WCDMA Air Interface

3/038 13 - EN/LZU 108 5306 Rev A WCDMA Air InterfacePart 3: 1 of 22

WCDMA Air Interface Training

Part 3CDMA Capacity Considerations


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


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


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


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


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.


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.


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?


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.


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:


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


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


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


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


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


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


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


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


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


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


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


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...

WCDMA Air Interface

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