IEEE 802.11 family – An Introduction
802.11 is an evolving family of specifications for Wireless local area network (WLAN). This has
been mainly developed by IEEE(Institute of Electrical and Electronics Engineers). WLAN is
analogous to LAN, but without wires. They transmit information through air using radio
frequencies. Their operating range is 2.4, 3.6 and 5Ghz.
WLANs are typically used in College campuses, Office buildings and also in private homes.
Although the initial cost is more than a LAN installation, it provides freedom of movement, as a
result of which it has gained a lot of popularity. Although they have low coverage area when
compared to LAN systems, they have very high data rate.
IEEE 802.11 Layers
The standard currently defines a single MAC layer which interacts with 3 physical layers,
namely


FHSS (Frequency Hopping Spread Spectrum) in the 2.4 Ghz Band.
DSSS (Direct Sequence Spread Spectrum) in the 2.4 Ghz Band.
IR (Infra-Red )
The Spread Spectrum techniques are used to avoid interferences from licensed and other nonlicensed users.
IEEE 802.11 Family
There are several specification in the 802.11 family :
 802.11-1997(802.11 Legacy) – Provides data rates of 1 Mbps & 2Mbps to be transmitted
via IR or Frequency hopping or DSSS techniques at 2.4Ghz.
 802.11a – Provides up to 54 Mbps in the 5Ghz band. It used OFDM encoding scheme
than Spread Spectrum techniques. This was the first wireless networking standard, but
802.11b was the first to be widely accepted.
 802.11b – Also referred to as Wi-Fi. Provides 11Mbps transmission in the 2.4Ghz band.
This uses only DSSS. This was the first to get accepted and became more popular.
 802.11g – Applies to wireless LANs over relatively short distances and provides 20+Mbps
in the 2.4 GHz band. This also uses the OFDM encoding scheme.
 802.11n – This was an improvement over the other standards. This standard used
Multiple Input and Multiple Output Antennas. With MIMO data rate up to 600Mbps is
achievable. It can work both in the 5Ghz and the 2.4Ghz range. It allows up to 4 spatial
streams.
 The other 802.11 standards are not very important. Their main functions were :
o 802.11e – Quality of service and prioritization
o 802.11f – Handover
o 802.11h – Power Control
o 802.11i – Authentication and Encryption
o 802.11j – Interworking
o 802.11k – Measurement reporting
o 802.11s – Mesh Networking
IEEE 802.11n in Wireless Standard
This was a draft version for almost 7 years until, it was approved by IEEE in September,09.
MIMO is the heart of 802.11n.
Range :


Most of the wireless technologies start to fade off at
around 100mts, MIMO is found to have a range of
around 300m – 3 times increase in range.
802.11n was designed to solve range issues with
techniques that minimize interference, optimize data
channels and increase the sensitivity of Wi-Fi devices.
While other WLAN standards perform at or below 1
Mbps at 300 feet, 802.11n can perform as high as 70
Mbps at 300 feet—a staggering difference
Speed:


With 802.11n, it opened up a whole new level of wireless experience. It has a throughput
that clocks around 150 Mbps or faster – 7 times faster than 802.11g.
At 300 feet, 802.11g performance plummets to 1Mbps. 802.11n networks operate at up to
70Mbps – 70 times faster than 802.11g.
Security:



802.11n uses WPA and WPA2 to secure the network.
WPA2, or Wi-Fi protected access, authenticates computers on a Wi-Fi network and provides
a complex algorithm for encrypting communication.
802.11n products using SecureEasySetup make activating security as simple as pushing a
button.
Products
802.11n products are used by companies like Linksys, NetGear, ASUS, Buffalo which deals with
Routers or Wireless Adapters and Acer, Dell, Hp, Lenovo which deals with Desktop/Notebooks.
The other companies that have implemented 802.11n in their products are D-Link, Fujitsu,
Gateway, Sony, Toshiba, U.S.Robotics.
