generations of cellular network

Generations of Cellular Network

Mobile Networks, EC-425

Cellular System Overview

Operation of Cellular Systems


Operation of Cellular Systems•Base Station (BS) – includes an antenna, a controller, and a number of receivers

– The controller is used to handle the call process between the mobile unit and the rest of the network

•Mobile telecommunications switching office (MTSO) – connects calls between mobile units•Two types of channels available between mobile unit and BS

– Control channels – used to exchange information having to do with setting up and maintaining calls

– Traffic channels – carry voice or data connection between users


Steps in an MTSO

Controlled Call between

Mobile Users


Steps in an MTSO Controlled Call between Mobile Users•Mobile unit initialization

– mobile unit turned on• scans and selects the strongest setup control channel used for this system

– Cells with different frequency bands repetitively broadcast on different setup channels (strongest signal and then BS antenna selected)

– Then a handshake takes place between the mobile unit and the MTSO controlling this cell, through the BS in this cell to identify the user and register its location

– As long as the mobile unit is on, this scanning procedure is repeated periodically to account for the motion of the unit.


Steps in an MTSO Controlled Call between Mobile Users•Mobile-originated call

– A call is originated by sending the number of the called unit on the preselected setup channel

– checks that the setup channel is idle by examining information in the forward (from the BS) channel.

– When an idle is detected, the mobile unit may transmit on the corresponding reverse (to BS) channel.

– The BS sends the request to the MTSO.


Steps in an MTSO Controlled Call between Mobile Users•Paging

– The MTSO sends a paging message to certain BSs depending on the called mobile unit number.

– Each BS transmits the paging signal on its own assigned setup channel.

•Call accepted– The called mobile unit recognizes its number on the setup channel

being monitored and responds to that BS, which sends the response to the MTSO.


Steps in an MTSO Controlled Call between Mobile Users•Ongoing call

– the two mobile units exchange voice or data signals, going through their respective BSs and the MTSO

•Handoff– the traffic channel has to change to one assigned to the BS in the

new cell

Second-Generation

• The second-generation (2G) mobile cellular systems use digital radio transmission for traffic

GSM D-AMPS (IS-136) CDMA (IS-95) PDC

2G o Global System for Mobile (GSM)o Digital- AMPS (D-AMPS)o Code Division Multiple Access

(CDMA)o Personal Digital Cellular (PDC)

Second-Generation

Key differences between 1st and 2nd Generation•Digital traffic channels

– Support digital data; voice traffic is first encoded in digital form before transmitting

• But the user traffic (data or digitized voice) must be converted to an analog signal for transmission between the mobile unit and the base station (air interface is analog)

•Encryption– it is a relatively simple matter to encrypt all of the traffic

to prevent eavesdropping because all of the user traffic, as well as control traffic, is digitized.

Second-Generation

• Error detection and correction– Very clear voice reception

• Channel access– each channel is dynamically shared by a number of users using

TDMA or CDMA

Global System for Mobile Communications (GSM)

• Developed to provide a common second-generation technology for Europe – the same subscriber units could be used throughout the continent

• The most popular standard, worldwide, for new implementations

• GSM first appeared in 1990 in Europe

GSM Network ArchitectureMobile Station (MS)•A MS communicates across the Urn interface (air interface) with BTS•Mobile Equipment includes:

• radio transceiver• digital signal

processors• subscriber identity

module (SIM)

GSM Network Architecture

• SIM is a portable device in the form of a smart card or plug-in module that stores • the subscriber's identification number,

• the networks the subscriber is authorized to use,

• encryption keys

• other information specific to the subscriber

• International Mobile Subscriber Identity (IMSI)– The IMSI is how the subscriber is identified to the network.

– Uniquely identifies the subscriber within the GSM global network.

– The IMSI is burned into the SIM card when the subscriber registers with PLMN service provider.

GSM Network Architecture• The IMSI is composed of three parts:• Mobile Country Code (MCC) - This number identifies which country the

subscriber's network is in. It has 3 digits.

Mobile Network Code (MNC) - This number identifies the home GSM PLMN of the subscriber (Cingular, T-Mobile, etc.). It has 2 or 3 digits. Some networks may have more than one MNC allocated to it.

Mobile Subscriber Identification Number (MSIN) - This number uniquely identifies a user within the home GSM network.

