MSIT 413: Wireless Technologies Week 2

Transcription

1 MSIT 413: Wireless Technologies Week 2 Michael L. Honig Department of EECS Northwestern University September

2 Wireless Standards: Our Focus Cellular LAN MAN PAN Sensor/IoT GSM CDMA2000 WCDMA UMTS 1xEVDO 1xEVDV 1G/2G/3G LTE (4G) 3GPP/3GPP2 5G WiFi a/ac/b/g/n WiMax Bluetooth ZigBee Z-Wave NFC Sigfox Neul LoRaWAN 2

3 Classification of Wireless Systems Cellular Wireless Local Area Networks (WLANs) Wireless Metropolitan Area Networks (WMANs) Wireless Personal Area Networks (WPANs) Sensor Networks 3

4 Comparison of Wireless Systems 4

5 Wireless Local Area Networks (WLANs) Very high data rates (up to 600 Mbps/user!) Low mobility within confined region (building or campus) Unlicensed bands Industrial, Scientific, Medical (ISM): 2.4 GHz National Information Infrastructure (UNII): 5 GHz Must accept interference, therefore uses spread spectrum signaling, or random access with collision avoidance. Family of standards (IEEE ) 5

6 Overview of Standard IEEE GHz 850 to 950 nm FHSS DS-SS Diffuse IR 2 Mbps 4GFSK 1 Mbps 2GFSK 2 Mbps DQPSK 1 Mbps DBPSK IEEE b Extension 5.5 Mbps DQPSK-CCK 11 Mbps DQPSK-CCK 6

7 Overview of Standard IEEE discontinued 2.4 GHz 850 to 950 nm FHSS DS-SS Diffuse IR 2 Mbps 4GFSK 1 Mbps 2GFSK 2 Mbps DQPSK 1 Mbps DBPSK IEEE b Extension discontinued 5.5 Mbps DQPSK-CCK 11 Mbps DQPSK-CCK 7

8 WLAN Family of Standards: : 2 Mbps (with fallback to 1 Mbps), 1997 & b: provides additional 5.5 and 11 Mbps rates in the 2.4 GHz band a: provides up to 54 Mbps in the 5 GHz band g: Supports roaming, higher rate, backward compatible with b n: High throughput amendment using multiple antennas (Multi-Input Multi-Output (MIMO)) ac: High throughput in 5 GHz band (> 1 Gbps) using wider bandwidth, multi-user MIMO 8

9 Additional Standards ad (WiGig): up to 7 Gbps in 60 GHz band (2016) ax: enhancement of 11ac, introduces OFDMA, scheduling e: QoS & Security Enhancements f: Inter Access Point Protocol (IAPP) h: Power Management for 5 GHz in Europe i: Security enhancements j: Enhancements to a for operation in Japan k: Radio resource management m: Technical corrections and clarifications u: Interfacing with external networks v: Upper layer interface for managing equipment 9

10 802.11a/b/g/n Comparison Comparison table 10

11 Integrated WLAN-Cellular Network Internet! Application Servers! BSC BTS" cellular! AP" Home! AP" AP" WISP/Operator Hotspot! AP" Ethernet" segments" Enterprise! MS As the user moves, different access choices become available.!

12 Extension to Cellular Handoff to Cellular (LTE) connection 12

13 Integrated WLAN/Cellular Network High data rates at hot spots covered by WLANs. Lower data rates elsewhere provided by cellular. Single account; single bill Roaming, session mobility Common applications and services Cellular traffic à WiFi offload 13

14 Classification of Wireless Systems Cellular Wireless Local Area Networks (WLANs) Wireless Metropolitan Area Networks (WMANs) Wireless Personal Area Networks (WPANs) Sensor Networks 14

15 Personal Area Networks (PANs) 15

16 Bluetooth: A Global Specification for Wireless Connectivity Wireless Personal Area Network (WPAN). Provides wireless voice and data over short-range radio links via low-cost, lowpower radios ( wireless cable). Initiated by a consortium of companies (IBM, Ericsson, Nokia, Intel) IEEE standard:

17 Bluetooth Specifications Allows small portable devices to communicate together in an ad-hoc piconet (up to eight connected devices). Frequency-hopped spread-spectrum in the 2.4 GHz UNII band. Range set at 10m. Gross data rate of 1 Mbps (TDD). 64 kbps voice channels Interferes with b/g/n Second generation (Bluetooth 3.0+) supports rates up to 25 Mbps. Competes with Wireless USB. 17

18 Wireless Challenges 18

19 Today Cellular terminology Interference and capacity (voice) Performance measures (SINR, user capacity) Blocking and grade of service Channel assignments 19

20 Cellular Concept Low power Transmitters Cellular Switch (MTSO) Location Database Microcells Handoff Enables frequency reuse! PSTN 20

21 Cellular Frequency Assignments E D F C A E B G D C E A B C D F A G B A G D C B A F G C G D A D B C F E G B cell cluster (contains all channels) co-channel cells 21

22 Example of Cellular Layout drive test plots Opensignal.com 22

23 Cellular Model Hexagonal cells Regular spacing Frequency reuse limited by co-channel interference. Received power decreases with distance. Note: Freq. i à group of channels (f 1,f 2,, f K ) Actual cell footprint will be irregular (depends on terrain, etc.) 23

24 Cellular Terminology Cell cluster: group of N neighboring cells which use the complete set of available frequencies. Cell cluster size: N Frequency reuse factor: 1/N Uplink or reverse link: Mobiles à Base station D B G A F C D B G E A F C D B G A F C Downlink or forward link: Base station à mobiles Co-channel cells: cells which are assigned the same frequencies cell cluster 24

25 Frequency Reuse (Ex) Given 50 MHz for Frequency Division Duplex (FDD) cellular 200 khz simplex channel (one direction) Cell cluster size N=3 Channel Bandwidth (BW) = Total available channels = Available channels per cell = 25

26 Frequency Reuse (Ex) Given 50 MHz for Frequency Division Duplex (FDD) cellular 200 khz simplex channel (one direction) Cell cluster size N=3 Channel Bandwidth (BW) = 2 x 200 = 400 khz Total available channels = 50,000/400 = 125 Available channels per cell = 125/

27 Co-Channel Cells For hexagonal model, N is restricted: N=i 2 +ij+j 2, where i, j are positive integers A A i=3, j=2 N=i 2 +ij+j 2 =19 A A A Other examples: i= j=1 è N=3 i=0, j=2 è N=4 i=1, j=2 è N=7 i=2, j=2 è N=12 A A 27

28 Example with i=j=1 (N=3) C A C A B A C B B A C C B C A B B A A C A C B A C B cell cluster Cell cluster shape also covers the plane. 28

29 Interference and Capacity As N increases: Interference Channels per cell Capacity As N decreases: Interference Channels per cell Capacity 29

30 Interference and Capacity As N increases: Interference decreases Channels per cell decrease Capacity decreases As N decreases: Interference increases Channels per cell increase Capacity increases Objective: choose the minimum N subject to acceptable interference levels. 30

31 Sources of Interference 31

32 Sources of Interference Other users Multiple-Access interference Multipath (reflections of signals) Other devices or systems (e.g., in unlicensed band) Categories: Co-channel (frequency bands coincide) Adjacent-channel power Note: as transmitted power increases, so does the interference Channel 1 Channel 2 32 frequency

33 802.11b/g/n Channels (2.4 GHz) Comparison table Channels: frequency à 14 overlapping (staggered) channels (11 in the U.S.) Center frequencies are separated by 5 MHz Bandwidth/interference controlled by spectral mask 30 db attenuation 11 MHz from center frequency 50 db attenuation 22 MHz from center frequency 33

34 Co-Channel Reuse Ratio 1 3 R D D R = 3N From hexagonal geometry As D/R increases, interference decreases (improved isolation between co-channel cells). 34

35 Small D/R: Co-Channel Reuse Ratio Small N, large number of channels/cell More interference (fixed cell size) Large D/R: Small capacity Less interference (improved call quality) Numerical examples: i=1, j=0 è N=1, D/R= 3 i= j =1 è N=3, D/R= 3 i=1, j=2 è N=7, D/R= 4.58 i=2, j=2 è N=12, D/R= 6 35