Comparison between different standards
Protocol
Release
Date
Frequency
(Ghz)
802.111997
802.11a
802.11b
802.11g
802.11n
1997
2.4
1999
1999
2003
2008
5
2.4
2.4
2.4 and 5
Throughput
(Mbit/sec)
Data Rate
(Mbs/sec)
Modulation
Technique
Range
(Outdoor)
0.9
2
DSSS
~100
23
4.3
19
144
54
11
54
600
OFDM
DSSS
OFDM
MIMO
~120
~140
~140
~250
No of
Spatial
Streams
1
1
1
1,2,3 or
4
 802.11b was the first standard to become popular in the market.
Pros - Lowest cost; signal range is good and not easily obstructed
Cons - Slowest maximum speed, home appliances may interfere on the unregulated frequency
band
 802.11a was created almost at the same time as 802.11b, but it was slow to gain
popularity
Pros- Fast maximum speed, regulated frequencies prevent signal interference from other
devices
Cons- Highest cost, shorter range signal that is more easily obstructed
 802.11g tried to combine the best of both 802.11a and b. It supports a bandwidth of
54Mbps, and uses 2.4 GHz for greater range.
Pros - Fast maximum speed, Signal range is good and not easily obstructed
Cons - Costs more than 802.11b, appliances may interfere on the unregulated signal frequency

802.11n improved over 802.11g in terms of bandwidth and number of antennas.
Pros - Fastest maximum speed and best signal range; more resistant to signal interference
from outside sources
Cons - Costs more than 802.11g; the use of multiple signals may greatly interfere with
nearby 802.11b/g based networks.
Review of MIMO techniques
MIMO unlike any other technology takes advantage of the multipath. It improves the
throughput by sending several data streams simultaneously.
Fig(ii) A 4X4 MIMO configuration and a 2X3 MIMO configuration
An NXM MIMO configuration has N transmitter antenna and M receiver antennas. Each
receiver receives signal from all the transmitter antenna via different paths. It is favorable if
these paths are spatially distinct.
 MIMO implements Spatial Multiplexing, i.e the ability to sent two or more signals at the
same time over the same spectrum. Each of these transmitted signals are called spatial
stream.
 MIMO implements Transmit Beamforming. It is a method to coordinate the signals sent
from the transmitter so that the signal at the receiver is drastically improved. When two
signals are sent from the receiver they are likely to get added at the receiver. This can be a
constructive or destructive form of addition. By carefully adjusting the phase of the signal
at the transmitter the SNR at the receiver can be improved. It effectively focuses the
transmit antenna on a single receiver.
 MIMO uses Diversity techniques to combat fading and multipath. The classical approach is
to use multiple antennas at the receiver and perform combining or selection and switching
in order or improve the quality of the received signal.
 Space Time Codes are used to achieve spatial diversity. STBC codes have advantages like it
is able to achieve full diversity with low complexity. Alamouti coding is the most popular
space-time coding used in the IEEE 802.11n.
 Spatial Expansion (SE), also called as cyclic expansion is the rudimentary method to map
small number of spatial streams to a large number of transmits antennas. SE works by




sending additional copies of the signal, like echoes, from different antennas. SE introduces
greater variability to the individual subcarriers so that the overall variability averages out.
Power Saving
o Spatial Multiplexing Power Save
There are two mode of operation under this namely, Static and Dynamic. Both works
on the same principle, i.e. all but one of the antennas of the client is turned off, then it
works as a 802.11a/g client configuration. So date is transferred only in one spatial
stream.
o Power Save Multi Poll
It improves over the Automated Power Save Delivery (APSD) mechanism as defined in
802.11e. A buffer mechanism is used here, wherein the client tells the access point to
buffer the incoming messages until it actually request for it. The information sent by
the client can act as a request for the access point to release the information in the
buffer. This greatly reduces the contentions between the client and access point thus
conserving power.
Backward Compatibility- 802.11n has a number of mechanism which allows it to be
backward compatible with 802.11a,b and g devices. It will continue to work in the mixedmode, until all the other technologies have been upgraded to 802.11n.
802.11n says its mandatory to have 2 transmit antennas and having or not having more
than 2 transmit antenna is optional and there should be 1 and 2 number of spatial streams
and its is optional to have 3 and 4 number of spatial streams.
802.11n says it’s mandatory to support BPSK, QPSK, 16-QAM and 64-QAM. It is optional to
support 256-QAM.
