School of Engineering and Technology
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1. INTRODUCTION
For nearly 35 years has been passed for inventing the wireless computer network. ALOHA
was the first computer networking systems developed at University of Hawaii. Later on many
protocols has been implemented by Federal Communications Commission (FCC). The Local
Area Network complies of few computers are connected with each other and shares
information in small area. Basically there are two types of computer network, wired and
wireless. This project is concern with wireless local area network (WLAN). The figure 1.1
shows a typical WLAN. [1][2]
Figure 1.1: a typical wireless local area network [3]
Components of WLAN are as follows [4]:

Any computing devices such as personal computers, lap tops PDA etc.

An access point also known as hub or router.

Modem is required for modulation and demodulation process.
Industry and academic research areas are attracted by Commercial off-the-shelf (COTS)
wireless equipments like IEEE 802.11(Institute of Electrical and Electronics Engineers)
WLAN product for low cost and high performance. [5][6][7]
There are several standards set by IEEE for 802.11 standards. They are as follows [8]:

802.11-1997

802.11a

802.11b

802.11g
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
802.11-2007

802.11n
Beng Final Year Project Report
For all these standards there are several channels operates by which data transfers.
Adjacent channel is the channel next to any particular channel. For an example bandwidth of
channel 2 in 802.11 b is 2417 MHz, it’s adjacent channels will be operating at 2422 MHz
(channel 3), 2427MHz (4), 2432MHz (channel 5) and so on Adjacent channel interference
(ACI) is cause by insufficient filtering, for an example partial filtering of unwanted modulation
products in FM Frequency Modulation) systems, inaccurate tuning, bad frequency control, in
either the reference channel or the interfering channel, or both. ACI also occurs by
interfering transmitters. [6][7]
1.1
AIMS & OBJECTIVES
Aims:
IEE802.11 b, n networks have sidebands which overlap with each other. The aim of this
project is to simulate the effect of interference between two wireless LAN access points
operating with various separation distances. By this
Objectives:
The objective is to produce a Matlab/Simulink model that predicts the amount of interference
that occurs between two IEE802.11b, -g or -n wireless LAN access points operating with
channel separations between 0 and 10 and various separation distances. Analyse the effect
when the channels are closer or far apart with each other.
1.2
CONTRIBUTIONS /RESEARCH
Several researches have been done for adjacent channel interference in 802.11 standards.
Out of these two studies are mentioned below.
A group of people from Technical University of Catalonia (UPC) Barcelona [9] has done a
high quality work on this area. The study was based on analysing the effect of adjacent
channel interference, when channels overlap with each other while transmission. A
theoretical model had been proposed which can determine the justification circumstances of
using adjacent channels. As shown in the figure 1.2, channel 3 causing an interference to
channel 1.
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Figure 1.2: matlab simulation, DSSS Receiver receiving interference between channel 1 and 3.
The study also shows that if frequency separation between two simultaneous transmission
increases, the Bit Error Rate (BER) decreases. They also proposed that to quantify
interference caused by transmission for partially overlapping channels then power spectral
density (PSD) of a filtered signal has to be compute. Here an assumption has been done;
the receiver is tuned at 𝑓𝑐 where f is the frequency with c channels apart. Also identical filter
is used in both of the transmitters. The frequency response of the filter is 𝐹𝑓𝑐 (𝑓), here f is
frequency (MHz). So interfering signals overlaping energy can be expressed as,
+∞
𝑃𝑖𝑛𝑡 = ∫−∞ 𝑃(𝑓). 𝐹𝑓𝑐 (𝑓 − 5𝑐). 𝐹𝑓𝑐 (𝑓)𝑑𝑓 [9]
As the transmission can be achive by Direct Spread Spectrum (DSSS) technique
or
Orthogonal Frequency Divission Multiplexing (OFDM), two values has been proposed. That
is for DSSS, P(f) = PDSSS(fc, 22) for OFDM 𝑃(𝑓) = ∑𝑘 𝑃sk(f). They concluded that to avoid
the interference the transmitters should be separated by 5 channels (5 x 5MHz i.e > 22 MHz).
The outcome of the research matches the theory. [9]
A demo Matlab simulink model has been created by the ‘math work’ by which brief analyse
can be done on bit error rate in circumstances on adjacent channel interferences. [10]
Research group from University of North Texas have researched on adjacent channel
interference based on signal to interference ratio (SINR) [11]. They proposed a mathematical
model which will assign channels on maximising Signal to Interfering Ratio (SIR). The model
is assigned to each Access point (AP) to minimize interference in AP but SIR in user’s
network will not be minimized. It has been considered that several APs are situated in single
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floor, a set of users access these APs. Power management in load is balanced so each user
could access only one AP at a time so it is assumed that the power management is fixed.
Each users received power level can be estimate by line of sight path loss mathematical
model.
𝑃𝐿(𝑑) = 𝑃𝐿0 + 29.4𝑙𝑜𝑔10 (𝑑) + 6.1𝑥𝛼𝑙𝑜𝑔10 (𝑑) + 24𝑦 + 1.3𝑥𝑠 𝑦 [11]
PLo=free space path loss
d= distance between user ‘m’ and AP ‘n’
𝑥𝛼, xs, y= independent Gaussian random variables of zero mean and unit variance.
A detailed study has been done and the implementation of the research based on different
scenarios where the number of Access Point (AP), distance between several AP, channel
separation, users changes. The study concludes that there will be no consideration given to
users for channel assignments rather that interference between APs to have an optimum
SIR [11].
A similar research had done a set of 802.11b transmitter and receiver is interfered by
Bluetooth [12]. This is a similar study since the operating frequency range of Bluetooth is
2402-2480 MHz with 79 bands.
[13] The study had been done both in laboratory and
outdoor. Software has been used to calculate SNR. In outdoor experiment it has been
observed that when the distance is smaller from a transceiver, interference increased and
packets loss is high. The figure 1.3 has been produced by which it can be analyzed the
interference level.
Figure 1.3: interference in Bluetooth in 802.11b at outdoor [12]
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The laboratory experiment set up comprises of a pair of DSSS and Bluetooth laptop.
Experiment setup had been fixed in such a way that all the transceiver will interfere with
each other. The results show that when signal to interference ratio (SIR) is low packet loos is
high. When the throughput of the receiving laptop goes high interference power decreases.
The figure 1.4 shows the results.
Figure 1.4: interference in Bluetooth in 802.11b at indoor [12]
Outdoor and indoor performance had been compared and concludes that at outdoor
performance goes much worst.
Additional Information about 802.11b problems:
There are total 14 channels in IEEE 802.11 ‘b’ standard. Out of these 14 channels, only 3
can be used, as shown in the figure 1.5.
The other channels are wasted because of
adjacent channel interference. There are manly two patterns for channel separation; 1st, 6th,
11thor 1st, 7th, 12th. The figure below shows how 14 channels are interfering with each other
and out of these 14 channels how 3 channels have been extracted by 5 MHz of channel
separation [8].
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Figure 1.5: two different ways of representing wanted channels are in 5 MHz separation in 802.11 [9][11]
Overlapping section:
As the figure a shown below, there are two access Point (AP), one is uses channel ‘n’ and
other uses channel ‘m’. At the far end of both of the AP; users can access signal properly.
But at the overlapping area of both AP, users can not access in an adequate way. At this
point the Bit Error Rate (BER) increases rapidly. In figure 1.6 a. describes if any use’s
position is in between two access point BER increases and figure 1.6 b. is a real life AP or
hub.
Figure1.6: a. Bit Error Rate (BER) increases when user’s position is in between two Access Point, b. Real life
access point or hub. [14]
Hidden node:
As shown in the figure 1.7, transmission ranges of A and C cannot communicate with each
other if they transmit data to B at the same time, in this case the frames may get corrupted.
[14]
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Figure 1.7: transmitter A & C communicate with B at the same time. [14]
This problem can be solved by adding two frames in Medium Access Protocol (MAC) layer,
as shown in figure 1.8. They are Request to Send (RTS) and Clear to Send (STS). One
transmitter sends RTS and the other transmitter and CTS is replied by destination
transmitter. Transmission suspended by the node that here RTS and CTS for a specific time
which has already indicated in RTS and CTS frames. [14]
Figure 1.8: RTS and CTS for hidden node problems [14]
These frames occupy a little space in MAC protocol. Station which hears RTS, delay
transmits until CTS frame. If the station does not hear CTS, then it transmits. The station
