CHAPTER 13 CELLULAR WIRELESS NETWORKS

Transcription

1 CHAPTER 13 CELLULAR WIRELESS NETWORKS These slides are made available to faculty in PowerPoint form. Slides can be freely added, modified, and deleted to suit student needs. They represent substantial work on the part of the authors; therefore, we request the following. If these slides are used in a class setting or posted on an internal or external www site, please mention the source textbook and note our copyright of this material. All material copyright 2016 Cory Beard and William Stallings, All Rights Reserved Wireless Communication Networks and Systems 1 st edition Cory Beard, William Stallings 2016 Pearson Higher Education, Inc. Cellular Wireless Networks 13-1

2 CELLULAR NETWORKS Revolutionary development in data communications and telecommunications Foundation of mobile wireless Telephones, smartphones, tablets, wireless Internet, wireless applications Supports locations not easily served by wireless networks or WLANs Four generations of standards 1G: Analog 2G: Still used to carry voice 3G: First with sufficient speeds for data networking, packets only 4G: Truly broadband mobile data up to 1 Gbps Cellular Wireless Networks 13-2

3 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) Cellular Wireless Networks 13-3

4 d = 3R 13.1 CELLULAR GEOMETRIES Cellular Wireless Networks 13-4

5 FREQUENCY REUSE Adjacent cells assigned different frequencies to avoid interference or crosstalk Objective is to reuse frequency in nearby cells 10 to 50 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 Cellular Wireless Networks 13-5

6 N = I 2 +J 2 + I J st I, J = 0, 1, 2, etc. D R = 3N D is minimum frequency reuse distance R is the cell radius N is the number of cells in a cluster D/R is the reuse factor 13.2 FREQUENCY REUSE PATTERNS Cellular Wireless Networks 13-6

7 APPROACHES TO COPE WITH INCREASING CAPACITY 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 wedgeshaped sectors, each with their own set of channels Network densification more cells and frequency reuse Microcells antennas move to buildings, hills, and lamp posts Femtocells antennas to create small cells in buildings Interference coordination tighter control of interference so frequencies can be reused closer to other base stations Inter-cell interference coordination (ICIC) Coordinated multipoint transmission (CoMP) Cellular Wireless Networks 13-7

8 13.3 CELL SPLITTING Cellular Wireless Networks 13-8

9 CELLULAR SYSTEMS TERMS Base Station (BS) includes an antenna, a controller, and a number of receivers Mobile telecommunications switching office (MTSO) connects calls between mobile units 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 Cellular Wireless Networks 13-9

10 13.5 OVERVIEW OF CELLULAR SYSTEM Cellular Wireless Networks 13-10

11 STEPS IN AN MTSO CONTROLLED CALL BETWEEN MOBILE USERS Mobile unit initialization Mobile-originated call Paging Call accepted Ongoing call Handoff Cellular Wireless Networks 13-11

12 13.6 EXAMPLE OF MOBILE CELLULAR CALL Cellular Wireless Networks 13-12

13 ADDITIONAL FUNCTIONS IN AN MTSO CONTROLLED CALL Call blocking Call termination Call drop Calls to/from fixed and remote mobile subscriber Cellular Wireless Networks 13-13

14 MOBILE RADIO PROPAGATION EFFECTS Signal strength Must be strong enough between base station and mobile unit to maintain signal quality at the receiver Must not be so strong as to create too much co-channel interference with channels in another cell using the same frequency band Fading Signal propagation effects may disrupt the signal and cause errors Cellular Wireless Networks 13-14

15 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 Cellular Wireless Networks 13-15

16 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 Cellular Wireless Networks 13-16

17 HANDOFF STRATEGIES USED TO DETERMINE INSTANT OF HANDOFF Relative signal strength Relative signal strength with threshold Relative signal strength with hysteresis Relative signal strength with hysteresis and threshold Prediction techniques Cellular Wireless Networks 13-17

18 13.7 HANDOFF BETWEEN TWO CELLS Cellular Wireless Networks 13-18

19 POWER CONTROL Reasons 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 co-channel interference, alleviate health concerns, save battery power In SS systems using CDMA, it s necessary to equalize the received power level from all mobile units at the BS Cellular Wireless Networks 13-19

20 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 BS makes power adjustment decision and communicates to mobile on control channel Cellular Wireless Networks 13-20

21 TRAFFIC ENGINEERING Ideally, available channels would equal number of subscribers active at one time In practice, not feasible to have capacity handle all possible load For N simultaneous user capacity and L subscribers L < N nonblocking system L > N blocking system Cellular Wireless Networks 13-21

22 13.8 EXAMPLE DISTRIBUTION OF TRAFFIC IN A CELL WITH CAPACITY 10 Cellular Wireless Networks 13-22

