【資料名稱】：Agilent Technologies

【資料作者】：Optimizing Your TDMA Network Today and T

【資料日期】：1099

【資料語(yǔ)言】：中文

【資料格式】：PDF

【資料目錄和簡(jiǎn)介】：


In this application note you will find
methods that can be used to easily
identify interfering base stations by
relying on a measurement receiver
to decode the DVCC of IS-136
TDMA channels. To properly
describe these methods,
background material is provided
in Sections 2, 3 and 4.
Section 1. Introduction to drive-testing
Section 2. Introduction to IS-136 TDMA
networks
Section 3. Channel planning basics
Section 4. Channel planning techniques
Section 5. Interference guidelines
Section 6. What is the DVCC?
Section 7. Method for adjacent channel
interference identification
Section 8. Methods for co-channel
interference identification
a. Clear channel
b. Wait for idle channel
c. Force idle channel
Section 9. Conclusion
Agilent Technologies
Optimizing Your TDMA Network Today
and Tomorrow
Interference Identification for IS-136
TDMA Wireless Networks
Application Note 1342


Section 1. Introduction to drive-testing
The growth and expansion of cellular and PCS networks continues at a
rapid pace throughout the world. To retain existing customers and
attract new customers, wireless service providers must maintain the
highest quality of service throughout their networks. Drive-testing
remains an essential part of the network life cycle, as an effective
means for continually optimizing network performance to maintain
customer satisfaction and reduce subscriber churn.
This application note provides an overview of how drive-test tools can
help optimize your TDMA-based cellular and PCS networks. These tools
allow you to turn-up networks faster, reduce optimization time, and
improve network quality of service.
Drive-test solutions are used for collecting measurements over a TDMA
air interface. The optimum solution combines network-independent RF
measurements using a digital receiver with traditional phone-based
measurements. A typical collection system includes a digital RF
receiver, phone, PC, GPS receiver and antennas. Refer to Figure 1.
Figure

Figure 1. Optimum drive-test solution consists of an integrated digital receiver
and phone. A GPS receiver is required for location information.


Optimization process
Optimization is an important step in the life cycle of a wireless network.
An overview of the optimization process is illustrated in Figure 2.
Drive-testing is the first step in the process, with the goal of collecting
measurement data as it relates to the user’s location. Once the data has
been collected over the desired RF coverage area, the data is output to
a post-processing software tool or mapping software such as MapInfo.
Engineers can use these tools to identify the causes of potential RF coverage
or interference problems and analyze how these problems can be
solved. Once the problems, causes, and solutions are identified, steps
are performed to solve the problem.
Figure 2 illustrates that optimization is an ongoing process. The goal is
to improve quality of service, retain existing subscribers, and attract
new ones while continually expanding the network.

Figure 2. The optimization process begins with drive-testing, moves to
post-processing, then requires data analysis, and finally action needs to be
taken correct the problems. Drive-testing is performed again to verify that the
actions were effective.


Section 2. Introduction to IS-136 TDMA networks
In recent years the number of wireless networks based on the IS-136
standard has grown considerably throughout both North America and
Latin America. Many of these networks evolved from the Advanced Mobile
Phone System (AMPS) standard. In anticipation of addressing
capacity concerns, the IS-54 standard was written, enabling TDMA
systems in the cellular frequency range (850 MHz). IS-54 based
networks were implemented, offering capacity relief to crowded AMPS
networks. As PCS frequencies became available, the IS-136 standard
emerged, providing the same TDMA operation as IS-54 in both the cellular
(850 MHz) and PCS (1900 MHz) bands. It also and provided the
mechanism for additional services. Today, mobile handsets are readily
available that can operate in TDMA and AMPS modes, and in both the
cellular and PCS bands.
For channel assignments, the original AMPS wireless networks relied
on Frequency Domain Multiple Access (FDMA). To transmit and receive
in FDMA systems, each user was assigned a dedicated frequency. In the
case of AMPS, each channel is 30 kHz wide and uses frequency modulation
(FM) to transport conversations. Since the amount of spectrum
owned by wireless service providers is limited to a fixed number of 30
kHz channels, each channel must be reused many times throughout a
network in order to provide enough channel capacity to satisfy
customer demand. Two base stations assigned to use the same channel
must be located far enough apart so that the channel users do not interfere
with each other. To provide satisfactory voice quality in a wireless
network, it is important to detect and fix situations in which base

