Second Generation Telecommunication Systems enabled voice traffic to go wireless. The data handling capabilities of second generation systems are limited, however, and third generation systems are needed to provide high bit rate services, capacity and other new services.
One of the third generation systems is referred as UMTS. WCDMA (Wideband Division Multiple Access) is the air interface for the UMTS system.
This report focuses on studying WCDMA system`s capacity, accomplished through simulation of the similar network.
functions are analyzed and final in chapter eight the simulation results are presented.
Introduction
The UMTS (Universal Mobile Telecommunications System) is for Europe a new system for mobile third generation (3G) and one of the 3G-standards that exist worldwide. This model has universal application, as it had and the second GSM mobile generation. The draft on the part of America is the CDMA-2000.[3]
This new system was discovered in order to become more efficient the exploitation of band species and in order to have higher data rates for the requirements of the users. During the same period that appeared the above requirements, we have the "explosion" of a second technology, the Internet, which requires high transmission rates. So manufacturers are beginning to seek a new standard that will be able to meet the new requirements. Two more reasons behind the search for a new system was the need for greater security against threats coming from outsider users «hackers», and the desire to create a single global standard, which ultimately aim was not achieved. In the case of mobile second-generation, GSM has the dominant hand, but many countries like Japan, a large part of America and other countries use competitive to GSM systems. So in 1992 the ITU (International Telecommunications Union), acknowledged the band around 2GHz, as the area where they could operate the third mobile generation. The mobile systems were reported in the third generation terminology as ITU IMT-2000 (International Mobile Telecommunications at 2000MHz). The role, therefore of a new generation of mobile is to create a structure that meets the needs of the time. And this can be achieved with the UMTS. UMTS adopts a new network segment, the UTRAN (UMTS Terrestrial Radio Access Network), whose technology is differentiated in relation to the GSM system and through it satisfies the first 2 requirements, efficient use of frequencies and faster data transfer.
The IMT-2000 consists of 4 essential systems and 2 technologies. These systems are;
• UMTS (UTRA-FDD και UTRA-TDD)
• CDMA2000
• DECT
• UWC-136 (EDGE)
The 2 technologies are;
 TDMA και CDMA
relevant to the multiplex in the time and scope of the Code, respectively.
The figure below shows the allocation of frequencies where third generation mobile can operate across regions.
Figure 1; Distribution of frequencies for third generation mobile.
As we can see the shape, Europe has chosen to focus on the terrestrial part of UMTS network (UMTS Terrestrial Radio Access Network - UTRAN);
• For FDD operation;
the frequency range 1920 - 1980 MHz for the upper link (paired).
the frequency range 2110 - 2170 MHz on the link below (paired).
• For TDD operation;
the frequency range 1900 - 1920 MHz (unpaired)
the frequency range 2010 - 2025 MHz (unpaired)
II. The UMTS Network
The term UMTS comes from the acronym for "Universal Mobile Telecommunications System" (UMTS). These developments in relation to capacity, speed data transmission and the availability of new services, consists the second generation of mobile networks. Today, more than sixty 3G/UMTS networks using WCDMA technology operating in 25 countries. The organization of the whole enterprise has established a special non-profit organization called Third Generation Partnership Project (3GPP) whose task is to monitor and steer developments in this technology area. Among the advantages of UMTS network it's distinguished the increased rate of transmission of data and simultaneous support for larger volumes of data and voice.In particular, the UMTS network in its initialphase offers theoretical data rates up to 384kbps in cases where there is increased mobility of the user. Conversely, when the user remains stationary, transmission rates increase significantly, reaching a value of 2 Mbps.
Figure 2; data rate transmission for various systems
It is estimated that in the future will be a further increase in the rate of data transmission. Already, the 3GPP standard has been set two new technologies. This is the High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), respectively. These technologies are essentially a development of UMTS, since it promises data transmission rates up to 14, 4 Mbps down-link (DL-downlink) and 5.8 Mbps on the upper link (UL-uplink).
Key features of WCDMA technology
The WCDMA (Wideband Code Division Multiple Access), was adopted as the most common wireless interface for third generation mobile. The requirements established by the 3GPP, in which telecom operators from Europe, Japan, Korea, America and China participate.
The main parameters of the WCDMA radio interface are;
- The WCDMA is a wideband Direct-Sequence Code Division Multiple Access (DS-CDMA) system, i.e. the bits of user information spread across a wide frequency range, multiplied by pseudo-random bits (called chips), and derived from respective codes of CDMA.