23 BLOCKING SYSTEM PERFORMANCE QUESTIONS Probability that call request is blocked? What capacity is needed to achieve a certain upper bound on probability of blocking? What is the average delay? What capacity is needed to achieve a certain average delay? Cellular Wireless Networks 13-23

24 TRAFFIC INTENSITY Load presented to a system: λ = mean rate of calls attempted per unit time h = mean holding time per successful call A = average number of calls arriving during average holding period, for normalized λ N = Number of channels A= lh ρ= server (i.e. channel) utilization A= rn Cellular Wireless Networks 13-24

25 FACTORS THAT DETERMINE THE NATURE OF THE TRAFFIC MODEL Manner in which blocked calls are handled Lost calls delayed (LCD) blocked calls put in a queue awaiting a free channel Blocked calls rejected and dropped Lost calls cleared (LCC) user waits before another attempt Lost calls held (LCH) user repeatedly attempts calling Number of traffic sources Whether number of users is assumed to be finite or infinite Cellular Wireless Networks 13-25

26 TRAFFIC MODEL AND GRADE OF SERVICE E.g., Erlang B: Infinite sources, LCC P is the blocking probability A N P N x 0 N! A x x! Cellular Wireless Networks 13-26

27 TRAFFIC MODEL AND GRADE OF SERVICE Cellular Wireless Networks 13-27

28 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 split in two to encourage competition Frequency reuse exploited Cellular Wireless Networks 13-28

29 DIFFERENCES BETWEEN FIRST AND SECOND GENERATION SYSTEMS Digital traffic channels first-generation systems are almost purely analog; second-generation systems are digital Using FDMA/TDMA or CDMA 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 Cellular Wireless Networks 13-29

30 GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS (GSM) FDMA/TDMA approach Developed to provide a common second-generation technology for Europe Over 6.9 billion subscriber units by the end of 2013 Mobile station communicates across the Um interface (air interface) with base station transceiver in the 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 Cellular Wireless Networks 13-30

31 13.9 OVERALL GSM ARCHITECTURE Cellular Wireless Networks 13-31

32 BASE STATION SUBSYSTEM (BSS) BSS consists of base station controller and one or more base transceiver stations (BTS) Each BTS defines a single cell Includes radio antenna, radio transceiver and a link to a base station controller (BSC) BSC reserves radio frequencies, manages handoff of mobile unit from one cell to another within BSS, and controls paging Cellular Wireless Networks 13-32

33 NETWORK SUBSYSTEM (NS) NS provides link between cellular network and public switched telecommunications networks Controls handoffs between cells in different BSSs Authenticates users and validates accounts Enables worldwide roaming of mobile users Central element of NS is the mobile switching center (MSC) Cellular Wireless Networks 13-33

34 MOBILE SWITCHING CENTER (MSC) 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 Cellular Wireless Networks 13-34

35 GSM RADIO LINK Combination of FDMA and TDMA 200 khz carriers Each with a data rate of kbps 8 users share each carrier Cellular Wireless Networks 13-35

36 GENERALIZED PACKET RADIO SERVICE (GPRS) Phase 2 of GSM Provides a datagram switching capability to GSM Instead of sending data traffic over a voice connection which requires setup, sending data, and teardown GPRS allows users to open a persistent data connection Also has a new system architecture for data traffic 21.4 kbps from a 22.8 kbps gross data rate Can combine up to 8 GSM connections Overall throughputs up to kbps Cellular Wireless Networks 13-36

37 ENHANCED DATA RATES FOR GSM EVOLUTION (EDGE) The next generation of GSM Not yet 3G, so called 2.G by some Three-fold increase in data rate Up to 3 bits/symbol for 8-PSK from 1 bit/symbol for GMSK for GSM. Max data rates per channel up to = 68.4 kbps per channel Using all eight channels in a 200 khz carrier, gross data transmission rates up to kbps became possible Actual throughput up to kbps. A later release of EDGE (3GPP Release 7) increased downlink data rates over 750 kbps and uplink data rates over 600 kbps Cellular Wireless Networks 13-37

38 WCDMA AND UMTS WCDMA is part of a group of standards from IMT-2000 Universal Mobile Telephone System (UMTS) Third-Generation Partnership Project (3GPP) industry organization 3GPP originally released GSM Issued Release 99 in 1999 for WCDMA and UMTS Subsequent releases were Release 4 and onwards Many higher layer network functions of GSM were carried over to WCDMA Cellular Wireless Networks 13-38