which hear CTS, it suspends the transmission until hearing the acknowledgement.[14] [4]
In the source station, a failure of the frame exchange protocol causes the frame to be
retransmitted. This is treats as a collision, and the rules for scheduling the retransmission
are described in the section on the basic access mechanism. To stop the MAC from being
monopolized attempting to send a single frame, retry counters and timers are there to limit
the lifetime of the frame. [14]
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1.3
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SOFTWARE
As the project work on simulation of adjacent channel interference in IEEE 802.11,
simulation has been done by ‘Matrix Laboratory’ shortly known as ‘Matlab’. For simulation in
matlab simulink is used. Matlab is a fourth generation computing language developed by
The MathWorks, Inc. [10]
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2. OVERVIEW OF THE 802.11B & N STANDARDS
IEEE 802.11b standards:
This standard implemented in to the market in mid of 1999. In general operational bandwidth
is 2400 -2483.5 MHz or 83 MHz, it has 14 channels. Each channel separation is 5 MHz apart.
For the effect of adjacent channel interference 1st, 6th, 11thor 1st, 7th, 12th channels can be
used. In this section mainly two standards ‘operating mode’ and ‘physical layers’ will be
discussed. [9][4][14]
Operating Modes [16]:
Two pieces of equipments are needed for IEEE 802.11b technology. One wireless station,
usually a PC which has Wireless network interface card (NIC) connected and a assess point
(AP). It is a bridge between the wireless and wired network. [15]
As shown in the figure 2.1, this standard basically has two types of operating mode, Ad hoc
mode and infrastructure mode. The network has a characteristic of at least one AP which is
connected with Base Service Station (BSS). It is an Extended Service Set (ESS) of two or
more BSS forms single sub network. It is because Wireless LAN (WLAN) needs access to
wired LAN for other services like printers, internet links, file servers etc which will operate in
infrastructure mode. [16]
Ad hoc mode is also known as peer to peer mode. It is also called Independent Basic
Service Set (IBSS). In this type of operation the wireless stations directly communicate with
each other without using any AP or any wired network. Ad hoc mode is usually useful for
easy installation such as airport, hotel rooms, convention centres etc. [15]
Figure 2.1: two operating mode a. Infrastructure mode, b. Ad hoc mode [14]
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Physical layers:
Two spread-spectrum radio method and diffuse infrared specification is defined by three
physical layers of 802.11. 2.4 GHz ISM (industrial, scientific and medical) band is specified
for radio based operation which is recognised by Federal Communication Commission
( FCC), MKK and ETSI etc. One more interesting point is 802.11 products do not require any
type of user; additionally the spread spectrum technique allows increase in reliability, boost
throughput. [4][14]
802.11 b standards have 4 types of transmission rates. They are 1, 2, 5.5, 11 Mbps. 1 and 2
Mbps system uses frequency hopping spread spectrum (FHSS) or Direct spread spectrum
(DSSS) techniques. FHSS and DSSS are fundamentally two different types of signalling
mechanisms and does not exchange with one another. 2.4 GHz band divides into 75 1-MHz
sub channels. When the sender and receiver agree on a hopping type, then only the data
can be sent over a series of the sub channels. Different hopping patterns uses with in a
network for each conversation and these patterns are designed to minimize the chance of
using same sub channels at the same time by the two senders. For a simple design FHSS
technique uses but has disadvantages of low transmission rate more than 2 Mbps.
According to the FCC regulation restriction sub channel bandwidth will be up to 1 MHz for
this restriction across 2.4 GHz band width, FHSS system spreads across 2.4 GHz band
width. This means they must hop very often; leads high hopping overhead. [4][14][15]
Where as in DSSS 2.4 GHz band is divided into 14 channels of 22-MHz Adjacent channels
overlap with each other where three of the fourteen channels are none overlapping. Over
this 22 MHz channel data is sent without any hopping with other channels. Clipping
technique is used to balance noise. [4][14][15]
The major advancement of the 802.11b is upgrading its transmission rate of 5.5 and 11
Mbps. To achieve these speed DSSS technique has been selected. The 802.11system has
interoperability with 1 and 2 Mbps
DSSS but not work with FHSS system. 802.11 DSSS
standard state that a Barker sequence (11 bit chipping of 10110111000, which will encode
all data and transmit over air. Every 11 chip series represents data (0 or 1) which is known
as symbol. Here the symbol rate is 1MSPS (million symbol per second) using Binary Phase
shift Keying (BPSK) technique. In 2 Mbps system Phase |Shift Keying (QPSK) technique is
used, which increases data rate and increase efficiency of using the particular bandwidth.
[4][14][16]
Complementary Code Keying (CCK) is used to increase the transmission rate. CCK
consists of 64 code word, where each code word consists of 8 bits. Every code word can
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distinguish by each other. 4 bit per carrier CCK is used in 5.5 Mbps systems and for 11Mbps
it gets just double i.e. 8 bit per carrier. QPSK technique is used for both the speeds. By this
way higher data rate can be achieved. In the noisy environment 802.11b uses dynamic rate
shifting where the data rate is adjusted automatically to compensate for a noisy channel. A
brief mathematical and logical theory about CCK is attached in appendix. Table 2.1 shows
the differences between for various parameters for different bit rates. [4][14][15]
Data
Code Length
Rate(Mbps)
1
11
Modulatio
Symbol
n
Rate(Mbps)
Bits/Symbol
Barker BPSK
1
1
Barker QPSK
1
2
sequence
2
11
sequence
5.5
8 CCK
QPSK
1.375
4
11
8CCK
QPSK
1.375
8
Table 2.1: various standards of 802.11b
IEEE 802.11n standards:
802.11n standard has been open since September 2002, but in October 2009 the standards
published. It operates with Multiple-Input and Multiple-Output (MIMO) mode, which means it,
has multiple antennas both in transmitter and receiver. The figure 2.1 shows an example of
MIMO. [17]
Figure 2.1: A simple example of MIMO operates in 802.11n [18]
In the figure above multiple transmitting and receiving antennas are in operation. Each
transmitter is transmitting to every receiver. The signals are described as S11, SS 21 SS 31 that
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means signal from first transmitter has been send to receiver 1, 2 ,3, ... so on so forth. It
implies for other transmitters and receivers also. Data rate has been increased from 11Mbps
(802.11b) to 600Mbps with 40 MHz channel bandwidth. [8]
It can be done by Spatial Division Multiplexing (SDM). In this technique multiple independent
data streams are spatially multiplexes and transfers at the same time within a spatial
channel/. As the spatial data flow increases, data throughput also increases. [15]
High data rate up to 600Mbps can be achieved by using four spatial stream which uses
channel bandwidth of 40 MHz. it uses several modulation scheme and spatial stream [19]
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Modulati
Spatia
Data Rate with 20 Data Rate with 40
on Type
l
MHz channel(Mbps)
Inde
Strea
x
m
800ns
MHz
channel(Mbps)
400ns
800ns
400ns
0
BPSK
1
605
7.2
13.5
15
1
QPSK
1
13
14.4
27
30
2
QPSK
1
19.5
21.7
40.5
45
3
16-QAM
1
26
28.9
54
60
4
16-QAM
1
39
43.3
81
90
5
64-QAM
1
52
57.8
108
120
6
64-QAM
1
58
65
211.5
135
7
64-QAM
1
65
72.2
135
150
8
BPSK
2
13
14.4
27
30
9
QPSK
2
26
28.9
54
60
10
QPSK
2
39
43.3
81
90
11
16-QAM
2
52
57.8
108
120
12
16-QAM
2
78
86.7
162
180
13
64-QAM
2
104
115.6
216
240
14
64-QAM
2
117
130
243
270
15
64-QAM
2
130
144.4
270
300
16
BPSK
3
19.5
21.7
40
45
64-QAM
4
260
288.9
540
600
..
31
Table 2.2: different parameters of 802.11n [19]
The APs and stations require confirming few capabilities such as spatial stream, channel
width, RF modulation type, guard interval etc. All these factors determines physical layer
data rate from 6.5 to 600Mbps. 77 possible factors determines data rate, this standard
specifies Modulation and Coding Scheme (MCS). MCS is a simple integer which is assigned
to every permutation of modulation, guard interval, coding rate, channel width, spatial
streams number. In the table above few broadly supported factors are given. [19]
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MCS: It determines how data will be sent over air.
Guard Interval: Time between each transmitted symbols. Guard Interval is important to
offset the effects of multipath which can cause Inter-Symbol Interference (ISI). [19]
802.11a/g standard uses 800ns of guard intervals, but 802.11n has both options of 800ns
and 400ns. If the guard interval is shorter, a probability increases for more interference and
less throughput. [19]
Unequal Modulation: as shown in the table 802.11n standards uses different modulation
techniques with various spatial streams. [19]
2.1
OVERVIEW OF THE MATLAB SIMULINK MODEL
(802.11B)
Several system parameters are used in this model which is given below:

