cellular networks
TRANSCRIPT
- 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?
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- D: Minimum distance between centers of cells that use the same band (co-channels)
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- R: Radius of a cell
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- d: distance between adjacent cells (d= 3 R)
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- N: Number of cells in a repetitious pattern (reuse factor)
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- D/R = (3N)
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- or D/d = N
5. Frequency Reuse
- Handout
6. Increasing Capacity
- Adding new channels:
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- add unused channels
- Frequency borrowing:
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- borrowed from adjacent cells
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- Frequencies can be also assigned dynamically
- Cell Splitting:
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- Original cells (about 6.5-13km in size) can be split to smaller cells (1.5 km minimum)
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- Powers of transmitters are lowered
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- Handoff becomes more frequent
- Cell Sectoring: A cell is divided into number of wedge-shaped sectors
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- Each sector is assigned a separate subset of cells channel
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- Directional antennas at the base station are used to focus on each sector
7. Operations Base Station Mobile TelecomSwitching Office(MTSO) 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.
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- One MTSO serves a single or multiple BSs
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- 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
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- Control channels: used to exchange information having to do with setting up and maintaining calls
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- Traffic channels: carry a voice or data connections between users
9. Typical Call - 1
- Mobile Unit Initialization:
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- The unit scans and selects the strongest set-up control channel
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- A handshake takes place between the mobile unit and the MTSO controlling this cell, through the BS in this cell
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- The scanning process is repeated periodically
- Mobile-originated call:
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- The number of the called unit is sent on the preselected set-up channel to the BS
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- The BS sends the request to the MTSO
- Paging:
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- The MTSO sends a paging message to certain BSs
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- Each BS transmit the paging signal on its own assigned set-up channel
10. Typical Call - 2
- Call accepted:
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- 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
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- MTSO sets up a circuit between calling and called BSs
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- MTSO selects an available traffic channel within each BSs cell and notifies each BS, which in turn notifies its mobile unit.
- Ongoing call:
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- While the connection is maintained, the two mobile units exchange voice or data signals, going through their respective BSs and the MTSO
- Handoff:
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- 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:
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- Network-initiated or mobile assisted
- Performance Metrics:
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- Cell blocking probability
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- Call dropping probability
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- Call completion probability
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- Probability of unsuccessful handoff
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- Handoff blocking probability
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- Handoff probability
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- Rate of handoff
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- Interruption duration
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- Handoff delay
12. Handoff: When to?
- Handout
13. First Generation (Analog Systems)
- Advanced Mobile Phone System (AMPS) developed by AT&T (early 1980s)
- Uses FDD scheme
- AMPS Parameters:
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- Base station transmission band = 869-894 MHz
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- Mobile unit transmission band = 824-849 MHz
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- Spacing between forward and reverse channel = 45 MHz
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- Channel bandwidth = 30 kHz
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- Number of full-duplex voice channels = 790
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- Number of full duplex control channels = 42
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- Data transmission rate = 10 kbps
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- 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 users 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:
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- Digital traffic channels: second generation systems provide digital traffic channels, supports digital data; voice traffic is encoded in digital form before transmitting
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- Encryption
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- Error detection and correction
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- Channel access: allows multiple users per channel using TDMA and CDMA
- Second Generation Cellular Telephone System:
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- GSM
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- Interim Standard 136 (IS-136)
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- IS-95
17. Standards
- GSM (Global System for Mobile Communications):
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- Introduced in 1990, TDMA-based
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- Transmission range: BS=935-960 MHz, MS = 890-915 MHz
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- Channel bandwidth = 200 kHz, 8 users/channel
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- Deployed widely in Europe, Asia, Australia, South America, PCS spectrum in US
- IS-136: (Also known as North American Digital Cellular)
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- Introduced in 1991, TDMA-based
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- Transmission range: BS = 869-894 MHz, MS = 824-849 MHz
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- Channel BW = 30 kHz, 3 users/channel
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- Deployed in North & South America, Australia
- IS-95: (Also known as cdmaOne)
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- Introduced in 1993, CDMA-based
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- BS and MS transmission bands = same as IS-136
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- Channel bandwidth = 1250 kHz, 64 users/channel
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- Deployed in North & South America, Korea, Japan, China, Australia
18. Global System for Mobile (GSM) Communications
- Services:
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- Teleservices: mobile telephony
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- Data services: packet-switched traffic
- User services:
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- Telephone services
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- Data services
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- packet switched protocols and data rates from 300 bps to 9.6 kbps
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- Supplementary ISDN services
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- Calling features, SMS, and cell broadcast
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- On-the-air privacy:
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- 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:
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- Base System Subsystem (BSS)
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- Network and Switching Subsystem (NSS)
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- 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)
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- Device including keypad, speaker, microphone, radio transceiver, Digital signal processor
- Subscriber Identity Module (SIM):
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- 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
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- Calls in GSM are directed to SIM
21. GSM Subsystems
- BSS: (also known as radio subsystem)
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- Provides and manages radio transmissions between MSs and Mobile Switching Center (MSC)
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- Manages the radio interface between all subsystem, reserves radio frequencies, manages handoff, controls paging
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- Each BSS consists of a Base Station Controllers (BSC) and multiple Base Transceiver Stations (BTS)
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- Connects the MS to the NSS via the MSCs
- NSS:
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- Manages the switching functions of the system and allows the MSCs to communicate with other networks such as PSTN and ISDN
- OSS:
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- 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
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- Carries traffic and maintenance data
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- Standardized for all manufacturers
- BSCs are physically connected to MSCs (A interface)
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- Uses an SS7 protocol called the signaling correction control part (SCCP) to support communications between the MSC and BSSs
24. NSS
- NSS:
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- Handles the switching of GSM calls between external networks and the BSCs
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- Responsible for managing and providing external access to several customer databases
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- MSC is the central unit in the NSS and controls traffic among all BSCs
25. Databases in NSS
- Home Location Register (HLR)
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- Contains subscriber information and location information for each user who resides in the same city as MSC
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- Each subscriber in a particular GSM market is assigned a unique International Mobile Subscriber Identity (IMSI)
- Visitor Location Register (VLR)
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- Temporarily stores the IMSI and customer information for each roaming subscriber who is visiting the coverage area of a particular MSC
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- 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)
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- Handles authentication and encryption keys for each of the subscriber in the HLR and VLR
- Equipment Identity Register (EIR)
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- Keeps the international mobile equipment identity (IMEI) that reveals manufacturer, country of production, terminal type
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- Helps in managing faults and thefts
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- 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:
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- Maintain all telecom hardware and network operations
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- Maintain all charging and billing procedures
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- 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:
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- Pilot (channel 0): allows MS to acquire timing, phase reference, and signal strength reference
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- Synchronization (channel 32): 1200 bps channel used by MS to obtain identification information (system time, code state, protocol version, etc.)
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- Paging (Channels 1-7)
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- 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
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- Supports up to 32 access channels and up to 64 traffic channels
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- The access channel is used by an MS to initiate a call, respond to paging, and for location update
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- Traffic channels is reverse links are mobile unique
29. Third Generation (3G) Wireless Networks
- Motivations:
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- Multi-megabit Internet access
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- Very high network capacity
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- VoIP
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- 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 calledUniversal Mobile Telecommunications Service (UMTS)