Section 2. Introduction to IS-136 TDMA networks
(continued)
Wireless networks based on the IS-136 standard operate using two
methods of access -- FDMA and Time Domain Multiple Access (TDMA).
The same 30 kHz channel bandwidth used in AMPS systems is used
here. Also as with AMPS networks, each channel must be used by
multiple base stations, so networks must be engineered to minimize
interference between base stations. In addition, three users share each
30 kHz channel. This is accomplished by dividing the channel into time
slots so each user of the channel has full use of the channel only onethird
of the time. Thus the TDMA network capacity is tripled as
compared to an AMPS network.

Figure 3. In AMPS systems, each conversation is assigned a 30 kHz channel. In
TDMA systems three conversations are assigned to each 30 kHz
channel. Each conversation uses the channel1/3 of the time.
Interference between base stations can exist since each channel is used
in more than one base station. If two base stations are using the same
channel simultaneously it can cause co-channel interference or if they
are using an adjacent channel simultaneously it can cause adjacent
channel interference. To maximize voice quality, both types of
interference must be identified and eliminated.
It is not difficult to identify adjacent channel and co-channel
interference when optimizing IS-136 TDMA wireless networks. This is
typically done using a phone-based drive-test system, which can identify
areas of high bit error. However, identifying the base station
transmitting the interfering signal can be a challenge. Using a drive-test
tool that has both phone- and receiver-based measurements allows the
user to not only identify that interference exist, but also which base
station is the source of the interference.
Section 3. Channel planning basics
TDMA wireless service providers need to assign channels to each base
station. The channel assignments must be made in a way that minimizes
interference. A channel table is used to assign channels for each of the
service provider’s geographical markets, based on the market needs.
Channel tables are commonly used to group channels into channel sets.
Channel sets are based on a channel reuse number (the number of cells
in which all channels will be used once, before they are reused in
additional cells). The channel reuse number determines the number of
columns in the channel table. Typically the number of columns is 3
times the channel reuse number. For example, for a network using
7-cell reuse (channel reuse number = 7) there are 21 columns in the
channel table (7 cells x 3 sectors per cell = 21 channel groups). A
network with 6-cell reuse would have 18 columns. A network using
12-cell reuse would have 36 columns. The smaller the channel reuse
number, the larger the number of channels per channel set. This is true
since each of the channels owned by an operator must occupy one spot
in the channel table. If the number of columns in the table is decreased
then the number of rows will increase. Conversely, if the number of
columns increases then the number of rows decreases. Using a smaller
channel reuse number requires a shorter distance between sectors that
reuse the same channels, since the number of channel sets (columns in
the table) is decreased. Tables 1 and 2 contain typical channel tables for
both the cellular and PCS bands.


Table 1. Typical A-band cellular channel table with 7-cell reuse
A1 B1 C1 D 1 1 E F1 G1 A2 B2 C2 D 2 2 E F2 G2 A3 B3 C3 D 3E3 F3 G3
333 332 331 330 329 328 327 326 325 324 323 322 321 320 319 318 317 316 315 314 313
312 311 310 309 308 307 306 305 304 303 302 301 300 299 298 297 296 295 294 293 292
291 290 289 288 287 286 285 284 283 282 281 280 279 278 277 276 275 274 273 272 271
270 269 268 267 266 265 264 263 262 261 260 259 258 257 256 255 254 253 252 251 250
249 248 247 246 245 244 243 242 241 240 239 238 237 236 235 234 233 232 231 230 229
228 227 226 225 224 223 222 221 220 219 218 217 216 215 214 213 212 211 210 209 208
207 206 205 204 203 202 201 200 199 198 197 196 195 194 193 192 191 190 189 188 187
186 185 184 183 182 181 180 179 178 177 176 175 174 173 172 171 170 169 168 167 166
165 164 163 162 161 160 159 158 157 156 155 154 153 152 151 150 149 148 147 146 145
144 143 142 141 140 139 138 137 136 135 134 133 132 131 130 129 128 127 126 125 124
123 122 121 120 119 118 117 116 115 114 113 112 111 110 109 108 107 106 105 104 103
102 101 100 99 98 97 96 95 94 93 92 91 90 89 88 87 86 85 84 83 82
81 80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65 64 63 62 61
60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40
39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19
18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 - - -
716 715 714 713 712 711 710 709 708 707 706 705 704 703 702 701 700 699 698 697 696
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 - - - - - - - - - - - - -
1023 1022 1021 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 - - - - - - - - -