Data rate: 11 Mbps

Packet size: 1024

Long preamble is used

Channel type: AWGN

Channel noise: 12dB

Frame base output
IEEE 802.11b standard has mainly 3 sub systems. They are transmitter, receiver and
channel. Each of the sub system’s working principle is discussed below.
1. Transmitter:
The very beginning block of the transmitter is ‘Random integer generator’ block. It generates
random number or it can be say as byte, 1024 byte has been set. Output of the first block
goes to ‘Convert bytes bits’ block. Here the generated bytes are converting to bits (1 byte =
8 bit, 1024 x 8= 8192 bytes). As the wireless transmission requires packets, the next block
assembles frames or packets.
As shown in the figure below, how Physical Layer Service Data Unit (PPDU) is sub divided
by the other sub frames. In the model PSDU is the information unit, Physical Layer
Convergence Procedure (PLCP) header and preamble has been generated with their
subsections. PLCP header and preamble are intelligent of the frame. Figure below shows
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simulink model of the ‘Framing by adding PLCP preamble and header’ sub blocks. As
802.11 standards include Physical Layer Convergence Procedure (PLCP) and (PSDU),
PLCP has two parts, PLCP preamble and header. Preamble is built on 2 sub frames,
Synchronization (Sync) and Start Frame Delimiter (SFD). Header performs 4 sub functions,
they are; signal, service, length, cyclic redundancy check (CRC). The combination of PLCP
header, preamble and PSDU is Physical Layer Service Data Unit (PPDU). The functions of
each sub frames is given below. The figure 2.2 shows 802.11 frame format and figure 2.3 is
the matlab simulink frame generation block which has been modulated.
Figure 2.2: division of a PPDU frame [4][14][16]

Sync: This section alters 1s and 0s. It alerts the receiver that a receivable signal is
currently there. The receiver synchronises with the input signal after detects the Sync.
128 bits or 16 bytes are allocated for this frame.

