15 research outputs found
Cooperative Diversity in CDMA over Nakagami−m Fading Channels
Spatial diversity can be employed by sending copies of the transmitted signal using
multiple antennas at the transmitter/receiver, as implemented in multiple-input multipleoutput
(MIMO) systems. Spatial receive diversity has already been used in many applications
with centralized systems where base station receivers are equipped with multiple
antennas. However, due to the power constraints and the small size of the mobile terminal,
it may not be feasible to deploy multiple transmit antennas. User cooperation
diversity, a new form of space diversity, has been developed to address these limitations.
Recently, user cooperative diversity has gained more attention as a less complex alternative
to centralized MIMO wireless systems. It revealed the ability to improve wireless
communications through reliable reception.
One common network of the user cooperation diversity is the direct sequence code
division multiple access (DS-CDMA) in which the Rayleigh fading channels are adopted
and the orthogonality between users is assumed. The Rayleigh fading channels are unrealistic
since they cannot represent the statistical characteristics of the complex indoor
environments. On the other hand, Nakagami-m fading model is well known as a generalized
distribution, where many fading environments can be modeled. It can be used to
model fading conditions ranging from severe, light to no fading, by changing its fading parameter m.
The bit-error-rate (BER) and outage probability of uplink cooperative DS-CDMA over
Nakagami-m has not been addressed in the literature. Thus, in this thesis, the performance
of both decode-and-forward (DF) and amplify-and-forward (AF) cooperative
asynchronous DS-CDMA system over Nakagami-m fading channels is investigated. The
Rake receiver is used to exploit the advantages of multipath propagation. Besides, multiuser
detection (MUD) is used to mitigate the effect of multiple-access interference (MAI).
We show that our proposed multi-user system achieves the full system diversity gain.
The first part of the thesis introduces a new closed-form expression for the outage
probability and the error probability of the DF cooperative DS-CDMA over asynchronous
transmission over independent non-identical Nakagami-m fading channels. The underlying
system employs MUD such as minimum mean square error (MMSE) and decorrelator
detector (DD) to achieve the full diversity. The aforementioned closed-form expression
is obtained through the moment generating function (MGF) for the total signal-to-noise
ratio (SNR) at the base station where the cumulative density function (CDF) is obtained.
Furthermore, we investigate the asymptotic behavior of the system at high SNR to calculate
the achievable diversity gain. The results demonstrate that the system diversity gain
is fulfilled when MUD is used to mitigate the effect of MAI.
In the second part of the thesis, we study the performance of cooperative CDMA
system using AF relaying over independent non-identical distribution (i.n.i) Nakagami-m
fading channels. Using the MGF of the total SNR at the base station, we derive the outage
probability of the system. This enables us to derive the asymptotic outage probability for
any arbitrary value of the fading parameter m.
The last part of the thesis investigates the optimum power allocation and optimum
relay location in AF cooperative CDMA systems over i.n.i Nakagami-m fading channels.
Moreover, we introduce the joint optimization of both power allocation and relay location
under the transmit power constraint to minimize the outage probability of the system.
The joint optimization of both power allocation and relay location is used to minimize
the outage performance of the system, thereby achieving full diversity gain
Advanced Trends in Wireless Communications
Physical limitations on wireless communication channels impose huge challenges to reliable communication. Bandwidth limitations, propagation loss, noise and interference make the wireless channel a narrow pipe that does not readily accommodate rapid flow of data. Thus, researches aim to design systems that are suitable to operate in such channels, in order to have high performance quality of service. Also, the mobility of the communication systems requires further investigations to reduce the complexity and the power consumption of the receiver. This book aims to provide highlights of the current research in the field of wireless communications. The subjects discussed are very valuable to communication researchers rather than researchers in the wireless related areas. The book chapters cover a wide range of wireless communication topics
Performanse bežičnog telekomunikacionog sistema u prisustvu n-m fedinga
In this thesis characteristics of wireless communication system operating over η-μ fading channel are considered together with diversity reception techniques which reduce the influence of η-μ fading on the system’s outage probability, average bit error rate, channel capacity, level crossing rate and average fade duration.
Performance improvement is very significant within radio systems operating into cell network configuration. Cell network configuration realization could be used for increasing capacity of wireless communication system. With the increase of number of cells, i.e. with reducing the surface area of each cell, channel capacity increases. With the increase of number of cells, co-channel interference level increase, which degrades system performance values. In this work the compromise between the system capacity and reception quality is inquired.
By applying diversity reception techniques, system performance values, degraded by the influences of slow fading, multipath fading and co-channel interference, are improved, so it is possible to reduce the cell area and to increase system capacity.
In the second part of Phd thesis, various distributions for modeling the envelope variations in fading channels have been presented, cases in which these models are used have been pointed out, and advantages and imperfections of corresponding models for corresponding propagation scenarios have been presented.
In third chapter, statistical characteristics of the first order of η-μ random variable, α-η-μ random variable and squared η-μ random variable are considered. For each mentioned variables, expressions for probability density function, cumulative distributive function, characteristic function and moments have been derived. Also for each observed case sum of two random variables, product of two random variables, ratio of two random variables, maximal value of two random variables and minimum value of two random variables have been determined. Obtained results are used for determining performances of wirelles reception with applied diversity technique for mitigation fading influence on system performances. Based on obtained expressions, graphs are depicted for probability density functions and cumulative distribution functions for the various values of propagation
environment parameters. Probability density function and cumulative distribution function values are also graphically presented for α-η-μ random variable in the function of α and μ parameter change.
For the purpose of performance analysis in η-μ fading environment, in the fourth chapter have been considered transformations of three η-μ random variables. Based on presented transformations of η-μ random variables, transmission performances estimation has been conducted, for the η-μ fading channel. Estimation of signal performances for the cases when diversity techniques are applied are carried out based on standard signal performance measures, i.e. outage probability (OP), average bit error probability (ABER), for observed modulation format and channel capacity. Graphically are presented ABER values for various values of system parameters when transmission is carried out with different modulation formats. By comparing obtained values it can be seen received signal performance improvement for the cases when diversity techniques are apliied over the reception case when there is no diversity technique applied.
In the fifth chapter statistical characteristics of the second order of η-μ radnom proccess, and random proccesses which represent various variations of η-μ radnom proccess, are considered. Brand new random proccesses, for describing fading in special channel conditions are fromed. For all this cases level crossing rates are determined.
In the sixth chapter of this Phd thesis are considered wireless communication systems with reception with applied diversity techniques for mitigating the influence of η-μ fading on system performances. Space diveristy technique has been used. Useful signals are accepted at the antennas, envelopes of these signals are combined and decision is made based on the signal values at the combiner outputs. System performances are determined for the cases of SC and MRC combining. For bouth cases probability density function and cumulative distribution function of the signal at the combiners outouts are derived, as well as the average bit error rate for the various used modualtion formats and level crossing rate. Results for ABER for various modulation formats are graphically presented as well as the improvement of the outage probability at the reception obtained by applying SC with two reception branches. In this part it has also been considered the case when bouth desired and interferring signal are described with η-μ distribution, as well as the case when desired signal has been described with η-μ distribution while interference has been described with κ-μ distribution.
In the seventh chapter macrodiversity sistem with SC reception and two MRC microdiversity combiners has been considered. At the inputs at the microdiversity combiners η-μ fading is present, while at the inputs as macrodiversity combiners slow Gamma fading is present. For this model of system it has been calculated probability density function, cumulative distribution function, characteristic function, moments, variance, outage probability and level crossing rate for the signal at the macrodiversity combiner output. Results obtained for level croosing rate at the macrodiversity combiner output are graphically presented