Mobile Handset Cellular NetworkCellular Network Basics There are many types of cellular services; before delving into details, focus on basics (helps navigate the “acronym soup”) Cellular network/telephony is a radio-based technology; radio waves are electromagnetic waves that antennas propagate Most signals are in the 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz frequency bandsCell phones operate in this frequency range (note the logarithmic scale)Cellular Network Base stations transmit to and receive from mobiles at the assigned spectrum Multiple base stations use the same spectrum (spectral reuse) The service area of each base station is called a cell Each mobile terminal is typically served by the closest base stations Handoff when terminals moveCellular Network Generations It is useful to think of cellular Network/telephony in terms of generations: 0G: Briefcase-size mobile radio telephones 1G: Analog cellular telephony 2G: Digital cellular telephony 3G: High-speed digital cellular telephony (including video telephony) 4G: IP-based “anytime, anywhere” voice, data, and multimedia telephony at faster data rates than 3G (to be deployed in 20122015)Evolution of Cellular Networks1G2G3G4G2.5GThe Multiple Access Problem The base stations need to serve many mobile terminals at the same time (both downlink and uplink) All mobiles in the cell need to transmit to the base station Interference among different senders and receivers So we need multiple access schemeMultiple Access Schemes Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA)3 orthogonal Schemes:Frequency Division Multiple AccessEach mobile is assigned a separate frequency channel for the duration of the call Sufficient guard band is required to prevent adjacent channel interference Usually, mobile terminals will have one downlink frequency band and one uplink frequency band Different cellular network protocols use different frequencies Frequency is a precious and scare resource. We are running out of it Cognitive radiofrequencyTime Division Multiple AccessTime is divided into slots and only one mobile terminal transmits during each slot Like during the lecture, only one can talk, but others may take the floor in turn Each user is given a specific slot. No competition in cellular network Unlike Carrier Sensing Multiple Access (CSMA) in WiFiGuard time signal transmitted by mobile terminals at different locations do no arrive at the base station at the same timeCode Division Multiple Access Use of orthogonal codes to separate different transmissions Each symbol of bit is transmitted as a larger number of bits using the user specific code Spreading Bandwidth occupied by the signal is much larger than the information transmission rate But all users use the same frequency band togetherOrthogonal among users2G(GSM)GSM Abbreviation for Global System for Mobile Communications Concurrent development in USA and Europe in the 1980s The European system was called GSM and deployed in the early 1990sGSM Services Voice, 3.1 kHz Short Message Service (SMS) 1985 GSM standard that allows messages of at most 160 chars. (incl. spaces) to be sent between handsets and other stations Over 2.4 billion people use it; multi-billion $ industry General Packet Radio Service (GPRS) GSM upgrade that provides IP-based packet data transmission up to 114 kbps Users can “simultaneously” make calls and send data GPRS provides “always on” Internet access and the Multimedia Messaging Service (MMS) whereby users can send rich text, audio, video messages to each other Performance degrades as number of users increase GPRS is an example of 2.5G telephony 2G service similar to 3GGSM Channels Physical Channel: Each timeslot on a carrier is referred to as a physical channel Logical Channel: Variety of information is transmitted between the MS and BTS. Different types of logical channels: Traffic channel Control ChannelDownlinkUplinkChannelsGSM Frequencies Originally designed on 900MHz range, now also available on 800MHz, 1800MHz and 1900 MHz ranges. Separate Uplink and Downlink frequencies One example channel on the 1800 MHz frequency band, where RF carriers are space every 200 MHz1710 MHz1880 MHz1805 MHz1785 MHzUPLINK FREQUENCIESDOWNLINK FREQUENCIESUPLINK AND DOWNLINK FREQUENCY SEPARATED BY 95MHZGSM ArchitectureMobile Station (MS) MS is the users handset and has two parts Mobile Equipment Radio equipment User interface Processing capability and memory required for various tasks Call signalling Encryption SMS Equipment IMEI number Subscriber Identity ModuleSubscriber Identity Module A small smart card Encryption codes needed to identify the subscriber Subscriber IMSI number Subscribers own information (telephone directory) Third party applications (banking etc.)