• International Mobile Equipment Identity (IMEI)– The IMEI uniquely identifies the Mobile Equipment itself. It is

essentially a serial number that is burned into the phone by the manufacturer. The IMEI is composed of three parts:

GSM Network Architecture• Type Allocation Code (TAC), 8 digits - This number uniquely

identifies the model of a wireless device. It is composed of 8 digits.

Serial Number (SNR), 6 digits - This number is a manufacturer defined serial number for the model of wireless device.

Spare (SP), 1 digit This number is a check digit known as a Luhn Check Digit. It is omitted during transmission within the GSM network.

On many devices the IMEI number can be retrieved by entering *#06#


Base Station Subsystem•BSS consists of:

– base station controller (BSC)

– base transceiver stations (BTS)

•BTS – defines a single cell, it includes:

• radio antenna

• radio transceiver (The GSM Recommendations allow for one BTS to host up to 16 TRXs. In the field, the majority of the BTSs host between one and four TRXs.)

• a link to a BSC.

– A GSM cell can have a radius of between 100 m and 35 km, depending on the environment.


• BSC– from a technical perspective, a small digital exchange with some

mobile-specific extensions

– The BSC was defined with the intention of removing most of the radio-related load from the MSC

– may be collocated with a BTS or may control multiple BTS units and hence multiple cells

– The BSC reserves: • radio frequencies

• manages the handoff of a mobile unit from one cell to another within the BSS

• controls paging.


Network Subsystem (NSS)•Handles Switching of calls between external

networks and the BSCs in the radio subsystem

• Responsible for managing and providing

external access to several customer databases.

•The central element of the NSS is the

mobile switching centre (MSC).

•MSC–It is supported by four databases that it controls:

• Home location register (HLR) database

• Visitor location register (VLR) database

• Authentication centre database (AuC)

• Equipment identity register database (EIR)

GSM Network Architecture• Home location register (HLR) database:

– The HLR stores information, both permanent and temporary, about each of the subscribers that "belongs" to it (i.e., for which the subscriber has its telephone number associated with the switching centre).

• Visitor location register (VLR) database: – One important, temporary piece of information is the location of the subscriber.

– Temporary stores IMSI and customer information for subscriber (subscriber information) who is visiting the coverage area of particular MSC.

– VLR is linked between several adjoining MSCs in particular geographic region

– For a call coming to the subscriber, the system uses the telephone number associated with the subscriber to identify the home switching center of the subscriber. This switching center can find in its HLR the switching center in which the subscriber is currently physically located.

– For a call coming from the subscriber, the VLR is used to initiate the call. Even if the subscriber is in the area covered by its home switching center, it is also represented in the switching center's VLR, for consistency.

GSM Network Architecture• Authentication center database (AuC):

– used for authentication activities, holds encryption keys

• Equipment identity register database (EIR) – keeps track of the type of equipment that exists at the mobile station

– It also plays a role in security (e.g., blocking calls from stolen mobile stations and preventing use of the network by stations that have not been approved).


Radio Link Aspects


Radio Link Aspects

•The GSM spectral allocation is 25 MHz for base transmission (935-960 MHz)- downlink and 25 MHz for mobile transmission (890-915 MHz)- uplink.

•There are radio-frequency carriers every 200 kHz, which provide for 125 full-duplex channels.•The channels are modulated at a data rate of 270.833 kbps•The ARFCN is a number that describes a pair of frequencies, one uplink and one downlink


Radio Link Aspects•Up = 890.0 + (ARFCN * .2) Down = Up + 45.0Example: Given the ARFCN 72, and we know the offset is 45MHz for the GSM900 band: Up = 890.0 + (72 * .2) = 890.0 + (14.4) Up = 904.40 MHz

Down = Up + Offset = 904.40 + 45.0 Down = 949.40 MHz

The uplink/downlink pair for GSM900 ARFCN72 is 904.40/949.40 (MHz)


TDMA

Format


TDMA Format•Trail bits – allow synchronization of transmissions from mobile units•Encrypted bits – encrypted data•Stealing bit - indicates whether block contains data or is "stolen"•Training sequence – used to adapt parameters of receiver to the current path propagation characteristics

– Strongest signal selected in case of multipath propagation

•Guard bits – used to avoid overlapping with other bursts

Note: Follow william stalling book as well

GSM Channel Types

• Two Types of Logical Channels

– Traffic Channels (TCH)• Carries encoded user speech and user data and have identical

functions and format on forward and reverse link.

