1. Introduction to Mobile Communications TCOM 552, Lecture #6 Hung Nguyen, Ph.D. 10 October, 2006

2. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 2Outline Cellular Communications Principles Control and Traffic Channels Frequency Reuse Hand-Off 1 st Generation Cellular Wireless Networks and AMPS

3. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 3 Cellular/PCS Evolution Bell Labs proposes cellular in 1968 First cellular system begins operation in early 1980’s using analog (AMPS) system, by AT&T -- First Generation systems (1G) –These are called cellular systems, in the 800 and 900 MHz range mostly. Large, bulky phones Digital system using TDMA starts early 1990’s (2G) –Both in 800 and 900 MHz, and around 1800 to 1900 MHz –These, along with GSM and CDMA below, sometimes labeled PCS, Personal Communications Systems (usually when the frequency range is in the 1800 and 1900 MHz range)

4. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 4 Cellular/PCS Evolution (cont’d) European GSM starts operation early 1990’s (2G) –Similar frequency range as TDMA but not exactly--- US and Europe usually differ –GSM used nearly worldwide, more users than US standards First CDMA operation 1996-1997 -- Qualcomm holds patents (2G) –Used by half of 2G systems in US, S. Korea, parts of Latin America First limited 3G operation in Japan in May 2001-- wideband CDMA mostly –Worldwide (semi) standardization agreed to in 1999, for high data rate services to 2 Mbps –New frequency allocation around 1800 MHz in most of world, with some variations –US allocations not made yet, probably 2002, best ones already being used

5. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 5 Cellular System Overview

6. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 6 Cellular Systems Elements Base Station (BS) – includes an antenna, a controller, and a number of receivers –Made up of Base Transceiver Station (BTS) and Base Station Controller (BSC) --- often one BSC controls multiple BTS’s Mobile telecommunications switching office (MTSO) – connects calls between mobile units -- Also called Mobile Switching Center (MSC) Two types of channels available between mobile unit and BS –Control channels – used to exchange information having to do with setting up and maintaining calls –Traffic channels – carry voice or data connection between users

7. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 7 Principles of Cellular Communications Base Stations cover an area called a cell Cell phones communicate with Base Station over wireless links Base Stations interconnected and handle users as they move from cell to cell Base Stations connected to PSTN to switch calls to/from cell phone users Wireless links involve radio communications between users and Base Stations. Cell phone is a radio. Used initially for voice but increasingly for data, email, Internet and later video

8. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 8 Cellular Network Organization Use multiple low-power transmitters (100 W or less) Areas divided into cells –Each served by its own antenna –Served by base station consisting of transmitter, receiver, and control unit –Band of frequencies allocated –Cells set up such that antennas of all neighbors are equidistant (hexagonal pattern) See Tutorial at –http://www.iec.org/tutorials/cell_comm/index.htmlhttp://www.iec.org/tutorials/cell_comm/index.html –http://www.iec.org/online/tutorials/cell_comm/http://www.iec.org/online/tutorials/cell_comm/

9. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 9 From IEC online

10. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 10 Cells usually adjacent to cover some selected area, often more along high traffic highways, downtowns, popular suburban shopping areas, business areas From IEC online

11. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 11 How Cellular Works A cellular telephone system provides a wireless connection to the PSTN for any user in the radio range of the system. It consists of: –mobile stations (cell phones) –base stations –the Mobile Switching Center (MSC) The base station provides a connection to the cell phone via wireless links or the ‘air interface’ The base station is the bridge between the mobile users and the MSC via phone lines or microwave links. The MSC connects the entire cellular system to the PSTN. The MSC, its interconnections and databases is called the ‘core network ’ MSC PSTN From iec.org

12. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 12 Cellular Architecture and Mobility Cellular Architectures Include –Mobiles –Base Stations In Adjacent Cells –Switching Centers: MSC’s –Interfaces to the PSTN –With Networked MSC’s A Key Factor is Mobility –Requires Handoff as User Moves from Cell to Cell –Requires Databases to Track User Mobility -- Used by MSC’s for Mobility Management Figure adapted from IEC cellular and wireless tutorials Other MSC’s

13. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 13 Cellular Channels Two-Way Cellular Communication relies on a variety of channels to ensure call connection and delivery. –Control Channels: Channels used as beacons for idle mobiles and access to traffic channels. Signaling channels sometimes grouped here also -- in AMPS these control channels are called forward control channel and reverse control channel –Traffic Channels: Channels used to support customer voice calls and messages. Separate forward and reverse channels –Signaling Channels: A type of control channel used to provide network and hand-off signals during customer communications -- in AMPS these are called ‘voice channels’ Base Station Forward Channel (downlink) Reverse Channel (uplink) Modified from iec.org

14. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 14 Cellular Control Channels The base station/mobile link Defined by the Air Interface which specifies several different channels. Example: (not AMPS) ­ Forward Access Channel (FAC) ­ Forward Traffic Channel (FTC) ­ Forward Pilot Channel (FPC) ­ Forward Synch Channel (FSC) ­ Reverse Access Channel (RAC) ­ Reverse Traffic Channel (RTC) Mobile User RAC RTC FAC FTC FPC FSC MSC From iec.org

15. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 15 Steps in an MSC Controlled Call between Mobile Users Mobile unit initialization (power on) Mobile-originated call (push send button) Base Station and MSC accept route call Paging -- to called user via his Base Station Call accepted by called user Ongoing call (traffic channels with signaling) Handoff as mobile unit moves to adjacent cell

16. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 16 Additional Functions in an MSC Controlled Call Call blocking (when no traffic channels available) Call termination (at end, re-assign the traffic channels to other calls) Call drop (when handoff does not work) Calls to/from fixed wireline subscribers –MSC acts like a PSTN switch, I/F to PSTN Calls to/from remote mobile subscribers –MSC has to find out where remote subscriber is and route call to Base Station there

17. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 17 Frequency Reuse Adjacent cells assigned different frequencies to avoid interference or crosstalk Objective is to reuse frequency in nearby cells but not adjacent ones –Multiple frequencies assigned to each cell –Transmission power controlled to limit power at that frequency escaping to adjacent cells –The issue is to determine how many cells must intervene between two cells using the same frequency

18. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 18 Sectoring permits more freq. reuse -- e.g. AMPS from 12 to 7, due to lower interference Fig. from iec.org SEVEN CELL REUSE Each cell number has a frequency allocation, so e.g., cell 1 only interfers with other cell 1’s, and so on. This is an example of 7 cell reuse, cluster sizes is 7 cells. Also used 3, 4 cell clusters, 12, 13, 19 CDMA does one cell reuse

19. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 19 From iec online Sectoring used to limit interference to sectors, thus increasing capacitySectoring

20. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 20 Mobile Radio Propagation Effects Okumura-Hata Propgation Model –Based on empirical data –Include base station and mobile heights –Equation 10.1 Example 10.2

21. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 21 Interference, Capacity, and Frequency Reuse Maximum number of users in cellular determined by interference levels that can be tolerated Min. SNR (=S/N) needed to receive a signal in interference become SINR (=S/(I+N)), where I is interference power But for maximum capacity I>>N, so (S/I)min is the minimum required S/I to make a receiver work –e.g., AMPS, (S/I) min for FM to work is 18 dB But in cellular I is proportional to S –I is due to signals (S) transmitted in nearby cells at same frequencies ---- but attenuated by being further away –So S/I determined largely by distance separations, D/R –D/R in turn determined by frequency reuse factor --- how often in cellular topology is the same frequency reused

22. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 22 FREQUENCY REUSE D/R= 4.6 for S/I=17-18 dB D R L= 7 ( see next page why)

23. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 23 Frequency Reuse L (Rappaport, others use N, N not noise) is number of cells in cluster –Frequencies not reused in cluster, but divided up, reused from cluster to cluster –L=7 in example If D=distance to next cluster and R is radius of cell –Hexagonal geometry yields L  1/3(D/R)^2 –For example, D/R=4.6, L=7 –AMPS used L=12 and L=7 –For D/R=3, L=3 --- D/R=6, L=12 S/I is related to D/R by D/R=>(6(S/I))^(1/n) –Because D/R determines relative distance to interferers compared to distance to wanted signal, n from 1/(R^n) propagation loss –For L=7, S/I=17.8 dB –Since AMPS requires 18 dB min. (for clear voice) it is close for L=7 and OK except worse cases near edge of cell--- sometimes L=12 was used --- it is better for TDMA and CDMA

24. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 24 Increasing Capacity, Frequency Reuse and Sectoring Frequency reuse determines capacity for a given technology –e.g., AMPS has 416 channels for a service provider-21 control channels so has 395 traffic channels --- With L=7 reuse each cell has 395/7= 56 and 3/7 channels max. capacity Capacity can be increased by –More spectrally efficient technology (e.g., AMPS to TDMA) –Better frequency reuse through various means to control interference Typically, Sectoring --- each cell divided into (e.g., 3) sectors –For L=7, this reduces the interference from adjacent cells to 1/3 of them Makes S/I=24.2 dB, so AMPS for sure well now Still AMPS not able to go to L=4 or 5 –GSM uses sectoring to reduce interference so reuse is 4 or 5 –CDMA uses sectoring to make reuse 1 (ie, L=1) Cell splitting (more, smaller cells, microcells -- more cells more capacity as 56 and 3/7 above is calls/cell), smart antennas

25. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 25 Base Station --- Diversity Receivers Each Sector RECEIVER A RECEIVER B Transceiver Mobile AUDIO out

26. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 26 Handoff Performance Metrics Cell blocking probability – probability of a new call being blocked Call dropping probability – probability that a call is terminated due to a handoff Call completion probability – probability that an admitted call is not dropped before it terminates Probability of unsuccessful handoff – probability that a handoff is executed while the reception conditions are inadequate

27. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 27 Handoff Performance Metrics Handoff blocking probability – probability that a handoff cannot be successfully completed Handoff probability – probability that a handoff occurs before call termination Rate of handoff – number of handoffs per unit time Interruption duration – duration of time during a handoff in which a mobile is not connected to either base station Handoff delay – distance the mobile moves from the point at which the handoff should occur to the point at which it does occur

28. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 28 Handoff Strategies Used to Determine Instance of Handoff Relative signal strength Relative signal strength with threshold Relative signal strength with hysteresis Relative signal strength with hysteresis and threshold Prediction techniques

29. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 29 Handoff Approaches Used Network Controlled Handoff -- NCHO –Network makes measurements and controls when handoff ocurs –In AMPS, when signal level from mobile falls below -100 dBm (or so, proprietary schemes) at serving basestation (BS), it looks for another BS and requests it to make measurements, when it’s -90 dBm there it hands off –Hysteresis, no immediate handoff, stabilizes for no ping pong –Done by MTSO Mobile Assisted Handoff –Mobile is making measurements and reporting them to BS, from its cell and adjacent cells, MTSO still controls it –Used in TDMA, GSM and CDMA

30. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 30 Power Control Design issues making it desirable to include dynamic power control in a cellular system –Received power must be sufficiently above the background noise for effective communication –Desirable to minimize power in the transmitted signal from the mobile Reduce cochannel interference, alleviate health concerns, save battery power –In SS systems using CDMA, it’s desirable to equalize the received power level from all mobile units at the BS IS-95 CDMA adjusts the power of each handset 800 times per second, thus minimizing interference

31. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 31 Types of Power Control Open-loop power control –Depends solely on mobile unit –No feedback from BS –Not as accurate as closed-loop, but can react quicker to fluctuations in signal strength Closed-loop power control –Adjusts signal strength in reverse channel based on metric of performance, usually power measured at BS –BS makes power adjustment decision and communicates to mobile on control channel –Used in AMPS and others

32. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 32 IS-41 -- Part 1 Established to provide automatic roaming services in US, preceded by similar system in GSM Open interface --- used to be no standards Roaming is being served by a System other than your Home System --- a license to operate in one area by one company defines a System -- for AMPS many Systems are controlled by one MTSO, some more Can use the PSTN using SS7 or ISDN, or dedicated trunks, it is at application layer (L7) in ISO, as it defines messages for the network management application related to roaming MSC’s is term used instead of MTSO for IS-41

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34. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 34 IS-41 -- Part 2 IS-41 allows roaming by providing 3 functions and 2 key databases HLR and VLR -- mobile location registers-- normally at MSC’s Exchange of messages between Visiting and Home MSC’s Mobility management –Allows users to be registered in visiting Systems, while having the Home System be updated so it can keep track of location –Handoff across MSC’s, forward, back, to third (as it proceeds) Authentication, through MSC messages Call Management, using HLR/VLR to find mobile –Serving MSC usually uses SS7 to set up path through PSTN

35. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 35 First-Generation Analog Advanced Mobile Phone Service (AMPS) –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 –Each band has two service providers (A and B) to encourage competition –Frequency reuse exploited

36. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 36 Cellular History 1983 FCC allocation 306 Metropolitan Statistical Areas (MSAs), 424 Rural Statistical Areas A Carrier B Carrier

37. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 37 AMPS Operation Subscriber initiates call by keying in phone number and presses send key MTSO verifies number and authorizes user MTSO issues message to user’s cell phone indicating send and receive traffic channels MTSO sends ringing signal to called party Party answers; MTSO establishes circuit and initiates billing information Either party hangs up; MTSO releases circuit, frees channels, completes billing

38. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 38 AMPS Standard –30 kHz channels in 50 MHz AMPS spectrum –Analog voice, FDMA, FM modulation –Control Channels are FSK –Reuse factor 7 usually with sectoring –416 channel pairs/ carrier (395 traffic) –Mobile to base 824-849 MHz (A and B providers) –Base to mobile 869-894 MHz (A and B providers) –25 MHz per service provider: 56 and 3/7 calls/cell –Spectral Efficiency -- 2.26 calls/MHz/cell

39. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 39 Cellular Power BASE Stations –BSAs: 100 - 500 watts –MSAs: 20 - 100 watts Mobiles: – 0.006 to 4 watts

40. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 40 AMPS Spectral Efficiency In 25 MHz there are 416 dual-30KHz channels, one uplink, one downlink 21 are used for control, leaving 395 traffic channels Since 7 cell reuse is needed, 395/7=56 and 3/7 are available in each cell (56 in 4 cells, 57 in 3). This means each cell can support 56 and 3/7 two-way conversations, or calls. Spectrum efficiency –is thus 56 and 3/7 calls per cell, with a total allocated bandwidth of 25 MHz, or 56 and 3/7 divided by 25, –equals 2.26 calls/cell/MHz Sometimes spectral efficiency done in calls/MHz/km^2 -- just figure how many km^2/cell, and divide into 2.26

41. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 41 AMPS Control and Signaling Channels -- Common and Dedicated Both done digitally, as Manchester coded FSK, as tones near the carrier, + and - 8 KHz deviation, at 10 kbps –Protected by block codes, BCH, repeated 5 and 11 times to make sure no errors Common channels, called control channels, used for control and paging information --- including the information to set up calls –Forward are broadcast channels (FOCC) –Reverse (RECC) are random access, controlled by status on FOCC Signaling channels while call is active called ‘voice channels’, forward and reverse, FVC, RVC, in band, inserted maybe 1 or 2 times/minute in blank and burst lasting 100 msec, used for handoff, power control, end

42. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 42 AMPS Terminal States/Functions Initialization –Terminal scan 21 control channels -- transmitted continuously by BS’s --- it locks on to strongest one, and usually uses it as a paging channel as well (or told where) –Control channels are reused on 21 factor basis, 21 BS’s, reuse freq*3 (# SAT’s -- SAT’s are supervisory tones, 3, inserted at 6 KHz), so terminal could access cell further away if stronger Idle –Monitors paging channel from cell, looks for its number Access –Requests traffic channel from BS, gets it and ‘voice channels’ Conversation –Still some signaling, in FVC/RVC, to control call

43. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 43 ID Codes and Channel Types ID Codes –MIN --- mobile ID #, its phone number, 10 digits –ESN -- electronic serial number on terminal, factory assigned –System ID -- SID, 15 bit, ID of System, used by mobile to check vs its SID, if different it’s roaming, otherwise at Home –SAT -- supervisory audio tone, in FVC/RVC, so mobile can tell which is his BS, BS can tell which are his terminals, analog, 3 of them –DCC -- like SAT, for digital control signals, dig. color code Channels are described at 3 levels: physical, logical, network –Physical is analog (FM), digital (FSK) –Logical is functions and format -- traffic, control, ‘voice’, and how bits are arranged –Network -messages -- e.g., call setup, handoff, etc

44. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 44 Network Operations –Call management (control sequences) –Mobility management Registration messages into databases, pages on most likely cells, updates of mobile location into HLR, VLR registrations, handoff –Radio Resource Management (BS channels mgt.) call admission, channel assignment, power control of terminals (8 levels), MTSO controlled handoff --threshold with hysteresis (-100 dBm to -90 dBm), also intracell –Authentication (but no privacy) phone number+serial number(on terminal), can be intercepted –Information Transport –OA&M

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46. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 46 Some Cellular System Parameters

47. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 47 Capacity Improvement

48. 10/10/2006 Hung Nguyen, TCOM 552, Fall 2006 48 Approaches to Cope with Increasing Capacity Needs More spectrum Adding new channels 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 Cell sectoring – cells are divided into a number of wedge-shaped sectors, each with their own set of channels --- permits greater frequency reuse Microcells and picocells – antennas move to buildings, hills, and lamp posts -- but not AMPS Higher spectral efficiency technology -- e.g., AMPS to TDMA or CDMA