Space Time Processing for Fixed Broadband Wireless
A. Paulraj
Gigabit Wireless&
Stanford University
ISART 6 -8 September, 2000
Boulder, CO
Essential Attributes of Broad Band Wireless (BWA) Network
• High Speed > 4-5 Mbps• High QoS / Availability
• DSL / Cable like
• Low Costs $$$• High capacity• High coverage• CPE and Infrastructure unit costs
• Friendly• Easy CPE Install• Environmental/Regulatory
• Scalability (Multi-cell)• Evolution to portability
BWA Network Architecture
• Multi-cell architecture • Low BTS antennas (50 - 150 ft)• Under-the-eave subscriber antennas
• Significant Non-LOS propagation (Rayleigh fading)
• Strong multi-path • Low Doppler channels
Mobile cellular-like network with all its challenges but with Mobile cellular-like network with all its challenges but with 50 Speed and 50 Speed and 50 QoS50 QoS
NAU = Network Access UnitBTS = Base Transceiver StationBSC = Base Station Controller
BWA Network
NAU
IP
NGU
BSC
BTS
BTS
BTS
Voice gateway Network
Operations Center
SMS
Access Mux
PSTN
IPGateway
NAU
NGU = Network Gateway UnitSMS = Subscription Management System
Data Rate & QoS Challenges
50 High Data Rate50 High Data Rate
50 QoS(0.999 Availability on Throughput, Delay, Jitter)
Capacity (Bits/Hz/Cell)
Coverage(Cell Radius)
0.4
0.2
0.4
Typical Propagation Scenario
BTS Directional Antenna
BTS
Ht 30’-150’
Ht 8’-20’
0.1 - 5 miles
CPE Directional Antenna
Distance to
mobile scatterer
s
100
101
102
-170
-160
-150
-140
-130
-120
-110
-100
Path Loss (Mean Level)
Sig
nal P
ower
(dB
) Super CellBTS ht: 1500ft
Macro CellBTS ht 150 ft
Range in km
Frequency: 2.5 GHzAnt Beamwidth: 90 deg
Super Cell: Free Space modelMacro/Micro Cell: Erceg’s model
K-Factor vs. Distance (Suburban BWA)
10-1
100
101
-20
-10
0
10
20
30
40
Distance in km
K-F
acto
r in
dB
ht = 15m, 90 deg. Rx antenna
hr = 3m
hr = 10m
GW and AT&T measured data similar
K = 0 is necessary assumption for reliable deployment
--- hr = 10m --- hr = 3m
RMS Delay Spread vs Distance (Suburban BWA)
10-1
100
101
-25
-20
-15
-10
-5
0
5
10R
MS
Del
ay S
prea
d in
Mic
rose
cond
s (d
B)
Distance in km
GW experimental data similar
C vs I Measurements
2 4 6 8 10 12 14 16 18 20-180
-170
-160
-150
-140
-130
-120Rx Ant 1
s
Pat
h Lo
ss [
-dB
]
C/I: 10.0212 dB
BTSCCI
2 4 6 8 10 12 14 16 18 20-180
-170
-160
-150
-140
-130
-120Rx Ant 2
s
Pat
h Lo
ss [
-dB
]
C/I: 6.632 dB
BTSCCI
Reuse factor 1
Reuse factor 1
20 40 60 80 100 120-180
-170
-160
-150
-140
-130
-120Rx Ant 1
s
Pat
h Lo
ss [
-dB
]
C/I: 8.3504 dB
BTSCCI
20 40 60 80 100 120-180
-170
-160
-150
-140
-130
-120Rx Ant 2
s
Pat
h Lo
ss [
-dB
]
C/I: 0.21909 dB
BTSCCI
Spatial Multiplexing (SM) Offers Higher Speeds
Each Eigen mode of the matrix channel can support a radio link. 11.4 1.5 speed up possible in 2 x 3 systems.
Tx RxH
Increasing Capacity/Coverage
• Coverage: Lower the C/N needed to close link - improves link budget
• Capacity: Lower C/I needed to close link - improves reuse factor– Use diversity, array gain, coding, etc to minimize
required C/N or C/I to support a target PER
– Use RLP and fragmentation to allow higher target PER
– Use spatial multiplexing to run parallel channels
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8100
120
140
160
180
200
220
Mode 6, Doppler 1Hz, MaxUtil 635 users, small files
link utilization [%]
me
an
th
rou
gh
pu
t [K
bp
s]
Mean Per 1% Mean Per 5% Mean Per 10%
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
500
1000
1500
2000
2500
3000
Mode 6, Doppler 1Hz, MaxUtil 635 users, big files
link utilization [%]m
ea
n t
hro
ug
hp
ut
[Kb
ps]
Mean Per 1% Mean Per 5% Mean Per 10%
- TCP windowing algorithm yields the difference in the throughput of ‘small’ and ‘big’ files
- Throughput = File size/ Mean transfer delay
Throughput vs Utiliz. (vs PER)
Increasing Coverage
• Diversity is critical (Tx antenna, Rx antenna, frequency, pol)
• Directional CPE antennas• Forward Error Correction (FEC) & ARQ• Increase BTS & CPE height
Coverage vs Required C/N
0 5 10 15 20 25 301
2
3
4
5
6
7
8
Ran
ge in
Mil
es
1 x 1, NO RLP2 x 2, RLP Required C/ N
CDF of Tx-Rx Diversity - Measured Data
-40 -30 -20 -10 0 10 2010
-4
10-3
10-2
10-1
100
Signal Power About Mean in dB
Pro
bab
ility
Tha
t S
igna
l Po
wer
< A
bsci
ssa
Before Combining After Combining
1 x 2
2 x 2
Probability of signal falling 10 dB below mean is 300 times lower with 2 x 2 compared 1 x 1
Increasing Spectrum Efficiency
• Reduce CCI to improve reuse - Increase signal diversity
- Interference averaging
- Antenna sectorization, beam forming, interference cancellation
• Reduce ACI to improve reuse
- Careful frequency planning
- Stringent emission masks, Rx filtering
- Interference suppression
• Use link adaptation to exploit available C/N or C/I levels• Use Radio Link Protocol to allow link to use high BER
C/ I After Combining - Measured Data
C /I = 10 dB
-40 -30 -20 -10 0 10 2010
-4
10-3
10-2
10-1
100
C/I About Mean in dB
Pro
bab
ility
Tha
t C
/I <
Abs
ciss
a
Before Combining
1 x 2
2 x 2
After Combining
Probability of C/I falling 15 dB below mean is 300 times lower with 2 x 2 compared to 1 x 1