36 Performance Measure: Signal-to-Interference-Plus-Noise Ratio (SINR) SINR = Received Signal Power Interference Power + Noise Power Expressed in db (10 log (SINR)) Typically, the interference power dominates (ignore noise) SINR à Signal-to-Interference Ratio (SIR or S/I) Total interference power is the sum over all interferers: More co-channel users à more interference 36

37 Why is SINR Important? 37

38 Why is SINR Important? The data rate depends on SINR: Data rate (bits/sec) actual curve depends on technology. SINR (db) Recall the Shannon rate: R = B log 2 (1 + SINR) B is bandwidth (Hz), rate is measured in bits per second Is it better to increase bandwidth or power? Why is SINR important for a voice service? 38

39 Why is SINR Important? For a voice service, R is fixed (say, around 10 kbps). This determines a minimum SINR that is required: Data rate voice rate SINR required SINR Smaller SINR à more co-channel users can be served Required SINR for voice users in cellular systems: 1G (AMPs): SINR 18 db G (GSM): SINR 12 db 16 2G (CDMA): SINR 7 db 5 3G (CDMA): SINR 3-5 db

40 First Tier Interference: Uplink Mobile-to-Cell Site Interference (Uplink) 6 co-channel cells (due to hexagonal cells) à 6 times the interference from a single cell 40

41 First Tier Interference: Downlink Cell Site-to-Mobile Interference (Downlink) 6 co-channel cells (due to hexagonal cells) à 6 times the interference from a single cell 41

42 Signal Attenuation distance d reference distance d 0 Reference power at reference distance d 0 Path loss exponent In db: P r = P 0 (db) 10 n log (d/d 0 ) P r (db) P 0 slope = -10n, n ~ 2 to 4 for urban cellular log (d 0 ) log (d)

43 First Tier Co-Channel Cells Signal power S P 0 (R/d 0 ) -n D First Tier Total Interference power I 6 P 0 (D/d 0 ) -n Therefore S/I = R -n /(6D -n ) = (D/R) n /6 1 R S/I (db) = 10n log (D/R) 10 log 6 = 10n log (3N) 1/2 10 log 6 Interfering Cell (will take n=4) 43

44 Same Principle for Other Wireless Devices Signal power S P 0 (R/d 0 ) -n D Total Interference power I 3 P 0 (D/d 0 ) -n Therefore S/I = R -n /(6D -n ) = (D/R) n /3 R S/I (db) = 10n log (D/R) 10 log 3 = 10 n log (3N) 1/2 10 log 3 (will take n=4) Interfering router 44

45 SIR vs. Frequency Reuse Signal-to-Interference Ratio (db) Cell cluster size N S/I (db) = 40 log (3N) 1/2 10 log 6 45

46 S/I Example Suppose desired S/I = 10 db = 10 From the graph, we can take N=3, that is, we need a 3-cell reuse pattern. But is this really adequate? 46

47 Worst Case Interference D Recalculating the S/I taking into account the different distances between co-channel cells would give an S/I < 10 db. D+R D D+R R D-R D-R To make sure the S/I 10 db, we must increase N à 4 (i=0, j=2), for which the S/I = 14 db. D However, even 14 db may not allow for additional Impairments due to different terrains, imperfect cell site location. Hence taking N=7 is safer Drawback? 47

48 Worst Case Interference D Recalculating the S/I taking into account the different distances between co-channel cells would give an S/I < 10 db. D+R D D+R R D-R D-R To make sure the S/I 10 db, we must increase N à 4 (i=0, j=2), for which the S/I = 14 db. D However, even 14 db may not allow for additional Impairments due to different terrains, imperfect cell site location. Hence taking N=7 is safer Changing N=3 à N=7 reduces the capacity by a factor of 3/7! 48

49 To Increase Capacity in Cellular Systems: 49

50 To Increase Capacity in Cellular Systems: Buy more spectrum Antenna sectoring Cell splitting Different cell configurations (zone microcell) Power control Migrate to higher efficiency systems 1G à 2G à 3G à 4G (LTE) 50

