Performance Enhancement of WLAN Using 802.11n and MIMO Technology
1
Performance Enhancement of WLAN Using 802.11n and
MIMO Technology
V. Vaithyanathan¹, Pethur Raj Chelliah² and Rathnakar Acharya³
1Dept.
of CSE, SASTRA University, Tanjavur
Technologies, Bangalore
3Alliance Business Academy Bangalore
E-Mail: 1vvn@it.sastra.edu, 2pethuru.chelliah@wipro.com, 3rathnakar.a@alliancebschool.ac.in
2Wipro
ABSTRACT: 802.11n is IEEE wireless standards that significantly improve throughput and range of Wireless Local
Area Networks(WLAN) compared with other 802.11 standards. It is expected to provide a throughput of over 100
Mbps, which is twice that of 802.11g. This difference is because it is designed to operate in both 5 GHz and 2.4 GHz
frequency band. The key issue in wireless communication is multi-path propagation. This multi-path propagation
occurs when signal bounces buildings, walls and other obstacles and arrives at the receiver at different times and
from different paths. If the time difference is large enough, the receiver gets confused and can’t interpret the signal
causing retransmissions and therefore reducing the speed and data rate of the 802.11 networks. IEEE802.11n takes
the advantages of multi-path propagation to increase throughput to speeds above 100 Mbps, by using MIMO or in
other words multiple transmitters and multiple receive (MIMO) antennas. It uses spatial diversity to induce multi-path
for the purpose of recombining the multiple signals to increase the signal gain, and channel multiplexing to send
multiple signals using multiple antennas, therefore multiplying speeds, increases the range and enhances the
performance.
Keywords—IEEE 802.11n, Wireless Local Area Network (WLAN), Multiple Input Multiple Output (MIMO), Spatial
Diversity.
INTRODUCTION
O
ne of the main issues for wireless communication by
WLAN is due to the unimpeded path from the
transmitter to receiver. The real world is neither flat nor
empty, however, and physical barriers have long been the
nemesis of WLAN performance. The natural and man
made obstacles impede wireless signals, resulting
distortions, multipath fading and retransmission as a
consequences and ultimately effecting signal integrity and
throughput. To compensate these earlier wireless
technologies used more power. To overcome these
limitations IEEE have effectively revised the standard. This
new standards are now taking advantages of diversity
conditions to improve the performance and speed of
WLAN.
This new standard is IEEE 802.11n, which uses similar
modulation formats as that of IEEE 802.11g but has the
added advantages of applying spatial processing to improve
the performance. The technique is known as Multiple Input
Multiple Output (MIMO). This uses multiple antennas both
at transmitter and receiver. When the standard is considered
it will include MIMO option such as transmit beam
forming, space-time block coding, cyclic delay diversity,
Maximum Ratio Combining (MRC) and intelligent antenna
selection. The use of MIMO technology in IEEE 802.11n
seems to be an attractive solution for the future wireless
systems. In this paper, our discussion incorporates the
analysis of performance enhancement of MIMO
architecture comparing to the Single Input Single Output
(SISO) system. The rest of the paper is organized as
follows: Section II provides the information about IEEE
802.11n standard. Section III we provide the overview of
MIMO architecture and its role to mitigate the multi-path
fading. Section IV gives the details about the performance
enhancement of WLAN using IEEE 802.11n standard and
MIMO with result and finally the conclusion.
IEEE 802.11N STANDARD
802.11n is an emerging IEEE standard that significantly
improves throughput and range compared to earlier 802.11
standards. Having higher data rate up to 100 Mbps, which
is twice the range of 802.11g, and also the backward
compatibility with 802.11a/b/g, IEEE 802.11n device can
communicate and inter-operate with legacy 802.11a/b/g
devices. In order to achieve the backward compatibility,
802.11n must support the three physical layer modulation
techniques used by the older standards; Direct Sequence
Spread Spectrum (DSSS), Complementary code keying
(CCK), Orthogonal Frequency Division Multiplexing
(OFDM).
802.11n includes many features that improve
throughput, range, reliability and efficiency. One of the
important changes of character 802.11n uses MIMO
technology to overcome the wireless propagation problem.
This is the first IEEE 802.11 standard to standardize use of
MIMO antenna design, to significantly improve throughput
