3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 1 of 22
WCDMA Air Interface Training
Part 3CDMA Capacity Considerations
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Cell Planning
• FDMA systems
Frequency Reuse Planning
SNR Link Budget
• TDMA systems
Frequency Reuse Planning
Timeslot Allocation
SNR Link Budget
• CDMA systems
Code Reuse Planning
SNR Link Budget
Interference Link Budget
3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 3 of 22
F1
F2
F7 F3
F6
F5
F4
F1
F2
F7 F3
F6
F5
F4F1
F2
F7 F3
F6
F5
F4
F1
F2
F7 F3
F6
F5
F4F1
F2
F7 F3
F6
F5
F4
F1
F2
F7 F3
F6
F5
F4
Frequency Reuse
• Basic Reuse Pattern of 7
F1
F2
F7 F3
F6
F5
F4
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Frequency Reuse
• Reuse Patterns of 4 , 3
F1
F2
F3F4
F1
F2
F3F4
F1
F2
F3F4
F1
F2
F3F4
F1
F2
F3F4
F1
F2
F3F4
F1
F2
F3F4
F1
F2
F3
F1
F2
F3
F1
F2
F3
F1
F2
F3
F1
F2
F3
F1
F2
F3
F1
F2
F3
F1
F2
F3
F1
F2
F3
F1
F2
F3
Reuse = 4 Reuse = 3
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Frequency Reuse
• Cell Sectorization
12
3
78
9
1011
1213
14
15
1617
18
1920
21
45
6
12
3
78
9
1011
1213
14
15
1617
18
1920
21
45
6
12
3
78
9
1011
1213
14
15
1617
18
1920
21
45
6
12
3
78
9
1011
1213
14
15
1617
18
1920
21
45
6
12
3
78
9
1011
1213
14
15
1617
18
1920
21
45
6
12
3
78
9
1011
1213
14
15
1617
18
1920
21
45
6
12
3
78
9
1011
1213
14
15
1617
18
1920
21
45
6
Reuse Pattern7/21
3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 6 of 22
CDMA Efficiency vs. FDMA, TDMA Multiple
Access Frequency
Reuse Carriers
per 5 MHz Users
per Carrier Users
per Cell [2,3]
AMPS FDMA 7 166 1 24
I S-136 TDMA 7 166 3 72
GSM TDMA 7[1] 25 8 28
I S-95 CDMA 1 4 35
(Theoretical) 18
(Practical)
140 (Theoretical)
72 (Practical)
cdma2000 (1x) CDMA 1 4 WWS WWS
WCDMA CDMA 1 1 WWS WWS
Notes:
[1] GSM also uses frequency reuse of 4, frequency hopping, and other techniques to optimize cell planning efficiencies.
[2] Users per cell is calculated in a 5 MHz carrier, assuming a frequency reuse of 7 for FDMA, TDMA; 1 for CDMA
[3] All FDMA, TDMA, and CDMA systems can make proportional capacity gains by using cell sectorization patternssuch as 7/21, 4/12, etc.
[4] Theoretical capacity of cdma2000 and WCDMA depends on Orthogonal Code allocations, and is therefore dependent on the mix of users and data rates. Practical capacity will depend on these and many other factors.
3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 7 of 22
CDMA Code Planning
PN1
PN2
PN7 PN3
PN6
PN5
PN4
PN1
PN2
PN7 PN3
PN6
PN5
PN4PN1
PN2
PN7 PN3
PN6
PN5
PN4
PN1
PN2
PN7 PN3
PN6
PN5
PN4PN1
PN2
PN7 PN3
PN6
PN5
PN4
PN1
PN2
PN7 PN3
PN6
PN5
PN4
PN1
PN2
PN7 PN3
PN6
PN5
PN4
CDMA Frequency Reuse: 1
PN Code Reuse Factor: 7
Sectorization: 3-Sector (7/21) Reuse
Codes available for code planning:
IS-95: 512*
cdma2000: 512*
WCDMA: 512
* IS-95 and cdma2000 use 512 discrete time offsets of the same PN code to facilitate cell planning.
