Curs 1 3G Core Network

3G Core Network Engineeringby René ANDREESCU

Curs 1

3G Core Network Engineering by René ANDREESCU

Why a third generation?


• The goal of the UMTS (universal mobile telecommunications system) is to deliver multimedia services to the user in the mobile domain. UMTS and multimedia services have a significant impact not only on the RF (radio frequency) network, but also on the core network architecture. Care must be taken to allow current GSM (global system mobile) operators to protect their infrastructure investments when their networks are upgraded to support UMTS.


Network Architecture


RAN Interfaces


Frequency Allocation






The core network handles call control and mobility management functionalities, while the UTRAN (UMTS terrestrial radio access network) manages the

radio packet transmission and resource management.Packet routing and transfer within the core network are supported by definition of new

logical network nodes called GGSN (gateway GPRS [general packet radio system] support node)

and SGSN (serving GPRS support node).

The GGSN is basically a packet router with additional mobility management features, and it connects with various network elements through standardized interfaces.

The GGSN acts as a physical interface to the external packet data networks (e.g., the Internet).

The SGSN handles packet delivery to and from mobile terminals.

Each SGSN is responsible for delivering packets to the terminals within its service area. GGSN and SGSN are capable of supporting terminal data rates up to 2 Mbps.


A UTRAN consists of one or more RNSs (radio network subsystems), which in turn consist of base stations (Node Bs) and RNCs (radio network controllers).

The RNS performs all of the radio resource and air interface management functionalities. The UMTS network architecture inherits most of its structure from the GSM model in the UTRAN.


Spectrum of Two WCDMA Carriers with 5 MHz Channel Spacing (Unlike GSM and TDMA, the same carriers can be used in all of the cells. Thus, the reuse factor for this

system is N = 1, while the GSM reuse factor is typically N = 4.)


WCDMA Carrier


• Duplex method: FDD/TDD• Channel spacing: 5 MHz• Carrier chip rate: 3.84 Mcps• Time slot structure: 15 slots/frame• Frame length: 10 ms• Modulation: QPSK• Detection: based on pilot symbols• Intra-frequency Handover: soft• Inter-frequency Handover: hard• Spreading factors:

Up-link: 4..256Down-link: 4..512

FDD


Used Modulations


In CDMA new channels are created by assigning more spreading codes. As long as the interference is low enough, we can open up a new channel for communication.


Spread Spectrum Techniques


Direct-Sequence – Spread Spectrum ( DSSS)


8 bit code8 bit code

DataData

Data x CodeData x Code

emissionemission

receivereceiveCodeCode

Data x CodeData x Code

Principles

3G Core Network Engineering

by René ANDREESCU

8-bit Code8-bit Code

Received signalReceived signal

Signal x CodeSignal x Code

Valid 1 Valid 0

IntegrationIntegration

Valid receiver


by René ANDREESCU

Received signalReceived signal

Other codeOther code

Signal x codeSignal x code

integrationintegration

Invalid receiver

In practice: Gold code 32 bits


by René ANDREESCU


x=1+X2+X5

Gold code generator basis





If we want to exploit the multi-path channel, the despreading becomes a bit more complicated ...

... but we gain frequency diversity.



• The jamming gain (J/C) tells us how much stronger a jamming signal can be, compared to the wanted signal:

This expression gives us a simple way of calculating how many users we can have in our system, if we regard the other users as jammers.

QUICK EXAMPLE:Assuming a spreading factor M=512 and an optimal processing gain of Gp=M, and a required (Eb/N0) of 10 dB for proper reception, we get

Hence, we can have 51 other users (with their own spreading codes and equal power) in our system.


• The jamming margin gives us a conservative measure on the number of users, since it assumes that we do not use any advanced detection scheme ... only despreading of each user and detection.

• Since a base-station has knowledge about the spreading codes of all users in a cell, it can detect all users jointly and thereby perform interference cancellation. This is called multi-user detection and requires high processing power of the base station.


• Since users in a cell are separated by codes, and transmit simultaneously in the same frequency band, we can use the same frequency band in all cells in a cellular system.

• An advantage of CDMA is that the establishment of new “channels” can be done as long as the interference is kept below a certain level. This gives a flexibility which we do not have in FDMA and CDMA.

