Transcript EE 364 Communication Theory Spring 2000

Improving Uplink Performance
by Macrodiversity Combining
Packets from Adjacent Access Points
Matthew C. Valenti
Assistant Professor
Lane Dept. of Comp. Sci. & Elect. Eng.
West Virginia University
Morgantown, WV
[email protected]
This work was supported in part by Cisco through the University Research Program
This presentation does not necessarily represent the views of Cisco.
copyright 2003
Motivation & Goals

Spatial diversity:
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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.
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Distributed diversity:
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
Combine signals received by adjacent access points.
Similar to soft-handoff in CDMA cellular networks.
Goal of this paper:
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Practical methods for achieving distributed diversity.
• Assume quasi-static Rayleigh fading channel.
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
Uplink of an infrastructure-based network.
• Example application to Bluetooth.
Conventional Antenna Arrays

With a conventional array, then elements are closely
spaced (/2) and connected through high bandwidth
cabling.
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Microdiversity.
Signals undergo different small-scale fading, but same largescale effects (path-loss and shadowing).
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Transmitter
Receiver
Distributed Antenna Array

With a distributed array, the antennas are widely
separated (e.g. different base stations) and
connected through a moderate bandwidth backbone.
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Macrodiversity.
Provides robustness against not only small-scale fading, but
also large-scale effects.
Receiver #2
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Transmitter
Backbone
Network
Receiver #1
Distributed
Turbo Multiuser Detection
extrinsic information
a priori likelihood
Bank of
Interleavers
LLR
y1 (t )
Bank of
Matched
Filters
y (1)
Log-MAP
Multiuser
Detector

Bank of
Deinterleavers
Bank of K
Log-MAP
Channel
Decoders
LLR
LLR
yM (t )
Bank of
Matched
Filters
y (M )
Log-MAP
Multiuser
Detector
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.
ˆ (q)
m
A Simple Approach to
Macrodiversity
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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.
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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.
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Each AP first detects and decodes the packet.
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• 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.
System Model
Access Points
mobile
station

Mobile station surrounded by ring of M access points.


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
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.
Propagation Model
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Quasi-static Rayleigh fading channel.
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


SNR constant for duration of a packet.
Varies from packet to packet.
Exponential random variable.
Path loss.

Received power at distance dm is:
F c IJF
d I
P G
H4d f KG
Hd J
K P  10
n
2
m
r
t
o
c
4
d m3 Pt
o
• Assuming path loss exponent n=3, free-space reference
distance do = 1 m, and fc = 2.4 GHz.
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Noise spectral density
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
No = 10-18 W/Hz
No shadowing.
Details of Bluetooth
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Modulation:
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Frequency hopping:
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625 sec slots.
79 frequencies in the hopping pattern.
Packet format:
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Nonorthogonal GFSK, 0.28  h  0.35
1 Mbaud symbol rate
Noncoherent detection
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
Packet Error Rate
100
10-1
M=1
10 dB gain
10-2
Uplink error performance
Mobile equidistant from
the M access points.
M=2
Larger gains at lower FER.
10-3
Gain diminishes with increasing M.
e.g. gain of 18 dB for M=6
M=3
M=6
10-4
5
10
15
20
25
Average Es/No in dB
30
35
Noncentral Mobile Station:
Uplink FER
Packet Error Rate
100
10-1
2.5 m
Access Points
M=1
mobile
station
10-2
5.9 dB
M=2
10-3
Absolute gains are now smaller.
But don’t diminish as quickly
with increasing M.
10-4
5
10
15
M=3
M=6
20
25
Average Es/No in dB
30
35
Comparison of
Transmit Power Requirements
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
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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Transmit power required
to achieve FER = 10-2 on
the uplink.
Most dramatic gains
when mobile is centrally
located.
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

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.
Network Considerations
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One access point serves as head AP.
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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.
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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.
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

Broadcast mode needed.
ARQ will need to be implemented in application.
Throughput Analysis
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Average throughput (in bps) is:
E[ R] 

where:
•
•
•
•

K
DE[ N ](625  106 )
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.
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Assuming ACK from head AP always received correctly:
E[ N ] 
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Imperfect ACK.
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1
1  pul
Packet error rate on uplink:
Prob. that packet is incorrect at all access points.
Head AP signals MS through fading channel:
E[ N ] 
1
pdl

(1  pul ) 1  pdl (1  pulh )
b g
Packet error rate on downlink:
Prob. that packet from head AP incorrect.
Prob. that uplink packet’s header is incorrect
at all access points.
Uplink Throughput:
Central MS & Perfect ACK
Average Throughput (kbps)
800
700
M=6
M=2
600
M=1
500
400
300
Larger gains at higher throughput.
At 500 kbps:
3.4 dB gain with M=2
6.7 dB gain with M=6
200
100
0
5
10
15
20
25
Average Es/No in dB
30
35
Uplink Throughput:
Central MS & Imperfect ACK
Average Throughput (kbps)
800
700
M=6
600
M=2
M=1
500
400
300
At 500 kbps:
3.2 dB gain with M=2
6.0 dB gain with M=6
200
100
0
5
10
15
20
25
Average Es/No in dB
30
35
Uplink Throughput:
Noncentral MS & Imperfect ACK
Average Throughput (kbps)
800
700
M=6
600
M=1
500
400
300
At 500 kbps:
0.3 dB gain with M=2
1.0 dB gain with M=6
200
100
0
5
10
15
20
25
Average Es/No in dB
30
35
Conclusions
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Benefits of macrodiversity combining:
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Disadvantages:
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Increased network complexity.
Increased traffic on backbone.
Reduction in number of served users.
Most applicable when low power mobile stations is of
top concern.
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Mobile station requires less transmit power.
Improves coverage in hard to reach locations.
Does not require complex MRC combining.
Additional diversity effect in shadowing.
Sensor networks.
Future work:
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Application to IEEE 802.11a
Downlink macrodiversity: Distributed space-time codes.
Virtual antenna arrays: Distributed diversity for ad hoc nets.
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