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Maciej Stasiak, Mariusz Głąbowski Arkadiusz Wiśniewski, Piotr Zwierzykowski

Modeling and Dimensioning of Mobile Networks: from GSM to LTE

Maciej Stasiak, Mariusz Głąbowski Arkadiusz Wiśniewski, Piotr Zwierzykowski

GSM

Modeling and Dimensioning of Mobile Networks: from GSM to LTE

GSM system – introduction 1/3

• GSM – Global System for Mobile Communication

• Operates in 900 and 1800 MHz

• Access to the radio link is based on frequency division

multiple access (FDMA) and time division multiple

access (TDMA)

• Each band available for the system is divided into

channels with bandwidth of 200 kHz

• For the GSM 900 system there are 124 available

channels (separate for the uplink and downlink

direction), and for the GSM 1800 374 channels


Feature/Bandwidth GSM 900 GSM 1800

Uplink (MHz) 890-915 1710-1785

Downlink (MHz) 935-960 1805-1880

Number of available channels

124 374

GSM system – architecture 1/6

• In the GSM system three basic subsystems can be

distinguished: o base station subsystem (BSS)

o core network (CN)

o user equipment (UE)

• Between particular elements of the system the

interfaces are defined


• Base station subsystem – includes system of base stations and their controllers

• Base station provide optimum radio coverage of a given area and communicates with user equipment over air interface

• The operation of the base station subsystem is controlled by the base station controller (BSC)

• This manages radio resources allocation, controlls the setting-up of calls, gathers results of measurements carried out by base station and mobile station

• The BSC is also responisble for power controll and handover controll

• Interface A enables the BSS system to be connected to mobile switching center (MSC)

• Interface Gb connects BSS with packet switching element


• The main elements of the core network are: o mobile switching center (MSC)

o visitor’s location register (VLR)

o home location register (HLR)

o authentication ceter (AUC)

o equipment identification register (EIR)

o serving GPRS support node (SGSN)

o gateway GPRS support node (GGSN)


• MSC basic task is to control and regulate services

provided by the system, circuit switching, and

gathering billing information

• VLR registry keeps information concerning mobile

stations available in the area of one, or several, MSC

switching centers

• SGSN is the equivalent of the MSC switching center of

packet switching

• GGSN is an interface between the mobile packet

network and external packet networks


• HLR is a central database that contains details of each

mobile phone subscriber authorized to use the GSM

core network and includes authorization data

• AUC generates sets of keys used in encryption of

transmission, identifies the mobile station and the

network, and controls and regulates the integrity of

transmitted data

• EIR is a data base that keeps a list of numbers

identifying a given mobile station – IMEI (International

Mobile Equipment Identity)

GSM system – time structure 1/3

• In the GSM system each carrier frequency is divided

into eight time slots

• Packet transmission is commenced every 4.615 ms and

single bit lasts 3.69 µs

• Typical packet, except access packet, has 148 bits,

thus its duration is about 546 µs

• The duration of a single time slot is 577 µs, wich allows

for maintaining a steady interval between successively

transmitted packets

GSM system – logical channels 1/4

• Logical channels can be divided into two categories: o control channels – are used to set up a connection in the radio network for

transmission of control data

o traffic channels – are used to transmit user data

• In the GSM system, speech signals are transmitted with

traffic channels (TCHs). Speech can be transmitted at

full rate, 13 kbps, or at half rate, 6.5 kbps


• Frequency correction channel (FCCH) – used by the mobile station to tune to the carrier frequency, a frequency correction burst is transmitted on the channel by generating unmodulated sine waves

• Synchronization channel (SCH) – transmits base station identity code (BSIC), which allows the mobile station to identify the base station and to convey synchronization information

• Broadcast control channel (BCCH) – used for transmission of control information such as: radio channel frequency used by a given cell, neighbor cell list, information on the paging channel, configuration of logical channels in the base station

• Paging channel (PCH) – with the PCH the base station initiates a connection with the mobile station


• Random access channel (RACH) – used by mobile station for initial

access to a system (with the RACH the mobile station initiates a

connection with base station)

• Access grant channel (AGCH) – used by the base station to assign resources to a mobile station requesting access to the network

• Stand alone dedicated control channel (SDCCH) – used to

provide a reliable connection for signaling and SMS messages, for

authentication, and to provide information on location update

• Slow associated control channel (SACCH) – supports the SDCCH

channel, used for sending network measurement reports and

information related to power control procedures

• Fast associated control channel (FACCH) – coupled with the speech channel, used for immediate transmission of information

related to, for example, cell handover

High Speed Circuit Switched Data 1/4

• High Speed Circuit Switched Data is an additional feature of the GSM network and was introduced in phase 2

• HSCSD technology enables a simultaneous application of several speech channels for a single data transmission link

• A connection can be set up that makes use simultaneously of n channels (time slots) in the radio interface, where n takes on the values n = 1; 2; ...; 8

• A HSCSD connection can be set up only when the mobile station is capable of using several radio channels simultaneously

• Additional modifications are needed in the BSS system that involve multiplexing of a component data stream in one 64 kbps channel of A interface