164
Mobile and Pervasive Computing (CoMPC–2008)
and range. 802.11n also incorporate many other features in
order to achieve superior performance. Comparison of the
different features and the throughput of IEEE 802.11a/b/g/n
are shown in the Table 1 and Fig. 1.
Table 1: IEEE 802.11 standard specifications
802.11a
54 Mbps
802.11b
11Mpbs
802.11g
54Mbps
802.11n
600Mbps
OFDM
DSSS /
CCK
RF Band
5GHz
2.4GHZ
DSSS
CCK
OFDM
2.4GHz
Available
B.W
No. of
spatial
streams
Channel
width
580 MHz
83.5MHz
83.5MHz
1
1
1
DSSS/
CCK/
OFDM
2.4GHz/
5GHz
83.5/580
MHz
1, 2, 3 or
4
20MHz
20MHz
20MHz
Max. data
rate
Modulation
20MHz40
MHz
Throughput (Mbps)
25
20
15
10
5
0
5
10
15
20
25
Simultaneous AP
802.11n
802.11a/g
Fig. 1: Average throughput/user
Fig. 2: Multi-path signal transmission
Increasing the number of MIMO transmitters and
receivers however holds even greater potential. As a simple
rule of thumb, the more radios you use the better is your
range and throughput up to a point we can obtain this
improvement at low cost in terms of both cost and power
consumption. By using multiple antennas both at
transmitter and receiver it boost the data transmission rate
and quality of wireless signals. In doing so MIMO takes the
advantages of the various reflections seen by the receiver.
Space Division Multiplexing (SDM) is one of the
techniques used in MIMO, which spatially multiplexes
multiple independent data streams transferred simultaneously within one spectral channel of bandwidth. MIMO
SDM can significantly increase data throughput as the
number of resolved spatial data streams is increased. Each
MIMO antenna requires a separate RF chain and Analog to
Digital Converter (ADC). This increasing complexity
ultimately translates to higher implementation cost and
higher performance systems are required. For each channel
the capacity increases as the channel bandwidth is
increased. It can be represented by the Shannon’s equation
C  B log2 1  SNR 
Where C is the channel capacity and B is the channel
Bandwidth. From the expression it is clear that theoretically
capacity increase as the bandwidth is increased. The graph
in Fig. 3 indicates the increase in capacity as the bandwidth
increases.
MIMO is a technology that uses multiple antennas at the
transmitter and receiver. MIMO exploits the fact that Radio
Frequency (RF) signals often reflected off of objects as in
Fig. 2 in their path, causing a phenomenon called multipath fading. Spatial multiplexing is one of the techniques
adapted in MIMO which helps to transmit multiple data
streams at the same frequency but over different spatial
channels. The spatial multiplexing takes advantages of the
multi-path phenomena to increase the effective channel
capacity without consuming additional spectrum. In effect
MIMO takes multi-path transmission and converts it from
signal impairment into a signal enhancement. MIMO
makes a channel more spectrally efficient because spatial
multiplexing increases the baud rate/hertz ratio.
Capacity (Mbps)
MIMO
400
350
300
250
200
150
100
50
0
0
'5
10
15
20
25
30
SNR (dB)
20MHz
40MHz
Fig. 3: Capacity with respect to Bandwidth
165
Performance Enhancement of WLAN Using 802.11n and MIMO Technology
Using wider bandwidth with OFDM offers significant
advantages when maximizing performance wider
bandwidth channels are cost effective and easily
accomplished with moderate increases in digital signal
processing (DSP).
PERFORMANCE ENHANCEMENT OF A
WLAN USING IEEE 802.11N USING MIMO
The use of MIMO in IEEE 802.11n, not only improve the
reliability in data transmission, it also enhances the data
rate over wireless channels. With multiple antennas both at
transmitter and receiver not only rejects fading; better yet it
actually harness the fading itself in favor of increased
throughput.
Consider the MIMO channel having multi-channel
propagation between the transmitter and receiver is shown
in the Fig. 4. Let M and N be the number of transmit and
receive antennas, respectively. The received signal in the
th
i antenna is given by
The above wireless channel is modulated as
The y  H x  n
Where H is the channel matrix and n is the channel
noise.
For transmit/receive beamforming with the diversity of
order MN, is considered as full diversity. On the other hand
the antenna gain is; max  M , N  antennagain  MN
The data transmission capacity bits/channel use of a
communication channel is the maximum throughput at
which data can be sent over the channel. While maintaining
a low probability of error, the capacity of a SISO channel
having one transmit and one receive antenna,
 E x2

