Cellular Network – Basis

Evolution & History of Telecommunication Systems & Cellular Networks • Frequency Reuses and Planning • Signal Quality Measurement: SIR/SNR • Power Control in Cellular Networks Cellular Network – Basis 1

What is a cellular network? • Why do we use cellular network for wide area communication, not the others? • If you are asked to design a wireless wide-area network, what will be your suggestion? Anything other than cellular? • What are the goals and performance requirements • High performance and support mobile units everywhere • Using a fixed infrastructure 3

Overview of Telecommunication Systems • 1G: Basic mobile telephony service • Based on analog cellular technology • I.e., American Mobile Phone (AMPS) and NMT in Europe • 2G: mobile telephony services for mass users with improved ciphering and efficient utilization of the radio spectrum • Digital cellular technology • i.e., GSM and CDMA • 2.5G: Mobile Internet/data services together with voice services • Packet switching technology adding into 2G • Providing mobile data services over 2G networks • i.e., GPRS (General Packet Radio Service) and EDGE • 3G: enhanced 2.5G services with improved mobile internet services and emerging new applications • CDMA2000 and UMTS (Universal Mobile Telecommunication System) • 4G (LTE/WiMAX) and XG: what will be the next? IP-based mobile networks? 4

Wireless Systems: Overview of the Development cordlessphones cellular phones satellites wireless LAN/MAN 1980:CT0 1981: NMT 450 1982: Inmarsat-A 1983: AMPS 1984:CT1 1986: NMT 900 1987:CT1+ 1988: Inmarsat-C 1989: CT 2 1991: CDMA 1991: D-AMPS 1991: DECT 199x: proprietary 1992: GSM 1992: Inmarsat-B Inmarsat-M 1993: PDC 1997: IEEE 802.11 1994:DCS 1800 1998: Iridium 1999: 802.11b, Bluetooth 2000: IEEE 802.11a 2000:GPRS analogue 2001: IMT-2000 2006: IEEE 802.16 WiMAX digital 2002: WCDMA/CDMA2000 TD-SCDMA 2009-2015: Fourth Generation (LTE) 2015 – 5G beyond? 4G – fourth generation (LTE): when and how? 5

Development of Mobile Telecommunication Systems CT0/1 FDMA AMPS CT2 NMT IMT-FT DECT IS-136 TDMA D-AMPS EDGE IMT-SC IS-136HS UWC-136 TDMA GSM GPRS PDC IMT-DS UTRA FDD / W-CDMA LTE (OFDM) HSPA IMT-TC UTRA TDD / TD-CDMA IMT-TC TD-SCDMA CDMA TD-LTE IS-95 cdmaOne IMT-MC cdma2000 1X EV-DO cdma2000 1X 1X EV-DV (3X) 4G 2G 3G 1G 2.5G The changes from FDMA -> TDMA -> CDMA->SD+TD+FD+CDMA+FH->LTE 6

Evolution of Telecommunication Systems Services From voice communication to voice and data communication Technologies From circuit switching to packet switching Point-to-point to Point-to-Many (Broadcast/Multicast) 7

Telecommunication Services • TRADITIONALLY, a (mobile) phone is for talking with another person • Phone: point-to-point talking services • NOW, mobile phones are more than phones • They are computing devices with networking capabilities • What can be done with them? • Various computing functions. Is talking a computing function? • As one of the components of a mobile computing system 8

Technological Changes • From 2G, 2.5G to 3G, 3.5G, then 4G/5G? • What are the differences? • Mainly voice communication (2G-CS) to voice and data (multimedia) to communication (3G-IP/PS) • What are the differences in performance requirements for voice communication and data communication? • Delay, traffic characteristics and accuracy requirements? • Data: could be highly bursty, large in volume for a short period of time. Encoded data are less affected to errors. Timing can be delayed in data transmission • Change from circuit switching (2G) to multiple channels and then to packet switching (3G) (what are the differences?) • Packet switching may achieve a higher bandwidth. Why—(L2)? • Change from point-to-point to point-to-many and multicast (why multicast?) 9

Telecommunication Services • Emails • Emails may be sent to another, including mobile phone users, who now hasn’t Internet email address? • Cheap and interoperable with Internet mail (connected to Internet) • What are the main differences in communicating using email and voice? • Web browsing • 2G provides limited text-based web browsing services with low-resolution graphics. 3G is better but has still many limitations • Trans-coding and adaptation of web contents to fit into handheld devices and limitations • Location-dependent services • Identify subscriber locations based on the connection points (base stations) • Location-specific web contents/services, i.e., location of banks and big sales coupons 10