39 WCDMA AND UMTS 144 kbps to 2 Mbps, depending on mobility High Speed Downlink Packet Access (HSDPA) Release to 14.4 Mbps downlink Adaptive modulation and coding, hybrid ARQ, and fast scheduling High Speed Uplink Packet Access (HSUPA) Release 6 Uplink rates up to 5.76 Mbps High Speed Packet Access Plus (HSPA+) Release 7 and successively improved in releases through Release 11 Maximum data rates increased from 21 Mbps up to 336 Mbps 64 QAM, 2 2 and 4 4 MIMO, and dual or multi-carrier combinations 3GPP Release 8 onwards introduced Long Term Evolution (LTE) Pathway to 4G, Chapter 14 Cellular Wireless Networks 13-39

40 UTRAN ARCHITECTURE RNS RNC: Radio Network Controller RNS: Radio Network Subsystem UE 1 Node B I ub I u RNC CN UE 2 UE 3 Node B Node B Node B Node B I ub I ur RNC UTRAN comprises several RNSs Node B can support FDD or TDD or both RNC is responsible for handover decisions requiring signaling to the UE Cell offers FDD or TDD RNS Prof. Dr.-Ing. Jochen H. Schiller MC

41 UTRAN FUNCTIONS Admission control Congestion control System information broadcasting Radio channel encryption Handover SRNS moving Radio network configuration Channel quality measurements Macro diversity Radio carrier control Radio resource control Data transmission over the radio interface Outer loop power control (FDD and TDD) Channel coding Access control Prof. Dr.-Ing. Jochen H. Schiller MC

42 CORE NETWORK: PROTOCOLS VLR RNS MSC GSM-CS backbone GMSC PSTN/ ISDN HLR RNS Layer 3: IP Layer 2: ATM Layer 1: PDH, SDH, SONET UTRAN SGSN GGSN GPRS backbone (IP) SS 7 CN PDN (X.25), Internet (IP) Prof. Dr.-Ing. Jochen H. Schiller MC

43 CORE NETWORK: ARCHITECTURE BTS A bis BSS I u VLR BSC MSC GMSC PSTN Node BTS B I u CS AuC EIR HLR Node B I ub GR Node B RNC SGSN G n GGSN G i Node B RNS I u PS CN Prof. Dr.-Ing. Jochen H. Schiller MC

44 CORE NETWORK The Core Network (CN) and thus the Interface I u, too, are separated into two logical domains: Circuit Switched Domain (CSD) Circuit switched service incl. signaling Resource reservation at connection setup GSM components (MSC, GMSC, VLR) I u CS Packet Switched Domain (PSD) GPRS components (SGSN, GGSN) I u PS Release 99 uses the GSM/GPRS network and adds a new radio access! Helps to save a lot of money Much faster deployment Not as flexible as newer releases (5, 6, 12) Prof. Dr.-Ing. Jochen H. Schiller MC

45 13.13 EVOLUTION OF CELLULAR WIRELESS SYSTEMS Cellular Wireless Networks 13-45

46 SPREADING AND SCRAMBLING OF USER DATA Constant chipping rate of 3.84 Mchip/s Different user data rates supported via different spreading factors higher data rate: less chips per bit and vice versa User separation via unique, quasi orthogonal scrambling codes users are not separated via orthogonal spreading codes much simpler management of codes: each station can use the same orthogonal spreading codes precise synchronization not necessary as the scrambling codes stay quasi-orthogonal data 1 data 2 data 3 data 4 data 5 spr. code 1 spr. code 2 spr. code 3 spr. code 1 spr. code 4 scrambling code 1 scrambling code 2 sender 1 sender 2 Prof. Dr.-Ing. Jochen H. Schiller MC

47 OVSF (ORTHOGONAL VARIABLE SPREADING FACTOR) CODING X 1,1,1,1 1,1 1,1,-1,-1 X,X 1 X,-X 1,-1,1,-1 1,1,1,1,1,1,1,1 1,1,1,1,-1,-1,-1,-1 1,1,-1,-1,1,1,-1,-1 1,1,-1,-1,-1,-1,1,1 1,-1,1,-1,1,-1,1, SF=n SF=2n 1,-1 1,-1,-1,1 1,-1,1,-1,-1,1,-1,1 1,-1,-1,1,1,-1,-1,1 1,-1,-1,1,-1,1,1,-1... SF=1 SF=2 SF=4 SF=8 Prof. Dr.-Ing. Jochen H. Schiller MC

48 BREATHING CELLS GSM Mobile device gets exclusive signal from the base station Number of devices in a cell does not influence cell size UMTS Cell size is closely correlated to the cell capacity Signal-to-nose ratio determines cell capacity Noise is generated by interference from other cells other users of the same cell Interference increases noise level Devices at the edge of a cell cannot further increase their output power (max. power limit) and thus drop out of the cell no more communication possible Limitation of the max. number of users within a cell required Cell breathing complicates network planning Prof. Dr.-Ing. Jochen H. Schiller MC