38
CHAPTER 2
LITERATURE REVIEW
2.1 MULTICLASS CALL ADMISSION CONTROL
TECHNIQUES IN W-CDMA
Huan Chen et al (2002) have proposed a call admission control
(CAC) scheme which gives preferential treatment to high priority calls, such
as and off calls, by pre-reserving a certain amount of channel margin against
the interference effect. It is called the interference guard margin (IGM)
scheme. The amount of guard margin is determined by the measurement
performed by the Resource Reservation Estimation (RRE) module in base
stations. Each RRE module dynamically adjusts the level of guard margin by
referencing traffic conditions in neighbouring cells based upon user’s
requests. A comprehensive service model is adopted to accommodate the
scenario of multiple services supported in the W-CDMA system. The service
model of consideration includes not only mobile terminal’s service rate
(source rate) but also different levels of priorities, mobility and rate adaptively
characteristics.
Their scheme has a dynamic call admission control based on two
concepts. First, a certain amount of interference-based guard margin (IGM) is
pre-reserved for the use of high priority calls. The amount of IGM is
dynamically adjusted by the resource reservation estimation module. Second,
the load curve is used to estimate the load increase as well as the interference
increase.
39
The concept of a guard margin illustrates, a new call to be admitted,
the total interference level should not exceed the upper bound of the
interference with threshold Ith that the system can tolerate. In addition to the
constraint of I’th, a lower priority call should comply with the augmented
constraint Ith. The margin between Ith and I’th is exactly the guard margin,
which provides the preferential treatment to high priority calls by limiting the
access to the low priority calls.
When a mobile terminal (MT) moves toward cell boundaries, the
neighbouring base stations (BS) receive stronger signal from it. Each of the
BS in the neighbouring cell will send messages to the main traffic switching
office (MTSO) to register itself as a handoff candidate for MT. The handoff
candidate registration (HCR) table is used in MTSO to maintain the
registration record and to inform the MT about where to handoff when its
signal fades. This table also provides very useful information to estimate
future handoff calls for a given cell. Before admitting a new or handoff call, j,
in a cell, dynamic resource reservation estimation scheme estimates the
interference guard margin IGM based on a weighted sum of estimated
minimum interference-increments according to the traffic profile for each
neighbouring active calls. In the CAC algorithm there are three types of
mobility, i.e. high, moderate and low mobility’s, are considered. By assigning
1, 2 and 4 units of speed for moderate and high mobility traffics, respectively.
The proposed CAC algorithm implies that a high speed MT is more likely to
handoff into the current cell even though it is farther away with respect to a
BS in comparison with a low speed MT.
The features of the proposed dynamic resource management are
summarized below:
40
Supports multiple service rates which are suitable for 3G
system such as wideband-CDMA (W-CDMA).
Supports rate adaptive characteristics for multiple services
with flexible QoS guarantees.
This scheme employs resource reservation estimation process
to adjust the level of reserved resources dynamically by
referencing the traffic condition in the neighbouring cells.
Bridges two concepts, guard channel (GC) and load curve
(LC) to provide the preferential treatment for high priority
calls.
Traffic mobility is considered in system model to achieve
better resource estimation results.
The system utilization is higher for traffic with the rate-adaptive
capability than that without the rate-adaptive capability under heavy traffic
loads. This can be explained by the fact that the system can provide calls with
a degraded service in terms of lower bandwidth when the system is congested,
thus increasing the overall system utilization. The use of weighting factor on
IGM increases the system utilization at risk of blocking more handoff calls.
Young-Long Chen et al (2007) have proposed a novel approach
which combines the CAC and power control mechanisms and operates in a
centralized control manner. The essence of the proposed centralized call
admission control (CCAC) scheme is to combine the two mechanisms and to
treat the call admission decision as an eigen-decomposition problem. In the
proposed approach, a new call is accepted only if the quality-of-service (QoS)
requirements of all the active links in the network can still be maintained. In
41
order to reduce the computational complexity of the eigen-decomposition
problem, they have proposed an additional scheme which uses a norm
operation rather than direct computation.
In their algorithm, it is assumed that all of the link gains between the
new call and the various base stations in the network can be estimated exactly
from the pilot signals broadcast during call setup initiation. Therefore, the
interference produced by the new call can be precisely predicted. When a new
call arrives, the decision to admit or reject the call is made depending on
whether or not there exists a power vector P which can guarantee every user’s
QoS requirement. The corresponding decision-making problem is formulated.
The QoS requirement is defined in terms of the bit error rate (BER) or the
frame error rate (FER). It is assumed that the BER or FER requirements can
be mapped into an equivalent SIR requirement for the modulation scheme.
Hence, when a new call arrives, it will be accepted only if the formulated
conditions are satisfied.
Malarkkan et al (2006) have presented functioning of call admission
control and resource reservation method based on the mobility of the users in
W-CDMA cellular systems. In order to assure the handoff dropping
probability, the mobility of the user was calculated based on a realistic
mobility model. The mobility calculation scheme was used to quote the set of
candidate cells into which the mobile may move in the near future and
calculates the similar value for each candidate cell.
The capacity of a CDMA system is limited by the total interference
it can tolerate, which is called the interference-limited system. Users with
different traffic profiles and attributes, such as the service rate, the signal-to-
Interference ratio (SIR) requirement, media activity, etc., different amount of
interference has been introduced to the system.
42
Malarkkan et al (2006) have analysed Fuzzy based Call admission
control scheme. They considered that the mobility information of the new
user requesting connection and already existing users, the type of service
request (real time or non-real time) and the load factor which is calculated
from the intra-cell interference and the inter-cell interference at the base
station. The QoS requirement of the non real time traffic is reduced to
accommodate more number of real time traffic users to improve the
performance of the admission control scheme.
