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# 3 cellular-networks

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### 3 cellular-networks

1. 1. Cellular Wireless Networks
2. 2. Introduction <ul><li>Cellular technology is the underlying technology for most mobile wireless communications and data transfers </li></ul><ul><li>Main concept: Use of multiple low-power transceivers </li></ul><ul><li>An area is divided into cells, each of which are served by a base station, consisting of transmitter, receiver, and a control unit </li></ul><ul><li>Adjacent cells are assigned different frequencies to avoid interferences and crosstalk. Cells at sufficient distance from each other can use the same frequency bands </li></ul><ul><li>The cells are hexagonal in shape </li></ul><ul><li>Each cell has a base transceiver – the transmission power is carefully controlled to avoid interferences with neighboring cells </li></ul><ul><li>About 10-50 frequencies are assigned to each cell </li></ul>
3. 3. Cellular Geometries <ul><li>Handout </li></ul>
4. 4. Frequency Reuse <ul><li>How many cells must intervene between two cells using the same frequency? </li></ul><ul><ul><li>D: Minimum distance between centers of cells that use the same band (co-channels) </li></ul></ul><ul><ul><li>R: Radius of a cell </li></ul></ul><ul><ul><li>d: distance between adjacent cells (d=  3 R) </li></ul></ul><ul><ul><li>N: Number of cells in a repetitious pattern (reuse factor) </li></ul></ul><ul><ul><li>D/R = √(3N) </li></ul></ul><ul><ul><li>or D/d = √N </li></ul></ul>
5. 5. Frequency Reuse <ul><li>Handout </li></ul>
6. 6. Increasing Capacity <ul><li>Adding new channels: </li></ul><ul><ul><li>add unused channels </li></ul></ul><ul><li>Frequency borrowing: </li></ul><ul><ul><li>borrowed from adjacent cells </li></ul></ul><ul><ul><li>Frequencies can be also assigned dynamically </li></ul></ul><ul><li>Cell Splitting: </li></ul><ul><ul><li>Original cells (about 6.5-13km in size) can be split to smaller cells (1.5 km minimum) </li></ul></ul><ul><ul><li>Powers of transmitters are lowered </li></ul></ul><ul><ul><li>Handoff becomes more frequent </li></ul></ul><ul><li>Cell Sectoring: A cell is divided into number of wedge-shaped sectors </li></ul><ul><ul><li>Each sector is assigned a separate subset of cells channel </li></ul></ul><ul><ul><li>Directional antennas at the base station are used to focus on each sector </li></ul></ul>
7. 7. Operations Base Station Mobile Telecom Switching Office (MTSO) Base Station Base Station
8. 8. Operations – cont. <ul><li>Base stations (BS) includes an antenna, a controller, and a number of transceivers, for communicating on the channels assigned to that cell </li></ul><ul><li>Each BS is connected to an MTSO. </li></ul><ul><ul><li>One MTSO serves a single or multiple BSs </li></ul></ul><ul><ul><li>MTSO assigns the voice channel to each call, performs handoff, and monitors the call for billing information </li></ul></ul><ul><li>Channels between the mobile units and the base stations </li></ul><ul><ul><li>Control channels: used to exchange information having to do with setting up and maintaining calls </li></ul></ul><ul><ul><li>Traffic channels: carry a voice or data connections between users </li></ul></ul>
9. 9. Typical Call - 1 <ul><li>Mobile Unit Initialization: </li></ul><ul><ul><li>The unit scans and selects the strongest set-up control channel </li></ul></ul><ul><ul><li>A handshake takes place between the mobile unit and the MTSO controlling this cell, through the BS in this cell </li></ul></ul><ul><ul><li>The scanning process is repeated periodically </li></ul></ul><ul><li>Mobile-originated call: </li></ul><ul><ul><li>The number of the called unit is sent on the preselected set-up channel to the BS </li></ul></ul><ul><ul><li>The BS sends the request to the MTSO </li></ul></ul><ul><li>Paging: </li></ul><ul><ul><li>The MTSO sends a paging message to certain BSs </li></ul></ul><ul><ul><li>Each BS transmit the paging signal on its own assigned set-up channel </li></ul></ul>