- The chip rate of 3,84 Mbps bandwidth demands of the carrier around 5 MHz The DS-CDMA systems with a bandwidth of 1MHz, as the system IS-95, often referred to as CDMA systems narrow bandwidth. The wide range of WCDMA supports high user data rates. The system may use multiple carriers of 5 MHz, to increase capacity. The distance of interest can be selected at 200 kHz between 4.4 and 5 MHz, depending on the interpolation between interests.
- The variable WCDMA supports high user data rates, in other words, the principle of bandwidth on demand (BOD) is supported adequately. Each user is within the duration of 10 ms, during which the data rate is kept stable. However, the capacity of data can change from context to context. Addressing the capacity is determined by the network to achieve maximum efficiency in data packet services.
- The WCDMA system supports two modes; Frequency division duplex (Frequency Division Duplex FDD) and time division duplex (Time Division Duplex TDD). The FDD technique, separate carriers used 5 MHz frequency for directions up and down link. The TDD technique is based on principles of FDD and was added to increase the performance of WCDMA base system.
Figure 3; Representation of FDD and TDD operation.
ï· In WCDMA mode supports asynchronous base stations, so unlike the IS-95 does not require the existence of a time reference signal, such as GPS.
ï· The WCDMA uses line scan in both directions up and down link. Although the use of consistent detection in the lower link has been made in the IS-95, but the use and direction over the coupling is expected to increase capacity and coverage in this direction.
ï· The WCDMA radio interface is designed so that advanced methods of making such smart, adaptive antennas can be used by the network administrator as a selection system to increase the coverage and / or capacity. Most second-generation systems, there is no provision for this with the result that such scenarios are applicable or used under significant restrictions, with limited potential for increased efficiency.
ï· The WCDMA has been designed to operate in conjunction with GSM. So "handovers" between GSM and WCDMA support in order to increase the efficiency of GSM coverage for the introduction of WCDMA.
The following figure includes services that can be served using the WCDMA system
Figure 4; Illustration of services covering the WCDMA system
IV.UMTS architecture
A UMTS network consists of 3 fields;
1. User Equipment (UE)
2. UMTS Terrestrial Radio Access Network (UTRAN)
3. Core Network (CN)
Fields UE and UTRAN consist of new protocols, which were designed with the needs of WCDMA.
In contrast, the CN is adopted from GSM. Below is the description of any field.
4.1 User's Equipment (UE)
The UE consists of 2 parts;
a) Mobile equipment (ME); A terminal used for radio. This takes place via the Uu interface, which is characterized by very high speeds (above 2Mbps).
b) UMTS Subscriber Identity Module (USIM);
This is a smart card containing the subscriber identity, performs algorithms certification, providing certification and encryption keys as well as some information necessary assistance.
V.UTRAN Network
The UTRAN network consists of 1 or more subnets RNS (Radio Network Sub-systems). An RNS is a subnetwork consisting of a base station controller RNC (Radio Network Controller) and a group of base stations (Node Bs). The RNS connected via the interface Iur, while base stations via the Iub.
RNC
The functions performed by an RNC are;
o (Radio Resource Control)
o (Admission Control)
o (Channel Allocation)
o (Power Control Settings)
o (Handover Control)
o (Macro Diversity)
o Encryption (Ciphering)
o (Segmentation / Reassembly)
o (Broadcast Signaling)
o (Open Loop Power Control)
There are three roles for the RNC to the base stations and mobile terminals.
1.For each base station is a Controlling RNC (CRNC) and it is this which leads the Iub interface .It is responsible for load controlling and congestion of the cells and carry out entry and distribution of the code to each new user who wants to connect to these cells.
Figure 5; CRNC
If a mobile connection - UTRAN uses information from more than one RNS, then RNCs involved have 2 different roles.
2. The Serving RNC (SRNC), which is unique for each UE, is the one who terminates the Iu interface for transmitting user information with the rest of the network, as well as the Radio Resource Control Signaling, which is the signaling protocol between the UE and UTRAN. . Performs the operation of Layer 2, but still made him the most important functions of the RRM, as decision for handover, power control of outer loop and correspondence of Radio Access Bearer with the corresponding transmission channels.
3. The Drift RNC (DRNC) is any other RNC other than the SRNC that controls the cells in which the user is located. Used in the case of handover. It doesn't perform functions or other Layer 2 functions of the SRNC, but it transfers data to other RNCS and Node-Bs, via the interface Iur and Iub.
