December, 1999 1 - 1Optimization v2.0 (c) 1999 Scott Baxter

Sector Traffic Loading EffectsSector Traffic Loading Effects

Case Study: Traffic Loading

December, 1999 1 - 2Optimization v2.0 (c) 1999 Scott Baxter

Runaway Class turns to Dark Side of the Force

� A major PCS operator often holds technical classes in an attractive conference center on the south side of Kansas City

� In early November, 1998, a CDMA performance optimization class realized it had a large number of mobiles on hand and decided to try to push a cell to the limit: to see just how far we could go in cell loading, and what would happen

� Data collection equipment was on hand to record the event from the mobile side

� System operations personnel were available to retrieve system-side statistics for the period

� Let’s see what happened!

December, 1999 1 - 3Optimization v2.0 (c) 1999 Scott Baxter

The BTS at the BTA Conference Center

� The classroom is about 500 feet northwest of the three-sector BTS

� The BTS’ gamma face is the dominant sector for the classroom, at PN 212.

Looking northwest

212

208

204

N

Classroom

BTS

from RFCAD

from IQAnalyzer

December, 1999 1 - 4Optimization v2.0 (c) 1999 Scott Baxter

What to Expect:Loading Effects on the Forward Link

� On the forward link, the overhead channels (Pilot, Sync, and Paging) remain constant

� Each new traffic channel consumes additional transmit power

� Total transmit power increases as traffic increases

� Ec/Io decreases as traffic increases (Ec stays the same, but Io is driven up)

Light Traffic Loading

Ec/Io = (2/4)= 50%

= -3 db.

2w

1.5w

Pilot

PagingSync I0EC

0.5w

BTSTransmit

Power

Heavily Loaded

Ec/Io = (2/10)= 20%

= -7 db.

2w

1.5w

Pilot

PagingSync

I0

EC

Traffic Channels

6w

0.5w

BTSTransmit

Power

December, 1999 1 - 5Optimization v2.0 (c) 1999 Scott Baxter

What to Expect:Loading Effects on the Reverse Link

� On a lightly-loaded sector, the noise floor is relatively low and an individual mobile can be heard at comfortably low power

� When the forward power goes up, each mobile’s open-loop power control will try to decrease mobile power output

� On a heavily loaded sector each mobile is competition against the others, so the BTS must raise each mobile’s power to remain competitive

� Closed Loop power control takes a “double hit” – correcting for both increased noise and the mobile’s incorrect power control instincts

Lightly Loaded Sector

ThermalNoise

Mobile

BTSReceivePower

Heavily Loaded Sector

ThermalNoise

Other

Mobiles

BTSReceivePower

Mobile

“I can hear it coming in the air tonight…..”--Phil Collins

December, 1999 1 - 6Optimization v2.0 (c) 1999 Scott Baxter

Light Traffic Loading

Heavily Loaded

BTS Loading Effects on the Reverse Link

� On the reverse link, receive power at the BTS increases when traffic increases

• BTS closed-loop power control must counter this trend, keeping each mobile competitive with the rest

� On the reverse link, the mobile responds inversely to BTS power output changes

• When traffic drives BTS power up, the mobile instinctively tries to power down

• BTS closed-loop power control must also counter this trend

� Mobile transmit power increases substantially during heavy-traffic periods! 2w

1.5w

Pilot

PagingSync

I0

EC

Traffic Channels

6w

0.5w

2w

1.5w

Pilot

PagingSync I0EC

0.5w

December, 1999 1 - 7Optimization v2.0 (c) 1999 Scott Baxter

All Phones in Idle Mode

Test Mobile C

all Begins

25+ Mobiles

Calls begin

Test Mobile call has ended,Other mobile calls continue

The Ground Shakes

December, 1999 1 - 8Optimization v2.0 (c) 1999 Scott Baxter

Test Mobile Receive Power

Average-76.5 dbm

With one user

Average-70.5 dbm

With max users

As expected, the additional calls increase the total power

output of the sector. This causes received power to

increase at the test mobile.