– Control Channels (CCH)• Carry sync and signalling command between BS and MS• Certain control channels are defined only for forward or reverse link.

GSM Channel Types

TCH (traffic)

CCH(control)

BCH

CCCH

Dedicated

2.4 kbps4.8 kbps

9.6 kbps

FCCH(Frequency correction)

SCH(Synchronization)

PCH(Paging)

RACH(Random Access)

AGCH(Access Grant)

SDCCH(Stand Alone)

SACCH(Slow-associated)

FACCH(Fast-associated)

Half rate 11.4kbps

Full rate 22.8kbps

Speech

Data

GSM Channel Types

• Traffic Channels (TCH)– Full rate ------ data within one TS per frame– Half rate ------ same time slot but sent in alternate frames– Traffic channel may not be in TS0 (serves as broadcast

station)

GSM Channel Types

• Full rate (FR) ------ data within one TS per frame– TCH/FS ------- The FR speech channel carries user speech

which is digitized at raw data rate of 13 kbps. Channel coding + digitized speech = FR speech channel carries 22.8 kbps.

– (TCH/F9.6)/(TCH/F4.8)/(TCH/F2.4)• carries raw user data which is sent at 9.6kbps (FEC coding

22.8kbps)• 4.8kbps (22.8kbps)• 2.4kbps (22.8 kbps)

GSM Channel Types

• Half rate (HR) TCH – TCH/HS ------- carry digitized speech at a rate half that of

FR. (carry 11.4kbps)

– (TCH/H4.8)/(TCH/H2.4)• carries raw user data which is sent at 4.8kbps (FEC coding

11.4kbps)• 2.4kbps (11.4 kbps)

GSM Channel Types

• Control Channels (CCH)– Broadcast Channels (BCH)

• Implemented only on certain ARFCN channels • Allocated timeslots in a very specific manners (TS0 of certain frames)

– Other timeslots available for TCH data or DCCH data– All eight frames on other AFCNs within the cell are available for TCH or DCCH

• Used forward link just• Provide sync for all MS inside the cell• Occasionally monitored by mobiles in neighbouring cells (received

power and MAHO decisions) • Broadcast Control Channel (BCCH)• Frequency Correction Channel (FCCH)• Synchronization Channel (SCH)

GSM Channel Types

• BCCH– Broadcast info such as

• cell and network identity• Operating characteristics of the cell

– Current control channels structure– Channel availability– Congestion

• Broadcast list of channels that are currently in use

• Note: Read rest of the channels detail from Rappaport book by yourself.

•

Speech coding

Speech coding

• The speech signal is compressed using an algorithm known as Regular Pulse Excited - Linear Predictive Coder– data from previous samples are used to predict the current

sample

2nd Generation CDMA (IS-95)• Transmission is in the form of DS-SS

– Use chipping code to increase data rate of transmission– Multiple access by assigning orthogonal chipping codes

Advantages of CDMA Cellular• Frequency diversity – frequency-dependent transmission

impairments (noise burst and selective fading) have less effect on signal

• Multipath resistance – chipping codes used for CDMA not only exhibit low cross correlation but also low autocorrelation

• Privacy – privacy is inherent since spread spectrum is obtained by use of noise-like signals

• Graceful degradation – system only gradually degrades to the point of an unacceptable error rate as more users access the system (noise level and error rate increases)

Drawbacks of CDMA Cellular

• Self-jamming – arriving transmissions from multiple users not aligned on chip boundaries unless users are perfectly synchronized

• Near-far problem – signals closer to the receiver are received with less attenuation than signals farther away.– Given the lack of complete orthogonality, the transmissions from

the more remote mobile units may be more difficult to recover.

– power control techniques are very important in a CDMA system.

• Soft handoff – requires that the mobile acquires the new cell before it relinquishes the old; this is more complex than hard handoff used in FDMA and TDMA schemes

Mobile Wireless CDMA Design Considerations

• RAKE receiver – when multiple versions of a signal arrive more than one chip interval apart, RAKE receiver attempts to recover signals from multiple paths and combine them– This method achieves better performance than simply recovering

dominant signal and treating remaining signals as noise


• Soft Handoff – mobile station temporarily connected to more than one base station simultaneously

Types of Channels Supported by Forward Link

• Pilot (channel 0) - allows the mobile unit to acquire timing information, provides phase reference and provides means for signal strength comparison

• Synchronization (channel 32) - used by mobile station to obtain identification information about cellular system (system time, long code state, protocol revision, etc.)

