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Week 13 lecture 1 Cellular Communications (I)

Week 13 lecture 1 Cellular Communications (I). Existing Network Infrastructure. Public Switched Telephone Network (PSTN): Voice Internet: Data Hybrid Fiber Coax (HFC): Cable TV. Market Sectors for Applications.

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Week 13 lecture 1 Cellular Communications (I)

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  1. Week 13 lecture 1Cellular Communications (I)

  2. Existing Network Infrastructure • Public Switched Telephone Network (PSTN): Voice • Internet: Data • Hybrid Fiber Coax (HFC): Cable TV

  3. Market Sectors for Applications • Four segments divided into two classes: voice-oriented and data-oriented, further divided into local and wide-area markets • Voice: • Local: low-power, low-mobility devices with higher QoS – cordless phones, Personal Communication Services (PCS) • Wide area: high-power, comprehensive coverage, low QoS - cellular mobile telephone service • Data: • Broadband Local and ad hoc: WLANs and WPANs (WPAN-Wireless Personal Area Network) • Wide area: Internet access for mobile users

  4. Evolution of Voice-Oriented Services FDMA – Frequency Division Multiple AccessNMT – Nordic Mobile TelephonyAMPS – Advanced Mobile Phone SystemGSM – Global System for Mobile CommunicationTDMA – Time Division Multiple Access IS-95 – Interim Standard 95CDMA – Code Division Multiple AccessPCS – Personal Communication SystemFCC – Federal Communication Commission

  5. Evolution of Data-Oriented Services HIPERLAN – High Performance Radio LANCDPD – Cellular Digital Packet DataU-NII – Unlicensed National Information InfrastructureGPRS – General Packet Radio Service

  6. Different Generations: 1G • 1G Wireless Systems: Analog systems • Use two separate frequency bands for forward (base station to mobile) and reverse (mobile to base station) links: Frequency Division Duplex (FDD) • AMPS: United States (also Australia, southeast Asia, Africa) • TACS: EU (later, bands were allocated to GSM) • NMT-900: EU (also in Africa and southeast Asia) • Typical allocated overall band was 25 MHz in each direction; dominant spectra of operation was 800 and 900 MHz bands. AMPS – Advanced Mobile Phone SystemTACS – Total Access Communication SystemNMT – Nordic Mobile Telephony

  7. 2G • 2G Wireless Systems: Four sectors • Digital cellular • GSM (EU/Asia): TDMA • IS-54 (US): TDMA • IS-95 (US/Asia): CDMA • PCS – residential applications • CT-2 (EU,CA): TDMA/TDD • DECT(EU):TDMA/TDD • PACS (US): TDMA/FDD CT-2 – Cordless Telephone 2DECT – Digital Enhanced Cordless TelephonePACS – Personal Access Communication System

  8. 2G (cont’d) • 2G Wireless Systems: Four sectors (cont’d) • Mobile data • CDPD shares AMPS bands and site infrastructure; • GPRS shares GSM’s radio system - data rates suitable for Internet • WLAN – Unlicensed bands, free of charge and rigorous regulations: very attractive! • IEEE 802.11 and IEEE 802.11b use DSSS physical layer; • HIPERLAN/1 uses GMSK; • IEEE 802.11a and HIPERLAN/2 use OFDM: next generation CDPD – Cellular Digital Packet DataGPRS – General Packet Radio Service DSSS – Direct Sequence Spread SpectrumGMSK – Gaussian Minimum Shift KeyingOFDM – Orthogonal Frequency Division Multiplexing

  9. 3G • 3G and Beyond • Purpose: develop an international standard that combines and gradually replaces 2G digital cellular, PCS, and mobile data services, at the same time increasing the quality of voice, capacity of the network, and data rate of the mobile data services. • Radio transmission technology of choice: W-CDMA • 3G was envisioned to provide multimedia services to users everywhere

  10. Summary Relative coverage, mobility, and data rates of generations of cellular systemsand local broadband and ad hoc networks.

  11. Satellite Regional Area Low-tier High-tier Local Area Wide Area Low Mobility High Mobility Heterogeneous Cellular Networks • Seamless mobility across diverse overlay networks • “vertical” hand-offs • software “agents” for heterogeneity management • IP as the common denominator?

