25-02-2013, 10:39 AM
Coexistence of Real Time and Best Effort Services in Enhanced Uplink WCDMA
Coexistence of Real Time.pdf (Size: 1.12 MB / Downloads: 17)
Abstract
The increasing use of data services and the importance of IP based services in
third generation mobile communication system (3G), requires the transmission
from the cell phone to the base station, i.e. uplink, to manage high speed data
rates. In the air interface for 3G in Europe, WCDMA, a concept for enhancing
the transmission from the cell phone to the base station, called Enhanced Uplink,
is being standardized. The overall goal is to provide high speed data access for
the uplink. One of the requirements is that the enhanced uplink channels must be
able to coexist with already existingWCDMA releases. For example, the enhanced
uplink must not impact seriously on real time services, such as speech, carried on
current WCDMA channels.
The purpose of this work is to study how the quality, coverage and capacity
of real time services carried on previous WCDMA releases is affected when introducing
the Enhanced Uplink in a WCDMA network. The main focus of the study
is thus to demonstrate the trade-off between voice and best effort performances.
Theoretical assessments and simulations show that the Enhanced Uplink has
many advantages over previous WCDMA releases. For example the Enhanced
Uplink yields a larger system throughput for all voice loads. The noise rise, i.e.
the ratio of total received power to the background noise power is being considered
as the resource. It is shown that user traffic carried on the Enhanced Uplink is
able to operate under a higher noise rise level as well as to get a higher throughput
per noise rise. The resource is hence more efficiently utilized.
Introduction
The introduction of the third generation mobile communication system (3G) opened
up new doors in wireless communication. The possibilities of transmitting all kinds
of data between cellular phones has increased enormously during the last decade.
The air interface standard for 3G in Europe, called WCDMA, makes it possible
to use the cellular phone for web surfing, e-mailing, interactive gaming, video
streaming and receiving several other data services like multimedia, video-clips
and pictures.
However, new features create new demands on the communication system.
In the WCDMA specifications a concept called high speed downlink packet access
(HSDPA) has been evolved. This concept makes it possible to transmit high speed
data from the base station to the cell phone, i.e. downlink.
The increasing use of data services and the importance of IP based services
also requires the transmission from the cell phone to the base station, i.e. uplink,
to manage high speed data rates. The standardization body for WCDMA is the
3rd generation partnership (3GPP). Within 3GPP a concept for enhancing the
transmission from the cell phone to the base station, called Enhanced Uplink
(EUL), is being standardized. The overall goal is to provide high speed data
access also for the uplink. One of the requirements that has been agreed upon
within 3GPP is that the enhanced uplink channels must be able to coexist with
already existing WCDMA releases. For example, the enhanced uplink must not
impact seriously on real time services, such as speech, carried on current WCDMA
channels.
Problem Statement
The master thesis assignment is to study how the quality, coverage and capacity
of real time services carried on previous WCDMA releases is affected when introducing
the Enhanced Uplink in a WCDMA network. Coverage is the geographical
area within which a user can connect to the mobile network with acceptable quality.
Capacity is the number of users or the total amount of data bits per time
instant that can be supported by the system.
Related Work
As far as we know, no previous work has been performed on evaluating simultaneous
voice and Enhanced Uplink users. However, a large amount of work has been
done on performance measures and evaluations of CDMA systems. The following
section will give an outline of such previous related work.
Quality is closely connected to both capacity and coverage measures. Capacity
is most often measured as a maximum throughput while still keeping the quality
requirements fulfilled, and coverage as a geographical area within which the quality
requirements fulfilled. Therefore no specific section for the quality measure is
given.
Capacity
Several publications deal with the problem of estimating the system capacity,
e.g. [8] and [19] consider an integrated voice/data WCDMA system. Both define
capacity as the maximum amount of users for which the probability that they are
satisfied is greater than some certain percentage. In the latter a user is defined
satisfied when the probability that the bit error rate is below some threshold, is
greater than some value. Since the bit error rate is directly related to the received
SIR, the satisfaction measure can also be formulated as the probability that the
received SIR is greater than some threshold. The capacity region, in terms of
number of users, is obtained for an integrated speech and long constraint delay
data system.
This measure is also exemplified in [8], however a more general method is
introduced. Furthermore the capacity is evaluated and optimized. It is shown
that the total system capacity is maximized when the per-service capacities for
all bearer services are equal. It is also shown that there is a linear relationship
between speech capacity and interactive data capacity.
Another common measure for data capacity is the system throughput, i.e. the
maximum possible transmission data rate (often in kbps) as a function of received
interference power or noise rise. This measure is used in [5] to evaluate the uplink
capacity gain in a WCDMA network due to faster scheduling. A capacity gain of
approximately 10% is shown, simply by reducing the packet scheduling interval
from 500 ms to 100 ms.
Coverage
A method of calculating the inter and intra cell interference in a UMTS system is
discussed in [18]. The interference is calculated by solving a system of fixed-point
equations, using an iteration algorithm. Furthermore, the result is used to reckon
the cell coverage area as the distance within which the outage probability is less
than a certain percentage.
An analytical approach to determine coverage probabilities are given in [17].
An algorithm is used where the interference at all cells, the transmission powers
of all mobiles and the probability function of received powers at the base stations
are calculated. From this the coverage probability for a given mobile is reckoned.
Another analytical approach is described in [14]. The uplink coverage is investigated
in a UMTS system under non-homogenous and moving traffic load. In
particular the coverage is investigated for different call assignment policies and
for a hot spot moving among the cells. Inter cell interference is also taken into
account. Some numerical results to verify the analysis are also given.
Third Generation Mobile
Communication System
The standard for third generation mobile communication system in Europe is
referred to as Universal Mobile Telecommunication Services (UMTS), adopted
by the International Telecommunication Union (ITU). The air interface used in
UMTS is Wideband Code Division Multiple Access (WCDMA). The following
sections will give an overview of WCDMA for UMTS and it’s evolution to the
Enhanced Uplink concept. More about UMTS can also be found in [6], [1] and
[10] and the basic principles of wireless communications are described in [3].
UMTS Network Architecture
The UMTS network consists basically of a core network, Radio Network Controllers
(RNC), base stations (Node B) and user equipments (UE). Figure 2.1
shows a schematic picture of the UMTS network and it’s elements.
The core network is the connection to an external network such as the internet
or the ordinary fixed telephony system. The UE can be a mobile phone or a
computer card. The RNC and Node B constitute the Radio Access Network
(RAN), or the connection between the UE and the core network.
Each RNC controls a number of Node Bs. The actual radio signal is transmitted
and received by a Node B. Each Node B supports one or a number of cells covering
a geographical area. If the antenna for example is of a three sector type, the Node
B consists of three cells. Each UE within a cell area is connected to a certain Node
B. The cells normally intersect near the cell borders, and UEs positioned in this
area are connected to more than one Node B.