Figure 6; SRNC and DRNC
NΟDE B
The base station of the network is called UMTS Node B and is responsible for the following functions;
o (Air interface Transmission / Reception)
o (Modulation / Demodulation)
o (CDMA Physical Channel coding)
o (Micro Diversity)
o (Error Handling)
o ΄ (Closed loop power control)
(Core Network-CN)
The CN is divided into 2 areas;
1. Field of switching circuit: Provides links to circuit switched services, such as existing telephone. ISDN and PSTN are examples of such services.
2. Field of packet switching: Provides links for switched packet services.
In the field of circuit switching are the following;
• Mobile services Switching Center (MSC); the switch serving for the user's equipment (UE) to its current location for circuit switched services (CS).
• Visitor location registers (VLR); is a database like the MSC serving the user's equipment (UE). The VLR function holds a copy of the profile of service users are visiting, as well as more accurate position information of the UE will assist in the system. Part of the network that penetrates through the MSC / VLR is often referred to as the CS domain (ownership CS).
• Gateway MSC; the switching point where UMTS PLMN interfaces with external circuit switched networks (external CS networks). All incoming and outgoing circuit switched connections go through GMSC.
While in the field of packet switching are as follows;
• GPRS Support Node (SGSN); typically used for Packet Switched Services (Packet Switched). To place the network accessed via the SGSN is often referred to as PS domain (ownership PS).
• Gateway GPRS Support Node (GGSN): The operation is similar to that of GMSC but in terms of Packet Switched services.
Some network elements like EIR, HLR, VLR and AUC are in 2 fields.
• The HLR (Home Location Register) is a database installed with the user's system that stores the master copy of the service user profile. The service profile consists, for example, from information on allowed services, forbidden roaming areas and information of additional services such as call forwarding and call forwarding number. Created when a new user is subscribed to the system and remain stored for as long as the subscription is active. For the purpose of routing incoming executions to the UE (call or SMS), the HLR also stores the position of the UE at the MSC / VLR and / or SGSN.
VI. WCDMA
Figure 7; Communication between two mobile terminals
The operation of the WCDMA is based on the deployment of process - recovery range.
We consider user data as a sequence bit, even if s (t), is modulated by BPSK and rate R. In the process of spreading each bit of user data is multiplied by a sequence of k bits, even sc (t).The last sequence is called spread code, and the bits of it are called chips. The period of chips, Tc, is considerably smaller than that of the user data bit Tb. The number of chips per user's bit is called spreading factor (SF). The data obtained from the multiplication are k-rate framework and exhibit the same random form as the spreading code. Thus we achieve spread spectrum by a factor k. The spread signal is the transmitted signal inside the wireless medium. The figure below shows the proliferation of user data bits (blue color), the sequence of chips (green) and the outcome of the process, i.e. the spread spectrum signal of the user.
Figure 8; Multiply user's bits with code spread
Figure 9; Illustration of the spread spectrum signal of the user
At this point I should mention that the rate of 3, 84 Mcps is chips and leads to a bandwidth of 5MHz.
On the side of the receiver, the signal obtained is multiplied by the same spreading code and under conditions of synchronization (the transmitted signal - the spread code) retrieve the original signal. The figure below describes the procedure performed on both side of the transmitter (spreading), and toward the receiver (dispreading).
Figure 11; Example of deployment and recovery of the signal of user by the SF = 8
Then the receiver completes every bit of the retrieved signal. The result of this process is a triangular pulse with minimum and maximum values-k and + k, respectively.
The WCDMA is designed to accommodate various services rates. At this point I observed that the processing gain decreases with increasing the bit rate of user data. Here are 4 examples of transmission rates with those agents spread.
• 16kbps , SF=256
• 32kbps , SF=128
• 64kbps , SF=64
• 128kbps , SF=32
VII. Simulation of WCDMA system
The scope of simulation;
The simulation of a WCDMA network is undoubtedly a useful tool that gives us information about how the network responds when operating under certain circumstances and according to specifications. I will examine the capacity of a system that uses conventional 3-sector antennas.
Flowchart system uses conventional antennas is figured below.
First we create a system of cells whose centers are located base stations. Then enter one to one user in the system and calculate the losses spread to every base station. Based on the losses we identify the station area which serves the user. Next step is to calculate the gain up the input of the user of the antennas in all sectors. Finally, we consider the criteria for accepting the connection and if the user is admitted to the system we apply a closed-loop power control, to the users of the central cell. This process is repeated several times. For the end we take the number of users that the system will serve.
NO
VIII.Conclusion
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