  12. Design Cellular Networks

  13. Cellular Network Architecture LocationRegister (Database) Radio Network Mobile Switching Center Base Station Controller MSC Backbone Wireline Network Mobile Terminal Base Station Cell

  14. BASIC ARCHITECTURE Home Location Register (HLR) BACKBONE TELEPHONE NETWORK Visitor Location Register (VLR) Mobile Switching Center MSC (MSC) VLR Mobile Terminal (MT) Local Signaling Long Distance Signaling

  15. Cellular Concept • The most important factor is the size and the shape of a CELL. • A cell is the radio coverage area by a transmitting station or a BS. • Ideally, the area covered a by a cell could be represented by a circular cell with a radius R from the center of a BS. • Many factors may cause reflections and refractions of the signals, e.g., elevation of the terrain, presence of a hill or a valley or a tall building and presence in the surrounding area. • The actual shape of the cell is determined by the received signal strength. • Thus, the coverage area may be a little distorted. • We need an appropriate model of a cell for the analysis and evaluation. • Many posible models: HEXAGON, SQUARE, EQUILATERAL TRIANGLE.

  16. Cell Shape R R R Cell R R (c) Different Cell Models (a) Ideal Cell (b) Actual Cell

  17. Size and Capacity of a Cell per Unit of Area and Impact of the Cell Shape on System Characteristics

  18. Cellular Concept - Example • Consider a high-power transmitter that can support 35 voice channels over an area of 100 km2 with the available spectrum • If 7 lower power transmitters are used so that they support 30% of the channels over an area of 14.3 km2 each. • Then a total 7*(30% * 35) = 80 channels are available instead of 35. 2 3 1 7 4 6 5

  19. Cellular Concept • If two cells are far away from enough that the same set of frequencies can be used in both cells, it is called frequency reuse. • With frequency reuse, a large area can be divided into small areas, each uses a subset of frequencies and covers a small area. • With frequency reuse, the system capacity can be expanded without employing high power transmitters.

  20. Capacity Expansion by Frequency Reuse • Same frequency band or channel used in a cell can be “REUSED’ in another cell as long as the cells are far apart and the signal strength do not interfere with each other. • This enhances the available bandwidth of each cell. • A group of cells that use a different set of frequencies in each cell is called a cell cluster.

  21. NUMBER OF CELLS IN A CLUSTER

  22. F7 F7 F7 F7 F2 F2 F2 F2 F6 F6 F6 F6 F1 F1 F1 F1 F5 F5 F5 F5 F3 F3 F3 F3 F4 F4 F4 F4 FREQUENCY REUSEExample:A typical cluster of 7 such cells and 4 such clusters with no overlapping area FREQUENCY REUSE DISTANCE D |------ | |D | | ----------

  23. RULE to Determine the Nearest Co-Channel Neighbors (with the same frequency set): Determining the Cluster Size j • To find nearest co-channel neighbors of a particular cell • Step 1: Move i cells along any chain of hexagons; • Step 2: Turn 60 degrees counterclockwise and move j cells AND REACH the co-channel. • i and j measure the number of nearest neighbors between co-channel cells • The cluster size, N, N = i2+ij+j2 i 3 2 1 4 If i =2 and j = 0, then N = 4 If i = 2 and j = 1, then N =7 1 2 3 2 1

  24. Frequency Reuse • The distance between 2 cells using the same channel is known as the REUSE DISTANCE D. • There is a close relationship between D, R (radius of each cell) and N (the number of cells in a cluster) -- details later D = (sqrt 3N) . R • The REUSE FACTOR is then D/R = sqrt (3N)

  25. Frequency Reuse • Let N be the cluster size in terms of number of cells within it and K be the total number of available channels without frequency reuse. • N cells in the cluster would then utilize all K available channels. • Each cell in the cluster then uses 1/N-th of the total available channels. • N is also referred as the frequency reuse factor of the cellular system.