Reference
[1] http://compnetworking.about.com/cs/wireless80211/a/aa80211standard.htm
[2] http://www.radio-electronics.com/info/wireless/wi-fi/ieee-802-11-standards-tutorial.php
[3] http://80211n.com/
[4] http://www.segfault.gr/uploads/papers/802.11family.pdf
[5]
http://www.ciscosystems.com/en/US/solutions/collateral/ns340/ns394/ns348/ns767/white_paper_c11
-427843_v1.pdf
[6] http://searchnetworking.techtarget.com/generic/0,295582,sid7_gci1271732,00.html
[7]
http://www.cisco.com/en/US/prod/collateral/wireless/ps5678/ps6973/ps8382/prod_white_paper0900
aecd806b8ce7_ns767_Networking_Solutions_White_Paper.html
[8]
http://www30.homepage.villanova.edu/phani.neelakantham/Comm%20Nets/Wireless%20Networking
%20802.11.htm
[9] http://searchnetworking.techtarget.com/generic/0,295582,sid7_gci1271732,00.html
[10] IEEE Journal on select areas in communication, VOL. 16, NO. 8, OCTOBER 1998 – “A SIMPLE TRANSMIT
DIVERSITY TECHNIQUE FOR WIRELESS COMMUNICATIONS” by Siavash M. Alamouti.
[11] http://en.wikipedia.org/wiki/802.11
Q2)
Part(i)
PLOTS
Rayleigh fading channel for No = 200
2
0
-2
Magnitude |h(t)|(dB) ---->
-4
-6
-8
-10
-12
-14
-16
-18
0
20
40
60
80
100
120
Sample (k) --->
140
160
180
200
Part(ii)&(iii)
Comparison of different MIMO configuration
0
10
For Mr=1
For Mr=2
-1
Symbol error rate ---->
10
-2
10
-3
10
-4
10
-5
10
0
2
4
6
8
SNR(dB)----->
10
12
14
COMMENTS
We see that a 2X2 MIMO configuration performs better than a 2X1 configuration in terms of Symbol
Error rate. The graph also proves that the performance of 2X2 increases with higher SNR.
MATLAB CODE
% %Jakes Simulator for M=8 to model Rayleigh Fading channel
clear all;
clc;
M=8;%no of oscillators
N=34;%No of rays
No=100000;%No of samples
NormDopplerFreq=0.00266;% fd,max.Ts (1)
%NormMaxDopplerFreq=0.1;
A=hadamard(8);
for k=1:1:No+1
val1=0;val2=0;val3=0;val4=0;
for n=1:1:M
Theta = (2*(pi)*n)/N;
Bn = ((pi)*n)/(M+1);
Alpha1=2*(pi)*n*2/(M+1);
Alpha2=2*(pi)*n*3/(M+1);
Alpha3=2*(pi)*n*4/(M+1);
Alpha4=2*(pi)*n*5/(M+1);
val1 = val1 +
A(1,n)*exp(1i*Bn)*cos(2*(pi)*k*NormDopplerFreq*cos(Theta)+Alpha1);
val2 = val2 +
A(2,n)*exp(1i*Bn)*cos(2*(pi)*k*NormDopplerFreq*cos(Theta)+Alpha2);
val3 = val3 +
A(3,n)*exp(1i*Bn)*cos(2*(pi)*k*NormDopplerFreq*cos(Theta)+Alpha3);
val4 = val4 +
A(4,n)*exp(1i*Bn)*cos(2*(pi)*k*NormDopplerFreq*cos(Theta)+Alpha4);
end
h1(k)=val1;h2(k)=val2;
h3(k)=val3;h4(k)=val4;
H1(k)=abs(val1); % Taking absolute value
H2(k)=abs(val2);
H3(k)=abs(val3);
H4(k)=abs(val4);
end
Pow1=0;Pow2=0;Pow3=0;Pow4=0; % Signal Average Power