• Another advantage of CDMA is that we can establish channels with

different spreading factors, allowing different data rates.


• The available radio resource is shared among users in a multiple access scheme.

• When we apply a cellular structure, we can reuse the same channel again after a certain distance.

• In cellular systems the limiting factor is interference.

• For FDMA and TDMA the tolerance against interference determines the possible cluster size and thereby the amount of resources available in each cell.

• For CDMA systems, we use cluster size one, and the number of users depends on code properties and the capacity to perform interference cancellation (multi-user detection).


Codes and their use


CDMA Equation

Important terms when talking about spread spectrum are the so-called spreading factor SF and the spreading gain SG.

SF describes the ratio of the information data rate (represented by the bit duration Tbit ) to the rate of the spreading code (represented by the chip duration Tchip )


Let us denote the signal level before despreading thechip energy to interference ratio Ec/I [dB]

and the signal level after despreading the bit energy to interference ratio

Eb/I [dB].

Than Ec/I [dB], Eb/I [dB] and SG are related by:

CDMA Equation


The factor OF [dB] describes the degree of orthogonalitybetween wanted user signal and interference signal.

The orthogonality factor (OF) for e.g. Gaussian noise equals 0 dB. Therefore, in a Gaussian noise environment the wanted user signal level is increased by an amount of SG dB.

CDMA Orthogonality


The reference sensitivity is the minimum receiver input power measured at the antenna port at which the bit error rate (BER) does not exceed a value of 10-3 .

This testcase determines the tolerable noise figure of the receiver front end.

The cumulative value of the incoming signal power is -106,7 dBm.

The wanted user signal level before despreading Ec/I is -117dBm

The reference channel is a 30 ksps channel which yields an SF of 128.

Reference Sensitivity Level Testcase


.

From the calculated link budget, the cell range R can be easily calculated for a known propagation model, for example the Okumura-Hata or the Walfish-Ikegami model.

In this case the Okumura-Hata model is used, because it offers better results for urban areas. The propagation model describes the average signal propagation in that environment, and it converts the maximum allowed propagation loss in dB to the maximum cell range in kilometers.

As an example we can take the Okumura-Hata propataion model for an urban macro cell with base station antenna height of 30m, mobile antenna height of 1.5m and carrier frequncy of 1950 MHz:

Propagation Model


.

Simulating the MORANS scenario (Mobile Radio Access Network reference Scenarios ) a second model can be used. The MORANS scenario offers path-loss information in a 25m grid to every base station. The model which was used to calculate the path loss matrices is a modified COST-Hata model:

MORANS Propagation Model

d Distance between receiver and transmitter in kmHeff Effective height of transmitter in mDiff Diffraction value according to DeygoutClutter Clutter dependent correction factor


.

The RLB as described previously does not offer the possibility to calculate the allowed path-loss for users with different services (speech, data, ...). That can be done using the load factor, which is calculated for uplink and downlink in different ways.

The term service mix means that several service profiles (so called ”bearers”) are defined, and the service mix is defined as the percentage of the subscribers who use these separate bearers. Table shows examples for these bearers.

Integration of Service Mix


.

The uplink load factor can be written as

As in Eqution can be seen, the service mix is not included. It can beintegrated by making a linear combination of the separate parameters,weighted by that leads to Equation.

Uplink Load Factor


.

Parameters used in uplink load factor calculation


.

The downlink load factor can be written as

As in Eqution can be seen, the service mix is not included. Therefor the individual services are integrated by linear combination, where is the percentage of subscribers, which ones are using carrier k. This relationship is shown in Equation.

Downlink Load Factor


.

Parameters used in downlink load factor calculation


.

The next Table shows an example for the extended RLB. In this example two groups, which use different carriers are presented. The parameters for the individual bearers can be found in Table of slide nr. 20

RLB and Service Mix

Integrating the load factor in the RLB can be done in the same way for up- and downlink using the following equation:


Cell Search


Open loop Power Control ( PC )



WCDMA Power Control


Gain of fast Power Control in WCDMA


Rx and Tx power versus interference levels


WCDMA Power Control in Soft Handover


Softer Handover


Soft Handover


Soft Handover with Iur connection

Date post:	23-Nov-2014
Category:	Documents
Upload:	mihai-rotaru
View:	121 times
Download:	2 times

Curs 1 3G Core Network

Documents