CSISO  log 2 1 
2
 E n 
P 

CSISO  log 2 1 
2 
 2 
Where P is the power of the signal transmitted
PE x
And
2
E x
Receiver
Transmitter
En
Fig. 4: MIMO System
2
2
 SNR 
Es
2
From the above expression it is clear that the capacity of
SISO system can be increased only if the transmission
power is increased. In case of Wireless MIMO
P 1 

CMIMO  log 2 det 1 
Q
2
 2 M 
M
M
yi   hij x j  ni where i = 1, 2, 3…N
j 1
hij is the fading corresponding to the path from transmit
antenna j to receive antenna i.
ni is the noise corresponding
to receive antenna i.
n1 
 y1 
 x1 
n 
y 
x 
Let
 2
 2
 2 
y  .  x  .  n  . 
 
 
 
. 
. 
. 
n N 
 yN 
 xM 
 
 
 
The channel matrix H is;
 h11 , h12 ,...........h1m 


 h22 , h22 ,..........h2 m 

H  .


.

 h , h ,........h 
NM 
 N1 N 2
2
where Power P   E x j the total transmission power
j 1
radiating from the transmit antennas
*

 HH ....ifN  M 

Q *

H
H
....
ifN

M




‘I’
is
the
identity
min M , N  min M , N
matrix
of
size
‘det’ is the determinant of the matrix.
Considering the Rayleigh distribution of the fading
yields
P 

CMIMO  min M , N  log 2 1 
2 
 2 
The multiplexing gain is; MultiplexingGain 
CMIMO
CSISO
Under the same transmission power p, the multiplexing
gain is MultiplexingGain  mM , N
166
Thus by using multiple antennas we can increase the
throughput. Using MIMO architecture the throughput can
be increased with a much reduced transmission power. The
upcoming IEEE802.11n Wireless LAN standard using
MIMO architecture along with OFDM and LDPC (LowDensity Parity Check) coding, where these LDPC codes are
highly efficient capacity approach codes, using LDPC code
helps to fulfil the high throughput potential of MIMO
system in a highly efficient manner.
CONCLUSION
802.11n is an emerging IEEE wireless standard, that
significantly imprives throughput and range compared with
older 802.11 standards. 802.11n is the only IEEE standard
that operates in either the 2.4 GHz and 5 GHz frequecy
bands, and it is the first to standardize the use of MIMO
architecture. 802.11n is backward compatible with
802.11a/b/g which means that a 802.1n device can
communicate and; interoperate with legacy 802.11devices.
802.11n may eventually become the dominant enterprices
LAN technolgoy. It is understood that both MIMO
technology and wider bandwidth channels will be required
to reliably satisfy the higher throughput demands of next
generation applications. At the same time, overall
throughput at the MAC Service Access Point (SAP) will be
enabled with new MAC features maximizing throughput
efficiency.
Mobile and Pervasive Computing (CoMPC–2008)
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