In the Fuzzy logic a set is defined without a crisp boundary. The
transition from “belong to the set” to “not belong to the set” is gradual, thus
representing the truth grade related to the definition of the concept. This
smooth transition is characterized by the so-called ‘Membership Functions’
that give set flexibility in modelling commonly used linguistic expressions,
like “the temperature is hot” or the “weather is warm”. A Fuzzy System
consists of a Fuzzifier, an Inference Engine, a Fuzzy Rule Base and a
Defuzzifier. The Fuzzifier transforms the values of the input parameters into
the fuzzy linguistic terms through a set of Membership Functions. These
fuzzy linguistic terms are the inputs of the Inference Engine, which will
perform the logic inference according to the Fuzzy Rule Base. The Fuzzy
Rule Base is constructed by the expert knowledge of the phenomenon
(admission control). The Defuzzifier converts the results of the inference into
the usable values for admission decisions.
The Fuzzy Reasoning is, also known as “approximate reasoning”, it
is an inference procedure that derives conclusions from a set of fuzzy rules
and known facts. It can be divided into four steps:
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Degrees of compatibility: compare the known facts with the
antecedents of fuzzy rules to find the degrees of compatibility
with respect to each antecedent Membership Function.
Firing strength: combine degrees of compatibility with
respect to antecedent Membership Functions in a rule using
fuzzy AND or OR operators to form a firing strength that
indicates the degree to which the antecedent part of the rule is
satisfied.
Qualified induced consequent Membership Functions:
apply the firing strength to the consequent Membership
Functions of a rule to generate a qualified consequent
Membership Function.
Overall output Membership Function: aggregate of all the
qualified consequent Membership Functions to obtain an
overall output Membership Function.
Bazil Taha Ahmed et al (2008) have specified the uplink capacity
and the interference statistics for a W-CDMA 3-D Air-to- Ground (AG)
cellular like network assuming imperfect power control and finite transmitted
power. The free space model of propagation was used to calculate the
intercellular interference. The uplink capacity has been considered for various
frequencies and situations. It has been shown that the effect of rain was to
reduce the uplink capacity and the maximum permissible cell radius. Also it
was shown that, the frequency of operation should be lower or equal to 2
GHz. For a frequency of operation of 2 GHz, the cell capacity can reach 70
voice users or 46 data users when the cell radius is 320km. The new
contribution of their work was the study of the effect of an imperfect power
44
control and the finite transmitted power on the uplink capacity of the Air-
Ground system for various values of outage.
Seong-Jun Oh et al (2000) have studied the radio resource allocation
problem of distributed joint transmission power control and spreading gain
allocation in a DS-CDMA mobile data network. The network consists of K
base stations and M wireless data users. The data flows which are produced
by the users are considered as best-effort traffic, in the sense that there are no
pre-specified restrictions on the quality of the radio channels. They are
interested in designing a distributed algorithm that attains maximal (or near-
maximal in some reasonable sense) aggregate throughput, subject to peak
power constraints.
Jyoti Laxmi Mishra et al (2007) have evaluated various types of call
admission control algorithm. The objective of their research was to improve
the same algorithm with multiclass users and multiservice using fuzzy logic.
El-Dolil et al (2008) have examined the trade-off in the uplink
direction using power-based Multi-Cell Admission Control (MC-AC)
algorithm. Multimedia services were measured with different QoS
requirements. Different traffic situations were also considered. Their
simulation results exposes that MC-AC algorithms has many advantages over
single cell admission control in terms of overall constancy of the system and
total system throughput.
In this system, to investigate the performance of multi-cell
admission control they have developed a model. This model is used when
heterogeneous traffic is considered. Their results proved that MC-AC
algorithm has many advantageous over SC-AC. It has more advantage in
terms of dropping probability, network stability, and total system throughput.
They have examined the trade-off between the dropping and blocking. As per
45
their system dropping an ongoing call is more annoying than blocking a new
one. Call dropping probability can be lowered without much increase in
blocking probability. Both homogenous and heterogeneous traffic are
considered in this model. They have achieved more capacity gain under
heterogeneous (hot around) traffic distribution. Their results show that high
bit rate services suffer from both higher blocking and dropping probabilities.
Andrej Husar et al (2003) have presented a synopsis for call
admission control schemes, which are a part of radio source management
which plays a vital role at user admission into the system. They have also
explained some admission schemes into the system and also they have
compared their performance.
In their scheme the call admission control performed only, if
required power control is achieved. Otherwise it will reject the call. They
have proved that the dropping probability is less than the quality threshold.
And it keeps the admitted load as high as possible.
2.2 ADAPTIVE SCHEDULING TECHNIQUES IN W-CDMA
Salman AlQahtani et al (2007) have proposed and analyzed an
efficient uplink-scheduling scheme in case of Radio Access Network (RAN)
sharing method. With reference to this new scheme as Multi-operators Code
Division Generalized Processor sharing scheme (M-CDGPS). It employs both
adaptive rate allocation to maximize the resource utilization and GPS
techniques to provide fair services for each operator.
The CDGPS discipline is adapted and extended in order to design a
new high performance GPS based scheduling scheme which can effectively
control the shared resources among W-CDMA multi-operators in an efficient
and fair manner. Efficiency means higher system utilization and fairness
46
means that each operator is guaranteed at least a capacity equals to its
capacity share specified in the SLA. Therefore, a multi-operator CDGPS (M-
CDGPS) rate scheduling scheme for the uplink W-CDMA cellular network is
designed and analyzed.
Their scheme employs both adaptive rate allocation to maximize the
resource utilization and M-CDGPS to provide fair services for each operator.