10. 10. Typical Call - 2 <ul><li>Call accepted: </li></ul><ul><ul><li>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 </li></ul></ul><ul><ul><li>MTSO sets up a circuit between calling and called BSs </li></ul></ul><ul><ul><li>MTSO selects an available traffic channel within each BS’s cell and notifies each BS, which in turn notifies its mobile unit. </li></ul></ul><ul><li>Ongoing call: </li></ul><ul><ul><li>While the connection is maintained, the two mobile units exchange voice or data signals, going through their respective BSs and the MTSO </li></ul></ul><ul><li>Handoff: </li></ul><ul><ul><li>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 </li></ul></ul>
11. 11. Handoff <ul><li>Handoff Initiation: </li></ul><ul><ul><li>Network-initiated or mobile assisted </li></ul></ul><ul><li>Performance Metrics: </li></ul><ul><ul><li>Cell blocking probability </li></ul></ul><ul><ul><li>Call dropping probability </li></ul></ul><ul><ul><li>Call completion probability </li></ul></ul><ul><ul><li>Probability of unsuccessful handoff </li></ul></ul><ul><ul><li>Handoff blocking probability </li></ul></ul><ul><ul><li>Handoff probability </li></ul></ul><ul><ul><li>Rate of handoff </li></ul></ul><ul><ul><li>Interruption duration </li></ul></ul><ul><ul><li>Handoff delay </li></ul></ul>
12. 12. Handoff: When to? <ul><li>Handout </li></ul>
13. 13. First Generation (Analog Systems) <ul><li>Advanced Mobile Phone System (AMPS) – developed by AT&T (early 1980’s) </li></ul><ul><li>Uses FDD scheme </li></ul><ul><li>AMPS Parameters: </li></ul><ul><ul><li>Base station transmission band = 869-894 MHz </li></ul></ul><ul><ul><li>Mobile unit transmission band = 824-849 MHz </li></ul></ul><ul><ul><li>Spacing between forward and reverse channel = 45 MHz </li></ul></ul><ul><ul><li>Channel bandwidth = 30 kHz </li></ul></ul><ul><ul><li>Number of full-duplex voice channels = 790 </li></ul></ul><ul><ul><li>Number of full duplex control channels = 42 </li></ul></ul><ul><ul><li>Data transmission rate = 10 kbps </li></ul></ul><ul><ul><li>Cell size = 2-20km radius </li></ul></ul>
14. 14. AMPS - Operation <ul><li>Each AMPS-capable cellular telephone includes a numeric assignment module (NAM) in read-only memory </li></ul><ul><li>NAM contains telephone number (provided by service provider) and the serial number of the phone </li></ul><ul><li>When turned on, the phone transmits its serial number to MTSO </li></ul><ul><li>MTSO maintains a database to take care of authentication and billing issues. </li></ul>
15. 15. Sequence of Events <ul><li>The subscriber dials a number </li></ul><ul><li>The MTSO authorizes and issues a message to the user’s cell phone indicating which traffic channel to use for sending and receiving </li></ul><ul><li>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 </li></ul><ul><li>When one party hangs up, the MTSO releases the circuit, frees the radio channels, and completes the billing information </li></ul>
16. 16. Second Generation (TDMA/CDMA) <ul><li>Goal: higher quality signals, higher data rates, and greater capacity </li></ul><ul><li>Advances: </li></ul><ul><ul><li>Digital traffic channels: second generation systems provide digital traffic channels, supports digital data; voice traffic is encoded in digital form before transmitting </li></ul></ul><ul><ul><li>Encryption </li></ul></ul><ul><ul><li>Error detection and correction </li></ul></ul><ul><ul><li>Channel access: allows multiple users per channel using TDMA and CDMA </li></ul></ul><ul><li>Second Generation Cellular Telephone System: </li></ul><ul><ul><li>GSM </li></ul></ul><ul><ul><li>Interim Standard 136 (IS-136) </li></ul></ul><ul><ul><li>IS-95 </li></ul></ul>