Table 2. Typical A-band PCS channel table with 12-cell reuse
A1 B1 C1 D 1 1 E F1 G1 H1 I1 J1 K1 L1 A2 B2 C2 D 2 2 E F2 G2 H2 I2 J2 K2 L2 A3 B3 C3 D 3 3 E F3 G3 H3 I3 J3 K3 L3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72
73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108
109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144
145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180
181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216
217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252
253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288
289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324
325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360
361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396
397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432
433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468
469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 - - - - -
Each column of a channel table is a channel set. Channel sets can be named in
a variety of ways. Two typical ways are to number the channel sets (1,2,3,…21)
and to use the following convention (A1,B1,C1,…A2,B2,C2,…A3,B3,C3…). Refer
to Tables 3 and 4 for channel set examples.
7

Table 3. Channel set A1 taken from A-band
cellular table, 7-cell reuse
A1
333
312
291
270
249
228
207
186
165
144
123
102
81
60
39
18
716
695
674
1023
1002
Table 4. Channel set B3 taken from A-band PCS
table, 12-cell reuse
B3
26
62
98
134
170
206
242
278
314
350
386
422
458
494
8
Table 5. A-band cellular channel table showing a designated use for each row
Designated use A1 B1 C1 D1 E1 G3
Analog control channel 333 332 331 330 329 313
Digital traffic channel 312 311 310 309 308 292
Digital traffic channel 291 290 289 288 287 271
Digital traffic channel 270 269 268 267 266 250
Digital traffic channel 249 248 247 246 245 229
Digital traffic channel 228 227 226 225 224 208
Digital traffic channel 207 206 205 204 203 187
Digital traffic channel 186 185 184 183 182 166
Digital traffic channel 165 164 163 162 161 145
Digital traffic channel 144 143 142 141 140 124
Digital traffic channel 123 122 121 120 119 103
Digital traffic channel 102 101 100 99 98 82
Digital traffic channel 81 80 79 78 77 61
Digital traffic channel 60 59 58 57 56 40
Digital traffic channel 39 38 37 36 35 19
Digital traffic channel 18 17 16 15 14 –
Digital traffic channel 716 715 714 713 712 696
Digital traffic channel 695 694 693 692 691 675
Digital control channel 674 673 672 671 670 –
AMPS 1023 1022 1021 1020 1019 1003
AMPS 1002 1001 1000 999 998 –
Each row in a channel table can be set aside for a particular use.
For example, a row is dedicated to analog control channels in mixed
AMPS/TDMA systems. In addition, rows may be set aside for AMPS
channels, Digital Traffic Channels (DTCs) and Digital Control Channels
(DCCHs). Please refer to Table 5 for an example of assigning dedicated
uses for each row in a channel table.