December, 1999 1 - 9Optimization v2.0 (c) 1999 Scott Baxter

Test Mobile Combined Ec/Io

Average-3.6 db

With one user

Average-6.8 db

With max users

Since the additional calls increase the total power

output of the sector, but the pilot power remains fixed, the Ec/Io at the test mobile decreases in proportion.

December, 1999 1 - 10Optimization v2.0 (c) 1999 Scott Baxter

Test Mobile Closed-Loop Power Control (TXGA)

Average-16 db

With only thisUser active

Average-6 db

while max usersactive

Since the additional calls increase the noise level at the BTS receiver, the BTS

must ask the test mobile to increase its transmit power output to keep up with the

crowd.

About 6 db of this increase is necessary to counteract the

mobile’s own open-loop instinct to power down due to increased BTS power.

The rest is needed to keep the mobile’s signal competitive at

the BTS.

December, 1999 1 - 11Optimization v2.0 (c) 1999 Scott Baxter

Test Mobile Transmit Power

Average-16 dbm

With this user only

Average-10 dbm

While max users active

Responding to the BTS closed-loop power control

instructions, the test mobile operates at a higher transmit power while competing with

many other users.

Why does all this data bounce around so much?

1. Random motion of users2. Rayleigh fading

3. Users’ varying vocoder rates4. Interference from elsewhere

December, 1999 1 - 12Optimization v2.0 (c) 1999 Scott Baxter

System-Side Data: Channel Element Usage

BTS DateStart Time End Time MOU CE

MOU Traffic CE/User

MOU Alpha

MOU Beta

MOU Gamma %SHO Max TCE

196 11/3/98 7:00:00 7:30:00 256.73 130.11 1.97 37.2 58.52 34.38 49.32 23196 11/3/98 8:00:00 8:30:00 265.42 145.49 1.82 45.22 62.49 37.78 45.18 17196 11/3/98 8:30:00 9:00:00 342.7 186.94 1.83 52.01 90.66 44.28 45.45 18196 11/3/98 9:00:00 9:30:00 317.5 172.02 1.85 43.67 79.94 48.4 45.82 21196 11/3/98 9:30:00 10:00:00 408.81 245.55 1.66 78.35 92.33 74.87 39.93 22196 11/3/98 10:00:00 10:30:00 288.33 138.41 2.08 46.61 60.9 30.91 52 16196 11/3/98 10:30:00 11:00:00 334.61 195.06 1.72 59.71 81.78 53.58 41.71 22196 11/3/98 10:30:00 11:00:00 289.53 161.27 1.8 60.04 60.48 40.75 44.3 18196 11/3/98 11:00:00 11:30:00 366.75 210.19 1.74 70.51 91.65 48.03 42.69 21196 11/3/98 12:00:00 12:30:00 299.25 156.26 1.92 53.34 63.01 39.91 47.78 18196 11/3/98 12:00:00 12:30:00 343.03 196.39 1.75 60.06 83.54 52.79 42.75 22196 11/3/98 13:00:00 13:30:00 327.2 225.23 1.45 71.01 78.72 75.51 31.16 31196 11/3/98 13:00:00 13:30:00 316.68 168.14 1.88 54.19 68.32 45.62 46.9 18196 11/3/98 13:30:00 14:00:00 270.9 163.34 1.66 57.55 55.8 49.99 39.7 18196 11/3/98 14:00:00 14:30:00 266.42 137.25 1.94 42.92 48.73 45.6 48.48 17196 11/3/98 15:00:00 15:30:00 323.56 193.92 1.67 56.77 79.3 57.85 40.07 20196 11/3/98 15:00:00 15:30:00 427.2 269.9 1.58 83.71 100.68 85.52 36.82 23196 11/3/98 15:30:00 16:00:00 316.61 191.03 1.66 56.15 82.61 52.27 39.66 21196 11/3/98 16:00:00 16:30:00 458.76 274.99 1.67 77.06 123.62 74.31 40.06 23196 11/3/98 17:00:00 17:30:00 444.98 244.12 1.82 81.45 94.16 68.51 45.14 24196 11/3/98 17:30:00 18:00:00 414.68 233.43 1.78 84.75 86.33 62.35 43.71 24196 11/3/98 18:00:00 18:30:00 354.47 180.47 1.96 66.13 74.77 39.57 49.09 19

