22 research outputs found
Low Correlation Sequences over the QAM Constellation
This paper presents the first concerted look at low correlation sequence
families over QAM constellations of size M^2=4^m and their potential
applicability as spreading sequences in a CDMA setting.
Five constructions are presented, and it is shown how such sequence families
have the ability to transport a larger amount of data as well as enable
variable-rate signalling on the reverse link.
Canonical family CQ has period N, normalized maximum-correlation parameter
theta_max bounded above by A sqrt(N), where 'A' ranges from 1.8 in the 16-QAM
case to 3.0 for large M. In a CDMA setting, each user is enabled to transfer 2m
bits of data per period of the spreading sequence which can be increased to 3m
bits of data by halving the size of the sequence family. The technique used to
construct CQ is easily extended to produce larger sequence families and an
example is provided.
Selected family SQ has a lower value of theta_max but permits only (m+1)-bit
data modulation. The interleaved 16-QAM sequence family IQ has theta_max <=
sqrt(2) sqrt(N) and supports 3-bit data modulation.
The remaining two families are over a quadrature-PAM (Q-PAM) subset of size
2M of the M^2-QAM constellation. Family P has a lower value of theta_max in
comparison with Family SQ, while still permitting (m+1)-bit data modulation.
Interleaved family IP, over the 8-ary Q-PAM constellation, permits 3-bit data
modulation and interestingly, achieves the Welch lower bound on theta_max.Comment: 21 pages, 3 figures. To appear in IEEE Transactions on Information
Theory in February 200
Proceedings of the Fall 1995 Advanced Digital Communication Systems
Coordinated Science Laboratory was formerly known as Control Systems Laborator
Nonlinear receivers for DS-CDMA
The growing demand for capacity in wireless communications is the driving force behind improving
established networks and the deployment of a new worldwide mobile standard. Capacity
calculations show that the direct sequence code division multiple access (DS-CDMA)
technique has more capacity than the time division multiple access technique. Therefore, most
3rd generation mobile systems will incorporate some sort of DS-CDMA.
In this thesis DS-CDMA receiver structures are investigated from the view point of pattern
recognition which leads to new DS-CDMA receiver structures. It is known that the optimum
DS-CDMA receiver has a nonlinear structure with prohibitive complexity for practical implementation.
It is also known that the currently implemented receiver in 2nd generation DSCDMA
mobile handsets has poor performance, because it suffers from multiuser interference.
Consequently, this work focuses on sub-optimum nonlinear receivers for DS-CDMA in the
downlink scenario.
First, the thesis reviews DS-CDMA, established equalisers, DS-CDMA receivers and pattern
recognition techniques. Then the new receivers are proposed. It is shown that DS-CDMA can
be considered as a pattern recognition problem and hence, pattern recognition techniques can be
exploited in order to develop DS-CDMA receivers. Another approach is to apply known equaliser
structures for DS-CDMA. One proposed receiver is based on the Volterra series expansion
and processes the received signal at the chip rate. Another receiver is a symbol rate radial
basis function network (RBFN) receiver with reduced complexity. Subsequently, a receiver is
proposed based on linear programming (LP) which is especially tailored for nonlinearly separable
scenarios. The LP based receiver performance is equivalent to the known decorrelating
detector in linearly separable scenarios. Finally, a hybrid receiver is proposed which combines
LP and RBFN and which exploits knowledge gained from pattern recognition. This structure
has lower complexity than the full RBF and good performance, and has a large potential for
further improvements.
Monte-Carlo simulations compare the proposed DS-CDMA receivers against established linear
and nonlinear receivers. It is shown that all proposed receivers outperform the known linear receivers.
The Volterra receiverâs complexity is relatively high for the performance gain achieved
and might not suit practical implementation. The other receiverâs complexity was greatly reduced
but it performs nearly as well as an optimum symbol by symbol detector.
This thesis shows that DS-CDMA is a pattern recognition problem and that pattern recognition
techniques can simplify DS-CDMA receiver structures. Knowledge is gained from the DSCDMA
signal patterns which help to understand the problem of a DS-CDMA receiver. It
should be noted that from the large number of known techniques, only a few pattern recognition
techniques are considered in this work, and any further work should look at other techniques.