  26. Capacity Expansion by Frequency Reuse • Assume each cell is allocated J channels (J<=K). If the K channels are divided among the N cells into unique and disjoint channel groups, each with J channels, then K = J N • The N cells in a cluster use the complete set of available frequencies. • The cluster can be replicated many times. • LetM be the number of replicated clusters andC be the total number of channels in the entire system with frequency reuse, then C is the system capacity and computed by C = M J N

  27. Cellular System Capacity - Example • Suppose there are K=1001 radio channels, and each cell is Acell = 6 km2 and the entire system covers an area of Asys = 2100km2. • Calculate the system capacity if the cluster size is N=7. • How many times would the cluster of size 4 have to be replicated in order to approximately cover the entire cellular area? • Calculate the system capacity if the cluster size is 4. • Does decreasing the cluster size increase the system capacity? Solution: • J=K/N=143, Acluster=N*6=42km2, M (# of clusters)=2100/42=50, C=MK=50,050 chs. 2. N=4, Ac=4*6=24km2, M=2100/24=87. 3. N=4, J = 1001/4 = 250 chs/cell. C = 87 * K= 87,000 chs. • Decrease in N from 7 to 4 increase in C from 50,050 to 87,000.  Decreasing the cluster size increases system capacity. So the answer is YES!

  28. Geometry of Hexagonal Cells (1)How to determine the DISTANCEbetween the nearest co-channel cells ? • Planning for Co-channel cells • D is the distance to the center of the nearest co-channel cell • R is the radius of a cell j D i 30o R 0

  29. Geometry of Hexagonal Cells (2) • Let Dnorm be the distance from the center of a candidate cell to the center of a nearest co-channel cell, “normalized” with respect to the distance between the centers of two adjacent cells, . • Note that the normalized distance between two adjacent cells either with (i=1,j=0) or (i=0,j=1) is 1. • Let D be the “actual” distance between the centers of two adjacent co-channel cells. D = Dnorm .

  30. Geometry of Hexagonal Cells (3) • From the geometry we have • From N and Dnorm equations

  31. Geometry of Hexagonal Cells (4) The actual distance between the center of the candidate cell and the center of a nearest co-channel is then: • For hexagonal cells there are 6 nearest co-channel neighbors to each cell (if cluster size = 7). • Co-channel cells are located in tiers. • In general, a candidate cell is surrounded by 6k cells in tier k. • For cells with the same size the co-channel cells in each tier lie on the boundary of the hexagon that chains all the co-channel cells in that tier.

  32. Geometry of Hexagonal Cells (5) • As D is the radius between two nearest co-channel cells, the radius of the hexagon chaining the co-channel cells in the k-th tier is given by k.D. • For the frequency reuse pattern with i=2 and j=1 so that N=7, the first two tiers of co-channel cells are given in Figure (next slide). • It can be readily observed from Figure that the radius of the first tier is D and the radius of the second tier is 2.D.

  33. Calculate Number of Cells in A Cluster • A candidate cell has 6 nearest co-channel cells. Each of them in turn has 6 neighboring co-channel cells. So we can have a large hexagon. • This large hexagon has radius equal to D which is also the co-channel cell separation. • The area of a hexagon is proportional to the square of its radius, (let =2.598), R D

  34. Calculate Number of Cells in A Cluster • The number of cells in the large hexagon is then • In general the large hexagon encloses the center cluster of N cells plus 1/3 the number of the cells associated with 6 other peripheral large hexagons. • Hence, the total number of cells enclosed by the large hexagon is • Thus we proved N = f(i,j) mentioned before

  35. Frequency Reuse Ratio • The frequency reuse ratio, q, is defined as q = D/R which is also referred to as the co-channel reuse ratio. Thus  q = D/R = sqrt(3N) Tradeoff • q increases with N. • However, a smaller value of N has the effect of increasing the capacity of the cellular system • But Smaller N can increase co-channel interference • Tradeoff on N

  36. We assume the size of all the cells is roughly the same, as long as the cell size is fixed, co-channel interference will be independent of transmitted power of each cell. The co-channel interference will become a function of q where q = D/R = sqrt (3N). (q is the CO-CHANNEL REUSE RATIO and is related to the cluster size). A small value of q provides larger capacity since N is small. For large q, the transmission quality is better, smaller level of co-channel interference. By increasing the ratio of D/R, spatial separation between co-channel cells relative to the coverage distance of a cell is increased.Thus, interference is reduced from improved isolation of RF energy from the number of cells per cluster N co-channel cells.

  37. Geometry of Hexagonal Cells (7) Furthermore, D (distance to the center of the nearest cochannel cell) is a function of NI and S/I (next lecture) in which NI is the number of co-channel interfering cells in the first tier and S/I = received signal to interference ratio at the desired mobile receiver.

  38. Questions

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