Cellular Networks No. 1 Seattle Pacific University Cellular Wireless Networks Common issues for wireless solutions Kevin Bolding Electrical Engineering

Cellular Networks No. 1Seattle Pacific University

Cellular Wireless NetworksCommon issues for wireless solutions

Kevin BoldingElectrical Engineering

Seattle Pacific University


Wireless Systems

H

Base

Network

Handsets – Portable mobile devices Base Station - Receiver

Network connects base stations


Wireless (Voice) System Design Goals

1. Transmit the voice signal without errors

2. Minimize the handset power

3. Minimize the handset cost

4. Maximize the handset/base station ratio

5. Stay within the allocated band/power

1st Generation: Analog (AMPS)

2nd Generation: Digital (GSM, CDMA)

3rd Generation: Better Digital (WCDMA, CDMA2000)


Second Generation Cell Phones

• GSM – Global System for Mobile Communications• Open international standard• 213 countries• 2 billion subscribers (82%)• All of Europe, much of Asia, much of Americas• Uses digital TDM / WDM

• CDMA – Standard owned by QualComm• 600 million subscribers (15%)• Largest in North America and Asia• Uses digital CDMA


Generic Cellular System

H

Base

H

HH

H

Base

H H

HH

H

H

Base

H

HH

H

H

Base

H

HH

H

Base Station

Controller

Base Station

Controller

Mobile Switching

Center

PSTN


Basic Issues for Mobile Communications

HBase

How do we manage multiple handsets communicating with one base?

H

H

H

H

BaseH

How do we manage handsets entering and leaving communication?

Base

Base

H

How do we manage handsets moving from base station to base station?

Base


Basic Issues for Mobile CommunicationsHow do we manage multiple handsets communicating with one base?

HBase

HH

HH

• Multiplexing (sharing the channel)

• Analog -

• Frequency-division

• Digital -

• Frequency-division and Time-division (GSM)

• Code-division (CDMA)

• We need at least:

• 1 send channel for each mobile

• 1 receive channel for each mobile

• 1 control channel



BaseH

How do we manage handsets entering and leaving communication?

• To initiate a call

• Mobile issues request on paging channel

• Receiving towers “discuss” who will answer

• To receive a call

• System must know where the mobile is

• Idle mobile periodically broadcasts on paging channel

• System broadcasts page signal on paging channel for all bases near mobile



Base

Base

H

How do we manage handsets moving from base station to base station?

Base

• Mobile uses idle slots to monitor control channels of nearby bases

• Keeps sorted list of the most powerful ones

• If error rate increases, mobile can either

• Increase power on same channel, same base

• Switch to a new base

• Handoff from base to base managed at higher level

• May be a soft handoff


Cells

•Circles don’t tile well…

Ideally, each base station serves a circular area

•Use Hexagons as approximations

17

65

23

4


Cell Channel Assignment Patterns

• We need to organize patterns for assigning channels to cells• Form a basic cell cluster that will be repeated

(tiled) to cover the entire service area • Each cell in a cluster is assigned a different set

of channels• More cells in cluster Fewer channels per cell

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R = Cell radius

D

R

D = Distance between co-channel neighbors

• Basic criterion – adjacent cells never have the same channel group• Keep cells with the same channel group as far

apart as possible• Use D/R ratio (Larger is better)

• Secondary criterion – adjacent cells have channels at least two channel groups apart• Channel 3 not adjacent to channels 2 or 4, etc.


C/I is Carrier-to-Interference Ratio• AMPS modulation characteristics

require 18 dB co-channel C/I over single interferer

• Between a pair of sites using same channel, three C/I regions exist:

• Site A C/I better than 18 dB• Neither site gives usable C/I• Site B C/I better than 18 dB

Co-Channel Interference Model

-120

-110

-100

-90

-80

-70

-60

Distance, km1 3 5 7 9 11 13 15 17 19 21 23 250

Site A Site B-50

C/I = 18 dB C/I = 18 dB

Good Service

Good ServiceInterference

RSSI,dBm

Frequency Reuse Scenario

D

R

Need a D/R that provides 18+dB C/I• D/R > 4 generally works


Cell Arrangement as a Function of N

• N=1 Lethal, works well in CDMA• Awful C/I: Every neighbor is co-

channel• Every neighbor cell is adjacent

channel too!• Center 1/3 of each cell OK, rest is

lost in horrible interference

1

11

11

11

2

1

• N=2 Better, but still lethal• Each cell still has 2 co-channel

neighbors• Each cell has 4 adjacent channel

neighbors

2

1

2

1

2

1

2

1 2

1


• N = 3 Better, but still lethal

• Co-channel neighbors are now spaced at D/R of 3.0 - better, but not 18 dB....