• A HSCSD connection can have a symmetrical configuration – the same number of speech channels is allocated for the uplink and the downlink direction – or a non-symmetrical configuration

• A non-symmetrical configuration is chosen when the subscriber requirements cannot be accomplished in a symmetrical configuration

• The maximum link transmission speed that can be achieved by the HSCSD technology depends on the number of channels used in the radio interface and on the applied coding

• Due to a necessity of transmitting all channels included in a HSCSD connection in one link 64 kbps in interface A, the transmission speed is limited to 57.6 kbps


Data rate performance in radio interface [kbps]

TCH/F4.8 TCH/F9.6 TCH/F14.4

4.8 kbps 1 N/A N/A

9.6 kbps 2 1 N/A

14.4 kbps 3 N/A 1

19.2 kbps 4 2 N/A

28.8 kbps N/A 3 2

38.4 kbps N/A 4 N/A

43.2 kbps N/A N/A 3

57.6 kbps N/A N/A 4

GPRS Packet Transmission 1/6

• The implementation of packet transmission in the GSM

system requires changes in the structure of the system

• New elements are included in the network – SGSN and

GGSN nodes

• According to the assumptions given in 3GPP

specification, GPRS should allow: o pulse data transmission in which time interval between individual moments of

transmission is considerably higher than the average transmission delay

o frequent transmission (several times per minute) of small amount of data (bursty

data transfer up to 500 octets)

o occasional transmission of large volumes of data


• The GPRS technology makes it possible to transmit data

in several channels. Within one packet connection, the

mobile station as well as the base station can make

simultaneous use of 8 time slots in a frame

• All users of a packet service can share resources

available for data transmission that are allocated, due

to the asymmetry of traffic, separately for the uplink

and for the downlink direction

• The radio interface resources can be dynamically

shared by speech service and packet data transmission

depending on the configuration of the network


• Following logical channels, responsible for transmission

of data and signalling information are introduced: o Packet Common Control Channel (PCCCH)

• Packet Random Access Channel (PRACH) – used by the mobile station to

initiate uplink transfer of user data or signalling information

• Packet Paging Channel (PPCH) – used to page a mobile station preceding

downlink direction packet transfer. The channel can also be used for

establishing speech connections

• Packet Access Grant Channel (PAGCH) – used in the packet transfer

establishment phase to send resource assignment to a mobile station

preceding packet transfer

• Packet Notification Channel (PNCH) – used to send point to multipoint

multicast notification information to a group of mobile stations preceding

multicast packet transfer


• Following logical channels, responsible for transmission

of data and signalling information are introduced: o Packet Broadcast Control Channel (PBCCH) – used to broadcast packet

system information

o Packet Data Traffic Channel (PDTCH) – allocated for user data transfer. Several

PDTCH channels can be allocated to a given mobile station. They can be

allocated temporarily to one or more mobile stations

o Packet Associated Control Channel (PACCH) – used to transmit signalling

information related to a given mobile station, such as those related to power

control or packet reception acknowledgement messages

o Packet Timing Advance Control Channel, Uplink (PTCCH/U) – used to ensure

that the correct timing advance is maintained for each mobile station. In the

uplink direction, the channel is used by a mobile station to send an access burst

o Packet Timing Advance Control Channel, Downlink (PTCCH/D) – used to send

packets in the downlink direction to assess the needed timing advance in order

to achieve frame synchronization


• Four coding schemes (CS) have been defined for the

GPRS transmission: CS-1, CS-2, CS-3 and CS-4

• Particular schemes are characterized by different user

data transmission speed and by a various degree of

error protection procedures

• CS-1 coding, due to its highest level of protection (error

correction), is used for channels with highest

interference and for signalling channels

• CS-4 coding enables the fastest data transmission

speed – 21,4 kbps for one channel, but has no

protection


Coding scheme Coding efficiency Data rate [kbps]

CS-1 1/2 9.05

CS-2 2/3 13.4

CS-3 3/4 15.6

CS-4 1 21.4

EDGE Packet Trasnmission 1/2

• Voice transmission in the GSM system and the GPRS packet

transmission uses the Gaussian minimum shift keying (GMSK)

modulation. This is a binary modulation with speed of 270.833

kbps

• EDGE technology achives higher speed value with the same

bandwidth of the radio channel thanks multivalue, eight-

level phase shift keying modulation (8PSK)

• EDGE uses nine coding schemes: MCS-1 – MCS-9

• Each coding scheme is characterized by a different data

transmission speed and data protection

• Transmission speed for one PDTCH channel with MCS-1

coding is 8.8 kbps, for MCS-9 coding scheme is 59.2 kbps

EDGE Packet Trasnmission 2/2

Coding scheme Coding efficiency Modulation Data rate [kbps]

MCS-9 1.0 8PSK 59.2

MCS-8 0.92 8PSK 54.4

MCS-7 0.76 8PSK 44.8

MCS-6 0.49 8PSK 29.6 27.2

MCS-5 0.37 8PSK 22.4

MCS-4 1.0 GMSK 17.6

MCS-3 0.85 GMSK 14.8 13.6

MCS-2 0.66 GMSK 11.2

MCS-1 0.53 GMSK 8.8

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