The resource allocated to each operator session is proportional to an assigned
weight factor as per the SLA specification. After the initial allocation of the
allotted capacity, M-CDGPS scheme uses the CDGPS service discipline to
dynamically schedule the assigned channel rates of one operator among the
traffic classes within that operator independently. Moreover, the system
performance measures in terms of bounded delay and buffer size are also
derived using the GPS performance model.
They have compared the performance between static and adaptive
M-CDGPS. They have compared both scheme in terms of system throughput
and delay. They have designed a system with three operators. They have
fixed same traffic load for two operators which is fixed one and they have
fixed different traffic load for the third operator. They have used a long range
of offered traffic. By that they have examined the system behaviour at low
and high traffic loads. They have shown that the throughput of adaptive rate
M-CDGPS is higher in case of using adaptive rate because of utilizing the
residual resources of other operators. Hence the system throughput increases.
They have also shown that the throughput is linearly increases as the offered
traffic increase. They have proved that, when the saturation point is reached,
the flow of third operator who is fixed with different traffic load is limited to
its minimum assigned rate.
47
Nidhi Hegde et al (2006) have designed a resource sharing of BE
applications with the RT applications in W-CDMA networks. Both the types
of traffic have flexibility to adapt the obtainable bandwidth but not like the
BE traffic. RT traffic requires strict minimum bounds on the throughput. They
have examined the performance of both BE and RT traffic and examined the
effect of reservation for some portions of the bandwidth for the BE
applications. Also they have presented a novel capacity definition associated
to the delay of BE traffic and showed how to calculate it.
They have considered best-effort (BE) traffic sharing the network
resources with real-time (RT) applications. They have shown that the BE
applications can adapt their instantaneous transmission rate to the available
one. Their meaningful QoS is the average delay. They have defined delay
aware capacity. Which is defined as the arrival rate of BE calls. Their system
can handle the expected delay of BE calls bounded by a given constant. They
have computed blocking probability of the RT traffic having an adaptive
Grade of Service (GoS) and the expected delay of the BE traffic for an uplink
multi-cell W-CDMA system. Thus their system yields the Erlang capacity for
former requirement and the delay capacity for the latter requirement.
They have assumed that RT traffic is subject to Call Admission
Control (CAC) in order to guarantee the minimum rates for accepted RT calls.
This implies that RT traffic may suffer rejections whose rate is then an
important QoS for such applications. In contrast, Best effort (BE) applications
can adapt their transmission rate to the network’s available resources and is
therefore not subject to CAC. The relevant QoS measure for BE traffic is then
the expected time delay of a call in the system.
They have considered BE traffic sharing the network resources with
RT applications. Their aim is to compute both the blocking (or rejection)
probability of the RT traffic as well as the expected delay of the BE traffic for
48
an uplink multi-cell W-CDMA system. RT calls need a minimum guaranteed
transmission rate, they are assumed to be able to adapt to network resources in
a way similar to the BE traffic. They have proved that duration of BE calls
depends on the dynamic rate assignment. They have proposed a probabilistic
model that takes these features into account and enables to compute the
performance measures of interest. And they have computed the blocking
probabilities and the average throughput per RT calls, the expected average
number of RT and BE calls in the system, and the expected delay of BE call.
They have modelled a resource sharing of BE applications with RT
applications in W-CDMA networks. Both type of traffic have flexibility to
adapt to the available bandwidth but unlike BE traffic, RT traffic requires
strict minimum bounds on the throughput.
Rekha Patil et al (2008) have proposed a Scheduling scheme which
aims to allocate the wireless channel to contending nodes. Through that they
reduced multiple access interference. At the same time each node is
guaranteed a minimum acceptable level of performance in terms of metrics
such as data rate, delay, probability of error etc.
Their scheduling algorithm admits the transmissions of static as well
as mobile users of multi service classes, in order to eliminate strong levels of
interference. That interference cannot be overcome by power control. They
have given priority for the mobile users when determining the admissible set
of users.
Remco Litjens et al (2002) have evaluated data and voice calls in
UMTS network. They have evaluated several scheduling scheme with
dynamic adaptation of transport channel rates. The have examined the
function of scheduling scheme according to the sharing of limited resources
for varying number of voice calls. It gives different traffic load to the
49
scheduling scheme. Based on the certain fairness objective they have
distributed the resources according to present load of a particular cell. They
have evaluated the analytical performance of each scheduling scheme.
They have evaluated the performance of the scheduling scheme in
the downlink of UMTS network. The network consists of integrated and
prioritized speech data and voice calls. They have derived closed form of
expression to apply optimal power for multiplexing of data transmission.
Along with this, it protects the performance of voice calls. They have
provided numerical examples for performance improvement of data and voice
calls by the utilization of scheduling scheme. The have assigned a minimum
power for voice and data calls to evaluate the performance and resource
sharing capabilities of UMTS network.
Leonardo et al (2003) have analyzed and presented a model trade-
off between the QoS metrics, blocking and dropping probability. They have
analyzed obtained performance is under different points conditions and they
exposed required aspects. They have analyzed known algorithms, in terms of
fairness and generality of the performance. They have taken realistic models
for mobility, data rate and discontinuous transmission (DTX) into account..
Their proposed approach to CAC is aware of mobility differences
among users. The mobility difference of the user is easily tracked by the base
stations. Their results proved that traditional approaches will give unfairness
results if users have different mobility patterns coexist in the same system.
Their Mobility-aware Interference-based CAC (MICAC) can provide much
better fairness performance compare to all other traditional users.