• Each cell has 6 adjacent channel neighbors - all neighbors are adjacent!!


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• N = 4 Better, but still lethal

• Co-channel neighbors are now spaced at D/R of 3.464

• Each cell has 4 adjacent channel neighbors

RD


• N = 5 Better, but not good enough• Co-channel neighbors farther away

• 2 at D/R of 3.0• 4 at D/R of 4.58

• Some cells have 2 adjacent channel neighbors, some have 3


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• N = 6 Better, but not by much• Co-channel neighbors farther away

• 2 at D/R of 3.464• 2 at D/R of 4.58• 2 at D/R of 6.0

• Some cells have 2 adjacent channel neighbors, some have 3


• N = 7 First arrangement that works in most propagation environments, giving 18+ dB C/I• Co-channel neighbors farther away

• 6 at D/R of 4.58

• Each cell always has 2 adjacent channel neighbors


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• N = 8 Better, but not worthwhile• Co-channel neighbors farther away


• Of the eight cells in the cluster, 2 have 2 adjacent-channel neighbors and 4 have 1 adjacent channel neighbor


• N = 9 Significant improvement

• Co-channel neighbors farther away• 6 at D/R of 5.20

• Out of 9 cells in cluster, 4 have 1 adjacent channel neighbor and 3 have 2 such neighbors


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• N = 10 Not impressively better

• Co-channel neighbors farther away• 2 at D/R of 4.58• 2 at D/R of 6.0• 2 at D/R of 6.06

• Out of 10 cells in cluster,• 6 have 1 adjacent channel neighbor• 3 have 2 adjacent-channel neighbors


• N = 11 Slightly better• Co-channel neighbors farther away


• Out of 11 cells in cluster, eight each have one adjacent channel neighbor


51

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2

910

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910

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910

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125

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10 11

12 • N = 12 Excellent but inefficient• Co-channel neighbors farther away

• 6 at D/R of 6.0

• No adjacent-channel neighbors


Frequency Reuse Implications of N• N is number of cells in frequency

reuse pattern and is critically important since it determines:• Capacity of an individual cell

• Channels per cell =

(total channels) / N• As N goes up, capacity

progressively decreases

• Interference• As N goes up, interference

becomes progressively less troublesome

Channelsper Cell* Min D/R

395198132997966564944

1.7321.7323.0003.4643.0003.4644.5834.5835.200

40 4.58336 4.583

N

12345678910

11

12 33 6.000

* Assumes use of 395 voice channels including expanded spectrum


Signal-to-Interference Ratios

Signal-to-Interference (Also known as Carrier-to-Interference) Ratio:

S/I = Signal Power / Interference Power

Signal-to-Noise+Interference (Or just Signal-to-Noise) Ratio:

S/(I+N) = Signal Power / (Interference Power + Noise Power)

In a cellular system, the main source of interference is Co-channel Inteference (CCI)

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For any regular hex tiling pattern, there are 6 co-channel neighbors.

CCI (total) = 6 x (CCI from individual interferer)


AMPS Cellular Bands

• Cellular telephony provides “full-duplex” communications• Two-way simultaneous conversation requires simultaneous voice paths in both

directions• 25 MHz band of frequencies used for mobile transmission (“Uplink”)• 25 MHz band of frequencies used for cell site transmission (“Downlink”)

• Cellular bands divided equally between two competing operators• A operator• B operator

824 835 845 870 880 894

869

849

846.5825

890

891.5

Uplink (“Reverse Path”)Downlink (“Forward Path”)

Paired Bands

Frequency, MHz

Uplink

Downlink


800 MHz Channel 800 MHz Channel Assignments by BandAssignments by Band

FunctionNo. of

ChannelsBand A

Channel numbersBand B

Channel numbers

Control channels

Voice channels

Voice channels,extended spectrum

Total number ofchannels per carrier

21

312

83

313-333

001-312

667-716991-1023

416

334-354

355-666

717-799

416


800 MHz Channel 800 MHz Channel Assignments by BandAssignments by Band

A, non-extended VCH

B, non-extended VCH

A, extended VCH

B, extended VCH

A, Control Channels

B, Control Channels

799717667666 716355312 3543340011023991 333313

Legend:

System A System B

VOICE CHANNELSCONTROLCHANNELSCHANNELS

VOICE CHANNELS

non-extendedexten-ded

non-extended extended

A’ B

VOICE

A”


A Band N = 7 Channel Sets

Channel Set 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21Designations A1 B1 C1 D1 E1 F1 G1 A2 B2 C2 D2 E2 F2 G2 A3 B3 C3 D3 E3 F3 G3

Control Ch. 333 332 331 330 329 328 327 326 325 324 323 322 321 320 319 318 317 316 315 314 313

Voice 312 311 310 309 308 307 306 305 304 303 302 301 300 299 298 297 296 295 294 293 292Channels 291 290 289 288 287 286 285 284 283 282 281 280 279 278 277 276 275 274 273 272 271

270 269 268 267 266 265 264 263 262 261 260 259 258 257 256 255 254 253 252 251 250249 248 247 246 245 244 243 242 241 240 239 238 237 236 235 234 233 232 231 230 229228 227 226 225 224 223 222 221 220 219 218 217 216 215 214 213 212 211 210 209 208207 206 205 204 203 202 201 200 199 198 197 196 195 194 193 192 191 190 189 188 187186 185 184 183 182 181 180 179 178 177 176 175 174 173 172 171 170 169 168 167 166165 164 163 162 161 160 159 158 157 156 155 154 153 152 151 150 149 148 147 146 145144 143 142 141 140 139 138 137 136 135 134 133 132 131 130 129 128 127 126 125 124123 122 121 120 119 118 117 116 115 114 113 112 111 110 109 108 107 106 105 104 103102 101 100 99 98 97 96 95 94 93 92 91 90 89 88 87 86 85 84 83 8281 80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65 64 63 62 6160 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 4039 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 1918 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

Expanded 1023 1022 1021Spectrum A' 1020 1019 1018 1017 1016 1015 1014 1013 1012 1011 1010 1009 1008 1007 1006 1005 1004 1003 1002 1001 1000

999 998 997 996 995 994 993 992 991

Expanded 716 715 714 713 712 711 710 709 708 707 706 705Spectrum A" 704 703 702 701 700 699 698 697 696 695 694 693 692 691 690 689 688 687 686 685 684

683 682 681 680 679 678 677 676 675 674 673 672 671 670 669 668 667

416Control 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Normal A 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 14 14 14A" 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 2 2 2A' 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 2 2 2 2

Total Voice 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 18 18 18 18

Set Channel Count Summary


Channel Set 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21Designations A1 BI C1 D1 E1 F1 G1 A2 B2 C2 D2 E2 F2 G2 A3 B3 C3 D3 E3 F3 G3

Control Ch. 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354

Voice 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375Channels 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396

397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666

Expanded 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732Spectrum 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753

754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795796 797 798 799

416Control 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Normal B 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 14 14 14B' 4 4 4 4 3 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4

Total Voice 19 19 19 19 18 19 19 19 19 19 19 19 19 19 19 19 19 19 18 18 18

Set Channel Count Summary

B Band N = 7 Channel Sets


Sector Cell Cluster – Frequency Reuse

B1

B2

B3

C1

C2

C3G1

G2

G3

A1

A2

A3

D1

D2

D3

E1

E2

E3

F1

F2

F3

120 degree sector

F1

F2

F3

Gain Pattern of 120-degree sector antenna

Directional sector antennas reduce the required D/R ratio


Sectoring Reduces the Interference

Only 2 of the 6 co-channelsinterfere now

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1A1C

1B1A1C

1B

1A1C

1B

1A1C

1B

1A1C

1B

1A1C

1B

1A1C

1B

Reduces CCI to 1/3 the previous level(reduced by 4.77dB)


Cell Splitting

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6

2

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4

1

A basic N=7 Frequency Reuse Plan with split cells

A basic N=7 Frequency Reuse Plan with split cells

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Documents

Cellular Networks No. 1 Seattle Pacific University Cellular Wireless Networks Common issues for wireless solutions Kevin Bolding Electrical Engineering