copyright 2005

Macrodiversity Packet Combiningfor the IEEE 802.11a Uplink

Shi Cheng and Matthew C. Valenti

Lane Dept. of Comp. Sci. & Elect. Eng.

West Virginia University

Morgantown, WV

mvalenti@wvu.edu

This work was supported in part by Cisco through the University Research ProgramThis presentation does not necessarily represent the views of Cisco.

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Outline

Macrodiversity vs. Microdiversity. Protocol for packet-level macrodiversity.

Application to 802.11a Throughput analysis Effect of imperfect ACK signaling Effect of channel estimation

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Microdiversity

With a conventional array, then elements are closely spaced (~/2) and connected through high bandwidth cabling.

Microdiversity. Signals undergo different small-scale fading, but same large-

scale effects (path-loss and shadowing).

ReceiverTransmitter

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Macrodiversity

With a distributed array, the antennas are widely separated (e.g. different base stations) and connected through a moderate bandwidth backbone.

Macrodiversity. Antennas far enough apart to provide independent channels. Provides robustness against not only small-scale fading, but

also large-scale effects.

Receiver #2Transmitter

Receiver #1

BackboneNetwork

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Network Topology

K access points (APs) along a ring of radius r. Master AP is the one closest to the mobile terminal.

Conventional system: Only master AP receives uplink signal. Macrodiversity system: All K AP’s receive uplink signal.

The other AP’s are supplemental.

AP #2

AP #3

MTPoint A

MTPoint B

MTPoint C

AP #1

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Packet-LevelMacrodiversity Protocol

Mobile transmission received by all K AP’s Packets are encoded with CRC code. Used by each AP to determine if packet correct.

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Packet-LevelMacrodiversity Protocol

If a supplemental AP correctly receives a packet, it forwards it over the backbone to the master AP.

If the master AP does not receive the mobile’s transmission correctly, then it will use one of the packets forwarded by the supplemental APs (if any).

Therefore,

the packet is accepted

if it is correct at any of

the K access points.

K

kkee pP

1

)(

Overall packet error

probability:

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Packet-LevelMacrodiversity Protocol

The master AP sends an ACK back to the mobile.

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Propagation Model

Quasi-static frequency selective Rayleigh fading. Impulse response:

L is the number of taps T is the tap spacing

• T = 50 nsec in 802.11a ’s are complex Gaussian with variance

rms is the RMS delay spread.

1

0

)()(L

Ttth

rms

rmsrms

LT

TT

e

ee

1

12

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Details of IEEE 802.11a

OFDM K=7 Convolutional Code. Adaptive modulation and coding

Rate Option Modulation Code Rate 6 Mbps BPSK 1/2 9 Mbps BPSK 3/412 Mbps QPSK 1/218 Mbps QPSK 3/424 Mbps 16-QAM 1/236 Mbps 16-QAM 3/448 Mbps 64-QAM 2/354 Mbps 64-QAM 3/4

K=1

K=2

K=3

Average CNR at AP #1

Pac

ket E

rror

Rat

e

MRC

Centrally located mobile.

6 Mbps rate option

1500 byte packet

50 nsec rms delay spread

Packet Error Rate:Central Location at 6 Mbps

Average CNR at AP #1

Pac

ket E

rror

Rat

e

6M

12M

24M

36M

54M

Influence of Rate Option

Centrally located mobile.

1500 byte packet

50 nsec rms delay spread

Average CNR at AP #1

Pac

ket E

rror

Rat

e

Point C

Point B

Point A

Receiver CSI available

Exponential path loss:

32

4

o

k

cor d

d

fd

cP

AP #2

AP #3

MTPoint A

MTPoint B

MTPoint C

AP #1

Influence of Mobile Location

6 Mbps rate option

1500 byte packet

tmto

m

cor PdP

d

d

fd

cP 35

2

1023.24

MAC Protocol

SIFS16 sec

DIFS34 sec

DIFS

Data Packet ACK

Distributed Coordination Function

ACK is sent only

by the master AP

14 bytes

Average CNR at AP #1

Thr

ough

put (

Mbp

s)

12M

6M

K=1

K=3

K=2

Throughput of 6 and 12 Mbps options

Centrally located mobile.

No receive CSI.

1500 byte packet

50 nsec rms delay spread

Average CNR at AP #1

Thr

ough

put (

Mbp

s)

54M

36M

K=1

K=2

K=3

Throughput of 36 and 54 Mbps options

Centrally located mobile.

No receive CSI.

1500 byte packet

50 nsec rms delay spread

Thr

ough

put (

Mbp

s)

Average CNR at AP #1 when MT is at A

Throughput of with Randomly Positioned Mobile

6 Mbps option.

Uniform location.

No receive CSI.

1500 byte packet

50 nsec rms delay spread

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Conclusions Benefits of macrodiversity combining:

Mobile station requires less transmit power. Improves coverage in hard to reach locations. Does not require complex MRC combining.

Disadvantages: Supplemental AP must signal quickly over the

backbone. AP might use different channels for different

sectors.