Code Division Multiple Access Cdma And Interference Computer Science Essay

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Mobile wireless communication has increased in popularity because it simplifies and revolutionizes communication. The success of mobile communications lies in the ability to provide instant connectivity anytime and anywhere and the ability to provide high-speed data services to the mobile user. Large number of users of various requirements and service demands appears as everything converging and also increases congestion and load imbalance scenarios in wireless cellular networks. Therefore, to meet the ever increasing and diverse user demands, an assumed large bandwidth third generation (3G) wireless networks have been designed to support heterogeneous mobile services ranging from high quality voice services, video on demand, inter-active computing and Internet services.

Universal Mobile Telecommunication System (UMTS) is a third generation mobile network designed for multimedia communication which enables person-to-person communication with high quality images and video. The Wideband Code Division Multiple Access (WCDMA) technology has been chosen as the air interface access mechanism in 3G networks. With 3G mobile network based on WCDMA, users (or MSs) can occupy the entire allocated frequency and time domain. Handovers are a key concept in providing the mobility. It makes it possible for a user to travel from one cell to another, without interruption or having a seamless connection. WCDMA applies at the radio air interface offers improved connection maintenance during the handover. Generally a handover is performed when the quality of the link between the base station and the mobile terminal on the move is decreasing.

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Cell breathing is a mechanism that attempts to keep the forward and reverse link handoff boundaries balanced by changing the forward link coverage according to the changes in the reverse link interference level. Reverse link handoff boundary is defined as the contour of mobile locations between neighboring cells where the received signal to noise ratios at the two base stations is the same. [1]

This paper focuses on the study of Cell Breathing in WCDMA Cellular Networks. The base station cell coverage may expand or shrink depending on the number of mobile station being served. To predict the cell coverage for the environment chosen the COST HATA model is used in this study. The actual received signal strength indicator (RSSI) obtained by conducting drive test measurement are compared to the received signal code power (RSCP) of the calculated using the propagation model. The error calculated in the measurements enables to determine the minimum discrepancy between RSSI and RSCP data. The data obtained from drive test will be analyzed using Matlab.

2. PROBLEM STATEMENT

Interference is a major factor that limits the coverage area of Code Division Multiple Access (CDMA) based networks and as a consequence the capacity in terms of density of Mobile Stations (MS) in a Cell. In order to keep the interference to a minimum it is important to have a precise and fast power control Users that are further away from Node-B (BTS in case of CDMA2000) have to transit signal with more power than those closer to Node-B, as the signal gets weaker the further it has to travel.

An increase in interference normally occurs when multiple users enters coverage area of a cell. This necessitates that the existing users at the edge of the cell to further increase their transmit power. If the UEs at the edge (border users) do not have the ability to transmit at a power that will overcome the noise floor, the users will be disconnected. As a consequence, the coverage area of the cell is reduced, as most distant users (border users) cannot communicate with the Node-B. This phenomenon is called Cell Breathing. This behaviour is particularly pronounced in CDMA networks and increases the complexity of network design.

In cellular network such as Wideband Code Division Multiple Access (WCDMA) cell breathing is the technique that can overcome the congestion load by changing the size of coverage cell either expanding or shrinking. This paper determines the cell breathing margin of urban environment in WCDMA. The cell boundaries of base station may expand or shrink when the number of mobile station is small or high.

3. RESEARCH OBJECTIVES

The objectives of this project are:

To analyze the cell breathing margin and coverage of a base station.

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To analyze the factors that can affect cell breathing such as drop calls, number of soft handover and cell coverage.

To evaluate the different between RSSI data from the drive test and calculated RSCP for both areas.

To study the implementation of COST HATA model to predict the coverage signal.

4. SCOPE AND LIMITATIONS OF STUDY

The analysis only based on one network operator.

This project would focus on the outdoor propagation model by using the theoretical semi-empirical path loss model approach.

The project only considers the downlink frequency (from the base station to mobile station).

The drive test is held in two different times and two different locations.

No repetition of drive test

5. LITERATURE REVIEW

Figure 1 : Cell Breathing Concept

Figure 1 shows the concept of cell breathing. Cell breathing is the constant change of the range of the geographical area covered by a cellular telephone transmitter based on the amount of traffic currently using that transmitter. When a cell becomes heavily loaded, it shrinks. Subscriber traffic is then redirected to a neighbouring cell that is more lightly loaded, which is called load balancing. The coverage versus plot graph below shows the example of the decreasing coverage whenever there is an increased of capacity.

Figure 2 : Coverage Versus Capacity Graph

Figure 3 : Urban Environment Coverage Concept

In an urban area especially at high level building environment, signal will reach through multipath propagation or reflection but it will be a poor quality signal. Mobile receive level will be good because there will be many signal come through but the quality will be poor because most of the signal quality is weak (pilot pollution). Call will often drop at this kind of environment.

Cell coverage can be defined by Ec/N0 which is the energy per chip over the noise, a measure of the quality of the signal. When Ec/N0 below target, there is no channel estimation and no call setup. Interference increases when traffic increases. The Received Signal Code Power is the received power on one DPCH, PRACH or PUSCH code after de-spreading, defined on the pilot symbols or in other words, it is the energy after processing. Cell coverage becomes smaller when traffic increases for the cell to breathe. RSSI is the Receive Signal Strength Indicator which is a circuit to measure the strength of an incoming signal or in other word it is a dB measure of the signal arriving at the antenna before gain from processing.

Several previous researchers adapt the concept of cell breathing in Wireless LANs. For example [3] applied cell breathing technique to balance the load between clients and access points (APs) by controlling the transmitted power of an APs in the network. They approved that cell breathing technique makes an AP available to reconfigure the cell boundaries by changing the transmitted power. On the other hand, the increasing and decreasing of transmitted power will affect the coverage size of AP.

The publication of cell breathing in Universal Mobile Telecommunication System (UMTS) is very limited. Nevertheless Kimmo Valkealahti, Albert Höglund, Jyrki Parkkinen, and Ari Hämäläinen from Nokia Research Center have studied about the cell breathing concept use in WCDMA radio network. The pilot energy-per-chip-to-total-wideband-interference-density ratio or Ec/Io is the main parameter involve in this research paper. They believe that the value change of common pilot power can improve the operability of the network and it is implemented with control software aiming for load and coverage balancing [4].

Another research concerning cell breathing concept was done by Jyoti Laxmi Mishra, Keshav. P. Dahal and M.A. Hossain. They focus on Call Admission Control using Cell Breathing Concept for Wideband CDMA. According to this paper, a current user in a call admission is affected when a new call is accepted due to cell breathing. They made the call admission decision by considering the CDMA uplink power, from mobile station (MS) to base station (BS) and discovered that due to frequency reuse, cell size is continuously reduced. This increases the number of handoffs of MS from one cell to another [5], [6].

6. METHODOLOGY

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Universiti Teknologi Mara (UiTM) has been chosen as the selected area for this project.

Drive test is done in two different times which is in the morning and afternoon to determine the cell breathing margin of a base station.