WiMAX Range and Throughput Measurements

Sponsored by the National Science Foundation 1March 15, 2011

WiMAX Range and Throughput Measurements

• Goals• Principal Elements• Process• Path Loss Measurements• Experiment Application Design• Connection Evaluation Steps• NEC Sector Antenna Tilt• Range and Throughput Measurements

– Plan– Results– Summary

• Conclusions and Next Steps

• Authors: Manu Gosain, Tony Michel, Tom Cahill, Harry Mussman


Goals

• Validate base station installation and configuration process– Provide comprehensive documentation

• Design an experiment to evaluate range and throughput– Document for use by other sites in evaluating their

expected range and throughput– Later: move to OMF/OML environment

• Evaluate range and throughput at BBN site– Compare to known calculations, measurements– Document for use by other sites in estimating their

expected range and throughput


Principal Elements

• Base station kit (BTS)– Utilizing NEC Profile C IDU and ODU

• Rooftop antennas– NEC 120deg sector– Commercial omnidirectional

• Anritsu spectrum analyzer, for measuring received power• Linux laptop with Intel 6250 WiMAX modem, acting as a mobile station

(MS)• BTS servers, including:

– ASN GW with WiMAX RF AggMgr (Case 1b)– Test host– I&M host

• Experiment application, running in: – MS (measurement script)– Test host (ping and iperf servers)– I&M host (report script)


Process

• 1) Conduct power measurements using Anritsu spectrum analyzer– Check for presence of Clearwire signal with Anritsu spectrum analyzer

• 2) Build and verify experiment application to conduct range and throughput measurements

• 3) Decide on best down tilt for NEC sector antenna – Estimate for electrical down tilt: 5deg– Options for mechanical down tilt: 10deg, 6deg, 4deg, 2deg (selected 4deg)

• 4) Conduct range and throughput measurements near BBN Technologies location in Cambridge, MA– Focus on line-of-sight, outside only (gives best case)– Keep nominal BTS configuration parameters

• Power set to +38dbm, the maximum allowed– Options for base station antenna:

• NEC sector base station antenna (at 4deg mechanical down tilt) • Omni-directional base station antenna

– Options for Linux laptop mobile station:• Internal Intel 6250 WiMAX modem, and internal antenna• External (USB-connected) 6250, with handheld omni-directional antenna


1) Power Measurements

• Power measurements using Anritsu spectrum analyzer– Measured with sector antenna, 6deg mechanical tilt– Near antenna: -34dBm– Point 41, 370ft: -59dBm (good signal)– Point, 520ft: -50dBm (good signal)– Point, 1190-ft: -79dBm (edge of coverage)

• Presence of Clearwire signal with Anritsu spectrum analyzer– On roof (line of sight): -60dBm– Point 47: -70dBm


.bbn.dataplane.geni.netBTS

ODU/IDU

ASNGW

“salamis” Test host

“argos”

NEC Base Station(BTS)

publicInternet

GREtunnelMobile Station (MS)Dell 1012 Netbook

WiMAX modem/antenna: 1) USB-connected Intel 6250/external omni 2) internal Intel 6250/ internal

4) Test targets: ping iperf

1) Range/throughput experiment script“tstats2”: Record location Scan/connect/chk RSSI Get IP via DHCP ping sequence iperf sequence Log results

2) Experiment Application Design

Antenna: 1) NEC sector 2) Omni

I&M host “black”

DHCP

2) WiMAX AggMgr service: Monitor GRE tunnels Collect BTS stats, chk RSSI Log results

3) Report script “report”: (manually gather logs from MS and BTS) Process logs Generate location summaries Generate run summary


Connection Evaluation Steps

• Step 1) Verify WiMAX connection occurs– See tunnel setup from BTS log– Check Down Link (DL) RSSI at MS– Check Up Link (UL) RSSI from BTS log

• Step 2) Verify MS get IP address via DHCP– Sometimes fails if UL is poor

• Step 3) Do a sequence of ping tests between MS and Test Host “argos”– Ping to argos, 10bytes, 10 times; check response within 1sec

window; log delays, % responses not within window (lost)– Ping to argos, 108bytes, 10 times; check response within 1sec

window; log delays, % responses not within window (lost)– Ping to argos, 1008bytes, 10 times; check response within 1sec

window; log delays, % responses not within window (lost)


(continued)• Step 4) Do a sequence of iperf tests between MS and Test Host “argos”

– Repeat 3 times– Use TCP– Use -d for double connection, separating DL and UL measurements– Throughput in Mb/s calculated from bytes transmitted within 60sec

interval– Print throughput in Mb/s to log– TCP parameters:

• use Nagle’s algorithm• window size and segment size per OS: 16kB• depth read/write buffer in socket, default: 8kB• max segment size: 1408B (MTU size) - 40B = 1368B

– Use of TCP gives conservative result, but typical of many applications


3) NEC Sector Antenna Tilt

• WiMAX antennas typically have a built-in electrical (down) tilt, and a variable mechanical (down) tilt

• Estimate for electrical tilt on NEC sector antenna, per specs: 5deg

• Options tried for mechanical (down) tilt: 10deg, 6deg, 4deg, 2deg– Too much down tilt “buries” the signals

close to the base station, and shortens range

– Too little tilt creates a blank spot near base station

– There is always a blank spot very near the base station (and within the building) caused by shadow of the building

• Chosen for mechanical tilt: 4deg– Throughput measurements showed range

at 4deg to be higher than at 2deg or 6deg


4) Measurements Plan

• Focus on line-of-sight, outside only (gives best case)– Points 41 through 48, in a straight line at center of 120deg sector pattern– Optional points 1 through 7, in orthogonal direction (with point 7 obstructed by

building), to verify expected 360deg omni coverage• Keep nominal BTS configuration parameters