Start Frame Delimiter (SFD): A particular fixed patterns of 0s and 1s of 16 bits or 2
bytes present at the beginning of each frame. The pattern is 1111001110100000
[4][14][16]

Signal. The data rate is identified by this sub frame. The length of the frame i s 1 byte.
For example, if the frame 00001010 for 1Mbps, 00010100 for 2Mbps, and so on. The
PLCP fields, however, are always sent at the lowest rate, which is 1Mbps. This
ensures that the receiver is initially uses the correct demodulation mechanism, which
changes with different data rates. [4][14][16]

Service: 1 byte field is always kept blank i.e. 00000000, for the future use. [4][14][16]

Length: This field verifies that how many microseconds will it takes for transmitting
the PPDU and the receiver uses this information data to decide the end of the frame.
2 bytes has been allocated for this sub frame. [4][14][16]

Frame Check Sequence: Detection of the probable errors in the header of the
physical Layer, it containing 16-bit of cyclic redundancy checks (CRC) result. The
MAC Layer also perform error finding on the PPDU contents as well. [4][14][16]

PSDU: It contents the actual information data of 8384 bits or 1048 bytes. [4][14][16]
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Figure 2.3: 802.11b frame generation in Matlab simulink [20]
As the frame gets ready, the next block is for modulation. Figure 2.4 is the matlab simulink
modulation subsystem block which has been modulated. The packets are divided in two
groups, one is Physical Layer Convergence Protocol (PLCP) preamble, header and other is
Physical Layer Service Data Unit (PSDU). Both parts will be modulated separately. As the
model uses long preamble the PLCP header, preamble is modulated by differential phaseshift keying (M-DPSK), here M stands for modulator and then Differential Binary Phase-Shift
Keying (DBPSK) techniques used respectively. The spreading technique is used to the
frame with a constant value of 11. The output frame length increased (11 x 192 = 2112 bits
or 264 bytes).
For PSDU, the whole bits are divided by 4 to create different phases. So for each phase it
has 2048 bits (1024 x 2= 2048). The phase from 0 to 3/2 π is modulate by Quadrature
Phase Shift Keying (QPSK) and for the region of 3/2 π to 2π by Differential Quadrature
Phase Shift Keying (DQPSK). The 4 constellation bits are produced for chip calculation. Chip
is a single bit of a pseudo noise sequence; here the spreading technique is applied. So total
number of chips are now 10304 (2112+8192=10304bits).
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Figure 2.4: subsystems of modulation block in matlab simulink [20]
The next block is for ‘upsample & pulse shape’. Here the chips are need to be in shape,
Finite Impulse Response (FIR) filter is used. Mathematical formula [21] of the filter is given
below:
𝑁
𝑦(𝑛) = ∑ 𝑎𝑖 𝑥(𝑛 − 1)
𝑖=0
Where, x(n)= input signal
y(n)= output signal
ai = filter co efficient
N= filter order.
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Figure 2.5: block diagram of FIR filter.[22]
Figure 2.5 is a block diagram of an FIR filter. Here Z-1 is delay and a1,2 filter coefficient.
802.11b operates in DSSS technique, for equalization in DSSS system mainly two methods
are used. One is FIR filter and the other is rake receiver. In the rake receiver instead of
putting dispread block after the FIR filter, it placed inside. The output will be same if
coefficient and delay unit are same. The FIR filter delays are almost equal but delay unit in
the rake receiver are not same. In the rake receiver usually strongest paths are kept
reserved and other coefficient replace by zero. FIR needs more Multiply and Accumulator
(MAC), but on the other hand rake receiver requires dispread operation. As 802.11b
operates in 11MHz, dispread is replaced by complex Walsh transform which follows by
searching a value of the size of 5 or 11Mbps in complex vector. So FIR filter is the ultimately
choice. [23]
Then the signal goes to ‘mix to centre frequency’ block where the centre frequency of each
channel will change according to the channel separation. The channel number and their
centre frequencies are given in table 2.3.
Channel number
Centre Frequency(MHz)
Channel number
Centre Frequency (MHz)
0
2400
8
2447
1
2412
9
2452
2
2417
10
2457
3
2422
11
2462
4
2427
12
2467
5
2432
13
2472
6
2437
14
2484
7
2442
Table 2.3: frequencies assign to separate channel number in 802.11
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The signal is ready to transmit. Before transmission one transmission power measurement
block is fixed to check the transmission power.
2. Channel:
IEEE 802.11b defines channel as Additive White Gaussian Noise (AWGN). AWGN Matlab/
simu
link block adds white Gaussian noise to an input signal which can be a real or
complex. The block adds real noise in input and at output it generates a real part when the
signal is real. For the complex signal it works same as describe above, i.e when the signal is
complex at input, it produces a complex output too. Few parameters of this block are
explained below. [10] www.mathwork.com
Initial seed: generates Gaussian noise.
Mode: specifies noise variance: Signal to noise ratio (Eb/No), Es/No, SNR, Variance from
mask, or Variance from port.
Eb/No (dB): ratio of bit energy per symbol to noise power spectral density (dB).
Es/No (dB): ratio of signal energy per symbol to noise power spectral density (dB), SNR (dB)
Number of bits per symbol: The number of bits in each input symbol.
Input signal power, referenced to 1 ohm (watts): square power of the input symbols or input
samples (watt).
Symbol period (s): The duration of a channel symbol, in seconds.
Channel capacity of AWGN channel can be defined by the following mathematical term. [21]
C= W log2 (1+SNR)
[21]
Where, C= channel capasity
W= channel band width
SNR= signal to noise ratio
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3. Receiver:
Working principle of the receiver is just opposite of the transmitter. The receive signal first
‘Rx front’ where the signal again mixes to its centre frequency and the pulses of the signal
reshape again. At this time mixing with centre frequency block acts as a logic operator. If the
local oscillator generates a complex carrier with compare to the receive signal then the
signal will meagre with the real part of the signal and both the frequency mixes. If the
channel size changes, the signal will pass through the complex carrier block rather than the
normal ‘pass through’ block and mixes with the original received signal. At the pulse
reshaping block, direct-Form II Transpose Filter has been used.
The signal then goes to ‘Sync to chip’ block. Here the signal is delayed by an integer number
which is sample correction. The signal is divided in to two parts, one goes to ‘Rx Signal
Aligned’ which goes to two scopes, eye diagram and scatter plot diagram respectively. The
other signal is down sample by the value of sample per chip. The signal is delayed by
several times and then it has been demodulated.
Demodulation technique is also follow almost similar way as modulation. The signal is
divided by PLCP header, preamble and PSDU frames. The PLCP header, preamble is
demodulated by same way as earlier. For PSDU signal Walsh code has been applied. This
code is used when an individual communication channel is defined individual. The Walsh
code is calculated by the Walsh function. It is consisting of sets of square pulses (which
allowed states are -1 and 1) so the transitions may only arise at fixed intervals of a unit time
step, the initial state is -1. [24]
The PSDU frame again merges with PLCP header, preamble and formed PPDU. The signal
then divided by two way, one goes to scope ‘Rx Bits’ to compare the BER; other goes for
‘deframing’ block, Where the whole packets again divides as PSDU, PLCP header and
preamble. And the model stops by a terminator.
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2.2 OVERVIEW OF THE MATBAL SIMULINK MODEL
(802.11N)
Working procedure of 802.11n matlab simulink model is completely different from 802.11b.