51 To Increase Capacity in Cellular Systems: Assign more spectrum Antenna sectoring Cell splitting Different cell configurations (zone microcell) Power control Migrate to higher efficiency systems 1G à 2G à 3G à 4G (WiMAX or LTE) 51

52 Sectorization (120 o ) Channels 1, 4, 7 Channels 2, 5, o 120 o 120 o Channels 3, 6, 9 Channels are divided into 3 groups, assigned to the different sectors. 52

53 Sectorization (120 o ) Two Interferers in First Ring per Sector 120 o 120 o 120 o Cell Site-to-Mobile Interference (Downlink) Mobile-to-Cell Site Interfaces (Uplink) Use directional antennas to reduce the number of interferers from 6 to 2. S/I increases by factor of 3 (about 5 db) 53

54 60 o Sectorization One Interferer in First Tier per Sector 60 o 60 o 60 o 60 o 60 o 60 o Cell Site-to-Mobile Interference (Downlink) Mobile-to-Cell Site Interfaces (Uplink) Number of interferers reduced from 6 to 1. S/I increases by factor of 6 (about 8 db). 54

55 60 o Sectorization: Worst Case Interference 2. D R R.. M D. 1 Even larger improvement relative to worst-case omni-directional antennas (about 11 db). 55

56 Disadvantages of Sectoring 56

57 Disadvantages of Sectoring Additional complexity Increased handoffs (can be accommodated at base station instead of cellular switch (MTSO), so not a major concern) Less effective in dense urban environments due to scattering of radio waves across sectors. Reduced trunking efficiency 57

58 Smart Antennas (Beamforming) Narrow beam focused on one user Different beams can use the same frequency! This is one type of Multi-Input Multi-Output (MIMO) technology. 58

59 To Increase Capacity in Cellular Systems: Assign more spectrum Antenna sectoring Cell splitting Different cell configurations (zone microcell) Power control Migrate to higher efficiency systems 1G à 2G à 3G à 4G (WiMAX or LTE) 59

60 Cell Splitting (6) (7) 2 (3) (4) (5) 6 (1) (2) Growing by Splitting Cell 4 Into Cells of Small Size Smaller cells è lower power, more channels available per unit area. 60

61 Mixed Micro/Macro Cells How to accommodate both pedestrian (low-mobility) and high-mobility users? Macro-cell (1-2 mile radius) High power (expensive) Transmitters Micro-cell (e.g., city block) Low power (inexpensive) transmitters Micro-cell overlay reduces capacity of macro-cellular network! 61

62 Umbrella Cell High mobility users communicate with large (high power base station). Must support handoff between macroand micro-cells. (Mobile speed must be estimated at base station.) 62

63 Extension to Cellular Handoff to Cellular connection 63

64 To Increase Capacity in Cellular Systems: Assign more spectrum Antenna sectoring Cell splitting Different cell configurations (zone microcell) Power control Migrate to higher efficiency systems 1G à 2G à 3G à 4G (WiMAX or LTE) 64

65 Zone Microcells Microwave or fiber optic link Zone Selector Base Station T x R x T x R x Remote Radio Heads T x R x Any channel can be assigned to any zone. No handoff between zones. Radiation localized, improves S/I. Highways, urban corridors. 65

66 Cellular Hierarchy 66

67 Remote Antennas / Radio Heads Blue circles represent remote antennas or radio heads. Signals from a cluster of antennas are processed by a Remote Central Processor. Envisioned as part of 5G. 67

68 To Increase Capacity in Cellular Systems: Assign more spectrum Antenna sectoring Cell splitting Different cell configurations (zone microcell) Power control Migrate to higher efficiency systems 2G à 3G à 4G (LTE) à 5G? 68

69 Power Control and System Migration Power control Advantages? 69

70 Power Control and System Migration Power control Minimizes interference Saves battery power Solves near-far problem (crucial in CDMA) 70

71 Power Control and System Migration Power control Minimizes interference Saves battery power Solves near-far problem (crucial in CDMA) Migration to 3G/4G/ extensions Target S/I decreases; enables increased frequency reuse IS-136 (2G) requires S/I > 12 db à N=4 GSM (2G) requires S/I > 9 db à N=3 IS-95 requires S/I > 7 db (N=1) CDMA 2000 requires S/I > 3-5 db (depending on mobility) 71

