Cellular Wireless Networks

1. Cellular Wireless Networks

2. Introduction • Cellular technology is the underlying technology for most mobile wireless communications and data transfers • Main concept: Use of multiple low-power transceivers • An area is divided into cells, each of which are served by a base station, consisting of transmitter, receiver, and a control unit • Adjacent cells are assigned different frequencies to avoid interferences and crosstalk. Cells at sufficient distance from each other can use the same frequency bands • The cells are hexagonal in shape • Each cell has a base transceiver – the transmission power is carefully controlled to avoid interferences with neighboring cells • About 10-50 frequencies are assigned to each cell

3. Cellular Geometries • Handout

4. Frequency Reuse • How many cells must intervene between two cells using the same frequency? D: Minimum distance between centers of cells that use the same band (co-channels) R: Radius of a cell d: distance between adjacent cells (d=3 R) N: Number of cells in a repetitious pattern (reuse factor) D/R = √(3N) or D/d = √N

5. Frequency Reuse • Handout

6. Increasing Capacity • Adding new channels: • add unused channels • Frequency borrowing: • borrowed from adjacent cells • Frequencies can be also assigned dynamically • Cell Splitting: • Original cells (about 6.5-13km in size) can be split to smaller cells (1.5 km minimum) • Powers of transmitters are lowered • Handoff becomes more frequent • Cell Sectoring: A cell is divided into number of wedge-shaped sectors • Each sector is assigned a separate subset of cells channel • Directional antennas at the base station are used to focus on each sector

7. Operations Mobile Telecom Switching Office (MTSO) Base Station Base Station Base Station

8. Operations – cont. • Base stations (BS) includes an antenna, a controller, and a number of transceivers, for communicating on the channels assigned to that cell • Each BS is connected to an MTSO. • One MTSO serves a single or multiple BSs • MTSO assigns the voice channel to each call, performs handoff, and monitors the call for billing information • Channels between the mobile units and the base stations • Control channels: used to exchange information having to do with setting up and maintaining calls • Traffic channels: carry a voice or data connections between users

9. Typical Call - 1 • Mobile Unit Initialization: • The unit scans and selects the strongest set-up control channel • A handshake takes place between the mobile unit and the MTSO controlling this cell, through the BS in this cell • The scanning process is repeated periodically • Mobile-originated call: • The number of the called unit is sent on the preselected set-up channel to the BS • The BS sends the request to the MTSO • Paging: • The MTSO sends a paging message to certain BSs • Each BS transmit the paging signal on its own assigned set-up channel

10. Typical Call - 2 • Call accepted: • The called mobile unit recognizes its number on the set-up channel being monitored and responds to that BS, which send the response to the MTSO • MTSO sets up a circuit between calling and called BSs • MTSO selects an available traffic channel within each BS’s cell and notifies each BS, which in turn notifies its mobile unit. • Ongoing call: • While the connection is maintained, the two mobile units exchange voice or data signals, going through their respective BSs and the MTSO • Handoff: • If a mobile unit moves out of range of one cell and into the range of another during a connection, the traffic channel has to change to one assigned to the BS in the new cell

11. Handoff • Handoff Initiation: • Network-initiated or mobile assisted • Performance Metrics: • Cell blocking probability • Call dropping probability • Call completion probability • Probability of unsuccessful handoff • Handoff blocking probability • Handoff probability • Rate of handoff • Interruption duration • Handoff delay

12. Handoff: When to? • Handout

13. First Generation (Analog Systems) • Advanced Mobile Phone System (AMPS) – developed by AT&T (early 1980’s) • Uses FDD scheme • AMPS Parameters: • Base station transmission band = 869-894 MHz • Mobile unit transmission band = 824-849 MHz • Spacing between forward and reverse channel = 45 MHz • Channel bandwidth = 30 kHz • Number of full-duplex voice channels = 790 • Number of full duplex control channels = 42 • Data transmission rate = 10 kbps • Cell size = 2-20km radius

14. AMPS - Operation • Each AMPS-capable cellular telephone includes a numeric assignment module (NAM) in read-only memory • NAM contains telephone number (provided by service provider) and the serial number of the phone • When turned on, the phone transmits its serial number to MTSO • MTSO maintains a database to take care of authentication and billing issues.

15. Sequence of Events • The subscriber dials a number • The MTSO authorizes and issues a message to the user’s cell phone indicating which traffic channel to use for sending and receiving • The MTSO sends a ringing signal to the called party. When the party answers, the MTSO establishes a circuit between the two parties and initiates billing information • When one party hangs up, the MTSO releases the circuit, frees the radio channels, and completes the billing information

16. Second Generation (TDMA/CDMA) • Goal: higher quality signals, higher data rates, and greater capacity • Advances: • Digital traffic channels: second generation systems provide digital traffic channels, supports digital data; voice traffic is encoded in digital form before transmitting • Encryption • Error detection and correction • Channel access: allows multiple users per channel using TDMA and CDMA • Second Generation Cellular Telephone System: • GSM • Interim Standard 136 (IS-136) • IS-95