– Power set to +38dbm, the maximum allowed• Options for base station antenna:

– NEC sector base station antenna (at 4deg mechanical down tilt), approx 90ft high

– Omni-directional base station antenna, approx 90ft high– Expect sector to work better than omni antenna within 120deg sector pattern,

since has higher gain• Options for Linux laptop mobile station (MS):

– External (USB-connected) 6250, with handheld large omni-directional antenna– Internal Intel 6250 WiMAX modem, and internal antenna– Expect large omni antenna to work better than internal antenna– Expect packet loss and throughput to vary from moment-to-moment, due to MS

position and multi-path propagation


(continued)

• For each option combination:– A) BS sector, MS omni antennas– B) BS sector, MS internal antennas– C) BS omni, MS omni antennas– D) BS omni, MS internal antennas

• For each point:– 41 – 48– option for C): 41 – 48 and 1 – 7

• Plot vs distance (mi) from base station to mobile station:– DL RSSI (db)– UL RSSI (db)– 1008byte pings, the % of responses not within window (lost)– DL iperf throughput, min and max over three attempts (Mb/s)– UL iperf throughput, min and max over three attempts (Mb/s)


Neighborhood ofBBN Technologies, Cambridge, MA


Photo of BBN base station and Concord Ave measurement points 41 - 48

4141 4

2424343

4444 4

545 4

646 4

747

BBNBase

Station0

BBNBase

Station0

0(0 mi,90 ft up)

BBNBaseStationAntenna

41(0.058 mi)

Center Parking Lot

42(0.085 mi)

Social SecurityEntrance

43(0.090 mi)

NEFawcett-Concord

44(0.097 mi)

SEFawcett-Concord

45(0.153 mi)

T (Bus)Stop

46(0.200 mi)

SWheeler-Concord

47(0.221 mi)

West edgerotary

48(0.254 mi)

East edgerotary

4848


Photo of BBN base station and Fawcett St measurement points 1 - 7

0(0 mi,90 ft up)

BBNBaseStationAntenna

1(0.040 mi)

Parking Lot

2(0.080 mi)

FawcettSt

3(0.110 mi)

FawcettSt

4(0.140 mi)

FawcettSt

5(0.180 mi)

FawcettSt

6(0.220 mi)

FawcettSt

7(0.230 mi)

FawcettSt (obstructed)

BBNBaseStatio

n0

BBNBaseStatio

n0 11

22

33

44

55

6677


A) Measurements results for BS with sector, MS with external omni antennas


B) Measurements results for BS with sector, MS with internal antennas


C) Measurements results for BS with omni, MS with external omni antennas


C2) Measurements results for BS with omni, MS with external omni antennas


D) Measurements results for BS with omni, MS with internal antennas


Measurements Summary

• RSSIs– DL RSSIs varies from -30db for a strong signal point, down to -

64db for a weak signal point; below that, the connection fails– UL RSSIs remained more constant, often close to -75db for a wide

range of points. Is this due to automatic WiMAX UL transmit power adjustments?

• Ping loss (1008bytes)– Measured delays are relatively constant (80 – 100ms) until link is

about to fail– For 1008byte pings, the % of responses not within window (lost)

increases quickly as link is about to fail; otherwise 0%– Good measure of overall connection quality


(continued)

• iperf Throughput– Use of TCP gives conservative result, but is typical of many

applications– Use of TCP results in significant variations over the 3 runs, due to

packet losses and retransmissions; need to consider both min and max

– As link gets poorer, the throughput eventually falls to zero– DL throughput is typically better than UL throughput, following

WiMAX convention– Best case DL throughput is over 10Mb/s– Best case UL throughput is approximately 1 Mb/s


(continued)

• Range:– Best range (to point 48, 0.254mi) seen with BS sector antenna and

MS handheld large omni antenna– Range is worse, as expected, with BS sector antenna and MS

internal antenna– Worst range (to point 46, 0.2mi) seen with BS omni antenna and

MS handheld large omni antenna– However, range is better with BS omni antenna and MS internal

antenna; why?– Expected packet loss and throughput to vary from moment-to-

moment, due to MS position and multi-path propagation, but not directly verified

– Range at points 1 - 7 comparable to range at points 41 – 47 verifies expected 360deg omni coverage

– Signal gone at point 7 obstructed by building


Conclusions and Next Steps

• Current measurements give range of approximately 0.25mi– How does this range compare with others?– What might be done to improve range?

• Other reported ranges:– Textbook gives calculated range of 0.6mi– Clearwire plots indicate their BS’s are approx 0.5mi apart– Univ Colorado plan calculates range up to 0.75mi– But, commercial services operate at higher power, and include

diversity at BS and sometimes diversity at MS– NYU Poly measurements?– UCLA measurements?– Univ Wisconsin measurements?


(continued)

• Consider to improve range:– Fix some mistake in BTS parameters– Modify BTS parameters to improve range by forcing reduced rate– Add diversity at BS (requires an extra ODU and an extra antenna)– Use vehicular omni antenna at MS (includes ground plane)– Add diversity at MS?– Tune up TCP and/or WiMAX parameters to improve throughput,

e.g., reduce iperf buffer length so packets fit within MTU– Turn ON ARQ or HARQ– Utilize for UDP traffic, and accept more lost packets– Can we get to 0.6mi?

• Expected to reduce range:– Use of MSs indoors– Leaves on trees starting in spring

WiMAX Range and Throughput Measurements

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