Telecommunication Services -- old apps • Java applications • Download and store a variety of dynamic applications and execute the applications in the handheld devices (portability) • Mobile games • Requiring a better operating environment and higher processing power • Full IP and Video-clip • 3G enables mobile users to obtain video contents • Real-time playback of videos (video stream data transmission) • High and reliable bandwidth, i.e., 384kps –uplink; 1Mbps--downlink • Multimedia mail • Mobile picture mail services. Formatting and presentation problems • Video phone • Built-in video camera for capturing videos in 3G • Full interoperability of various video phones • How to process the images? Any compression? 11

New Telecommunication Services • Any more??? Such as … • Tracking of mobile objects, i.e., cars and animals • Remote Control and surveillance, i.e., home and field management • Shopping, marketing, logistic services, inventory control, IoT (Internet of Things) … • What will be the future mobile computing applications? Whatever you want to do or want someone to do for you … • Try to think wider, mobile communication and computing provides a mean to overcome the restrictive due to physical separation • Android based Mobile Phones, Apple iPhones 12

2015: approx. 4.88 bn Mobile Phone Subscribers Worldwide 2010: approx. 4 bn 2008: approx. 3.3 bn 2009: >4.6 bn 1600 1400 1200 GSM total 1000 TDMA total CDMA total Subscribers [million] PDC total 800 Analogue total W-CDMA 600 Total wireless Prediction 400 200 v 13 0 1996 1997 1998 1999 2000 2001 2002 2003 2004 year

Wireless Mobile Communication Layer • service location • new applications, multimedia • adaptive applications • congestion and flow control • quality of service • addressing, routing, device location • hand-over • authentication • media access • multiplexing • media access control • encryption • modulation • interference • attenuation • frequency Application layer Transport layer Network layer Data link layer Physical layer

Frequencies for Radio Transmission twisted pair coax cable optical transmission 1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz visible light VLF LF MF HF VHF UHF SHF EHF infrared UV VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency Frequency and wave length:  = c/f wave length , speed of light c  3x108m/s, frequency f 16

General Frequency Ranges • Microwave frequency range • 1 GHz to 40 GHz • Directional beams possible • Suitable for point-to-point transmission • Used for satellite communications • Radio frequency range • 30 MHz to 1 GHz • Suitable for omnidirectional applications • Infrared frequency range • Roughly, 3x1011 to 2x1014 Hz • Useful in local point-to-point multipoint applications within confined areas 17

Cellular Networks • In a cellular network, the service area is considered as a number of connected cells • Do you see the cell boundaries? No. Why? • How to define the boundary of a cell? Virtual (no physical boundary) and operational (out of the boundary => no service) • What is a cell? • In each cell, there is a base station (transmitter) to connect with the mobile stations currently within the cell • The base stations are front-end units responsible for channel allocation and mobile station management • One cell -> one base station -> several channels => multiple users • Each area (cell) normally belongs to a cell • Could it belong to more than one cell? Yes. • Does it allow overlapping in cell area? Why not 18

Cellular Networks Radio coverage, called a cell Base station f f If a mobile station is far away from a base station, it cannot communicate with the base station using radio signals => cell boundary A mobile station may receive signals from more than one base stations f2 19

Signal Propagation Ranges sender transmission distance detection interference Fr. Schiller • Transmission range • Communication possible • Low (acceptable) error rate • Detection range • Detection of the signal possible • No communication possible • Interference range • Signals may not be detected • Signal adds to the background noises 20

Cellular Networks • Why cellular? Could be any other structures/architectures? • Combining space and frequency multiplexing • Each channel has a fixed bandwidth and frequency band (frequency multiplexing) • Usually a fixed number of channels per cell • For a given service area, more cells => more channels • “Near-by” cells should not use same frequency bands • A frequency band can be reused after a suitable distance D • D interference efficiency of reuse, and vice versa • Cells modeled as polygons conceptually. Could it be circular? • Approximating circles (is it really a polygon?) • Do you see hexagons? No. Logical only • What is the distance between the centre of the cell (base station) and a corner? (next) 21

Geometric Representation • Cells are commonly represented by hexagons. • Why hexagon? • How about circle? • How about square, or triangle? 22

Hexagonal cells cover all areas Circular cells have some gaps (uncovered areas) R Fixed sizes Could be variable 23