demand. Only a few channels are
used in areas of low mobile phone density, while all the channels in the
channel set are used in areas of high mobile phone density. Some
sectors with very high usage may require assignment of additional channel
sets in order to have enough channels to handle customer demand.
Channel plans can be designed so channel sets are assigned once in
each reuse cluster (once every 7 cells in a system using a reuse number
of 7). Figure 4 shows several clusters of cells. Each cell cluster is
shaded the same way. Cells that use the same channel are labeled with
the same number. However, cells aren’t always built on a grid with an
equal distance of separation. Terrain and traffic loading pose challenges
to a standard reuse plan. Channel sets are not always assigned in reuse
clusters, but are instead assigned where they will cause the least
amount of interference.
Figure 4. Cell clustering.
9
Section 4. Channel planning techniques
(continued)
Most TDMA infrastructure allows the channels in a base station to be
utilized in order according to an assignment list. This means that the
first channel in the assignment list is the first channel to be assigned a
call. If another call is set up it will be assigned to the second channel in
the assignment list and so on. Using this method, the first channel in the
assignment list will have the highest amount of usage and the last
channel in the assignment list will have the least amount of usage. The
last channels in the assignment list will only be used during the periods
of time when the sector is busiest.
When assigning multiple sectors of a network to use the same channel
set, alternating channel assignment can be used to minimize the
probability of co-channel interference. Alternating channel assignment
takes two of the closest sectors and assigns them the same channel set,
but their channels are assigned in alternate directions. One sector will
use the lowest channel number in the channel set then gradually
increase to higher channel numbers, until there are enough channels assigned
to meet the traffic demand. The other sector (assigned with the
same channel set) uses the highest channel number in the channel set,
then gradually decreases to lower channel numbers until there are
enough channels assigned to meet traffic demand. If the two sectors
don’t require all the channels in the channel set be used, then some
channels used at one sector will not be used at the other sector. There
will be no co-channel interference between the sectors for these
channels -- consisting of the highest and lowest channel numbers in the
channel set. This method is not as beneficial if either of the sectors
require all channels in the channel set.
10
Section 4. Channel planning techniques
(continued)
The previous example of two of the closest sectors being assigned to
the same channel set, helps illustrate the power of using alternating
channel assignment and assignment lists in conjunction. One sector is
given an assignment list, which starts with the lowest channel number
then ends with the highest used channel number in the channel set. The
other sector’s assignment list is just the opposite, starting with the highest
number in the set and ending with the lowest assigned channel.
Using this method minimizes the probability of co-channel interference
between the two sectors. Table 6 illustrates the assignment list and
alternating channel assignment techniques.
Table 6. Channel plan for channel set A1
Set A1 Sectors assigned to use channel set A1
Channel number 2γ 6β 18α 23β 34α 40α 44α 53β
333 1 21 1 1 1 1
312 2 20 2 2 2 2
291 3 19 3 3 3 3
270 4 18 4 4 4 4
249 5 17 5 5 5 5
228 6 16 6 6 6 6
207 7 15 7 7 7 7
186 8 14 8 8 8
165 9 13 9 9 9
144 10 12 10 10 10
123 11 11 11 11 11 11
102 10 12 10 12 12
81 9 13 9 13 13
60 8 14 8 14 14
39 7 7 15 7 15 15
18 6 6 16 6 16
716 5 5 17 5 17
695 4 4 4 18
674 3 3 3 19
1023 2 2 2 20
1002 1 1 1 21
The first column of Table 6 contains the channel numbers in the A1
channel set. Each additional column contains data for a sector assigned
to use channel set A1. The numbers listed under the sector name in the
table represent the position in the assignment list for the respective
channel. Each sector of a base station is designated as the alpha (α),
beta (β) or gamma (γ) sector. Notice that all the sectors are assigned to
the channel set but they don’t necessarily use all the channels in the
channel set. Sectors 6β and 53β must handle a large amount of traffic
and therefore utilize all the channels in the channel set. Sector 2γ only
needs to use channels 333-123. If demand increased at sector 2γ then
channels 102-1002 could be used. Notice that sector 40α has low
demand, only requiring the use of 7 channels. Sectors 2γ and 34α use
the alternating channel assignment technique.
11
Section 4. Channel planning techniques
(continued)
They both require 11 channels but the channels are assigned at opposite