17. Standards • GSM (Global System for Mobile Communications): • Introduced in 1990, TDMA-based • Transmission range: BS=935-960 MHz, MS = 890-915 MHz • Channel bandwidth = 200 kHz, 8 users/channel • Deployed widely in Europe, Asia, Australia, South America, PCS spectrum in US • IS-136: (Also known as North American Digital Cellular) • Introduced in 1991, TDMA-based • Transmission range: BS = 869-894 MHz, MS = 824-849 MHz • Channel BW = 30 kHz, 3 users/channel • Deployed in North & South America, Australia • IS-95: (Also known as cdmaOne) • Introduced in 1993, CDMA-based • BS and MS transmission bands = same as IS-136 • Channel bandwidth = 1250 kHz, 64 users/channel • Deployed in North & South America, Korea, Japan, China, Australia

18. Global System for Mobile (GSM) Communications • Services: • Teleservices: mobile telephony • Data services: packet-switched traffic • User services: • Telephone services • Data services • packet switched protocols and data rates from 300 bps to 9.6 kbps • Supplementary ISDN services • Calling features, SMS, and cell broadcast • On-the-air privacy: • The digital bit stream sent by a GSM transmitter is encrypted according to a specific secret cryptographic key that is known only to the carrier. The key changes with time for each user.

19. GSM System Architecture • Three subsytems: • Base System Subsystem (BSS) • Network and Switching Subsystem (NSS) • Operation Support Subsystem (OSS) • Mobile station (MS) is considered a part of BSS • Refer to the GSM Architecture (handout)

20. Mobile Station • Mobile Equipment (ME) • Device including keypad, speaker, microphone, radio transceiver, Digital signal processor • Subscriber Identity Module (SIM): • A pluggable memory device that stores information such as subscriber identification number, the network and countries where the subscriber is entitled to service, privacy keys, and other user-specific information • Calls in GSM are directed to SIM

21. GSM Subsystems • BSS: (also known as radio subsystem) • Provides and manages radio transmissions between MSs and Mobile Switching Center (MSC) • Manages the radio interface between all subsystem, reserves radio frequencies, manages handoff, controls paging • Each BSS consists of a Base Station Controllers (BSC) and multiple Base Transceiver Stations (BTS) • Connects the MS to the NSS via the MSCs • NSS: • Manages the switching functions of the system and allows the MSCs to communicate with other networks such as PSTN and ISDN • OSS: • Supports the operations and maintenance of GSM; monitor, diagnose, troubleshoot all aspects of GSM system

22. Base Station Subsystem (BSS) • The MSs communicate with the BSS over the radio interface • BSS consists of one or more BSCs which connect to a single MSC • Each BSC typically controls about several hundred Base Transceiver Stations (BTS) • BTS may be colocated with the BSC or remotely distributed • Mobile handovers between two BTS under the control of the same BSC are handled by the BSC – reduces switching burden on MSC

23. GSM Interfaces • Um: radio interface • Abis: interface connecting a BTS to a BSC • Carries traffic and maintenance data • Standardized for all manufacturers • BSCs are physically connected to MSCs (A interface) • Uses an SS7 protocol called the signaling correction control part (SCCP) to support communications between the MSC and BSSs

24. NSS • NSS: • Handles the switching of GSM calls between external networks and the BSCs • Responsible for managing and providing external access to several customer databases • MSC is the central unit in the NSS and controls traffic among all BSCs

25. Databases in NSS • Home Location Register (HLR) • Contains subscriber information and location information for each user who resides in the same city as MSC • Each subscriber in a particular GSM market is assigned a unique International Mobile Subscriber Identity (IMSI) • Visitor Location Register (VLR) • Temporarily stores the IMSI and customer information for each roaming subscriber who is visiting the coverage area of a particular MSC • Once a roaming mobile is logged in the VLR, the MSC sends the necessary information to its HLR so that the calls can be appropriately routed • Authentication Center (AUC) • Handles authentication and encryption keys for each of the subscriber in the HLR and VLR • Equipment Identity Register (EIR) • Keeps the international mobile equipment identity (IMEI) that reveals manufacturer, country of production, terminal type • Helps in managing faults and thefts • Implementation is optional to the service provider

26. OSS • OSS supports one or several operation maintenance centers (OMC), which are used to monitor and maintain performance of each MS, BS, BSC, and MSC • Functions: • Maintain all telecom hardware and network operations • Maintain all charging and billing procedures • Manage all mobile equipments in the system

27. Second Generation CDMA (IS-95) • Forward link: Up to 64 logical CDMA channel – each occupying 1228 kHz bandwidth • Types of channels: • Pilot (channel 0): allows MS to acquire timing, phase reference, and signal strength reference • Synchronization (channel 32): 1200 bps channel used by MS to obtain identification information (system time, code state, protocol version, etc.) • Paging (Channels 1-7) • Traffic channels (8-31, 33-63) up to 14.4 kbps

28. IS-95 (continued) • Reverse Link: Up to 94 logical CDMA channels, each occupying 1228 kHz bandwidth • Supports up to 32 access channels and up to 64 traffic channels • The access channel is used by an MS to initiate a call, respond to paging, and for location update • Traffic channels is reverse links are mobile unique

29. Third Generation (3G) Wireless Networks • Motivations: • Multi-megabit Internet access • Very high network capacity • VoIP • Integration of multiple technologies • ITU formulated a plan to implement a global frequency band in the 2000 MHz range that would support a single, ubiquitous wireless communication standard for all countries – called International Mobile Telephone (IMT-2000) • 3G evolution of CDMA – cdma2000 • 3G evolution of GSM – wideband CDMA (W-CDMA), also called Universal Mobile Telecommunications Service (UMTS)