17. 17. Standards <ul><li>GSM (Global System for Mobile Communications): </li></ul><ul><ul><li>Introduced in 1990, TDMA-based </li></ul></ul><ul><ul><li>Transmission range: BS=935-960 MHz, MS = 890-915 MHz </li></ul></ul><ul><ul><li>Channel bandwidth = 200 kHz, 8 users/channel </li></ul></ul><ul><ul><li>Deployed widely in Europe, Asia, Australia, South America, PCS spectrum in US </li></ul></ul><ul><li>IS-136: (Also known as North American Digital Cellular) </li></ul><ul><ul><li>Introduced in 1991, TDMA-based </li></ul></ul><ul><ul><li>Transmission range: BS = 869-894 MHz, MS = 824-849 MHz </li></ul></ul><ul><ul><li>Channel BW = 30 kHz, 3 users/channel </li></ul></ul><ul><ul><li>Deployed in North & South America, Australia </li></ul></ul><ul><li>IS-95: (Also known as cdmaOne) </li></ul><ul><ul><li>Introduced in 1993, CDMA-based </li></ul></ul><ul><ul><li>BS and MS transmission bands = same as IS-136 </li></ul></ul><ul><ul><li>Channel bandwidth = 1250 kHz, 64 users/channel </li></ul></ul><ul><ul><li>Deployed in North & South America, Korea, Japan, China, Australia </li></ul></ul>
18. 18. Global System for Mobile (GSM) Communications <ul><li>Services: </li></ul><ul><ul><li>Teleservices: mobile telephony </li></ul></ul><ul><ul><li>Data services: packet-switched traffic </li></ul></ul><ul><li>User services: </li></ul><ul><ul><li>Telephone services </li></ul></ul><ul><ul><li>Data services </li></ul></ul><ul><ul><ul><li>packet switched protocols and data rates from 300 bps to 9.6 kbps </li></ul></ul></ul><ul><ul><li>Supplementary ISDN services </li></ul></ul><ul><ul><ul><li>Calling features, SMS, and cell broadcast </li></ul></ul></ul><ul><li>On-the-air privacy: </li></ul><ul><ul><li>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. </li></ul></ul>
19. 19. GSM System Architecture <ul><li>Three subsytems: </li></ul><ul><ul><li>Base System Subsystem (BSS) </li></ul></ul><ul><ul><li>Network and Switching Subsystem (NSS) </li></ul></ul><ul><ul><li>Operation Support Subsystem (OSS) </li></ul></ul><ul><li>Mobile station (MS) is considered a part of BSS </li></ul><ul><li>Refer to the GSM Architecture (handout) </li></ul>
20. 20. Mobile Station <ul><li>Mobile Equipment (ME) </li></ul><ul><ul><li>Device including keypad, speaker, microphone, radio transceiver, Digital signal processor </li></ul></ul><ul><li>Subscriber Identity Module (SIM): </li></ul><ul><ul><li>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 </li></ul></ul><ul><ul><li>Calls in GSM are directed to SIM </li></ul></ul>
21. 21. GSM Subsystems <ul><li>BSS: (also known as radio subsystem) </li></ul><ul><ul><li>Provides and manages radio transmissions between MSs and Mobile Switching Center (MSC) </li></ul></ul><ul><ul><li>Manages the radio interface between all subsystem, reserves radio frequencies, manages handoff, controls paging </li></ul></ul><ul><ul><li>Each BSS consists of a Base Station Controllers (BSC) and multiple Base Transceiver Stations (BTS) </li></ul></ul><ul><ul><li>Connects the MS to the NSS via the MSCs </li></ul></ul><ul><li>NSS: </li></ul><ul><ul><li>Manages the switching functions of the system and allows the MSCs to communicate with other networks such as PSTN and ISDN </li></ul></ul><ul><li>OSS: </li></ul><ul><ul><li>Supports the operations and maintenance of GSM; monitor, diagnose, troubleshoot all aspects of GSM system </li></ul></ul>
22. 22. Base Station Subsystem (BSS) <ul><li>The MSs communicate with the BSS over the radio interface </li></ul><ul><li>BSS consists of one or more BSCs which connect to a single MSC </li></ul><ul><li>Each BSC typically controls about several hundred Base Transceiver Stations (BTS) </li></ul><ul><li>BTS may be colocated with the BSC or remotely distributed </li></ul><ul><li>Mobile handovers between two BTS under the control of the same BSC are handled by the BSC – reduces switching burden on MSC </li></ul>