The figure 2.6 is 802.11n and the working procedure has been discussed.
Figure 2.6: 802.11n matlab simulink model [10]
As it is discussed earlier, this standards works on MIMO basis, variable data generates
automatically. Data goes for modulation at the modulator bank. In the modulator bank not
only the transmission bit enters but a feedback from ‘adaptive modulation control’ block
where signal to Noise Ratio (SNR) threshold, hysteresis factor and bit rate are determined.
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The bits and feedback from adaptive modulation control subdivided by 4 parts, and enters to
another 4 sub modulator bank known as ss1, ss2, ss3, ss4. In each modulator BPSK, QPSK,
16 and 24 Quadrature Amplitude Modulation (QAM) techniques are used. And output signals
join
together and signals are again modulate by Orthogonal Frequency Division
Multiplexing techniques and then bits are sending for framing.
As mentioned earlier, a frame consists of information bits and intelligent bit that ensures the
correct destination of the frames. In this standard, preambles are generated by Pseudonoise sequences (PN sequence) generator. PN sequences are binary sequences bits which
display noisy signals which are difficult to decode for unauthorized user. It has randomness
properties.
After framing the signal goes to ‘Space Time Block Coding’ (STBC) block is used to improve
reception which spreads the spatial streams diagonally from multiple antennas or to transmit
same signal in a different cyclic shift [23] the next block is ‘antenna map’ where the each
signals are divided in to four part and subcarriers are added with each signal. Then each
signal is transforms to Infinite Fast Fourier Transform (IFFT) and cyclic prefix is added. The
frames are then multiplex by OFDM technique, and transmitted through channels using four
spatial streams which have much wider band width of 40 MHz. This bandwidth is much wider
than 802.11 a, b and g.
The next blocks are in receiver subsystem. The receiver always works just opposite to the
transmitter. Here the signals are demultiplex by OFDM technique. The cyclic prefix is
removed and FFT function is used to retrieve a clean received signal. In MIMO detection
block works opposite to antenna map block. Then the frames are break up and demodulates.
After demodulation the signal is read to receive. Several scopes such as SNR for four
channels signal visualise scope has been used to analyse the transmission performance.
Model’s transmission parameters can also be changed by ‘simulation setting’ block.
In overall 802.11n is a complex system rather than a, b or g but the transmission rate can be
achieve much wider from 6.5 Mbps to 600Mbps, which is much wider than other standards.
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DESIGN CRITERIA (802.11b)
In the section description of the working procedure has been discussed. A number of steps
have been completed to achieve the ultimate goals. Here few of steps have been discussed.
1. Interfering transmitter with AWGN channel
To achieve the aim of the project an interfering transmitter has been added with different
characteristic of wanted transmitter, figure 2.7 shows the wanted and interfering transmitters
in matlab simulink. The channel allocated for the interfering transmitter also have different
characteristic. It is because both the transmitters and channels are not identical. The
parameters are given in the table 2.4.
Parameters
M-ary numbers
Initial seed
Sample time
Frame based output
Sample per frame
Output data type
Wanted transmitter’s random
number generator
255
12345
PPDU_frame_period/Packet size
Yes
Packet size
Double
Interfering transmitter’s random
number generator
63
13425
PPDU_frame_period/Packet size
Yes
Packet size
Double
Table 2.4: parametters of 802.11b
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Figure 2.7: full model with wanted, interfering transmitters, receiver, channels, scopes and meter [20]
The figure 2.7 shows the model which has been modified and simulated.
2.
Decibel gain:
A decibel (dB) gain block has been set just after the interfering channel. This gain is acts as noise to the wanted
transmitter. At first minimum gain will be applied (-60dB, 1 x 10-6 in linear unit). And gradually the gain will be
increased. The aim is to analyse the effect in BER by increasing interfering transmitter’s gain.
3.
Addition of two transmitter:
The wanted and interfering transmitters have been added. It is done to analyse the effect in BER as shown in the
figure above. The two signals are then connected to the receiver.
4.
Phase Frequency offset: The model had an inbuilt function by which channel could be changed from
various setup. Unfortunately that function did not work properly. For channel separation ‘Phase
Frequency Offset’ block has been used. Here only the frequency offset option has been used. This block
can shift the operating frequency to reduce interference. The mathematical equation for frequency offset
is given below:
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𝒇 .𝓟
𝑳
𝒇𝒐𝒔 = 𝒇𝒄 + 𝟏𝟐
fos = offset in Radio Frequency(RF)
fc = center frequency in operation
p = an integer
fL = line frequency.
5. Frame delay: As in real world, two WiFi transmitters do not transmits data at the
same time; a frame delay block has been added to the transmitter to make the model
more realistic.
6. Scopes & meters:
In this model mainly 3 types of scopes has been used. They are FFT spectrum scope,
Discrete-Time Eye Diagram Scope and Discrete-Time Scatter Plot Scope. The BER meter
has been used to calculate how many error bit is received by the receiver when the wanted
signal is interfered by the other transmitter in different level of noise.
Figure 2.8 shows the output of the adding signal (wanted and interfering signal) will be
observed by the FFT spectrum analyser. Which will show the channel separation of wanted
and interfering signal. Figure below shows two signals with certain channel separation.
Figure 2.8: two separate signals with different channel separation by FFT spectrum scope [20]
Discrete Time scatter plot is used to analyse the phase also the noise level of the received
signal. Figure 2.9 shows an example of scatter plot scope with a noise free and noisy signal.
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Figure2.9: an example of scatter plot scope. [20]
In communication engineering the "eye diagram" is used to analyse the performance if the
signal. It is not an accurate analysis like BER meter; though overall performance can be
conclude by eye diagram. A detailed study on eye diagram is attached in appendix. Figure
2.10 shows an eye diagram (simulation result)
Figure 2.10: eye diagram [20]
Note: in real world transmitters may transmits millions of packets at a time and will never
transmits 1024 bytes. This value has been chosen for simplicity. Depending on the
configuration of computer simulation time may vary. But in real world for transmitting 162
frames it may take fraction of seconds. Realistic time for simulation taken for transmitting
162 frames is 1x10-4- second.
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3.0 RESULTS & ANALYSIS:
In this part mainly BER performance has been analysed. Basically BER in three scenario
has been analysed, the amount of interference that occurs between two IEE802.11b
wireless LAN access points which operates with channel separations of 0 and 10 and
various separation distances of 25, 20 and 15 MHz.
Wanted transmitter operates in 2417 MHz of centre frequency. Interfering transmitter
channel centre frequency has been shifted 25MHz to 15MHz (5th to 3rd channel separation).
The gain (noise for wanted transmitter) of interfering transmitter has been changed from 5dB to -1dB (0.316228 to 0.794328 in linear units). Wide range of interfering gain has not
taken since the variance of the values are too high, which will affect the results. Several BER
results have been achieved by inputting different parameters which are shown in the table
3.1 to 3.5.
3.1