for iter=1:1:No+1
Pow1 = Pow1 + realpow(H1(iter),2);
Pow2 = Pow2 + realpow(H2(iter),2);
Pow3 = Pow3 + realpow(H3(iter),2);
Pow4 = Pow4 + realpow(H4(iter),2);
end
Pow1 = Pow1/No+1;Pow2 = Pow2/No+1;
Pow3 = Pow3/No+1;Pow4 = Pow4/No+1;
h1= h1/realsqrt(Pow1);
h2= h2/realsqrt(Pow2);
h3= h3/realsqrt(Pow3);
h4= h4/realsqrt(Pow4);
for iter=1:200
H1(iter)= 10*log10(H1(iter)/sqrt(Pow1));
H2(iter)= 10*log10(H2(iter)/sqrt(Pow2));
H3(iter)= 10*log10(H3(iter)/sqrt(Pow3));
H4(iter)= 10*log10(H4(iter)/sqrt(Pow4));
end
k=1:1:200;
figure(1);
plot(k,H1(k),'x',k,H2(k),'*-',k,H3(k),'-o',k,H4(k),'-');
grid on;
xlabel('Sample (k) --->');
ylabel('Magnitude |h(t)|(dB) ---->');
title('Rayleigh fading channel for No = 200');
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Mr=1;Mt=2;
SNR=1:1:13;
BER=zeros(1,12);
Noise=rand(2,No+1);
for iter=1:1:length(SNR)
P=10^(SNR(iter)/10);
display(P);
error1=0;
Sinitial=sqrt(Mt)*eye(2);
Yinitial=sqrt(P/Mt)*Sinitial*[h1(1);h3(1)];
H = zeros(Mt,Mr);
N = zeros(Mt,Mr);
for k=1:1:No
l=randi([0 3],1);
H=[h1(k+1);h3(k+1)];
Cl=sqrt(2)*[exp(1i*l*(pi)/2),0;0,exp(1i*l*(pi)/2)];
St=sqrt(1/Mt)*Cl*Sinitial;
Y1=sqrt(P/Mt)*St*H+Noise(:,k);
Sinitial=St;
%Receiver end
min=99999;
for iter1=0:1:3
Cl_rec=sqrt(2)*[exp(1i*iter1*(pi)/2),0;0,exp(1i*iter1*(pi)/2)];
Y_rec=Y1-(sqrt(1/Mt)*Cl_rec*Yinitial);
Yrec=trace(Y_rec'*Y_rec);
if(Yrec<min)
min=Yrec;
lestimate=iter1;
end
end
Yinitial=Y1;
if(l~=lestimate)
error1=error1+1;
end
end
BER(iter)=error1/No;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Mr=2;Mt=2;
SNR=1:1:13;
BER1=zeros(1,12);
Noise1=rand(2,No+3);
for iter=1:1:length(SNR)
P1=10^(SNR(iter)/10);
display(P1);
error2=0;
Sinitial=sqrt(Mt)*eye(2);
Yinitial=sqrt(P1/Mt)*Sinitial*[h1(1),h2(1);h3(1),h4(1)];
H = zeros(Mt,Mr);
N = zeros(Mt,Mr);
for k=1:1:No
l=randi([0 3],1);
H=[h1(k+1),h2(k+1);h3(k+1),h4(k+1)];
Cl=sqrt(2)*[exp(1i*l*(pi)/2),0;0,exp(1i*l*(pi)/2)];
St=1/sqrt(Mt)*Cl*Sinitial;
Y2=sqrt(P1/Mt)*St*H+Noise1(:,[k k+1]);
Sinitial=St;
%Receiver end
min=999999;
for iter1=0:1:3
Cl_rec=sqrt(2)*[exp(1i*iter1*(pi)/2),0;0,exp(1i*iter1*(pi)/2)];
Y_rec=Y2-(1/sqrt(Mt)*Cl_rec*Yinitial);
Yrec=trace(Y_rec'*Y_rec);
if(Yrec<min)
min=Yrec;
lestimate=iter1;
end
end
Yinitial=Y2;
if(l~=lestimate)
error2=error2+1;
end
end
BER1(iter)=error2/No;
end
figure(2);
grid on;
xlabel('SNR (db) --->');
ylabel('Symbol error rate ---->');
title('Comparison of different MIMO configuration');
semilogy(SNR,BER,'-x',SNR,BER1,'-o');
legend('For Mr=1','For Mr=2');