In practice, IS-95 consecutive codes are avoided in order prevent potential time overlap of transmissions from widely spaced cells. The maximum number of available codes is therefore 256.
3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 8 of 22
CDMA Capacity• What determines the capacity of an FDMA system?
• What determines the capacity of a TDMA system?
• What determines the capacity of a CDMA system?
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CDMA Capacity
• Factors influencing CDMA capacity
Eb/N0 (Energy per bit - to - Noise ratio) Limited by transmit power; All base station transmissions share a single transmit power budget
Eb/I0 (Energy per bit - to Interference Ratio) Uplink: SSMA interference from mobiles in same cell, mobiles in other cells Downlink: SSMA interference from surrounding base stations, distant base stations Excessive interference associated with imperfect power control Spread Spectrum Processing Gain is reduced at higher data rates
Multipath reflections, doppler shift, near-far ratio, obstructions, etc.
3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 10 of 22
CDMA Capacity
• Example of Eb/N0 and Eb/Io capacity limitations
Cell 1
Cell 2
MS1
MS2
MS3
Cell 1 cannot accommodate MS3 because:
Cell 2 cannot accommodate MS2 because:
3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 11 of 22
CDMA Capacity
• Digital SNR: Eb/No
bb R
SE Energy per bit (Eb)
equals the average signal power (S) divided by the data bit rate (Rb)
pbb
b GSNRR
B
N
S
NR
S
N
E
00
1
Energy per bit (Eb) - to - Noise RatioThe Signal-to-Noise Ratio (SNR) times the SSMA Processing Gain
B
NN 0
Noise power density (N0)The total noise power in the signal bandwidth, divided by the signal bandwidth
3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 12 of 22
CDMA Capacity
• Eb/No vs. Probability of Error (BPSK, QPSK)
0 2 4 6 8 10 12 1410-12
10-10
10-8
10-6
10-4
10-2
100
Eb/No (dB)
Pe (
Pro
bab
ilit
y o
f b
it e
rror)
For QPSK Modulation, an Eb/N0 of ~ 7.5 dB is required to achieve a Pe of 10-3
02
1
N
EerfcP b
e
3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 13 of 22
CDMA Capacity
• Uplink Capacity Limit due to SSMA Interference
InterferingSignals
Desired Signal
1
1
MI
S
Signal-to-Interference RatioThe signal-to-interference ratio for any single user. (Assumes perfect power control; all users received at the same power)
pb
bb GMR
B
MBI
E
I
E
1
1
1
1
)/(0
Energy per bit (Eb) - to - Interference RatioThe Signal-to-Interference ratio times the SSMA Processing Gain
3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 14 of 22
CDMA Capacity
• Uplink Capacity Limit due to SSMA Interference
0/1
IE
GM
b
p
Solving for M (The number of users that can be accommodated at a given Eb/I0):
0/ IE
GM
b
p
Assuming that M >> 1;
The number of users equals the SSMA processing Gain divided by the desired Eb/I0
fb
p
vIE
GM
1
/ 0
Taking into account Voice Activity Factor:
Voice Activity Factor ranges from 0.35 to 0.65
3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 15 of 22
CDMA Capacity• Theoretical Uplink Capacity Example (one MS, one cell)
455.0
1
62.5
128310
Pe
M
Gp = 128 Spread Spectrum Processing Gain; vf = 0.5
Case I: Desired Pe = 10-3 ; Eb/I0 = 7.5 dB = 5.62
Case II: Desired Pe = 10-2 ; Eb/I0 = 5.1 dB = 3.23
Approximately 45 SSMA users can co-existin a single-cell system
with a Pe of 10-3 if each has a processing gain of 128
795.