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They have modelled Call Admission Control instantaneous
threshold algorithms for 3G. The Mobility aware Interference based Call
Admission Control improves the performance of network. It is used in
instantaneous heuristic evaluation of the network. Their results may be
globally good, improvement under the aspect of fairness may still be
necessary. They have tested different systems under the aspects of sensitivity
to their parameters and fairness. They have identified that the performance
can be improved by a better model of the drop policy. In drop policy along
with mobility class, global blocking and dropping probability, but even the
behaviour with respect to each mobility class is considered for evaluation.
Jean Marc Kelif et al (2005) have considered W-CDMA system
with two types of calls. They are, real time (RT) and non-real time (NRT)
calls. Real time calls have dedicated resources, and data. Non-real time (NRT)
calls are treated using a time-shared channel (such as the HDR or the
HSDPA). They have reserved some resources for the NRT. The NRT further
assign the given resources to RT. They have controlled the grade of service
(GOS) of RT to handle and admit more RT calls during congestion period. It
degrades the existing user to provide service to more RT calls. They have
considered both uplink and downlink. Through this they have examined the
blocking probability and sojourn time of NRT calls. They have tested further
the conditional expected sojourn time of a data connection, size and the state
of the system. They have examined and created a framework to handle
handover calls.
They have further multiplexed NRT calls using time. It avoids the
amount of interference and increasing the available average throughputs. This
51
kind of multiplexing is used in High Speed Downlink Packet Access
(HSDPA). It is a high speed downlink data channel.
They have proposed a simple model to analyze the capacity
requirement of a call of given class. It is performed only when the call of class
uses the given grade of service (transmission rate). They have proposed a
control policy to combines admission control together with a control of the
grade of service (GoS) of real-time calls. They have computed the Key
performance measures by quasi-birth-and-death (QBD) process. They have
obtained the call blocking probabilities and expected transfer times, already
available for the uplink case. They have further obtained another important
performance measure for both uplink and downlink. That is expected transfer
time of a file conditioned on its size. They have analyzed the influence of the
control parameters on these performance measures. They have further
examined the model to handle handover calls.
Remco Litjens et al (2002) have evaluated data and voice calls in
UMTS network. They have evaluated several scheduling scheme with
dynamic adaptation of transport channel rates. The have examined the
function of scheduling scheme according to the sharing of limited resources
for varying number of voice calls. It gives different traffic load to the
scheduling scheme. Based on the certain fairness objective they have
distributed the resources according to present load of a particular cell. They
have evaluated the analytical performance of each scheduling scheme.
They have evaluated the performance of the scheduling scheme in
the downlink of UMTS network. The network consists of integrated and
prioritized speech data and voice calls. They have derived closed form of
expression to apply optimal power for multiplexing of data transmission.
52
Along with this, it protects the performance of voice calls. They have
provided numerical examples for performance improvement of data and voice
calls by the utilization of scheduling scheme. The have assigned a minimum
power for voice and data calls to evaluate the performance and resource
sharing capabilities of UMTS network.
2.3 POWER CONTROL TECHNIQUES IN W-CDMA
Dejan Drajic (2002) has formulated some adaptive technique to
enhance power control technique in W-CDMA. The fast power control for
downlink is a very important part of the system. The aim is to keep the
received power per bit at a satisfactory level. It is well known that the power
control can be very efficient when the receiver velocity is relative small. On
the other hand, turbo codes with long inter-leaver are expected to combat
successfully with short deep fades appearing during the fast moving of the
receiver.
In CDMA system power control (PC) has a strong effect on the
interference experienced by the receiver, and hence it directly affects its
performance. The compensation of fading channels and changes in the
transmitted powers of interfering users are recognized as the main
characteristics of power control. However, it might cause problems on the
adaptation of equalizers. The power control in W-CDMA is SIR (or SINR)
based. In the model, due to exceptional complexity of such an approach, a
simplified model based on signal-to-noise ratio (SNR) was implemented.
Here, perfect knowledge of channel is assumed and powers of other
interfering users and CPICH are held constant. Also, the power control model
compensates only for the fading channel of the desired user. The
characteristics and parameters of model are: the power range of desired user is
limited ±10dB around nominal Eb/N0 level, and the received Eb/N0 is
53
averaged over last two pilot symbols in a slot, and compared to the nominal
Eb/N0. The transmitted power is changed 1dB step for the next slot to
compensate for the difference in Eb/N0 within limit of power range. In the
generation of power control command for the next slot, the transmitted power,
channel coefficients and noise power are known, and detection of power
control commands in base station is assumed to be error free.
Siamak Naghian et al (2002) have proposed a novel dynamic step
size power control method to improve the performance of UMTS/W-CDMA
cellular system. The proposed method utilizes dynamic step-size power
control commands, received SIR/power, and mobile handset location
assistance data. Furthermore, dynamic inter-operation between the power
control, admission control, and handoff control is evolved to improve the
convergence of the proposed power control mechanism. Based on the output
of this interoperability, a multi-step transmit power edge setting is proposed.
In the uplink power control, on the network side, the radio access
control and the base station are involved in the controlling part of the power
adjusting process. The Admission Control and the Power Control entities of
the radio access controller set the signal quality targets, which includes
SIR_max, SIR_opt_max, SIR_opt_min, and SIR_min. This can be based on
the traffic information available in AC, signal strength, SIR, access priorities,
location assistance information, and so on.
First, the mobile station receives the power control commands from
the base station. It registers the forthcoming power control command into the
command bit register. The change history can also be stored there, including
data on the latest power control commands, step-sizes, and possibly the
handset coordinates. The mobile station goes through the power control
command values, step-sizes, and possibly location assistance data, included in
the change history. If the power control command or step-size stream is even,
54
meaning that the power level is not radically changed but is kept stable, then
there is no considerable need for changing the transmit power.
