Cellular Networks

1. Cellular Networks

2. Overview Data Rates 2 Mbps 3G (144Kbps to 2Mbps) 1 Mbps 100 Kbps 2.5G (10-150Kbps) 10 Kbps 2G (9.6Kbps) 1 Kbps 1G (<1Kbps) 1980 2010 1990 2000 Years

3. Cellular networks: From 1G to 3G • 1G: First generation wireless cellular: Early 1980s • Analog transmission, primarily speech: AMPS (Advanced Mobile Phone Systems) and others • 2G: Second generation wireless cellular: Late 1980s • Digital transmission • Primarily speech and low bit-rate data (9.6 Kbps) • High-tier: GSM, IS-95 (CDMA), etc • Low-tier (PCS): Low-cost, low-power, low-mobility e.g. PACS • 2.5G: 2G evolved to medium rate (< 100kbps) data • 3G: future Broadband multimedia • 144 kbps - 384 kbps for high-mobility, high coverage • 2 Mbps for low-mobility and low coverage • Beyond 3G: research in 4G

4. Issues Vital to cellular • Frequency allocation • Licensed • Many providers • Multiple Access • Many users • Wide area of coverage • Traffic management • Location management • High mobility (in cars, trains) • Multiple suppliers • Handoff management, roaming • General principles • Handled differently by different generations

5. Multiple Access Techniques: How to allocate users Session4 Session2 Session3 Session3 Session4 Session1 Frequency Session2 Frequency Session1 Time Time Time Division Multiple Access (TDMA) 2G TDMA 3G TDMA Frequency Division Multiple Access (FDMA) 1G Cellular (AMPS) All sessions based on a code Frequency Time 2G CDMA (IS-95) 3G CDMA Code Division Multiple Access (CDMA)

6. Cell 1 A Cellular Network Public Switched Telephone Network (PSTN) Mobile Telephone Switching Center (MTSC) Cell 2 HLR VLR Base Transceiver Station (BTS) Mobile User HLR = Home Location Register VLR = Visitor Location Register Cordless connection Wired connection

7. Overview of Location Services • Cell-id based location. • assigned an id of the cell that you are in. • cell-id is stored in a database. • As you move from one cell to another, you are assigned a different cell-id and the location database is updated. • most commonly used in cellular networks. (HLR, VLR) • Neighborhood polling: Connected mobile units only move to adjacent cells • Angle of arrival (AOA). the angle at which radio waves from your device "attack" an antenna is used to calculate the location of the device. • Time taken. In this case, the time taken between the device and the antenna is used to calculate the location of the device. • Network assisted Global Positioning System (GPS). a GPS chip is installed inside a phone and thus the location of the user is tracked.

8. Cell 1 Public Switched Telephone Network (PSTN) Mobile Telephone Switching Center (MTSC) Cell 2 HLR VLR Cellular System Handoffs (typically 30 mseconds): 1. At any time, mobile station (MS) is in one cell and under the control of a BS 2. When a MS leaves a cell, BS notices weak signal 3. BS asks surrounding BSs if they are getting a stronger signal 4. BS transfers ownership to one with strongest signal 5. MTSO assigns new channel to the MS and notifies MS of new boss

9. Frequency Reuse • The concept of frequency reuse is based on assigning to each cell a group of radio channels used within a small geographic area • Cells are assigned a group of channels that is completely different from neighbouring cells • The coverage area of cells is called the footprint and is limited by a boundary so that the same group of channels can be used in cells that are far enough apart

10. Frequency Reuse • Cells with the same number have the same set of frequencies Frequency Reuse

11. f2 f7 f3 f1 f6 f4 f5 Frequency Reuse using 7 frequencies allocations f2 f7 f3 f1 f6 f4 f2 f5 f2 f7 f3 f7 f3 f1 f2 f1 f6 f4 f7 f3 f6 f4 f5 f1 f5 f6 f4 f5 Each cell is generally 4 to 8 miles in diameter with a lower limit around 2 miles.

13. Problem with Smaller Clustersize Interfering cells are closer by when clustersize is smaller.

16. 0G Wireless • Mobile radio telephones were used for military communications in early 20th century • Car-based telephones first introduced in mid 1940s • Single large transmitter on top of a tall building • Single channel used for sending and receiving • To talk, user pushed a button, enabled transmission and disabled reception • Became known as “push-to-talk” in 1950s • CB-radio, taxis, police cars use this technology • IMTS (Improved Mobile Telephone System) introduced in 1960s • Used two channels (one for sending, one for receiving) • No need for push-to-talk • Used 23 channels from 150 MHz to 450 MHz

