Encoding and error control

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).

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

Bits sent per second= data rate

Bits sent on two seconds  = 2+4+6+4

=16 bits  

16 bits is in the Interval of 2 seconds so in one second  it will be 8bits.

1. b) Explain 3 types of suitable encoding techniques could be used to encode the status string.

Non return to zero 

in this technique one level of voltage zero corresponds to another which corresponds to One

Pseudoternary

Involves encoding the ones with flat lines and zeros with negative and positive voltages alternating

Non return to zero invert

Encoding depends on the bit that is before. It should have some starting point

1. 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%.  
2. Write the status string in binary for this instance

Status string= readings in accelerometer + motor status functionality + battery level status + obstacle detection in ultrasound reading

Binary convertion

Ultrasound= 48cm

Binary = 110000

Acceleration = 5m/s²

 Binary = 0101

Motor status = 1111

Level of the battery 75/100 which is 0.75

Binary = 0.11

Combined status string will be a sequence of the individual strings status

= 0101+110000+1111+0.11

= 0101110000111111

1. Represent the status string on ASK, FSK, and PSK encoding techniques

FSK

FSK is the switching of the sinusoidal career wave frequency, depending on the digital input signal

Bit string = 010111000011111

FSK is achieved through changing the frequency of the sinosidal carrier according to the digital input signal 

ASK

Bit string = 01011100001111

Carrier wave is transmitted when the input is one but it is not transmitted when the input is 0

PSK

A phase shift of 180 degrees takes place whenever there is a change in the input signal e.g from 1 to 0 or from 0 to 1 

1. d) Calculate the CRC for the status string derived in c) with polynomial divisor 11001101

First step

Input bit sequence = Data word

010111000011111 =

No of divisor digits -1= additional digits

                  8-1 = 8

  7

Second step

The seven digits are added to the data word to form augmented dataword

Augmented data word= data word + additional digit

                                  = 0101110000111111+ 0000000

                                   =  01011100001111110000000

Third step

Augmented data is changed to polynomial

01011100001111110000000

X²¹+X²⁰+X¹⁹+X¹⁸+X¹⁶+X¹⁵+X¹⁴+X⁹+X⁸+X⁷+X⁶+X⁵+X⁴

Forth step

The divisor is changed in to polynomial

11001101

X⁷+X⁶+X³+X²+1

Fifth step

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

Sliding window flow control

The sender and receiver need to have a window of frames. Every particular frame is numbered according to the sliding window for a window of size p , the number 0 to p-1 is given to the frame. Many frames can be sent by the sender as long as it fits in a window. A slide is when a receiver sends an acknowledgement after receiving the frames sent up to a certain point. For example p=8  the size of the window is (p-1) 7 .

Stop and wait Flow control

For every frame that has been sent the sender has to wait for an acknowledgement because the other frame can only be sent after the frame has been received.This process goes on until the sender sends an (EOT)End of Transmission frame . 

Types of Suitable Encoding Techniques

Vertical Redundancy Check (VRC)

Parity bit which is aredundant bit  is applied in every data unit in order for the total number of 1’s to be even. 

Checksum

Two algorithms are in place the checksum generator at the receiver and checksum generator at the sender. The following steps are followed by the sender the data units are divided in to sections then units . The sections are then added using 1’s compliment so as to get the sum. The checksum is achieved when the sum is completed after that the data is sent with the checksum. The unit received is divided into sections each of specific bits therefore an addition is done of the section using 1’s complement to get the sum the end of the calculation the data is only accepted if it is zero. (Thomas, Sharma, Banerjee, Lee, & Dalal, 2013) 

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 various applications.

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

TDM(Time Division Multiplexing)

This is a technique which allows the whole frequency bandwidth to be accessed but for a short period of time. Co-channel interference occurs when two different transmissions overlap

FDM( Frequency Division Multiplexing)

FDM allows the channel to occupy a fraction of the frequency. Channels are defined through the center of the bandwidth and the frequency. Example of this technique is radio and television signals (Holma & Toskala, 2009)

CDMA(Code Division Multiple Access)

Channels sharing frequency simultaneously occurs in this technique. An example of CDMA is GPS(Global Positioning System). CDMA uses analog to digital conversion together with spread spectrum technology (Palanki, Khandekar, & Sutivong, 2014) .

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 has better performance and tolerance furthermore it allows more subscribers in, multiple cellular application. The signal used in OFDM is subdivided into small chunks called sub-carriers which are supposed to be orthogonal during transmission this gives room for consistency and reach of the waves in real time (Hwang, Yang, Wu, Li, & Li, 2009).

1. c) IEEE 802.11ac wireless LAN standard uses 40 MHz total bandwidth in OFDM scheme.
2. For 48 subscribers what should be the subcarrier bandwidth ( fb ) ?

 fb= allowable number of sub carriers

Channel bandwidth = 40MHz

Allowable subcarriers = 128 sub carriers

fb=40MHz/128subcarriers

= 0.3125MHz

= 312.5 KHz

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

 T= 1/f(b)

fb= 0.3125

T=1/0.3125

=0.0032 micro seconds

iii. Explain how OFDM overcomes the issue of inter symbol interference (ISI)  

It modulates data in the frequency domain over short subcarriers and this results to long OFDM symbols which are longer than the delay spread. A cyclic prefix which has an appropriate length is chosen to avoid inter-OFDM-symbol interference (Thomas et al., 2013)

Part III: Wi-Fi

1. a) Design the network specifying the locations of access points and distributions system showing the backbone network.

1. b) Calculate the BSS and ESS sizes

Room 1

Area= L*W

=10*10

=100m²

 BSS size=100m²

Room 2

Area= L*W

=10*10

=100m²

 BSS size=100m²

Room 3

Area= L*W

=10*10

=100m²

 BSS size=100m²

Room 4

Area= L*W

=10*10

=100m²

 BSS size=100m²

Room 5

Area= L*W

=10*10

=100m²

 BSS size=100m²

Lounge and reception

Area= L*W

=20*10

=100m²

 BSS size=100m²

Extended Service Set(ESS)

ESS= Total coverage area of all the access points connected to the Distribution switch

=Area(Lounge+room5+room4+room3+room2+room1)

= (200+100+100+100+100+100)

=700m²

1. c) Calculate the throughput for the DS

 The actual data passing through the network per second

For every room throughput(100*8)mbps

=800mbps

The rooms are five so(800*5)

=4000mbps

Lounge thoughput (100*25)

=2500mbps

Total throughput = 4000+2500

= 6500

1. d) Recommend a suitable IEEE substandard for the network and give reasons for your choice

 IEEE wifi is good for the above setup that is IEE 802.11ac because it allows video streaming , the buffering is less and surfing will be faster.

1. e) Recommend suitable security strategies for the network

Use only wpa2 encryption, WPS to be turned off, make a point of changing Access point password.

References

Holma, H., & Toskala, A. (2009). LTE for UMTS: OFDMA and SC-FDMA based radio access. John Wiley & Sons.

Hwang, T., Yang, C., Wu, G., Li, S., & Li, G. Y. (2009). OFDM and its wireless applications: A survey. IEEE Transactions on Vehicular Technology, 58(4), 1673–1694.

Palanki, R., Khandekar, A., & Sutivong, A. (2014). Code division multiplexing in a single-carrier frequency division multiple access system. Google Patents.

Thomas, D. A., Sharma, P., Banerjee, S., Lee, S.-J., & Dalal, A. C. (2013). Method and system for content downloads via an insecure communications channel to devices. Google Patents.