72 Will 4G Satisfy Projected Demand? 72

73 Increases in Cellular Capacity Capacity (Spectral Efficiency 73

74 Trunking and Grade of Service (GoS) Idea (trunking): Allocate channels on a per-call basis. Select from pool of available channels. 74

75 Trunking and Grade of Service (GoS) Idea (trunking): Allocate channels on a per-call basis. Select from pool of available channels. Number of Calls in progress System capacity Example: set of channels is (f 1, f 2, f 3 ) service times 1 time call arrival f blocked or 2 f 3 f 2 f 2 f 1 f 1 f delayed call 75 1

76 Grade of Service (GoS) GoS measures: Blocking probability Probability that delay > time T (e.g., maximum acceptable delay for voice call) GoS pertains to busiest hour (e.g., rush hour) GoS depends on:?? 76

77 Grade of Service (GoS) GoS measures: Blocking probability Probability that delay > time T (e.g., maximum acceptable delay for voice call) GoS pertains to busiest hour (e.g., rush hour) GoS depends on: Number of channels Call arrival rate λ Average holding time H Traffic engineering problem: determine the number of channels so that GoS meets some target performance (e.g., Prob of blocking < 2%). 77

78 Traffic Intensity Traffic intensity is defined as λ H, and measured in Erlangs. Example: λ=1 call/minute, H= 1 minute, λ H = 1 Erlang è on average, users request 1 channel λ H = ½ Erlang è there are no requests for channels more than 50% of the time Departing, or carried traffic Offered traffic ( load ) λ H = A Channel assignment f 1 f. 2 C channels f C Offered traffic is not the same as the carried traffic, due to blocking! 78

79 Erlang B Formula (1917) Formula for computing blocking probability assuming: Blocked calls disappear Requests for channel arrive according to a Poisson random process (inter-arrival times are independent, exponentially distributed) Applies to infinite user population C channels available Exponentially distributed service time Blocking probability formula: 1 Prob(call lasts < t secs) 1-e -t/τ 3 minutes time Blocked calls cleared formula (A = λh) 79

80 Erlang B Curves 80

81 Erlang C Formula Blocked calls enter a queue First come, first served Call is blocked if queuing delay D > T Formula for probability of blocking given in text. 81

82 Example Take P B = 2%, C=5 è Offered load A < 1.7 Erlangs If each user is busy 1/10 of the time (0.1 Erlang/user), Total # users < 17 (maximum) For C=10: A < 5 Erlangs or 50 users For C=100: A=88 or 880 users Observation: A/C (or users per channel) increases with C (e.g., 10 groups of 10 channels can support only 500 users) Trunking efficiency increases as C increases 82

83 Trunking Efficiency Refers to the traffic intensity (Erlangs) that can be supported given a fixed number of channels and a target blocking probability. For a fixed blocking probability: Trunking efficiency improves with the number of channels. è Best to pool as many channels as possible. 83

84 Defined as Voice Capacity [Total Total traffic carried (Erlangs) bandwidth (MHz)] Total area (km 2 ) Example: SE = 2 Erlangs/MHz/km 2 è 1 MHz supports 2 Erlangs/km 2 (Note that 1 km 2 may correspond to a cell.) If on average, each user is active 1/10 of the time, then 1 Erlang corresponds to 10 users. Traffic per cell depends on 84

85 Defined as Voice Capacity [Total Total traffic carried (Erlangs) bandwidth (MHz)] Total area (km 2 ) Example: SE = 2 Erlangs/MHz/km 2 è 1 MHz supports 2 Erlangs/km 2 (Note that 1 km 2 may correspond to a cell.) If on average, each user is active 1/10 of the time, then 1 Erlang corresponds to 10 users. Traffic per cell depends on: Number of channels Grade of Service (e.g., typically 2%) S/I requirement (determined by cluster size N) 85

