Kerdock code is provided, Examples of QOS's are presented for small lengths. Some examples of quaternary QOS's drawn from Family A are also included.X1172sciescopu
Secure OFDM System Design for Wireless Communications
Wireless communications is widely employed in modern society and plays an increasingly important role in people\u27s daily life. The broadcast nature of radio propagation, however, causes wireless communications particularly vulnerable to malicious attacks, and leads to critical challenges in securing the wireless transmission. Motivated by the insufficiency of traditional approaches to secure wireless communications, physical layer security that is emerging as a complement to the traditional upper-layer security mechanisms is investigated in this dissertation. Five novel techniques toward the physical layer security of wireless communications are proposed. The first two techniques focus on the security risk assessment in wireless networks to enable a situation-awareness based transmission protection. The third and fourth techniques utilize wireless medium characteristics to enhance the built-in security of wireless communication systems, so as to prevent passive eavesdropping. The last technique provides an embedded confidential signaling link for secure transmitter-receiver interaction in OFDM systems
Efficient Radio Resource Allocation Schemes and Code Optimizations for High Speed Downlink Packet Access Transmission
An important enhancement on the Wideband Code Division Multiple Access
(WCDMA) air interface of the 3G mobile communications, High Speed Downlink
Packet Access (HSDPA) standard has been launched to realize higher spectral
utilization efficiency. It introduces the features of multicode CDMA transmission
and Adaptive Modulation and Coding (AMC) technique, which makes radio resource
allocation feasible and essential. This thesis studies channel-aware resource
allocation schemes, coupled with fast power adjustment and spreading code optimization
techniques, for the HSDPA standard operating over frequency selective
channel.
A two-group resource allocation scheme is developed in order to achieve a
promising balance between performance enhancement and time efficiency. It only
requires calculating two parameters to specify the allocations of discrete bit rates
and transmitted symbol energies in all channels. The thesis develops the calculation
methods of the two parameters for interference-free and interference-present
channels, respectively. For the interference-present channels, the performance of
two-group allocation can be further enhanced by applying a clustering-based channel
removal scheme.
In order to make the two-group approach more time-efficient, reduction in
matrix inversions in optimum energy calculation is then discussed. When the
Minimum Mean Square Error (MMSE) equalizer is applied, optimum energy allocation
can be calculated by iterating a set of eigenvalues and eigenvectors. By
using the MMSE Successive Interference Cancellation (SIC) receiver, the optimum
energies are calculated recursively combined with an optimum channel ordering
scheme for enhancement in both system performance and time efficiency.
This thesis then studies the signature optimization methods with multipath
channel and examines their system performances when combined with different
resource allocation methods. Two multipath-aware signature optimization methods
are developed by applying iterative optimization techniques, for the system
using MMSE equalizer and MMSE precoder respectively. A PAM system using
complex signature sequences is also examined for improving resource utilization
efficiency, where two receiving schemes are proposed to fully take advantage of
PAM features. In addition by applying a short chip sampling window, a Singular
Value Decomposition (SVD) based interference-free signature design method is
presented
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
Detecção multiutilizador do domĂnio da frequĂȘncia para sistemas DS-CDMA
Doutoramento em Engenharia ElectrotĂ©cnicaOs Sinais de Espalhamento de Espectro de SequĂȘncia Directa exibem propriedades cicloestacionĂĄrias que implicam redundĂąncia entre componentes de frequĂȘncia espaçadas por mĂșltiplos da taxa de sĂmbolo.
Nesta tese, Ă© apresentado um cancelador de interferĂȘncia multiutilizador (Cancelador por translação na frequĂȘncia - FSC) que tira partido desta propriedade. Este cancelador linear opera no domĂnio da frequĂȘncia no sinal espalhado de tal forma que minimiza a interferĂȘncia e ruĂdo na saĂda (CritĂ©rio do MĂnimo Erro Quadrado MĂ©dio). AlĂ©m de testado para o caso de antena Ășnica, sĂŁo avaliadas as performances das configuraçÔes de antenas mĂșltiplas para o caso de beamforming e canais espacialmente descorrelacionados considerando sistemas sĂncronos e sistemas com
desalinhamento no tempo dos perfis de canais (ambos UMTS-TDD). Essas configuraçÔes divergiam na ordem da combinação temporal, combinação espacial e detecção multiutilizador. As configuraçÔes FSC foram avaliadas quando concatenadas com o PIC-2D. Os resultados das simulaçÔes mostram consideråveis melhorias nos resultados relativamente ao RAKE-2D
convencional e PIC-2D. Foi atingida performance prĂłximo ao RAKE de utilizador Ășnico quando o FSC foi avaliado concatenado com PIC-2D em quase todas as configuraçÔes. Todas as configuraçÔes foram avaliadas com modulação QPSK, 8-PSK e 16-QAM. Foi introduzida codificação Turbo e identificou-se as situaçÔes da vantagem de utilização do FSC antes do PIC-2D. As modulaçÔes 8-PSK e 16-QAM foram igualmente testadas com codificação.Direct Sequence Spread Spectrum signals exhibit cyclostationary properties which imply redundancy between frequency components separated by multiples of the symbol rate. In this thesis a Multiple Access Interference
Canceller (Frequency Shift Canceller - FSC) that explores this property is presented. The linear frequency domain canceller operates on the spreaded signal so as to minimize the interference and noise at the output (Minimum
Mean Squared Error Criterium). Moreover the FSC was tested with single antenna, the performance of multisensor configurations for the cases of beamforming and uncorrelated spatial channels was evaluated considering both synchronous and time misalignment systems (both UMTS-TDD). Those configurations diverge in temporal combining, spatial combining and multiuser detection order. The FSC configurations were concatenated with PIC-2D structure and evaluated. The simulation results show considerable improvement relative to the conventional RAKE-2D and PIC-2D receiver. A performance close to the single user RAKE case was a achieved when it (FSC) was evaluated jointly with PIC-2D. All the configurations with modulation QPSK, 8-PSK and 16-QAM were evaluated. Turbo Codes were introduced and it was identified the situations which it is advantageous to use the FSC before the PIC-2D. The 8-PSK and 16-QAM modulations were equally tested with coding