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Part I: Encoding and error control
Sam is a computer science student and working on a new micro robot design. The robot continuously sends every two seconds a status string comprising the accelerometer reading (4 bits), ultrasound obstacle detection (6 bits), motor functionality (4 bits) and
battery power level (2 bits).
a) Calculate the data rate required for robot to remote controller communication
b) Explain 3 types of suitable encoding techniques could be used to encode the status string.
c) At a certain instance, the sensors provided data as acceleration 5 m/s2, obstacle at 48 cm, all motors working (i.e. motor functionality 1111), and battery is 75%.
i. Write the status string in binary for this instance
ii. Represent the status string on ASK, FSK, and PSK encoding techniques
d) Calculate the CRC for the status string derived in c) with polynomial divisor 11001101
e) Briefly explain some other error control and flow control techniques that Sam can use to ensure the accuracy of robot control
Part II: Multiplexing and multiple access
Multiplexing plays an important role in communication as it allows the combination of multiple streams together. Different multiplexing techniques are used for variousapplications.
a) Explain TDM, FDM and CDMA multiplexing techniques with suitable diagrams
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
c) IEEE 802.11ac wireless LAN standard uses 40 MHz total bandwidth in OFDM scheme.
i. For 48 subscribers what should be the subcarrier bandwidth ( fb ) ?
ii. Propose a suitable subcarrier bit time of T to achieve orthogonality
iii. Explain how OFDM overcomes the issue of inter symbol interference(ISI)

## Part I: Encoding and error control

Part I: Encoding and error control

1. a) Calculate the data rate required for robot to remote controller communication

Data rate= number of bits sent/ second

Total number of bits send = 16

Time= 2seconds

16/2

8mbs

1. b)Explain 3 types of suitable encoding techniques could be used to encode the status string.
2. Bipolar AMI encoding: a no line signal represent a zero and a one is represented by positives and negatives.  In this technique, there is no loss of synchronization with a long string ones (Maeda, 2014)
3. Manchester encoding: there is a transition at the middle of one bit. low to high represent a one while high to low shows a zero. The transition serves as a clock and data. This encoding is used in 10Mbps Ethernet (Amiri & Ali, 2013)
• Pseudo-ternary encoding:  One is represented by the absence of a line signal. An alternating negative and positive indicates zero. There is no loss of synchronization with a long string of zeros.

c)

1. Write the status string in binary for this instance

Convert  all the parameters  to a binary

5m/s = 0101

48cm = 110000

Motor functionality = 1111

75%= 0.11

Combine all the binary to form one string

0101110000111111

1. Represent the status string on ASK, FSK, and PSK encoding techniques
1. d) Calculate the CRC for the status string derived in c) with polynomial divisor 11001101

Dividend= 0101110000111111

Divisor = 11001101

Step 1

= divisor no -1

= 8-1

= 7 units.

Step 2

Add additional digits to the data word to form the argument data word

= 01011100001111110000000

1. Briefly explain some other error control and flow control techniques that Sam can use to ensure the accuracy of robot control

Flow control techniques

1. Stop and wait control

When a frame is sent the sender wait for an acknowledgment. When the acknowledgment s received then the next frame is sent.. the process continues till the end of the transmission. In this techniques, the receiver indicates when its ready to receive data in each frame(Gomez, Demirkol, & Paradells, 2011).

1. Sliding window flow control

Both the receiver and the sender have a window frame. Frames are numbered according to the sliding window. Many frames from the sender as many as the window can hold are sent. the receiver when it has received enough frames sends an acknowledgment all frame slide. Once the acknowledgment is sent, the receiver indicates the number of next frames it expects to receive(Zhang, Zhang, Ma, & Kan, 2010).

Error control techniques

1. Error detection:   it uses the redundancy concept that is adding extra-bits to detect errors at the destination. when the checking function performs the error detection action on the stream of the bit, the data portion is either accepted or rejected.
2. Vertical redundancy check: this technique a parity bit which is called a redundant bit is appended to every unit of data. This makes the number of the 1’s together with the redundant bit become even. If the bits are already even the parity nit added is a 0 (Wu & Sutardja, 2011)

Part II: Multiplexing and multiple access

1. Explain TDM, FDM and CDMA multiplexing techniques with suitable diagrams
2. TDM

is it a method for transmitting and receiving an independent signal over a path. It allows sync of switches on each end of transmitting line. method of transmitting and also receiving an independent signal over a path of the signal by synchronizing switches at each end of the transmission line. It is used when the bit rate(Yazdani, 2008)

used in digital signals and also be applicable in analog multiplexing. The domain of time is divided into subsequent slots of time which have a fixed length.  A TDM frame is made up of, one-time slot per subchannel and a synchronization channel alongside error correction, and synchronization.  One cycle  does error correction and also synchronization(Elliott, 2011)

1. FDM

Frequency Division Multiplexing was used for connections over a long distance to the multiplex thousands of voice signals via a coaxial cable system. It is an analog multiplexing technique which combines continuous signal. It is used when the combined bandwidth of the signal being transmitted is less than the link bandwidth(Glisic & Leppänen, 2012)

Signals are produced by sending diverse device- modulated carrier frequencies and the modulated signals are put together to form one signal which is transmitted over the channel.

