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Near-far resistance of parallel interference cancellation detector in a multirate DS-CDMA systems
Abstract
In this paper, the performance of the hard decision (HD) parallel
interference cancellation (PIC) detector (HD-PIC) is studied in a
multirate direct-sequence code division multiple access (DS-CDMA)
system. The performance is measured in terms of bit error rate, in an
additive white Gaussian noise (AWGN) channel under the impact of
imperfect delay estimation and the near-far conditions. The performance
results are evaluated for a single-rate and a three-rate system.
Monte-Carlo computer simulations have been used to generate the results
... However, if the transmission power of the interfering signal is too strong, the spreading factor alone is not enough to solve the near-far problem. The resistance to the near-far problem is is an active research problem [3,28,55]. ...
... Of course, if the transmission power of the interfering signal is too strong, the spreading factor alone is not enough to solve the near-far problem. This is an active research problem and many solutions have been proposed to detect multiple users such as minimum mean square error (MMSE) or parallel interference cancelation (PIC)[3,28,55]. ...
Future mobile communication systems are expected to provide a wide variety of services from high quality voice to video through high data rate wireless links supporting seamless roaming among different systems any place and any time. This chapter presents a vision of a system beyond third generation mobile communications, which comprises a combination of factors such as development of consumer market, communications and computer industries.
This paper presents a throughput-delay analysis of a centralized unslotted DS/CDMA packet radio network using rate adaptive channel load sensing protocol (CLSP). CLSP is an access control protocol used to limit the contentions, i.e. multiple access interference (MAI) of the CDMA unslotted ALOHA radio channel, under a certain threshold to improve the system throughput. A mobile-location (space) based rate adaptive extension of CLSP is proposed to: (i) compensate the near-far effects and transmitter power control (TPC) difficulty of datagram packet transmissions under changing channel conditions; (ii) enhance the system performance and coverage; (iii) increase the energy-efficiency of the mobile unit. An analytical method is developed based on a multi-rate loss network model, of which each system load state is described with a two-state hidden Markov model (HMM) taking into account the impacts of TPC inaccuracy, spatial user distributions and channel attenuation. The results show that the proposed rate/space adaptive CLSP system gains significant improvement in throughput-delay tradeoffs compared to the fixed rate counterpart with the same coverage, offered traffic and QoS requirements. Also, in the new system the mobile terminal can operate with much lower maximum power.
This paper presents the throughput-delay analysis or a centralized unslotted DS/CDMA packet radio network using rate adaptive channel load sensing protocol (CLSP). CLSP is an access control protocol used to limit the contentions of CDMA unslotted ALOHA radio channel under a certain threshold to improve the system throughput. The rate adaptive extension of CLSP is proposed (i) to compensate the near-far effects and power control difficulty of connectionless packet transmissions under changing channel conditions; (ii) to enhance the system performance and coverage. The analytical model is developed based on a multi-rate loss network model taking into account the impacts of spatial user distributions and radio propagation attenuation. The results show that the rate adaptive CLSP system gains significant improvement in throughput-delay tradeoffs compared to the fixed rate counterpart with the same coverage, offered traffic and QoS requirements.
Communication networks using code division multiple access (CDMA)
include applications where several packets of information are
transmitted synchronously and simultaneously over a common channel.
Consideration is given to the problem of simultaneously demodulating
every packet from such a transmission. A nonlinear detection scheme
based on a linear complexity multistage multiple-access interference
rejection algorithm is studied. A class of linear detectors is
considered as constituting the first stage for the multistage detector.
A bit-error probability comparison of the linear and multistage
detectors is undertaken. It is shown that the multistage detectors are
capable of achieving considerable improvements over the linear
detectors, particularly in near-far situations, i.e., in the
demodulation of weak signals in the presence of strong interfering
signals. This problem has been of primary concern for currently
operational CDMA systems
This paper presents the impact of timing error on the performance
of multirate direct sequence code division multiple access (DS-CDMA)
systems based on variable spreading factor (VSF) using multiuser
detection techniques such as the parallel interference cancellation
(PIC) detector. The performance is evaluated for a single-rate and a
three-rate system using Monte-Carlo simulation in the additive white
Gaussian noise (AWGN) channel without multipath fading. The bit error
probability (BEP) of the matched filter (MF) and the PIC detector with
and without timing errors are compared
Wideband DS-CDMA has gained a lot of attention for the third
generation wireless systems (UMTS and IMT-2000). On the other hand,
multiuser detection has been under intense research since the
formulation of optimum multiuser detector by Verdu in 1986. For the
third generation wireless systems, several wideband CDMA schemes with
multiuser detection have been proposed. This paper presents an overview
of developments in the field of multiuser detection for DS-CDMA.
Finally, possible future research directions are suggested
Spread spectrum-based code division multiple access (CDMA), has
taken on a significant role in cellular and personal communications. We
concentrate on direct sequence CDMA (DS/CDMA). We show that there is a
natural modification of the present systems that is potentially capable
of significant capacity increases. By “natural modification”
we mean a modification that can be made conceptually clear, not that it
is easy to implement. Indeed, the optimal multiuser detector is much too
complex and most of the present research addresses the problem of
simplifying multiuser detection for implementation. The objective of the
article is to make the basic idea intuitive and then show how
investigators are trying to reduce the idea to practice. We also
indicate multiuser receiver structures with potentially acceptable
levels of complexity and address potential obstacles for achieving
theoretically predicted performance in practice. As a result of these
investigations, an answer to the following question is expected. Is
there a suboptimal multiuser detector that is cost effective to build
with significant enough performance advantage over present day systems?
A definitive answer is not yet available
Consideration is given to an asynchronous code-division
multiple-access environment in which receiver has knowledge of the
signature waveforms of all the users. Under the assumption of white
Gaussian background noise, the authors compare detectors by their worst
case bit error rate in a near-far environment with low background noise,
where the received energies of the users are unknown to the receiver and
are not necessarily similar. Conventional single-user detection in a
multiuser channel is not near-far resistant, and the substantially
higher performance of the optimum multiuser detector requires
exponential complexity in the number of users. The authors explore
suboptimal demodulation schemes which exhibit a low order of complexity
while not exhibiting the impairment of the conventional single-user
detector. It is shown that there exists a linear detector whose
bit-error-rate is independent of the energy of the interfering users. It
is also shown that the near-far resistance of optimum multiuser
detection can be achieved by a linear detector. The optimum linear
detector for worst-case energies is found, along with existence
conditions, which are always satisfied in the models of practical
interest