If the power control command or step-size stream is not even, the
process advances to the step, where it is checked whether the power control
command stream is uneven, in other words if only one set of the power
control commands is repeated often. In that case the larger step-size could be
considered to compensate the variations of the transmit power.
If the power control command or step-size stream is not even nor
uneven but they are repeated irregularly, then the process advances directly to
the step where a fast power control without delay takes place. Location-aware
mobile station is able to predict the possible environmental changes, which
may affect the radio channel and avoid phenomena such as “corner effect”
when making the power control decision.
Subba Rao et al (2011) have proposed an adaptive power control
mechanism for multimedia traffic in W-CDMA networks. Their algorithm
uses two most recent Transmit Power Control (TPC) commands to compute
the Adaptive Factor (AF) based on a predefined Adaptive Control Factor
(ACF). With introduced another parameter, Power Determining Factor (PDF)
based on the data traffic rate to determine the power. Based on this parameter,
the power is increased or decreased. Depending on the traffic rate, the PDF
factor is updated i.e., if the observed traffic rate is high, then it will increase
the parameter and subsequently increases the power and if it is low, then the
parameter will be decreased and correspondingly the power also.
The aim of a power control procedure is to ensure an adequate SIR
for all mobiles in a system by a simple algorithm. The algorithm requires
feedback information from the receiver and based on that, the transmitting
power is adjusted at transmitter side. The information regarding the feedback
55
is delivered by means of the feedback information. The bandwidth of the
feedback is expensive and so the amount of information required by the
procedure must be kept minimal.
There are outer and inner loop power controls. The outer-loop
power control is performed by Radio Network Controller (RNC). Based on
some particular measured parameters such as BLER (Block Error Rate), the
outer loop power control adjusts the SIR target. The required BLER depends
on radio conditions and service types. According to the conditions and service
types, the individual SIR target of each mobile will be set. The requirement of
SIR target will be higher if a mobile moves quickly resulting in rapid change
in radio channel and vice versa. In addition, different service types require
different BLER. The SIR targets of data services are higher than the voice
service as the data service requires lower BLER than voice services. The
frequency of SIR target updating is 10-100 Hz.
Inner loop power control mechanism adjusts the transmitted power
to maintain the received SIR equal to the SIR target at the receiver. In 3GPP
specifications, each W-CDMA frame of length 10ms consists of 15 time slots
and each of which consists of one bit of the power control command called
“Transmit Power Control” or TPC. TPC is a command, which increases or
decreases the transmitting power. The transmitter will be controlled to
increase OR decrease the transmit power, so that the power will always be
oscillated even if the received SIR is close to the SIR target. The step size will
be increased if same TPC commands are detected or else the updated step is
very large, the step size will then be decreased.
The power control step size is adapted by multiplying a factor called
Adaptive Factor (AF) with the fixed step size. This factor will be updated
according to received TPC commands. The proposed algorithm increases the
step size i.e. AF when the mobile detects the same sequence of TPC. The
56
same sequence of TPC commands will occur when rapid changes of power
are required, such as when the radio channel condition variants rapidly and
continuously. The proposed algorithm uses two most recent TPC commands
to compute the AF based on a predefined ACF. The ACF is a constant chosen
by the networks.
In their proposed algorithm, they have introduced a new parameter,
Power Determining Factor (PDF) based on the data traffic rate to determine
the power. Based on this parameter, value is determining whether the power is
increasing or decreasing. Depending on the traffic rate, the PDF factor is
updated. If the received message is HCM, then it will increase the parameter
and subsequently increases the power and if the message is LC, then the
parameter will be decreased and correspondingly the power also.
Ch. Sreenivasa Rao et al (2012) have proposed a power controlled
call admission control scheme for handoff in the advanced wireless networks.
The incoming call measures the initial interference on it and then the base
station starts transmitting the packets to the new call. The new call is rejected
when the interference reaches a threshold value. Whenever an existing call
meets the power constraint, the transmit power is decremented based on the
traffic class and incoming call obtains this information by monitoring the
interference received on it. The convergence of the power control algorithm is
checked and the power levels of all incoming calls are adjusted.
The power constraint for admitting a new call is determined using
the present uplink interference, increased uplink interference, and the target
threshold. The power decrement value is calculated for both the real-time
packets and the non-real time packets. Using this algorithm, the call
admission is processed based upon the power constraints.
57
The QoS Based Adaptive Admission Controller (QAAC) technique
measures the channel quality and separate queues for maintaining each class
of service. The basic concept of QAAC algorithm is to simultaneously
provide transmission priority and space priority for the data flows of the same
end-user. The algorithm tries to minimize the number of the sessions that are
blocked due to insufficient resources in the target network.
The power constraint for admitting a new call depends upon the
present uplink interference, increased uplink interference, and the target
threshold. When the target threshold exceeds the sum of the present uplink
interference and the increased uplink interference then the call cannot be
admitted.
Aguero et al (2009) have presented new methods to improve control
with respect to the strong non-linear effect. Therefore they have proposed
adaptive control architecture for inner loop power control, having three
degrees of freedom. Their three degrees of freedom were used to address
errors resulting from channel/interference variations, quantization of the
power increments and saturation of the power increments. They have also
added a simple adaptation mechanism to their design so that non-stationary
channel gain and interference models could be tracked. A disadvantage of
their adaptive scheme was that information about the channel interference
model has needed to be transmitted from the base station to the mobile unit.