Cellular Networks • A cellular network consists of two parts: • A fixed infrastructure which is connected by a high performance reliable fixed network • Front-end: base stations <= fixed network => backend: other servers • Mobile stations (user handsets) • Mobile stations are mobile while base stations are stationary • Mobile stations within a cell are freely to move (roaming) • Mapping: mobile station location => cell ID (base station) • Handoff/handover • Mobile stations may move from one cell into another while in connection (connect to a new base station) • In this case, a new channel is needed to be allocated from the base station of the new cell to maintain the connection • How about for the case of not in connection? Registration 24

Cellular Networks Mobile stations Stationary base stations Fixed architecture of the telecommunication systems Uneven distribution of base stations  Why? 25

Cell Sizes • Cell size: 0.1 – 30 Km (radius). Not a constant. How to determine? • Determined by the designer based on connection workload • Macro cell • Large cell for sparsely populated area (lesser users) • Micro cell • Small cell for densely populated area (more users) • More cells=> more channels for the same area size • Lower transmitter power to reduce physical cluster size (cell size) • What will be the problem if macro cell for more users places? • Umbrella cell (hierarchical cell) • Two (or multiple) levels: macro cell over multiple micro-cells • To reduce number of handoffs for fast moving vehicles • What are the benefits and problems of umbrella cells? Handoff decision and channel allocation 26

Umbrella cell: A macro cell on top of 7 micro cells. A mobile station can choose to connect to the micro cell or the macro cell. How to make the decision? 7 x micro cell channels = total micro cell channels 7 cells with similar size Each cell has a base station for connecting (channel allocation) with the mobile stations within the cell Receiving a new connection, should it be assigned to the micro cell or macro cell? 27

Tradeoffs of Cellular Networks • Advantages of cellular networks • Higher capacity: implementing space division multiplexing to allow frequency reuses to support higher bandwidth and more mobile stations • Less transmission power: Mobile stations are not far from their base stations. The power for communication can be minimized • Local interference only: Having shorter distance between mobile station and base station results in a less serious interference problem (various types of interferences and short term fading) • Robustness: Decentralized system with multiple base stations for connection with mobile stations => more fault tolerant • Disadvantages of cellular networks • Infrastructure needed: require a complex infrastructure to connect all the base stations (switching and routing) • Handoff needed: moving from one cell into another cell • Frequency and area planning to maximize the bandwidth for communication 28

Cellular Concepts for Mobile/Cellular Networks • Can the cellular network concepts be applied to other mobile networks? Yes. Any examples? • Mobile object managements • The service area is divided into region, i.e., LAN segments and grids in ad hoc networks • Fixed base stations for managing areas for connection: mobile objects => connected to a fixed base-station. Then, location of mobile objects = location of the base stations. • Dynamic base stations (i.e., in ad hoc network): the base stations may be mobile => the cell is mobile too. • Then … what is a cell? Dynamic changing cell location => difficult to tell the location of mobile objects and difficult to manage the resources. 29

Frequency Reuses and Planning How to assign frequencies to be used in each cell (Frequency Multiplexing) To maximize the total system bandwidth and minimize the degree of interferences 30

Area Planning & Frequency Assignment • Area planning problem: • How to divide the service area into cells? • Objective: to minimize the number of call blocking, channel utilizations and handoff operations (reduce ping-pong effect) • Call blocking: the connection request of a mobile station is rejected due to no spare channel • What is the size for each cell? • Smaller cells => more cells => more base stations => more operation and management cost but more channels => more users => lower call blocking probability • Channel utilization will be lower if many channels but only a few users (installation/management of channels/base stations are expensive) • What is the frequency band to be assigned to a base station for channel assignment => frequency assignment problem 31

Area Planning & Frequency Assignment f1 f2 f1 f4 f2 f3 f4 f3 How many cells? What is the size of each cell? What is the frequency band used in each cell? Remember: Neighboring cells need to have different frequencies f1 f2 f4 f3 f5 32

Frequency Assignment and Reuses • Two basic ways to assign frequencies to a base station from a given frequency band assigned to the system • Fixed frequency assignment: • Certain frequencies are assigned to a certain cell • Problem: different traffic load in different cells • Dynamic frequency assignment: • Base stationsassigned frequencies depending on the frequencies currently using in neighbor cells (real-time assignment) • More capacity in cells with more traffic • Assignment can also be based on interference measurements or a controller maintain a pool of channels • How and when to do the assignment? On request 33

Frequency Reuses Radio coverage, called a cell f f2 f The same frequency can be reused in different cells, if they are far away from each other Assign frequency on demand f1 f2 f3 f4 Controller Frequency pool 34

