Modeling and Dimensioning of Mobile Networks: from GSM to LTE

Maciej Stasiak, Mariusz Głąbowski Arkadiusz Wiśniewski, Piotr Zwierzykowski Modeling and Dimensioning of Mobile Networks: from GSM to LTE

GSM

Maciej Stasiak, Mariusz Głąbowski Arkadiusz Wiśniewski, Piotr Zwierzykowski 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 GSM system – introduction 2/3

Feature/Bandwidth GSM 900 GSM 1800

Uplink (MHz) 890-915 1710-1785

Downlink (MHz) 935-960 1805-1880

Number of available 124 374 channels GSM system – introduction 3/3

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 GSM system – architecture 2/6

• 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 GSM system – architecture 3/6

• 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) GSM system – architecture 4/6

GSM system – architecture 5/6

• 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 GSM system – architecture 6/6

• 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 – time structure 2/3

GSM system – time structure 3/3

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 GSM system – logical channels 2/4

GSM system – logical channels 3/4

• 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 GSM system – logical channels 4/4

• 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 High Speed Circuit Switched Data 2/4

• 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 High Speed Circuit Switched Data 3/4

Data rate performance in radio TCH/F4.8 TCH/F9.6 TCH/F14.4 interface [kbps] 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 High Speed Circuit Switched Data 4/4

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 GPRS Packet Transmission 2/6

• 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 GPRS Packet Transmission 3/6

• 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 GPRS Packet Transmission 4/6

• 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 GPRS Packet Transmission 5/6

• 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 GPRS Packet Transmission 6/6

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