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Wireless:Facts and Fiction
Benjamin Friedlander
Department of Electrical Engineering
University of California at Santa Cruz
Wireless Communications and Signal Processing Lab
Phone: 831-459-5838
Focus of Talk
Fixed broadband wireless access Physical layer (range, throughput) Performance claims vs. “Physics of
wireless” Wireless systems vs. single links Guidelines for sorting fact from fiction
How to estimate range?
Link Budget - #1 Radiated Power: 50dBm
Transmit power: 30dBm Transmit Antenna Gain: 20dB
Required Power at Receiver: -83dBm SNR for 70MBPS: 22dB Noise floor: -94dBm (NF = 5dB) Receive Antenna gain: 16dB (-) System losses: 5dB Link Margin: 0dB
Maximum Pathloss: 133dB Free space 30miles: 133.7dB
From “WiMAX’s technology for LOS and NLOS environments” a WiMax Forum publication
Urban & Suburban Scenarios Free space conditions are rare Most links will not have line of sight
Non Free Space Pathloss
10-1
100
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80
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120
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range [miles]
loss
[dB
]n=2n=3
n=4n=5
Conclusion #1
Range of 30 miles may be achieved in special / rare situations
Typical ranges will be MUCH smaller
Range vs. Coverage
Range Point-to-point communication Basestation to a specific subscriber
Coverage Point-to-multipoint Subscribers in random locations Specified probability of service
Cell Size vs. Range
2
4
6
30
210
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180 0
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180 0
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180 0
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180 0
Building Penetration
Additional propagation loss for building penetration
Typically: 10 – 20 dB Impact on range: reduce by x2 – x3
Conclusion #2
To serve a large population of users with high reliability the density of base stations needs to be significantly higher than predicted by the range calculated earlier (fractions of mile)
So what about throughput?
Rate vs. Range
200 300 400 500 600 700 800 900 10000
10
20
30
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Range [meters]
Rat
e [M
BP
S]
Rate vs. Range, gexp=3.5
Throughput vs. Cell size
200 300 400 500 600 700 800 900 10000
10
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Cell Radius [meters]
Rat
e [M
BP
S]
Throughput vs. Cell Size, gexp=3.5
Factors further reducing effective throughput Interference Bursty traffic Other …..
Conclusion #3
Effective throughput is much smaller than indicated by maximum data rates
Performance is dominated by low data rate users
So how can we measure system performance?
Measuring System Throughput
Measuring Throughput
Monitor data flowing in/out over time and record peak throughput (heavily loaded system) [bits/sec]
Normalize throughput by Total coverage area [bits/sec/m2] Total bandwidth [bits/sec/Hz] Total number of users [bits/sec/user]
Cost per MBPS (equipment, install, operation)
Range & Throughput Summary
Always ask for link budget 30 mile range at 70MBPS possible (barely) Backhaul range likely to be on order of a mile
(highly dependent on actual locations) Typical cell size a fraction of a mile Cell throughput (point to multipoint) will be
much less than 70MBPS
Some final words
Communication systems (modulation, coding, etc.) are well developed.
Future improvement are likely to be incremental, not revolutionary
For given resources (BW, power) it is practically impossible to have 10X improvement
We have more technology than we know what to do with
For additional information
Please contact [email protected] Related talks:
Smart antennas, MIMO, transmit diversity – so how many antennas do we need?
Multiaccess methods: TDMA, FDMA, CDMA, OFDMA – so what comes next?
Wireless in the wild west: operating in the unlicensed spectrum.
Communicating on the move – mobility and its limitations
The amazing story of ultra-wideband