They have proposed a novel architecture for inner loop power
control of CDMA cellular systems. This algorithm uses uplink of the W-
CDMA system as an example. These methods are of general validity for
power control of CDMA cellular systems. The architecture has three degrees
of freedom. They have developed this system to address simultaneously three
performance limiting factors. (i) the effect of SIR changes due to channel and
interference variations, (ii) the effect of quantization of the power increments
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and (iii) step size issues. In this scheme they have used simple adaptation
mechanism. Through simulation they have shown that the scheme
consistently working in other related architectures. They have shown that the
adaptation mechanism yields a globally stable closed loop system. The
performance of the proposed architecture is shown that it reduces the variance
of the control signal, prior to its quantization.
Markus Laner et al (2010) have analyzed the un-coded BER
parameter, with respect to its possible contribution to an improvement of the
uplink OLPC algorithm. Their research was performed by means of extensive
measurements in a live network which proved that the actual implemented
algorithm converged slowly. They have also founded that the reason was due
to the QoS estimated by CRC and as the un-coded Bit Error Ratio (BER) hold
information about the QoS, this parameter could be used to increase
convergence speed of their OLPC. They have also presented a statistical
model of the control path of the OLPC which considered the un-coded BER
information.
The have proved that large-scale measurements have shown that the
uplink OLPC algorithm in UMTS networks, has a convergence time in the
order of a short call duration. It cannot follows quickly changing SIR
demands of a user in movement. They have provided a detailed analysis of the
relations between target SIR, BLER, and un-coded BER for static and moving
users, respectively. Their results revealed that the un-coded BER can be used
to multiply the OLPC convergence speed. Further they have proposed a new
OLPC algorithm which makes use of this parameter. They have simulated
using different time with different kind of user to check SIR value. They have
checked the user in movement, static user and user with short duration call.
They have showed that the algorithm reduces the mean target SIR with
independent of the user mobility.
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Tajje-eddine Rachidi et al (2004) have presented a QoS aware
mechanism for power control and Handoff in 3G W-CDMA networks. They
have also presented two mechanisms named Quality of Service aware Power
Control (QaPC) and QoS aware prioritization of Handoffs (QaHO) which
were based on the class of service, bit rate and Service Degradation
Descriptor (SDD) as enabling QoS parameters. They have also obtained the
numerical results using a W-CDMA and UMTS compatible test bed which
showed that their proposed QoS aware mechanism significantly improved the
QoS contract upholding for premium mobile users, as well as increased
resource utilization, while improving the acceptance of soft handoffs.
In two phases the adaptation algorithm resolves congestion. In case
of congestion handling and in case of SHO admission these two phases are
applied differently. User Equipment (UE) contains a QoS profile as specified
by their QoS framework. Their profile comprises the required bit rate, traffic
class, and the Service Degradation Descriptor. They take values between 0
and 5 for SDD. The user with larger SDD value is considered as to degraded
or dropped. They have proposed two phases to perform this operation. They
are as follows,
The Degradation Phase
This phase is based on the service degradation descriptor. The active
connection which has highest SDD value is degraded based on their
bandwidth. In this phase they have degraded such connection with 384Kbps
bit rate requirement to 144Kbps. Similarly 144Kbps is swapped for 64Kbps,
and 64Kbps is swapped for 16Kbps. 2 Mbps and 16Kbps connections are not
degraded in their schema.
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The Dropping Phase
This phase is working based on willingness of the user. This phase
is invoked only when willingness received from the user. Then their
connections were degraded. After dropping the call congestion persists in this
phase.
2.4 INTERFERENCE REDUCTION TECHNIQUES IN W-CDMA
Derong Liu et al (2007) have presented a simple interference
cancellation technique for the downlink of wideband code division multiple-
access (W-CDMA) systems in multipath environment. With the same
knowledge required by a RAKE receiver, the present method acts as an
equalizer and cancels the interfering multipath signals from the received
signal to retrieve the orthogonality property of the received signal. The
present receiver has a simple structure and it has significant performance gain
against the RAKE receiver. In addition, the noise enhancement is negligible
when there is a line of sight path or the channel power delay profile has an
exponential decaying form.
In their work, a new technique for multiple access interference
(MAI) cancellation based on successive cancellation of interfering multipath
signals from the received signal is presented. Their method has the capability
to completely cancel the MAI with only a slight increase in complexity
compared to the conventional RAKE receiver. The three basic assumptions
often used in some literature under which the application of their method is
feasible include:
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There is a guard interval at the beginning of each transmitted
frame so that there is no interference between two consecutive
frames, though this guard interval may used for different
purposes;
The multipath delays are integer multiples of the chip interval,
this assumption is easily achieved by sampling the data in
multiple integer of the chip rate; and
There exists a line of sight path (a relatively strong first path),
or the channel power delay profile has an exponential
decaying form.
Pon Rattanawichai et al (2011) have presented the FPGA (Field
Programmable Gate Array) implementation of self interference cancellation
technique based on an adaptive LMS (Least Mean Square) algorithm. By
using the FPGA Virtex@6 HW module, the field data measured through a RF
repeater is adopted to improve a signal quality and to reduce oscillation of the
system due to the feedback interference signal coming from transmit antenna
of a W-CDMA radio repeater.
The main parts of their system are composed of:
“New Weight Vector” block: obtaining an internal value
named “Input Values” to be used further in the coefficients
update.
“ICS Input” block: buffering the input received signal in
FPGA after performing ADC conversion.
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“Filter Error Calculation” block: canceling the interference
feedback signal yF using the estimated feedback signal yE.
“ICS Output” block: buffering the cleaned output signal
before performing DAC conversion.
“Normalization” block: suppressing the amplitude signal to an
effectively sufficient level in order to accelerate the
convergence rate of the LMS algorithm.
Jraifi Abdelouahed (2012) has proposed the system in which, radio
frequency interference is caused by occupying exiting radio frequency
resource, improper configuration of network by different operators, cell
overlapping external interference sources and electromagnetic compatibility.