RESULTS:
BER for 25 MHz frequency separation:
Tx Power (W)
0.919
0.919
0.919
0.919
0.919
Variable
interference (dB)
-5
-4
-3
-2
-1
Total no. Of transmitted
bits
8190
8190
8190
8190
8190
Error bit received
3
86
254
538
1375
BER
0.000256
0.0105
0.03101
0.06569
0.1678
Table 3.1: 25 MHz of frequency separation

BER for 20 MHz frequency separation:
Tx Power (W)
0.919
0.919
0.919
0.919
0.919
0.919
Variable
Total no. Of transmitted
interference (dB)
bits
Error bit received
-5
8190
0
-4
8190
0
-3
8190
9
-2
8190
59
-1
8190
303
0
8190
934
BER
0
0
0.001099
0.007204
0.037
0.114
Table 3.2: 20 MHz of frequency separation
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BER for 15 MHz frequency separation:
Tx Power (W)
0.919
0.919
0.919
0.919
0.919
0.919
0.919
0.919
0.919
0.919
0.919
Variable
interference (dB)
-5
-4
-3
-2
-1
0
1
2
3
4
5
Total no. Of transmitted
bits
8190
8190
8190
8190
8190
8190
8190
8190
8190
8190
8190
Error bit received
0
0
0
0
0
0
0
22
96
367
942
BER
0
0
0
0
0
0
0
0.002686
0.01172
0.04481
0.115
Table 3.3: 20 MHz of frequency separation

BER for zero channel separation:
Tx Power (W)
0.919
0.919
0.919
0.919
0.919
Variable
interference (dB)
-5
-4
-3
-2
-1
Total no. Of transmitted
bits
8190
8190
8190
8190
8190
Error bit received
21
86
254
538
1374
BER
0.002564
0.0105
0.03101
0.06569
0.1678
Error bit received
0
0
0
0
0
BER
0
0
0
0
0
Table 3.4: zero channel separation

BER for 10 channel separation:
Tx Power (W)
0.919
0.919
0.919
0.919
0.919
Variable
interference (dB)
-5
-4
-3
-2
-1
Total no. Of transmitted
bits
8190
8190
8190
8190
8190
Table 3.5: 10 channel separation
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3.2 ANALYSIS:

Analysis of BER verses signal to interference ratio for various separation distances of
25, 20 and 15 MHz.
The results above shows that while increasing in noise level for 25 MHz of frequency
separation BER increases. For 20 & 15 MHz of separation the phenomena are same. The
point to be noted, decrease in BER is not same at different channel separation. BER verses
interfering gain graph is plotted, which is shown in figure 3.1.
Figure 3.1: BER vs. Signal to interference Ratio (dB) in various channel separation
A logarithmic graph is used since the BER values are too large. It has been analysed that as
signal to interference ratio increases, BER decreases gradually. Performance of the system
gets better as BER decreases. There are several points in the graph. It would be easier if
one to two reference points are set up. At 3 dB of x-axis 3 separation point has been
intersected. It can be analysed that as channel separation increases BER decreases. At 3dB
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of x-axis for 25 MHz separation BER is around 1x10-16, for 20 MHz separation BER is 1x10-19
and for 15 MHz separation BER is around 1x10-29.
The same types of values are seen for 2 and 1 dB signal to interference ratio. From this part
it can be conclude that the theoretical results matches with this simulation.

