Wireless LAN Performance Analysis
Presented By Nikos Ioannou
Wireless LAN
Architecture
• Access Points
• Clients
Types of wireless LAN
• Peer-to-peer
• Bridge
• Wireless Distribution System
Wireless LAN
IEEE Standards
802.11b (Wi-Fi)
802.11a
Both define specific physical (PHY) layers
IEEE Standards (Summary)
Performance Analysis in Wireless
LANs
WLAN Propagation Overview
Available Spectrum and Efficiency
Signal Range Overview
Power Considerations Overview
Throughput Overview
Metrics
Performance analysis using reference model
Performance analysis using measured data
WLAN Propagation Overview
Three primary modes for Electromagnetic signal
• Reflection
• Diffraction
• Scattering
For indoor environment the signal does not
predictably lose energy so is difficult to calculate
Path Loss.
Available Spectrum
IEEE 2.4 GHz 802.11b
• Uses DSSS Modulation
• Requires 22MHz band for one network
• With 83.5 MHz band will supports 3 non-
overlapping simultaneously Wi-Fi Networks
Available Spectrum (cont)
IEEE 5GHz 802.11a
• Uses OFDM Molulation
• Requires 16.6 MHz band for one network
• With 300MHz of spectrum supports 12 non-
overlapping simultaneously networks
Signal Range Overview
The signal range of every wireless system is
governed by the following variables
• RF power transmit level
• Required Es/N0 (Signal Energy required to
recover the transmitted symbol)
• Environment
• Signal Propagation
Signal Range Overview (cont)
One of the fundamental differences between
communications operating at 2.4 and 5GHz is the
achievable communication range between the AP
and the station. Holding variables above constant
2.4 GHz frequency offer roughly double the range
of those operating in the 5GHz band
Signal Range Overview
Power Considerations
The limitation in range is caused by the more
severe path loss of the 5GHz spectrum
By increasing the power of a 5GHz system
approximately 4 times can achieve ranges
similar to 2.4GHz systems
802.11a system (5GHz) is more power
efficient over small areas and
802.11b(2.4GHz) is more efficient over
greater distances
Throughput Overview
Metrics
Range is the greatest distance from an AP at which the minimum
data rate demodulated with an acceptable BER
Coverage is the resulting cell size or square meters per AP
Rate weighted coverage is the bit rate with respect to area
covered or (Mb/sec*m²)
Transmit Power upper bound (for 802.11a 16.02dBm in 5.15-
5.25GHz, 23.01dBm in 5.25-5.35GHz and for 802.11b maximum
of 30dBm)
Receiver Sensitivity
Noise and Interference (ACI,CCI)
Bit error Rate
Receiver Sensitivity
For IEEE 802.11b receiver should be able to detect -
76dBm with BER of min 10e-5 in the absence of ACI. If
ACI is present the receiver must be able to detect -
70dBm
For IEEE 802.11a as follows
Bit Error Rate
IEEE 802.11b for BER better than 10e-5 then min S/N
IEEE 802.11a for BER better than 10e-5 then min S/N
The Model
For indoor environment the signal power at the
receiver SRx is related to the transmit power
TRx as shown below (this model will be used as the
reference analysis model)
Where C=speed of light, f=center frequency, N: path
loss coefficient. ITU recommends N=3.1 for 5-GHz
and N=3 for 2.4-GHz
Analysis using reference model
(Range Comparisons)
IEEE 802.11b (with N=3)
• With EIRP of 30dBm max range=154m
• With EIRP of 19dBm max range=66.4m
• With EIRP of 15dBm max range=48.4m
IEEE 802.11a (with N=3.1)
• With EIRP of 18dBm range=14m with 54Mbits /s
• With EIRP of 23dBm range=30m with 54Mbits/s
Analysis using reference model
(Practical Deployments)
To cover greater areas networks designers
deploy multiple APs in a hexagonal cellular
arrangement (like cellular telephony)
We assume also that only one station is
transmitting in each cell
Spectral leakage (l) from one adjacent
channel is -23dB for a 802.11a network and
-34.40dB for a 802.11b network
Analysis using reference model
(Practical Deployments :3cell net)
For 802.11b (we have only 3 frequency channels) one of the 3 cells
experiences ACI from the other two cells. The greatest interference noise
power (IN) is experienced by a user located at point G. Desired signal
power is P(Rx). Therefore SINR=(2f )^-1 which is 31.29dB means all data
rates are supported.
The size of the network below can be calculated by finding the radius of
each hexagon. Using the min Rx sensitivity of -70dBm an 15dBm solution
can cover 7397m² at 11Mbs while a 19dBm solution 13670m²
Analysis using reference model
(Practical Deployments :3cell net)
For 802.11a the designer can choose channels 1,5,8
for cells I,II,III so there is no ACI. With EIRP
23.01dBm for cells I and III and 16.02dBm for cell II
and min Rx sensitivity we have a total coverage of
2730m² at 54Mbits/s (as the required rate)
Alternatively if the required rate is 12 Mbits/sec with
EIRP 23.01dBm the total area covered becomes
17650m² (SINR=f^-1)
Analysis using reference model
(Practical Deployments :8cell net)
In an 8-cell 802.11a network all APs transmit at the same power of
16.02dBm and the greatest interference is experienced by a station
belonging to cell IV located at G. The sources of interference are the two
ACs one at 2R and the other at root 7R. The SINR is given below giving
about 31.49dB means that a rate of 54Mbs is accessible throughout the
network. The total area covered is 2082m².
Analysis using reference model
(Practical Deployments :8cell net)
Similarly a 802.11b can be used to cover the same geometry. Again the
most interference is experienced by a station located at vertex G. This leads
to co-channel at 2R, a co-channel at root 7R, two adjacent at 2R, two
adjacent channels at R and one adjacent channel at root 7R. Therefore the
least SINR is shown by the equation below or about 7.45dB. With this
result the 11Mbits/s is accessible throughout the network. A 15dBm
solution can cover 19727m² while a 19dBm will cover 36453m²
Performance analysis using
Measured Data
The measured performance data collected in a typical
office environment 265 foot by 115 foot rectangular with
conference rooms closed offices and walls as well as
semi-open cubible spaces.
For the 802.11a system data was sent between two PC
cards one as fixed AP and the other as a mobile station
with output power of 14dBm.
For the 802.11b system comprised as AP and a PC card
with output power of 15dBm
Data Link Rate results (for test
environment with one AP and a
mobile Station)
Throughput Results (for test
environment with one AP and a
mobile station)
System Capacity under CCI using
measured data
The analysis will be for the following 8-cell systems
System Capacity refers to the throughput of an entire WLAN
comprised of many cells
System Capacity under CCI using
measured data
Co-Channel interference is less for 802.11a systems than 802.11b
due to the presence of more channels
Average Cell Throughput under
CCI using measured data
Measured performance data from single AP-User inputted
into a system capacity model proposed by NEC1 to evaluate
the system capacity of the 8-cell WLAN system above
System Capacity under CCI using
measured data
System capacity as the average cell throughput (shown above)
multiplied by the number of cells.
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