Mobile communication interference is common-frequency, adjacent-
frequency, out of band spurious emission, inter-modulation emission and
blocking interference. He has developed an analytical expression of the Signal
to Noise Ratio (SNR) which is used to predict some parameters. By fixing the
SNR to a specific value, he has extracted easily information on the optimal
numbers of users.
He has adopted a micro-zoning architecture to compute the (S/I)
ratio. This analytical result is used to predict the number of users per micro-
zones. This result is of crucial importance because it contributes positively in
improving the quality of services. Furthermore, if one success to integrate
some system parameters into the theoretical result then his approach could
also be used for other applications.
Anis Masmoudi and Sami Tabbane (2006) have proposed two
systems called uniform and non-uniform traffic environment in interference
of W-CDMA. It presents an analytical work which provides exact derivations
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of the general F-factor, CDF and PDF distributions laws, mathematical
expressions in W-CDMA systems with one interferer. Their approach doesn’t
depend on any assumption or values range. They have also investigated the
unequally-loaded cell case referring to the more general non-uniform mobile
distribution. They have proved that this model refines the planning process
and thus increase the quality and capacity of a cellular network.
In Uniform traffic distribution, all the cells are assumed to be
equally loaded, i.e. the traffic and users distribution are uniform within each
cell, allowing the deployment of a regular grid of base stations (BS) or nodes
B. Propagation conditions are assumed to be the same all over the studied
area. In such conditions, by considering a simplified scenario with only two
nodes B (one is the serving node B and the other is its neighbor interferer) –
the required transmitted powers for the two nodes B are the same. Therefore,
the power of both interfering and serving nodes B, really used, can be
mathematically simplified when calculating f-parameter which becomes
depending only on different path losses as expressed later in another node.
Total transmit power reflects the equally-loaded cell case, independently of
any power control on the downlink, considered as perfect in this modeling.
Thus, in each link, the transmitted power is the one needed to ensure to the
mobile the required quality corresponding to the received SIR (Eb/N0)
necessary for the used service. So, the downlink transmitted power to each
mobile is not constant since its position varies as well as the propagation
conditions from one mobile link to another.
Non-uniform traffic distribution occurs with non-uniform traffic
environments or at the borders of two different traffic environments (e.g.,
urban and sub-urban). Overloaded cells size will shrink (cell breathing
phenomena) compared to unloaded ones. The size reduction of the most
loaded cell of the network is due either to uplink capacity limitations or to
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downlink coverage limitation. The first case occurs when the overloaded cell
traffic density increases and reaches the uplink pole capacity limit. The
maximum number of users depends on many factors such as service type
mixture, required quality of service, traffic and service distribution, and the
maximum allowed uplink noise rise.
Their study is particularly useful for choosing the convenient value
of f-parameter to consider in the loading equations or in the generic downlink
SIR expression and suitable for input static simulations and thus an accurate
design of UMTS cellular networks in different load conditions. In fact, their
analytical approach gives accurate modeling of f-parameter distribution and
thus corrects values that should be used in simulations required for a
preliminary planning process both in equally and unequally-loaded cell cases.
Moreover, their study is not valid only for regular grid cells, but it applies also
to irregular networks which operators are trying to optimize. In fact,
unequally-loaded cells due to non-uniform users distribution and cell
breathing in W-CDMA networks are based on cells with different sizes, and
thus on irregular cell grid principle.
The analytical calculations performed are very long and not
straightforward even after simplification to one interferer. Here, the
calculations have not been done directly considering multiple interferers
because the sum distribution is not as trivial as it seems. The f-parameter PDF
expression with shadowing has also been established using convolution
product and has been plotted by simulations and differentiated with that
without shadowing in various propagation environments. Shadow correlation
has also been taken into account.
Maan Al-Adwany et al (2011) have proposed interference canceller.
In W-CDMA base station receiver, the BER can be considerably reduced by
using the proposed interference canceller. Initially, they have extracted the
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TDMA interference through the use of a low pass FIR filter. They have
constructed a threshold circuit to eliminate the residual W-CDMA signal that
passes with the extracted TDMA signal. Finally, they have also evaluated the
performance of W-CDMA uplink system for UMTS mobile communications.
The idea their proposed interference canceller is to extract the
TDMA interference through the use of a low pass FIR filter which is matched
to the TDMA bandwidth (2.048 MHz), and then subtract the extracted TDMA
signal from the received composed signal (TDMA+W-CDMA). Hence, it is
possible to get a W-CDMA signal with approximately no TDMA
interference.
The threshold circuit is used to eliminate the residual W-CDMA
signal which passes with the extracted TDMA signal. The filter has a cutoff
frequency of 2.048 MHz which is matched to the TDMA system bandwidth.
Also, when designing the filter, the group delay of the filter is to be taken into
consideration. It must not have a large group delay (group delay= {filter
order – 1}/2). In their design, they select a reasonable filter order of seven;
which gives a group delay of three sample periods.
Since the signal will suffer a group delay of three sample periods
when it passes through the designed FIR filter, this delay must be
compensated by adding a delay line (z-3) for the purpose of signals alignment.
Also, it is worthwhile to mention that all the delays and their compensations
are taken into account throughout the simulation.
Muhammad Suryanegara et al (2006) have examined the
interference on W-CDMA system that caused by CDMA2000 network. The
have used several scenarios and calculation models were applied to measure
impacts on uplink and downlink scheme. Their results showed that
interference from CDMA2000 influence W-CDMA performance
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significantly. Distance between coexistence terminals (MS and BS), their
number, and guard band frequency determined minimum allowed received
power at W-CDMA BS and SIR in W-CDMA MS, which lead to capacity
degradation.
