Wireless Network and Communication: Encoding, Error Control, Multiplexing, and Wi-Fi

This article covers encoding and error control techniques like NRZ-I, Manchester encoding, and differential encoding. It also explains multiplexing techniques like TDM, FDM, and CDMA, and how OFDM is different. The article also includes a Wi-Fi network design example and calculations for data rate, CRC, and subcarrier bandwidth. Suitable for students of wireless communication courses.

WIRELESS NETWORK AND COMMUNICATION
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Part I: Encoding and error control
a) Calculate the data rate required for the robot to remote controller communication
Formulae for data rate is Number of bits passing through the network in bits per second.
Number of bits sent=Sum of individual bits
=4+6+4+2
=16
But that is in 2 seconds
therefore dividing by 2 to get in bits/second
=16/2
=8bps
b) Explain 3 types of suitable encoding techniques could be used to encode the
status string.
The robot design system could need an encoding scheme that is efficient in bandwidth utilization
and more effective for wireless transmission. The following will be more suitable for encoding the
bit strings.
1. NRZ-I(Non-Return to Zero Invert)
In NRZ-I signal encoding, we have to start from the start, all we have to do is to turn to the
opposite side if a one is found, otherwise, we don't have to do anything. If we get constantly zeros,
we have to stay on that side. This is illustrated in the diagram below,

2. Manchester encoding
It uses different curves to show whether it is a zero and another curve, the curves are shown below
zero
One
The ones are encoding with a high then lowered voltage while the zero is encoded with a high then
switched to a low voltage. This is illustrated in the example below,
3. Differential encoding
In D-man encoding, we make an inversion whenever we get one input from the high voltage to low
voltage and we make no inversion whenever we found a zero input. This is illustrated below

c)
i. Write the status string in binary for this instance
The status string is the array of the individual bits sent at the time instance.
First, converting all the values to binary notation gives the following output,
Accelerometer reading =5m/s2
Converting it to binary gives 0101 which is a 4-bit data output
Ultrasound detects an obstacle at 48cm. Converting this into binary notation yields the following
bits
110000 which is a 6-bit input
The motor status report is already in binary and it is 1111
The level of the battery is given at 75% which is equivalent to 0.75 decimal notation. Converting it
into binary, we get 0.11
The aggregated status stream is the array of the individual output data which is shown below
Status string = 0101+110000+1111+0.11
which concatenates to 0101110000111111
therefor the status string in binary is 0101110000111111
ii. Represent the status string on ASK, FSK, and PSK encoding techniques
i)ASK
In ASK, the amplitude of the carrier signal is modulated according to the value of the digital signal.
When a single bit is transmitted, Then a carrier signal is sent, if zero is transmitted then there will
be no carrier signal. In other words, the amplitude of the carrier signal will only be seen whenever
there is a one in the digital signal when zero is transmitted, there will be no carrier signal. That is
why ASK is also known as On-Off Keying.
Carrier wave is shown below

Wave, after ASK, is shown below
ii) FSK
In FSK, two different carrier signals will be used. Here we shift the carrier frequency from a
particular frequency to a different frequency when logic one or zero is sent. If you have to transmit
logic one through FSK, you will be transmitting logic one in a particular carrier frequency and for
transmitting logic zero, you will be transmitting the logic signal in a different carrier frequency
Carrier wave is shown below
The signal after FSK is shown below
iii) PSK
In phase shift keying the data or the message is conveyed by changing the phase of the carrier
signal. In PSK, the digital signal gets represented in a bipolar signal that is for two discrete values in
the digital signal zero and one, You have two different poles, for the one you have +v and for zero
you have -v. Here whenever there is a change in bit transmitted either from logic zero to logic one
or logic one to logic zero, then we change the phase of the carrier signal by 180o, and when there is
no change in digital input, no change is done to the carrier signal
Carrier wave is as shown below