• Paging (channels 1 to 7) - contain messages for one or more mobile stations

• Traffic (channels 8 to 31 and 33 to 63) – the forward channel supports 55 traffic channels

Forward Traffic Channel Processing Steps

• Speech is encoded at a rate of 8550 bps• Additional bits added for error detection• Data transmitted in 20-ms blocks with forward error

correction provided by a convolutional encoder• Data interleaved in blocks to reduce effects of errors• Data bits are scrambled, serving as a privacy mask

Forward Traffic Channel Processing Steps (cont.)

• Power control information inserted into traffic channel• DS-SS function spreads the 19.2 kbps to a rate of 1.2288

Mbps using one row of 64 x 64 Walsh matrix• Digital bit stream modulated onto the carrier using QPSK

modulation scheme

Note: Must follow book William Stalling for reverse traffic channels and for more understanding

2.5G (GSM)• “Generation 2.5” is a designation that broadly includes all advanced

upgrades for the 2G networks• Generally, a 2.5G GSM system includes at least one of the following

technologies: high-speed circuit-switched data (HSCSD), General Packet Radio Services (GPRS), and Enhanced Data Rates for Global Evolution (EDGE).

2.5G: GPRS Architecture (1)

SGSN: Serving GPRS Support NodeGGSN: Gateway GPRS Support Node

GPRS Mobile Stations:New Mobile Station are required to use GPRS packet data services.

2.5G: GPRS Architecture (11)GPRS Base Station Subsystem:

•Each BSC requires the installation of one or more Packet Control Units (PCUs) and a software upgrade.

•The PCU provides a physical and logical data interface to the BSS for packet data traffic.

•The BTS can also require a software upgrade but typically does not require hardware enhancements.

•At the output of the BSC, the traffic is separated; voice is sent to the MSC per standard GSM, and data is sent to a new device called the SGSN via the PCU over a Frame Relay interface.

GPRS Support Nodes:

Following two new components, called GPRS support nodes (GSNs), are added:

Gateway GPRS support node (GGSN):

•GGSN acts as interface between the GPRS backbone and the external packet data network (PDN).

•Converts the GPRS packet coming from the SGSN into proper packet data protocol (PDP) format (i.e. X.25 or IP) before sending to the outside data network.

•Similarly it converts the external PDP addresses to the GSM address of the destination user. It sends these packets to proper SGSN. For this purpose the GGSN stores the current SGSN address of the user and his profile in its location register.

•Also performs the authentication and charging functions.

Serving GPRS support node (SGSN):

•The Serving GPRS Support Node is responsible for authentication of GPRS mobiles, registration of mobiles in the network, mobility management, and collecting information for charging for the use of the air interface.

3rd Generationprovide

fairly high speed wireless communications to support multimedia, data, and video in

addition to voice

ITU’s View of Third-Generation Capabilities

• Voice quality comparable to the public switched telephone network

• 144 kbps data rate available to users in high-speed motor vehicles over large areas

• 384 kbps available to pedestrians standing or moving slowly over small areas

• Support for 2.048 Mbps for office use• Symmetrical / asymmetrical data transmission rates• Support for both packet switched and circuit switched data

services

ITU’s View of Third-Generation Capabilities

• An adaptive interface to the Internet to reflect efficiently the common asymmetry between inbound and outbound traffic

• More efficient use of the available spectrum in general• Support for a wide variety of mobile equipment• Flexibility to allow the introduction of new services and

technologies

Alternative Interfaces

3rd Generation (3G)• 3G is created by ITU-T and is called IMT-2000.• The ITU accepted the following proposals as IMT-2000 compatible

– IMT Multicarrier (IMT-MC; also known as CDMA2000)– IMT Direct Spread (IMT-DS; also known as UTRA FDD);– IMT Time Code (IMT-TC; also known as UTRA-TDD/TD-SCDMA

“narrowband TDD”)– IMT Single Carrier (IMT-SC; also known as UWC-136)– IMT Frequency Time (IMT-FT; also known as DECT)