Analysis of BER verses signal to interference ratio for 0 channel separation (co
channel interference):
As mentioned earlier the centre frequency of the wanted transmitter is 2417 MHz. To make
the interfering transmitter’s frequency at same frequency offset has to be set at -25MHz.
frequency cannot be negative. But as the model has been designed, -25MHz should be set
to achieve the correct result. The explanation of this topic is attached in the appendix. The
graph/ figure 3.2 shows the analysis.
Figure 3.2: BER vs. Signal to interference Ratio (dB) in various channel separation
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It can be analyse that at zero channel separation signal to interference ratio is higher than
adjacent channel interference. As in adjacent channel at 3dB of x-axis the BER is around
1x1029 but in co channel interference at -3dB point of x-axis signal to interference ratio is
around 1x10-75. This result also matches the theory. It is obvious that if an interfering channel
completely overlaps the wanted channel, the BER goes tremendously high. Few picture of
wanted and interfering channel’s spectrum has been taken, which is attached in the
appendix.

Analysis of BER verses signal to interference ratio for 10 channel separation:
The 10th interfering channel is an adjacent channel with respect to wanted channel. This
separation is too far from the wanted transmitter. To achieve this, frequency offset up has to
be set at 25 MHz i.e. the centre frequency of the interfering transmitter is 2467 MHz. The
results for this separation have been mentioned in the above section. BER is zero for the
range of interfering noise signal. So no graph could be plot. As theory says, minimum
channel separation should be at least 5. As the interfering transmitter is set too far from the
wanted transmitter, there is no chance of BER, which also matches with theory.
Along these analyses, for a DSSS system interference would be around 20 to 25dB. But
according to this model simulation it shows if the interfering transmitter interferes at 1 to 5 dB,
a significant BER occurs, which certainly suspicious. This could happen if wanted transmitter
and receiver is not tuned properly.
3.2

FURTHER DEVELOPMENTS:
Research on simulating the 802.11b model in different sets of noise level can be
done which will be a vast and different area of studies.

Similar analysis can be done for 802.11n standard i.e study on BER for several
channel separation between wanted and interfering transmitter is different level of
interference.