Totals for BTS 196 9783.79 5348.84 1.83 1760.71 2109.54 1478.61 45.33 31

The number of channel elements active on this BTS reaches its highest value for the day during the 30-minute period of our experiment.

December, 1999 1 - 13Optimization v2.0 (c) 1999 Scott Baxter

System-Side Data: BTS Blocks

Cell Start DateStart Time End Time

Blocks No TCE

Blocks No Fwd

Blocks No Rev

SHO Blk No TCE

SHO Blk No Fwd

SHO Blk No Rev

Succ Calls

Succ SHO

196 11/3/98 8:00:00 8:30:00 0 0 0 0 0 0 66 988196 11/3/98 8:30:00 9:00:00 0 0 0 0 0 0 112 934196 11/3/98 9:00:00 9:30:00 0 0 0 0 0 0 126 907196 11/3/98 9:30:00 10:00:00 0 0 0 0 0 0 160 1099196 11/3/98 10:00:00 10:30:00 0 0 0 0 0 0 77 853196 11/3/98 10:30:00 11:00:00 0 0 0 0 0 0 121 1009196 11/3/98 10:30:00 11:00:00 0 0 0 0 0 0 102 924196 11/3/98 11:00:00 11:30:00 0 0 0 0 0 0 132 905196 11/3/98 12:00:00 12:30:00 0 0 0 0 0 0 102 885196 11/3/98 12:00:00 12:30:00 0 0 0 0 0 0 105 852196 11/3/98 13:00:00 13:30:00 0 20 0 0 0 0 172 1018196 11/3/98 13:00:00 13:30:00 0 0 0 0 0 0 97 913196 11/3/98 13:30:00 14:00:00 0 0 0 0 0 0 117 744196 11/3/98 14:00:00 14:30:00 0 0 0 0 0 0 83 953196 11/3/98 15:00:00 15:30:00 0 0 0 0 0 0 132 924196 11/3/98 15:00:00 15:30:00 0 0 0 0 0 0 149 1103196 11/3/98 15:30:00 16:00:00 0 0 0 0 0 0 119 828196 11/3/98 16:00:00 16:30:00 0 0 0 0 0 0 129 1064196 11/3/98 17:00:00 17:30:00 0 1 0 0 0 0 128 1044196 11/3/98 17:30:00 18:00:00 0 0 0 0 0 0 129 914196 11/3/98 18:00:00 18:30:00 0 0 0 0 0 0 96 979

Totals for BTS 196 0 21 0 0 0 0 3140 28102

The BTS experiences 20 cases of blockage due to no forward power available during the 30-minute period of our experiment. The only other time during the

day when it experienced ANY such blocks was 17:00-17:30, when there was only one despite traffic levels actually higher than during our experiment.

December, 1999 1 - 14Optimization v2.0 (c) 1999 Scott Baxter

System-Side Data: BTS Blocks, Access Failures

Site Call Call % Total % Tot BTS %BTS Acc. %Acc. Screen %Scr. Calls %Att. Succ. Succ. Block Block Block Block Fail Fail Calls Calls Drop Drop

===== ===== ===== ===== ===== ===== ===== ===== ===== ===== ===== ===== ===== =====196X 55 54 98.18 1 1.82 0 0 0 0 0 0 0 0196Y 111 110 99.1 0 0 0 0 1 0.9 0 0 4 3.64196Z 95 93 97.89 1 1.05 1 1.05 1 1.05 0 0 0 0

The sector hit by our experiment shows the worst BTS blocks and Access Failures.