They have examined the effect of adjacent channel interference on
the W-CDMA system uplink and downlink scheme. The have used 3G
coexistence network, the presence of CDMA2000 system influenced W-
CDMA as a environment to check the performance. They have shown that the
interference generated by CDMA2000 MS increased the minimum allowed
received power at W-CDMA BS. It leads to reduction of W-CDMA coverage
and cause a significant capacity loss. In downlink scheme, they have shown
that the interference due to CDMA2000 BS influenced the performance of W-
CDMA communication system which can be seen from significant reduction
of SIR value. The over all outcome of their experiment is effect of
interference depends on several factors including distance between W-CDMA
and CDMA2000 terminals (BS and MS), number of CDMA2000 MS,
interference power, and guard band frequency. They have used a guard band
to reduce the effect of interference.
2.5 GAP ANALYSIS
In 2002, Huan Chen et al (2002) have proposed a call admission
control (CAC) scheme which gives preferential treatment to high priority
calls, such as and off calls, by pre-reserving a certain amount of channel
margin against the interference effect. This is the interference guard margin
(IGM) scheme. In the dynamic IGM scheme, the resource reservation module
is used to dynamically reserve an interference margin for the use of potential
high priority calls by referencing the traffic condition and mobile users’
traffic profile in neighboring cells. Their fixed and dynamic IGM schemes
outperform the non-priority scheme in the overall objective function.
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In 2007, Young-Long Chen et al (2007) have proposed a novel
approach which combines the CAC and power control mechanisms and
operates in a centralized control manner. Their approach outperforms
conventional call admission methods both in terms of its blocking rate and its
outage rate. The main benefit of their proposed CCAC method is that it takes
the entire link conditions into account such that their minimum SIR
requirements are maintained when a new call is admitted.
In 2006, Malarkkan et al (2006) have analysed Fuzzy based Call
admission control scheme. The fuzzy approach can overcome measurement
errors; mobility and traffic model uncertainty, and avoid the requirements of
complex mathematical relations among various design parameters. The fuzzy
CAC scheme can achieve QoS satisfaction in terms of the outage probability,
and achieve lower new call blocking probability, lower handoff call dropping
probability when compared with the SIR based CAC schemes without fuzzy.
In 2007, Salman AlQahtani et al (2007) have proposed and analyzed
an efficient uplink-scheduling scheme in case of Radio Access Network
(RAN) sharing method. Their proposed adaptive rate MCDGPS scheduling
scheme improves both system throughput and average delays.
In 2002, Siamak Naghian et al (2002) have proposed a novel
dynamic step size power control method to improve the performance of
UMTS/W-CDMA cellular system. In their method, the multi-edge threshold
concept, dynamic step-size, power-control commands/steps-based history data
have been used. Their proposed dynamic step-size power control method can
be more efficient than a conventional one due to its capability to deal flexibly
with both slow and fast fading of the transmission signal of the W-CDMA
system. Also they have utilized the location information jointly to combat
transmit power deterioration.
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In 2004, Dejan Drajic (2004) has formulated some adaptive
technique to enhance power control technique in W-CDMA. They have
shown that the power control and channel coding complement each other
when mitigating the effects of fading. In CDMA system power control has a
strong effect on the interference experienced by the receiver, and hence it its
performance is directly affected.
In 2011, Subba Rao et al (2011) have proposed an adaptive power
control mechanism for multimedia traffic in W-CDMA networks. Their
power control algorithm reduces the power consumption of multimedia
traffic. They introduce a new parameter, Power Determining Factor (PDF)
based on the data traffic rate to determine the power. Based on this parameter,
they are determining whether the power is increasing or decreasing.
Depending on the traffic rate, the PDF factor is updated.
In 2012, Ch. Sreenivasa Rao et al (2012) have proposed a power
controlled call admission control scheme for handoff in the advanced wireless
networks. In their scheme, the hybrid priority queuing mechanisms for call
requests and efficient scheduling a power controlling mechanism have been
used in the handoff process. Their power control technique provides efficient
handoff in the 4G networks by not degrading the QoS parameters like
throughput and delay.
In 2007, Derong Liu et al (2007) have presented a simple
interference cancellation technique for the downlink of wideband code
division multiple-access (W-CDMA) systems in multipath environment. Their
method acts as an equalizer and cancels the interfering multipath signals from
the received signal to retrieve the orthogonal property of the received signal.
Their algorithm is not complex and does not consume much time since each
information bit can be detected after the interfering chip cancellation is done
in a bit interval. The simplicity of their algorithm with the perfect cancellation
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of MAI comes at the expense of noise enhancement. Although this noise
enhancement causes a loss of few dBs in SNR compared to the single user
system, the performance of the receiver, as the simulation results show, is
much higher than the RAKE receiver and is free from any error floor.
In 2011, Pon Rattanawichai et al (2011) have presented the FPGA
(Field Programmable Gate Array) implementation of self interference
cancellation technique based on an adaptive LMS (Least Mean Square)
algorithm. Their algorithm achieves a high efficiency and the similar
performance in comparison with the implementation result. By the FPGA
implementation, their designed technique provided more flexibility than
existing methods.
In 2011, Maan Al-Adwany et al (2011) have proposed interference
canceller. They have been shown that the W-CDMA and TDMA systems can
work in the same cell and hence, it is possible to increase the cell capacity as
the problem of cross interference between the two systems can be reduced
using the proposed interference canceller. In W-CDMA base station receiver,
the BER can be considerably reduced by using the proposed interference
canceller. On the other hand, in TDMA base station receiver, the effect of W-
CDMA interferers can be reduced by increasing the transmitted power of the
TDMA mobile station.
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