Analysis on throughput, user load, channel capacity including various access points
in 802.11n standard. This will be a realistic research. Now a day 802.11n is taken
over 802.11b. So research on 802.11n will be a high quality of work.
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Industrial development/ research:
All the research areas listed above will be helpful to develop a better system than
802.11n.
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4. Conclusion
Throughout the whole year this particular project has being carried out. The main aim of this
project was to create a model by which the BER performance can be analysed under various
circumstances such as different channel separation of the interfering transmitter, different
level of interference in interfering transmitter etc. At this very end several conclusions can be
given. Few important points will be discussed here.
According to theory, minimum 5 channel separation should be set for an error free
transmission. As the interfering transmitters channel will be closer apart BER will be
increased. By the simulation results it can be analysed that the model which has been
created it is working perfectly. But in simulation or real world, by matching one aspect the
final results cannot be conclude. As the system runs by DSSS techniques it says that the
power (gain) of the of the interfering transmitter need to be much higher than the results
which are recorded by the simulation. It is very suspicious the model is not working properly.
Previously many times this model behaved extremely different, the built-in channel
separation key had some problems.
After various research that problems has been solved. It cannot be concluding at this
moment that why the results show different that it is expected. Most probably the tuning
frequency of the wanted transmitter and receiver is not matched though it was set properly.
At the end it can be said that the given problem is achieved at the end of the project. After
completion this work a wide gain of knowledge is achieved in this area.
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5. References
[1] http://www.fcc.gov/
[2] http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=4045588
[3] www.impulseportal.com/gigabitnetworks.asp
[4] IEEE Standard for Information technology— Telecommunications and information
exchange
between
systems—
Local
and metropolitan
area
networks—
Specific
requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer
(PHY) Specifications, IEEE , 3 Park Avenue , New York, NY 10016-5997, USA , 12 June
2007
[5] Aguayo, D., Bicket, J., Biswas, S., Judd, G., and Morris, R., “Link-level Measurements
from an 802.11b Mesh Network,” SIGCOMM 2004, Aug 2004
[6] Jain, K., Padhye, J., Padmanabhan, V. N., and Qiu, L., “Impact of Interference on Multihop Wireless Network Performance,” ACM MobiCom 2003, September 2003
[7] Li, J., Blake, C., De Couto, D. S. J., Lee, H. I., and Morris, R., “Capacity of Ad Hoc
Wireless Networks,” ACM MobiCom, July 2001
[8] www.ieee.xplere.org
[9] Villegas E. G, Aguilera E. L, Vidal R, Paradells J, Effect of adjacent-channel interference
in IEEE 802.11 WLANs, Technical University of Catalonia (UPC),.
[10] www.mathworks.com
[11] Haidar M, Ghimire R, Rizzo H. A, Akl R, Chan Y, Channel Asignment In an IEEE 802.11
WLAN Based On Signal to Interference Ratio, University of North Texas, USA
[12] Ratish J. Punnoose, Richard S. Tseng, Daniel D. Stancil, Experimental Results for
Interference between Bluetooth and IEEE 802.11b DSSS Systems, Carnegie Mellon
University, Pittsburgh
[13] Newton, Harold. (2007). Newton’s telecom dictionary. New York: Flatiron Publishing.
[14] ergenergen@eecs.berkeley.edu, Ergenergen M, (2010), 802.11b standards , [email],
Mahmud, 11-01-2010 IEEE 802.11 Tutorial , ergenergen@eecs.berkeley.edu, Mustafa,
University of California Berkeley June 2002.
[15] IEEE 802.11b Wireless LANs Wireless Freedom at Ethernet Speeds, technical papers
(2003) www.3com.com
[16] http://spacehopper.org/
[17] http://www.wi-fiplanet.com/
[18] www.80211n.wifinetnews.com
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6. Bibliography
[4] IEEE Standard for Information technology— Telecommunications and information
exchange between systems— Local and metropolitan area networks— Specific
requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical
Layer (PHY) Specifications, IEEE , 3 Park Avenue , New York, NY 10016-5997,
USA , 12 June 2007
[5] Aguayo, D., Bicket, J., Biswas, S., Judd, G., and Morris, R., “Link-level
Measurements from an 802.11b Mesh Network,” SIGCOMM 2004, Aug 2004
[6] Jain, K., Padhye, J., Padmanabhan, V. N., and Qiu, L., “Impact of Interference on
Multi-hop Wireless Network Performance,” ACM MobiCom 2003, September 2003
[7] Li, J., Blake, C., De Couto, D. S. J., Lee, H. I., and Morris, R., “Capacity of Ad Hoc
Wireless Networks,” ACM MobiCom, July 2001
[9] Villegas E. G, Aguilera E. L, Vidal R, Paradells J, Effect of adjacent-channel
interference in IEEE 802.11 WLANs, Technical University of Catalonia (UPC),.
[11] Haidar M, Ghimire R, Rizzo H. A, Akl R, Chan Y, Channel Asignment In an
IEEE 802.11 WLAN Based On Signal to Interference Ratio, University of North Texas,
USA
[12] Ratish J. Punnoose, Richard S. Tseng, Daniel D. Stancil, Experimental Results
for Interference between Bluetooth and IEEE 802.11b DSSS Systems, Carnegie
Mellon University, Pittsburgh
[13] Newton, Harold. (2007). Newton’s telecom dictionary. New York: Flatiron
Publishing.
[15] IEEE 802.11b Wireless LANs Wireless Freedom at Ethernet Speeds, technical
papers (2003) www.3com.com
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7. Appendices
1. Complementary Code keying:
For the optimum bit rate, two 11 bit barker codes used to be use. CCK is developed, instead
of using barker code, 64 bytes unique complex code gives much more efficiency. So up to 6
bit could be represented by any code. These sets of code have unique mathematical
property which allows correctly differentiate from one other by the receiver. It would be
achieve in presence in multipath interference and noise. [25]
In 802.11b uses CCK modulation to transmit data in a symbol consists of eight chips. Each
chip is a complex Q-PSK bit pare and the chip rate is 11Mchip per second. In 11Mbps of
transmission mode four and eight bits are modulated on eight chip of symbols, starts from c0,
c1… c7 according to the formula [25]given below.
Here bits which are modulates, they determines phi 1 to phi 4. For 11Mbps encodes 8 bits
per carrier. This speed used DQPSK modulation techniques. DPQSK uses four phases (0,
π/2, π, 3π/2 and 2π radian) to encode two bits in the identical space as BPSK encodes one.
At this position increase in power is recommended. Since FCC regulation the power in the
output is 1 watt.[25]
2.0 Eye diagram:
In digital communication the eye diagram is the oscilloscope display by which the signals
performance may be observed. This method is not 100% accurate like BER. In BER
mathematical values are expected which are accurate. But by visualising an eye diagram
accuracy cannot be reached at 100%. The figure below an example of eye diagram is given.
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Figure: an example of eye diagram [21]
Since the stream of the symbol generates from a random source, sometimes it can be
negative. So the resolution of the oscilloscope shows us some patterns which is similar to an
eye. The wider eye opening refers signal’s condition is better, noise have not effected much
to the signal. If filtering in the system is not active, eye shape will be in box shape. In the
above figure if the width of DA increases, it can be conclude that ISI decreases. There are
several measurements can be obtained by the eye diagram. They are as follows
Amplitude measurements:
[10]
:
Time measurements:
-Eye Amplitude
-Deterministic jitter
-Eye crossing amplitude
-Eye crossing time
-Eye crossing percentage
-Eye delay
-Eye height
-Random jitter
-Eye level
-Peak to peak jitter
-Eye SNR (Signal to noise ratio)
-Quality Factor
-Vertical eye opening
As the width of the mentioned area will increase, the signal will more effected by the
noise.The notations of the eye diagram of the figure [21] above are as follows:

ISI: Inter Symbol Interference

NM: Noise Margin (SNR at sampling point)

ST : Sensitivity to time error
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
Optimum sampling time

Jitter: time variance of a periodic signal
Beng Final Year Project Report
3.0 Various simulation results:
The spectrum scope in this model has a built in frequency setup characteristic. It shows
negative frequencies. In practice which is impossible. The interpretation of this dissimilar
characteristic is explained by the following figure.
Figure: interpretation of negative frequency
The figure above interprets about negative frequency shows in spectrum scope. Total
numbers of channels are 14. The middle channel is referred as zero frequency in MHz. Both
of the side channels is referred in negative and positive frequencies.
Figure: interfering signal close with wanted signal
The channel separation between two signals is 4.
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SIMULATION OF ADJACENT CHANNEL INTERFERENCE IN IEEE802.11 WLAN
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Figure: co channel simulation
The figure above is a simulation result when both channels are in zero channel separation.
As separation is zero, both channels completely overlap with each other for which BER
increases rapidly
Figure: two channel are separated by 10 channel separation
The figure above is the simulation result where both channels are separated by 10 channels.
At this situation the BER is zero. According to the theory, channels must be separated by at
least 5 channels.
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SIMULATION OF ADJACENT CHANNEL INTERFERENCE IN IEEE802.11 WLAN
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.
Figure: BER meter
The above subsystem is BER meter, this subsystem is calculates three different types of bits.
It calculates PLCP preamble and header, also PSDU. In this simulation PSDU error rate is
more important than others since PSDU frame contains all the information data.
Figure: eye diagram when channel separation is zero with 4 dB interference
The figure above shows an eye diagram when channel separation between wanted and
interfering channel separation is zero. i.e. co channel interference.
Job schedule:
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SIMULATION OF ADJACENT CHANNEL INTERFERENCE IN IEEE802.11 WLAN
School of Engineering and Technology
Beng Final Year Project Report
Gantt chart:
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SIMULATION OF ADJACENT CHANNEL INTERFERENCE IN IEEE802.11 WLAN
School of Engineering and Technology
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SIMULATION OF ADJACENT CHANNEL INTERFERENCE IN IEEE802.11 WLAN