Signal after PSK
d) Calculate the CRC for the status string derived inc) with the polynomial divisor
11001101
CRC is a technique used for error detection, we are given a data word to be sent and a divisor
First, we append a few bits typically zeros to the data word bits. The number of zeros appended
should be one less the number of bits in the divisor
data word=0101110000111111
divisor=11001101
therefor number of zeros to append to data word=8-1
=7 zeros
After getting the CRC and we append it back to the word, then the remainder should be zero. So
basically in the beginning, if we divide the word by the divisor and we get some non-zero
remainder, it means there is an error which happened while transmitting. And if we get a zero
remainder, it means there is no error
The calculation is shown below,

e) Briefly explain some other error control and flow control techniques that Sam
can use to ensure the accuracy of robot control
Flow control
Flow control will allow the sender to always follow the receiver hence the name closed loop
protocol, which means the sender will always send packets only if the receiver is in a position to
receive it. The following flow control methods are suitable for the robot design
1. Stop and Wait for flow control
This is the simplest form, the sender sends frame number one, and the frame number one will get
received by the receiver, Once the frame number one is processed by the receiver, it will ask for
frame number two by sending acknowledgment number 2. The sender sends frame number two, the
receiver shall receive frame number two. Once the receiver is done with processing frame number
two, it will send acknowledgment number three requesting for the sender to send frame number
three. In a nutshell, the sender sends one packet and then stop and wait until the receiver sends an
acknowledgment signaling it has finished processing the frame and the sender shall send the next
frame. The main reason why it is suitable for the robot design is that it is simple to design and
implement. The illustration below summarises the concept

The only problem with the above method is the delay time for the acknowledgment to be sent and
this lowers the efficiency. Another suitable approach to work around the issues is to have the sliding
window protocol in use.
2. Sliding windows flow control
The sliding window is the theoretical concept that can be implemented using the following way
GBN(Go Back N)
The basic idea behind this scheme is that we use a sliding window, meaning sender can send frames
without receiving acknowledgment but once the source realizes one of the frames was not received
or was received in a gabbled form, it will resend, not only that frame but also the frames that follow
it. This is true even if the destination actually received some of those frames
The following are critical in the design of Go Back and N, first the sender window size in going
back N should be greater than 1. If it is one then that is just a stop and wait for protocol and will
have no meaning.
The following illustrates the concept

Error Control
If data is being transmitted from the transmitter to the receiver, there is a channel. And in this
channel, there is much possibility of the noise being added to the data being sent. This will distort
our data and that we don't want. Therefore error detection and correction is very important in data
communication. The following methods of error control apart from CRC could be used in the robot
design.
Hamming code
This was developed by R.W Hamming and one of the very easy to implement. Mostly a seven-bit
Hamming code is used but one can use any number of bits. The following rules are applied to get
the bits.
The parity bits and the data bits are sent together. The parity bits are used to detect the errors. For
example, if the data bit is 4 and parity bit is 3, the Hamming code will be 7 bits.
Decide the position of the data bits and the position of the parity bits (Chang, Lin, & Chung, 2012).
This can be done by following this formula
2n {where n=0,1,...n} will be the position of the parity bits
D4 D3 D2 p3 D1 p2 p1
7 6 5 4 3 2 1
20=1 (Position 1 is parity bit)
21=2(Position 2is a parity bit)
22=4(Position 4 is a parity bit)
23=8(Position 8 is a parity bit but we don’t have position 8)
The remaining positions shall be a data bit
The arrangement above is the Hamming code and will be sent to the receiver to do the analysis for
error detection by checking whether the bits are even or odd depending on the parity bit.
2. Backward error control
FEC is a way to improve link performance
The receiver will check the error, detect the error then it will send information to the sender and ask
the sender to retransmit the frame once again a process known as an automatic repeat request

Forward error control
This is based on the received data and not a retransmission of data. Once the error has been detected
using the various algorithm, the receiver can use the various additional bits also known as parity
bits to perform error correction. The algorithm uses the principle of Hamming distance to gauge the
probability of which bit flip to make to correct the error (Ahmad & Habibi, 2008).
Part II: Multiplexing and multiple access
a)
Whatever radio spectrum we have, it is always of limited bandwidth. Every subscriber should get a
chance to use the limited bandwidth. Multiplexing is used to allow many mobile users have access
to the limited spectrum. The various methods are explained below
a) Explain TDM, FDM and CDMA multiplexing techniques with suitable diagrams
FDMA
In FDMA, Every user can use the same frequency band at the same instance of time. Whenever we
have a base station, it can take any number of users at the same time but use different frequencies.
In a nutshell, multiple frequencies are accessing the same base station in the same instance of time.
The only drawback to this setup is whenever we have frequency slots adjacent to each other, There
will be a chance of interferences called co-adjacent channels interference. To avoid that, for every
frequency slot we give a guard band. These guard bands are not in any way participating in the
transmission of data This is illustrated in the diagram below.