ends of the channel set, so that only the 11th channel (channel 123) has
the possibility of co-channel interference. Sectors 18α and 40α also use
the alternating channel assignment technique; in this case there is no
overlap of channels and therefore no possibility of co-channel
interference between the two sectors. The assignment list technique is
used with sectors 6β and 53β. They both require 21 channels and so the
potential for co-channel interference between the two sites is present
on all channels. The amount of interference in minimized by using the
lower channels in the table first at 6β and by using the upper channels
in the table first for 53β.
Since 11 is the last position in the assignment list, the probability that
2γ and 34α will be using channel 123 at the same time is minimized
since channels with a lower position in the assignment list will be
used first.
Figure 5 shows where each of the sectors in Table 6 might be located
on a site map. It also shows which sectors are using the A1 channel set.
This diagram is useful in determining which sectors are likely to
interfere with each other based upon their proximity. Information about
the orientation of each sector is also conveyed.
Figure 5. Base station location with channel set reuse emphasized. All shaded
sectors use the A1 channel set, according to Table 6.
12
Section 5. Interference guidelines
This section gives quantitative measures for describing interference.
Two types of self-imposed interference are present in TDMA systems --
adjacent channel and co-channel interference. When channel planning,
it is important not to use adjacent channel groups too close together because
the two channels can interfere with each other. There is no guard
band between 30 kHz TDMA channels, so the network designer must
implement a guard band by properly assigning channels. How close
together can sectors be which use the same channel set? When frequency
planning, the carrier channel must always be stronger than any
adjacent channel. If one of the two adjacent channels is ever stronger
than the serving channel, voice quality will begin to degrade (bit errors
will be induced). To quantify this condition the carrier to adjacent ratio
can be used (C/A). The C/A is found by subtracting the dBm value of the
adjacent channel from the dBm value of the serving channel. If the C/A
is less than or equal to zero, voice quality will begin to degrade.
Interference on the same channel as the serving channel must also be
considered. In this case the signal coming from any base station other
than the serving base station (the one communicating with the phone)
must be 17 dB lower than the serving signal. If it is higher, voice quality
will begin to degrade. Planning adequate isolation between sectors that
use the same channel set will ensure interfering signals are 17 dB lower
than the serving signal. The carrier to interference ratio is used to quantify
co-channel interference. The C/I is found by subtracting the dBm
value of the interfering signal from the dBm value of the serving signal.
If the C/I is less than 17 dB then voice quality will begin to degrade.
Section 6. What is the DVCC?
The Digital Verification Color Code (DVCC) is a signal sent from the
base station to the phone, then from the phone back to the base station.
When the phone transmits the DVCC on the uplink it must send the
DVCC that it received from the base station on the downlink. The phone
cannot use prior knowledge of the DVCC. It is not allowed to send the
correct DVCC on the uplink if it received the wrong DVCC on the downlink.
If the correct DVCC is not received by the base station on the
uplink then the phone call may be handed off or dropped after a period
of time specified by the network operator. The DVCC is heavily error
coded before it is transmitted so that the DVCC can be correctly
decoded despite some amount of bit error. In cases when the heavily
protected DVCC code is not making it through correctly on both the
downlink and uplink, then some impairment must be present which is
causing bit errors.
Typically the same DVCC is assigned to all sectors of a base station. The
DVCC is transmitted on both Digital Control Channels (DCCH) and on
Digital Traffic Channels (DTC). Since there are 255 unique DVCCs, there
is usually great distance between base stations that use the same DVCC.
Each of the 255 DVCCs are assigned to a base station before having to
reuse the DVCC for an additional base station. Since the same DVCC is
typically used for all sectors of a base station, the DVCC reuse will
occur only in systems having more than 255 base stations. Once a DVCC
must be used in more than one base station, the base stations can be
isolated by a large distance, thus having little chance to interfere with
each other.
13
Section 7. Method for adjacent channel
interference identification
When there is adjacent channel interference, bit errors occur and voice