17. First-Generation Cellular • Advanced Mobile Phone Service (AMPS) invented at Bell Labs and first installed in 1982 • Used in England (called TACS) and Japan (called MCS-L1) • Key ideas: • Exclusively analog • Geographical area divided into cells (typically 10-25km) • Cells are small: Frequency reuse exploited in nearby (not adjacent) cells • As compared to IMTS, could use 5 to 10 times more users in same area by using frequency re-use (divide area into cells) • Smaller cells also required less powerful, cheaper,smaller devices

19. Cell Design E D F E A D F C G A B C G E B D F A C G B • Cells grouped into a cluster of seven • Letters indicate frequency use • For each frequency, a buffer of two cells is used before reuse • To add more users, smaller cells (microcells) are used • Frequencies may not need to be different in CDMA (soft handoff)

20. Cellular Network Organization • Cell design (around 10 mile radius) • Served by base station consisting of transmitter, receiver, and control unit • Base station (BS) antenna is placed in high places (churches, high rise buildings) - • Operators pay around $500 per month for BS • 10 to 50 frequencies assigned to each cell • Cells set up such that antennas of all neighbors are equidistant (hexagonal pattern) • In North America, two 25-MHz bands allocated to AMPS • One for transmission from base to mobile unit • One for transmission from mobile unit to base

21. Approaches to Increase Capacity • Adding/reassigning channels - some channels are not used • Frequency borrowing – frequencies are taken from adjacent cells by congested cells • Cell splitting – cells in areas of high usage can be split into smaller cells • Microcells – antennas move to buildings, hills, and lamp posts

22. Security Issues with 1G • Analog cellular phones are insecure • Anyone with an all band radio receiver can listen in (many scandals) • Theft of airtime: • all band radio receiver connected to a computer • can record 32 bit serial number and phone number of subscribers when calling • can collect a large database by driving around • Thieves go into business - reprogram stolen phones and resell them

23. Second Generation Cellular • Based on digital transmission • Different approaches in US and Europe • US: divergence • Only one player (AMPS) in 1G • Became several players in 2G due to competition • Survivors • IS-54 and IS-135: backward compatible with AMPS frequency allocation (dual mode - analog and digital) • IS-95: uses spread spectrum • Europe: Convergence • 5 incompatible 1G systems (no clear winner) • European PTT development of GSM (uses new frequency and completely digital communication)

28. Advantages of Digital Communications for Wireless • Voice, data and fax can be integrated into a single system • Better compression can lead to better channel utilization • Error correction codes can be used for better quality • Sophisticated encryption can be used

29. Differences Between First and Second Generation Systems • Digital traffic channels – first-generation systems are almost purely analog; second-generation systems are digital • Encryption – all second generation systems provide encryption to prevent eavesdropping • Error detection and correction – second-generation digital traffic allows for detection and correction, giving clear voice reception • Channel access – second-generation systems allow channels to be dynamically shared by a number of users

30. Integrating Data Over Cellular • Direct access to digital channel • Voice and data using one handset • PCS 1900 (GSM-1900) • 9.6 kbps circuit switched data • 14.4 kbps under definition • Packet mode specified • Short message service • IS-95-based CDMA • 13 kbps circuit switched data • Packet mode specified • Short message service

31. GSM (Global System for Mobile Communications) • Completely designed from scratch (no backward compatability) • Uses 124 channels per cell, each channel can support 8 users through TDM (992 users max) • Some channels used for control signals, etc • Several flavors based on frequency: • GSM (900 MHz) • GSM 1800 (called DCS 1800) • GSM 1900 (called DCS 1900) - used in North America • GSM 1900 phone only works in North America. • In Europe, you can transfer your SIM (Subscriber Identity Module) card to a phone of the correct frequency. This is called SIM-roaming.

38. GSM (2G-TDMA) • Circuit mode data • Transparent mode • Non-transparent mode using radio link protocol • Data rate up to 9.6kb/s • Short message service • Limited to 160 characters • Packet mode data: Plans for GSM Phase 2+ • Architecture specification very detailed (500 pages) • Defines several interfaces for multiple suppliers

40. Mobile Station and Base Station Subsystem (BSS) Mobile station • Mobile station communicates across Um interface (air interface) with base station transceiver in same cell as mobile unit • Mobile equipment (ME) – physical terminal, such as a telephone or PCS • ME includes radio transceiver, digital signal processors and subscriber identity module (SIM) • GSM subscriber units are generic until SIM is inserted • SIMs roam, not necessarily the subscriber devices BSS • BSS consists of base station controller and one or more base transceiver stations (BTS) • BSC reserves radio frequencies, manages handoff of mobile unit from one cell to another within BSS, and controls paging

41. Network Subsystem Center Mobile Switching Center (MSC) is at core; consists of several databases • Home location register (HLR) database – stores information about each subscriber that belongs to it • Visitor location register (VLR) database – maintains information about subscribers currently physically in the region • Authentication center database (AuC) – used for authentication activities, holds encryption keys • Equipment identity register database (EIR) – keeps track of the type of equipment that exists at the mobile station