TDMA
In TDMA, the users are divided with respect to the time slot. All the users are accessing the
frequency of the band but at different time intervals. The only challenge in this setup is that there
must be proper synchronization with the transmitter\
CDMA
In this method, a user is allocated a unique sequence and the transmitter encodes the user data with
its unique code and then the receiver, the receiver decodes the data by using the unique code for the
particular user. It uses the spread spectrum algorithm. Every user in CDMA gets a unique code
which is known as chipping sequence. The codes are orthogonal, which means if these codes are
multiplied together, it will give zero. The code is multiplied at with the data at the transmitting point
a process known as spreading and the data plus the code get transmitted together. At the receiver,
the reverse process, de-spreading is used to multiply the code with the received input signal.

b) Main air interface in the change from 3G to 4G and WiMAX. Explain how
OFDM is different from above techniques and the special feature of signals
used by OFDM
Orthogonal frequency division multiple access is like the traditional frequency division multiple
access, but the carriers are all generated by a single transmitter in a special way that allows them to
be squeezed closer to each other and span a much higher bandwidth, up to 200MHz. This means we
can reduce or eliminate the use of guard bands and pack more bits per hertz hence increasing the
efficiency without much change to the existing systems.
This is possible due to the special features in OFDM.OFDMA subcarrier signals are rectangular
pulses in the time domain and they have sine(f)/(f) or sync(f) in the frequency domain. This set up
makes them orthogonal hence non-interfering with each other.
c
i. For 48 subscribers what should be the subcarrier bandwidth ( fb)?
FO=B/N
Fo= Fundamental frequency
B=The total bandwidth

N=Total number of subcarriers
Fo=?
B=40
N=48
B=Fo/N
=40/48
=0.833MHz
= 833KHz
ii. Propose a suitable subcarrier bit time of T to achieve orthogonality
T=1/B
=1/0.833
=1.2s
iii. Explain how OFDM overcomes the issue of intersymbol interference (ISI)
OFDM signals exploit the property of orthogonality that allows them to be closely packed together
without the ISI issue. This removes the use of guard bands in reducing orthogonality.

Part III: Wi-Fi
a) Design the network specifying the locations of access points and distributions system
showing the backbone network.
Room 1
Room 2
Lounge and
Reception
Room 3 Room 4
Room 5
10 m
10 m
10m
10m
10m

BSS=Area covered by each wifi hotspot
Area room 1=L*W
=10*10
=100m2
Area room 2=L*W
=10*10
=100m2
Area room 3 =L*W
=10*10
=100m2
Area room 4=L*W
=10*10
=100m2
Area room 5= L*W
=10*10
=100m2
Area lounge =L*W
=20*10
=200
ESS=Set of all the area covered by the individual wifi access points
=(100+100+100+100+100+200)
=500+200
=700m2
c) Calculate the throughput for the DS
Throughput=Sum of individual throughput
8*100=800(Each room throughput)
for five rooms=5*800

=4000
Add the lounge 25*100
=2500
Total throughput =4000+2500
=6500mbps
d) Recommend a suitable IEEE substandard for the network and give reasons for
your choice
The best-recommended IEEE substandard will be the 802.11ac that provides higher data rate and
gives a high number of users per channel band contrary to the earlier standards that provided a
limited number of active connection.
e) Recommend suitable security strategies for the network
Details about our personal lives are being beamed out over wifi including our financial transaction,
Security the networks has become a huge priority
Secure the wifi with a strong password to ensure all access to the network is secured and encrypted.
Use WPA2 wireless encryption standards to increase the security of the data while in transit. WPA2
encryption is less vulnerable than WEP and WPA. WPA2 set it mandatory to use the AES
encryption method
Turn WPS convenience button for the business network
References
Ahmad, I., & Habibi, D. (2008). A proactive forward error control scheme for mobile WiMAX
communication.
Chang, R. Y., Lin, S.-J., & Chung, W.-H. (2012). New space shift keying modulation with
Hamming code-aided constellation design. IEEE Wireless Communications Letters, 1(1), 2–
5.