quality degrades. Many receiver-based drive-test tools are available to
help identify that the adjacent channel C/A guideline (C/A >0) has been
violated. In Figure 6 the horizontal axis is used for channel number
and the vertical axis is used for signal strength (in dBm). The figure
illustrates an adjacent channel interference problem since channel
212 is stronger than the serving signal, channel 211. The channel
number of the serving channel can be determined using a phone-based
drive-test tool.
Figure 6. Adjacent channel interference example.
14
Bit errors will occur on channel 211 due to the strong signal present on channel
212. Knowing that the interfering signal is on channel 212 is useful. A channel
plan can now be used to determine the source of the strong signal on channel
212. First, examine the channel table for channel set D3, of which channel 211 is
a member. For this example assume that the serving sector is 5β. Notice that the
techniques of alternating channel assignment and assignment list and channel assignment
are used in Table 7.
Table 7. Channel plan for channel set D3
Set D3 Sectors assigned to use channel set D3
Channel number 3γ 5β 30α 39β 45α 70α 89α 103β
316 1 16 1 1 1 1
295 2 15 2 2 2 2
274 3 14 3 3 3 3
253 4 13 4 4 4 4
232 5 12 5 5 5 5
211 6 11 6 6 6 6
190 7 10 7 7 7 7
169 8 9 8 9 8 8
148 9 8 9 8 9 9
127 10 7 7 10 7 10 10
106 11 6 6 11 6 11 11
85 5 5 12 5 12 12
64 4 4 13 4 13 13
43 3 3 14 3 14 14
22 2 2 15 2 15 15
1 1 1 16 1 16
15
A strong signal on channel 212 is causing interference on channel 211.
The channel plan for the E3 channel set can be used to determine which sectors
are assigned to use channel 212. In examining Table 8 for the E3 channel set,
notice that sectors 81α and 158β are assigned to the E3 channel set, but do not
use channel 212.
Table 8. Channel plan for channel set E3
Set E3 Sectors assigned to use channel set E3
Channel number 17α 26β 38β 49γ 81α 109γ 158β 234γ
317 1 1 1 1 1
296 2 2 2 2 2
275 3 3 3 3 3
254 4 13 4 4 4 4
233 5 12 5 5 5 5
212 6 11 6 6 6 6
191 7 10 7 7 10 7 7
170 8 9 8 8 9 8 8
149 9 8 9 9 8 9 9
128 10 7 10 10 7 10 7 10
107 11 6 11 11 6 11 6 11
86 12 5 12 5 12 5 12
65 13 4 13 4 4 13
44 14 3 14 3 3 14
23 15 2 15 2 2 15
2 1 16 1 1 16
The channel plan in Table 8 shows that the sectors in Table 9 are potential
interferers since they all use channel 212. Which of the potential interferers is the
actual interferer? The answer to this question depends on many factors. Using
knowledge of the base station locations, terrain, antenna heights and
orientations, each potential interferer can be evaluated. The results will be as
illustrated in Table 9. Some sectors can be ruled out based on network specific
knowledge, such as 38β and 49γ in Table 9. Other sectors are highly likely to
interfere, as illustrated by sectors 17α and 234γ in Table 9. The remaining sectors
are more difficult to classify, such as 26β and 109γ below. Additional steps must
now be taken to determine the interferer. Typically additional drive-testing is
required to gain additional information needed to determine the source of the
interference. In this example additional investigation would need to be
conducted for the following sectors: 17α, 26β, 109γ and 234γ.
Table 9. Potential interferers on channel 212
Potential interferers Interference
on channel 212 likely?
17α Yes
26β Maybe
38β No
49γ No
109γ Maybe
234γ Yes
16
Figure 7. Adjacent channel interference with DVCC decode.
The process of identifying the interfering base station is simplified if a
drive-test tool is able to provide the channel number, power level of the
interfering signal and the DVCC of the interfering signal. Figure 7 shows
a strong signal on channel 212, which is adjacent to the serving channel
211. In addition, the DVCC of the signal on channel 212 is shown on top
of the bar. In this case the DVCC is 234.
Knowing the DVCC, a new table can now be constructed to help
identify the interferer. All the sectors using channel 212 are listed, along
with the DVCC assigned to each of those sectors. From Figure 7, the
DVCC of the interferer was identified as 234. Using the following table it
is clear to see that 234γ is the signal transmitting the interfering sector
since it is the only sector assigned both channel 212 and DVCC 234. No
guesswork or additional investigation is required since the interferer
has been uniquely identified. Using this method reduces drive-test and
investigation time. Now that the offending base station has been
identified, steps can be taken to solve the interference problems. These
steps may include channel plan changes, antenna height or orientation
changes, and others.
Table 10. Potential interferers on channel 212
Potential interferers DVCC Interferer?
on channel 212
17α 17 No
26β 26 No
38β 38 No
49γ 49 No
109γ 109 No
234γ 234 Yes
17
Section 8. Methods for co-channel
interference identification
To illustrate a process for identifying co-channel interferers refer to