1. CDMA

## a) Data rate calculation

This technique is pure digital referred to as spread spectrum. it spreads a digital form of an analog signal over a wide bandwidth at a lower power level. This is called direct sequence spread spectrum (DSSS). The compressed digitized voice signal in serial form is transmitted by processing it in an XOR together with a chipping signal at a higher frequency. It is 95 standards, A 1.2288MBIT/S and transmits at 13kbits/s. (Yazdani, 2008)

A pseudo-random code generates a  signal which it assigns a unique code to the users within the channel. The code is transmitted over a bandwidth of 1.25MHz. The produced signal is a low powered level appears like noise.

1. 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

OFDM system has the effective bandwidth.

The system mostly has peaked to average power ration.

It provides a higher data rate and is used for broadband internet service.

OFDM has more interference on the multi-path which can be reduced by use of the high-end algorithm.

The bandwidth is divided into many narrow-band channels and each is allocated to subscriber hence it supports more channel(Srikanth, Pandian, & Fernando, 2012)

1. c)
2. For 48 subscribers what should be the subcarrier bandwidth ( fb)?

F(b)= the bandwidth of the channel/number of subcarrier allowed

The bandwidth= 40MHZ

sub carriers=128

F(b) =40MHZ/128 subcarrier

=0.3125MHz

=312.5KHZ

1. Propose a suitable subcarrier bit time of T to achieve orthogonality

Bit time = 1/fb

Fb = (312.5khz)

=1s/312.5Khz

Bit time = 0.0032 microsecond

iii. Explain how OFDM overcomes the issue of inter-symbol interference

The OFDM solve the problem of inter-symbol interference by modulating the data in the frequency domain over the short sub-carriers and this make the OFDM long enough, it is longer than the delay spread. To avoid inter OFDM symbol interference a cyclic prefix of appropriate length chosen.

Part III: Wi-Fi

1. a) Design the network specifying the locations of access points and distributions system showing the backbone network.
 Room Capacity of Users Bandwidth/user in Mbps Aggregate bandwidth in Mbps Room 1 8 100 800 Room 2 8 100 800 Room 3 8 100 800 Room 4 8 100 800 Room 5 8 100 800 Lounge and reception 25 100 2500 total 65 100 6500
1. Calculate the BSS and ESS

BSS(Basic Service set) is the area covered  by an access point

Room 1 Room 2

Area= length *width Area= eighth *width

= 10*10 = 10*10

=100m2 =100m2

Room 3 Room 4

Area= length *width Area= length *width

= 10*10 = 10*10

=100m2 =100m2

Room 5 lounge and reception

Area= length *width Area= length *width

= 10*10 = 20*10

=100m2 =100m2

ESS is the total area of the covered by the various BSS

ESS=(Room1 +room2+Room3+Room4+Room5+Lounge)

= 100+100+100+100+100+200

=700m2

1. Calculate the throughput for the DS.

The actual data passing through  the network/ second

For each room throughput= 8*100mbps

The rooms through put = 5*800mbps

Lounge and reception throughput = 25*100mbps

Lounge +rooms = 2500+4000

= 6500mbps

1. Recommend a suitable IEEE Substandard for the network and give a reason for your choice.

IEEE 802.11ac. this substandard is the next generation for WIFI. It will give three times faster connection that the substandard IEEE 802.11b/n. this WIFI standard allows for use of multiple users. It offers fast surfing, smooth video streaming and less buffering (Ong et al., 2011).

1. Recommend suitable security strategies for the network

The name of the default network should be changed that is the SSID.

The password should be strong unique and secure for the wireless network.

Use WPA2 encryption for the network for additional security.

Turn off the Wi-Fi while not in use. (Aggelou, 2008)

References

Aggelou, G. (2008). Wireless Mesh Networking. McGraw-Hill Professional.

Amiri, I. S., & Ali, J. (2013). Data signal processing via Manchester coding-decoding method using chaotic signals generated by PANDA ring resonator. Chinese Optics Letters, 11(4), 041901.

Elliott, B. B. (2011). Time division multiple access for network nodes with multiple receivers. Google Patents.

Glisic, S. G., & Leppänen, P. A. (2012). Code division multiple access communications. Springer Science & Business Media.

Gomez, C., Demirkol, I., & Paradells, J. (2011). Modeling the maximum throughput of Bluetooth low energy in an error-prone link. IEEE Communications Letters, 15(11), 1187–1189.

Maeda, T. (2014). Information processing apparatus, encoding method, and a frame synchronization method. Google Patents.

Ong, E. H., Kneckt, J., Alanen, O., Chang, Z., Huovinen, T., & Nihtilä, T. (2011). IEEE 802.11 ac: Enhancements for very high throughput WLANs. In Personal indoor and mobile radio communications (PIMRC), 2011 IEEE 22nd international symposium on (pp. 849–853). IEEE.

Raghavan, S. H., Holmes, J. K., & Maine, K. P. (2009). Code division multiple access enhanced capacity system. Google Patents.

Srikanth, S., Pandian, P. M., & Fernando, X. (2012). Orthogonal frequency division multiple access in WiMAX and LTE: a comparison. IEEE Communications Magazine, 50(9).

Wu, Z., & Sutardja, P. (2011). System and method for correcting errors in non-volatile memory using product codes. Google Patents.

Yazdani, N. (2008). Multi-Frequency Time-Division multiple-access (MF-TDMA) resource packing. In Military Communications Conference, 2008. MILCOM 2008. IEEE (pp. 1–8). IEEE.

Zhang, D., Zhang, R., Ma, J., & Kan, Z. (2010). Method, system and device for controlling a transmission window size. Google Patents.

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