Pattern recognition techniques can reduce the complexity of existing DS-CDMA receivers
while maintaining performance, leading to novel receiver structures
Iterative multiuser receivers for coded DS-CDMA systems
The introduction of cellular wireless systems in the 1980s has resulted in a continuous and
growing demand for personal communication services. This demand has made larger capacity
systems necessary. With the interest from both the research community and industry in wireless
code-division multiple-access (CDMA) systems, the application of multiuser detection (MUD)
techniques to wireless systems is becoming increasingly important. MUD is an important area
of interest to help obtain the significant increase in capacity needed for future wireless services.
The standardisation of direct-sequence CDMA (DS-CDMA) systems in the third generation of
mobile communication systems has raised even more interest in exploiting the capabilities and
capacity of this type of technology. However, the conventional DS-CDMA system has the major
problem of multiple-access interference (MAI). The MAI is unavoidable because receivers
deal with information which is transmitted not by a single source but by several uncoordinated
and geographically separated sources. As a result, the capacity of these systems is inherently interference
limited by other users. To overcome these limitations, MUD emerges as a promising
approach to increase the system capacity.
This thesis addresses the problem of improving the downlink capacity of a coded DS-CDMA
system with the use of MUD techniques at the mobile terminal receiver. The optimum multiuser
receiver scheme is far too computational intensive for practical use. Therefore, the aim of this
thesis is to investigate sub-optimal multiuser receiver schemes that can exploit the advantages
of MUD but also simplify its implementation. The attention is primarily focused on iterative
MUD receiver schemes which apply the turbo multiuser detection principle. Essentially this
principle consists of an iterative exchange of extrinsic information among the receiver modules
to achieve improved performance.
In this thesis, the implementation of an iterative receiver based on a linear MUD technique and
a cancellation scheme over an additive white Gaussian noise (AWGN) channel is first proposed
and analysed. The interference analysis shows that good performance is achieved using a lowcomplexity
receiver structure. In more realistic mobile channels, however, this type of receiver
suffers from the presence of higher levels of interference resulting in poor receiver performance.
The reason for this is that in such scenarios the desired signals are no longer linearly separable.
Therefore, non-linear detection techniques are required to provide better performance. With
this purpose, a hybrid iterative multiuser receiver is investigated for the case of a stationary
multipath channel. The incorporation of antenna arrays is an effective and practical technique to
provide a significant capacity gain over conventional single-antenna systems. In this context, a
novel space-time iterative multiuser receiver is proposed which achieves a large improvement in
spectral efficiency and performance over multipath fading channels. In addition, it is shown that
this architecture can be implemented without a prohibitive complexity cost. The exploitation
of the iterative principle can be used to approach the capacity bounds of a coded DS-CDMA
system. Using the Shannonâs sphere packing bound, a comparison is presented to illustrate how
closely a practical system can approach the theoretical limits of system performance
The deep space network
The objectives, functions, and organization of the deep space network are summarized. Progress in flight project support, tracking and data acquisition research and technology, network engineering, hardware and software implementation, and operations is reported. Interface support for the Mariner Venus Mercury 1973 flight and Pioneer 10 and 11 missions is included
Radar Technology
In this book âRadar Technologyâ, the chapters are divided into four main topic areas: Topic area 1: âRadar Systemsâ consists of chapters which treat whole radar systems, environment and target functional chain. Topic area 2: âRadar Applicationsâ shows various applications of radar systems, including meteorological radars, ground penetrating radars and glaciology. Topic area 3: âRadar Functional Chain and Signal Processingâ describes several aspects of the radar signal processing. From parameter extraction, target detection over tracking and classification technologies. Topic area 4: âRadar Subsystems and Componentsâ consists of design technology of radar subsystem components like antenna design or waveform design
The Fifth NASA Symposium on VLSI Design
The fifth annual NASA Symposium on VLSI Design had 13 sessions including Radiation Effects, Architectures, Mixed Signal, Design Techniques, Fault Testing, Synthesis, Signal Processing, and other Featured Presentations. The symposium provides insights into developments in VLSI and digital systems which can be used to increase data systems performance. The presentations share insights into next generation advances that will serve as a basis for future VLSI design