23. 23. GSM Interfaces <ul><li>Um: radio interface </li></ul><ul><li>Abis: interface connecting a BTS to a BSC </li></ul><ul><ul><li>Carries traffic and maintenance data </li></ul></ul><ul><ul><li>Standardized for all manufacturers </li></ul></ul><ul><li>BSCs are physically connected to MSCs (A interface) </li></ul><ul><ul><li>Uses an SS7 protocol called the signaling correction control part (SCCP) to support communications between the MSC and BSSs </li></ul></ul>
24. 24. NSS <ul><li>NSS: </li></ul><ul><ul><li>Handles the switching of GSM calls between external networks and the BSCs </li></ul></ul><ul><ul><li>Responsible for managing and providing external access to several customer databases </li></ul></ul><ul><ul><li>MSC is the central unit in the NSS and controls traffic among all BSCs </li></ul></ul>
25. 25. Databases in NSS <ul><li>Home Location Register (HLR) </li></ul><ul><ul><li>Contains subscriber information and location information for each user who resides in the same city as MSC </li></ul></ul><ul><ul><li>Each subscriber in a particular GSM market is assigned a unique International Mobile Subscriber Identity (IMSI) </li></ul></ul><ul><li>Visitor Location Register (VLR) </li></ul><ul><ul><li>Temporarily stores the IMSI and customer information for each roaming subscriber who is visiting the coverage area of a particular MSC </li></ul></ul><ul><ul><li>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 </li></ul></ul><ul><li>Authentication Center (AUC) </li></ul><ul><ul><li>Handles authentication and encryption keys for each of the subscriber in the HLR and VLR </li></ul></ul><ul><li>Equipment Identity Register (EIR) </li></ul><ul><ul><li>Keeps the international mobile equipment identity (IMEI) that reveals manufacturer, country of production, terminal type </li></ul></ul><ul><ul><li>Helps in managing faults and thefts </li></ul></ul><ul><ul><li>Implementation is optional to the service provider </li></ul></ul>
26. 26. OSS <ul><li>OSS supports one or several operation maintenance centers (OMC), which are used to monitor and maintain performance of each MS, BS, BSC, and MSC </li></ul><ul><li>Functions: </li></ul><ul><ul><li>Maintain all telecom hardware and network operations </li></ul></ul><ul><ul><li>Maintain all charging and billing procedures </li></ul></ul><ul><ul><li>Manage all mobile equipments in the system </li></ul></ul>
27. 27. Second Generation CDMA (IS-95) <ul><li>Forward link: Up to 64 logical CDMA channel – each occupying 1228 kHz bandwidth </li></ul><ul><li>Types of channels: </li></ul><ul><ul><li>Pilot (channel 0): allows MS to acquire timing, phase reference, and signal strength reference </li></ul></ul><ul><ul><li>Synchronization (channel 32): 1200 bps channel used by MS to obtain identification information (system time, code state, protocol version, etc.) </li></ul></ul><ul><ul><li>Paging (Channels 1-7) </li></ul></ul><ul><ul><li>Traffic channels (8-31, 33-63) up to 14.4 kbps </li></ul></ul>
28. 28. IS-95 (continued) <ul><li>Reverse Link: Up to 94 logical CDMA channels, each occupying 1228 kHz bandwidth </li></ul><ul><ul><li>Supports up to 32 access channels and up to 64 traffic channels </li></ul></ul><ul><ul><li>The access channel is used by an MS to initiate a call, respond to paging, and for location update </li></ul></ul><ul><ul><li>Traffic channels is reverse links are mobile unique </li></ul></ul>
29. 29. Third Generation (3G) Wireless Networks <ul><li>Motivations: </li></ul><ul><ul><li>Multi-megabit Internet access </li></ul></ul><ul><ul><li>Very high network capacity </li></ul></ul><ul><ul><li>VoIP </li></ul></ul><ul><ul><li>Integration of multiple technologies </li></ul></ul><ul><li>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) </li></ul><ul><li>3G evolution of CDMA – cdma2000 </li></ul><ul><li>3G evolution of GSM – wideband CDMA (W-CDMA), also called Universal Mobile Telecommunications Service (UMTS) </li></ul>