86 Effect of Sectorization Does sectorization increase user capacity (Erlangs) per cell? 86

87 Effect of Sectorization For fixed N, sectorization Increases the S/I Reduces trunking efficiency Example: 90 channels per cell, 120 o sectors gives 30 channels per sector, which reduces the number of Erlangs that can be supported for a given blocking probability. Can we use sectorization to increase user capacity? Yes, must also reduce N Increases channels per cell è increases user capacity Increases interference, lowers S/I Sectorization then increases S/I 87

88 Channel Allocation Consider GSM: 25 MHz (simplex), 200 khz channels à 125 channels (1 is used for control signaling) How to allocate channels to cells? Suburb (lightly loaded) Train station Shopping mall 88

89 Channel Allocation Consider GSM: 25 MHz (simplex), 200 khz channels à 125 channels (1 is used for control signaling) How to allocate channels to cells? Objective: equalize blocking probability (target is around 2%) Fixed channel allocation assigns fixed set of channels to each cell Drawback? Suburb (lightly loaded) Train station mall 89

90 Channel Allocation Consider GSM: 25 MHz (simplex), 200 khz channels à 125 channels (1 is used for control signaling) How to allocate channels to cells? Objective: equalize blocking probability (target is around 2%) Fixed channel allocation assigns fixed set of channels to each cell. Drawback: traffic is time-varying Suburb (lightly loaded) Train station mall Dynamic channel allocation varies the number of channels per cell, depending on the load. 90

91 Dynamic Channel Allocation Channels already in use 1,3 2,6 new call: search for channel 1,3,4 5 91

92 Dynamic Channel Allocation Channels already in use 1,3 2,6 4 new call: search for channel 5 1,3,4 Channels assigned from complete set of available channels Each user searches for a channel with high SINR (low interference) No frequency planning! 92

93 802.11b/g/n Channels Comparison table Channels: overlapping (staggered) channels (11 in the U.S.) Center frequencies are separated by 5 MHz Bandwidth/interference controlled by spectral mask 30 db attenuation 11 MHz from center frequency 50 db attenuation 22 MHz from center frequency 93

94 Dynamic Channel Allocation: b/g Channel 6 Channel 11 Channel 1 94

95 Dynamic Channel Allocation: b/g Channel 6 Channel 11 Channel 1 interference Add new router Channel 6 95

96 Dynamic Channel Allocation: b/g Switches to channel 1 Channel 11 Channel 1 interference Channel 6 96

97 Dynamic Channel Allocation: b/g channel 1 Channel 11 Switches to channel 6 interference Channel 6 97

98 Dynamic Channel Allocation: b/g channel 1 Channel 11 channel 6 interference Switches to channel 1 Dynamic channel assignment becomes unstable! 98

99 Overlapping Channel Assignment channel 1 Channel 11 channel 6 power Interference (less than before) Interference (less than before) Channel 3 Channels: frequency 99

100 Channel Allocation Objective: equalize grade of service (blocking probability) over coverage area à Allows increase in subscriber pool. Fixed Channel Assignment (FCA): channels assigned to each cell are predetermined. Separate channels within a cell to avoid adjacent-channel interference Nonuniform FCA: distribute channels among cells to match averaged traffic load over time. Channel borrowing: borrow channels from neighboring cell Temporary: high-traffic cells return borrowed channels Static: channels are non-uniformly distributed and changed in a predictive manner to match anticipated traffic Dynamic Channel Assignment (DCA): channels are assigned to each call from the complete set of available channels Must satisfy S/I constraint Channels returned to pool after call is completed Can be centralized (supervised by MSC) or distributed (supervised by BS) Distributed DCA used in DECT 100

101 FCA Low complexity Better under heavy traffic Sensitive to changes in traffic Variable grade of service Higher probability of outage Suitable for macro-cellular systems (e.g., cellular) Low call setup delay Requires careful frequency planning Centralized assignment FCA vs. DCA DCA Moderate/High complexity Must monitor channel occupancy, traffic distribution, S/I (centralized) Better under light/moderate traffic Insensitive to changes in traffic Stable grade of service Low probability of outage (call termination) Suitable for micro-cellular systems (e.g., cordless) Moderate/high call setup delay No frequency planning Assignment can be centralized or distributed 101