CDMA Design Considerations• Bandwidth – limit channel usage to 5 MHz• Chip rate – depends on desired data rate, need for error

control, and bandwidth limitations; 3 Mcps or more is reasonable

• Multirate – advantage is that the system can flexibly support multiple simultaneous applications from a given user and can efficiently use available capacity by only providing the capacity required for each service

CDMA2000 lx EV-DO• Also known as CDMA/HDR (CDMA high data rate) and as

IS-856 (1st 3G deployed)• To offer near-broadband packet data speeds for wireless

access to the internet• The Ix prefix refers to its use of Ix (1 times) the 1.2288

Mcps spreading rate of a standard IS-95 CDMA channel• Geared toward the use of IP for packet transmission and for

Internet access.

CDMA2000 lx EV-DO• A well-engineered lxEV-DO network delivers average

download data rates of between 600 kbps and 1.2 Mbps during off-peak hours

• between 150 kbps and 300 kbps during peak hours• Instantaneous data rates are as high as 2.4 Mbps

CDMA2000 lx EV-DO• The lxEV-DO design focuses on integration with IP-

based networks

CDMA2000 lx EV-DO• Because it is data-only, the transmission scheme can be

optimized for data transfer and need not support voice requirements– voice frames are short, typically on the order of 20 ms, in order to

minimize delays. • use of short frames increases overhead, resulting in reduced efficiency

– In a data-only network, longer average delays can be tolerated, and QoS facilities can be used to accommodate transmissions that require tight delay values.

• a data-only network can use longer frames, reducing overhead.

CDMA2000 lx EV-DO• In a typical data-only application, the amount of traffic from

the network to the user significantly exceeds user-to-network traffic– To optimize throughput and make the best use of the available

bandwidth, 1xEV-DO sends and receives at different data rates. • Download (forward channel) rates vary from 38.4 kbps to 2.4576 Mbps and

• upload (reverse channel) rates vary from 9.6 kbps to 156.3 kbps.

CDMA2000 lx EV-DO• A major difference in a data-only design as compared to a

voice-optimized– Voice-optimized systems use power control (path loss and/or

fading, close to the base station reduce power)

– In contrast, 1xEV-DO alters the data rate rather than the power when signal levels change (forward and reverse channels).

3G: UMTS

• Universal Mobile Telecommunications System (UMTS)• UMTS is an upgrade from GSM via GPRS or EDGE• The standardization work for UMTS is carried out by Third Generation

Partnership Project (3GPP)• Data rates of UMTS are:

– 144 kbps for rural– 384 kbps for urban outdoor– 2048 kbps for indoor and low range outdoor

• UMTS Band – 1900-2025 MHz and 2110-2200 MHz for 3G transmission– In the US, 1710–1755 MHz and 2110–2155 MHz will be used instead,

as the 1900 MHz band was already used.

UMTS Architecture

UMTS Network Architecture• UMTS network architecture consists of three domains

– Core Network (CN): Provide switching, routing and transit for user traffic– UMTS Terrestrial Radio Access Network (UTRAN): Provides the air

interface access method for user equipment.– User Equipment (UE): Terminals work as air interface counterpart for

base stations.

UTRANUTRAN• Wide band CDMA technology is selected for UTRAN air interface

– WCDMA• Base stations are referred to as Node-B and control equipment for Node-B

is called as Radio Network Controller (RNC).

UMTS Network Architecture: UTRAN

WCDMA•WCDMA is a wideband Direct-Sequence Code Division Multiple Access (DS-CDMA)system, i.e. user information bits are spread over a wide bandwidth by multiplying the user data with quasi-random bits (called chips) derived from CDMA spreading codes.•The chip rate of 3.84 Mcps leads to a carrier bandwidth of approximately 5 MHz which is wider than IS-95 (CDMA), that’s why “Wide”.

Functions of Node-B are:• Air Interface Tx/Rx• Modulation/Demodulation

Functions of RNC are:• Radio Resource Control• Channel Allocation• Power Control Settings• Handover Control• Ciphering• Segmentation and reassembly

generations of cellular network

Technology

mobile unit number

mobile users mobileoriginated

mobile communications

mobile users ongoing

g mobile cellular systems

bts mobile equipment

mobile network code

mobile units exchange