1
23.3
128210
Pe
M
3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 16 of 22
CDMA Capacity
• Uplink Capacity in a multi-cell network
1
1
0 MG
I
Ep
b
Required Eb/No in a single-cell network:
fp
b
vfMG
I
E
11
1
0
Required Eb/No in a multi-cell network:Interference from other cells during soft handover is given by f;
fbMAX vfIE
GpM1/
1
0
Inter-cell interference factor increases with thestandard deviation of path loss; typical values of (f) range from 0.4 to 2.6 (V. Garg, “IS-95 and cdma2000”, Table 13-1)
Also Taking into account other cell interference
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CDMA Capacity
• Example 1: Uplink Capacity in a multi-cell IS-95 networkSSMA Bandwidth (B) 1.25 MHz Intercell Interference f = 0.6 (typical for 3-way soft handover, path loss deviation 8 dB)Voice Activity Factor Vf = 0.5Required Pe 10-3 (Eb/I0 = 7.5 dB = 5.62) for 9.6 kbps
10-4 (Eb/I0 = 8.5 dB = 7.62) for 14.4 kbps
28)5.0)(6.01)(62.5(
11286.9,
kbpsMAXM
Case I: Data rate = 9.6 kbps; Gp = 1.2288 Mcps / 9.6 kbps = 128
Case II: Data rate = 14.4 kbps; Gp = 1.2288 Mcps / 14.4 kbps = 85.33
15)5.0)(6.01)(1.7(
1854.14,
kbpsMAXM
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CDMA Capacity• Example 1: Uplink Capacity in WCDMASSMA Bandwidth (B) 3.84 MHz (equal to W-CDMA chip rate for this example)Intercell Interference f = 0.6 (typical for 3-way soft handover, path loss deviation 8 dB)Voice/data activity 0.5Required Pe 10-3 (Eb/I0 = 5.62)
28)5.0)(6.01)(62.5(
112815,
kbpsMAXM
Case I: Data rate = 30 ksps; Gp = 3.84 Mcps / (60 kbps/2) = 128
1)5.0)(6.01)(62.5(
1492.1,
MbpsMAXM
Case II: Data rate = 960 ksps; Gp = 3.84 Mcps / (1.92 Mbps/2) = 4
Many other factors will affect actual capacity... see next page
3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 19 of 22
CDMA Capacity
• Other factors influencing capacity
Other Factors that increase capacity: Cell Sectorization Antenna Gain Antenna Diversity Soft Handover Macrodiversity Gain Use of higher-strength error protection (e.g., turbo coding) Statistical multiplexing of packet data users
Other Factors that decrease capacity: Imperfect Power Control Downlink Interference from other Base Stations Absorption (body, terrain, structural, atmospheric...) Use of lower strength error protection on high-speed channels Multipath fading Frequency-selective fading
3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 20 of 22
CDMA Capacity
• Downlink Capacity
Fundamental Capacity Limitation is available RF transmit power One RF power budget must be split between all Mobile Stations! Fixed portion of RF power Budget allocated to Pilot, Broadcast, Paging channels
SSMA interference from other Base Stations Growing problem in Microcellular and Hierarchy topologies
Traffic channel power is allocated based on Mobile Station needs More power allocated to distant MS’s; less to nearby MS’s IS-95B provides 20msec downlink power update for 14.4 kbps data rate WCDMA, cdma2000 use fast power control on the downlink traffic channels
3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 21 of 22
Soft Blocking
• Soft vs. Hard Blocking FDMA/TDMA: When all frequencies and/or timeslots are used, calls are blocked CDMA: When interference levels are too high, calls are blocked
Making enough money?
Adjust Interference
Limit
Yes
No
Yes
New User requests Access
New User requests Access
Interference within limits?
GrantAccess
Block Access
NoAdmissionControl
Interference within limits?
• Send users to other cells, frequencies, or systems
• Reduce non-CBR data rates
• Reduce packet data throughput Reduce error protection
• Ignore downlink TPC from MS
• Selectively drop callsYes
No
Load(Congestion)Control
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Summary: CDMA Capacity
• Summary
Cell capacity not determined by size of Orthogonal Code set!
Uplink capacity is usually interference-limited
Downlink capacity limited by both RF power allocation to traffic channels and interference from other base stations
• Experience from IS-95
Typical per-sector capacity of 13 to 16 mobile stations Some operators report up to 20 mobiles per sector on a single CDMA carrier
DL capacity sometimes higher, sometimes lower than UL capacity
• And in the final analysis...
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