Channel Reuse • Total number of channels are divided into K groups. • K is called reuse factor or cluster size. • Each cell is assigned one of the groups. • The same group can be reused by two different cells provided that they are sufficiently far apart. 35

f3 f3 f3 f2 f2 f1 f1 f1 f3 f3 f2 f2 f2 f1 f1 f3 f3 f3 f2 f2 f2 f1 f1 f1 f3 f3 f3 h2 h2 h1 h1 g2 g2 h3 h3 g2 g1 g1 g1 g3 g3 g3 Frequency Planning f2 f3 f7 f5 f2 3 cell cluster f4 f6 f5 f1 f4 f3 f7 f1 Note the neighboring cells using different frequencies (separated by one cell distance) If signal strength is strong, the separation should be larger f3 f2 f6 f2 f5 7 cell cluster Which one is better? K = 3 or 7? More channels 3 cell cluster with 3 sector antennas (directional) The signals are not propagate evenly in all directions From Schiller 36

Coordinate System Use (i, j) to denote a particular cell i and j are integers called shift parameters, i.e., how many cells it has been shifted from the starting cell to the ending cell i: number of cells moved horizontally j: number of cell moved vertically from the current cell Example: Cell A is represented by (2,1) A j i 37

Reuse Factor • Consider a cluster with C number of duplex channels for use in K cells • C = c x K, where c is the number channels allocated to each cell • C is obtained by frequency division (including the guard space) • To increase the capacity (number of channels), the clusters are replicated. The degree of replication refers to the number of times a cluster is replicated • A higher degree of replication (smaller clusters) supports more users • If cluster size K is reduced (i.e., from 7 to 3) without changing the cell size, more number of clusters will be required to cover the area • Thus more channels but the degree of interference could be higher • I.e., C = 105, K = 7, c = 15; K = 3, c = 35 • K = (i + j)2 – i x j (how? See next slide) • For different values of i and j, K could be 1, 3, 4, 7, 9, 12, … 38

Coordinate System－How K is calculated? Consider distance between cell a and b k =  3R x = i k cos 30o = i  3 R cos 30o = 3/2 i x R y = i  3 R sin 30o =  3/2 i x R D2 = (3/2 i x R)2 + ( 3/2 i x R +  3 j x R)2 = 9/4 i2R2 + 3R2 (i2/4 + i x j + j2) = 3R2 (i2 + j2 + i x j) D = (3R2 (i2 + j2 + i x j)) = R (3 (i + j)2 – i x j)) b D j*k R k y i*k 30o a x 39

Distance Formula where (i,j) R D Reuse factor i and j are integers. The possible values for i and j are 0, 1, 2, 3, … Then, K = … 40

Distance Formula K = 4 Each cell = C / 4 K = 7 Each cell = C / 7 K = 3 Each cell = C / 3 C is a constant. Its value depends on the frequency band assigned to the cluster. Then, the band is divided into channels separated by guard frequencies 41

Signal Quality • The signal quality depends on the ratio between signal power and interference (noise) power • This is called signal-to-interference ratio (SIR) Interference from the i-th interfering BS. 43

Signal-to-Noise/Interference Ratio • Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission • Typically measured at a receiver • Signal-to-noise ratio (SNR, or S/N; SIR) • A high SNR means a high-quality signal, low number of required intermediate repeaters • SNR sets upper bound on achievable data rate 44

Unit of SIR (SNR) SIR is measured in dB (decibel) . SIR in dB = 10*log10(SIR in absolute value). For example: Y dB = 10*log10X – We want to find X Then X= 10Y/10. 20 dB = 102 = 100. SIR= 20~25 dB --- Excellent Signal SIR = 15 ~ 20 dB --- Good Signal SIR< 12 dB --- Relatively poor signal 45

Propagation Model • The received signal power depends on the distance between the transmitter and the receiver. • P0 is the power received at a reference distance d0. •  is called the path loss exponent. • Typically, 2 ≤  ≤ 5. 46

Worst-case Analysis • Assumption: • The user is located at the corner of a cell, i.e., d = R • The worst case Di is the distance between the centers of the reference cell and the i-th interfering cell. 47

Reuse factor: K = 7 48

Find SIR R D • Consider only the 1st tier of interfering cells • Note: in real cases, more factors needed to be consider. What are they? 49

Reuse Distance • How far apart can two users share the same channel? • It depends on whether signal quality is acceptable or not. • The larger the distance between the two users who share the same channel, the better the signal quality. • How to measure signal quality? 50

Cellular Network – Basis

Cellular Network – Basis

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