copyright 2003

Improving Uplink Performanceby Macrodiversity Combining

Packets from Adjacent Access Points

Matthew C. Valenti

Assistant Professor

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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Motivation & Goals

Spatial diversity: Used to mitigate the effects of fading and interference. Usually implemented with an antenna array at each access

point.• Each mobile station associates with a single access point.• Presence of other nearby access points is ignored.

Distributed diversity: Combine signals received by adjacent access points. Similar to soft-handoff in CDMA cellular networks.

Goal of this paper: Practical methods for achieving distributed diversity.

• Assume quasi-static Rayleigh fading channel. Uplink of an infrastructure-based network.

• Example application to Bluetooth.

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Conventional Antenna Arrays

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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Distributed Antenna Array

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

Macrodiversity. Provides robustness against not only small-scale fading, but

also large-scale effects.

Receiver #2Transmitter

Receiver #1

BackboneNetwork

Log-MAPMultiuserDetector

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Distributed Turbo Multiuser Detection

M.C. Valenti and B.D. Woerner, “Iterative multiuser detection, macrodiversity combining, and decoding for the TDMA cellular uplink,”IEEE Journal on Selected Areas in Commun., vol 19, pp. 1570-1583, Aug. 2001.

LLR

LLR

LLR

extrinsic informationa priori likelihood

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A Simple Approach to Macrodiversity

Consider a mobile station that is equidistant from two (or more) access points.

The signal transmitted by the mobile is received by all access points within close proximity.

Could be maximal ratio combined (Hanly 1996). However, MRC has drawbacks:

• Requires accurate channel estimates.• Soft decisions must be passed over backbone network.• Vulnerable to interference.

Instead, we take a post-detection approach. Each AP first detects and decodes the packet.

• Error detection code used to determine if it is correct.• Correct packets are forwarded over backbone to the “head” AP.

Packet is accepted by the network if it is correct at any AP.• Retransmission is necessary only when incorrect at all APs.

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System Model

Mobile station surrounded by ring of M access points. Ring has a diameter of 10 m. For typical current-generation networks, 1 More densely deployed networks could have M>3.

Bluetooth network. Data is transmitted on the uplink using DH5 packets. Acknowledgements on the downlink using DH1 packets.

Access Points

mobilestation

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Propagation Model

Quasi-static Rayleigh fading channel. SNR constant for duration of a packet. Varies from packet to packet. Exponential random variable.

Path loss. Received power at distance dm is:

• Assuming path loss exponent n=3, free-space reference distance do = 1 m, and fc = 2.4 GHz.

Noise spectral density No = 10-18 W/Hz

No shadowing.

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Details of Bluetooth

Modulation: Nonorthogonal GFSK, 0.28 h 0.35 1 Mbaud symbol rate Noncoherent detection

Frequency hopping: 625 sec slots. 79 frequencies in the hopping pattern.

Packet format: 72 bit access code 54 bit packet header Payload

• DHx = high rate (no FEC) code• DH5 = 2712 user data bits.• DH1 = 216 user data bits (but we just use for ARQ).• CRC code for error detection.

Centrally Located Mobile Station:Uplink FER

5 10 15 20 25 30 3510-4

10-3

10-2

10-1

100

Average Es/No in dB

Pa

cke

t Err

or

Rat

e

M=1

M=2

M=3

M=6

Uplink error performanceMobile equidistant fromthe M access points.

Larger gains at lower FER.

Gain diminishes with increasing M.e.g. gain of 18 dB for M=6

10 dB gain

Noncentral Mobile Station: Uplink FER

5 10 15 20 25 30 35Average Es/No in dB

10-4

10-3

10-2

10-1

100

Pa

cke

t Err

or

Rat

e

M=1

M=2

M=3

M=6

Access Points

mobilestation

2.5 m

Absolute gains are now smaller.But don’t diminish as quicklywith increasing M.

5.9 dB

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Comparison ofTransmit Power Requirements

Transmit power required to achieve FER = 10-2 on the uplink.

Most dramatic gains when mobile is centrally located. Order of magnitude reduction of Tx power by just using a

second AP. Central location corresponds to poorly covered regions in

the network. Gains also possible without central location.

Number of

access points

Central Location

Noncentral

Location

1 2.95 mW 373 W

2 282 W 96 W

3 121 W 58 W

4 78 W 43 W

5 58 W 36 W

6 47 W 31 W

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Network Considerations

One access point serves as head AP. Head AP is usually the one closest to the mobile. All others act as supplemental APs.

Each supplemental AP that receives a correct packet forwards it to the head AP.

Increases the traffic on the (wired) backbone. Extra delay may be needed at the head AP.

Only the head AP sends out an acknowledgement to the mobile station.

In Bluetooth, MS is master and the APs are slaves. Broadcast mode needed. ARQ will need to be implemented in application.

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Throughput Analysis

Average throughput (in bps) is:

where: • N is the number of transmissions by the mobile station • E[.] is expectation.• K is number of data bits (=2712 for DH5)• D is number of round-trip slots (=6 for DH5/DH1).

Perfect ACK. Assuming ACK from head AP always received correctly:

Imperfect ACK. Head AP signals MS through fading channel:

E RK

DE N[ ]

[ ]( )

625 10 6

E Npul

[ ] 1

1Packet error rate on uplink:Prob. that packet is incorrect at all access points.

E Np

p

p pul

dl

dl ulh

[ ]( ) ( )

1

1 1 1b gPacket error rate on downlink:Prob. that packet from head AP incorrect.

Prob. that uplink packet’s header is incorrectat all access points.

Uplink Throughput:Central MS & Perfect ACK

5 10 15 20 25 30 350

100

200

300

400

500

600

700

800

Average Es/No in dB

Ave

rag

e T

hro

ug

hpu

t (kb

ps)

M=1M=2M=6

Larger gains at higher throughput.At 500 kbps: 3.4 dB gain with M=2 6.7 dB gain with M=6

Uplink Throughput:Central MS & Imperfect ACK

5 10 15 20 25 30 350

100

200

300

400

500

600

700

800

Average Es/No in dB

Ave

rag

e T

hro

ug

hpu

t (kb

ps)

M=1M=2M=6

At 500 kbps: 3.2 dB gain with M=2 6.0 dB gain with M=6

Uplink Throughput:Noncentral MS & Imperfect ACK

5 10 15 20 25 30 350

100

200

300

400

500

600

700

800

Average Es/No in dB

Ave

rag

e T

hro

ug

hpu

t (kb

ps)

M=1

M=6

At 500 kbps: 0.3 dB gain with M=2 1.0 dB gain with M=6

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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. Additional diversity effect in shadowing.

Disadvantages: Increased network complexity. Increased traffic on backbone. Reduction in number of served users.

Most applicable when low power mobile stations is of top concern.

Sensor networks. Future work:

Application to IEEE 802.11a Downlink macrodiversity: Distributed space-time codes. Virtual antenna arrays: Distributed diversity for ad hoc nets.

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