Table 11. While drive-testing with a phone-based tool, severe bit error
problems are encountered on channel 314 of sector 194α. Using a
channel plan for channel set B2 in a system with a reuse number of 12,
all of the sectors that are assigned channel 314 can be identified.
Table 11. Channel plan for channel set B2
Set B2 Sectors assigned to use channel set B2
Channel number 3α 37β 68β 100β 141α 164β 194α 203β
494 1 1 1 1 1 1
458 2 2 2 2 2 2
422 3 3 3 3 3 3
386 4 4 4 4 4 4
350 5 5 5 5 5 5
314 6 6 6 6 6 6
278 7 7 7 7 7
242 8 8 1 8
206 9 9 2 9
170 10 5 10 3 10
134 11 4 11 4 11
98 3 12 12
62 2 13 13
26 1 14 14
18
Once identified, a list of potential interferers can be built. Refer to Table
12 for the list of interferers. Notice that even though sectors 68β and 141α
are assigned to the B2 channel set they are not potential interferers since
they are not assigned channel 314. Each potential interferer can be
evaluated using knowledge of the base station locations, terrain, antenna
heights and orientations. The results of this evaluation are illustrated in
Table 12 below. Some sectors can be ruled out, such as 3α. Other sectors
are highly likely to interfere, as illustrated by sectors 37β and 100β in
Table 12. The remaining sectors are more difficult to classify, such as 164β
and 203β in Table 12. Additional steps must now be taken to determine the
interferer. Additional drive-testing would typically be required to gain
additional information needed to determine the source of the interference.
From Table 12 additional investigation would need to be conducted for the
following sectors: 37β, 100β, 164β and 203β. No further investigation
would be needed if the DVCC of the interferer was known, since the
interferer would be uniquely identified. For example, if the DVCC was
known to be 37, then 37β would be the interferer.
Table 12. Potential interferers on channel 314
Potential interferers DVCC Interferer?
on channel 314
3α 3 No
37β 37 Yes
100β 100 Yes
164β 164 Maybe
203β 203 Maybe
Section 8. Methods for co-channel
interference identification (continued)
The DVCC of the interferer can be determined using one of three methods:
clear channel, wait for idle channel or force idle channel. All three
methods involve monitoring all the channels in the serving channel’s channel
set. These channels are monitored for their power level and their
decoded DVCC value. To determine the interferer in Table 12, the B2 channel
set must be monitored using a drive-test tool with a display like Figure
8. In all three methods it may be necessary to monitor the B2 channel set
over a period of time by driving through the area of interference. The
Figure 8 shows all the channels of the B2 channel set across the horizontal
axis. Signal strength (in dBm) is shown on the vertical axis. The decoded
DVCC value of each channel is shown above the channel’s bar.
Figure 8. Sample plot of channel set B2 channels.
19
Clear channel method
The first method, clear channel, involves looking for a clear channel,
one that is used at the potentially interfering base station, but not at the
serving base station. In Table 11, sector 100β fits this description. Sector
100β has radios assigned on channels 242, 206, 170, 134, 98, 62 and 26.
All of these channels are clear channels with respect to 194α. At the
serving base station, 194α, no radios are assigned to any of these
channels so none of them will ever be in use at 194α (this can be
determined using the table for channel set B2 shown in Table 11). All of
the clear channels can be monitored using a measurement receiver
capable of decoding DVCC. If the receiver is able to decode a DVCC of
100 on any of the clear channels, then the power level of signals coming
from sector 100β can be compared with the power level of signals
coming from 194α, used to determine an inferred carrier to interference
ratio (C/I). The C/I will be accurate if all channels in a sector are set to
transmit at the same power level. This practice is followed in nearly all
cases, except when downlink power control is implemented. In Figure
9, since the receiver is able to decode a DVCC of 100 on channel 242, it
is certain that the signal is coming from sector 100β. If the signal from
sector 100β on the clear channel (channel 242) is strong enough to
interfere with the signal from 194α (the inferred C/I is less than 17 dB),
then it can be inferred that 100β is interfering with 194α on channels
494-278 (the channels that are in use at both sites). Thus sector 100β is
extremely likely to be the source of the interference on channel 314.
Figure 9. Co-channel interferer identification using clear channel method.
20
Inferred C/I of 5 dB{
Wait for idle channel method
The next method, wait for idle channel, applies to potentially
interfering base stations that have channels that completely overlap
with the serving base station. In the example shown in Table 11, sector
37β fits this description since there are no channels in use at 37β that