42. GSM Location Services 6 2 9 Terminating MSC Gateway MTSC BTS 1 10 Public Switched Telephone Network (PSTN) 10 10 8 9 10 7 5 3 4 VLR HLR 10 5 6. Call routed to terminating MSC 7. MSC asks VLR to correlate call to the subscriber 8. VLR complies 9. Mobile unit is paged 10. Mobile unit responds, MSCs convey information back to telephone 1. Call made to mobile unit (cellular phone) 2. Telephone network recognizes number and gives to gateway MSC 3. MSC can’t route further, interrogates user’s HLR 4. Interrogates VLR currently serving user (roaming number request) 5. Routing number returned to HLR and then to gateway MSC Legend: MTSC= Mobile Telephone Service Center, BTS = Base Transceiver Station HLR=Home Location Register, VLR=Visiting Location Register

43. GSM Protocol Architecture CM CM MM MM BSSMAP BSSMAP RRM BTSM BTSM RRM SCCP SCCP LAPDm LAPD LAPD LAPDm MTP MTP Radio 64 Kbps 64 Kbps Radio 64 Kbps 64Kbps Base Station Controller Mobile Station Base Transceiver Station Mobile Service Switching Center MM = Mobility Management MTP = Message Transfer Part RRM = Radio Resources Management SCCP = Signal Connection Control Point BSSMAP = BSS Mobile Application part BTSM = BTS management CM = Connection Management LAPD = Link Access Protocol, D Channel

44. Functions Provided by Protocols • Protocols above the link layer of the GSM signaling protocol architecture provide specific functions: • Radio resource management: controls setup, termination and handoffs of radio channels • Mobility management: location and security (MTSO) • Connection management: connects end users • Mobile application part (MAP): between HLR,VLR • BTS management: management base system

45. 2G CDMA Cellular IS-95 is the best known example of 2G with CDMA Advantages of CDMA for Cellular • Frequency diversity – frequency-dependent transmission impairments have less effect on signal • Multipath resistance – chipping codes used for CDMA exhibit low cross correlation and low autocorrelation • Privacy – privacy is inherent since spread spectrum is obtained by use of noise-like signals • Graceful degradation – system only gradually degrades as more users access the system

46. Drawbacks of CDMA Cellular • Self-jamming – arriving transmissions from multiple users not aligned on chip boundaries unless users are perfectly synchronized • Near-far problem – signals closer to the receiver are received with less attenuation than signals farther away • Soft handoff – requires that the mobile acquires the new cell before it relinquishes the old; this is more complex than hard handoff used in FDMA and TDMA schemes

47. Types of Channels Supported by Forward Link • Pilot (channel 0) - allows the mobile unit to acquire timing information, provides phase reference and provides means for signal strength comparison • Synchronization (channel 32) - used by mobile station to obtain identification information about cellular system • Paging (channels 1 to 7) - contain messages for one or more mobile stations • Traffic (channels 8 to 31 and 33 to 63) – the forward channel supports 55 traffic channels

48. Forward Traffic Channel Processing Steps • Speech is encoded at a rate of 8550 bps • Additional bits added for error detection • Data transmitted in 2-ms blocks with forward error correction provided by a convolutional encoder • Data interleaved in blocks to reduce effects of errors • Data bits are scrambled, serving as a privacy mask • Power control information inserted into traffic channel • DS-SS function spreads the 19.2 kbps to a rate of 1.2288 Mbps using one row of 64 x 64 Walsh matrix • Digital bit stream modulated onto the carrier using QPSK modulation scheme

49. Enabling Technologies 3G 2 Mbps CDMA2000 3XRTT (UMTS) CDMA Migration Wireless Network Evolution to 3rd Generation W-CDMA (UMTS) 1G-2G Migration 500 kbps TDMA Migration 2.5G EDGE 150 Kbps CDMA-2000 1XRTT 100 Kbps GPRS 2G 50 Kbps 10 Kbps 1G IS-95 GSM 1 Kbps AMPS 1999 2000 2001 2002 2003 1980

50. Alternatives to 3G Cellular • Major technical undertaking with many organizational and marketing overtones. • Questions about the need for the additional investment for 3G (happy with 2.5G) • Wireless LAN in public places such as shopping malls and airports offer options • Other high-speed wireless-data solutions compete with 3G • Mobitex low data rates (nominally 8 Kbps), it uses a narrowband (2.5KHz) as compared to 30 KHz (GSM) and 5 MHz (3G). • Ricochet: 40 -128 kbps data rates. Bankruptcy • Flash-OFDM: 1.5 Mbps (upto 3 Mbps)