are not also in use at 194α. In this case there is no clear channel to
monitor, instead all overlapping channels are monitored (channels 494-
314). During times when one of the overlapping channels is not in use at
the serving base station, 194α, that channel can be monitored by the
receiver. If the receiver can decode a DVCC of 37 on the channel while
it is not in use at 194α, then the signal level can be compared to other
channels that are currently in use at 194α to infer a C/I value. In order
for the inferred value to be correct all radios assigned to a sector must
be set to the same power level. If the C/I is less than 17 dB then it
almost certain that 37β is the interfering sector causing bit error
problems on channel 314. This method requires the user to monitor
channels over time, waiting for channels at 194α to go idle and hoping
that while idle at 194α, the same channel is active at 37β. This method
can be used on any of the overlapping channels (494-314).
Figures 10 and 11 illustrate this method. Notice in Figure 9 that channel
314 is currently active at sector 194α. This is apparent since a DVCC of
194 is displayed. In Figure 11, channel 314 is no longer active at sector
194α but a signal is present. The signal has a DVCC of 37 so it must be
coming from sector 37β, which is the only sector to use channel 314 and
DVCC 37.
21
Figure 10. Co-channel interferer identification using wait for idle method, channel 314 is
busy at the serving sector, 194α.
Figure 11. Co-channel interferer identification using wait for idle method, channel 314 is
idle at the serving sector 194α but in use at 37β, note that the DVCC of channel 314 changed
from 194 to 37.
22
Inferred C/I of 15 dB{
Force idle channel method
Finally, force idle channel is the last method. Force idle channel applies to
potentially interfering base stations with channels that completely overlap
with the serving base station, as in the wait for idle channel method. The
force idle channel method is the same as wait for idle channel method
except that it does not require that the user wait for channels to go idle at
the serving base station and hope that the same channel will be active at
the interfering base station. Instead a channel is temporarily forced to be
idle at the serving base station and the same channel is temporarily forced
to be active at the potentially interfering base station. Force idle channel is
quicker than the wait for idle method, but requires the user to send control
commands to the serving base station and to the potentially interfering
base stations in order to force the channels idle and active respectively.
Force idle channel method can be used if there is high channel usage at
the serving base station, making the wait for idle method ineffective.
All three methods discussed can be effective in determining the DVCC of
the interfering base station. Using these methods saves time over methods
that don’t involve the use of decoded DVCC because they provide the user
with the ability to definitely determine the base station transmitting the
interfering signals. Once the interfering base station has been identified,
steps can be taken to solve the interference problems. As with adjacent
channel interference, these steps may include channel plan changes,
antenna height or orientation changes, and others.
Section 9. Conclusion
Adjacent and co-channel interference problems in IS-136 TDMA wireless
networks are not difficult to identify using a phone-based drive-test tool.
However, identifying the base station which is the source of the
interference can be difficult and time consuming without the use of
identification methods that rely on a receiver-based drive-test tool capable
DVCC decode. The methods which have been presented for interferer
identification can save time and lead to significant voice quality
enhancement in IS-136 TDMA wireless networks.
23
We offer application notes
that span many of today’s RF
network issues:
• Optimizing your CDMA Wireless
Network Today and Tomorrow.
Using Drive-Test Solutions
Application Note-1345 (literature
number 5968-9916E).
• Optimizing your GSM Network
Today and Tomorrow. Using
Drive-Testing to Troubleshoot
Coverage, Interference, Handover
Margin and Neighbor Lists.
Application Note-1344 (literature
number 5980-0218E)
For specific examples of how
the Agilent Technologies drivetest
solutions are used to
solve optimization problems:
• Spectrum and Power
Measurements Using the Agilent
CDMA, TDMA and GSM Drive-Test
System Product Note (literature
number 5968-8598E)
For additional Agilent
Technologies TDMA drive-test
information:
• E7474A TDMA Drive-Test System
Configuration Guide
(literature number 5968-5861E)
• E7474A TDMA Drive-Test System
Technical Specifications
(literature number 5968-5556E)
• Indoor Wireless Measurement
System Product Overview
(literature number 5968-8691E)
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